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Survey of Telephone Switching
Chapter 5

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The step-by-step system of telephone switching has limitations due to its direct dial control method of operation and the small trunk groups that step-by step switches can hunt over. The effort to overcome these led to the development of the panel system. This equipment contains a circuit called a "sender" which is capable of storing up a dialed number and controlling the selection of the circuit elements required to reach that number.

At the time development of the panel system was begun, the real possibilities of mechanical telephone switching design as now known were not realized. It was believed that, at best, the mechanical equipment could only imitate the human efforts of the operating force, and the panel system was designed along direct lines of existing manual operation. The line finder and district frames comprised the functions of the "A" switchboard, and its selectors the answering and calling cords. The sender circuit replaced the operator and the incoming and final frames the incoming "B" switchboard, and incoming trunk circuits.

Apparatus Design

A large terminal bank was designed to utilize the efficiency to be gained by the use of large trunk groups. This bank was in the form of a panel about 3 feet long and 14 inches high, made up of 300 metal strips with projecting terminals, each strip alternating with an insulating strip, as shown in Figure 5-1.

Each metal strip had soldering terminals at either end for cable terminations. It is from this panel multiple bank that the system received its name, Panel Type Machine Switching System. This bank could accommodate 100 3-wire circuits, either trunks or lines, which was a big increase over the 10-trunk limit of the step-by-step system. Each set of terminals appears 30 times in the front and 30 times in the rear for a total of 60 appearances.

Five of these banks are mounted vertically, one above the other, to form a frame giving a capacity of 500 trunks in a vertical column, multipled 60 times around the frame.

To reach these terminals a rod type selector was designed, equipped with 5 multiple brushes to contact the projecting terminals. One selector was equipped for each appearance of terminals on the multiple bank, 60 in all, with 30 selectors in front of the frame and 30 in the rear of the frame. The provision of a contact multiple brush for each bank limited the up and down travel of the selector brush over the terminals to only 14 inches to reach all terminals.

Wiring inside the hollow selector rod connected 5 brushes in multiple. Each multiple brush (Figure 5-2) had a device to keep the contacts spread apart when the brush was normal or not in use, so that the selector could ride up and down without the contacts touching the bank terminals. When desired this device could be actuated or "tripped" to permit one of the brushes to make contact with the terminals as the selector rode up and down.

The up and down travel of the selector required a flexible connector to maintain electrical circuit connection to the multiple brushes. This was accomplished by means of a commutator shown in Figure 5-3. The commutator is made of insulating material with brass strips imbedded as shown. In addition to maintaining the electrical connections for the tip, ring and sleeve leads, the commutator signals the control equipment the exact location of the multiple brushes on the multiple bank terminals. Pulses are generated as the commutator brush travels over the commutator segments and are sent back to the sender for controlling the selections. This type of pulsing is known as "revertive pulsing". A source of power is required to move this 5 foot long selector up and down the multiple banks. This is furnished by motor driven cork rolls located at the bottom of the frame. These rolls are in continuous operation, one required for updrive and another revolving in an opposite direction for downdrive. The final frame has two updrive rolls, one at regular speed the other at low speed for unit selection. The bottom of the selector rod terminates in a flat strip of punched metal called a rack. In front of the cork rolls is located a clutch, one per selector, with the selector rack between the clutch and the rotating cork rolls. Figure 5-4 shows a clutch for a final frame.

The operation of the clutch presses the rack against the rolls and the selector is driven upwards, until the clutch is released. At the moment of release a small pawl on the clutch engages the rack and maintains it in position.

Located between the selector rod and the multiple bank is a smaller solid rod, which does not move up or down but may be rotated. Attached to this rod are 5 flexible metal fingers called trip fingers. (Figure 5-5).

A trip magnet located at the top of the clutch, Figure 5-4, can rotate the trip rod and trip fingers through an arc of about 90 degrees. The selector is first driven upwards from 1 to 2 inches under control of the commutator and the common control equipment, then the trip magnet is operated and a trip finger will engage the tripping mechanism of a selected multiple brush in accordance with the height of the updrive. Only one brush will be so engaged and on the next upward movement of the selector this brush will be tripped and make contact on the terminals of one multiple bank only. (Figure 5-6)

A selector in making selections will first updrive for brush selection then the proper brush will be tripped and updrive for group selection will be made after which an idle trunk will be selected to the office desired.

The circuits which control the movement of panel type selectors are necessarily complicated, and their operation requires the making and breaking of a large number of connections. Some of these connections are established or broken by means of relays. To reduce the number of relays required, a special form of switch, called a "sequence" switch, has been developed which in general performs the functions of a group of relays (Figures 5-7 and Figure 5-8).

The sequence switch consists of discs of insulation on each side of which metal battens are riveted and these discs are mounted on a shaft up to 21 in number. The switch is rotated by the power driven disc shown at the left end. This switch makes it possible to utilize certain relays over and over again and breaks up what appears to be complicated circuits into a number of local circuits.

A typical panel type frame is shown in Figure 5-9.

Progress Of A Call

The train of selection in a panel office (Figure 5-10) does not, as in a step-by-step office, advance in synchronism with the dialing. When the receiver is removed a line finder selects the calling line at the line finder frame. The line finder circuit and calling line are then associated with a sender and the subscriber receives dial tone. The subscriber dials and a setting is registered in the sender. By means of a decoder that part of the sender setting which represents the office code directs the selection of a trunk to the proper office. This selection is made at the district frame or at an office frame reached through the district frame. On a call to a manual telephone, connection is made over a trunk to a call indicator "B" board position and the numerical or line number setting of the sender, controls the call indicator display. On a call to a dial telephone, connection is made over a trunk to an incoming frame at the terminating office. The sender setting which represents the "thousands" and "hundreds" digits controls the selection at the incoming frame of a path to a final frame containing the particular group of 500 terminals in which the called number appears. The "hundreds" setting again, along with the "tens" and "units" setting determine the selection of the called line terminal on the final frame. When this connection is made the called station is rung and the ringing signal comes back to the calling station.

Equipment Layout

Line Finder Frame - The line finder frame provides means for associating subscribers' lines with the central office equipment used in establishing connections. The frames are of two general types, the older known as a 300 point frame contains fifteen multiple banks and twenty sets of terminals per bank and the newer or 400 point frame contains ten multiple banks and forty sets of terminals per bank. In either type, capacity is provided for sixty selectors together with their associated mechanism. Subscribers' lines terminate on the multiple banks and have an appearance before a number of line finder selectors. These lines, together with all the selectors which have access to them, constitute a line finder group.

The multiple banks of a line finder group are split. This forms two subgroups designated "A" and "B". Terminals in subgroup "A" number from the bottom up while those in subgroup "B" number from the top down (Figure 5-11). The start circuit will start an idle line finder in the particular sub-group having the calling line on the bottom half of the bank. This arrangement reduces the hunting time and consequently increases the speed of connecting a subscriber's line to a sender.

In addition to using line finder groups of full frame capacity, different size groups can be made by using split frames. The size of the group is controlled by the holding time and the calling rate of the various classes of lines served.
Four distinctions in classes of service are made in grouping lines due to the varying circuit complications arising in the line finder district selector units. These groupings are classed as follows:

(1) Flat rate service only.

(2) Message rate individual and PBX. Flat rate service can also be assigned.

(3) Message rate, two-party, with equipment for registering on two-party lines. Flat rate and message rate individual lines can be assigned on these frames.

(4) Coin box, with equipment for controlling the disposition on the coin.

District Frame - The district frame provides outgoing routes for the originating call. At this frame appear trunks to other offices, to operator or desk positions, or to office frames containing trunks to other offices.

The district frame consists of five multiple banks of 100 terminals per bank and capacity for 60 selectors together with their associated mechanism. Each bank is made up of ten sets of terminals consisting of eight sets of 11 terminals each and two sets of six terminals each. The last terminal in each set is wired as an overflow terminal, leaving 90 terminals in each bank available for assignment as trunks or paths. In case more than ten trunks are required for a trunk group the overflow terminal is so wired that it will test busy instead of giving an overflow. In this way it is possible to establish trunk groups of more than 10 trunks and to arrange for a selector to hunt over the entire group for an idle trunk.

In case it is necessary to secure more outgoing trunk capacity, the district multiple may be either subgrouped or graded, or both. Additional trunks may also be obtained by adding an office multiple.

When subgrouping is employed the trunks in each subgroup are multipled through the banks of sufficient district frames to provide a load for that subgroup. The number of subgroups depends, of course, upon the size of the entire group as determined by the amount of traffic to be handled.

When grading is used the trunk multiple is divided into groups of individual and common trunks. A certain number of adjacent individual and common trunks are wired so that they may be readily converted into either type and are therefore known as convertible trunks. The individual trunks are subdivided into two or more subgroups and are assigned to different portions of the entire group. They are on the lower numbered terminals in the groups, thus being first choice on the frames on which they are assigned. The common trunks are multipled in front of all the district frames and are on the higher numbered terminals in the group. The convertible trunks are used either as individual or as common trunks, depending upon the size of the trunk group required. With this arrangement the size of a trunk group can be increased by cross-connections without disturbing the cabling arrangement on the individual frames.

Office Frame - Office frames provide additional outgoing trunk capacity when the requirements exceed the capacity of the district frame. The banks are similar to the district frames. Office frames are separated into a number of groups known as office multiples. Each office multiple contains out going trunks to a particular group of offices and consists of one or more office frames. In large exchanges it may be necessary to provide several office multiples with a number of office frames in each multiple.

Incoming Frame - The incoming frame is the terminating point for incoming trunks to make connection between and provides a path to the final frames. It also supplies the proper ringing current to ring the called station when the line had been selected on the final frame. This frame is similar to the district frame in that it contains five banks of 100 terminals per bank and capacity for 60 selectors. It differs, however, in that each bank is made up of four groups of terminals consisting of 24 trunk terminals and one overflow terminal. Thus, 500 terminals are arranged to provide 20 groups or choices, each of which has access to equipment serving 500 numbers, or a maximum capacity of 10,000 numbers or terminals for a full sized unit. Figure 5-12 shows the trunking between incoming and final frames and association of peg count and overflow registers. Due to circuit requirements separate groups of incoming selectors are provided for handling calls from dial, manual and toll offices.

When 24 trunks are insufficient to carry the traffic to any particular final choice, grading, similar to that employed on the district frame, is used.

Final Frame - The final frame is the last step in the connection of a call to a dial telephone. This frame is similar to the district and incoming frames in that it contains five banks of 100 terminals per bank and capacity for 60 selectors. However, instead of the multiple banks of trunk terminals, this frame has multiple banks of subscribers' terminals. On district, office and incoming frames the selector picks an idle terminal in a particular group while on the final frame, the selector picks a particular terminal corresponding to a subscriber's number, that is, it is of the particular-terminal selecting type rather than of the trunk hunting type. In the case of a PBX or multiline group the wiring is so arranged that the final selector selects the first idle trunk in the PBX group.

Normally, one final frame with capacity for 60 final selectors is sufficient to handle calls for a particular group of 500 numbers. However, more than one frame can be provided if conditions require more than 60 selectors. For example, if more than 60 and less than 90 final selectors are required, one final frame bridged with one-half of another constitutes a final choice. Similarly, two final frames can be bridged together, giving a maximum of 120 final selectors for a choice. Also, if less than 60 selectors are required, two-thirds or one-half of a frame can be utilized to make up a final choice.

Panel Link - The panel system utilizes a link as a means for associating a sender with a line finder-district selector unit.

The link is a double-ended type of selector and is mounted on a link frame which has capacity for 30 links. One end of the link has access to a sender selector bank with capacity for 100 sets of terminals and the other end has access to a district finder bank with capacity for 20 or 40 sets of line finder-district selector circuits. The circuits are so arranged that a district finder pre-selects the idle line finder which will be used on the next call. When the call is originated the line finder picks the subscriber's line and the sender selector connects it with an idle sender.

The link frames are designated "A" and "B" corresponding to the line finder subgroups. Normally subscribers in subgroup "A" will be served by line finders and links in subgroup "A" and subscribers in subgroup "B" will be served by line finders and links in subgroup "B". However, if all the line finders or links in either subgroup are busy and a call originates in that subgroup, the line will be served by a line finder and link in the other subgroup, if one is available.

Sender - Decoder - The sender controls the setting up of a connection through the office. It registers the pulses from the subscribers dial and controls the movements of equipment necessary for completing the connection. A sender, to complete a call, must function in conjunction with either a translator or a decoder to set up the necessary relays to pick a trunk group to the desired point. The holding time on decoders being extremely short, very few are required. A minimum of three and maximum of six will be sufficient for any size of office. The decoders are available to all senders, regardless of class of service.

The sender leaves the connection and is available for another call as soon as the called station or operator is rung or a paths busy or line busy signal is introduced.

Call Indicator - As in step-by-step areas, call indicators are required in panel areas to give dial subscribers access to manual offices. When a panel dial subscriber calls a manual subscriber, the dial equipment automatically selects an outgoing trunk to the manual office. This trunk terminates on a cord and plug at a call indicator position. The trunk is seized at the originating end and the sender sends PCI (panel call indicator) pulses to set up the proper relays to display the called number on a call indicator display plate. The call is completed by insertion of the plug of the trunk into the multiple jack of the called line.

Translator Type Panel System

The original panel system design had the following features which were found undesirable:

a. Ground on the cutoff relay, which under certain trouble conditions could create a fire hazard.

b. Line finder frames with 15 multiple banks of 20 terminals each for a total capacity of 300 lines. The 15 banks required 15 brushes on each selector rod, which was too much load for the friction rolls without spring compensation and much maintenance.

c. The sender selector was a 200 type rotary selector limited to 22 senders, which was too small a group for efficiency.

d. The senders had eight 200-type selectors used as registers, which required considerable maintenance to follow high speed dials.

e. The senders were designed for one class of service only. Coin service required a coin type sender which had to be selected a second time to collect or return coin.

f. The pulse machines were very troublesome, required considerable maintenance and the translator selectors and banks wore out rapidly. Its code limit was 200 codes.

Decoder Type Panel System

A redesign of the panel system eliminated the translator system troubles as follows:

a. Ground on the cutoff relay was replaced with battery on the cutoff relay.

b. The line finder frame was designed with 10 banks of 40 terminals each with a frame capacity of 400 lines and 10 multiple brushes per selector.

c. The panel link frame was designed to connect the line finder districts to senders by links, using a panel multiple to terminate the senders. This permitted 100 senders to a group, reducing the total senders required.

d. The 200 type selectors in the senders were replaced with all relay registers for storing of the dialed digits.

e. The senders were designed for 4 classes of service. The coin control features were removed from senders and made a part of the coin district circuit.

f. The pulse machines and translators were replaced by an all relay decoder with a capacity for 800 codes.

However, in spite of these improvements, the panel system has inherent limitations. Noise due to the multiplicity of moving contacts, and wear on multiple brushes, commutators, sequence switch brushes, etc. make the maintenance expense high. The trunk-by-trunk hunt for an idle circuit and the establishing of a connection from line finder to district to office to incoming to final was much too slow and required considerable equipment to maintain.

The addition of Automatic Message Accounting (AMA) to existing panel frames is not practical and direct distance dialing could be added but at a cost that made it questionable. Therefore, the panel system was finally abandoned in favor of the No. 1 Crossbar system.

Panel System "A" Switchboards

When the panel offices were installed a local "A" switchboard was provided for each building. The outstanding feature of these switchboards was a universal type cord circuit permitting its connection to either high- low or reverse battery type supervision. The sleeve of the outgoing trunk jack indicated the type of supervision desired. The various types of switchboards in use are briefly as follows:



Key Pulsing



Dial Key


8 panel - narrow keyshelf


Listening Key


8 panel -
Wide keyshelf


Listening Key

On Call Cord

9 panel -


Listening Key

On Both Cords

9 panel -
(lower units)

These switchboards are also being used for Decentralized Toll Operation, which consists of outward toll and TX service only.

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