Survey of Telephone Switching

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The No. 5 crossbar system was originally developed to' fill a need for a system especially suitable for isolated small cities and for residential areas on the fringes of large cities. The design of the system was influenced by the special characteristics of telephone traffic in these regions. The percentage of calls completed to subscribers in the same office was expected to be relatively high. It was recognized that the system would have to interconnect with existing offices of all types. In addition, the tendency toward extension of dialing areas indicated that a local system with some provision for tandem or intertoll switching would simplify trunking, relieve the load on regular tandem offices, and reduce backhaul. This means that in certain cases, where toll requirements are small, the No. 5 office could be established as a toll center with control switching point (CSP) features. Finally, the new concept of extended subscriber dialing demanded automatic recording of call details.

The system was designed to meet requirements as outlined above, and also has built into it other features which adapt it to new concepts of telephone service. Improvements and added features have widened the application of the No. 5 equipment. It is presently being used in almost all areas including metropolitan business exchanges and rural centers of about 2000 lines or more.

The No. 5 crossbar system is first of all a highly efficient local telephone switching system which can operate with all present local, tandem, and toll switching offices, except that it cannot direct calls through a panel 2-wire office selector. Table A shows the usual kinds of pulsing, or manner of operation, for the various combinations of No. 5 crossbar and connecting offices. Where more than one type of pulsing is available, the preferred type is shown first.

No. 5 crossbar has several features that distinguish it from previous systems. The more important of these features are:

Because of its flexibility and full provision in digit number and type of pulsing, the system lends itself to new development. No. 5 crossbar offices are presently serving as FACD (foreign area customer dialing) points at several locations. CAMA (centralized automatic message accounting) is under development and will enable the No. 5 office to handle customer-dialed long distance traffic from step-by-step offices and from multi-party subscribers in its own area. Field trials are planned for an electronic subscribers line concentrator, for which a No. 5 crossbar office will serve as the control office.

The system is presently under going design changes incorporating new apparatus developments, notably the wire spring and dry reed type relays. These design changes result in savings of floor space and of holding time of circuits. An example is the dial tone marker where the new design comprises one 23 inch bay instead of two. Holding time is reduced from 377 to 220 milliseconds and power consumption is reduced by 56 percent. It may be noted that the reduced holding time may represent a large saving in those cases where a reduced number of circuits is required as compared to the older design.

The equipment in a No. 5 crossbar office can be classified into two general types: the talking path equipment and the common control equipment. The talking path equipment consists of line link and trunk link frames and trunk circuits.

The common control equipment, which makes up the remainder of the equipment in the office, does the following jobs:

The common control equipment is used for relatively short periods of time in setting up connections, and therefore it consists of a relatively small quantity of highly complex circuits, designed for great reliability and equipped to detect trouble during any stage of setting up the call.

Figure 7-1 shows typical calls set up through the talking path, with the temporary connections made by the common control equipment in setting up the calls shown by dotted lines. Figure 7-2 shows the common control equipment in greater detail.

The Talking Path - The No. 5 system, unlike other familiar systems, employs a single switching train to handle all types of calls; incoming, outgoing or switched through. In addition, the connection of the subscriber to the dial register circuit is also made through this switching train. This eliminates the line link and sender link controllers required in the No. I crossbar system and transfers their functions to the marker.

The switching train is made up of two basic elements, the line link frame and the trunk link frame. Subscriber lines and incoming trunks for through switch calls terminate on the line link frames; trunks and originating registers terminate on the trunk link frames. The line link and trunk link frames are interconnected by "junctors" which give each line link frame access to all of the trunk link frames. The basic switching device used on the frames is the crossbar switch. Because of the use of AMA charging, and message registration over the sleeve wire, the No. 5 crossbar line link frames use the economical 3-wire crossbar switch exclusively.

Line Link Frames - The crossbar switches on the line link frame are divided functionally into line switches and, junctor switches (Figure 7-3). Subscriber lines are connected to the line switch verticals and junctors to the junctor switch verticals. Line links, which are merely connecting wires, interconnect the line switches and junctor switches. The basic line link frame is a 2-bay framework with each bay mounting ten 200 point, 3-wire switches. The ten switches on one bay are used as combined line and junctor switches, and provide terminations for 100 junctors and 100 lines. The ten switches on the other bay are line switches which provide terminations for 190 additional line s and ten "no test" verticals used to obtain access to busy lines. The basic line link frame, as described, thus provides 290 line terminations and 100 junctor terminations. Since the calling rate and holding time habits of subscribers may vary widely in different areas, provision is made for adding supplementary line switch bays of either 200 or 100 lines to increase the capacity for lines. In the U and Y (older) version capacities of 290 to 590 lines in steps of 100 lines are available.

With the introduction of the wire spring line link frame, a feature whereby line switch bays may be split between two line link frames was introduced. This permits smaller increments in line link frame size and closer adjustment to expected load requirements. In the wire spring frames, sizes from 190 to 590 lines in steps of 50 lines are available.

A 490 line frame is illustrated schematically in Figure 7-3. The diagram shows how the lines are divided into vertical and horizontal groups and vertical files, for purposes of identification to the markers.

Line links appear on the horizontals of the switches; ten line links on each switch. These ten line links are distributed among the ten junctor switches, one link to one horizontal on each of the ten junctor switches. This system of line links permits each line on a line link frame to reach any one of the 100 junctors serving that frame.

In addition to the switches, line relays and control relays are mounted on the frame. The hold magnets on the line switches are equipped with off-normal springs which serve as cut-off relays for the line circuits.

A feature of this line link frame is that the same frame can serve customers who have various classes of service; for example, coin, flat rate, and message rate customers can have their lines all terminating on the same frame. A maximum of thirty classes of service can be served on a frame and this is also the maximum that can be served by one marker group.

Junctors - Each line link frame has 100 junctor terminations which are used to connect to all the trunk link frames in the office. Since each trunk link frame has 200 junctor terminations for connecting to all line link frames, the ratio of line link frames to trunk link frames in an office generally is 2:1.

There are no half frames. (In an office with thirteen line link frames, there are usually seven trunk link frames.) However, conditions peculiar to a particular office may cause some variation in this ratio.

The 100 junctors from each line link frame are divided into approximately equal groups, with one group from each line link frame going to each trunk link frame. The number of junctors in a group depends on the number of trunk link frames in the office. The number of junctors per group is determined by dividing the 100 junctors by the number of trunk link frames. However, there is a limiting factor; for efficient service no group can contain less than ten junctors.

When there are ten or fewer trunk link frames in an office, each junctor group has ten or more junctors. For example, in an office with eight trunk link frames and sixteen line link frames, each junctor group contains either twelve or thirteen junctors. Figure 7-4 illustrates the junctor distribution for two trunk link frames and four line link frames.

However, in an office with eleven to twenty trunk link frames, each junctor is multipled to two trunk link frames in order to have at least ten junctors per group. For example, in an office with twenty trunk link frames and forty line link frames, each junctor group contains ten junctors. Figure 7-5 illustrates the junctor distribution for twenty trunk link frames and forty line link frames. In this case, the number of junctors in a group is determined by dividing 100 by the number of pairs of trunk link frames.

Trunk Link Frames - The trunk link frame is made up of trunk switches, junctor switches, and relays for marker access to the frame. Trunks and originating registers, which register the called number, are connected to the trunk switches. The junctors from the line link frame are connected to the junctor switches. The trunk and junctor switches are interconnected by trunk links, as illustrated in Figure 7-6.

The system of trunk links that permits any outlet or trunk on a trunk link frame to be connected to any one of the 200 junctors serving that frame is similar in principle to that used on line link frames. The number of trunk links, which is 200, is the same as the number of junctors. The trunk links run from vertical to vertical, the junctors being connected to the horizontals of the junctor switches and the trunks to the horizontals of the trunk switches. In order to terminate twenty junctors on the horizontals of one 200 point switch. it is necessary to split the horizontal multiple into a left-hand and a righthand half-switch. Each half-switch so formed has one link to each of the ten trunk switches. The horizontals of the trunk switches are not split. Thus the trunk is accessible to all the junctors on the frame, but only to either the left or right junctors on any one channel test.

Extension Trunk Link Frame - As stated previously, when eleven to twenty trunk link frames are involved, each junctor is multipled to two trunk link frames in order that each junctor group contain a minimum of ten junctors. This requirement reduces the junctor capacity of the basic trunk link frames by 50 percent, and it is necessary to provide additional junctores for each trunk link frame. These additional switches are mounted on the extension trunk link frame which, if equipped initially, is placed adjacent to the junctor switch bay of the trunk link frame. The extension frame consists of a framework with ten 200 point, 3-wire switches the same as the junctor switches on the trunk link frame. These switches have a capacity for 200 junctors which, with the 200 junctors on the trunk link frame, provide a total of 400 junctors for the combination.

Trunk Switches - The ten trunk switches on the trunk link frame are 6-wire switches. They furnish locations for 160 trunks. Each switch has locations for sixteen trunks on eight of its levels (horizontals). Figure 7-7 illustrates how this is accomplished.

The 6-wire switches are so arranged that each one of levels 2 to 9 terminates on two 3-wire trunks. One trunk on each level is connected to one set of three wires of the horizontal multiple and designated appearance A, and the other trunk to a second set of three wires and designated appearance B.

The trunk link is wired to the first three non-multiple terminals of level 1 and the last three non-multiple terminals of level 0. Each operation of the trunk switch requires two select magnet operations. Either 0 or 1 select magnet must be operated to direct the trunk link to the proper 3-wire connection of the vertical, and in addition, the select magnet associated with one of the levels 2 to 9 where the trunk appears must be operated. The levels 0 and 1 are directing levels. The eight appearances on each switch that are selected by horizontal 0 are called A appearances and those selected by horizontal 1 are called B appearances.

Channels - A channel is a combination of a line link, a junctor, and a trunk link that can be formed, by crosspoint closures, into a chain that interconnects a line and a trunk. Each line link, junctor, and trunk link consists of a tip, ring, and sleeve lead with a switch appearance at each end.

The ten or more junctors in a group connecting a line link frame with a trunk link frame are distributed over the ten junctor switches of both the line link and trunk link frames, the junctor switch number being the same on both ends for each junctor. There are ten line links serving each particular subscriber line on the line link frame, and these are also distributed over the ten junctor switches.

There are twenty trunk links serving each particular trunk on the trunk link frame, and these are also distributed over the ten junctor switches. Thus, when a particular line and a particular trunk for a unit consisting of, for example, twenty line link and ten trunk link frames are considered, there are ten channels available for a connection. These channels are numbered according to the junctor switches on which they terminate, as illustrated in Figure 7-8. An idle channel is selected by testing the ten channels at the same time. For job sizes other than the above, there are more than ten channels available. For example, in a 10 line link and 5 trunk link frame job, there are twenty channels provided, as illustrated in Figure 7-9. In these cases, additional tests are made when an idle channel is not found in the first ten channels tested.

Trunks - A distinguishing feature of the switching train is that, unlike other local systems, supervisory and charging circuits are not an integral part of the switching train, as in the case of the panel district selector or the No. 1 crossbar district junctor circuit. The supervisory and charge features are functions of the various trunk circuits which connect to the trunk levels of the trunk link frames. By this arrangement trunks may be provided in type and quantity as dictated by the requirements of a particular job. Trunk relay circuits are cabled to their appearances on the trunk link frames, out-sender link frames and incoming register link frames. For this reason, changes in the frame locations of existing trunk circuits must be made by the Western Electric Company installer.

A study is being made of the economic desirability of providing a trunk distributing frame for No. 5 crossbar, which would permit trunk relocations by cross-connection rather than by recabling.

This section describes in general terms the functions of the various control frames used in No. 5 crossbar. Figure 7-2 shows in block form, the relationship between the various control frames and the switching frames in the talking path.

Markers - The marker is the most active piece of common control equipment in the office. It is used one or more times in the completion of every call. Different offices have various numbers of markers depending on the size of the office and the amount of traffic. All the markers and their associated equipment serving up to a maximum of 20,000 numbers make up a marker group. (Arrangements are available to increase the number capacity beyond 20,000 per marker group, providing the line capacity of forty line link frames is not exceeded.)

There are three types of markers: (a) combined, (b) dial tone, and (c) completing. The combined marker performs all the marker jobs listed below; the dial tone and completing markers divide the jobs of the combined marker between them. The dial tone marker is used exclusively on dial tone jobs and the completing marker performs all the other jobs. Economic and traffic conditions determine whether an office has a single group of combined markers or a subgroup of dial tone and a subgroup of completing markers. In general, the combined marker will prove more economical only for very small installations such as those requiring three or less markers on the combined basis.

The dial tone and completing markers are independent of each other; individual office requirements determine how many markers of each type are required. The marker normally completes each of its various functions in less than one second. Therefore, a small number of markers can serve a large office.

Special Features in the Marker - Two markers (0 and 1) in a group of combined or completing markers are usually equipped with special features for handling certain test calls. These calls are set up by operators, testmen, or maintenance men and are of the following types:

The regular subscriber line tests, ground and continuity, which are performed by the marker during normal operation are cancelled on all these calls.

Optional Features for Markers - If required by a particular installation, the combined or completing markers may be equipped with additional equipment frames to provide optional features as follows:

Originating Registers - Originating registers furnish dial tone to subscribers and record the digits that are dialed. After dialing is completed, the called number is transmitted from the register to the marker. These registers also make party test to determine whether a tip or ring party is making the call. Originating registers appear on trunk link frames and one is connected to the subscriber's line by the combined or dial tone marker when the customer lifts the receiver off the hook. A No. 5 crossbar office which includes any coin lines must have all the originating registers in the office arranged for coin operation.

Pretranslators - Pretranslator circuits may be provided in offices located in areas where some calls require the dialing of more digits than others. The originating register circuit may be arranged to seize the pretranslator after either the second or third digit has been dialed. From these digits, the pretranslator determines how many more digits the register should expect before seizing a marker.

Where the volume of calls of this nature is not great and the numbering- plan is not too complex, pretranslation can take place in the originating register. The register can be arranged to determine how many digits it should receive from the first digit or from a limited combination of the first and second digits.

For more complex numbering plans, a separate pretranslator circuit is provided. This circuit is called in by the pretranslator connector when the first two or three digits have been set in the originating register. The pretranslator determines from these digits how many more should be dialed and tells the register that it must wait for these before it calls in a marker.

On calls to stations where a party letter is part of the directory number, the register has to wait for an extra digit. This situation is known as stations delay. The pretranslator recognizes stations delay from the dialed code and informs the register to wait for a possible additional digit.

A uniform numbering plan without party letters or ringing digits eliminates the necessity for pretranslation.

Number Groups - The number group translates subscriber directory numbers into line equipment locations of subscriber lines. (The line equipment location identifies the line link frame location of a subscriber line.) The number group also supplies the proper ringing control information and other in' formation concerning the called number, such as whether it is in a terminal hunting group or in a physical or theoretical office.

A number group frame serves 1000 consecutive directory numbers. For example, number group frame 1 contains directory numbers whose numericals are 1000 to 1999. Therefore, the total amount of directory numbers, in multiples of 1000, to be equipped, determines how many number group frames are required.

The number group is also used on tandem or toll through-switched calls. On these calls the number group supplies the marker with the line link frame location of the trunk seeking a path through the office. The trunks may be given regular subscriber directory numbers for use by the marker in entering the number groups. However, since this uses up available numbers (two subscriber numbers per trunk served) it may be better to furnish separate trunk number groups (two required) where the quantity of toll or tandem trunks is high. The trunk number groups are not part of the subscriber number series.

Outgoing Senders - An outgoing sender is employed on all calls requiring pulsing to connecting offices. The marker transfers the required digits of the called number to a sender which is connected to an outgoing trunk. (The function of the sender is to furnish the pulses which control the operation of the switching equipment in the connecting office.) The type of connecting office (step-by-step, panel, manual, or crossbar) determines what kind of sender should be used to transmit the called number. Therefore, four different types of outgoing senders are provided in a No. 5 crossbar office, as listed below:

The multifrequency outgoing sender requires an a-c supply of six different frequencies. These are used in various combinations of two each for the digits 0 to 9 and the start and end signals.
Outgoing Sender Links - Outgoing sender links connect outgoing and intermarker group senders to outgoing trunks. Information from a sender to a trunk is transmitted through this sender link.

One sender link frame mounts ten 200 point crossbar switches. The four types of outgoing senders, MF, DP, RP, PCI, and intermarker group senders may be located on one sender link frame. The senders connect to horizontals, the trunks to verticals of the switches. Control is by the markers.

Incoming Registers - Incoming registers record the pulses on calls received over incoming trunks from operators or connecting offices. Since these pulses are incoming from various types of offices, the following different incoming registers are provided to record them:

Incoming Register links - The incoming register links connect incoming trunks to incoming registers. This connection is made without the use of a marker. Information from incoming trunks to incoming registers is transmitted through these links.

The link frames use 200 point crossbar switches to connect a maximum of ten incoming registers of one type to trunks connected to verticals of the switches. The number of trunks served by a group of ten registers can be increased by associating several frames into a "link group".

Cross-connections associated with the individual trunks advise the selected register of the trunk's class and trunk link frame number. If the trunk is used for through switching, a trunk number will also be derived in the register. These facts about the trunk will later be given to the marker to enable it to set up the call.

When the incoming register link frame handles calls dialed directly by subscribers in step-by-step offices, the connection from the trunk to the register through the crossbar switch may not be closed when pulsing of the first digit starts. For this type of call an early "by-link" path is provided through trunk and register preference relays to start registration of the first digit before closure of the final pulsing path through the link switch.

Intermarker Group Senders - The intermarker group sender is used for traffic between two different No. 5 crossbar marker groups housed in the same building. It serves in two capacities: as an outgoing sender for the calling marker and as an incoming register for the called marker. These Senders transfer information from one marker group to the other by means of connectors rather than by pulsing.

Connectors - A connector is a relay-type switching device for interconnecting, for a short interval of time, two equipment elements by a relatively large number of leads.

A specific method is used in naming these connectors. If more than one type of equipment can originate action toward another type, the connector is named according to both the originating and terminating action; for example, in connectors such as the line link marker connector with the word "marker" in the title, the action terminates in the marker and is originated by the line link frame. The originating circuit must be mentioned because other circuits can originate action toward the marker. Other connectors of this class are the originating register marker connector and the incoming register marker connector.

Similarly, when only one type of equipment can originate action toward another type, the connector is named according to where the connector action terminates. For this reason the connectors from markers to other frames do not contain the word "marker" in the title. Connectors in this class are the line link, trunk link, number group, and outsender connectors.

Coin Supervisory Circuits - Coin supervisory circuits handle all the Coin operations except those taken care of by the originating register. One of these circuits, which are in a common group and are mounted on relay racks, is connected to a trunk that is serving a coin call. The duties of this circuit are to collect the coins at the end of a completed call for which a charge is made, and to effect coin return when the call is not completed or is one for which no charge is made.

Coin Supervisory Links - These links connect coin trunks to coin supervisory circuits. The frame is similar to the incoming register link frame and the circuit arrangements are the same. However, because the holding times of coin supervisory circuits with coin trunks are very short, a group of ten coin supervisory circuits can serve as many as 600 trunks.

Message Register Frames - Calls involving one message unit may be recorded by AMA equipment or on message registers. If message registers are used, they are mounted on message register frames.

Message registration is accomplished over a single-sleeve lead which permits line link frames with 3-wire switches to serve all classes of lines. The message register service charging arrangement involves a cold-cathode vacuum tube. Selective operation of either a tip-party or a ring-party register on 2 party lines is obtained.

The message register operating principle is shown in simple form in Figure 7-10.

A message register power supply circuit is required, to furnish 60 cycle a-c voltages used to fire the cold cathode tubes in series with each message register and operate the registers. Supplied to the registers are voltages differing in phase by 180 degrees for tip and ring party registers. The power supply also furnishes to message register trunk circuits two positive half-wave voltages differing in phase by- 180 degrees, but in phase respectively with the two voltages supplied to the tip and ring registers. At the time a charge is to be made, the trunk, having been given a party indication, applies one or the other positive half-wave voltage to the sleeve of the connection. Although both the tip and ring registers are connected to this sleeve by cross-connections at the MDF, only one of the cold cathode tubes fires and permits its register to operate. This will be the register connected to the supply voltage which is 180 degrees out of phase with the voltage applied by the trunk circuit.

Automatic Message Accounting (AMA) Equipment - The description of the AMA features are included in another section. A brief listing of the equipment is given here for reference.

Transverters receive from outgoing senders the details of an outgoing call: called number, calling party's line location and party, message billing index, and the number of the recorder serving the outgoing trunk being used on the call. The transverter converts this information into a proper form for recording, and then seizes the recorder and controls perforation of this information in the AMA tape.

Recorders serve as controls for the perforators which punch the tapes. One recorder and its associated perforator serve a maximum of 100 AMA trunks.

Translators are used by the transverter to translate the calling line location and party information into the subscriber's directory number for recording on the tape.

Call Identity Indexers supply a two-digit number identifying the trunk used on a call. This number is included in each entry associated with the call. The outgoing AMA trunk, through the call identity indexer, seizes its recorder to perforate entries for timing of the call, one entry at the time the called party answers, another at the time of disconnect.

The Master Timer keeps a running record of the time by month, day, hour, minute and tenths of minute. The running record of the time within each hour is supplied to each recorder for use in recording the answer and disconnect entries. At the turn of each hour, the timer supplies the new hour number for perforation by each recorder. The month, day and hour are included in tape identification entries perforated to permit cutting of the tape.

Foreign Area Translators - A foreign area translator frame and associated connectors contain circuits which operate in conjunction with the markers to permit routing calls to other national numbering areas if there is more than one trunk route available to the numbering area. Arrangements are provided for translation into a maximum of six foreign areas. Where only one route is available to each numbering area, or one combined route is available for a number of areas, the marker can route calls to them without using the foreign area translator. However, if different AMA charge treatment is required for two or more destination codes reached over a single route to a foreign area, the foreign area translator will be required.

This part outlines, without circuit detail, the method of operation on some typical calls through a No. 5 crossbar office.

Calls involving subscribers are of four general types: intraoffice, reverting, outgoing and incoming. A dialing connection is established from the subscriber to an originating register for the first three types of calls.

Through-switched calls are completed from a line link appearance of the incoming toll or tandem trunk. For these calls a 'dialing connection through the line and trunk link frames is not required. Instead, the recording of dialing or other pulsing is by the incoming registers, which also may supply dial tone if required by any trunk groups.

Certain features of marker operation are similar for all types of calls. For the sake of brevity, description of these features are omitted in the outlines of the calls here given. However a general statement of the features may be of interest:

Dial Tone Connection (Figure 7-11) - The steps in establishing a dial tone connection are as follows:

Outgoing Call (Figure 7-12) - An outgoing call requiring outpulsing is set up as follows:

Tandem Call (Figure 7-15) - A call is illustrated routing from dial office X to dial office Z, with tandem switching through an intermediate No. S crossbar office Y.

Local Completion - If the call received over an incoming tandem trunk is for a number in the local No. 5 office, the marker recognizes this by the office code received. The marker sets up a connection from the trunk link appearance of the tandem trunk to the subscriber's line on a line link frame. This connection is the same as a regular incoming call.

Toll Call - A through toll call is handled the same as the tandem call described above with one additional feature. If the marker is unable to establish the desired connection from one line link appearance of the incoming intertoll trunk, it obtains from the second number group the alternate line link frame appearance and makes another try. If this attempt also is unsuccessful, the marker can advance to an alternate route if there is one, or if not set the intertoll trunk for reorder signal.
Local completion to the home marker group is handled as described for the tandem call.

Reverting Call -, A special type of intraoffice call is one that occurs between two subscribers on the same line. This is known as a reverting call. Since the local office code has been dialed by the subscriber, the call starts out as a regular intraoffice call and an intraoffice trunk is selected. On all intraoffice calls the marker compares the line location obtained from the number group (called number) with its record of the line location of the calling line. If the two. line locations match, the marker recognizes that the call is to a party on the same line as the calling subscriber. The marker will then "route advance" from the intraoffice trunk group and will select a reverting call trunk if the class of service of the calling line permits this type of service. The reverting trunk has special provisions for signaling the two subscribers.

If the class of service of the line does not permit reverting calls, the route advance will be to a tone trunk to return the busy signal to the calling party, or to an operator trunk on which the call can be handled and ticketed.

Terminal Hunting - The sleeves of non-PBX lines are not accessible to the marker for a busy test of the line until after the line link frame has been seized and several connecting relays operated. For a PBX hunting group, this method of busy test would not be satisfactory since several lines in the hunting group may be busy when a call is made to the PBX. For this reason, when the marker gets an indication from the number group that the number called is in a hunting group, it prepares for terminal hunting. A sleeve connector relay in the number group is operated, giving the marker access to the sleeves of ten consecutive numbers, including the directory number. These number sleeves are cross-connected at the main distributing frame to the sleeves of the various line locations assigned to the PBX group. The marker is thus able to make a simultaneous test of ten numbers, or less for small groups, and selects one which is idle. The marker then proceeds to obtain a line location for the selected number. If more than ten numbers are required in the hunting group, additional sleeve connector relays are used to connect to additional sleeves in groups of ten.

For very large hunting groups, two methods for saving hunting time are available. These are the "block select" and the PBX allotter features. Both of these methods are based on providing busy indication to the markers on the basis of complete tens blocks. With the block select method, the tens block containing the directory number is tested first. If an idle line is not found on this block, the marker passes over any succeeding blocks which indicate busy and selects a block of ten that has at least one idle line. With the PBX allotter feature, the hunting group can be distributed over several number groups. The marker is always directed to a tens block having one or more idle lines in it. Both methods require an auxiliary relay per PBX trunk.
Other Features and Operations

Many of the features described in this section represent selection options. The decision as to proper arrangement for individual office cases should be based on coordination between engineering, plant, traffic and commercial. While traffic loads and operation arrangements may frequently dictate the selection, the equipment engineer is responsible to see that all factors have been properly evaluated before final decision is made.

Operation with Switchboard - The switchboard providing the required operator services for the No. 5 crossbar office may be in the same building or in a building at a distance from the crossbar office. Where the No. 5 office handles purely local traffic, it is common practice to use a remote switchboard for operator services because the amount of this traffic does not ordinarily justify a local switchboard. Features are available which economize on the number of cable pairs required for the switchboard trunks. These include concentrating circuits for permanent signal and coin-supervisory circuits, and provision of operator trunks with E and M lead signaling suitable for use on carrier circuits.

Where the amount of switchboard traffic requires it, as in the case of a No. 5 crossbar office arranged for toll center operation, the switchboard is in the same building. This arrangement makes possible some additional features of the No. 5 system.

Marker Pulse Conversion (MPC) - This is a feature of the No. 5 system which permits operator positions located in the same building as the No. 5 equipment and equipped for sending only MF pulsing, to employ direct trunks for reaching offices which require the receipt of dial pulsing or revertive pulsing.

This feature uses the same MF incoming registers and DP or RP outgoing senders as are used for other calls through the office and thus eliminates the need for switchboard senders. Pulse conversion calls do not switch through the line and trunk link frames and therefore do not use up capacity in the switching train.

Trunks using the MPC feature may be toll switching trunks for switchboard use only, or they may be outgoing or two-way intertoll trunks having a switchboard appearance, or two-way operator office trunks.

Since the operator selects the trunk manually at the switchboard, only the digits required at the connecting office are key pulsed by the operator.

A pulse conversion trunk first operates as an incoming trunk by seizing a MF incoming register and passing pulses to it. Later, it acts as an outgoing trunk by connecting to a dial or reverting pulse outgoing sender and transmitting the required type of pulses to the called office. A call using this feature is shown in Figure 7-16.

The calling subscriber reaches the DSA operator in the usual manner and tells her the called number. From this information, the operator plugs into a trunk arranged for pulse conversion outgoing to the called office (connection 1). Through an incoming register link, the trunk is connected to a MF incoming register (connection 2) and transmits the trunk class mark indication to it. The operator then keys the number after receiving a start dialing signal.

The incoming register seizes a marker through an incoming register marker connector (connection 3) and transmits the above information to it. From the trunk class mark, the marker learns that this is a pulse conversion job and determines whether a dial or revertive pulse outgoing sender is required. It then seizes the proper sender through an outgoing sender connector and transmits the called numericals to it (connection 4). The outgoing sender is connected to the trunk through a sender link (connection 5), and after making trunk test, outpulses the digits to the connecting office. The talking connection is established over the trunk (connection 6). The crossbar control equipment is released and the operator supervises the call.

The trunk link appearance of the pulse conversion trunk shown in Figure 7-16, is required in setting up the sender link connection and is also used in setting the pulse conversion trunk for reorder in case all senders of the required type are busy. For trunks used from the switchboard only, this trunk link frame appearance may be a "bunched" appearance, that is one trunk link termination may be used in common by several pulse conversion trunks. Bunching of trunks on a trunk link appearance is done to save trunk link terminations. It may be done only when completion of calls from the trunk link appearance is not required.

Through-Switched Calls - Through-switched calls are of two general types, tandem and toll. The distinction is not in the method of switching, which is fundamentally the same for the two classes, but rather in the type of traffic handled. Tandem class traffic may originate from local dial system offices, from other machine tandem points, or from local operators, and call destinations are restricted to specific areas. Intertoll traffic may originate from other toll offices and from operators, and be routed to points anywhere 'within the nationwide toll system.

Trunks for tandem service are one-way. Intertoll trunks may be one-way or two-way, and the outgoing and two-way trunks are usually given an appearance on a toll or combined toll and DSA switchboard located in the same building as the No. 5 crossbar office. Tandem trunks do not provide for passing re-ring signals whereas this feature is provided for intertoll trunks. The transmission path for the toll trunks is set up on a 600 ohm impedance basis, while for the tandem trunks this impedance is 900 ohms.

The two types of traffic differ in the method of handling supervision through the No. 5 crossbar office. Incoming tandem trunks usually are switched to outgoing local trunks which are also available to the subscribers. The outgoing trunk must be arranged for tandem completion. On a tandem usage of the outgoing trunk, the tip and ring conductors are cut through in the outgoing trunk so that reverse battery supervisory signals are passed directly from the distant trunk circuit to the supervisory relay in the incoming tandem trunk.

Incoming intertoll calls are switched to outgoing or two-way intertoll trunks, to toll completing trunks, and to toll information and toll switchboard trunks. On these connections, supervision is passed through the switches by means of a simplex circuit over the tip and ring of the talking path. Simultaneous signaling in both directions is accomplished. The intertoll and toll completing trunk circuits are provided as required to accept any type of supervision from a connecting trunk or toll line and convert this supervision to the simplex type required for the connection through the No. 5 equipment.

It is necessary that incoming calls for through switching be identified by class marks as tandem or toll, so that connections can be made to outgoing trunks of the proper types.

If the trunking plan requires that incoming tandem trunks be given access to intertoll trunks, the intertoll trunks must be provided with auxiliaries which convert the simplex supervision to reverse battery. These auxiliaries may also be used to give subscribers in the No. 5 office access to intertoll trunks.

The former method of transmission level control, by means of switching pads associated with intertoll trunks, has been superseded by the later method of providing a fixed pad in any toll connecting trunk having less than 2 db loss without the pad. The pads are provided in the various trunk circuits as indicated in the trunk tables.

Trunks incoming to the No. 5 office for tandem switching are given an appearance on a line link frame similar to a subscriber's line appearance, so that connections can be set up through the line link switches to trunks outgoing from the office. Intertoll trunks are given two such line link appearances on different frames to improve the chances of finding a channel to a selected outgoing trunk. Trunks of both types usually have a trunk link appearance for completing calls in the local marker group.

The amount of switched-through traffic that can be economically handled by a No. 5 crossbar office is limited, because the higher calling rate of the trunks uses up the load capacity of the unit at a much higher rate than subscriber lines. Equipment elements primarily designed for subscriber traffic may become inefficient for traffic originating predominantly from trunks. A line link frame, for example, may reach the limit of its load capacity with a relatively small number of trunks, leaving a large quantity of line terminations that cannot be used.

The tandem switching features are especially useful where a change in type of pulsing is required to handle traffic between two offices. As an example a panel or No. 1 crossbar office may use a No. 5 office as a tandem point to reach a step-by-step office. The No. 5 office in this case receives revertive pulses, using a RP tandem register, and sends out dial pulses. Similarly, the No. 5 office can accept dial pulses from a step-by-step office and convert them to any other type of pulsing required for a particular routing.

Manual Calls - If any lines in the No. 5 office require manual service, a simple method of providing this service is available. The manual line is given a class of service which the originating register recognizes when the dialing connection is set up. Instead of returning dial tone, however, the register immediately connects to a marker and signals it to route the call to an operator. A separate group of trunks to the operator may be used, or the call may be sent to the regular "0" operator group. In the latter case a tone signal may be employed to advise the operator that the call is from a manual station.

Intermarker Group Operation - The capacity of a marker group is presently limited to forty line link frames, twenty trunk link frames and 20,000 numbers. When any of these capacities are exceeded by the requirements of an exchange area served from one building, a second marker group may be established in the building. A special method of handling calls between the two marker groups, known as intermarker group operation, is provided. This method provides for handling calls between the two marker groups without using pulsed digits. The medium of exchange is the intermarker group sender which plays a dual role, acting as a sender in one marker group and as an incoming register in the other.

Coin Junctor Operation - Coin junctor operation is a plan provided to enable the handling of coin traffic over non-coin outgoing trunk groups to connecting offices. The advantage gained by this method is the elimination of small inefficient trunk groups for coin calls to each destination. A saving is made not only in trunk relay equipment equipped for coin supervision and timing, but also in outside plant, the latter consideration being the more important in most cases.

To provide coin junctor operation, the marker group must be arranged for tandem switching and the non-coin outgoing trunks to be given coin access must have the tandem completing feature.

Figure 7-17 shows the handling of coin traffic by both the direct coin trunk method and by the coin junctor method. As described later, some situations may dictate use of both methods in one office.

For either method of operation, the coin customers will have been connected to an originating register where his call is recorded. If direct coin trunks are used, the register then calls in a marker which sets up connection "d" and attaches an outgoing sender of the proper type for pulsing to the distant office. The coin outgoing trunk has access through the coin supervisory link to coin supervisory circuits for collection and return of coins and for access to operators for overtime or trouble conditions.

When the call is to be handled by the junctor method, the marker sets up connection "a" from the coin line to a coin junctor, which is one of a group used to reach several distant offices. The marker attaches an outgoing sender, usually MF, to the coin junctor and records in the sender the number called. The marker then releases itself. The sender then signals the junctor to call in an incoming register. After the incoming register is connected, the sender pulses the called number to it, including the office code of the called destination and the numericals of the called number. The sender then releases. The incoming register, acting as it does on tandem calls through the office, now calls in a marker to set up connection "c" from the line link appearance of the coin junctor to a non-coin outgoing trunk to the destination dialed. An outgoing sender of the required type is connected to the outgoing trunk and the called number is transferred to it from the incoming register. The marker then releases itself and the sender outpulses the called number over the trunk to the distant office. Supervision of the call is placed under control of the coin junctor which also is given access to the coin supervisory equipment. This permits concentrating the coin features in a relatively small group of coin junctors rather than a larger collection of outgoing coin trunk circuits.

The coin junctor method requires double handling of the call by markers and senders, and ties up two channels through the switching train. For these reasons the exclusive use of this method may not be economical in all cases. For example, if there is an appreciable amount of coin traffic to a particular destination it may be more economical to have a high usage coin trunk group to that office. These direct trunks will handle most of the coin traffic to that point, while coin junctors may serve as an overflow routing to that office and also handle all coin traffic to other points.

AMA Junctor Operation - AMA junctor operation is similar to the coin junctor operation and can be employed for subscriber calls originating in the No. 5 office and completing via outgoing interoffice trunks and also via outgoing intertoll trunks. The latter type of call will be encountered with foreign area customer dialing (FACD) and may also be encountered on the long haul calls within the home numbering area (HAGD).

AMA Junctor Operation with Outgoing Interoffice Trunks - Where trunk groups handle a small percentage of AMA billed calls and the rest are noncharge, the AMA junctor method can be used to permit more economical equipment arrangements. From the standpoint of equipment arrangements, the AMA junctor is assembled from an AMA outgoing trunk and an incoming tandem trunk or auxiliary junctor.

On a call using the AMA junctor arrangement, the subscriber is connected to the outgoing AMA trunk in the regular manner. The out-trunk and out-sender call in the AMA equipment to record the necessary details of the call. The associated incoming tandem trunk or auxiliary junctor, then calls for an incoming register and the digits are transferred to it. The call is then routed through the frames the second time, like any other tandem call, to the tandem-type non-AMA out-trunk.

AMA Junctor Operation with Outgoing Intertoll Trunks - As outgoing intertoll trunks are primarily used by operators, they do not contain AMA arrangements nor provision for supplying talking battery. Accordingly, when subscribers are given direct access to these trunks, the required AMA and talking battery supply arrangements are obtained by associating an auxiliary trunk with the intertoll trunk. Where trunk groups handle a small percentage of subscriber dialed calls compared with operator-handled traffic, it may be more economical to use AMA junctor operation than to furnish the auxiliary trunks. In this case, the equipment arrangements of the AMA junctor are assembled from an AMA outgoing (loop signaling type) trunk and an incoming intertoll (reverse battery signaling type) trunk or intertoll auxiliary junctor. Subscriber access to intertoll trunks by the two methods (AMA auxiliary, AMA toll junctor) is illustrated in Figure 7-19.

The method of operation for the AMA junctor for intertoll trunks is similar to that described for interoffice trunks above, except that the toll class is indicated to the incoming register and marker for the second trip through the switches.

Message Register Junctor - In offices using message registers, a junctor method similar to those described can be used if the percentage of calls handled on a message rate basis is low enough to justify use of the junctor operation.

Physical-Theoretical Office Operation - The No. 5 crossbar marker group is arranged to handle a maximum of 20,000 directory numbers. A minimum of two office codes must be assigned when an office is equipped with more than 10,000 numbers. The two divisions of maximum 10,000 numbers each are called office A and office B.

If required, either or both of the 10,000 number series may be subdivided into three arbitrary groups of numbers called physical, theoretical, and extra-theoretical. Each of these subdivisions will be assigned a distinct office name. The principal reasons for using theoretical offices in a marker group are (a) to permit rate discrimination between the number subdivisions. (b) to minimize subscriber number changes when, for example, a No. 5 office replaces more than one office, or where subscribers who will ultimately be served by new equipment, may be initially assigned as a theoretical office in an existing marker group. When the new equipment is ready, the subscribers with the theoretical office code are transferred without number change to the new equipment.

When discrimination is used, the subscriber numbers associated with the physical, theoretical, and extra-theoretical office codes are assigned in increments of 100 numbers and in any ratio desired. The hundreds blocks assigned to any one of these subdivisions need not be consecutive. There is one restriction, namely, that an increment of 100 numbers cannot contain numbers of two or more subdivisions.

When discrimination is not used, any of the hundreds blocks may be arranged to complete the connection when the numericals are preceded by any one of the several office codes in the group of 10,000 numbers.

The discrimination feature matches a physical or theoretical indication received from an incoming trunk with an indication from the number group for the particular block of 100 numbers called. If the comparison indicates that the called number should not be accessible to the particular trunk because of rate discrimination, the call is routed to intercepting.

Direct Routes - In a small -or moderate sized exchange area, generally each local central office has direct trunks to all other central offices in that area. The trunks are usually provided on a one-way basis; that is, traffic in each direction is handled over a separate group of trunks.

When the marker is setting up an interoffice call, it determines whether a trunk link frame has idle trunks before seizing the frame. After a frame has been seized, the marker can test a maximum of twenty trunks at one time on the one frame. Since each frame is, therefore, limited to a maximum of twenty trunks per route, the total number of trunks per route in the marker group is limited to twenty times the number of trunk link frames. For example, if office X has five trunk link frames, the marker can determine if any of 100 trunks to office Y are idle by the single trunk link frame test.

In the example given on the preceding page, if the 100 trunks of office X (five trunk link frames) are not adequate to handle the amount of traffic to office Y, then more than 100 (but not more than 200) trunks are provided. The trunks are divided into two subgroups, 100 trunks maximum per subgroup. These subgroups are spread over the five trunk link frames, a maximum of twenty trunks of each subgroup on a frame.

Note that the subgroup in such an office cannot have more than 100 trunks because the marker is limited to testing twenty trunks after the frame has been seized. To use both subgroups effectively, the marker allots calls into each subgroup. This is done by testing one subgroup on one call to office Y and the other subgroup on an alternate call. If the marker tests for idle frames with idle trunks in one subgroup but finds no trunks available, it tests the second subgroup in an effort to complete the call successfully before routing it to overflow.

Alternate Routes -The alternate route principle is a combination of direct and tandem routing. Direct trunk groups are supplied from office X to office Y, but these groups are intentionally made inadequate to carry all of the traffic during heavy loads. Office X will always attempt to route calls over the direct group to office Y first, and therefore, the high load on that group keeps it working efficiently. When all of the trunks of the direct group are in use and another call is originated to the same destination, the marker in office X will attempt to complete the call over the direct group, but in this case will find all the trunks busy. The marker then attempts to complete the call over the alternate (tandem) route in the manner of a tandem call.

In large exchange areas where trunking becomes more involved, more than one alternate route may be available for a call; for example, office X can reach office Y over the direct route trunks, and also through tandem offices I and 2.

The markers in office X route calls for office Y over the direct route trunks as long as any of them are idle. Additional calls are then routed to office Y through tandem office 1. The route through tandem office 1 is called the first alternate route. If both direct route and first alternate route trunks test all busy, the markers in office X attempt to route additional calls for office Y through a tandem office 2 (second alternate route).

The No. 5 crossbar system can handle three alternate routes, in addition to the direct route. If a call cannot be completed over any of these routes , overflow tone is returned to the calling subscriber.

Digit Deletion, Prefixing and Code Conversion - Features are provided for changing the code digits registered in an originating or incoming register to a different set of digits for outpulsing to the next office. The changes are accomplished by deleting any number of the initial consecutive digits up to a maximum of six, and by prefixing arbitrary digits up to a maximum of three. Either deletion or prefixing may be used alone, or a combination of deletion and prefixing may be used.

Present arrangements for MF pulsing provide for prefixing either "11" or one arbitrary digit which may be any digit 0 to 9, or the combination of "11" followed by one arbitrary digit. For DP pulsing former arrangements were the same as for MF pulsing. Later provisions, requiring a special code conversion bay, allow one, two, or three arbitrary digits for DP. Prefixing of three arbitrary digits for MF pulsing will be provided in the wire spring MF out-sender.

Deletion is used on outgoing calls over direct trunk groups. For example, an office code plus numericals (ABX-1234) are received by the marker from the originating or incoming register and all seven digits are transferred by the marker into an MF or DP out-sender. If the call is to a single unit office, only the numericals are needed at the terminating point. The marker therefore instructs the sender to "delete 3", and the sender outpulses only "1234". If the call were to a multi-unit terminating office, the sender would be instructed to "delete 2" and the sender would then outpulse "X-1234".

If there were an alternate route for these direct routes through a tandem office, it would be necessary to outpulse all seven digits in order to provide routing information to the tandem office. For the alternate route call, the marker instructs the sender "delete none".

An example of the use of code conversion is shown in Figure 7-18. A call is made through the "A" No. 5 crossbar office to the Davenport 6 office at "C". The call is routed through a step-by-step intertoll train at "B". The routing through the intertoll switches requires the digits 1-6 to reach the trunk group to "C". The required conversion from DA6 (326) to 1-6 is made in the No. 5 machine by deletion of two digits (32) and prefixing of the digit "1". Note that the registered office code digits (326) are used to route the call through the No. 5 crossbar office, and that the converted code (16) is then pulsed forward to route the call through the step-by-step intertoll selectors. The called line numericals are then pulsed by the No. 5 crossbar sender to control the selectors and connectors in the "C"office.

Direct Distance Dialing (DDD) - Provision of AMA charging features permits customer dialing of toll calls to points in the home area within the limitations of the trunking plan of the office. Toll calls may be routed through a toll switching point such as 4A, crossbar tandem, of step-by-step intertoll office. If the No. 5 office itself has any intertoll trunk routes, its subscribers may be given access to the intertoll trunks by means of auxiliaries or AMA toll junctors.

The design of the No. 5 crossbar system permits extension of customer dialing to all numbering plan areas with a minimum of modification and addition to the equipment of the office. The additions and modifications which may be required in an office to prepare for DDD include:

The area code is used by the marker to select a trunk group, either to a switching center in the area dialed or to an intermediate toll switching point which will select a route to the area dialed. Where more than one route is available from the No. 5 crossbar office to a particular area, the marker must have access to a foreign area translator to determine which route to use to the particular office called.

Figure 7-19 illustrates methods of giving customers access to the intertoll network for direct distance dialing.

Foreign Area Translation - Prior to the provision of the foreign area translator, the marker could look at only the three digits of the area code on calls to a foreign numbering area; hence all calls to a given area had to be routed over the same tandem or toll office trunk group. This restriction can result in indirect routing and inefficient use of outside plant; particularly when the No. 5 office is near the dividing line 'of two numbering plan areas.

With foreign area translators, six digits translation is permitted. The first three digits, the area code, are used by the 'markers to select a foreign area translator (FAT). The three digits following the area code are then given to the FAT which translates them and returns to the marker a route indication for the particular office represented by these digits. By this means calls to a foreign area may be routed through more than one tandem office or toll center, or over direct trunks to local offices in the other area. Translation for a maximum of six foreign numbering areas with a total of 100 route indications is provided.

An example of the use of foreign area translation is shown in Figure 7-20. The "A" No. 5 crossbar office is located near the boundary of the 916 area. Calls for most points in the 415 area will be routed through the "D" No. 4A crossbar toll office. However, a percentage of the calls to the 415 area will be to towns just over the line and these calls would involve a long backhaul if handled through "D". These calls are therefore given a more direct routing through a No. 5 crossbar office at "B" which serves as a toll switching point. Since this represents a second route into the 415 area, translation of the three digits following the area code will be required to determine whether to send a call through "D" or "B".

The figure shows the routing of two typical calls from an "A" custom-3r. On the first call, the customer dials a telephone in "C" which has a directory number XX2-1234. Since this is a foreign area call, the "A" subscriber will dial the area code 415 ahead of the directory number XX2-1234. At the No. 5 office the marker handling the call will refer the office code digits XX2 to a foreign area translator for area 415, and will be instructed by the FAT to route the call to the "B" trunk group. The marker will tell the out-sender to delete the area code digits, and the sender will outpulse XX2-1234. At "B", a through switching operation will route the call to the XX2 trunk group and cause the numericals 1234 to be outpulsed.

On the second call, the customer dials a "D" telephone number, 415-XX1-1234. When the FAT looks at the office code digits (XX1), it tells the marker to route the call over the trunk group to "D". The area code will be deleted from the outpulsing.

On either of these calls an alternate route may be set up through the "E" 4A toll office. If this is done, it is necessary to restore the area code digits so that the call can be recognized at "E" as a call for the 415 area.

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |

Survey of Telephone Switching
Chapter 7

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 | | Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |

Survey of Telephone Switching Chapter 7

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 | | Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |

Survey of Telephone Switching
Chapter 7

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |