A Data Communication Historical Series
By Bob Pollard
A brief History of Message Switching Centers:
In the beginning Switching Centers were simply central locations placed geographically where the closest users could send messages to the central location (center) and then the messages would be forwarded to the necessary distant location. These were all handled manually, hand delivered locally, and delivery (transmission) to a distant location involved retyping the message.
Centralized message processing:
The Postal Telegraph Company and Western Union Telegraph Company established a network of centralized, manual, Centers interconnecting most large cities for the purpose of providing a telegram service across the nation. All local or nearby terminals (small Cities) were terminated into the large City Switching Centers, when and where possible. In most instances, prior to 1910, it was necessary to manually retype and transmit a particular message received in one city in order to relay the message to another city. In other words a message received via a Teletypewriter (Teletype) required the message to be retyped on another Teletype that was connected to the appropriate destination or, up through the middle of the 1950s, the message in many instances was relayed utilizing the Morse code system.
Every Town and City, especially if a Post Office or Train Station existed, had a telegraph office. The small locations would accept a written telegram from an individual and then would transmit (relay) the message to a central location within their area, usually a medium to large City, or in some cases, directly to the addressed location. This could be via Morse code or a Teletype or even hand carried.
Medium to large Cities had local offices at various locations within the City. If the local office(s) were located on the ground floor of the Central office a message would be transferred via a pneumatic tube to the Teletype area. Messages to be locally delivered took the reverse route and were sent via the Morse code or a printing Teletype, or delivered by a messenger to the appropriate address within the City.
Between 1910 and 1920 the introduction and improvements in the paper tape perforator / reperforator and paper tape transmitter gradually eliminated the need to retype messages. Each line was terminated through a tape perforator and as the message was received each character (bits) would be punched in the paper tape. This tape could then be placed in a paper tape transmitter for the addressed destination for transmittal. An office in a medium to large city would have rows of Teletypes, paper tape perforators and transmitters operated by individuals who routed and sent messages (telegrams) and processed the local received messages.
In Western Union systems most of the final destination Teletypes printed the message on a narrow tape (1/4 to 5/16 wide) that was glued on the Telegram form for delivery in the local area. A Tape ‘gummer’ was used for this purpose. These were composed of a bottle filled with water about 6 inches long by 2 inches in diameter with a device on the end that allowed the tape to be fed through over a felt surface saturated with water. At the end was a sharp edge for tearing off the tape in order to make it fit on the telegram form and terminate the message at the end. All of this activity required a great deal of manual labor.
Prior to 1910, private companies were also experiencing message relay problem where the message had to be retyped for forwarding to another destination. For example: Most connections were on a point-to-point basis. A company probably would have a connection from its New York office to the Philadelphia office, then to their Pittsburgh office and maybe to Chicago. A message sent from New York to Chicago would be routed through Philadelphia and Pittsburgh in order to reach the Chicago office. The message would be manually handled (retyped) at Philadelphia and Pittsburgh in order to relay it to Chicago. This retyping of the message was not very efficient and many errors occurred.
If a Company connected every City to each other so they could communicate directly it would involve many lines (circuits) and would be very costly. Also if two terminals were communicating with each other they could present a busy signal, depending on the type of equipment, to all other terminals. For example, a group of terminals with direct connections between each terminal could appear as shown in Figure 1-1.
Manual Message Switching Centers:
The message Switching Centers provided the capability to eliminate the point to point connections and also performed a message store and forward function. Most importantly Switching Centers helped reduce the high cost of leasing or constructing the necessary lines for connecting all the terminals over great distances. Multipoint terminal mechanical-electrical selection features were also developed so terminals could be individually selected for either sending or receiving messages.
Paper Tape punch and Transmitter-Distributor:
One version of the first and early paper tape motor driven transmitter-distributor used sensing fingers riding on top of the tape. These sensing fingers would drop into the punched holes as the tape passed under them. This in turn closed electrical contacts, which were in series with the sending distributor. The line of code holes was not exactly perpendicular to the centerline of the tape, which allowed sequential reading for serial transmission to the line.
The Distributor, line-sending part of the transmitter, was a brush and commutation (commutator) unit, with the segments consisting of round button contacts. The Receiving Distributor was on a parallel shaft, with small round buttons, which detected the received code pulses near the center of the button. Small stationary pigtail brushes (brass) made contact with the buttons as they rotated at a pre-determined speed. A range- finder (adjustment) system was used to rotate the faceplate back and forth in order to maintain the best contact point for operation.
The later Paper tape transmitter-distributor using the five bit code (Baudot / variations) used parallel pins that rose through the punched holes from underneath, closing contacts in the process. The five bit character holes were punched across (column) the tape, which allowed each column to represent a character. Tape driver (feed) holes were impressed in the tape by a star wheel type mechanism with sharp points at the same time the character holes were punched by the perforator. The Teletype perforator could also print a representation of the punched characters so the person handling the tape could easily read the information punched in the tape. Otherwise the operator had to decipher the punched holes.
The perforator was the message-receiving device and the transmitter was the sending device. The transmitter could be associated with or a part of the tape perforator device or a stand-alone unit. Normally when the perforator and transmitter were installed in a common console they represented the send and receive side of the connected office. The perforated (punched) tape allowed the person forwarding the message to tear off the received punched tape from the perforator and place it in the appropriate transmitter for forwarding to the next destination. This led to the central switching system concept for a network of offices (terminals), along with trunk lines between central switching centers. Large companies and the military services established centralized message switching centers that accepted input messages from groups of connected offices, and then in turn forwarded the messages to the proper connected destination. This destination could be another distant switching center or a local connected terminal (office).
The term Transmitter-Distributor indicates the transmitter reads the character bits in parallel from the punched (perforated) tape, but sends the character bits in series to the line (circuit/facility). The distributor part of the transmitter performs this parallel to serial conversion function. Refer to the illustrations under the Baudot code transmission functions in the ‘Printing Telegraphy and Teletype’ or in the ‘Early Transmitter / Distributor / Multiplex devices’ section. Maintenance personnel called the Transmitter-Distributor unit a ‘TD’ or Transmitter.
Figure 1-2 illustrates how a switching center helped solve the high cost problem of connecting all the terminals together, which was shown in Figure 1-1 and eliminates many of the message relay problems. Also since messages would be received at the center and then forwarded to the designated destination none of the terminals would appear busy to other terminals. Of course direct conversations would not be possible with a store and forward message system. Each additional distant switching center would be connected by trunk lines between the centers. This provided direct communication between centers for forwarding messages.
Each message switching center(s) had banks (rows) of perforators and transmitters for receiving and sending messages. The received punched tape was torn off the perforator and hand carried to the appropriate transmitter. In some cases due to a backlog or problems the messages would be held for future delivery (transmission). Many times the messages (paper tape) were hung on a flexible accordion wire tape holder in the ‘last-in’ position and would then be placed in the transmitter in the order of receipt, thus assuring ‘First-In’, ‘First-Out’ for the assigned destination. This operation was referred to as a ‘torn tape’ message-switching center and could be considered the first ‘store and forward’ message-handling center.
Following the invention of the Teletypewriter, Tape Perforator, Paper tape Transmitter and, later, the establishment of message switching centers it became apparent that working with DC voltage was simply not practical. DC voltage was not suitable for long distances due to the necessity to continually upgrade the signal and restore the voltage level due to the voltage quickly dropping in value along the line. Also it was difficult to allow more than one terminal to transmit simultaneously on a shared line. The early electrical-mechanical ‘Multiplex’ unit allowed shared transmission, but the speed of transmission for each terminal could be reduced based on the transmission speed of the common line and equipment reliability. This possible terminal speed reduction occurred because the characters from two or ‘more’ terminals were interspersed on the common shared line. A character from Terminal ‘A’ would be sent then a character from terminal ‘B’, then ‘A’, then ‘B’ until the messages were completely transmitted. This electrical-mechanical shared line multiplexing was still around in the 50’s, but was slightly more sophisticated and was used by Western Union who used the term ‘Veriplex’ to identify these devices. The modern electronic multiplex devices were not available during this period.
A resolution to this DC voltage problem became available when technological developments provided the capability to utilize Telephone type facilities (circuits/lines) for data (message) transmission. The ‘Carrier’ modulating device for transmission over AC (analog) lines was developed and implemented. Use of these Carrier devices allowed many terminals to share one line without interfering with each other. The ‘Carrier’ was the first MODEM type unit to be placed in service since it converted a DC signal to an AC signal and then back to a DC signal. Information concerning the Carrier system is provided in the Carrier section.
Semi-automatic Message (data) Switching Systems:
Western Union (W U) systems are primarily used for discussion because W U was one of the major companies involved in data communications/message switching systems. In the early days, prior to the breakup of AT&T, government regulations separated the companies providing voice and data communications. A T& T had most of the voice systems and Western Union had most of the data (message) communications systems.
An exception to these Government regulations was AT & T’s ownership of the TWX message switching system, established in 1931. Western Union owned a similar type system in the Telex message switching system, established 1958. Both TWX and Telex was dial up systems using the same nomenclature: ‘Teletypewriter Exchange’ and they both used a modified ‘Baudot’ code set. Users leased the Teletype equipment, and in addition, were basically billed (charged) based on the dial up connection time. All connections were accomplished through a dial up method much in the same manner as dialing on the telephone. All dialed connections resulted in a direct mechanical connection between two terminals. This method of utilizing a shared switching system for connecting between two terminals provided a low cost message-switching environment for businesses. TWX was eventually sold, in 1966, to Western Union because of Government regulations and was modified to use the ASCII code set.
Plan 2A/2B through Plan 20 – semi-automatic systems:
These Message Switching Systems were replacements for the ‘torn tape’ message handling process and were implemented in the late 1930’s. These store and forward message switching centers were more automated and eliminated the necessity for tearing the punched tape off the perforator. These systems were developed to process Western Union Telegrams, but were also made available to other business and the government. The Plan 2A/2B through 20 allowed the operators to accomplish a cross-office connection through the use of a 9-conductor patch cord. This provided an electrical path so the received perforated tape, which was fed to a transmitter, could to be sent directly to the sending side re-perforator and that punched tape was fed to an associated sending transmitter. This 9-conductor cord allowed the character bits to be transmitted cross-office in parallel and were used in conjunction with a built in patch panel. This eliminated the ‘torn tape’ requirement. The operator would check the routing indicator(s) (address) on a message and patch the receive position to the proper send position reperforator. The receiving perforators punched tape fed into a tape storage bin and then to a cross-office (center) transmitter. When a cross-office connection was established, via the patch cord, the transmitter was started and the message was sent to the sending position reperforator. The character bits were transmitted in parallel during the cross-office transfer. The send side reperforator tape fed into a tape storage bin and then to the send transmitter. The term ‘reperforator’ was used because the received message, via the receive perforator (punched tape), was then transmitted to a cross-office reperforator (re-punched) for delivery. The term ‘ Message Switching Re-perforator Center’ or ‘ Reperforator Center’ was used by many to identify these switching centers.
Plan 21 through Plan 51 (Reperforator Centers):
These Message Switching systems used more sophisticated methods for routing received messages cross-office for transmission. This was accomplished through the use of electrical-mechanical rotary switches, relays and push button switches. This allowed received messages to be routed cross-office by pushing a button switch. The received message fed from the perforator to the cross-office transmitter, and upon receiving a complete message, the operator would push the appropriate cross-office connection switch (button). After a connection was established the transmitter would start and transmit the message to the selected cross-office send side reperforator.
The differences between the Plan 21 and future systems up to the Plan 51 would be found in the electrical-mechanical operational design concepts. The Plan 21 system used noisy rotary switches along with multiple contact relays, where the later systems did away with the rotary switches and utilized all electrical-mechanical, multiple contact, relays and other electrical components. No modern day electronics, such as, transistors and other solid state components were used since they were not available at this time.
Plan 51 Switching Centers:
The Plan 51 was a semi-automatic punched paper tape message switching center, with patch cord facilities, push-button panels and relay switching matrices. The Plan 51 system was used by Western Union, various military bases and was also used in some civilian applications.
All messages received (input) at the center were via a paper tape printer-perforator which created a printed and punched paper tape based on the message content; the printer-perforator punched and printed on ‘chadless’ tape. Chadless tape implied that the holes punched in the tape did not result in the hole remains (Chad) being scattered all over the equipment or floor. When the holes were punched in the tape only three quarters of the hole was punched which resulted in the Chad being retained or connected to the tape by a small section of Chad. This allowed the transmitter mechanical pins to penetrate the holes in the tape and read the tape because the Chad was pivoted up, but still connected to the paper tape. The Baudot (modified) code was used and the character bits representation was punched across (column) the tape, which required a tape width that would accommodate five bits.
The paper tape from the perforator was fed from a roll of paper tape contained on a reel. After the tape was printed and punched it fed directly into (down) a paper tape chute into a temporary storage bin, then fed under a tape tension unit and across a paper tape transmitter, and then onto a tape storage reel immediately to the left of the transmitter. The transmitter would feed any idle characters between messages automatically and then stop upon detection of the Start of Message/Message address indicators. A white ‘beehive’ Message Waiting Lamp above the transmitter would illuminate to indicate when a message is present for routing, which alerted the operator. In addition a light illuminated the transmitter area allowing the operator to read the routing indicator(s) (address). If the message contained a single destination routing indicator, the appropriate push button would be depressed, which would cause the message to be routed to the proper cross-office (center) destination. A multiple addressed message required a different operator procedure. This cross-office connection was accomplished through the associated electrical-mechanical switching matrices. Once the electrical relay connection was accomplished the transmitter would begin sending the message to the addressed cross-office re-perforator associated with the addressed destination. This punched tape was fed into a tape storage bin and then to the sending (output) transmitter. As the tape fed (stepped) through the transmitter (receiving and sending side) it was routed to a paper tape take-up reel. The name ‘Reperforator Message Switching Center’ was derived, from the function of re-perforation (duplication) of messages transmitted from the receiving position to the sending position (cross-office).
Other procedures and switching options were available to the Operator for the handling of multiple address indicators and other special requirements.
Each connected tributary required an assigned receive (input) perforator and a send (output) transmitter position. A tributary could have both send and receive capability (full duplex), but may only require a send or receive service.
The Baudot (modified) code set was used for all messages. Since the Baudot code did not have Start of Message (SOM) or End of Message (EOM) symbols it was necessary to use a sequence of characters for those purposes. Common usage was “ZCZC” for the SOM and “NNNN” for the EOM. When a complete message was received (at the center), EOM present, the message could be routed to its destination, based on the routing indicator(s). Each message would include, as a minimum, a SOM, Address field (indicators) and an EOM.
Example of the switching equipment used for the Semi-Automatic message switching systems