Background
for Telephone Switching
2nd Edition (Revised and Expanded)
Chapter 5
User Oriented Features
OUTLINE
OBJECTIVES:
The objectives of this chapter are:
-
To describe the principal user
features of modern telephone systems
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To show the differences in
business and residential features
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And to organize features in
certain categories to simplify understanding them.
PREVIEW QUESTIONS:
As you read, watch for the answers to the
following important questions:
1. How does the user tell the
phone system what feature is wanted?
2. How does the phone system tell
the caller what it is doing?
USER
ORIENTED FEATURES
The application of stored program
control to central office switches in the mid 60s and to PBXs a
decade later, coupled with cheap RAM and ROM memory to allow almost
infinite program storage, led to an eruption of supposedly new
features that is still going on. Actually, few of these features are
new; what has come about is a variety of different ways to provide
older features that had either been done manually, in switch
hardware, or in special telephone sets. Whether the features are new
or not, the stored program approach to meeting long-standing needs
has turned out to be nothing less than revolutionary.
Because more than 70% of the telephone
lines in the United States are residential, it is not surprising
that most CO switching system designers have concentrated on
residential telephone service and have ignored the much more
interesting world of business communication. Even a small PBX must
provide many features not required by residential lines; these
usually come about because business phones tend to serve groups of
people who use the telephone to facilitate the way they work
together. Residential phones, in contrast, are usually associated
with a single line used by one person at a time. In this chapter, we
will consider features in certain logical groupings so that their
fields of applicability, for both individuals and groups, can be
appreciated.
User Features and Telephone Sets
Telephone sets, the terminals used by
subscribers not only to talk to one another but to signal a
switching system's control, went through various developments in
connection with system features. Vast amounts of energy were
expended in efforts to control these features with inexpensive 2500
type telephone sets. Although partially successful in residential
service, more satisfactory solutions emerged for business
situations.
Single line sets.
Providing a multiplicity of features such as hold, park, camp-on,
transfer, add on, forward, etc., etc., was relatively easy; the
difficulty lay in finding a way for the caller to tell the system
what was wanted. The switch-hook flash, described in Chapter 3, was
the basis of certain common features. To deal with call-waiting, to
be described in detail below, the user flashed the switch-hook to
put the existing call on hold and access the new incoming call.
Features like consultation/conference/transfer also used the flash
to put an existing call on hold, but returned dial tone to the
flasher to indicate a new call could be originated.
To go beyond this, "feature codes" had
to be assigned to features so that the caller could dial them up as
though they were a telephone call. Almost immediately, PBX systems
began boasting of a hundred or more features, ignoring the fact that
users might not care to memorize so many feature codes. To reduce
human memory requirements, the Rolm Corporation had telephone sets
built for its PBXs with feature names and codes inscribed on the set
itself, as shown in Fig. 1. To put a call on hold, one would flash
the switch-hook and dial *9. To reconnect to the held call, *1 would
be used. Other manufacturers used plastic overlays for dials or key
pads to assist the user in remembering the codes for the most common
features.
The "stupid" flash to active the
"smart" signaling of the keypad or dial was harder for subscribers
to use than PhD system designers in their ivory towers had imagined.
As a result, a flash button, which sent a flash of fixed duration,
whether the caller pecked it or held it down, was quickly made an
optional feature, particularly for phones intended for use with PBX
and Centrex systems.
The next addition was a message
waiting lamp. Although the message lamp had been available for years
with Hotel/Motel telephones, system designers finally conceded that
message centers existed in most other kinds of enterprises, and that
the value of a message center was greatly enhanced if users were
made aware of messages waiting to be picked up. Traditional message
waiting lamps for 2500 phones were neon bulbs that drew a very small
current but required a fairly high voltage to operate; current
practice is to replace neon bulbs with light emitting diodes (LEDs).
In either case, a special line card is needed in most PBXs to
activate the indicator, although some PBXs use approaches such as
applying the message waiting signal via the ringing access
circuitry, making an extra line card unnecessary.
Finally, cheap memory made it possible
to store telephone numbers economically within the telephone set
itself, in spite of arguments favoring economies of scale when bulk
memory in the switch was used. Repertory dialing swiftly became
available, allowing station users to place calls by pushing a button
associated with the desired number. After several years, it dawned
on the industry that repertory dialers could send feature codes as
easily as telephone numbers, and several families of "feature
phones" sprang up. In addition to digits, a "number" stored in the
phone's memory could also include a switch-hook flash and an ability
to detect dial tone. Thus the push of a button might activate a
feature code consisting of Flash/detect dial tone/*/9. To save
money, a pause was often substituted for the ability to detect dial
tone before sending digits. Programmable feature phones could work
on any make of PBX, but some feature phone manufacturers made up
special phones with feature codes for specific PBXs stored in ROM.
This eliminated the need for programming, or re-programming after
power failure.
Even with such virtuosity, there were
many basic telephone functions which single line phones were
incapable of handling. For example, they could not help a secretary
screen a call for a principal who was busy on another line,
communicate the nature of the new call to the principal, and then
allow the latter to continue with the existing call or go to the new
one.
Key telephone systems.
Although not generally realized by switch designers developing their
multiplicity of features between 1970 and 1985, it was unusual for a
PBX or Centrex system to use single-line telephone instruments.
Typically, more than 90% of PBX and Centrex lines terminated on key
telephones which provided the features business customers needed.
Although several sources suggest that, in many PBX and Centrex
installations, as few as 30% of the station users actually needed
key telephone features, in that 30%, key was hard to shake. The cost
of key systems, plus the cost of making moves and changes, was
considerable, but there was no way single line sets, even with
infinite features, could facilitate secretarial help with screening
incoming calls and placing outgoing calls for principals.
To solve this problem, telephone
companies and interconnect vendors alike engaged in an extensive
public relations campaign insisting that executives should place and
answer their own calls. They were not particularly successful.
Direct inward dialing increased the need for secretarial screening,
and even without it, few business telephone customers were
interested in giving up perks to make life easy either for telephone
designers or their own support personnel.
Key telephone equipment, usually
referred to as 1A2 after the designation used in the days of the
Bell System, provides relatively simple functions such as putting
one line on hold while answering another, operating a buzzer to
signal a secretary, etc. Key systems place no demands on the
associated PBX or CO switch (other than the release of the holding
bridge, discussed in Chapter 3). They allow one telephone to pick up
several lines and one line to be answered by several telephones.
They provide a visual display to show the status of each line: idle,
ringing, talking and on hold. They permit conferencing when several
phones pick up one line at the same time, and they also allow one
telephone to pick up two lines at once by the simple (but not
recommended) expedient of removing the mechanical interlocks that
normally permit only one button at a time to be depressed. Separate
intercoms are common key telephone features, and intercom signaling
is distinctive when compared with the regular telephone bell.
Intercom signaling, buzzers controlled by the dial or push buttons,
can also be used for other purposes.
Fig. 2 shows a highly simplified
schematic of a 1A2 key telephone system. In such systems, wires were
traded for sophisticated circuitry, with 25 pair cables serving
common six-button sets. Each line from the PBX is a single pair
which terminates in its own line control equipment called a KTU for
Key Telephone Unit. KTUs, along with the cross-connect field that
allows the cable from each phone to be associated with various lines
and features, are housed in a KSU, or Key Service Unit, usually
located with the PBX if the area served by the PBX is small, or in a
wiring closet near the telephones if the area served by the PBX is
large.
From a human factors standpoint, key
telephone systems evolved over a 30 year period into something close
to perfection; users have no difficulty in mastering their
operation, even without formal training. The key system works the
same way on all calls, being independent of the switching equipment;
intra-switch calls, incoming calls, outgoing calls, tie-trunk calls
are all handled the same way by the user. It is not unusual to have
key systems without a PBX, but a PBX without the functions provided
by key systems is nearly unusable in the business world.
Key systems can be arranged in many
ways, but two of the most common patterns are boss-secretary and
principal-plus- assistants. In the former, two lines are shared by
the boss and the secretary; they are in hunt so that if the boss's
line is in use, an incoming call will complete to the secretary's
line. If the secretary's line rings when the boss's line is idle,
the call was intended for the secretary in the first place. The
secretary can screen incoming calls and handle their disposition
over an intercom; the secretary can also establish outgoing calls
for the boss and again use the intercom to report that the call is
ready. Either boss or secretary can always put a given call on hold
to use the intercom; both lines can be on hold at the same time, if
desired.
In the principal-plus-assistants
pattern, three or more lines are in hunt. All are available to the
assistants who normally answer and try to deal with incoming calls,
and also to the principal who is only called in (via intercom) when
necessary. The principal can pick up the several lines, but will
often have a private line, unavailable to the assistants, which may
be one of a boss-secretary pair used as described above. With a
number of lines and perhaps two intercoms, one to the group and one
to the secretary, and line buttons with visual displays, key
telephone systems make many complex procedures simple.
A third pattern, often used by small
businesses in place of a PBX, is to have all lines appear on all
phones. Key telephones with 10, 20, or more buttons are available,
and any line can be answered at any phone. A receptionist usually
answers incoming calls and uses an intercom to announce them to the
called parties. Paging is often made part of such a system,
augmenting the intercom, enabling the called party to pick up
wherever he or she may be. A major advantage of this kind of system
is that a user can put a call on hold and pick it up again at the
same or any other phone. Note that systems of this sort never
transfer calls. Different people on different phones can connect to
a given line by simply depressing that line's button on their phone.
Electronic key telephones.
All things considered, electronic key telephone sets appeared on the
scene none too soon. Early system trials showed electronic key
systems to be uneconomical in stand-alone form, but when a stored
program PBX system took on the functions of the KSU, the picture
changed drastically. Ultimately, of course, the cost of control
electronics dropped low enough to allow very small PBXs, disguised
as electronic key systems, take over where 1A2 had once reigned
supreme.
In passing, it should be noted that
today there is no difference between electronic key systems and
PBXs. Both have a switching matrix, a control, line cards for lines
to individual telephones, and trunk cards to the serving switch. In
1A2, however, there was no switching matrix; the desired line was
selected by the called party who operated switch contacts associated
with the line button in the set itself. As a result, line numbering
was a function of the switch on which the key system homed. Only
when a separate intercom was included (as it often was) did an
individual telephone set have an identity independent of the serving
PBX or CO. In an "electronic key system," all phones have (but may
not choose to use) an identity similar to the 1A2 intercom station.
The makers of stand-alone feature
phones, by bringing two or more lines to their sets and adding a
hold feature and sophisticated electronic lamping, are competing
with PBX-like electronic key systems by offering "KTU-less" phones
which look to PBXs and CO switches like regular 2500 sets but which
provide most key system features.
The general approach in all electronic
key telephones (other than the above KTU-less sets) is to provide
one voice channel and a separate signaling channel between the set
and the line-card. Line (or feature) buttons send signals to the
system control, and the control sends back signals to light lamps or
operate other visible or audible displays. Actual talking
connections are made by the matrix itself, connecting a particular
call to the set's talk path. When a second person wants to join the
call, as would be done with 1A2 by simply pushing the button for the
active line and bridging on, the user still pushes the button but
the switching matrix combines the new phone, the old phone, and the
outside connection via a conference bridge.
Northern Telecom's SL-1 PBX in 1975
was one of the first systems on the market using an integrated
electronic set (see Fig. 3); although AT&T and several other
companies quickly followed suit, it took almost a decade for the
great majority of PBXs to fall in line and relieve customers of the
need to add ancient 1A2 key systems to supposedly innovative new
PBXs.
The original SL-1 set required three
pairs: one for voice using the standard 2-wire transmission circuit
of the 2500 phone, one for signaling and control, and a third for
the power required by add-on modules such as speaker-phones. The
third pair could pick up power at a local wiring closet, and did not
have to run back to the line card in the PBX. Power for the basic
telephone set was transmitted via the voice and control pairs; note
that the set, its control electronics in particular, was powered
independent of the switch-hook.
The basic SL-1 set had a total of 25
digital signaling buttons: 12 in the standard 3x4 array similar to
that used by DTMF and three below the signaling pad to provide
volume up and volume down on the equivalent of the bell and one to
provide the traditional hold function. The remaining ten, eight of
which were paired with LED lamps, were in a strip along the side of
the set, and could be programmed as required to pick up specific
lines or to activate various features.
Whenever any of these buttons or the
switch-hook were activated, the signaling circuit, which monitored
them, passed the information along to an interface in the PBX line
card at 2.4 kBps. Because signaling was full duplex, the PBX was
able to send back information that caused LED lamps to display which
line had been picked up or which feature was in force. The ringing
signal was a digitally generated audio tone connected via the
switching matrix to the talk path; because the answer signal came
back on the signaling path, all the ring-trip problems of
traditional systems were eliminated. Later generations of electronic
set removed ringing from the voice path and activated a ringer with
a message via the signaling channel.
The common control, upon finding a
particular button had been depressed, learned from information
associated with the matrix port (and stored in system memory) what
purpose the button served; it was then able to carry out the desired
function. Because feature and line pick-up information in the
system's memory could easily be changed by data processing
techniques, modifications could be made without visiting the set
itself, or even the PBX site. Updating labels for the buttons, of
course, required someone at the set; when more buttons were needed,
add-on units could be attached to the set without changing the
system wiring.
Northern Telecom later changed the
name of its PBXs to "Meridian," and came out with a new line of
Meridian telephones which did the analog-to-digital conversion in
the set itself. This digital voice signal was multiplexed into a bit
stream that included signaling and data; by sending half the time in
each direction, but twice as fast while sending, only a single pair
was needed between set and line card. In the same time frame, Rolm
developed the Rolmphone, again using single pair wiring but sending
digital bit streams simultaneously in both directions, separating
them at each end with hybrids and echo cancellation. AT&T and NEC,
among others, used 2-pair wiring to digital proprietary sets,
somewhat similar to the proposed wiring from the ISDN S/T interface.
ISDN sets themselves, if and when they arrive on a standardized
basis, might replace proprietary sets but, because they may require
local power, will require special measures to achieve reasonable
overall reliability.
From the user's point of view, many
systems now offer electronic telephone sets that, with their
separate signaling channel, can provide 1A2 features and a vast
array of other features as well, available at the push of a suitably
labeled button. It is possible to escape switch-hook flashes, second
dial tones, trick call progress tones, and the need to memorize long
complicated lists of feature codes that were, for a time, "popular."
Liquid crystal displays (LCDs) are now
common in electronic telephone sets, displaying prompts to help
implement complex features and other information from the switch's
common control. "Soft function keys," defined by text in the LCD,
can be used to activate a wide variety of features, changing as
needed by circumstances. Some systems even use LCDs to show the
extension number associated with a given soft key, making it
unnecessary to provide or change labels for set buttons. LCDs can
also display messages. They can, for instance, show a person busy on
the phone the name or number of a new caller trying to get through.
Names of outside callers can, in some systems, be entered via the
attendant console.
Numbering Plans for Business
The great majority of PBXs have less
than 400 lines; as a result, dialing internal connections typically
uses two or three digits, although larger systems may require four
or even five. To reach the outside world, access codes are usually
used: dial 9 for outside, for instance, or 8 for a private network.
The digit 0 is usually reserved for the console attendant.
In addition to access codes, a PBX or
Centrex numbering plan includes feature codes so conventional
telephones can access features as discussed above. Access and
feature codes must not overlap extension numbers or each other. But
these problems are relatively simple compared to those encountered
by electronic key sets. With traditional 1A2, one telephone set
would pick up several lines, and one line would usually appear on
several telephone sets. There was no particular reason why a
telephone set had to be associated with one of its lines more than
another (except for billing when sets were rented by the month), or
why a particular line might single out one of several telephone sets
to be its very own. And, of course, the switch didn't care, because
it treated each line as a separate entity, without regard to the
sets beyond the KTU. As a result, the communication manager could
have more sets than lines in one group, and more lines than sets in
another, if such arrangements were needed.
Although there have been some
exceptions, most switches which support electronic telephones
require a one-to-one match between matrix port and set so that the
bit stream between the two can maintain synchronism and the
signaling path is always available for use in either direction. That
is, two or three electronic sets are not usually bridged across the
line to the switch, and no set has two or more lines reaching
different matrix ports. Rather, there is a talk path and a signaling
path on the physical channel between set and line card.
As a result, there is no "line" to
which an extension number, dialed by a caller, must be assigned. An
extension number may appear on several phones, but each of those
phones has only one talk-path. Thus the system has to have a
different means for alerting several telephones and then, when one
of them answers, connecting the call appropriately and manipulating
the lamps or other signals on the remaining phones. Clearly, each
phone and its associated matrix port are represented by an equipment
number, but there is no longer an obvious one-to-one relation
between an equipment number and a directory number. The
relationships among a directory number which can be dialed, the
system memory for that number which includes COS and translation to
equipment number(s), and the variation of COS as a function of
equipment numbers reflecting properties of the phones on which the
called number appears can become quit intricate.
In general, most PBX designers have
insisted that each phone represent a "prime line," and, often, that
every extension number be a prime line on some telephone. The first
requirement says that there cannot be more phones than there are
extension numbers in a group (one cannot have six telephones served
by three extension numbers, for instance); this is no great problem
because extension numbers have no cost* while matrix ports, wires,
and telephone sets do. If you have the set, there is little to be
saved by not assigning it a number. The second requirement, however,
can make a great deal of trouble. It eliminates "dummy numbers"
which might be used for pilot numbers in hunt groups, parking
orbits, and a variety of other non-physical identities which a
business switch may need to address. It also says you cannot have a
second (perhaps private) number on your electronic phone.
[*Footnote: Note that telephone
companies do, however, rent seven-digit DID numbers.]
The best approach would be to allow
extension and dummy numbers to exist as memory elements which relate
to actions the switch must perform; with the dialed number, the
program can then look up the next step which may require action at
one equipment number, several equipment numbers, or at no equipment
number at all. However, it is also necessary to separate extension
and dummy numbers from access and feature codes; thus
pre-translation as well as translation is needed.
With multi-line telephone sets, both
class of service and class of restriction add interesting
requirements to business numbering plans. As will be discussed,
ringing and delayed ringing are a function of both the set and the
called extension number. Further, call pick-up may be a feature on a
secretary's phone but not the boss's, even though both phones serve
the same extension numbers. Should restriction be a function of the
phone, or the particular extension number? If the secretary's line
is restricted and the boss's is not, can the boss call on the
secretary's line? Can the secretary make a forbidden call on the
boss's line? If the set rather than the extension number carries the
restriction, can the secretary set up a restricted call which is ok
for the boss to make? If CDR records which set placed the call and
which set was ultimately involved, class of restriction can give way
to other types of accountability, but only after the call is made.
Each PBX and Centrex system appears to have made different decisions
as to how features, restriction and numbering relate on electronic
telephone sets; the customer selecting such equipment must exercise
great care if unpleasant surprises are to be avoided.
The ISDN BRI, with its NT1 interface
acting like a KTU, may well be able to handle several different
terminals simultaneously; however, it seems quit evident the
relationship between the two B channels for circuit switching and
the D channel for packets on a given line will have to be uniquely
related to the identity of their matrix port as will both the
identity and classification of terminals served by that port. To use
BRI-compatible phones for multi-line service behind PBX and Centrex
systems will require more cooperation among the various switch
designers than has yet been in evidence.
Hard and soft hold
The term "hold" covers several basic
features needed by telephone systems; the general idea is to hold or
retain a connection when normally the system might think the call is
over. As we have seen in Chapter 3, "joint holding" allows certain
emergency switchboards to maintain a connection even if the caller
hangs up, and "calling party hold" allows the called party to hang
up one phone and go to another on the same line without losing the
connection (if the move can be effected before the system times
out). However, the most common use of the hold feature is to allow
one party to leave a call with the intent of returning to it, or
having someone else "pick it up."
In addition to this principal
function, hold can also be an auxiliary feature used to help other
features work. "Stupid-smart signaling" takes advantage of the
switch-hook flash to put a call on hold while a new call or some
feature is signaled to the switch. The hold operation so invoked is
often called "soft hold," and also provides the caller with dial
tone. Some systems offer the caller a feature code for "hold" to
allow dismissal of dial tone and the signaling receiver while
maintaining the other party on hold. This is similar to "hard hold,"
where the caller pushes the hold button on a 1A2 key telephone or
its electronic emulator; if the caller with such a phone now wants
dial tone, a different line-select button must be pushed.
When 1A2 key systems are used behind
electronic switches capable of soft hold upon receiving a
switch-hook flash, calls on soft hold can be "hidden" when hard hold
is subsequently applied. To minimize this problem, many designers
have removed the switch-hook flash from electronic telephones, and
recommend against its use on lines to 1A2 key systems. An example
will be provided below in connection with call-waiting.
Ringing and alerting
As was discussed in Chapter 3, ringing
can be varied to produce a number of features. "Distinctive ringing"
has two forms: the first tells the called party something about the
incoming call, perhaps differentiating an outside call from an
intra-PBX or intercom call, while the second makes one phone (or one
line) ring with a different sound from another, useful when a called
party has to use audible cues to identify his or her own phone from
those of others. The first form is usually a function of the switch,
while the second is often a function of the actual ringer in the
telephone set.
Calling number ID can be thought of as
an advanced form of distinctive ringing, presenting the called party
with the number and/or the name of the calling party. Calls internal
to a PBX, where name is associated with number in system memory, can
do this easily, but calls from another switching system must depend
on something like SS7 to carry the calling party's name forward. In
time, this will be commonplace; when the caller's name can be
displayed instead of his or her number, many of the privacy problems
associated with calling number ID will be less pressing.
Answering bureaus and voice mail
systems need the number and name of the callED party to respond
properly to ringing. Usually, the name is associated with the number
internal to the answering system, making the called number alone
sufficient for generating the proper response to the caller.
Party line ringing lets the switch
ring one particular station out of several on the line or, if all
are rung, to provide a code that identifies the particular caller
wanted. Party line ringing can be arranged to provide intercom
connections among several phones on a residential line, or to let a
particular family member be identified, but the D channel of the
ISDN BRI will expand this capability by allowing specific terminals
to be selected for either outside or intercom calls.
Almon Strowger's first dial system at
LaPorte, Ind., in 1892, had no busy test. If two or three people
wanted to call the same party at the same time, they would all
connect to that line for conversation. Doubtless some just listened,
and this lack of privacy was almost instantly recognized as a
difficulty to be corrected.
Manual switchboards, of course, have
had a busy test for a very long time. One of the most common ways
was to use the battery on the sleeve lead of a busy line; the
operator touched the tip of a cord circuit to the sleeve at the jack
in the multiple and heard a click in her head-set when the line was
busy. Battery on sleeve, of course, was holding the cut-off relay
operated to disconnect the line relay that had detected off-hook.
Electromechanical automatic systems
have used battery or ground (depending on design) on sleeve to cut
off the line relay, to hold the matrix path operated and to provide
a busy mark. The system, upon discovering a busy mark, could return
busy (or ATB) tone to the caller. Obviously, status of busy lines
and trunks and segments of busy matrix paths need not be stored on a
third wire, provided at considerable expense just for this purpose.
In computer-controlled systems, as mentioned in Chapter 1, busy and
idle status are stored entirely within the system memory, and busy
testing and path hunts are carried out at electronic speeds without
even looking at the hardware. In such systems, it is important to
insure that the "map" in memory is updated regularly; it must
correspond to the current state of the real world it represents.
In most older systems, a line is
either busy or idle (recall that a line being rung tests busy even
though it is on hook). In many military systems, however, as well as
most of the more modern commercial switches, a line can be busy in
several different ways. It can, for instance, be busy at any one of
several levels of priority; calls with higher priority will be
permitted to break in, while calls of lower priority will not.
Similarly, a line may be busy with one or more other calls waiting
in queue. Clearly, a line that has a call waiting in addition to a
call in progress is busier than an active line without a waiting
call.
A new problem surfaces when electronic
key telephone sets are used. Each of these sets, as discussed above,
has an independent control channel plus a talk path (with perhaps a
second channel for data). It can have several extension numbers
appearing on pick-up buttons with appropriate lamping, while a given
extension number can appear on buttons on several electronic sets. A
typical question is how to define the way the system interprets and
reports a busy set as opposed to a busy extension number. How does
the secretary, for instance, know if the boss is on the phone when
the boss has additional numbers available, some not appearing on the
secretary's phone? There is, as yet, no generally agreed upon
resolution of such problems and indeed, a clear interpretation is
not always possible.
A line, matrix port or telephone set
that is in trouble should be made busy by the system, although
routine testing may detect clearance of the trouble and put the line
back in service automatically. Receiver off hook (ROH) is a typical
cause of such troubles, and "line lock-out" is often provided after
a time-out so that the system will not tie up a signaling receiver
waiting for instructions from the customer. Usually a howler tone is
returned to the telephone set to attract the user's attention; when
the misplaced handset is returned to the cradle, or when some
maintenance action clears the trouble, the "make busy" condition is
terminated and the line can originate and receive calls.
Busy verification (no-test)
All systems require some means for
operators or test personnel to make sure a line that tests busy
really is. In SXS systems, "no-test" connectors were provided,
identical to others in their group except for the elimination of the
busy-test function. These connectors bridged a line to permit it to
be monitored, either by listening or using instruments. Crossbar
systems provided no-test verticals to make similar connections. In
digital systems, simple bridging is, of course, not practical, and a
conference connection is required to allow both sides of a
connection to be monitored. Ideally, a digital no-test connection
should be made via a device which monitors each side of the
connection independently so that fax, data and image transmissions
will not be disturbed by the signal processing involved in
conferencing. A no-test connection is available only to authorized
telephone company personnel; customer access is prohibited.
In older PBXs, the station multiple at
the attendant's switchboard provided a convenient point for
connecting to any line. Consoles eliminate the station multiple,
requiring switched access to extensions for all calls. In addition
to increasing the size of the switching matrix, consoles also impose
a new requirement for busy verification. There are many ways to
accomplish this.
Busy override
Busy override, also called "executive
right of way" and "barge-in," is a PBX or Centrex feature similar to
no-test. It allows users with the proper priority to complete calls
to other lines, whether those lines are busy or not. A tone is
provided to the existing conversation to warn one party that a new
call is coming in and the other to be prepared to by placed on hold.
If some users are class-marked to apply barge-in, it is evident that
others will be class-marked for privacy to prevent barge-in. With
multiple-level busies, as when calls are waiting, barge-in rules
have to be carefully thought out.
Barge-in should not, of course, extend
beyond a PBX or Centrex to outside stations on the public network,
although traveling class marks, via CCIS, could be used effectively
within a private tie-trunk network, either civilian or military.
Priority features in private networks extend to trunks as well as
lines, and permit callers with high priority to take over facilities
in use at a lower level. Again, the programming for such features
becomes intricate.
Privacy
There is more to the privacy problem
than anti-barge-in class-marks. With cord boards, there was always
the possibility of listening in from the attendant positions, and
some systems evolved ingenious circuitry to lock out the switchboard
appearance once the connection was established. Recall by
switch-hook flashing, in such systems, permitted the attendant to
reconnect to the call.
In more modern systems, certain user
transfer approaches, used either by the attendant or another
station, permit the transferring party to stay with a call after the
other two parties think transfer has been completed. This problem
will be discussed in connection with station dial transfer and
consultation hold, later in this chapter. "Data privacy," used in
connection with camp-on and call-waiting, will be covered in the
next section.
Yet another aspect of privacy concerns
those who do not, for the moment, wish to receive incoming calls.
The traditional approach is to take the phone off hook, causing the
switching system to believe it has a "permanent signal" (see Chapter
8). A better approach is to allow the customer to signal his
intention to the system and allow it to return an appropriate
message rather than audible ring to incoming calls. "Call
forwarding" (discussed below) to voice mail can also work well here.
Camp-on and call-waiting
Camp-on is a PBX feature associated
with incoming calls completed via the console attendant when the
called station is busy. The attendant "parks" the call on the busy
line; if the existing call ends within a timed interval, the called
party's line is rung and the camped-on call is completed. After the
timed interval, an unanswered call is returned to the attendant who
may inquire if the caller wants to try another line. The time-out
interval is usually about 30 seconds.
"Camp-on with indication" gives the
called party an audible signal to warn that a call is waiting. It is
desirable that only the called party hear this signal; on
established intra-switch calls, the ambiguity produced in both
parties thinking they have a camped-on call can be annoying.
Camp-on produces the obvious question
of how many calls can be stacked waiting for a given extension, and
what is to be done with calls that exceed that number. Usually, only
one call is stacked, and the attendant gets a busy response when a
second is attempted. A related problem concerns hunt-groups. Should
the call camp on to the desired line, or hunt to another line in the
group? To put it another way, should camp-on (and call-waiting) be
mutually exclusive with hunting or call forwarding? Many feel it
should, particularly when the several extension numbers have
appearances on multi-line sets so that the called party can put the
existing call on hold and take the new call on a second "line."
Call-waiting is similar to camp-on,
but is initiated by another caller rather than an operator or
attendant. It is an excellent feature for residential service,
provided party lines are not involved, and is effective on business
lines that terminate on single line sets. As with camp-on, it tends
to be mutually exclusive with hunting, call forwarding, and
multi-line sets.
As presently provided, a called party
who is already on the telephone hears a call-waiting tone when a
second call comes in. As with camp-on, only the called party should
hear the tone to prevent the other party from also responding. He or
she makes an apology and flashes the switch-hook to put the other
party from the original connection on hold, and the new caller is
connected. Subsequent switch-hook flashes allow the called party to
alternate between the two calls. This feature is sometimes called
"broker call" or, more picturesquely, "flip-flop"; it obviously
gives the user the equivalent of two incoming lines where only one
is actually provided. Telephone companies usually charge residential
customers a high fee for this service rather than use it to increase
revenues by completing more calls.
If call-waiting is offered on a line
terminated in a 1A2 key system, and that line is put on hard hold
(to permit making or answering a call on another line, for
instance), two calls rather than one may actually be held with only
one having a lamp indication; the second call, on soft hold, may
escape detection completely. The only real solution to calls on
"hidden hold" is electronic telephone sets with a separate signaling
channel and efficient displays, or simply barring soft-hold features
on multi-button sets.
Call-waiting should be avoided on
party lines because any user with a high enough calling rate to need
it should have an individual line in the first place. Further, busy
testing can be complicated. The system would have to know which
party on a party line is engaged in the existing call. If it is the
called party, call-waiting could be used, but if it is not, the
caller should be given busy tone. With an ISDN BRI, the signaling
channel will presumably handle the connection to any of the devices
on the S/T interface, sending an appropriate signal via the D
channel whether the terminal is busy or idle, with various options
for announcing a waiting call.
Central office call-waiting should be
avoided on multi-line groups to PBXs simply because there is no way
to know if the PBX's CO trunk is connected to the one desired person
out of many behind the PBX. Similarly with key systems; is the boss
on the secretary's line and has the secretary just answered a new
call on the boss's line? If so, which line gets the call-waiting or
camp-on signal? If the PHONE of the called party can be identified
independent of extension number, camp-on or call-waiting have a
better chance of being useful, although a multi-line sets tend to
make them unnecessary.
The stacking problem with call-waiting
and camp-on can get out of hand. A better solution appears to be the
use of several "appearances" of the SAME extension number on an
electronic telephone, and allow the user to manipulate the hold
button and the line pick-up buttons to select the particular call
desired. With several different buttons, even though similarly
labeled on both a principal and a secretarial set, it is possible to
deal with the situation; where real confusion reigns is when single
line phones are allowed to have a number of calls waiting or on hold
in a "push and pop" stack, as is available on some PBXs. There are
times when simply returning a busy signal is not a bad alternative.
With call-waiting, the new caller does
not hear busy prior to answer; rather, ringback tone is returned. If
the called party cannot leave the existing connection for one reason
or another, the calling party may assume the called party is absent
rather than busy with another call. Spouses and bosses can sometimes
misinterpret what the telephone system is telling them.
Another problem with camp-on and
call-waiting is the tone they inject into the existing connection.
If that call happens to be data or facsimile, the whole transmission
can be ruined. Many systems offer "data privacy" as a feature which
can be activated for a given call. This usually requires giving the
system a special feature code as part of call set-up. A better
practice is to block camp-on and call-waiting from lines that are
normally used for fax or data.
Automatic call-back
Automatic call-back started as a PBX
feature that let a caller reaching a busy extension on an intra-PBX
call instruct the PBX to call back (with a distinctive ring) as soon
as that extension became idle. The PBX, detecting answer on
call-back, would then ring the called party, formerly busy, with the
new call. Like camp-on and call-waiting, automatic call-back works
best by being associated with a particular telephone rather than an
extension number which might be in use on any of several phones.
Again, call stacking and hunt-group
problems appear. New problems are introduced by the fact that the
calling party may be busy on another call when the called line
becomes free; or, the calling party may not pick up the phone
quickly on the callback, and the called party may, in the meantime,
originate a new call. Although useful, this feature is another that
both the designer and the customer should approach with extreme
caution.
Call-back is as easy to provide for
residential customers on a single CO switch as on a PBX; however,
with one switch serving a number of office codes, and many CO
switches in metropolitan areas, it is hard for the user to know when
it works and when it doesn't. CCIS within a LATA (local access and
transport area) solves the problem by allowing frequent
"conditional" retries without tying up trunks in the process; when
the called line free, the originating switch can then arouse the
caller and try for real. Sometimes marketed under the name "Repeat
Call," this feature can be quite useful. When local and long
distance companies allow their SS7 networks to cooperate, the value
of the feature will be much enhanced.
Manufacturers of telephone instruments
are not waiting for such sophistication; sets with automatic dialers
can often be set to retry every ten seconds or so. Called "busy
busters," these telephone can drive a CO switch or PBX crazy by
generating a large number of unpaid call attempts, each of which
requires as much effort from switch controls as does the successful
completion for which the telephone company is finally paid.
Another current residential feature
called "Return Call" is somewhat similar to automatic call-back
(Repeat Call). The called party, reaching the ringing phone just as
the caller abandons, can dial a feature code to initiate call-back
(within the same LATA); the calling number has been stored in memory
associated with called line for just this eventuality. Some
telephone sets arranged for Calling Number ID can display call
attempts to the called party who can then use the stored numbers to
return those calls which seem important. Note that a call
originating in the public network via a PBX trunk could not be
called back effectively; without knowing the extension, the caller
would be blocked at the PBX console. Thus an answering machine or
voice-mailbox may provide a more satisfactory solution by allowing
the caller to provide the call-back information needed, or leave a
message making call-back unnecessary.
An amusing aspect of Return Call comes
in connection with wrong numbers. If the caller suddenly realizes
that he or she has dialed the wrong number and hangs up, the
incorrect called party can call back to the confusion of both,
generating revenue for the telephone company to make up for wasted
effort dealing with busy busters.
Two of the biggest problems in using
the telephone are busy lines and lines that don't answer. Then,
there are calls to wrong numbers, and those that tell the caller the
number dialed is non-existent. Any modern switching system must make
an effort to deal with these problems; camp-on and the related
features described above are a help, but additional solutions made
possible by computer control and SS7 are far easier and more general
than the manual or electromechanical approaches of the past.
Call Transfer by Attendant and User
When someone calls a business large
enough to need a PBX, it is likely that the identity of the desired
party is not known. And even when it is, personnel changes,
departmental responsibilities and the like may make it necessary to
contact one or more other people. As a result, any call incoming to
a PBX must be considered a candidate for a transfer. Central offices
do not, as a rule, transfer calls; this is a major difference
between PBXs and COs, and caused no small confusion in the early
days of CO-based Centrex.
Attendant transfer.
Early PBX attendants answered incoming calls on ring-down trunks to
a cord board which acted either as a stand-alone PBX or as an
attendant position for the SXS PBXs which were nearly ubiquitous
prior to 1975. In either case, the attendant used cords to complete
the connection between the incoming trunk and the called extension.
When the called extension answered, the cord lamp went out; upon
hang-up, it came on again, steady, to tell the attendant to pull
down the connection. If the called extension turned out to be the
wrong one, the called party flashed the switch-hook. Upon seeing the
flash, the attendant re-entered the call, obtained the necessary
information, and moved the cord to the appropriate jack in the
multiple.
Because outgoing and intra-PBX calls,
dialed directly by the user, bypassed the switchboard and were not
available to the attendant, attendants could only transfer incoming
calls. When consoles were first used with electromechanical
switching, each incoming trunk circuit had a lamp and button on the
console to permit direct access by the PBX attendant (key per trunk
operation; see Chapter 6), the attendant could bridge on to the
trunk in response to ringing or a flash, obtain the necessary
information, and signal the common control to extend the call from
the trunk circuit to the desired extension.
In large PBXs, where there were too
many incoming trunks to give each an appearance on the console and
more than one console was usually needed, access switches provided
"switched loop" connections to consoles. Because all this was
expensive, outgoing trunks, reached by dialing 9, were not provided
with console access. A caller needing help in setting up a call
would dial 0, appear at the console on a special group of attendant
trunks, give the necessary information, and hang up. The attendant
would then set up the call on an incoming trunk and call the user
back. Such procedures reinforced the perception that only incoming
trunks need have operator access and, as a result, transfer
capability.
It took some years for designers to
realize that limiting transfer to incoming calls, imposed by
obsolete hardware, was not a basic system requirement. The present
approach is to treat the console like a regular electronic telephone
set, complete with a signaling channel and suitable displays, and
switch all trunks to it via the switching matrix. The attendant can
then obtain the name of the called party and transfer the call,
extend an outgoing call and drop off, and perform a variety of other
services. Stored program systems work easily this way, and can give
attendants access to any type of call, incoming, outgoing, or intra.
When a call requesting service is
connected to the console, it is desirable for the console to present
to the attendant an indication of the type of call (recall or new,
dial 0, etc.) and display the identity of the call's source.
Further, it is often desirable in multi-console operations to return
to the original console if possible.
Because a three-way connection is
often needed during transfer (calling party, called party, and
attendant), connections via the switching matrix may be handled as
though they were conference calls. In two-wire systems, three
parties can be bridged together without difficulty, but in digital
systems, which must be four-wire, conference circuitry is necessary.
Attendant and operator access will be discussed further in Chapter
6.
Station dial transfer.
With attendant transfer only, the transferrer flashes the
switch-hook to ask the system to add the attendant's position in a
three-way conference so that the attendant can drop the wrong party
and carry out the transfer operation. It is not much harder for the
system to put the calling party on "consultation (soft) hold" and
connect the transferrer not the attendant but to a signaling
detector which returns a special dial tone. This "recall dial tone"
is similar to regular dial tone except for being pulsed three times
before becoming steady; it lets the transferrer know the system has
interpreted the hook flash correctly rather than as a hang-up
followed by a new origination.
With recall dial tone, the transferrer
can dial 0 for a connection to the attendant and give verbal
instructions, or dial directly the extension number of the person
who should have received the call in the first place. The
transferrer can announce the call to the new person in complete
privacy, flash the switch-hook again and hang up; the held party is
now connected to the new party and the transfer is complete. If the
transfer is refused, the new party is supposed to hang up so that
the held party can be re-connected to the transferrer.
There are three problems with this
approach: the transferrer may not choose to hang up after transfer,
the new party may not choose to hang up after consultation hold, or
the new party may be busy or may not answer. In the first two
instances, the party who is assumed to have hung up can simply keep
quiet and listen to the following conversation, a serious breach of
privacy. Busy and ring-no-answer pose a series of control problems
that complicate system design.
An alternative approach was used by
some PBXs. They assumed transfer was more important than private
consultation, and immediately established a three-way connection
between the calling party, the transferrer, and ringback or busy
tone on behalf of the new line. Thus the caller was not on hold,
remained in contact with the transferring party, and could hear the
call progress tone at once. This approach allowed the transferrer to
drop off without announcing the call, just as a console attendant
would, as soon as ringback was heard, or to flash the switch-hook to
drop busy tone without losing the connection to the calling party.
Early systems which used the consultation hold procedure would lose
the outside connection if they hung up before the new party
answered; later systems, taking advantage of the greater flexibility
of stored program control, associate the held call with the call
progress tone when the transferring party hangs up or goes on to
another call. In general, it is hoped that the transferring party
will, upon hearing busy tone, flash the switch hook to dismiss it
and return to the outside caller.
If the new extension does not answer
after a timed interval, and the outside caller, listening to
ringback, does not abandon, the call should be routed to a console
or else returned to the extension that tried to transfer. If the
console is closed after hours, there is no alternative in most cases
except the transferring extension. Now another stacking problem is
encountered; suppose the transferring extension is on another call,
quite possible when a feature such as Universal Night Answer (to be
discussed) is provided. The determined outside caller may very well
be left connected to ringback until the original extension becomes
idle.
If the line to which the call is to be
transferred is busy, a whole new array of problems appears. Should
the call hunt to an idle line or use call forwarding if available,
or should call-waiting with indication be used? Should the called
party have to terminate the present call to respond, or should it be
possible to put the present call on hold? How does one reject the
new call and go back to the old call after talking to the
transferring party? These functions, easily accomplished with
multi-line key telephone sets, become very intricate when attempted
with switch features activated from single line phones. Yet finding
the called party busy is quite likely. The problem cannot be
ignored, nor is there any one solution that is generally accepted.
In most approaches to transfer and/or
consultation hold, a three-way conference is a natural and often
unwanted by-product. To provide some protection for privacy, a
conference tone, perhaps like the familiar recording "beep," may be
desirable at regular intervals. This will not prevent listening in,
but will at least provide warning.
When electronic telephone sets with
feature buttons are available, separate buttons for transfer,
conference and drop are often provided. These additional signals
tell the system control exactly what is wanted and, when used
correctly, insure privacy and prevent lost calls (hit transfer, dial
the new number and hang up, or else hit conference, dial the new
number to announce the call or consult, then hit conference again
for three-way or drop to return to the original caller). However,
they require vastly more training than is necessary when 1A2
Station dial transfer, along with
consultation-hold and add-on conference, was built into
electromechanical PBX trunks as a single feature and, to save money,
was usually supplied on incoming CO trunks only, as has been
discussed. Now that stored program control has freed the user from
trunk hardware, and trunk hardware from functions which can be
programmed, all features should work the same way, independent of
the route to the connected party (generally unknown to a PBX user).
But when these features are used in connection with outgoing calls
that require assistance from a telephone company operator, an
additional problem arises: the PBX must now be able to tell when the
operator is wanted rather than the attendant, and act accordingly.
There is also a need to discriminate between the local telephone
company's operator and the operator provided by a particular long
distance carrier.
A similar problem exists when small
PBXs, posing as electronic key systems, are used behind a Centrex
(or large PBX) to provide business features. As was mentioned in
Chapter 3, the ability to recall the key system control to activate
its features and the Centrex control to activate Centrex features
must be differentiated. Because most such small systems are used to
support electronic key telephones, transfer can usually be effected
internally using the hold button and then the intercom to call the
new party who then pushes the line pick-up button for the designated
Centrex extension. Thus a separate button, labeled "Flash," can be
used to instruct the PBX to apply a flash at the trunk circuit
terminating the line from the Centrex switch. This will get recall
dial tone from the Centrex, and the user can key in the Centrex
extension number for transfer/consultation/3-way or a feature code.
It must be kept in mind that, when Centrex can put an extension on
soft hold with a switch-hook flash, the held line will be invisible
to the user of the a key telephone system, electronic or
conventional.
Consultation hold and three-way
conference are good features for residential lines on a modern CO,
but station dial transfer poses charging problems. When consultation
and conference take place, the person invoking the feature can be
charged for the additional connection, but if a call were to be
transferred to a distant telephone, should the transferring party,
who hangs up, have to pay for the following long distance
connection? The problem becomes more interesting if the transferring
party has one long distance carrier, and the transferred party
another. In general, residential lines are not allowed to transfer,
even when they have consultation hold and three-way conference
capability.
Transmission may also limit the extent
to which these features can be used. Within one PBX, Centrex system
or local CO, the only real problem may be OPX or FX lines to a
distant city. But when trunks between switches in either private or
public networks are used, and both ends of a conversation add on one
or more parties, volume and echo control may become difficult. With
ISDN, 4-wire telephones, digital transmission and advanced
conference bridges may solve these problems, but for the present,
the caller is taking a risk.
Trunk answer from any station
This PBX feature, also called UNA for
"universal night answer," allows any station, or certain selected
stations, to answer incoming calls from the CO. When the console is
closed, one or more night bells with a distinctive ring are
activated where the night-answering extension users can hear them.
Any telephone can respond by coming off-hook and dialing the
appropriate feature code. The incoming call is then completed to
that telephone. With station dial transfer, the answerer can then
pass the call on to someone else, if necessary. It seems to be
almost a law of nature that if two people are working late, the
person responding to the night bell will be the wrong one.
It has not escaped the notice of
astute communications managers that, with trunk answer from any
station and the transfer feature, console attendants can be
eliminated, along with the expensive hardware needed to support
them.
Call pick-up, paging and parking
Trunk answer from any station is
useful in DID (and Centrex) systems only when someone dials the
company's directory number--a call that would normally appear at the
console in the first place. A bigger problem is calls, whatever
their source, that ring unanswered at unattended telephones. Call
pick-up solves this problem. Pick-up is like UNA except that the
bell associated with the particular line is heard. Any other line
class-marked to be in the same pick-up group can come off-hook and
dial a feature code to answer the call. Human-factors studies show
that users are quite willing to learn this procedure; apparently
they will do anything to silence a ringing telephone. Depending on
building lay-out, related to who can hear which telephones, a number
of pick-up groups can be provided. Ideally, if you can hear it, you
can answer it (company politics permitting).
In its simplest form, pick-up requires
the user to dial only the pick-up code to answer a ringing line. If
two lines are ringing simultaneously, the user has no control over
which one the system chooses to deliver. A variation, often named
"directed call pick-up," requires the user to add an extension
number after the pick-up code to specify which call is desired. This
implies, of course, that the user can tell which phone is ringing in
the first place.
When a user can specify which line is
to be answered, other features become possible. In particular,
message centers, with special displays which light when any covered
phone is ringing, allow agents to pick up the particular line. With
repertory dial feature phones, pushing the button related to the
ringing line can send a flash, the directed pick-up code, and then
the particular number. When electronic telephone sets are used, a
button may be provided for each pick-up group covered, along with a
lamp to blink when any line in that group is ringing. Of course, 1A2
emulation, with a lamp and button for each covered line, is also
common. Depressing the group or line button brings the call to the
answering phone, but spares the user from having to think about
pick-up as a special feature.
Electronic phones with lamps and
buttons for many extensions are often used by several secretaries
screening calls for a number of principals. Such phones have useful
ways of setting ringing for each secretary. For instance, a call for
the secretary's principal may ring immediately, while "delayed
ringing" may be used on calls for other principals to induce their
secretaries to answer first; the extension lamps, of course, will
blink on all phones. The general idea is "if your phone rings,
answer it," using this more sophisticated version of call pick-up.
(Each principal's phone may be set not to ring at all except for
secretarial intercom, or be given delayed ring as a safeguard for
when the secretary is not available.)
When a person is frequently away from
his or her desk, as a doctor doing rounds in a hospital, a
programmer in the computer room, or a floor-walker in a department
store, paging may be the only way to complete certain calls. Often
the PBX console attendant will have access to an external paging
public address (PA) system, usually by pushing a console button;
station users may also have dial-up access to paging if suitably
class-marked. "Zoned" paging may also be supplied, with different PA
systems for different areas. When privacy is important, code calling
can be used. Here the attendant supplies a number identifying the
person wanted to external equipment, and a chime sounds
appropriately. Dr. 23, for instance, would respond to two notes
followed by three notes.
Response to paging or code calling is
usually via some form of call pickup. While a person's line is
ringing, paging can be used with a message such as "Dr. Jones, pick
up your phone" using directed call pick-up. When a feature called
"park" associates a call with a given line without ringing it,
directed pick-up can again retrieve the call while keeping the noise
level down. Paging often depends on "parking orbits," numbers
without a phone which can be associated with a call awaiting pickup.
Depending on system design, the paged person can either dial the
parking orbit's number directly, or dial 0 for the console attendant
who will then make the connection. When many simultaneous calls are
being completed via paging, finding and identifying an idle parking
orbit is sometimes a problem; use of the called party's extension
number is much simpler, and reduces the load on the console
attendant.
The park feature, combined with
pick-up, is also handy for those who may answer a call at one point
but find they have to go to a different location (perhaps for
information or privacy) to continue. This is, of course, a somewhat
more complex way of putting a line on hold at one phone and picking
it up at another in the same 1A2 grouping.
Line and trunk hunting
SXS selectors, upon reaching the
"level" defined by a given digit, would go into a rotary mode and
hunt for an idle path (called a "trunk" in SXS parlance) among the
ten on that level. In PBXs, the 9 level on the first selector was
usually reserved for trunks to the CO, and the 0 level, for trunks
to the switchboard or console. Often, the 8 level was used for
access to a private tie-trunk network. Rotary stepping worked by
sequentially testing the sleeve lead of each terminal and, if a busy
condition was found, moving on to the next.
In a connector, which completed to the
called line, a similar busy-test-and-step routine was available if a
"strap" or piece of wire was in place between two terminals related
to a line which was supposed to hunt. This rudimentary form of class
marking enabled stepping to the next line if the desired line was
busy, and could be installed as required. Typically, pairs of
adjacent numbers would be reserved for "boss-secretary" operation;
if the boss's line was busy, the selector would continue to the
secretary's line.
Because SXS numbering was fixed and
dictated by the switches, only adjacent numbers in the same tens
group (starting with 1 and ending with 0) could be in hunt groups
(note that 3459 could hunt to 3450). Because jack numbering was also
fixed on cord boards, hunt groups were shown there by a line painted
under the jacks. Although "level hunting connectors" were available
to serve line hunt groups larger than 10, such groups were seldom
encountered. Trunk groups, however, were often larger than 10, and
various ingenious methods such as "graded multiples" and rotary
out-trunk switches (ROTS) were devised to get around the limitations
of SXS switches.
Although SXS is obsolete, its
continues to cast its shadow into modern PBX and Centrex design. We
still dial 9 for outside and 0 for the console attendant, we find
hunting differentiated from the more general "call forwarding," to
be discussed below, and we find many situations where consecutive
extension numbering or related matrix port locations are assumed
necessary when there is no reason for such limitations to continue.
In a modern switching system, a hunt group is simply a list of
extension numbers in memory; if one is busy, the control tries the
next and the next until the list is exhausted or an idle port is
found. Line hunting and trunk hunting follow the same process; there
is now no limit to the number of ports that can be in a given hunt
group, or the order in which numbers must be searched. In general,
directory numbers are related to matrix ports by translation which
can be quite arbitrary.
Terminal and circular hunting.
In terminal hunting, the switch starts looking for an idle line at a
given terminal, and hunts from that terminal to the end of the
group, just as in a SXS connector. If the terminal addressed is the
first in the group, all lines will be examined until an idle line is
found, and busy tone will be returned only if all are busy. If some
line in the middle of the group is addressed by the caller, hunting
will start at that line, and only seek an idle line in the part of
the group that remains, even if lines before the starting point are
idle.
Circular hunting assumes the last line
in a hunt group precedes the first line, and no matter where hunting
starts, the system will search in circular order over the entire
group, if necessary, only returning busy if all lines in the group
are in use. Programming includes precautions that the hunt will only
go around the circle once.
Pilot number hunting.
With pilot number hunting, a group of lines or trunks is identified
by a pilot number which is different from numbers associated with
its individual members; the group may be arranged for either
terminal or circular hunting as desired. In both instances, the
system will hunt over all the lines or trunks in the group; however,
circular hunting can be arranged to start where the last hunt left
off, while terminal hunting always starts at the same point. As a
result, one has the option of "equalizing the load" on all terminals
in the hunt group, or allowing the first choice to "cream skim" and
carry appreciably more traffic than lines or trunks "in the back of
the grade." With two-way trunk groups, terminal hunting is usually
used, one end hunting from 1 to N, and the other hunting from N to
1.
Because many PBXs can only support
extension numbers that a have physical implementation, their use of
pilot numbers (and parking orbits) is blocked; the first extension
in a hunt group, however, can be used as though it were a pilot
number, making terminal hunting almost inescapable (circular hunting
in such instances is then available as part of an extra-cost ACD
option package). In CO switches, where hunt groups are often trunks
to a PBX, the tradition of giving each trunk its own telephone
number, primarily for billing purposes, is sometimes abandoned to
slow the saturation of the North American numbering plan.
With terminal hunting, a first-choice
trunk where supervision works but transmission does not (a "killer
trunk," discussed further in Chapter 8) is a major problem. Users
will continually be assigned to it, only to hang up when they
discover it does not work. In a period of high traffic, some other
unlucky person will probably be stuck with the killer trunk when an
earlier user retries; after hours, this kind of aid usually isn't
available. One recourse is to use a second phone to tie up the
killer trunk, and then make a call which is forced to use the next
trunk in the group. On the other hand, circular hunting, starting
each hunt where the last left off, greatly reduces the probability
of a user being assigned the bad trunk on successive calls.
Other types of hunting.
With stored program control, hunting can be arranged to do many
interesting things that SXS was never able to consider. For
instance, a person might be a member of two or three different
groups hunted in several different ways: a secretary might be
reached on terminal hunt when the boss's line is busy, and on
circular hunt when a pilot number for all secretaries in the
accounting department is addressed.
A particular feature called
"secretarial hunting" or "next number hunting" has found
considerable favor. Here, the memory for each directory number has
associated with it the next number in its hunt group. Because the
next number for several different executive phones can be the same
secretarial phone, one secretary can answer for several different
principals when their lines are busy. Obviously, several strings of
next-number hunt groups can flow together into another next-number
hunt group. In such arrangements, it is desirable to display the
original called number on the phone that finally rings.
Hunting as a function of
caller. There are many instances,
particularly in connection with Automatic Call Distributors, where a
different hunt (or call forward) procedure is desirable for calls
arriving on different trunk groups, for calls arriving on the same
trunk group but as the result of the caller dialing a different
number, and as a result of the calling number itself. Further, it
may be necessary to display the called and/or the calling number to
the person who ultimately gets the call.
If each CO trunk group has its own
directory number, a PBX or ACD can arrange hunting over one or more
groups of agents in different ways, depending on which trunk group
was addressed. However, many network features are available today to
deliver calls, particularly those dialed as 800 numbers, to
different trunk groups at different locations as a function of
traffic, time of day, emergency operation, etc. Thus the actual
number dialed rather than trunk group on which a call arrives may be
the factor which directs hunting. Finally, it may be desirable to
make hunting patterns be a function of the number from which the
call was placed. DNIS, for "dialed number identification service"
and Calling Number ID both take advantage of the ISDN D channel to
deliver the dialed number or the number from which the customer
dialed, respectively, to the destination PBX, Centrex or ACD. With
this information, the destination switch can then effect the desired
hunting pattern.
The number of the caller, once
delivered to the called switching system, is often sent on to an
associated computer which uses it to pull up the caller's file from
a data base. The switch can then connect the voice call to an
agent's telephone and the caller's file to the agent's terminal. The
logical way to do this would be with the two B channels in a digital
PBX or a CO switch equipped for ISDN, but data base access, more
often than not, uses a local area network. However these two related
connections are established, calling a data base system from someone
else's phone can lead to confusion.
Step Call.
A feature called "step call," related to hunting, is found in some
PBXs. It allows an inside caller or console attendant to instruct
the system to step to the next number if busy is encountered. In
electromechanical systems, where hunting had to go to the next
consecutive extension number, and people in a given work group
happened to have adjacent numbers, this feature was fairly easy to
implement and sometimes useful. Another application was in finding
an idle parking orbit for use with paging. In modern stored program
systems, such a feature, based on hunt groups stored in memory,
might still have utility under some circumstances. With end-to-end
signaling, such as DTMF, selected outsiders could also be given the
"step code," assuming a signaling detector is present when busy tone
is returned.
Uniform call distribution.
The use of circular hunting to deliver calls to agents is the basis
of uniform call distribution, one of the major features of an
Automatic Call Distributor or ACD. ACDs started out in the telephone
company as hardware to distribute directory assistance calls to
available operators; similar needs exist in reservation systems and
other service functions. The coming of 800 numbers and the
popularizing of credit cards and private delivery services have also
made ACDs vital in telemarketing.
It is obvious that clever programmers
can devise more intricate hunting algorithms than merely picking up
a pilot-number hunt at the next number in a circular list. It is not
particularly difficult to deliver the next call to the agent who has
handled the least traffic in the last interval, for instance,
although often the next agent to come free, regardless of activity,
will get the first call waiting in a queue. Adding the queue to the
hunting operation when all ports are busy is, of course, another
step in the sophistication of UCD.
The other principal feature of an ACD
is providing detailed reports to supervisors, usually in real time,
for both agents and hardware, as discussed in Chapter 2. An ACD may
either be a stand-alone system, or a particular set of features
programmed into a PBX or Centrex. The major difference in an ACD and
a regular telephone switch lies in the level of concentration
possible; ACD agents are, almost by definition, on the phone all the
time; further, many ACD calls have very short holding times. Thus
both a non-blocking switching matrix and a powerful control system
are indicated, and the ratio of trunks to agents often exceeds 1:1.
In a UCD group, it should be possible
to bypass unoccupied positions; further, because an agent may occupy
any position, and different agents will use the same position,
particularly when several shifts are scheduled, agent identity
should be made known to the system. Typically, a personal
identification number may be keyed in when an agent sits down,
followed by a deactivate code when the position is vacated. A
ring-no-answer situation should also take a position out of service,
and notify the supervisor. It many kinds of ACD operation, it is
necessary to give agents time to write up orders or some similar
function when the call is over. A station may be made busy for a
standard timed interval after the completion of a call, or readiness
for the next call may be keyed in by the agent.
The use of 800 numbers on national
television can make the arrival of calls anything but random. Even
without this stimulus, more calls may be present than there are
agents to serve them. Thus incoming calls must be placed in queues
and the callers reassured with suitable recorded messages. "Music on
hold" is a way of letting callers know that the system hasn't
forgotten them. (Some PBX and Centrex systems, when they include UCD
as a separate feature, differentiate between music on hold and music
on queue.)
Usually, the outside party is seeking
a function rather than a particular individual. However, there are
instances, particularly when the ACD function is built into a PBX or
Centrex system, when it may be necessary to complete a call to a
specific agent. In such instances, dialing the agent's extension
should get the individual, while calls arriving at the same phone
via UCD should come via a pilot number. In general, removal of a
phone from a UCD group should not bar incoming calls to the specific
extension number: distinctive ringing for UCD vs. calls to an
individual can be helpful in this context, as can the various
displays on electronic telephones.
When there are several groups of
agents, each specializing in a given function, it may be desirable
to equalize the load when one group is busy and the other has agents
available (assuming, of course, that both groups are suitably
trained). Thus uniform call distribution may add various group
overflows, sometimes under the control of a supervisor, to queuing,
hunting, and connection to recorded announcements.
An interesting variation in UCD,
pioneered by Northern Telecom, takes advantage of properties of
multi-button electronic telephone sets. A given extension number
appears on an illuminated line pick-up button on a number of
multi-button sets. When a call arrives for that extension number,
all the sets ring, but the first to answer gets the call. If 1A2
emulation were used, the extension would show busy at all of its
illuminated buttons. However, a different mode of operation is
possible: the extension becomes busy only at the phone that took the
call. A second call coming in to the same extension number will ring
on the remaining sets, will be answered by one, and the rest will
show idle. Only when all the appearances of the extension are in use
on different calls will busy be returned to additional callers. This
approach delivers incoming calls uniformly to all members of a group
of agents and requires only a single extension number. However,
because a call is ringing on several phones at once, it is possible
for drones in the work force to sit back and let the hard workers
take most of the calls.
Traffic balancing.
For many types of switching matrix, hunt groups must be organized
with care to prevent traffic overloads. Line hunt groups, and UCD
groups in particular, tend to have higher occupancy than individual
lines, and when large hunt groups are needed, other lines served by
the same switches may find themselves locked out. The classic
example is afforded by using a SXS PBX to access a computer. With
ten computer ports "in rotary," all addressed by a single number,
computer access is assured as long as a port is free. However, a
shelf of connectors serves 100 lines; if all ten connectors are tied
up with long-holding-time calls to the computer, 90 other lines are
not going to be able to receive any calls at all. (What was actually
done to prevent such situations was to come off selector rather than
connector levels, and access computer ports via trunk circuits).
Unless the matrix is non-blocking,
examples such as the above can occur in even the most modern PBX and
central office switches. Trunk hunting groups can be equally
dangerous in digital switches where the matrix has no separate line
and trunk side. However, traffic can be balanced by scattering
members of a heavily used hunt group over a number of line groups,
averaging their traffic with more lightly used lines. Such
scattering also makes hunt groups more reliable in that failure of
any one line group will only take out a part of the hunt group. One
of the few disadvantages of accessing T-spans directly, as with
ISDN, is that scattering is not possible and all trunks in a given
T-span are driven by the same part of the associated switch.
No-hunt test calls.
Just as the no-test operation must be provided to permit maintenance
calls to complete to selected lines without a busy test, a no-hunt
connection is necessary to permit test calls to be made to specific
lines within a hunt group. Even when a line is only available to
users via a pilot number, means must be provided for test access
individually. The most common approach is to have the test position
dial in a test code followed by the pilot number and then the number
of the circuit within the group.
Tests of this sort are most meaningful
when agent stations are conventional phones or key systems where
each number has its own line. With electronic phones, where "lines"
of the hunt group are represented by illuminated buttons controlled
via signaling channels, different tests are appropriate. Analog CO
trunks and tie-trunks require both no-hunt and no-test access;
digital trunks multiplexed into groups where there is no hardware
specific to an individual trunk can often be checked by observing
framing bits or the use of other procedures which deal with the
group as a whole.
Call forwarding
There was a time when Call Forwarding
could be differentiated from hunting in two ways: first, it worked
on ring-no-answer instead of busy, or else as a direct substitute
for the called line without even testing for busy, and second, it
was user activated rather than wired or programmed into the system.
Time and experience have changed both of these factors. Today, many
systems allow hunting to work after a given number of rings, and
call forwarding to take place upon discovering busy; further, it has
been found highly desirable in many instances to remove call
forwarding from user control. Because call forwarding implies more
flexibility and generality than hunting, its name will probably
survive to cover the functions offered by both features.
Intercept.
Intercept is the probably the most important form of call forwarding
in a central office context. The purpose of CO intercept is to
minimize the number of calls that cannot be completed because of
customer moves, disconnections, number changes, etc. Such calls are
diverted to an intercept operator (or robot) who provides the caller
with updated information. Because somewhere between 15% and 20% of
all lines in a given central office may be on intercept at any one
time, and telephone companies cannot bill for calls not completed,
the economic importance of a good solution cannot be overemphasized.
Today, most central offices are
programmed to connect the caller to a trunk to an automatic
intercept machine after transmitting it the called number. The
intercept machine can then return a recorded announcement to tell
the caller that the line is no longer in service and, if it has the
information, the new number and its location. Because telephone
numbers are a relatively scarce commodity, they must be reassigned
fairly quickly; informing callers via an intercept system helps
achieve this goal.
In PBX and Centrex systems, where a
telephone number is often associated with a particular department or
function rather than an individual, and where the cost of making
moves and changes must be borne by the customer, the situation is
somewhat different. With Centrex systems, where customers reserve
some numbers for future expansion and may otherwise have some
numbers unassigned, the company name may be identified in a recorded
announcement, or the call diverted to the customer's console
attendant. In PBXs, the usual procedure is to direct unassigned
numbers to the console. In either instance, however, the call is
considered completed and the calling party is billed. If an
intercept message without mention of the company name or other
advertising is the only one possible, billing the calling party can
be avoided under existing regulations.
Call forwarding all calls.
This was the first of the customer call forwarding features, and was
widely advertised for both CO and PBX use. Before leaving the phone,
the user instructs the system to forward all incoming calls directly
to some other number. This is usually done by dialing the
appropriate feature code followed by the new number. (Outgoing calls
are not affected and can be made at any time.) Upon returning, the
user dials a cancel code. This feature can be used by people going
next door for the evening, or by managers of small businesses who
want their customers to be able to reach them at home after hours.
Obviously, the person forwarding the call should pay for the second
leg of the connection, particularly if it is long distance.
Call forwarding all calls is not
without its pitfalls. If a number of people forward their calls to
the same location, such as a conference room or a lodge hall, the
recipient may be overloaded. If call forwarding can be invoked from
a phone other than the one involved, competitors can steal calls; if
it can be invoked only from one phone, that phone must be revisited
to activate each change in destination. If complex security codes
are used, they can be forgotten or stolen. On top of all this, the
innocent caller may be confused upon reaching the "wrong" party, and
office clowns may call forward in a circle so that no party can be
reached. It is common practice to limit the number of call
forwarding "hops" permitted, and to program defensively against
circles.
Perhaps the worst problem is failure
to cancel. The person receiving forwarded calls cannot turn them off
and, upon calling the forwarding party, will get busy because the
call is forwarded right back to the calling phone. When the person
forgetting to cancel doesn't get any calls for two or three days and
reports trouble on the line, the service visit to cancel call
forwarding can be fairly expensive.
There are several solutions to this
problem, the simplest being to provide a "splash ring" at the called
phone each time a call is forwarded. Upon hearing this "ding," the
user will be prompted to cancel. Similarly, "broken dial tone" can
be supplied whenever the phone comes off hook. Another approach is
to cancel call forwarding all calls at midnight, awkward for those
who want their calls to go to an answering service while they are on
a two week vacation.
Most PBX and Centrex systems allow the
console attendant to cancel call forwarding, but where more heroic
measures are required, both applying and canceling call forwarding
can be made the exclusive prerogative of the attendant who also
keeps a log. Many modern PBXs can keep such logs automatically,
displaying them on demand on the attendant console's VDT.
Call forwarding within one switching
system, whether CO or PBX, is relatively simple. The switch checks
its memory tables and connects to one line rather than another. But
if the call is to be forward to another switch, voice mail or
intercept system, "call forwarding off net" must be used. Typically,
this feature is needed when a PBX or Centrex user in the city wants
to forward business calls to a residence in the suburbs. Now the PBX
must select an outgoing trunk, outpulse the residence number, answer
the incoming call and connect it to the outgoing trunk. There may be
transmission problems with two connections back to back in this
manner, and the calling party will be charged for the call to the
PBX, even if the suburban residence does not answer. However, this
particular feature has been found useful in making connections to
voice mail systems; the outpulsed number can be the identity of the
voice mail box for the person invoking call forwarding.
On occasion, a principal will use call
forwarding to direct all calls to a secretary to prevent
interruptions. The secretary receiving these forwarded calls should
be able to call the principal in case of emergency; when provided,
this is referred to as "call forwarding override." A separate
intercom function can do the same job.
Call forwarding on busy and no answer.
These features closely resemble hunting, but require great care in
their application. As with call forwarding all calls, it is often
desirable to arrange the system to only allow the console attendant
and/or the telecommunication manager to apply and remove them;
putting them in the hands of station users invites disaster. Another
question is whether call forwarding on busy should be combined with
call forwarding on no answer, or whether these features should be
separate. Finally, when hunting is offered as a separate feature,
how should these call forwardings and hunting interact?
By and large, call forwarding on busy
and no answer, the combined feature, is used with single-line
telephones where secretarial screening of incoming calls is not in
effect. If an incoming call finds the line busy or rings N times
with no answer, it can be moved to a message center or voice mail.
Residential phones can use this feature as effectively as Centrex
and PBX extensions, particularly as a means of accessing voice mail.
Call forwarding on no answer, as a
separate feature, is often used with lines in a hunt group; this
lets a call use hunting to reach the secretary if the boss is on
another call, but if the secretary can't answer for some reason, or
if neither the boss nor the secretary is available, the call can be
sent to a message center after N rings. Generally N should be no
more than 3; there may be a few rings at the message center, and
business callers tend to be impatient.
From the above, call forward on busy,
if used in place of hunting, is different from call forward on no
answer: different destinations are required. Just to complicate the
issue still more, different destinations may be chosen for outside
vs. inside calls, calls from specific numbers, etc. Putting such a
variety of choices, each with its own magic feature code, at the
disposal of the station user wastes endless hours in training, and
even then does not work well. It is far better to train the
customer's communication manager to do this programming from a
central terminal or console. The system must store such programming
in non-volatile memory (often magnetic tape or a hard or floppy
disk) so that it can reload after a system or power failure.
Because call forwarding is often left
to the user, various safeguards are frequently built into the
system's software. Some systems will allow only one type of call
forwarding to be in effect at a time. This requires canceling call
forwarding on no answer, for instance, before activating call
forwarding all calls during a vacation interval, followed by another
cancel and reapplication at the end of the vacation period.
Other limitations, already mentioned,
include the number of "hops" permitted. Some systems allow only a
single hop; this lets two people swap offices for the day and still
get their calls, something that might not happen if incoming calls
ricocheted from one phone to the other indefinitely, no matter which
number was called. Call forward on busy, if used as "next number
hunting," should allow multiple hops, but should return busy if it
finds itself hunting in a circle. Call forward no answer should
avoid going down a hunt group and ringing each line three times
before moving on to the next.
Each manufacturer seems to have
different ideas about how these features should be designed and
used; the customer, if fully aware of the possibilities available,
can often obtain highly satisfactory service. All too often,
however, the user finds out too late that there is some quirk in
system programming that prevents a desired action.
When calls are forwarded to a message
center, or from several principals to one secretary, it is desirable
for the answerer to know who the call was originally intended for.
If the secretarial or message center electronic telephone has enough
call pick-up buttons, one can be used as a "call forwarding target"
for each line (or group of lines) covered (note the similarity to
directed call pick-up), even if the called number is on a
single-line 2500 set. Because the number of buttons available is
always limited, and because it is possible that a call may have
hopped past several intermediate people before reaching a message
center, modern systems often show the extension number or even the
name of the called party in a liquid crystal display on the message
center phone, delivered via the phone's digital signaling channel.
This saves buttons and makes the response of the message center much
more useful.
Voice mail systems have, since about
1990, changed the way hunt and forwarding groups are set up in PBX
and Centrex systems. With stored program control, hunting and call
forwarding can easily be arranged to eliminate busy and
ring-no-answer situations, delivering a call to one of several
alternates if the principal is not available. Unfortunately, even
the best secretaries or message center agents have difficulty taking
lengthy messages, and sometimes even take down short messages
incorrectly.
Voice mail has no such problems. Voice
mail can, like a telephone answering machine, take a message on
ring-no-answer; it can also take a message if the called line is
busy, and can even take two or more messages at the same time. The
caller can leave a message in his or her own words, as lengthy and
complex as necessary, and the machine will deliver it as recorded,
playing it back as often as desired. Thus today, if a principal is
busy on another call or does not answer, the next choice is usually
NOT a secretary but a voice mail-box, which greets the caller in the
principal's own voice.
Even so, there are times when only a
human will do, and most voice mail systems give the caller verbal
cues as to how a human can be reached, instead of or in addition to
the mailbox. This requires a DTMF signal from the caller to the
voice mail system, and the voice mail system to transfer the call to
another PBX or Centrex extension. As has been discussed, this
requires the switch and the voice mail controls to be in close
communication. If voice mail ports simulate electronic telephone
sets, either proprietary or ISDN BRI, appropriate communication is
simple via the D Channel; if voice (or fax) mail systems insist upon
looking like 2500 sets, they have to use stupid-smart signaling and
even then may not be able to do the entire job. Even so, hackers
find the link between voice mail systems and PBXs prime targets in
their quest for free long distance, computer access, etc.
In any kind of telephone system, the
caller has to identify the called party to the system on each call.
In the days of SXS, one could usually complete a local call by
dialing 4 or 5 digits. With the coming of DDD in the 1950s, seven
digit numbering became standard for local calls, with a three-digit
area code added for long distance. Later, a preliminary 0 or 1 had
to be added to identify the following three digits as an area code
rather than an office code, bringing to 11 the number of digits for
a long distance call, and soon even local calls will require an area
code. Overseas calls, of course, require even more digits. One might
assume that reducing the number of digits dialed, particularly for
frequently called numbers, would be a popular feature.
Abbreviated and repertory dialing
Abbreviated dialing was offered with
great fanfare in the 1960s as one of the many features made possible
by the huge amounts of inexpensive memory in central office switches
with stored program control. In the next decade, technology dropped
the price of memory enough to allow PBXs to offer a similar feature.
Not only was the number of digits required of the user reduced, but
dialing was faster and more accurate (recall that a machine can dial
a 10 digit DTMF number in one second, although a human cannot).
To be most effective, abbreviated
dialing should allow a user to contact frequently called numbers by
dialing a minimum number of digits: one digit for 10 or fewer, two
digits for 100, etc. However, the switch must have some way to
differentiate one or two digits identifying the called party from
the first digits of a conventional telephone number, feature code or
member of a dial intercom group. A time-out (typically 3 seconds or
so) is frequently used, but this tends to defeat the purpose. To
eliminate delay, an initial feature code or an end-of-dialing
signal, often the # of DTMF, is commonly used, although this leaves
rotary phones with a problem. Use of one of the ten digits can be
arranged not to conflict with the numbering plan in a PBX or Centrex
system, but at the expense of future flexibility.
Rolm's solution, as we have seen in
Fig. 1, is to use #3 as a feature code to access "station speed
call," a list of phone numbers private to one telephone, and #4 for
"system speed call," a list of frequently called numbers which are
shared by a group of phones. A two-digit feature code followed by
one or two digits identifying the called party suggests abbreviated
dialing should not be used for intra-PBX calls which typically need
only three or four digits in the first place. However, for placing
outside calls where the system must dial 9, detect dial tone or time
out, and then dial a number consisting of 7, 11 or more digits, the
utility of abbreviated dialing can be appreciated.
The memory technology which made
abbreviated dialing a natural for computer-controlled switching
systems also allowed telephone sets to store frequently called
numbers directly, and to send them at the push of a single button.
The term "repertory dialing" is often used to differentiate between
a telephone list stored in a telephone set and an abbreviated
dialing list stored in switch memory. As has already been discussed,
feature codes can be stored in repertory dialers as easily as
telephone numbers. Repertory dialing, one of the most useful and
least expensive features which can be provided, is not limited to
telephones; facsimile machines and personal computers also provide
it. As a result, switch-based abbreviated dialing has not developed
as rapidly as its early proponents had hoped.
One advantage of abbreviated dialing
over repertory dialing is the availability of "common lists" shared
by a group of people. The numbers in common lists can include
suppliers, customers, and other frequent business contacts. Further,
with the right kind of program design, common list abbreviated
dialing can be used to control long-distance toll abuse: restriction
is applied to all long distance calls dialed in the regular way, but
access to a common list allows abbreviated dialing to complete
legitimate long distance calls.
Whether abbreviated or repertory
dialing is used, the customer should be free to change items on a
personal list without the help of telco personnel, and without
delay. Similarly, authorized personnel in corporate communications
departments should be able to change common lists. Unfortunately,
these changes are sometimes more complicated than might be expected;
an advantage of system-based abbreviated dialing is that changes can
be made from a central point by trained personnel, reducing or
eliminating the need to continually train and retrain station users.
A complication produced by private
list abbreviated dialing is the need for the caller to use one
number from his or her own phone, and a different number when
calling from any other. A repertory dialer, with a separate button
for each frequently called party, eliminates the need to memorize
abbreviated numbers. Note that repertory dial phones send the entire
telephone number, and thus are independent of the serving switch.
Although pushing a single button is
usually more convenient than using an abbreviated number, extra
buttons cost money. Thus some systems, both switch-based and
telephone-based, use liquid crystal displays to step through the
names in the phone list, allowing the user to send the stored number
when the right name appears in the window. In some systems, the
called party's name can be spelled out using the letters on the DTMF
key pad; this is easy to do and the system can usually find a match
in a limited list quite quickly. To call "John," for instance, the
caller would depress 5646, but if no other name started with a J, K
or L, the name "John" would pop up in the window after the first
digit. Countless schemes of this sort have been devised.
On electronic telephone sets,
line-select buttons and repertory dial buttons are more similar than
might be thought. Systems can be arranged so that, if an extension
is idle, pushing its button on another set will establish a
connection, while if the extension is in use, pushing its button on
another set will cause that set to "bridge on." Extra set buttons
can also be used for repertory dialing of outside numbers; because
the system usually does not know the status of distant called lines,
the lamp associated with the button serves no purpose. Some
electronic telephones such as the NEC Dterm sets used with the NEAX
2400 PBX have two sets of buttons, one with lamps (for line pick up)
and one without (for repertory dialing).
Hot line.
Hot line is similar to abbreviated
dialing but only one called number is stored; when the phone is
taken off hook, the system establishes a connection to that number
with no user dialing at all. An arrangement of this sort is much
more reliable than a permanent private line, simply because the full
alternate routing and maintenance capabilities of the switched
network are available to each connection. It is usually less
expensive, because the connection is paid for on a call by call
basis plus a local line charge (if on a CO rather than a PBX).
Hot lines, implemented with a
repertory dialer rather than switching system memory, are frequently
used as "public" lines or courtesy phones in airports and train
stations, limiting user access to selected hotels, reservation
counters and car rental companies; low cost and ease of control are
factors here. Whether set or system memory is used, hot lines can
provide an instant connection between a store and a warehouse,
broadcast control room and transmitter, etc.
Evidently, hot-line service could
easily be activated on a dial-up basis by residential customers; a
baby-sitter could simply pick up the phone for connection to where
the children's parents have gone for the evening. In this kind of
service, a time-out would be needed before establishing the
connection to permit a cancel code or other telephone number to be
entered.
Direct-in Line and Direct Department
Calling
These two PBX features are quite
similar and are a variation of hot line. In the former, one outside
trunk has a hot-line relation with an internal extension, while in
the latter, a specific outside trunk group has a hot-line relation
with a group of internal extensions, usually in some version of
hunting from a pilot number. In either instance, an incoming call,
ringing at the PBX trunk, is detected by the system control which
causes ringing to be sent to the appropriate extension. When that
extension answers, a hot line matrix connection is established,
incoming trunk to line.
Although the principal reason for
having private lines behind a PBX is to provide calling capability
direct to the CO if and when the PBX fails, there are several
reasons for giving up this approach to reliability and making a
switched connection through the PBX. First, with electronic PBX
phones, the only way the same phone can pick up a private line as
well as internal extension numbers is to bring the CO line to the
PBX as a trunk. Second, when the connection is made via the PBX, the
full range of PBX features, including transfer, conference,
forwarding, etc., is available. And finally, if the phone happens to
be compatible with CO signaling, power (or system) failure transfer
can still provide reliability in case the PBX dies. Note that 1A2
phones have this compatibility, but most electronic PBX multibutton
sets do not; ISDN phones using the S/T interface will neither meet
the BRI U interface nor the PRI.
Direct department calling is usually
used incoming only, its purpose being to bypass the PBX console
attendant and deliver calls directly to the department in question.
If the department is for sales, service, or something else that
might use the UCD feature profitably, there is no point in delaying
such calls at the console. Direct department calling uses regular CO
trunks, and thus is available even when DID is not supported by the
local CO. Even when DID is available, eliminating the need for the
CO to outpulse the called number using dial pulses can speed up
operations. With Centrex, of course, there is no need for either
feature, because extensions and hunt groups are already functions of
the central office switch.
Night connections.
A "night and through" connection for
after-hours use at a PBX can be thought of as a dial-up hot line,
working like a direct-in line from a regular CO trunk when the
console is closed. The attendant can either set up each connection
as needed before closing the board, or a group of night connections
can be programmed into the system and activated by simply putting
the console in the night mode. This is the better procedure for
those who generally work late; their contacts can then call them by
dialing "their" specific trunk number. Again, if DID is available,
night connections are not necessary.
In the days before consoles replaced
cord boards, the attendant would simply use PBX cord circuits to
connect selected extensions to CO trunks. This meant, of course,
that the PBX extension was now directly connected to the CO for
outgoing calls as well as incoming, and could not dial intra-PBX
connections or access WATS lines or other special facilities.
Electromechanical systems with consoles had various ways of setting
up night connections, most of which were relatively complicated
compared to putting up a pair of cords. Today, most of these
approaches are of historical interest only.
Note that night connections are not
mutually exclusive with UNA. The latter is used with the first
choice trunks from the CO, and later-choice trunks are used for
night-and-through. Out-of-hours traffic being what it is, there is
seldom any overflow from directory number to night-and-through.
There is no particular reason why the
hot-line night connection must stay in the PBX. Arranging it to work
like call forwarding off net can reach executives at home or via a
tie-trunk network. It is even possible to arrange for the remote
answerer on a private network to invoke functions such as transfer
internal to the PBX which first received the call (See Chapter 6).
Add-on conference and
consultation-hold have been discussed in connection with station
dial transfer. User acceptance, however, leaves much to be desired.
At present, it is very likely that most consultation while another
party waits is done via conventional or electronic key telephone
sets; the first party is put on hold and the consultation call is
placed on a second extension number. The user can alternate between
the original and the consultation call by operating the hold button
and then depressing the pick-up button for the line desired. Some
skilled multi-line executives can have three or four calls going at
once.
Conferencing is most often carried out
by having several phones on each end of a conversation connected to
the line. This is common in residential service where only one line
is provided; it is even more convenient in business where
conventional 1A2 key telephone sets allow a given line to be picked
up on several different phones by depressing the appropriate line
button. Such bridged connections tend to impair transmission,
particularly on long distance calls, and telephone companies do not
guarantee transmission under these circumstances, but there is
usually no problem if only one or two phones are added.
Electronic phones, of course, depend
on the switching matrix to set up connections via conferencing
circuitry even when they emulate the bridged connections of 1A2. An
advantage of three-way and larger add-on conferences, whether the
instruments are electronic or conventional, is that conferees are
not limited to those whose numbers appear on multi-button sets. Even
single-line sets can establish conferences fairly easily by
instructing the switching matrix to carry out the appropriate
connections.
Many patents have been issued for
analog conference bridges, circuits that use amplifiers and hybrid
coils to match impedances at two-wire interfaces and maintain signal
levels as the number of conferees increases. Most of these circuits
depend on return-loss balance at each port. Because this is not
particularly good, there are limits to the amount of gain that can
be provided and the number of phones that can be connected together.
To minimize the return-loss problem, conferencing is better done at
a four-wire point, something quite natural for digital switches.
From a four-wire point of view,
conference bridges must listen to the transmit side of all
participants, combine the signal that results when two or more speak
at the same time, and feed the resultant signal to the receive side
of each line after subtracting out that line's transmit signal.
Whether analog or digital systems are
involved, conference transmission is well understood; the problem of
control can sometimes be more difficult. Conferees must be added one
at a time, with attention given to the proper procedure for the
conference originator to follow when a busy line or a ring-no-answer
condition is encountered. It is generally desirable to speak
privately to each new conferee before addition to the group, so that
he or she will know what is going on; it is also desirable to
minimize the time the ever-growing group has to wait before
festivities can commence.
One interesting application of
abbreviated dialing is to call up an entire conference with one
code. For regularly scheduled conferences, this can be a great time
saver, but dealing with busies and no-answers, and providing
instruction to the called parties (particularly when some have
screening secretaries) can be a problem. If everybody knows the
conference time, a "meet me" conference, where all conferees dial
the number of the conference bridge at the same time, can save
set-up time and eliminate the busy and no answer problems of
abbreviated dialing. However, a meet me conference may very well
have unauthorized attendees.
Once the call is in progress, the lack
of visual cues and even positive identification of the speakers can
lead to human-factors difficulties beyond the scope of telephone
switch design. Unless some very rigid protocol can be adopted, large
telephone conferences can easily degenerate into chaos. Often,
techniques surprisingly similar to polling or token ring, used in
local area data networks, emerge. Northern Telecom and others have
done some work to use computers associated with voice-data ports to
provide a visual indication of who is present in the conference, and
also to allow transmission of data and graphics to other members of
the group. Such approaches work best within one PBX or Centrex
system, where all the ports are identified, but with the
availability of SS7, similar conferences should be possible via the
public networks.
In audio conferencing using two-wire
facilities, hang-up supervision can be tricky; the system must
monitor each line and, when hang-up is detected, it must terminate
that line's port on the conference bridge quickly. Sometimes it is
desirable to drop all remaining connections when the conference
originator hangs up. The problem is compounded when the conference
circuitry is associated with a PBX where answer and hang-up
supervision from the public network may not be available to the PBX
control. Two-wire CO trunks also inject needless transmission
difficulties. Digital trunks, preferably with a separate signaling
channel (ISDN PRI), should solve these problems nicely.
A variation on conferencing may be
called "dial up alert." Volunteer fire departments, ambulance
squads, etc., need a service of this sort. When the siren, a pager
beep or other trouble signal is heard, members dial the "alert
number." Someone or a special recording at a central point repeats
over and over the fire address or other instructions. Multiple
callers are bridged on in a listen-only connection, just as to tone
distribution in a digital switch, remain until the needed
information is acquired, and then drop off.
In a two-wire analog system, either a
two-way conference bridge or a one-way broadcast amplifier with one
input and several output ports can be used; a relatively large
number of people can be handled with a modest number of ports.
Callers hear audible ringing until a port comes free; the system has
to be smart enough to connect them in first-come first-served order,
as with a UCD queue. Because most digital systems can allow an
almost unlimited number of ports to listen to a given source,
digital systems are at an advantage in providing such services. They
might equally well be used to provide general information to the
public in emergencies such as earthquakes, floods, etc.
As of the early 1990s, video
teleconferencing is just becoming available with pictures compressed
into two DS0 channels (112 or 128 kBps) between conference rooms. To
connect multiple conference rooms, the equivalent of a conference
bridge will be needed, but it will obviously not make a linear
combination of all pictures. More likely, it will monitor which
source is providing audio, and will send its picture to all other
conference rooms, while allowing the speaking party to continue to
view the previous image, presumably that of the person being
addressed. As long as pictures can be transmitted via the same
circuit-switched bit streams that handle voice and data, both PBX
and CO switches can be expected to enter this field. Standards for
combining several T-carrier voice channels to provide a broader
digital bandwidth are now available for better picture quality, and
better compression algorithms can be expected to reduce the
bandwidth needed. This may require smarter control in most switches,
but should be easily accommodated.
The increasing availability of
powerful PCs on office desks and in the home is providing an
additional approach to picture phone and video teleconferencing. A
PC is already an intelligent display; addition of a camera as a
peripheral device and appropriate software can turn it into a
picture phone terminal at a relatively small incremental cost. Still
pictures (slow-scan TV) are easily sent over the analog voice
network like fax (9.6 kBps). When DS0 channels are available, more
or higher quality still pictures can be sent, and compressed
full-motion video similar to that discussed above is practical.
Video conferencing with a separate window on the PC screen for each
connected party is already more than a novelty at trade shows.
At the turn of the last century, the
better hotels were not at all convinced that a phone in every room
was a good idea. To crack the market, the local telephone company in
Chicago made the Congress Hotel "an offer it couldn't refuse" and a
very early PBX was installed on terms extremely favorable to the
hotel. For other hotels, it was hard to avoid the same kind of deal,
and for many years hotels got a real bargain under the guise of
acting as an agent for the telephone company in providing service to
their joint customers.
Hotels, in many ways similar to
hospitals and colleges, differ from other businesses. In particular,
their station users are divided into two distinct classes: customers
and staff. Customer phones generate relatively light traffic mostly
with the outside world, and are quite similar to residential phones
on a telephone CO. Staff phones, on the other hand, are much more
like regular business phones, generate high intra-switch as well as
outside traffic, and require features such as those provided by 1A2
key systems. Customers tend to use the phones in the evenings, while
staff use them during the day; customers outnumber staff by a wide
margin, so average traffic is about the same, day and evening, and
relatively light. Finally, hotels and motels, at least, really do
act as agents to collect telephone charges from their customers.
Prior to deregulation, "commissions" for this service often came
close to covering the monthly rental of PBX equipment supplied by
the telephone company.
When interconnect became legal, the
light traffic and relative simplicity of hotel/motel service made it
a natural target for the new vendors. There was some friction when
the hospitality industry continued to expect commissions and free
telephone directories from the telephone company from whom they were
no longer renting equipment, but interconnect apparently won the
day, even before the divestiture of the Bell operating companies
from AT&T. And even though hotel service was relatively simple to
provide, stored program control made a number of features standard.
Numbering plans for hotels
It is traditional and often logical
for PBX extension numbers to agree with hotel or motel room numbers
(architectural numbering). However, the arrangement of numbers as
dictated by hotel floor plans is seldom ideal from a communications
standpoint. Usually, the first digit of a room number identifies the
floor and the remaining digits the room on the floor. Because there
are often more than ten and fewer than 100 rooms on any given floor,
many "holes" are left in the numbering.
Downtown hotels usually have one floor
above another (often omitting 13 as a designation); however, if the
hotel is more than ten stories high, the first digit for floors one
through nine either has to be 0 (which conflicts with console
access) or else the system has to be able to differentiate between
such rooms 101 and 1010. If room numbering starts at 200 or 300,
allowing lower floors for lobby, offices, meeting rooms, etc.,
duplication of the first digit does not occur until the 20th or 30th
floor. However, the system must be prepared to handle both 3 and 4
digit room numbers if the building is more than 10 stories high.
Motels are usually no more than two or
three stories high, and expand in additional buildings. Relating
building, floor and room to an extension number sometimes requires a
truly creative approach.
A related numbering plan problem comes
from the desire to have various hotel services available by dialing
a single digit (a form of abbreviated dialing, but without a feature
or access code). If the building is only two or three stories high,
the digits 4, 5, 6, and 7 can be used for room service, front desk,
mail, call girl, etc. The digit 8 was, in the past, used for access
to the long distance operator; 9 provides telco access, sometimes
restricted to local calls, and 0 reaches the console attendant.
Sometimes an access digit is used for room extensions: dial 7 plus
the room number. This leaves the digits 1 though 6 for special
services, and allows all digits to be used to identify floors, as
long as the system can differentiate between rooms on floors 1 and
10, 2 and 20, etc.
One of the major advantages of a
stored program system is flexibility in numbering. Unfortunately,
some early systems chose to develop non-flexible numbering plans, no
better than SXS, to save a little memory. Thus changes in hotel
plans during construction, which the flexibility of the computer
should have been able to accommodate, posed major problems. Even
when hotel room numbering is known completely, full flexibility for
the telephone numbering plan can sometimes turn out to be of great
importance, and should be insisted upon by the customer.
Charging, billing and restriction
A bill, including telephone charges,
must be ready when a hotel guest checks out. Providing such timely
charging information to the customer is one of the major
requirements of hotel/motel service. Hospitals work pretty much the
same way, although the average stay may be longer. Colleges usually
require telephone charges to be included in the monthly bill for
student room, food, etc.; here the telephone system is expected to
provide billing information in a format compatible with the
institution's property management computer system (PMS).
Traditionally, electromechanical
message registers behind the hotel front desk, one per room, counted
message units for local calls accessed by dialing 9 (CO trunks for
toll calls, reached by dialing 8, did not operate message
registers). When central offices and PBXs were SXS, the battery
reversal on each dial 9 trunk could reach back through the PBX to
score the calling room's message register appropriately. With
crossbar and ESS switches at the CO, most telephone companies made
available a third wire with each CO trunk to the PBX for message
register operation. This, of course, required special trunk circuits
at the hotel PBX to extend the signal to the room's message
register.
Today, of course, electromechanical
message registers are no longer used in new equipment. With memory a
natural function of the switch control, system memory becomes the
logical way to store local call counts for each room. Some systems
have a special console for the cashier and/or the front desk to call
up telephone and other room information at check-out time, and then
reset call counters. The same console can be used to activate or
deactivate other hotel phone features, as will be discussed below.
In small PBXs, the regular console often adds these duties to its
functions.
With deregulation of the telephone
industry, and the requirement that customers own the equipment on
their own premises, many hotels opted to handle their own charging;
various approaches are used such as a flat fee added to the room
rent for local telephone service, a charge per local call whether it
is answered or not (because there is usually no answer supervision),
or both. This has not generated customer good will. Although some of
the newer COs can provide answer supervision as a special option on
analog CO trunks, it appears that ISDN with its separate signaling
channel will be necessary to make accurate answer supervision
universal.
Toll calls billed to the room require
more effort. Although some telephone companies and long distance
carriers have provided per-call billing information via a technique
identified by the acronym HOBIC, where billing information is
delivered to the hotel from the CO via a teletypewriter or other
data link, many hotels now handle their own billing, estimating
answer time. Most PBXs generate CDR records on a per-call basis;
these records are delivered via an RS-232C port to a PC equipped
with software to calculate the charge per call based on called
number and duration, add it to the call record, and accumulate call
records by room number for later use. In small hotels and motels,
the PC may actually handle the entire hotel billing, running the CDR
program in the background and allowing room, movie, restaurant and
other charges to be entered from its keyboard. In larger hotels, a
separate PMS computer may be required; if so, it can interrogate the
PBX as needed to get call records accumulated by room number.
Guests who wish to charge long
distance calls to their telephone credit cards have to be able to
reach their own long distance carrier, which may be different from
the one used by the hotel; the hotel may add a surcharge for the use
of its PBX when such calls are made. The additional signaling
capability of ISDN should make carrier selection and other billing
needs simple, but perhaps at a cost higher than some hotels are
willing to pay.
Directory and message center.
When station users are transitory,
outside callers will almost always ask for them by name rather than
number. As a result, built-in telephone directory systems (discussed
in Chapter 2) which can be updated in real time are of particular
value in hotels and similar institutions. Obviously, the additional
ability to search by first name, company affiliation, home address
and other clues, typical of data base operation, can be a big help
to the console attendant.
The directory can also be helpful at
the message center, where guests who cannot be located must be
identified accurately so that their messages, once recorded, can be
delivered to them. Storing messages typed in at a keyboard rather
than transcribed with a quill pen is highly desirable, and can
easily be designed into the PBX. The message center must also be
able to activate appropriate message-waiting lamps to make sure
guests know that messages await them. Voice mail systems can be used
in hotels, but there is much to be said for human message centers
where privacy and accurate delivery are available for current
guests, expected guests, and those who have already departed.
Other hotel/motel features
Wake-up and privacy.
Hotels require a number of specialized features (originally handled
manually) such as wake-up and privacy. Providing manual wake-up
service from the switchboard or console, when a large number of
people might want to be awakened at about the same time, either
implies a delay before all rooms can be contacted or an excessive
number of attendants. With automatic wake-up service, the room
number and time are entered into the PBX memory, perhaps when the
guest checks in, perhaps later, and the PBX rings the phone and
connects a recorded wake-up announcement, usually including a time
check, when the phone is answered. If the phone is not answered, the
system can be arranged to alert the console attendant. The wake-up
signal should have the option of being a one-time occurrence or
repeating every morning while the guest remains in the hotel. In any
event, it should be canceled at check-out.
Some hotels may wish to offer
uninterrupted sleep at night, at least as far as the telephone is
concerned. With manual switchboards, all this required was suitable
training for the attendant, but with an automatic switch, privacy
requires that, after a given hour, internal calls be forwarded to
the console rather than rooms desiring not to be disturbed; the
console can override in emergencies.
Room status.
Room status is a feature that would be hard to provide manually. It
starts out simply enough by changing the class of service of a room
phone from restricted to internal calls only, when the room is idle,
to having outside access as soon as a guest checks in. Obviously, at
check out, when the billing information is obtained, the phone is
changed back to restricted.
The next step is to provide on
request, at a front desk display, further information such as
whether the room is being made up or is ready to rent. This
information is generated by the maid dialing a code into the system
upon entering a room, and another upon completing the job. Where a
separate PMS is used, this information, like billing, is passed from
the PBX to the computer. Similarly, efforts have been made to
combine environment control with the telephone, accessing room air
conditioners, TV sets, and other heavy users of electricity via the
PBX wiring and control intelligence already present.
Paging.
Paging, preferably with several zones (dining room, swimming pool,
lounge, etc.), is another natural service for hotels. With an
on-line directory and paging, it should be possible to locate any
guest fairly quickly and easily.
Consoles for hotel service.
PBX systems for small hotels usually
have only one console for answering incoming calls and handling the
features described above; however, a different face-plate is often
provided so that labeling will be suitable to hotel/motel service
and match the special programming option the hospitality industry
requires. The need to display calling numbers to room service and
the bell captain suggests additional simple displays are highly
desirable.
Another option is to have a special
small console, for hotel features only, to augment the main console.
This console is used to read and reset message registers, read room
status, display calling room numbers, and control message waiting
lamps. The coming of electronic telephone sets with their displays,
lamps, buttons and flexible programming is tending to make special
consoles unnecessary. Paging is usually handled from the main
console. Were directory, message storage or PMS features are
included in the PBX, a video display terminal of some sort is highly
desirable.
Consoles will be discussed further in
Chapter 6.
Click Here for
Answers
1. How are business telephone features
different from those needed by residential customers?
2. Is it possible to use only single
line telephones in a business context?
3. How do single line phones invoke
features?
4. What is meant by "1A2"?
5. Name two basic 1A2 user patterns.
6. How do PBX electronic phones differ
from 1A2? How do electronic key systems differ from 1A2?
7. List some differences between PBX
and telco CO numbering plans.
8. What factors must a business
numbering plan relate?
9. What is a "prime line?"
10. What is the difference between
hard hold and soft hold?
11. How does joint holding differ from
calling party hold?
12. Describe two kinds of distinctive
ringing.
13. What is a "no-test" connection?
14. What is the difference between
Camp-on and Call-waiting?
15. How does a "busy buster" differ
from automatic call-back?
16. Give some reasons why a PBX should
be able to transfer only calls incoming from a CO.
17. Suggest some privacy problems to
check on your PBX.
8. What are the two kinds of call
pick-up feature?
19. What is a "parking orbit?"
20. What is the difference between
"park" and "hold?"
21. How does DNIS differ from Calling
Number ID?
22. Suggest some differences in
hunting and call forwarding.
23. Suggest some ways for call
forwarding to offer variety.
24. What is the difference between UCD
and ACD?
25. What is the difference between
abbreviated and repertory dialing?
26. Why is DDC to a UCD group faster
than DDD?
27. Suggest reasons why digital
switches can provide better conferencing than analog switches.
28. Name some important hotel/motel
features.
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