A data Communication Historical Series
By Bob Pollard
Technical Control (T C or Tech. Control)
The term ‘Technical Control’ is a military designation for facilities (line, circuit, and channel) termination equipment, MODEM(s), jacks and patch panels, and all the necessary test and control equipment. Similar equipment is used in the private sector, and therefore, a discussion of one will describe the other. estern Union maintenance personnel would refer to a Technical Control facility as a ‘Wire and Repeater’ Center (room).
Typical 1940’s Western Union Telegraph test and switch board
Most military T C facilities are associated with a large message-switching center. In the private sector, such as Western union, the Wire and Repeater rooms (centers) are located in most large cities with interconnecting trunk lines between cities and the local circuits terminate into the closest Wire and Repeater room. The military uses ‘Crypto’ equipment for data security purposes and again the private sector utilizes similar equipment for scrambling and decoding the data bit stream.
A Tech Control facility would have the necessary equipment to perform the following functions:
· A cable connection (cross cut mainframe) facility for connecting outside local or distant lines (circuits) to the internal equipment interfaces.
· Jack Matrix for line and equipment testing and rerouting of failed lines and equipment.
· MODEM and other Device racks for mounting the required MODEM(s) and devices.
· Crypto interface facilities. The Crypto equipment would be located in a high security area.
Cable connection Mainframe:
The cable connection mainframe would normally be located in the general area of the T C. The mainframe is the point where all connections between the outside terminal equipment and lines from distant points are connected to the internal T C and center equipment. Each outside line enters the building via a cable from the Communications Company (Telephone / Vendor Company) and each individual line is terminated at the mainframe on an assigned terminal block connection(s). The T C has test and patching jack sets assigned to each line, and these are also terminated on a terminal block connection(s). Wire straps are then connected between the line terminal blocks and the appropriate jack sets in T C. MODEM(s) are handled in a similar manner. They are assigned jack sets in T C that are associated with particular pre-assigned lines. This is accomplished via straps on the terminal block(s). A secure cable facility is routed between T C and Crypto.
Technical Control facility:
The technical Control facility consists of many panels of jacks (plug-in jacks) that are individually associated with a line or piece of equipment. A line and associated MODEM would have jacks next to each other. The jacks are internally wired (connected) so the line and MODEM are normally connected to each other. This normal connection could be broken if a patch cord was inserted into the jack. The patch cord will break the original connection and route the circuit through the patch cord. The other end of the patch cord would be inserted into a substitute equipment/line jack. This allows for substitution of failed lines or equipment by patching a good line in place of a failed line or a failed MODEM could be replaced by a good one via a patch cord. Other associated jacks provide a means of testing lines and equipment without breaking the connection between the line and MODEM.
In addition to the jack matrices Tech Control has many switches, meters, patch cords and the necessary test equipment to trouble-shoot failed lines and equipment. Different patch cords are used that allow flexibility for testing or connecting equipment together. For example: a black cord connects a 2-wire circuit directly, jack tip to tip and jack ring to ring. A red cord reverses the connection, tip to ring and ring to tip. This reversal allows equipment to be connected back to back. In other words the DC side of a MODEM could be connected to the DC side of another MODEM. This connection reversal connects the send leg of one MODEM to the receive leg of another MODEM and in the reverse direction receive leg to the send leg. This referred to as a ‘bust back’ or ‘back to back’ connection.
This ‘back to back’ connection is done to test the line and/or MODEM(s) or extend a line. In a test mode, full duplex line, any data transmitted by the distant end is routed back to them because the received signal is patched to be sent back on the other MODEM, which is the distant end receive line. This type of patch could also be used to extend a line (cut through) to another office.
Jacks and patch cords:
Figure 1 illustrates how individual jacks are connected to the send and receive side of a Carrier device, a multiplex unit, a tuner, an amplifier or pad (power reducer) and the send and receive lines, which are telephone channels; two individual telephone channels, one for send (2 wires) and one for receiving (2 wires), are required. Since the Carrier, multiplex unit (modulator or demodulator) and frequency tuner are connected to telephone lines an amplifier is used on the Carrier receiving line because the power level is 17 dB lower than the normal Carrier operating power level. On the send side to the telephone line the power level would be reduced 17 dB. This power level change is necessary when the connection is to a telephone line. The tuner and modulator are necessary because the Carrier device is functioning on a sub channel, a small frequency portion of the total available telephone channel bandwidth (usable 3300-hertz).The sub band (individual) frequency is generated through the combined Carrier device modulator and tuner, which means the telephone channel, does not provide the analog signal (frequency or power.
The Carrier device is used for discussion since it requires many jacks to serve all the necessary functions and test facilities, which illustrates how complex a jack matrix can be for some devices. The MODEM device does not require the same number of jacks since the power amplifier, receive side, and pad (power reducer), send side, are not used. Also the modulator / demodulator and tuner sections are electrically combined in a different manner.
The following is a component description of Figure 1. The subject jacks would be located adjacent to each other within the total panel of jacks.
1. The frequency tuner, which is necessary for modulation or demodulation of the carrier frequency. 1A is part of the tuner.
2. The carrier frequency (oscillator) for the modulator or demodulator.
3. On the receive side, to the carrier device, this would be a power amplifier.
4. On the send side, from the carrier device, this would be a ‘pad’ (power reducer).
5. The jack connected to the carrier device for transmitting (send) to the telephone line.
6. The jack connected to the carrier device for receiving from the telephone line.
7. The jack connected to the frequency tuner on the transmitting (send) line and strapped to jack 5 to complete the circuit.
8. The jack connected to the frequency tuner (1A) on the receive line and is strapped to jack 6 (to receive carrier) to complete the circuit.
9. The jack connected to the frequency tuner on the telephone channel side and is strapped to jack 10 to complete the circuit.
10. Jack 10 is connected to the amplifier (3) on the receiving side of the line. On the send (transmitting) side of the line jack 10 is connected to a ‘pad’ (power reducer) (4). Jack 10 is strapped to jack 9 to complete the circuit.
11. Jack 11 is connected to the amplifier (3) on the receiving side or the pad (4) on the send side. Jack 11 is strapped to jack 12 to complete the circuit.
12. Jack 12 is connected to the telephone channel (line), one for sending and a second one for receiving since sending and receiving functions must be separated, which requires two channels (lines). Jack 12 is strapped to jack 11 to complete the circuit.
13. Jack 13 is a jack used for connecting test equipment; includes an isolation pad (coil).
The patch cords used to plug into these jacks use a ‘tip’ and ‘ring’ that makes contact with the two halves of the jack and breaks the internal jack strapped connections. The two halves are necessary since the circuit (line) uses two wires all the way from the carrier device to the telephone channel, which also uses two wires. There would be two wires for sending and two wires for receiving, making a total of four wires utilized from end to end for a full duplex circuit.
Patch cords are used for replacing faulty equipment, rerouting a line or for test purposes. If a patch cord was plugged into jack 5 (send) the connection would be broken between jack 5 and jack 7, which would disconnect the Carrier device from the tuner. The open end of the patch cord could then be plugged into different tuner (another jack 7), which would result in a replacement of the original tuner. The other side of the replacement tuner would have to be patched into the original line on the pad side, using the jacks represented by 9 (replacement tuner) and 10 the original ‘pad’ connection. The end result would be a patch cord between jack 5 (original Carrier) and a jack 7 (replacement tuner) and a patch cord between a jack 9 (replacement tuner) and jack 10 of the original ‘pad’, which is connected to the original telephone channel.
There are different types of patch cords, such as a three-wire patch cord that would be used in jacks that are connected to three circuits for various reasons. The patch cord and jack would use ‘tip’, ‘ring’ and ‘sleeve’ connections. The red patch cord (reversed) was covered earlier.
Figure 2 shows one end of a black patch cord; the other end would be duplicated.
Figure 3 shows a typical T C service and control console.
Figure 4 shows several racks of Jack Panels, with patch cords plugged into several jacks and hanging below the jacks.
A Technical Control can be a fairly simple arrangement of patch panels, racks of MODEM(s) and other equipment, a cabling mainframe, test equipment and a monitor and control console. Or a Tech. Control can be an automated very sophisticated facility.
In the 1980’s and into 2000 most large message switching centers had large T C facilities. But, due to the proliferation of the Internet, large message (data) switching centers were phased out during the 1990’s and early 2000’s since it was more economical to use the world wide communication facilities offered by the WWW / Internet. This eliminated many T C facilities on military bases and in the private sector. The Internet ‘Routers’ (super computers) would now be the strategically located message (data) routing systems. The Router facilities use a similar more modernized type of ‘automated’ testing, circuit rerouting and control facility.
When encryption devices are used messages (data) are encrypted during transmission before entering the DC side of the MODEM and are decrypted at the receiving end on the DC side of the MODEM. This encrypted and decrypted data is routed through the jack matrices of Tech. Control.
A typical Terminal to Terminal communication via a data/message Switching Center would occur and be connected as illustrated in Figure 5, a simple sequence; all connections are routed through the T C jack panel matrix. A review of Figure 19-6A and Figure 19-6B will provide a more complete picture of the actual path a data signal would take.
Terminal (send) > > > CRYPTO (encryption) > > > (DC) MODEM (AC) > > >Telephone line or leased line > > > (AC) MODEM (DC) > > > CRYPTO (decryption) > > > Communications switching Computer > > > CRYPTO (encryption) > > >(DC) MODEM (AC) > > > Telephone line or leased line > > > (AC) MODEM (DC) > > > CRYPTO (decryption) > > > Terminal (receive).
Figure 6 illustrates how a 1970’s through 1990’s automated Tech Control would be configured and the types of equipment that would be used.