gec.gif (1164 bytes)GEC  MX-1

G.E.C. MX-1
Military Communications System

The MX-1 Military Communications System is an advanced telecommunications switching system employing stored program control of digital switching. The system has been designed to CCITT standards and is compatible with existing public exchanges, private branch exchanges and conventional telephone instruments.

The system provides digital interfaces at both the trunk and subscriber level. This allows the formation of totally digital networks supporting digital data interfaces and digital telephones. Interfaces to conventional telephones and analogue networks are also available.

The wide variety of MX-1 interfaces allows the connection of many other devices such as data terminals, radios, consoles and recorders. The flexibility of the software
control allows easy adaptation for more specialised military terminals.

The total design of MX-1 provides an extensive range of features and services for the military user. The stored program control of each switch and the use of common channel inter-processor network signalling allows the formation of a distributed secure military network. Each switch is capable of local and tandem switching. Network architecture is made flexible through the use of automatic route selection.

MX-1 also provides all the features of modern Private Automatic Branch Exchanges and Key Telephone Systems, and the efficiency of Centralised Telephone Operator and Central Network Management.

System Description

The MX-1 system consists of two functional areas, the Common Equipment and Peripheral Equipment.

The Common Equipment comprises the central processor and the Network Circuits that provide the digital switching. The Peripheral Equipment is connected to the Common Equipment by 2 Mbit/s multiplex loops which carry the digital control and voice messages between the Network and Peripheral Equipment.

The central processor is responsible for the stored program control and consists of the Central Processor Unit (CPU) and its memory. These units are duplicated to provide high reliability. The memory holds the operating programs and the configuration data relating to a particular exchange.

A cartridge-type magnetic tape unit is supplied for high speed loading of the operating programs and data, and to provide non-volatile memory storage. All data entered into the system memory is duplicated so that, in case of power or control failure, the complete system software program and data are available for automatic reloading.

Instructions and data to the system, such as feature assignments and maintenance instructions are usually entered from a standard Teletypewriter (TTY) unit or Visual Display Unit (VDU). Output messages are printed on the TTY or a printer. The data terminal may be located with the exchange or at a remote location.

The system provides multiple (CCITT V24) TTY interfaces each of which may be defined individually. This allows different functions (e.g., traffic information, call detail recording and maintenance data) to be dealt with by different terminals and at different locations if required. System security is maintained by various levels of password access.

The Network Circuits contain the MX-1 digital switch. These are connected to all other Network Circuits by 2.048 Mbits/s digital loops multiplexed into thirty-two 64-kbit/s channels.

Up to four shelves of Peripheral Equipment may be connected to a single multiplex loop. This allows for 40 peripheral circuit pack positions per loop or 160 subscribers, 80 trunks or an equivalent combination. The traffic capacity of each loop is governed by the Grade of Service required.

Not all multiplex loops are used to serve peripheral shelves. Services such as conferencing, and tone generation are also provided via multiplex loops.
One conference circuit can control up to 30 conferees e.g. ten different simultaneous 3-way conferences or five 6-way conferences. The maximum size of a conference is limited by the speech transmission plan.

The Peripheral Equipment consists of subscriber line and analogue trunk circuits. With the exception of the interface to the digital telephone and data terminal adaptor, each circuit converts analogue speech or other speech band signals (e.g. modems, facsimile machines) into digital signals for digital switching. The conversion is made by a single encoder/decoder (CODEC) per line or trunk into 8-bit CCITT A-law pulse code modulation (PCM).

2-wire and 4-wire analogue trunk circuits are provided which cater for a wide range of signalling systems.

The Digital Trunk Network Circuits provide a direct connection to a CCITT 2.048 Mbit/s trunk circuit and incorporate common channel signalling. This advanced inter-processor signalling technique ensures that telephony features and access security are maintained at a network level.
2.048 Mbit/s interfaces are also supplied for MX-1 Remote Peripheral Equipment where a group of subscribers is located at a distance from the exchange equipment.


The capacity of the MX-1 system can be defined in terms of numbers of subscribers, numbers of trunks, traffic carrying capability, or more usually combinations of these parameters.
Each MX-1 system may have up to 80 Network Circuits. The traffic carrying capacity of each Network Circuit is a function of the Grade of Service (COS) required and is typically of the order of 16 Erlangs. In large military networks the overall COS must be apportioned depending on the network architecture and routing, and the traffic per Network Circuit is adjusted accordingly.

There are two basic types of MX-1 equipment cabinet: Common Equipment and Peripheral Equipment Cabinets. A modular approach has been used to ensure that MX-1 is economical from 100 line systems to systems with thousands of lines. Network Circuit and Peripheral Circuit shelves allow any type of module to be placed in any position on their respective shelves. Systems may therefore be tailored exactly to each user's requirement. Systems may also easily be expanded in service, usually by adding further Peripheral Cabinets. The software control ensures that such extensions cause minimum disruption to service.

Not all Network Circuits are used to carry traffic. Usually one in eight Network Circuits is used to provide tone and conference Services. On purely tandem switching exchanges with digital trunks no Service Circuits or Peripheral Equipment are required.


A line circuit is provided to interface with a standard single-line rotary dial telephone. This circuit can also be used with standard DTMF push-button telephones (such as the GEC Keyphone 91). Standard line feed and ringing voltage is provided so that any conventional apparatus such as telephone answering sets, dictation equipment, modems, facsimile machines and loop extenders can be used with the MX-1 system if desired.

The MX-1 telephone is an electronic unit designed specifically for the MX-1 system to provide single line and multi line service as well as to improve significantly custom calling feature operation.

The basic set has a 3 x 4 push-button digit pad and 13 non locking keys. A speaker is provided beneath the handset for tone ringing, on-hook dialling, and direct voice calling.
Most of the feature keys have an associated Light Emitting Diode (LED) indicator to show the calling line or confirm the operation of a feature.
Add-on key/lamp modules are available to expand the capacity of the basic set as more features are required. Ten- and twenty-key modules can be added to provide up to 60 additional keys. A loudspeaking (handsfree) module may also be added on the left hand side of the set. These modules may easily be added in service.

The basic set is also available with a 16-character LED display. This is used to show calling line identity, dialled digits, the contents of repertory dialling stores, time and date etc. The display is particularly useful for central enquiry and automatic call distribution features.

The feature assignment of the 10-button key modules is flexible and may be individually tailored to each user's requirements. Feature operation is very simple since it only requires the operation of an assigned key to cause immediate action. This naturally leads to fast and efficient communication compared with a traditional telephone where features are controlled by dialled codes.

The complete feature telephone is connected to the exchange by two-pair wiring, which also carries the power for the basic set. The power for the add-on feature modules may be from a local AC transformer or on a third pair from the exchange. The latter would take benefit from any standby power arrangement for the exchange.

MX-1 feature phone
From left to right - Loudspeaking Module, Basic Set and 20 lamp/key Module

The MX-1 Digital Feature Telephone is connected to the system by a single pair using 'burst-mode' transmission. The digital transmission provides a 64 kbit/s channel connected to a CODEC in the telephone for speech transmission and an 8 kbit/s channel for signalling. An additional 8 kbit/s channel is also available for low speed data applications.

The use of 'burst-mode' transmission facilitates end-to-end 'four wire' digital transmission. This leads to improved speech transmission that is uniform between all subscribers on a network. The use of digital transmission greatly improves crosstalk performance and allows the use of digital encryption units to achieve a secure military network.

The MX-1 digital telephone provides all the features of the MX-1 feature telephone. In addition the 8 kbit/s signalling channel offers the capability to provide alphanumeric displays and telemetry functions such as intruder alarms and smoke detector interfaces.

The MX-1 operator's console has been designed with all the features necessary for fast and efficient call handling. Consoles may be local to each exchange or grouped on a network basis to provide a central telephone operator service. A combination of the two is often most convenient.

The console uses switched loop operation; six loops are provided. Incoming calls are presented in order of arrival or can be manually selected from any of the incoming call types displayed on the console.

The console is equipped with five 10-button non-locking key strips for call control and feature activation. LEDs are associated with keys that require visual indicators. A sixteen digit LED display is also provided.

An expansion module can be added to expand the console features and a busy line indicator module may also be fitted. The complete console is connected to the MX-1 system by a six-pair cable, which also includes the power feed.

Operators Console with added 10 key Module


MX-1 provides a comprehensive range of data switching features. Traditionally data transmission has been effected by the use of modems over analogue lines. To achieve an acceptable error performance high speed modems (e.g. 9.6 kbauds) incorporate advanced modulation techniques and adaptive equalisers and are expensive.

The MX-1 data terminal adaptor (DTA) provides a simple interface between the digital data and the digital switching of the MX-1. Utilising the same line card and transmission as the digital feature telephone a duplex data connection of up to 64 kbauds may be established.  The DTA is shown to the right.

The switched path between two DTAs is code transparent which allows the use of any data protocol, including X25 packet data, to be transported. The DTA provides the standard CCITT V24, X20 and X21 interfaces and caters for all common data speeds (e.g. 200, 600,1200, 2400, 4800, 9600, 19200 bauds).
Data terminals are assigned their own directory numbers which may be dialled from the key pad on the DTA. Call progress is shown by LED indicators. Tone ringing is used to indicate the arrival of a call, which may be connected by pressing the answer key. An auto-answer feature is provided for use with computer ports.

The DTA is compatible with the range of add-on modules used by the MX-1 feature telephones and similar features and direct calling keys may be assigned. The ring back when free feature is often useful when trying to gain access to a well used group of computer ports or a word processor.
The X20 and X21 interfaces include a protocol that allows call selection, progress and calling line identification to be handled by the data terminal equipment.
Advanced military data networks nowadays take advantage of packet switching techniques. DTA interfaces may be used to provide circuit switched connections to a Packet Assembler/Disassembler (PAD). DTA interfaces may also be connected to the packet switches themselves; permanently held calls may then be established to provide 64 kbit/s data bearers between the packet switches at each location of a network. The data bearers may be set up by the packet switch, using X21 protocol or automatically by MX-1 under the control of system management.

MX-1 therefore provides a truly integrated voice and data network. The DTA avoids the use of modems and offers high data speeds of up to 64 kbauds at a low error rate. All the major system features used for voice calls are available to data calls leading to fast, efficient and secure data communication.
In networks that are not completely digital MX-1 allows the use of a 'modem pool'. On data calls over analogue lines, a modem can be inserted from the pool automatically without special action by the user.


The flexibility of the MX-1 system stems from the fact that the central control is a computer. The system features and services are defined by a software program rather than circuit wiring. Features can be added, deleted or modified by changing the software program rather than hardware.

The software program, which defines the system operation along with customer related information and maintenance diagnostics, is held on a magnetic tape cartridge and in the system random access memory. The customer data which defines the number and types of terminals, network capacity, trunks, classes of service, etc. is unique to each installation and is arranged in data blocks. The program and data are loaded into the MX-1 system memory via the magnetic tape unit. The cartridge tape is kept in the magnetic tape unit as a non-volatile data source.

Changes in customer data are made with a standard teletypewriter (TTY) or visual display unit (VDU) and printer. All changes made in memory are duplicated on the cartridge tape. In case of power or system failure, the program and complete data are automatically reloaded in about two minutes from the cartridge tape.
The MX-1 system provides its own record of traffic in terms of system and feature usage. These are printed out at defined intervals on the teletypewriter or VDU. A Call Detail Recording program is also available that records more detailed information of individual calls.

Changes in the customer data, as well as the monitoring of the system operation and traffic measurement can be made remotely. This allows Network Management Centres to have overall responsibility for the operation of a group of exchanges. The Network Management Centre personnel would also monitor the performance of the network as a whole in terms of its trunk routes and other resources such as packet switches.


The MX-1 system has been designed for unattended operation and no preventative maintenance is required. Corrective maintenance consists of fault detection, isolation and replacement of the faulty plug-in module. Fault detection equipment and programs are built in and allow self checking by the system. If a fault is detected, an appropriate alarm is generated which is accompanied by a diagnostic message on a printer. Most equipment circuit pack faults will also be indicated by an LED situated on the front of each module.

Using system indicators and diagnostic routines contained on the magnetic tape cartridge, most faults can be quickly traced and isolated. These routines may be called up automatically by the CPU, or the CPU can be instructed to execute required diagnostic programs from the local teletypewriter or remotely from the Network Management Centre. Repair consists of replacing the defective equipment, followed by tests to ensure the equipment is operating satisfactorily.

Operating personnel require no special electronics training to install and maintain MX-1. The inherent reliability of MX-1 and the speed of fault detection and repair ensures a high system availability.


GEC Telecommunications provides a comprehensive support for MX-1 in terms of technical publications, training, spares, hardware module repair and software maintenance.
Each MX-1 system is supplied with its own set of handbooks detailing installation, operation and maintenance. The GEC training school currently operates training courses for installation and commissioning, and system administration. Documented courses may be supplied.

Economic quantities of spare modules and equipment may be held at each site and at central stores locations. Hardware repair of faulty modules may be undertaken by GEC or on Automatic Test Equipment supplied to the maintenance authority. Facilities for producing magnetic tape cartridges and customer configuration data are also available.


MX-1's modular design makes for ease of installation and facilitates further expansion of existing MX-1 systems. The common equipment is usually located adjacent to the peripheral equipment; however by means of a PCM link the peripheral equipment may be sited up to forty miles away. Individual extensions are served from the peripheral equipment via the building distribution wiring.

The equipment cabinets may be arranged back to back or side by side. They may be placed against a wall or left free standing. The actual layout for a site will depend on the requirements of the installation design authority. Normally the common equipment cabinet is placed in the middle of the cabinet arrangement to minimise interconnecting network cable lengths. Expansion of the system beyond the initial cabinets is by the alternate provision of peripheral cabinets with and without power.

Extensions and trunks are connected to the individual circuits on the peripheral equipment shelves via the MDF using standard 25-pair distribution cables and connectors. Four cables are required to connect each peripheral shelf to the MDF. Each network loop is connected between the common and peripheral equipment by using an 18-pair distribution cable.

Each cabinet with a power shelf requires a separate single-phase three-wire, commercial power supply. Each power source is wired and fused independently and capable of delivering 20 amperes.

A standby battery ensures that communications are maintained in the event of mains failure. The battery may be charged by the rectifiers in the MX-1 system.


Exchange and System Features
Alarm Indication
Automatic Route Selection Automatic Daily Routines Busy Line Display
Call Detail Recording
Call Restriction Services
Central Administration and Maintenance
Central Operator Service
Closed User Groups
Covert Monitoring
Digital Data Transmission with Code Transparency Digital Trunks and Common Channel Signalling Digitone Calling
Dual Central Control
Emergency Services
Emergency Transfer of Calls Emergency Call Treatment Emergency Extension
Non-Busy Number
Flexible Numbering Plan
Multiple Console Operation Night Service
Direct Extension
Night Answer Telephones
Night Bells (Trunk Answer from any Station) Operator Service Panel
Public Exchange Compatibility Precedence and Pre-emption Remote Administration
Remote Peripheral Equipment
Satellite Working
Standby Power
Standby Ringing
Tandem Switching
Tie Lines (Inter PBX Lines)
Trans-network Services and Facilities

Operator Console Features
Automatic Timed Recall Call Splitting
Control of Trunk Access (Operator Assured Exchange Line Access)
Display Time and Date
Incoming Call Identification
Interposition Calling Intrude
Operator Call Waiting
Position Busy
Public Operator Recall Route
Busy Identification Secrecy
Selective Answering
Time and Date Display

Subscriber Features
Call Transfer
Call Forward Busy
Call Forward on No Reply
Call Waiting
Dedicated Calling Extension
Departmental Hunt
Direct Dialling Inwards
Direct Dialling Outwards
Do Not Disturb
Extension to Extension Call
Extension Group Hunting
Intrusion Inhibition
Manual Extension
Mixed DTMF, Dial, and Digital Telephones Operator Call In
Long Line Extension
Distinctive Ringing

Subscriber Facilities (using access codes or feature keys)
Access to Paging
Access to Centralised Dictation Access to Recorded Announcement Authorisation Codes
Call Pick Up
Call Forward
Charge Account Codes Executive Intrusion Follow Me
Refer Back
Ring Back - When Free
Short Code (Abbreviated) Dialling (Common Lists and Individual Lists) Short Code Control
Save (Repeat Number Dialled)

Additional Facilities Available With MX-1 Sets
Automatic Call Distribution
Autodial (user programmed)
Display (Including Incoming Call Identification)
Display Time and Date
Handsfree Operation (Using Loudspeaker Attachment) Headset Operation
Immediate Access
Multiple Appearance Directory Numbers
On Hook Dialling
Push-button Access to Facilities
Privacy Release Release
Private Line (Hot line) also available from GEC 746 telephones
Visual Indication of Call and Feature Status
Voice Calling and Buzzing
Volume Control

Taken from G.E.C. MX-1 Military Communications System - Technical Appreciation Catalogue (1981)


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Last revised: October 08, 2010