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Survey of Telephone Switching
Chapter 4

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As the name implies, the term "step-by-step" is descriptive of the manner in which the call progresses. Each letter or numeral dialed by the subscriber or operator causes a separate mechanism, called a switch, to function to advance the call through the various steps required for completion. The number of switching steps or stages is directly related to the number of subscriber or trunk codes to be reached by direct dialing. The trunk codes are required for access to central offices outside the originating office.

Field of Use

Step-by-step systems are quite flexible in that they may be used for local dial service in communities requiring about 100 lines, or for large central offices requiring 10,000 lines or more.

When intertoll dialing was first introduced, step-by-step arrangements were readily adapted to this new concept in telephone service. Therefore, step-by-step intertoll systems have been used in many toll centers, arranged for two-way or one-way incoming intertoll dial operation. Recent crossbar systems, with their many advantages, are now replacing some of the larger step-by-step intertoll systems. Most of the future intertoll systems will be of the crossbar variety except for certain situations such as small isolated toll centers, or toll centers with local step-by-step systems where step-by-step intertoll arrangements may be installed for economy reasons.

Basically speaking, all step-by-step systems are alike; however, certain circuit requirements vary with the size of the system. In addition, some features which are desirable in larger offices are unnecessary in smaller, less complicated units. Therefore, various types of systems have been developed which provide adequate yet economical service - always a Bell System objective - for their particular field of application.

Originally, the No. 1 step-by-step system was the only type available for local dial service and it was designed primarily for large central offices of 10,000 lines or more. As conversion to dial operation progressed, the need for smaller type systems became apparent. As a result, the 350-A and 360-A systems were developed. Certain undesirable features inherent to the 360-A system resulted in development of an improved small dial office, designated the 355-A type, which is widely used throughout the Bell System for small communities.

No. 1 and 350-A Type Offices

These types of offices have been used, in the past, for providing dial service to large or medium size areas of population. Today, No. 5 crossbar is utilized for the majority of these locations but where it cannot be justified, economically or otherwise, step-by-step units are established.

The 350-A was originally designed for unattended operation - all toll and auxiliary service was to be furnished from a distant toll center. The No. 1 was designed for use at toll centers. As time elapsed this distinction has largely disappeared.

Switching Device - The basic element of the system is the "step-by-step" switch and its bank. For circuit reasons the number of banks, stepping control, etc. may vary, but essentially these are "building block" variations of the basic mechanical arrangement. The switch without its bank is illustrated in Figures 4-1 and 4-2. Its essential parts include lower cover plate, frame, magnets, shaft, ratchets, wipers, pawls, dogs, commutator, test jack, plug, fanning strip, spring assemblies and cover.

Banks are associated with each switch. Each bank consists of ten rows of terminals in a partial circle (Figures 4-3 and 4-4) so placed that the switch steps upward to select the row and rotates to select the terminal. Terminals are furnished either single for "100" point or in pairs for "200" point banks. The number of banks associated ' with each switch varies depending on the purpose of the switch and the number of leads required - a 100 and a 200 bank for three wire, two 200 banks for four wire, etc. The numbering plan for the terminals on the bank is shown in Figure 4-5. This conforms to the numbering plan followed on the dial.

Commutators (Figure 4-6) are provided on switches that must hunt for a level (line finders level hunting connectors, etc.). They control the vertical stepping to the desired level. Bank wipers (Figure 4-7) make contact with the bank terminals to complete the paths through the switch.

The switch steps by means of ratchets and pawls (Figure 4-8 and 4-9). The pawls are operated by magnets (Figure 4-10 and 4-11.) Dogs under control of a release magnet hold the switch when the pawls fall back during stepping or when the desired point is reached. To restore the switch to normal the release magnet (Figure 4-12) disengages the dogs, the shaft spring (Figure 4-13) returns the shaft to its rotary normal position, and gravity restores it to the vertical normal position.

Line Finder - The function of the line finder is to locate or find the calling subscriber's line and extend that line to a first selector.

In general, the line finder has a capacity for serving 100 or 200 subscriber lines (Figure 4-14). The line finders and their associated line relays constitute a line finder group. Standard line finder units, which may or may not be fully equipped, provide group sizes of 16, 20 and 30 line finders for the 200 line unit (Figure 4-15). The 100 line unit is furnished in the 10 line finder size only.

Ordinarily, line finder units are segregated into the following classes of subscribers' lines:

a. Flat rate lines
b. Message rate lines
c. Prepayment coin lines

The mixing of more than one class may sometimes be economically desirable where there are only a relatively few lines of one class, or to provide for unusual conditions. However, prepayment coin lines cannot be combined in the same group with either flat or message rate lines.

The line finders in a group have their multiple banks slipped one level between adjacent line finders (Figure 4-16), thus giving each subscriber an equal chance of having. a line finder locate his line on the lowest level in the bank and in the shortest time.

Selectors - Selectors are switches arranged to step vertically in response to dial pulses and to rotate automatically while hunting for an idle trunk (Figure 4-17).

In general there are two types of selectors: namely, local and toll. Local selectors are used in three ways for all stages of selection: (a) between line finders and connectors for intra-office calls, (b) between incoming trunks (with or without trunk circuit equipment) and connectors for incoming interoffice calls, and (c) between line finders and outgoing trunks for inter-office calls. For incoming inter-office use, the local selectors are designated incoming selectors. Toll selectors are provided for the completion of incoming toll calls, either subscriber or operator dialed.

Local selector stages are known as first, second, etc. selectors depending upon the numbering plan. The first selector is always arranged to return dial tone to the subscriber upon seizure by its associated line finder. Three-wire banks are generally used with local selectors. Calls routed through the local selectors may encounter an all paths busy condition: that is, all outgoing trunks on a particular level are busy. Then the selector will return either 60 or 120 interruptions per minute (IPM) tone. Older offices may be arranged for 60 IPM; however, 120 IPM is now standard.

Toll selectors are available in a variety of types, with some requiring special mounting arrangements and different circuit conditions. All toll selectors except toll preceding selectors, use four-wire banks since a fourth wire is needed to return certain supervisory signals to the source. Trunks from toll switchboards, toll preceding selectors, step-by-step intertoll systems, or No. 4 type toll systems can terminate on a toll transmission type selector. With this switch the called station ringing is under control of the preceding equipment and the transmission features are built in the selector. This switch can also be equipped with coin control features required for prepay operation.

Toll intermediate selectors are used in all stages between the toll transmission selectors and connectors. Also, they may be used in lieu of toll transmission selectors to terminate four-wire switching trunks from toll switchboard or intertoll systems located in the same building as the dial equipment. In this case, the transmission features are incorporated in a trunk circuit which is cross-connected ahead of the selector. Another type of toll selector is known as the A-B toll transmission selector. It provides all the features of a toll transmission selector except controlled ringing.

Toll preceding selectors are used immediately preceding toll transmission or A-B toll transmission selectors, and generally provide common trunking to all offices in a building.

Selector shelves can either be ten or twenty selector capacity. The banks of ten selectors are multiplied to form a bank unit. Two bank units are required for the twenty type shelf. Moreover, since the larger capacity shelf is principally used in larger offices it has a special outgoing trunking scheme (Figure 4-18) known as a distributing terminal assembly.

Trunking Arrangements - First selectors, in response to the first digit dialed, provide routing via any one of ten trunks on a bank level to another selector group or to trunks to another office. Prepayment coin service makes it essential to segregate the flat rate (FR) and coin box (CB) first selectors into separate trunking groups as the zero level is used for trunks to the switchboard and separate FR and CB trunk groups are necessary. Also, the first level is used for access to a service code group of selectors which are again split into FR and CB groups. From these service code switches paths are available to information, repair service, local test desk and any other specialized service that may be necessary. Since these various codes are usually 11X, the service code selectors are arranged for single digit absorbing which permits the switch to restore to normal after the first digit is dialed and then proceed in a regular manner. As an example, for Code 113 the first digit 1 would register on the first selectors and the second digit 1 would step the service code selector vertically one step, but being arranged for digit absorbing , it will release and restore to normal; then the next digit 3 will register and route the call. The remaining levels of the first selectors provide trunk routes to other offices, to second selectors or to connectors.

Additional digits are registered on succeeding stages of selectors. The number of stages is dependent on the number of digits in the office. However, the tens and unit digits are always registered in the connector.

Graded Multiple - The selector will seek an idle trunk, on any bank level but its choice of trunks on that level is limited to ten. Wherever trunk groups to another office or to a succeeding selector group in the local office exceeds 10 trunks, a trunking scheme known as graded multiple is employed.

Graded multiple permits certain trunks, known as common trunks, to appear before all selectors in all divisions of a trunk subgroup. The remaining trunks are divided into additional subgroups and are known as individual trunks. As the individual trunks are always the lower numbered trunks, they are first choice trunks to the selectors in the divisions in which they are located. In order to reduce the selector hunting time and also to reduce wear on switches, a reversal in the common trunks is usually placed between selector divisions at the middle of the subgroup. The tenth trunk is always multipled straight for all divisions in a subgroup for last trunk busy registration.

Digit Absorbing - As impetus was given to the 2L-5N program, it became apparent that means should be developed to somehow reduce the number of selector stages needed for the seven dial pulls required by the 2L-5N scheme. As the outcome of this development, widespread use of various digit absorbing features came into existence. Although the following discussion deals with local selectors, various types of toll selectors are available with digit absorbing features.

The local single digit absorbing selector (SDA) has been available for some time and it formed the background for the double digit and two-digit absorbing types. The double digit absorbing selector (DDA) will absorb any digit repeatedly or once-only and both actions may be operative on different levels of the same switch. Therein lies the main differences between the single and double digit types since the SDA can only be arranged for one or the other of the actions and not both on the same switch. Any level not arranged for absorption can be blocked (arranged to return the all paths busy tone) until once-only absorption has occurred. In this case the once only absorption prepares the switch to operate in a normal manner on any subsequent digits dialed; thus it is known as the unlocking feature. Both the SDA and DDA types can be arranged for this feature. On the single digit absorbing, selector levels arranged for repeated absorption cannot be used for trunking. Circuit modifications can be applied to SDA selectors to arrange them for double digit absorbing.

Recently, a new selector, the two-digit absorbing selector, was made available. This switch will absorb no more than two successive digits but its main advantage is that the treatment accorded a particular level on the first digit dialed has no bearing on, and places no restriction on, the condition imposed on that same level for the second digit dialed. An innovation on this switch is the provision of no such number tone (NSNT) on blocked levels rather than all paths busy tone. Both the first digit and the second digit, if the first was absorbed, can be treated in any one of three ways;

a. Absorb once on any specified level.

b. Cut through on any specified level.

c. Block on any specified level and return NSNT to the calling party.

Repeated absorption is not possible on this switch. However, this new type offers more freedom of choice of office codes and permits more flexibility than has heretofore been possible on other switches.

On any digit absorbing selector, the various features are accomplished by means of a normal post assembly (Figure 4-19) and a cam placed on the normal post of the selector. As the shaft raises to a particular level, the cam raises simultaneously, and if any of the right, left or both teeth are bent, they will operate the associated spring assemblies to activate the switch accordingly.

Connectors - The -connector (Figure 4-20) is the last step in the train of selection and responds to the tens and units digits (last two digits) dialed to locate the called station. This step-by-step switch performs many functions; namely, applies a predetermined ringing signal-to the called line; returns audible ringing to the calling line and will return a line busy signal (60 IPM tone and/or flash) if a busy line is encountered. It also furnishes talking battery to the transmitters of the called and, on local calls, the calling parties.

Each connector group (Figure 4-21) consists of one or more shelves, usually eleven switch positions, a bank multiple for 100 terminals and any quantity of connector switches, depending upon traffic volume. One switch position, normally the first, mounts a test connector.

Connectors are of two general types; those used to serve non-consecutive subscribers' lines and those used to serve consecutive lines of a P.B.X. or multi-line subscriber. The latter is known as the hunting connector and is available in either the rotary or level hunting type.

The rotary hunting connector is arranged to hunt over a predetermined group of terminals on one level only, and connect to the first set of idle terminals. Level hunting connectors normally are arranged to hunt over all terminals on a predetermined group of levels, and to connect to the first set of idle terminals in the hunted group.

Miscellaneous Switch Frame Equipment

Repeaters - In order to establish a connection between a step-by-step office and another dial office located in a separate building, it is necessary to furnish some means for repeating dial pulses between the offices. If the connection is used for two-way traffic, a two-way dial repeater or trunk is used at both offices, while one-way connections generally use an outgoing repeater only at the originating office.

Figure 4-22 shows the basic trunking arrangements for two-way and one-way inter-office trunking between two local step-by-step offices.

The ordinary outgoing repeater will not re-transmit the dial pulses with the same precision as those originating from a dial. As the conductor loop resistance between offices increases, due either to the length or to the size of the conductors, repeater changes are necessary to compensate for this increased resistance. This may be accomplished by using battery and ground pulsing and supervision instead of the normal loop arrangements. However, when the resistance reaches a certain point, even these measures are unreliable and a special repeater, known as a pulse corrector must be introduced at the incoming end.

Repeaters can either be relay rack units or switch-mounted. The common types are usually switch-mounted, (Figure 4-23) and located on universal switch frame shelves of twelve or fourteen capacity. A small four-position shelf is also available for mounting switch type repeaters on relay rack bays.

Test Distributors - These are switches which are arranged to step vertically, and generally horizontally, under control of the digits dialed by the originating source. Their banks are wired to the test connectors, thus permitting access to all subscriber stations in an office.

Two distinct types are available. One is used for switchboard operators to dial a particular station and listen in to determine if it is busy; thus the name verification. The other is used by the test desk to test subscriber stations and is known as a test distributor.

Reverting Call Selectors - Reverting call selectors are provided to enable one station on a party line to dial another station on the same line. They are switch-mounted and located on ten position shelves which fit on universal frames.

An associated interrupter circuit, also switch-mounted, is located on the same shelf and a test set unit is located on a relay rack bay.

Rotary Out Trunk Switch Equipment - Trunks to other offices must be carried in outside plant and where the distance is at all great it is highly desirable to keep the number of trunks between offices to a minimum. When local inter-office dial trunk groups reach about thirty trunks a device known as rotary out trunk switches (ROTS) can be used to reduce the number of outgoing trunks required and still. provide adequate service. Another field of application is for concentrating recording trunks to the switchboard in order to reduce switchboard multiple requirements.

A brief discussion of the switch itself is in order. It uses a 206 type selector (Figure 4-24) which differs radically from the regular step-by-step mechanism. The switch has 22 sets of terminals on its bank and a series of brushes which engage the terminals. The brushes are mounted on a shaft which is driven by a ratchet and pawl mechanism actuated by a magnet. Any trunk terminating in the brushes will, therefore, have access to 21 outgoing terminals or trunks. The twenty-second terminal is used for traffic register control. Outgoing trunks are connected to the banks of these switches and trunks from step-by-step selector banks to the brushes (Figure 4-25). Each ROTS, when not in use, pre-selects an idle trunk from those connected on its bank so that it will always be holding an idle trunk when connected to a selector level.

Intercepting Facilities - These facilities provide means for intercepting calls to non working connector terminals, vacant selector levels and plug-up lines (lines in trouble). These calls can be routed either to mechanical intercepting equipment or to operators in the same or distant building. Intercepted calls are concentrated over a common group of outgoing trunks.

The concentrating unit is a trunk finder unit similar to a line finder. Trunks from connector terminals (terminal-per-station), selector levels and plug-up lines are terminated on the banks. Each trunk finder is connected to an outgoing intercepting trunk. The trunk finders, being step-by-step, are equipped with normal post springs which function to cause a lamp on the switchboard to light on intercepted calls from trouble lines. Thus trouble lines must be segregated according to bank level groups.

Intertoll Dialing

The dialing of toll calls by either a subscriber or an outward operator constitutes what is known as intertoll dialing. Switching equipment is located at toll centers to route this traffic according to the dialed information. All of these locations are arranged to serve outward and inward terminal traffic and, in some cases, through traffic. With step-by-step toll centers, outward traffic is routed by the operator directly to intertoll trunks appearing on the switchboard. For access to toll switching trunks she may either use trunks on the switchboard or may dial through the switching equipment. The inward terminating equipment is arranged to automatically distribute incoming intertoll traffic to operator services and to all central offices served by that toll center.

Through switching can be defined as the switching of traffic at intermediate centers from incoming to outgoing intertoll trunks. The switching may be on a manual or automatic basis. Where step-by-step through switching is employed outgoing intertoll trunks are multipled on switch levels in addition to their appearance on the switchboard.

New intertoll systems arranged for through switching will generally employ cross-bar equipment because of its many advantages such as alternate routing, code conversion, varied types of pulsing and common control apparatus. Moreover, most of the new terminating systems at toll centers will be equipped similarly for the same reasons. Obviously, step-by-step has limited application for this purpose. Some of the situations which might justify step-by-step in lieu of crossbar are:

a. Small toll centers where local manual equipment is to be replaced with step-by-step equipment.

b. The establishment of intertoll dialing at toll centers already equipped with a step-by-step equipment.

A typical intertoll trunking diagram for a step-by-step train is shown in Figure 4-26.

The step-by-step intertoll system has the following peculiarities in circuit operation which are different from local step-by-step systems.

a. Selector simplex pulsing which must be converted to loop pulsing for operating regular toll selectors in the local dial offices.

b. All incoming terminal intertoll trunks must be arranged for automatic connector ringing, which eliminates the need for through ringing from the originating office.

c. Transmission feature (talking battery to the called party) must be provided for those switching trunks terminating in a local dial office.

d. Start dialing signals.

Requirements for Direct Distance Dialing - The introduction of direct distance dialing, has made-it necessary to adopt certain minimum requirements on a system wide basis. Of prime importance from an equipment standpoint is the information transmitted to the originating caller when the call is not completed due to a busy condition or reaching an improper code or an unassigned number.

Step-by-step systems must have three requirements to permit inward direct distance dialing. First, vacant codes and unassigned station numbers should be connected to intercepting facilities, either recorded announcement or operator terminated. Second, two letter five digit numbering must be in effect. Third, proper tones and flashes must be applied if a busy condition is encountered.

Step-by-step intertoll dial systems designed exclusively for operator dialing will transmit only busy flash (120 IMP) if there is an all-paths-busy on the intertoll selectors. This is unsatisfactory for subscribers' calls since they depend 'on a tone signal to indicate a busy condition. Thus, at intertoll switching centers, the intertoll trunks to step-by-step tributary points should be arranged for tone application. Eventually, the design of intertoll step-by-step selector circuit may be revised to incorporate both the tone and flash (120 IPM) condition.

Toll completing selector trains should be arranged for tone and flash (120 IPM) for all paths busy while local completing trains should have selectors equipped with 120 IPM tone only. Where the toll completing trains are of the local type modifications can be made to provide the above requirement.

When a station busy is encountered on a toll call it should be indicated by tone and flash (60 IPM) and a local call requires tone only of the same interruption.

Another indication that should be provided is 30 IPM flash and tone for overflow of intertoll switching trunks, which is generally referred to as a no circuit condition. This signal is used in lieu of the 120 IPM signal on intertoll step-by-step through selectors only.

Selectors - The only step-by-step switch utilized in the intertoll system is the selector. Once again the number of steps is directly related to the number of digits required for the trunking to the various central offices homing on the toll center. These selectors respond to dial pulses in the same manner as ordinary local dial selectors but, whereas local dial selectors are loop pulsed, these are pulsed by a simplex arrangement. Figure 4-27 illustrates a simplified intertoll pulsing path and how it is applied to the intertoll selector.

Certain supervisory conditions are treated slightly different than local arrangements and thus require an additional switch lead. So all intertoll selectors are four-wire.

Basically, there are two broad sub-divisions of intertoll selectors. One group, known as incoming and auxiliary selectors consists of two types of selectors. One is a conventional selector providing the regular features and the other' is a more complex type having many optional provisions. Among these are digit absorbing (repeatedly and/or once only), stop dial signals and ringdown control. All of these features are associated with bank levels and are therefore controlled by normal post cam adjustments.

The other broad category comprises those selectors with the transmission feature. First, there is the regular intertoll transmission selector which receives dial pulses on a simplex basis and converts them to loop pulses for repeating to regular toll intermediate selectors. The other type, besides having the transmission feature, can also be arranged for use as a regular intertoll auxiliary or incoming selector and is known as a combined intertoll and transmission selector. Both of these selectors can be arranged for digit absorbing.

Outgoing Trunks - For all distant dial tributary trunk groups, an outgoing trunk is needed to convert the simplex pulsing to either loop or composite signaling. In addition, if the outlying equipment is arranged for controlled ringing, the outgoing trunk must apply an immediate ring condition when the called station is reached and found idle. Therefore there are a variety of these circuits, all relay rack mounted, which will permit many different circuit arrangements for any type of inter-office signaling.

355-A Community Dial Office

Being a step-by-step system, the three basic components of step-by-step switching, namely line finders, selectors and connectors are once again employed in these offices.

Provision has been made for handling the various classes of service commonly required in community dial areas and, in addition to the traffic features usually necessary, optional facilities are also available for numerous features for which there is a limited demand. However, in the interest of economy, certain refinements used in the large type step-by-step offices have been excluded in the 355-A type. The principal simplifications are:

a. The elimination of distributing terminal assemblies.

b. The use of universal switch frames which mount all types of switch units.

c. An entirely different line finder unit design eliminating certain costly circuit features inherent in the large office line finder unit.

d. Alarm system designed with a minimum of features and elimination of the office alarm frame.

e. No aisle and frame lighting.

The above simplifications result in some limitation in the field of use, but in general it has been the objective to apply these offices to small unattended locations requiring less than 1,000 lines in the ultimate. In certain instances, where a relatively simple numbering plan can be employed and the commercial growth data indicate very nominal growth over an extended period, it may be desirable to consider a 355-A unit for slightly larger ultimate requirements. When planning to utilize a 355-A office in a border line case, the importance of careful evaluation, especially in the long range view, cannot be overemphasized. Due to the equipment arrangements, serious difficulties may be encountered in the future if the central office expands rapidly. For example, lack of distributing terminal assemblies complicates graded multiple trunking schemes necessary in larger offices. Also, building penalties may occur due to the shorter equipment frames, resulting in less equipment capacity per square foot of building space.

All switch frames are the universal type. They mount all switch frame shelves and nearly all types of equipment can be located on some type of switch shelf.

Line Finders - The line finder units accomplish the same purpose as the. large office type but are not interchangeable. Either 100 or 200 subscriber line units are available. The 200 line unit can be obtained in two line finder capacities, either 16 or 20 while the 100 point is available with ten finders only. Smaller quantities of line finders can be equipped on any unit dependent upon traffic volumes.

A major difference between this unit and the large office type is the segregation of various classes of subscribers' lines. In the 355-A office line finder groups may be arranged for the following classes of subscribers' lines:-

a. Flat rate lines.

b. Message rate lines.

c. Postpayment coin lines.

d. Prepayment coin lines.

e. Pre-postpayment coin lines (Provisional).

Provision is made for combining two or three of any of these classes in the same group where desirable with the exception of prepay coin lines which have to be segregated because of circuit differences and differences in operating methods at the operator office and toll boards. Normal post cams are necessary for all line finders in any line finder unit containing more than one class of service. These cams are used to extend class indications to the first selectors which in turn extend them to the trunk for conversion of distinct tone s.

Whereas the large office multiple banks are slipped one level between adjacent line finders on a group, the 355-A unit splits the upper shelf and lower shelf of line finders into Group B and Group A respectively. A complete reversal of the banks is inserted between groups A and B, which normally limits the hunting of a line finder to the lower five levels.

An outstanding feature on the 355-A line finder unit is the lockout feature. On lines arranged for lockout service, if a permanent signal condition occurs the lockout feature will prevent a line finder and first selector from being held out of service. This is very desirable for offices where the maintenance force is located at some distant point. The various line finder units are all obtainable with either 20 or 100 per cent of the line circuits arranged for lockout.

Prepayment coin lines and message rate lines are treated similar to those in large step-by-step offices by utilizing relay rack type trunks or auxiliaries.

Postpay coin lines require a special relay circuit which splits the connection between the calling coin line and the called subscribers when the called party answers. When the connection is split a tone is sent back to the calling subscriber which is his signal to deposit the coin. After the deposit of the coin or coins the relay circuit establishes the connection between the two parties.

Pre-postpayment coin service is not considered standard but is available for special offices where it is deemed advisable. The circuit itself is quite costly and circuit features inherent in the design may cause service irregularities.

Selectors - Certain of the local selectors are unlike their large office counterparts while others are identical and thus interchangeable. Where used, toll selectors are entirely the same and can be used for either large offices or 355-A offices.

The disparity in local selectors is primarily in the first selectors If discriminatory features are extended from the line finder groups, a four-wire first selector is necessary to extend this to the selector level trunk circuit. A non-digit absorbing local selector is available for this purpose which is not available for No. 1 or 350-A offices. But if either single or double digit absorbing is required by the numbering plan, the ordinary types apply. At the present time, the two digit absorbing selector cannot be provided in 355-A offices because it has no optional features for connecting to the 355-A alarm system.

In lieu of the regular toll selector train, a four-wire local train may be used. With this arrangement, a four-wire switch as used in the regular toll train is connected to the incoming toll trunk. However, the supervisory signals are transmitted over the fourth conductor to the trunk circuit which relays them to the originating source. This differs from the regular toll train and thus special trunks, selectors and connectors must be used. Each item is somewhat less expensive under this arrangement and thus represents an overall capital savings. However, with the local train, coin service is limited to postpay operation since local trains are not arranged for coin control or controlled ring features.

Selector level trunking is very similar to larger offices. The selector trunking pattern is obtained by cross-connections between the bank terminal strips and the outgoing trunks terminal strips mounted on the end of the selector shelves.

Connectors - All types of classes listed under the No. 1 and 350-A offices are available for 355-A offices.

As far as mounting and shelf arrangements are concerned, very little difference exists between either the large office or 355-A office methods.

Trunk Equipment - Since every phase of 355-A design has been directed toward simplicity and lower cost the inter-office trunks have been reduced to the minimum and shorn of all special features wherever possible.

Two-way trunks of the loop signaling variety are usually switch mounted on twelve position shelves. Among these are the interlocal dial and operator office types.

One-way trunks or repeaters can be either switch or relay rack mounted. All repeaters associated with No. 1 or 350-A offices can be used in these offices.

Composite signaling trunks, either one-way or two-way are generally relay rack type.

Test and Verification Distributors and Reverting Call Selectors - These items are identical in all step-by-step offices.

Intercepting Concentration Equipment - These offices are usually too small to warrant the cost of the large office concentrating equipment. Therefore a small rotary type unit, utilizing 22 point selectors for a maximum of 21 incoming intercepting trunks, is available. It also provides for a maximum of three outgoing trunks to the intercepting service desk or switchboard. Trouble intercepting is not available with this simplified arrangement. The mechanical intercepting mentioned previously has made this concentrating arrangement obsolete.

Alarm and Ringing Equipment - Originally, the alarm relays and ringing equipment were mounted on a compact shelf situated on a switch frame. Recently, this arrangement was re-designed and segregated into two separate components, an alarm relay rack bay designated MAB (Maintenance Alarm Bay) and a ringing plant relay rack bay coded 806-F.

The alarm circuits are considerably less complicated than the larger type and the audible and visual signals have been eliminated. In place of these, an alarm checking terminal is provided together with means for extending the alarm condition to the master office. An operator or maintenance man at the master office may dial through the regular step-by-step train to a connector terminal assigned for alarm checking purposes and terminating in the alarm equipment. The presence or absence of various tones on this terminal.will indicate whether trouble exists and also its nature.

Any one of three methods can be employed to transmit the alarm indications to the master office. One is the alarm sender which routes alarm conditions via the operator office trunks. This method does not require separate cable pairs, and consequently may be desirable to conserve outside plant facilities. The other two schemes send the alarm indications to two lamps located either in the PBX cabinet or the switchboard. One lamp signifies minor troubles and the other lamp indicates major troubles. The separate lamp method located on the PBX cabinet is usually the preferred method of operation.

Class of Service Tones - Certain classes of service, such as postpay or prepay coin lines, and message rate lines, require distinctive indications on calls to the operator and may require different access on selector banks. These can, of course, be obtained by segregation of line finders, selectors and trunks. This method is followed in the 355-A office in the case of prepay coin lines. For other classes of service, however, provision has been made for handling combinations of classes through the same switches and trunks to obtain equipment economies and to simplify balancing of traffic.

Classes of service are indicated to operators by the presence or absence of a momentary tone on plug-in. The operator may cause the tone to be repeated by replugging. Where two classes are involved, such as postpay coin and non coin, the former will be indicated by low tone. Where three classes are involved, as postpay coin, message rate and non coin, the second class. will be indicated by rapidly interrupted low tone.


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| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |