Answers to Review Questions

• Chapter One

• Chapter Two

• Chapter Three

• Chapter Four

• Chapter Five

• Chapter Six

• Chapter Seven

• Chapter Eight

Chapter 1

1. Interconnection, billing, calling features, operator access and test access.

2. The first switch connects the callING line to a trunk to the second switch, and that switch connects the trunk to the callED line.

3. The switching matrix and the system control.

4. Space, frequency and time division.

5. Packet switching.

6. It uses channels separate from those used for conversations, and operates at much higher speeds using data transmission techniques.

7. 500 ports can generate, at most, 250 simultaneous conversations. With 125 possible, concentration is 2:1. If the probability of that many simultaneous conversations occurring is very low, there is little motivation to spend money to make more possible.

8. In a system made up of line groups and a group selector, the line groups interface ports and provide concentration, while the group selector provides distribution of calls among the line groups.

9. An electronic matrix, because the interface to the outside world has to be in place at all times; traditional functions cannot usually be switched in on an as-needed basis via the matrix as can often be done with a metallic switch, but must always be present.

10. Electronic, so that port circuits tailored to each phone or trunk can all interface the matrix which can then act as a converter from one port to another. Note that signaling is similarly converted via the system control.

11. Because they need amplifiers and/or "pair gain" (that is multiplexing to put many channels on one transmission medium), both of which are best implemented with one-way systems.

12. Probably not. Microwave, for instance, usually develops more channels to sell for a given amount of bandwidth by using analog rather than digital techniques. Because bandwidth is limited, this makes a problem. There are some long-haul digital systems on coaxial cable, but optical fiber is much less expensive and more reliable. Possible conclusion: no single mode optical fiber, no ISDN.

13. Toll switches were four-wire to match the their trunks, eliminating hybrids and the echoes they caused.

14. (a) 15 switches, 20 inputs per switch: 300 inputs and 300 outputs. (b) Each primary and tertiary switch has 20x10=200 crosspoints and there are 30 of them. Each secondary switch has 15 inputs and 15 outputs for 225 crosspoints and there are 10 of them. Thus the total number of crosspoints is 200*30+225*10=8250. (c) For 300 inputs and 300 outputs, we would need 90,000 crosspoints. If we have 300 2-wire ports, a triangular matrix will need 44,850 crosspoints. If we have 300 4-wire ports, we will still need only 44,850 crosspoints, but each crosspoint will have twice as many contacts so that it can connect each port's talk path to the other's listen path.

(d) Primary and tertiary switches, still 15 of each, would have to have 39 horizontals. Thus they would have a total of 20*39*15*2=23,400 crosspoints. There would be 19 secondary switches, still 15x15, adding another 4275 crosspoints for a grand total of 27,675. A lot less than 90,000, or even 44,850.

15. Just unplug the phone at the old location and plug it in at the new one. The radio signal on the coax will find it wherever it is.

16. 128, because each connection needs two time slots, one for each direction of transmission.

17. Each primary and each tertiary switch would have 20 8-bit words to match the time slots on its incoming or outgoing highway. We would probably have 20 time slots divided into three phases, one write and two read on the primaries, and two write and one read on the tertiaries, where the read phases on the primaries match the write phases on the tertiaries. There would only be one physical secondary switch, 15x15, but it would be the equivalent of 40 because of the different connections possible in each time slot. We'd probably make the secondary handle all 8 bits in parallel in a given time slot, which would make it a little larger.

18. T-S. Time switching is provided first, followed by space. Thus any time slot on any input can reach any time slot on any output.

19. A shared codec must work at a much higher speed, and requires relatively elaborate circuitry to effect the sharing. Further, noise can sometimes leak over from one line to the next via the shared codec. With a codec per line, the switch sees only digital signals; thus it can switch digital signals whether they coded voice, data, image, or whatever.

20. A terminal sending a packet of information inserts a "header" containing the called terminal's address at the header's start. The packet header is read by electronic "stunt boxes" in all terminals as it goes past them; the stunt box for the called terminal, when it sees its own address in the header, causes the packet to be copied for storage or display.

21. Yes, but delay and echo can be problems. Packet networks are at their best with bursty data; speech, while "half duplex," generates a lot of packets.

22. SXS set up a connection one switch (one digit) at a time, each digit eliminating 90% of the lines available; upon connecting to the called line, busy test was made. 5XBAR used the whole number to make the matrix connection all at once, but made busy test on the called line before trying to make the connection. Early SPC machines used a single processor to handle connections; today, distributed control, usually in a hierarchy, is used, although there is a strong trend toward autonomous and equal line groups.

23. One of the several claims in the 5XBAR patent. A connection was set up through the matrix to an originating register to get the called number; once the number was obtained, that connection was discarded, and a new one set up to complete the call. This gave 5XBAR great flexibility, but "wasted" the initial connection to the OR.

24. It provided an inexpensive way to tell a dial pulse from a hang-up, and an inter-pulse interval from an interdigital interval on each SXS switch. In its day, quite remarkable.

25. Translation, storage of calls-in-progress detail, storage of system data base including, in particular, Class of Service for each port. Above all else, can provide good administrative access to the program and its data base. The system can be upgraded by changing the program, not the hardware.

26. No.

27. Operation and Administration. Operation handles the Recording, Completing, and Modifying functions for each call, while Administration deals with management and maintenance; in general, the latter is much the larger.

28. In certain kinds of manual systems, there were two kind of operators: Recording and Completing. Analogy with their tasks has been used here. The Recording process detects originations, returns dial tone, and obtains the called number. It also keeps track of the calling number, class of service, etc. The Completing process takes this information and establishes the connection, including finding the appropriate outgoing trunk, connecting to it, and outpulsing, or making the busy test on the called line, ringing it or returning busy tone. The Modifying process consists of watching for hangup as well as requests for features. All three processes participate in billing. Recording gets the basic information, Completing gets answer time, and Modifying gets hangup time.

29. Not really. Some have referred to the use of a powerful computer to interrogate all lines and trunks five times a second to determine switch-hook status as "fanning the air" because so much effort and computer "real time" is taken away from other perhaps more important functions while so few changes of status are found. However, when computers were very expensive, there was nothing to which this task could be delegated. Today, of course, scanning is usually handled by small front-end processors which transmit only pertinent information up the control hierarchy. When each line group brings its own front end processing, system growth has less impact on central control equipment.

Chapter 2

1. Per-call, per circuit and per system. RAM, pROM and ROM respectively can be used, but it is common to use RAM for everything, with backup on a hard disk.

2. The called number is most important, but the calling number is needed for call-back and billing, and COS is needed to see if the call can be completed and how. For billing, answer and hang-up time will be collected. The system also has to keep track of the path through the switching matrix.

3. COS or Class of Service defines the features and services available to a line (or trunk). COS will tell the kind of signaling the expect on an incoming call, and the kind of ringing or outpulsing needed for an outgoing call. COS also lists restriction and routing information which can be used, what kind of telephone set is on the line, what features (conference, transfer, pick-up, etc.,) it has, etc. Trunks may be one way in or out or two-way and, without CCIS, may have different kinds of signaling and supervision, all part of COS.

4. If there are 6 things that can be in or out, there will be 6 class marks, or 2¶6¶ = 64 classes of service. However, a great many more than 6 class marks may be needed to define the features and privileges of a PBX or CO line.

5. A generic program is complete, but may be only partially activated, while a building block program is installed with only the parts present that the customer has contracted for.

6. One approach is for the control to deal with "lists" or queues of lines ready for some particular operation. The computer will run down each list in turn, providing the list's operation to each member. One list might be for connecting dial tone, while another might be for making a matrix connection.

7. A more powerful processor can be used, the system can be designed to have multiple processors sharing the load, front end processors can deload the central processor, certain kinds of routine tasks can be suspended during peak traffic periods, etc.

8. Translation is the process of obtaining one symbol that stands for another. Here, we are particularly concerned with the relation between telephone directory numbers and equipment numbers (location of the line on the switching matrix), and the relation between route codes and destination codes. The first kind of translation allows any line to be associated with any matrix port, while the second allows the same directory number to be used independent of how calls are routed.

9. In electromechanical systems, COS had to be related to the equipment number because certain matrix ports provided certain functions. In electronic systems, where flexibility is much greater, COS can be associated with the directory number rather than the equipment number. This allows a simple change in translation take a line's entire feature set to a different matrix port. As with most things in telephony, however, not all designers do it this way.

10. A theoretical office is one whose office code may be established on a nearby existing switch until a new switch is installed to serve the area. Today, an RSU can easily be installed in the area where the new office will someday be, and later become part of the new office.

11. Pre-translation consists of examining each digit as dialed by the user. Its main purpose is to try to figure out how many more digits to expect, and when enough digits are available to allow the system to take action.

12. It is not always possible to tell how many digits are coming. If a user dials 0, for instance, it may be followed by a telephone number or a second 0 for the long distance operator. In private networks, an end of dialing digit is sometimes used, but in public networks, failure to receive another digit in a timed interval (typically after receiving a 0) will tell the system the user has finished dialing. When abbreviated dialing is provided by a CO, # can usually be used as an end of dialing signal to eliminate the time-out (see Chapter 5).

13. A route code tells the telephone system how to set up a connection. A destination code tells the system where the call is supposed to go. In the days of SXS, they were the same. Today, the user dials the destination code into the switch which then translates it to find the appropriate outgoing trunk group.

14. To keep the user from dialing until the system is ready.

15. It did not have to associate a digit receiver with the trunk before signaling could commence.

16. No. CO switches today can serve up to 100,000 lines, and four digits, from 0000 to 9999, can identify only 10,000. But not all numbers are used, more than 10 office codes are frequently found in one central office switch, and several switches can be in one building in a metropolitan areas.

17. Not necessarily. Some PBXs, when set up for apartment houses, let customers dial other apartments with * for an access code, but allow outside calls to be dialed as usual. That is, an access code is used for inside rather than outside calls, but it must not conflict with outside numbering.

18. Area codes used to have 0 or 1 as a middle digit, and office codes originally did not. Now, if a CO sees a preliminary 1, it has reason to believe that the three digits which follow will be an area code and not an office code.

19. Three digit screening, either for restriction or routing, looks only at the office code or the area code. Six digit screening looks at both area code and office code. It should be noted that PBXs and Centrex sometimes need to look at all 10 digits to block calls to particular numbers while allowing calls to other lines with the same office code.

20. It can be arranged to prevent ring-around-a-rosie routing when used with simple switches and signaling. It also acts to concentrate traffic from one area to another so that direct trunks will not have to be provided between all pairs of switches at a lower level in the hierarchy.

21. No. CCIS allows more signaling information to be sent to prevent calls shuttling back and forth among two or three nodes, digital technology allows much bigger tandem and toll switches to be built, and optical fiber makes more direct trunks economical. Although there are no more direct long distance trunks between local central offices, toll networks have a great many more direct trunks than they used to, and the hierarchy is much flatter.

22. In a single switch based on progressive control, the entire connection is set up to the called line and then the busy test is made. With common control, the busy test is made first, and then the connection is set up all at once through the matrix. With intelligence associated with modern CCIS signaling systems, it is now possible to test for busy first, then, if the called line is free, select trunks between switches and instruct all the switches to make trunk-to-trunk connections at the same time.

23. Restriction was originally tariffed to be performed on the customer's premises, while toll diversion was a CO feature. More important, restriction, which can be based on three or six digit screening, can select or block calling areas with a great deal more flexibility than toll diversion which simply blocks toll calls.

24. Yes, but it drives the users nuts, forcing them to find the correct access code that picks the trunk group that is not restricted for a particular call. It is much more satisfactory to combine restriction with ARS and let the switch do the work, based on called number and caller COS.

25. With cut-through, the user behind a PBX dials an access code and is connected to the CO (or another PBX), into which additional digits are dialed directly. This saved memory, important in electromechanical switches. Senderized operation is preferred in electronic switches for a number of reasons, but "access tandems" often use cut-through to let a called number flow through to the customer's long distance carrier of choice without stopping off at the tandem.

26. Holding time for DTMF is very short; overlap would greatly increase it.

27. It was invented before inexpensive solid-state memory was available, and a switch had to deliver the information it had while it had it so that its relay memory used could be made available for other calls. Thus three entries were made, one with the calling and called numbers, the second with answer time, and the third with hang-up time, all related by the same trunk number. This practice has been copied extensively in computer-controlled systems where it is not appropriate.

28. Flat rate billing for local calls was much easier for CO switches to provide in electromechanical times. Customers also liked it, because they could "make all the (local) calls they wanted, free." But usage sensitive billing, not difficult for electronic switches, lets the basic monthly charge be lower.

29. ANI is automatic number identification, usually used with CO switching systems to identify the calling line for billing. ANI had to be added to many electromechanical switches, but is built into most computer controlled electronic switches. AIOD is automatic identified outward dialing, used in connection with Centrex and PBXs. Originally, AIOD identified a PBX or Centrex extension making a toll call and passed the information on to the CO where the billing record was kept. Today, when there are many long distance companies and some choose to do their own billing rather than have it done for them by the local exchange carrier, AIOD is used in connection with passing the calling number forward for billing, or even for passing it on to the called party for calling number ID.

30. Traffic relates to the use of specific facilities such as trunks or digit receivers, independent of a call's source or destination. Billing information relates caller to called for distance and duration, independent of the trunks actually used to make the connection.

31. If you divide CCS by 36 to get Erlangs, the answer is 6.11. Obviously, it is impossible to have 6 trunks carry 6.11 calls, on the average, during the busy hour or any other time.

32. ACDs handle heavy traffic, frequently use queuing, and use human beings as well as inanimate objects such as trunks. Trunks do not have to take lunch breaks or be paid overtime. Further, trunks are always available, while ACD agents frequently have to become unavailable so that they can write up orders, etc. Today, large companies may have ACDs in several different parts of the country, and traffic is routed to one or another depending on load. ACDs are even more sensitive to variations based on day of week, week of month, and month of year than other traffic-carrying facilities.

33. There is no point in gathering and presenting data if nobody looks at it. Drawing the wrong conclusions is also a disadvantage. Presenting data properly is an art well worth the trouble of learning, although few engineers or programmers seem to agree.

34. It has a computer, scanner and distributor capabilities, and a spider-web of wires, often handling digital signals, into the whole area. Energy and property management and security can use these capabilities which are already present.

Chapter 3

1. A pair is needed to balance out noise and to minimize the effects of ground potential difference; two pairs are more costly than one.

2. Tip, ring and sleeve. Tip and ring go to the telephone, but sleeve exists only internal to electromechanical offices. Tip, ring, and (if used) sleeve run from the matrix port to the MDF; tip and ring continue from the MDF to an RJ-11 or similar jack at the customer's premises.

3. BRI-U interface wiring at an NT is a single pair that goes back to the switch; it can go outside. Wiring at the S/T interface goes from the NT to the telephone set or other station equipment, consists of two pairs, one for each direction of transmission, and must NOT be exposed to outside environment.

4. Series resistance, leakage resistance, capacitance, and characteristic impedance. Series resistance limits the flow of current, leakage resistance draws current even when the phone is one hook, capacitance is like leakage resistance but for alternating currents only; it shorts out high frequencies more than low frequencies. Characteristic impedance is what you measure between tip and ring at the central office.

5. A bridged tap is tip and ring extending beyond where the user's CPE is connected. A loading coil is an inductance inserted into the line to improve frequency response. Data and ISDN customers and suppliers care, because unexpected inductance and capacitance at points in a line can destroy high speed digital transmission.

6. Metallic matrices are usually 2-wire, and expect to meet trunks on a 2-wire basis. Thus hybrids are on the trunk side, and usually part of the trunk. A digital switching matrix has only 4-wire paths, so it usually interfaces trunks on a 4-wire basis. A 2-wire line, to connect to a digital matrix, must have a hybrid as part of its line circuit, along with a codec, supervisory sensor, etc.

7. In principal, the digital matrix with the line-side hybrid, because the hybrid's terminating network can be tailored to match the characteristic impedance of the line it serves.

8. By supervision, signaling, ringing and ring-trip, and transmission.

9. Higher usage implies less concentration in the switching matrix. Better service and special features are also demanded. But "value of service" is usually used to justify higher rates.

10. An off-prem extension is a telephone located remote from the PBX on which it homes. An FX line is from a CO other than the one that would normally serve the customer. FX lines are often used as CO Trunks to PBXs which need access to specific COs. Centrex, where a CO switch performs PBX functions, tends to confuse these definitions because all its extensions are off-prem.

11. The complexity they require of the CO switch for billing and ringing is out of all proportion to the savings they are supposed to produce. They also have privacy problems and tend to be busy with one party when an incoming call arrives for another.

12. Transmission, supervision, signaling, alerting, release at the end of the call.

13. Transmission, coded into digital words on a B channel, will be independent of distance. D signaling channel will handle all the other requirements directly, and do even more.

14. A bridged extension is a second (or third or fourth) analog telephone connected to the same tip and ring. Digital sets cannot be bridged the way analog phones can, but the various functions produced by bridging can be emulated.

15. A more sensitive detector will be more subject to false originations, and will have difficulty releasing in the presence of leakage current.

16. Usually to provide an extra dc signaling path to a PBX or coin phone. Ground on tip is easier to detect than dial tone, open on tip gives a positive indication that a call is over, and the tip can reach coin control circuitry in a coin phone even if the phone is on hook.

17. Probably not if the calling number is displayed and system records can show the name and address of the calling party.

18. No. However they, and other electronic phones, may use their signaling channel to tell their switch that the customer has pushed a button labeled "flash."

19. SXS systems reverse battery on the customer's line, and occasionally certain modern systems copy this practice in special line circuits to provide a premium service. With ISDN BRI and PRI interfaces, the D channel will take answer and hangup supervision to the CPE as a matter of course.

20. To provide accurate timing on customer generated bills, and to release various CPE when the call ends.

21. Try it and see. All 5XBAR and 1ESS offices where designed to handle it.

22. It is faster than dialing and can go anywhere a voice signal can go. It also has well known properties that make it easy to detect and relatively hard to confuse with speech.

23. Ringers which respond to it are relatively inexpensive, and now that they are owned by customers, retiring them from service will be difficult.

24. Special procedures must be used to be sure a valid answer signal, often much smaller than the ringing signal and difficult to differentiate, can turn off ringing upon answer while false tripping is prevented. The telco usually requires that a line have no more than five ringers on it to minimize the probability false tripping.

25. An audio tone that results when the power ring signal is detected in the telephone set by electronic circuitry rather than a conventional ringer; an audio tone from the CO that is amplified by a speaker in the telephone; an audio tone that is generated in the telephone set upon receiving a command via a signaling channel.

26. A reverting call is one from one party to another on the same party line. Such a connection might be used from one bridged extension to another and sold as "intercom service."

27. An ISDN line can support up to 8 different devices, sharing two talk paths for circuit switched connections and a D (signaling) channel which can also carry packet messages. There are various ways devices on the same BRI can communicate with each other; the D channel, like a packet network, can be shared by a number of users on one BRI.

28. Call progress tones can be permanently assigned to specific matrix time slots; any number of line or trunk circuits can be set to listen to these time slots and deliver the call progress tone independently to their transmission facilities. If the latter are analog, the codec in the line or trunk circuit will convert the digital signal to an audible tone which the customer can hear.

29. Usually, those that distribute high voltages like ringing and coin control, and provide test access. Separate matrix-connected circuits related to signaling and supervision may also fade out as D channels take over. Conference circuits are gone in some systems, and tone circuits may be replaced by generators in telephone sets activated by D-channel messages. Ultimately, voice announcements and music on hold may be all that is left in the area of service circuits. However, as functions such as voice recognition, voice mail, etc., become popular, they may be located on a separate switch called a Service Circuit Node, and shared by a number of COs, reducing administration and maintenance costs by taking advantage of digital transmission and signaling.

Chapter 4

1. One telecommunication channel between two switching centers. But note that in the future, trunks will have different bandwidths and traffic designs will be more difficult.

2. When it is a CO trunk to a PBX. Note that an FX line is a CO trunk from a PBX to a distant CO.

3. With each end in a CO and most of the length protected, leakage resistance from one wire to another is much higher.

4. Adding electronics at each end of an existing cable is usually cheaper today than adding new cables without electronics; not having to dig up streets to increase duct space for copper or optical fiber is a bigger saving.

5. With the right kind of digital, multiplexed trunks can terminate directly on the switch, omitting individual circuit boards on the carrier system, individual trunk circuits on the switch, and hybrids, signaling, A/D conversion, etc., that are needed to make a digital carrier system look like a metallic trunk to a 2-wire analog switch.

6. Out-of-band signaling was not affected by talk-off, but required signaling leads as well as voice leads to be cross-connected when several channels were connected back to back to make up a single trunk. In-band SF signaling requires signaling equipment only at the two ends of a connection.

7. Only one of the two switches can seize a one-way trunk, while both switches can seize two-way trunks. Two-way trunks are usually more efficient, but the double-seizure problem has been difficult to deal with in the past.

8. When one-way trunk circuits have been installed. Software alone cannot turn one kind of trunk circuit into another; however, if a 2-way circuit (E&M) is installed, the software can use it as required. Be sure to change the software at both ends of the trunk, however.

9. Regular CO trunks provide battery and ground from the CO and the PBX provides a closure or ground for seizure. A DID trunk circuit at the PBX, however, provides battery and the CO provides a tip-ring closure. DID trunks use automatic signaling (often dial pulses) incoming, and are one-way, while regular CO trunks can send automatic signaling but cannot receive it, and can be used two-way.

10. To send off-hook to the distant office, the M lead goes from ground to battery. At the far end, the E lead goes from open to ground. Between the M lead at one trunk circuit and the E lead at the other, a great variety of signaling equipment may be used. SF was popular on analog trunks, but T-Carrier trunks use specific bits in their digital bit streams. A pulse link repeater must contain two relays, one operated by the M lead of trunk circuit 1 while the other is operated by the M lead of trunk circuit 2. That is, when the M lead from the trunk circuit provides battery, the relay operates, and when it provides ground, the relay releases. Each relay needs a single contact so that, when it operates, it can connect ground to the E lead of the opposite circuit, and when it releases, it removes the ground.

11. It can read and modify the signaling bits in the trunk bit stream directly. It can operate like E&M, without any of the traditional hardware. In modern T-carrier, each trunk has a 1.67 Kbps "bit-robbed" data link built-in for signaling; effective use of this data channel has never been attempted, although on-hook, off-hook and dial-pulsing at 10 pps use it regularly. CCIS permits bit-robbed signaling to be abandoned.

12. A momentary off-hook signal sent by the called end of a trunk to the calling end to announce that it is ready to receive signaling information. Wink start performs the same function as dial tone, but is supposedly easier for machines to detect, particularly when using relays.

13. Glare occurs when two senders, one at each end, seize a trunk simultaneously, and each waits for the other to send a wink start signal. The best solution is to use one-way trunks so the situation doesn't come up, or to use CCIS to let the two switches talk intelligently to each other. Use of dial tone as a start-sending signal is also possible but seldom employed. Complex time-outs as described in the text can lead to reasonably good operation if one must use 1930s techniques.

14. DTMF is used on lines, MF on trunks. MF is a 2 out of 6 code, using frequencies spaced 200 hz apart between 700 and 1700; DTMF is a 1 out of 4 AND 1 out of 4 code (NOT a 2 out of 8 code), using geometrically spaced frequencies in a low band and a high band.

15. Bit robbing refers, in T-carrier PCM, to taking one bit from voice coding to use for supervision in one frame out of six. Digit robbing refers to reducing an 8 bit voice sample to 7 bits because the incoming and outgoing superframes through a digital switch are not aligned.

16. CCIS is much faster than MF, not subject to talk-of, is two-way, can be used at any time, eliminates per-trunk signaling and supervisory equipment, eliminates digit senders and receiver, and can be used for a variety of other information.

17. TLP stands for Transmission Level Point, a point of reference where signal levels at different points in a central office can be meaningfully compared.

18. VNL is Via Net Loss, the minimum loss that a trunk must have to control the subjective effects of echo.

19. Return loss balance and longitudinal balance. The former refers to impedance matching to prevent echo, while the latter deals with the impedance from tip and ring to ground, a factor which affects the pick-up of noise.

20. Voice transmission would work fine using such techniques, although the cost of administering it would far exceed any possible value obtained. Any signal other than PCM-encoded voice, however, including non-modem data, compressed video, etc., would be converted to garbage.

21. To deliver bits or bytes from source to destination unchanged. This should be easier than preserving the exact shape of an analog signal and delivering it at exactly the right level.

Chapter 5

1. Residential features usually serve a single line, while business features relate lines serving groups of people.

2. In general, it appears not. At least 30% of business phones require the ability to handle several extensions.

3. The user must flash the switch-hook to get a signaling receiver, and then dial a feature code.

4. 1A2 identifies an early business telephone system, used stand-alone or behind PBX and Centrex systems, to provide business features. Sets had a Hold button and additional buttons for line pick-up or certain features.

5. Boss-secretary and Principal + Assistants.

6. They have no KSU; they are controlled directly by the PBX and use the PBX switching matrix to make various connections. 1A2 took each line to one or more sets; EKTS have a talk path and a signaling path and can emulate 1A2 when designers choose to follow this approach. Often, they work in other ways. Electronic key systems are actually small PBXs which terminate PBX, Centrex or CO lines on trunk circuits and telephone sets, both electronic and conventional, on line circuits. Electronic key systems have a switching matrix to make connections requested by the user, and usually have a common control.

7. PBX numbering plans need not have 7-digit numbers internally, but must have access codes and feature codes in addition to extension numbers. PBXs also need dummy numbers.

8. Extension numbers, both real and dummy, must be related to memory locations which explain their function, and to matrix ports and telephone sets and/or buttons on telephone sets. Feature codes and access codes must not overlap extension numbers.

9. An extension number assigned to an electronic telephone set as its principal identification. This allows a port on the matrix to have a one-to-one relationship with a directory number and telephone set, even though a prime line extension number can appear on other telephones where it is not a prime line.

10. Soft hold puts the existing connection on hold and returns dial tone to the activating phone to permit it to obtain another connection or some system feature. Hard hold puts the existing connection on hold without providing a dial-tone connection. When used with single line phones, a feature code can dismiss dial tone to convert soft to hard hold. In some systems, hanging up a single line phone on either soft or hard hold will cause it to immediately ring so that the held connection will not be forgotten.

11. Calling party hold allows the called party to go on hook briefly without losing the connection so that transfer to another phone can take place. A time-out limits the on-hook interval so that a calling party cannot tie up the called phone indefinitely. Joint holding says that the connection will be maintained as long as either the calling or called party stays off hook. This is helpful in emergency services.

12. One lets you know something about where a call is coming from (as intra-PBX vs outside), while the other helps you tell which phone (or line) is ringing.

13. A connection made to a line without a busy test; it is usually done to test the line or verify that it is in a valid connection.

14. Camp-on is usually set up by a console attendant, while call-waiting is usually a caller feature. Camp-on often does not inform the called party it is active, while call-waiting gives the caller a beep and includes a means for switching between the new and old call.

15. Automatic call-back uses system signaling to find out if the called party is still on a previous call. A busy-buster keeps on trying over and over, tying up control facilities and trunks on each attempt. This is no problem for the caller, and costs him or her nothing. It is a problem for the telephone company, and can require excessive facilities for non-paid traffic.

16. From the telephone company point of view, only incoming calls need to be transferred; an intra-PBX call getting a wrong number should be redialed just as is done by residential callers in the public network, and people should dial their own outgoing calls rather than burden their secretaries with such trivia. Needless to say, business customers do not agree.

17. See if somebody transferring a call can stay on, undetected, in a three-way conference. See what happens when someone contacted via consultation hold does not hang up at the end of consultation. See what call waiting does to fax. Be very careful when DISA ports, Automated Attendant and Voice Mail Systems are implemented (see Chapter 6).

18. Group and directed. Group pick-up requires only a pick-up feature code or signal and lets you answer a call ringing in your pick-up group. Directed call pick-up requires a feature code plus the extension number you want to answer. Both are easier with an electronic telephone set, particularly when it has a lamp and button for each phone or group to be picked up and can display the name of the called party.

19. A dummy extension number to which a call can be assigned while waiting for a response from a party being sought by paging or code calling.

20. A call can be "parked" on a line by an attendant or some third party without ringing, and can be retrieved at that line or some other if necessary with directed pick-up; this is helpful in connection with paging. A call can usually be placed "on hold" only after being answered, and can only be picked up by another telephone set if that set has a button labeled with the held extension's number.

21.DNIS sends the number the caller dialed to the system which receives the call. Calling number ID gives the called system the number from which the call was placed.

22. Usually, hunting is system programmed rather than station programmed. Hunting is generally used to provide another option for answering a call if a line is busy, while forwarding can work with busy or no-answer. These distinctions are gradually fading out, but current systems do not always do what is expected.

23. Calls can be forwarded differently if they originate within the PBX or come from outside. They may have different destinations if the called line is busy or if it does not answer. Specific calling numbers can even be marked for specific destinations as opposed to the destination for general incoming calls.

24. UCD is a hunting algorithm for delivering incoming calls evenly to extensions in hunt groups. It may be part of a the program for a PBX or an ACD. ACD stands for Automatic Call Distributor, and refers to a system which specializes in handling incoming calls distributed by UCD. An ACD will also have extensive real-time FADS displays to facilitate system management. An ACD does not usually waste space providing regular PBX extensions, but specializes in handling heavy traffic, typical of that offered to airline reservation systems, telemarketing systems reached via 800 numbers, etc.

25. Not everyone makes this distinction, but abbreviated dialing uses switching system memory and requires the caller to dial a shorter number which the system translates to a longer number needed by the telephone network. A repertory dialer stores called numbers internally and allows the user to push a button or otherwise to cause a phone number (or feature code) to be transmitted from the telephone set or dialer itself. Repertory dialers with 10 to 50 buttons have been widely marketed; they are usually set up to store as many as 22 digits per called number.

26. DDC does not require the CO to outpulse the called extension number. Until recently, most COs used dial pulsing tell connected PBXs which extension a DID call was for.

27. Digital systems have to be four-wire; thus the "talk" and "listen" sides of each connected line are separate, and return loss is less of a problem. Even when the caller arrives on a two-wire facility, the balance network in the hybrid associated with the line can be selected to provide a better balance. In the not too distant future, all connections will be four-wire, telephone set to telephone set, eliminating echo paths almost entirely. Digital conference bridges can combine signals by using digital addition and subtraction rather than analog blending. Because any number of conference ports can listen to a digital signal without changing its amplitude, gain and loss with their variations and noise are eliminated.

28. The ability to bill guests on a daily basis is vital, for both local and long distance calls. Further, the guest must be able to use a credit card or a long distance company different from that of the hotel. "Architectural numbering," to relate the telephone extension to the room number, is expected, as is "single digit dialing" for hotel services. Wake-up, room status, paging, directory, message center with message waiting lamps on phones, etc., are also important.

Chapter 6

1. Telephonists are telephone professionals who act as an interface between the caller and the telephone system. Operators are employees of the telephone company, while attendants are employees of the business customer. Agents are people who staff ACDs, are employed by telephone customers, and usually interface the caller with to computerized data base or similar system rather than a telephone system.

2. The best. One or more human beings.

3. Identify calling and called party or trunk; trace; show hunt groups; provide busy test, test access, and transfer; indicate call type; split a connection; provide camp-on, barge-in, call forwarding as well as trunk selection and queuing and various billing functions.

4. Have a direct trunk group to an operator accessed by dialing 0, or 411 or 114 for directory assistance); have the operator system switch an access path from each trunk to an appropriate position; have the local or toll switch connect the caller to an operator position; same, but use a BRI to interface the operator.

5. A combination of data terminal to interface a telephonist to the control of an associated switching system and a telephone to provide access to the caller and, perhaps, the called party.

6. DSS is direct station selection, a PBX console feature that helped an attendant complete calls faster than using a rotary dial. Each extension had a status lamp and a push-button.

7. A KPT console has a trunk appearance (a status lamp and a pick-up button) for each trunk which the attendant can access directly. A switched-loop console has one or more appearances on the console which, like lines, can have a trunk switched to them. Usually displays identify the trunk (or extension dialing 0) to the attendant. After answering the call, the attendant can transfer it to the appropriate extension or trunk as required.

8. A switched loop console is not limited by the number of buttons available to a maximum number of trunks it can support.

9. With the coming of the ISDN BRI, it appears that future consoles need be nothing more than a telephone and a PC with suitable programming. However, where cost is a major factor, a simple traditional console with buttons and lamps may be more appropriate. Even so, a proprietary electronic PBX telephone set or an ISDN phone for use with BRI will eventually turn out to be both cost effective and perfectly adequate for most console needs, and will make a separate data channel available if one wants to add a PC.

10. A main-satellite system answers directory number calls at the main location, and completes calls to satellites via tie-trunks. A CAS system allows each location to have its own directory number, but uses release link trunks (RLTs) to switch incoming calls to the centralized attendants and allow the attendants to control transfer at the PBX where the call enters the system. Note that both approaches become unnecessary when modern "distributed" PBX systems are used.

11. Require users to enter identity numbers and/or security codes. A more effective approach is to also require them to enter their phone number and have the system call them back. We may hope for voice-print identification to become practical in the not too distant future.

Chapter 7

1. A frame is usually open, while a cabinet is enclosed.

2. Frames. Electronics usually goes in cabinets.

3. Safer, need no rolling ladders, make room for cable runs, reduce floor loading, easier to move into place.

4. They protect components, provide electrical and acoustic shielding, block dust, and help control cooling.

5. Wiring can go directly from the MDF to cable troughs on top of the cabinets.

6. Often "stackable" shelves, each complete with its own power and access connectors, are called "cabinets," and several, one on top of another, are called a "stack." Note also that a PBX, complete in one shelf, will refer to that shelf as a cabinet.

7. Electromechanical systems were usually built in frames, with devices readily available for adjustment. Electronic systems are composed of chips on PCBs plugged into backplanes in cabinets. There are few adjustments, field repair is almost impossible, and replacement in the field is limited to PCBs. Further, design emphasizes the minimization of connections among chips, boards, shelves and cabinets rather than minimizing the number of "gates" in each logical operation. In advancing beyond electromechanical systems, the demands on mechanical designers have become much more exacting.

8. Control of temperature and humidity are probably the most important for electronic systems, which says reliable power is as much needed for the air conditioning system as for the switch itself. Clean air (no smoke, dust, etc.) and freedom from vibration and physical shock are also important, and protection must be provided against dangerous electrical signals from the outside world. Proper illumination is also required to meet the needs of craftspeople who must work on the system. All too often, electronic systems such as computers and telephone switches must be designed not to radiate electrical noise to each other and other electronic equipment in the general vicinity. Proper grounding is a basic requirement.

9. Small systems are usually 100 lines or less, medium sized systems range from 100 to 400 lines, while large systems may go to 5000 lines or more. For larger systems, which are relatively rare, Centrex is sometimes used or a CO switch may be bought and adapted to PBX use. Some manufacturers even provide PBX versions of their CO switches.

10. Individual port groups are added as needed, while the group selector is expanded to terminate their multiplexed links. To grow, the group selector often expands very much like an electromechanical "link type system" with junctors in multiplexed bundles spread from incoming to outgoing switching elements.

11. To provide a location designed specifically for changing connections from customer lines to line circuits on the switch, trunks to signaling and transmission circuits, etc. Once wiring is brought from equipment to permanent locations on a distributing frame, cross-connects can be made easily without fear of damaging the equipment itself.

12. The MDF or main distributing frame.

13. Today, a special protection frame is often used and cables are run from it to the Vertical side of the MDF. Not too long ago, protection devices (heat coils and carbon blocks) were mounted directly on the MDF.

14. In general, the old open wire (uninsulated) lines on glass insulators have been retired, replaced by aerial or underground cable. This automatically protects customer lines from many hazards, and has led to the elimination of heat coils. Lightning and other over-voltages are now protected by gas tubes or solid state devices rather than carbon blocks.

15. Apparently, when all the class marks associated with the old port must be inserted at the new port.

16. When the new port to be used is implemented with a different kind of line card, as when an ISDN phone is to replace an analog 2500 type set. Clearly, the ISDN phone requires a different type of line card in addition to software programmed COS and features.

17. JGFs are usually specific to a given kind of switching matrix, and arranged to facilitate growth of that kind of matrix only; they often switch a number of junctors at the same time. Other DFs are general purpose, and do not depend on the type of switch or switches they serve.

18. A major advantage of digital switching is that it can match directly with digital transmission. This allows trunk circuits on the switching matrix, stand-alone signaling circuits, and individual trunk ports on transmission systems to be omitted, along with the IDFs and TDFs formerly used to provide the once required cross-connects.

19. A Digital (Access and) Cross-connect System is like an MDF in that it allows DS0 digital channels multiplexed into a T-span to be cross-connected directly without being demodulated and brought back to analog. This is particularly important when a T-span from a customer includes regular CO trunks which must go to a local switch, tie-trunks which have to bypass the local CO switch and go directly to a long distance carrier, and data lines which must to be connected to some other transmission or switching mechanism. A DACS can rearrange these channels as required, under control of a remote terminal, maintaining the digital format from end to end.

20. Because a large battery can provide a convenient source of reliable DC power to operate the system when the commercial power fails. Reliable AC power systems using large flywheels and gasoline or diesel generators are available, but such systems are expensive and intricate by comparison with a battery, and have a harder time taking over when needed; the battery is always floating across the DC line, ready to pick up the load instantly. Of course a battery plant to store power for more than about 4 hours of operation gets cumbersome and should be supported by a gasoline or diesel-powered generator to replace commercial power during prolonged outages. Chemical storage of energy is far more compact than electrical, as Henry Ford proved to Thomas Edison.

21. No. There is usually a 5 to 10% voltage drop when the battery, floating across the DC bus, goes from trickle charge to discharge. There can be a similar rise when AC power from an external source comes back on. Note that electronic switching systems are usually designed to have frame filters and shelf power converters which shield their loads somewhat from these fluctuations.

22. The phone must be compatible with the CO trunk to which it is to be connected. That is, it must be capable of running from power received over the CO trunk, and its signaling, ringing, supervision and transmission must be compatible with the trunk. Because most PBX CO trunks are ground start while extension lines are loop start, power failure transfer must include some means of converting from ground to loop start while connecting the extension line to the trunk and disconnecting both from the PBX.

23. An alarm fuse is one that provides a signal to an external alarm when it blows. Such fuses are particularly useful in providing automatic switching systems with trouble indications. See Chapter 8.

24. In general, yes. Lack of dial tone, for instance, can lead the customer to believe the whole system is down. A system needs to know if the signal is actually being generated or if an open circuit (such as a jumper omitted at the MDF) or a short is preventing it from reaching the customer. It is never enough for a system to decide to do something; it should, whenever possible, check to see that such things have been or at least can be done.

Chapter 8

1. You better believe it.

2. A switch, whether local, tandem or toll, has access to most of the transmission facilities that have to be tested. Further, a modern switch has the built-in intelligence to carry out and record for administrative purposes most changes required by operations and maintenance.

3. Lines are still mostly copper pairs and must be tested for leakage, continuity, etc. Trunks, however, are almost always multiplexed into carrier systems, making the switch's access at a level different from the physical transmission medium. Another difference becoming increasingly important in testing is the nature of signaling, which even with ISDN will stay with the line while it will migrate into CCIS for trunks.

4. The power source and distribution. It may also protect the fused device, but not necessarily.

5. They are designed to call attention to themselves when they blow; they can operate signal lights or gongs, and can also control a scanpoint for the switching system to inspect and identify to maintenance forces.

6. Usually, by timeouts in the switching system. If a line shows off-hook but nobody dials after x seconds, the DP or DTMF receiver must be freed and made available to others. Similarly when enough digits are not received. Note that permanent signals and partial dials are among the most important indications an automatic switch can receive to enable it to deal with its own problems.

7. A condition in which the system knows a permanent signal exists but no path through the switching matrix is provided. Thus no signaling receiver, tone source or connecting path is tied up, but if the line goes on-hook and then comes off-hook again, the system is expected to respond with dial tone.

8. First of all, make it busy so that it will not be seized by paying customers. Then find the problem and fix it.

9. With SF signaling rapidly being phased out in favor of CCIS, massive trunk seizures due to a cable cut are much less likely than in the past. On the other hand, cable cuts are not decreasing in frequency, and digital trunks have a whole new array of problems that the switches on each end should know about. These poblems almost alwlays involve the entire trunk group and not just one particular time slot, so CGA will apparently be of considerable importance for years to come.

10. A trunk with many short calls may be a killer trunk, one that can be seized by will not carry the call. Long holding time may imply a stuck trunk, or simply somebody faxing a 200 page document with a 300 Bps modem.

11. A procedure to allow emergency calls to get through when a disaster or other event causes people to generate far more calls than they usually make "individually and collectively at random."

12. One procedure is to make sure some change that the control can detect will take place as a result of the order, and set up the program so that if the change is not detected before timeout, maintenance procedures can be instituted.

13. By being designed to detect failures when they occur, and to substitute working equipment until the failure can be fixed.

14. One way is to have two identical units, one in standby while the other does the job. A second approach is to have one standby unit associated with a number of working circuits, along with the capability of inserting it in place of one that fails. A third approach simply provides a number of circuits to share the load, with provision for making a bad circuit busy. This means the others have to work harder, of course. Note that all of these techniques assume that maintenance will be informed of the failure so that the bad part can be fixed.

15. One can always switch audio tones through briefly for continuity testing, but carrying an extra bit or two along with the switched signal (for parity checking, supervision, etc.) and injecting test patterns of bits rather than audio tones can provide the additional information needed.

16. You have only tested inward transmission: you can hear them, but you don't know if they can hear you. With SF and MF trunk signaling, you used to know that audio was at least getting out, but with CCIS, that may not be the case. Thus a loop-around test line is desirable.

17. Automatic Line Insulation Testing will probably be more important for ISDN lines than for lines to a 2500 type telephone. Sending high speed pulses over a line will require more knowledge than just low leakage resistance. Presumably, ALIT will have to expand to identify other electrical parameters such as characteristic impedance and bridged taps which are important to ISDN.

18. Usually master control deals with the small number of major subsystems while the line and trunk test panel deals with very large numbers of less complex items. As a result, a master control may be optimized for one or two people, while line and trunk test panels may have to permit five or more people to work simultaneously under some circumstances.

19. My opinion should be obvious, but most designers disagree with me. Let me know what you think.

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Copyright 2006 Lee Goeller. All Rights Reserved.