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Survey of Telephone Switching
Chapter 3

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The first step in telephone switching was to gather the subscriber lines into one location and then provide means for an operator to connect any two subscribers who desired to talk. Thus was born the first "manual" switching system. For many years most of the telephone switching was handled by operators. With improvements in mechanical switching the manual system has declined until today it is of minor importance in handling local calls. There still remains a small portion of Pacific System local traffic so handled.

The first commercial switchboard made its appearance in 1878 with a capacity of eight lines and twenty-two subscribers. Other manufacturers followed with switchboards using various means of cross connections such as switches, brass pegs, double ended cords, etc. In 1881 the Western Electric Co. brought out the "Standard" switchboard which was the forerunner of the boards later used throughout the Bell System. This board was made in sections, designed for growth and equipped with line signals, jacks, clearing out signals, key shelf with cords and push button type keys for ringing and listening. As the demand for telephone service grew, the Standard boards were placed side by side in sufficient number to care for the subscribers. The trunking problem that this raised caused the re-design of the board to include multiple.

All the early boards were magneto. That is, line drops were rung down by the subscriber turning the crank of a magneto and electrical energy for talking was supplied by local batteries at the subscriber set.

Local Switchboard

No. 1 Switchboard - The development of the small incandescent lamp, repeating coil, small jack and impedance type cord circuit resulted in the No. 1 common battery switchboard. This board was developed shortly after the turn of the century and is still in use today. As the number of central offices increased, the board finally evolved into an "A" or subscribers switchboard for originating calls and a "B" or trunk switchboard for terminating calls, with trunks connecting the two boards. This arrangement places a maximum of 10,500 multiple jacks within reach of an operator.

The "A" board section is arranged for three operator positions and has eight panels for jack equipment (Figure 3-1 Front View Subscriber or "A" Switchboard). The face of the "A" switchboard contains the lamps and jacks associated with the subscriber lines. A line lamp and answering jack and an ancillary line lamp and jack are provided for each subscriber line. The ancillary is placed seven panels away from the primary towards the growth end of the board, thus bringing each line in reach of several operators. Line and cutoff relays are associated with the answering jack to light and extinguish the line lamp. Above the answering jacks are the outgoing trunk jacks (OGT) to which are connected trunks to "B" boards throughout the exchange area. These jacks are multipled throughout the board and are available to every operator.

Each position is equipped with cord pairs which the operators use in handling calls. Each cord pair consists of an answering cord, calling cord, supervisory lamps, listening and ringing key and necessary relay equipment. In addition, cord pairs will have coin collect and return keys, message register keys, special ringing keys, etc. dependent upon the class of service being handled in the position.

The "B" board section is arranged for two operator positions and seven panels that will hold 10,500 subscriber's jacks (Figure 3-2 Front View of Trunk Switchboard). These jacks are multipled into each section and make up what is known as "subscriber multiple". Each operator then has access to all numbers on the board.

The "B" position is equipped with 48 to 50 single ended cords with plugs, each of which is connected to an incoming trunk from an "A" board. A guard and disconnect lamp is furnished with each cord and plug. To complete a call it is necessary for the "A" operator to inform the "B" operator of the number being called. Two methods have been used to accomplish this. The older is the call wire method where the "A" operator presses a key in her position which connects her to the telephone circuit of the "B" operator who then assigns the trunk and places the plug in the multiple. With the newer or straightforward method, the "A" operator selects an idle trunk and plugs in. The trunk automatically connects to the "B" operator's telephone circuit and the number is passed. The trunk disconnects from the "B" operator's telephone circuit when the plug is placed in the multiple.

For flexibility the answering jacks and multiple are cross connected on the distributing frame. When an operator plugs into a line, battery on the sleeve of her cord operates the cut-off relay which removes the line relay and its answering lamp from the circuit. The battery on the sleeve also provides a busy indication on both the "A" and "B" boards to prevent double connections.

A variation of the "A" and "B" board arrangement which was used in small offices is the combination board (Figure 3-3 "Combination" Board Position Arranged for Party Line Ringing). This is an "A" board with multiple appearing in the upper part. With this arrangement one operator handles the entire call including ringing the called subscriber. Space limitations for the multiple restrict this to small offices.

No. 10 Switchboard - These boards which were plentiful in the Pacific System a few years ago were designed for single office areas. They are like the No. 1 combination board in having the answering jacks and multiple in the same panels. The multiple is on a 4-panel basis and is cross-connected to the answering jacks at the distributing frame. The cord circuit is essentially the same as in the No. 1 board. The multiple jacks and answering jacks are of the cut-off type with the ring in series with the winding of the line relay. When the operator plugs in, the line relay is removed from the circuit and the line lamp retired. Battery on the sleeve provides a busy indication.

Toll sections are usually in line with the local sections but carry toll multiple in the lower portion of the panels instead of answering jacks.

Special No. 9 Switchboard - This board is a Pacific System design and should not be confused with the Bell Laboratories designed No. 9 board. These boards are essentially the same as the No. 10 board except that line lamps are directly associated with the subscriber's multiple, thus eliminating the need for separate answering jacks. Cut-off jacks are used to retire the line lamp.

No. 11 Switchboard - This board was developed shortly after World War I, and was intended for use in either single office or multi-office areas.

Multiple line lamps are associated directly with the subscribers multiple and provide distribution of the originating traffic. When larger line capacity is required a second line-up of switchboard is provided and the multiple inverted as shown in Figure 3-4 (Distributing Frame and Switchboard Cabling Two Lines of Subscriber Switchboard (Adjacent)). Still larger capacity can be obtained by adding a trunk board (Figure 3-5 Distributing Frame and Switchboard Cabling Three Lines of Subscriber and One Line of Trunk Switchboard). The maximum capacity is 10,400 subscribers' lines.

No. 12 Switchboard - The No. 12 board is a common battery board of low capacity and low cost serving both common battery and magneto lines. All operator positions are equipped to handle toll traffic as well as local. The board has a capacity of 640 subscriber lines with designation strip or 1440 without designation strip. Line relays are not used for normal loops, instead, a special line lamp which operates over a wide range of voltages is hooked in series with the subscribers loop. Cut-off jacks are used to remove the lamp when the operator's cord circuit is connected to the line.

An important feature is the "packaged unit" approach on this board. Only two floor plans are available and multiple is ordered on a "J" unit and list basis. Cable of length determined by the floor plan s attached to the multiple and terminal strip at the factory.

Toll Switchboards

In the early days of telephony, interurban or toll traffic was handled by the local operators. As toll traffic increased, specialists known as toll operators came into the picture. These operators first were located on the local board and completed calls through the multiple direct to the Subscriber. As multi-office exchanges developed toll operating was concentrated into special boards whose major function was to connect "toll lines" and trunks to the local board.

No. 1 Type Toll Switchboard - The No. 1 toll switchboard circuits have changed so radically over the years that the board of today resembles the original board only in general appearance.

The section contains two operator positions with a calculagraph between. The face equipment contains incoming and outgoing trunk multiple and answering jacks for special types of service. The switchboard comes in three sizes known as "High", "Intermediate" and "Low". The keyshelf is the same height in all sizes but the jack space varies.

The cords in the older type boards were designed for use with switching, recording completing or other trunks to local offices on one end and with ringdown intertoll trunks on the other. The cords were therefore arranged for D.C. supervision and dialing on the "trunk" end and A.C. or ringdown supervision without dialing on the "toll" end. With the increase in toll dialing these cords have been replaced with "A" type cords which provide D.C. supervision and dialing on both cords. The "A" type cords can be equipped for repeated or combined loop and repeated dialing, multifrequency key pulsing, D.C. key pulsing or combined M.F.-D.C. pulsing.

No. 3 Type Toll Switchboard - As intertoll dialing forced the No. 1 toll board to be modified for "A" type cord, the cord circuits became complex and difficult to maintain. To overcome this a new type of board known as the No. 3 was designed. In this board the relays used for signaling, ringing, etc. were removed from the cord circuit and placed in the trunk circuit. This resulted in a very simple cord circuit hardly more than a pair of wire connecting the front and rear cord, with a means of bridging the operator's positional circuit for talking, dialing, etc. The front and rear cords are alike and can be used indiscriminately to connect to trunk or toll line.

Placing the signaling relays in the trunk circuit resulted in equipment savings. Incoming signals are of two kinds: supervisory signals when the trunk is connected to a cord circuit and incoming trunk signals when the trunk is not so connected. The relay equipment for the incoming signal must be associated with the trunk, while the equipment for the supervisory signal may be associated with either the trunk or the cord circuit,. In toll boards there are generally more cords than trunks, so a saving is made by placing as much as possible of the equipment in the trunk circuit. Still further savings follow since a single set of relays will respond to both line and supervisory signals instead of one set for each kind of signal.

Maintenance of signaling equipment is also facilitated. Since a given signaling relay has to function with only the particular trunk to which it is permanently connected, it can be adjusted for a narrower range of conditions than it would have to meet were it to be connected indiscriminately to any trunk.

The No. 3 board underwent minor modifications: principally smaller type jack, finish, etc. and has emerged as the 3C board. This board is composed of separate upper and lower framework assemblies. The upper unit is available in three heights "High" (jack panel opening 2' 9") "Medium" (jack panel opening 1' 9-5/8") "Low" (jack panel opening 11-5/8"). The lower unit has either a 30" or 40" keyshelf height. The sections are either three or nine panels wide to accommodate either one or three operator positions. There is also space to locate calculagraphs between alternate positions.

A combination known as 3-CL composed of low keyshelf and high upper unit has become the "standard" for new installations of medium or large toll centers. The three operator position sections are usually used for this board.

The 3CF board is used in small installation where the switchboard is in the same building as the community dial office. It is available only in the low upper, low keyshelf, single operator position sections.

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