Mobile Exchanges
by John Chenery

MNDX No.341

Over the years, a variety of mobile exchanges were built to provide extra exchange capacity where it was most needed and to give emergency cover in the event of a disaster. Typical equipments fitted ranged from special UAX variants, Non-Director to TXE2 as well as containerised Digital and customer switch replacement modules.

[Pictured is  MNDX No.341]

| EXIT to Inland Switching |

The very first Mobile Automatic eXchange (MAX) ...

The very first Mobile Automatic eXchange (MAX) was brought into service in 1938 by the Postmaster General, Major G.C. Tryon, M.P. who made the inaugural call from Post Office Headquarters to the chairman of Essex County Council in Chelmsford. After the ceremony, the exchange was then put to work in North Weald, Essex where it replaced a small manual exchange.

Choose a decade to study:    

1960s

1970s

1980s

1990s

 2000s

The MAX

The early MAXs housed No. 12 type equipment with provision for 100 lines-90 subs. and 10 junctions. The mobile had many special features:-

Trailer Design

Once on site, the pneumatic tyred wheels would usually be replaced by heavy cast steel feet, bolted directly onto the wheel axles.

Exchange Design

MAX12s were often used to provide service during the conversion from manual working while the permanent auto exchange was being constructed. Typical setting-up time including diversion of local cabling was 2 weeks. 

MAX13s were the mobile version of the UAX13 and needed 2 trailers (?)

The Sixties
Whereas, MAXs were used predominantly for rural areas and to aid the conversion from manual to auto working, the demand for telephone service was increasing and a larger capacity expedient was needed. Two new style mobiles were designed; the MNDX  (a subscribers' unit) and the MTX (a tandem unit).  In the early Sixties, an initial distribution of 30 units was made to the Regions with more on order.

The New Mobiles

Mobile Non-Director Exchanges (MNDXs) and Mobile Tandem Exchanges (MTXs).

Key features of the new mobiles were:

The first Mobile Non-Director Exchange (MNDX) called Crompton was brought into use in February 1963 as relief to Shaw auto exchange in Lancashire.

MNDX1 was produced in 1962 by the Post Office Factory at Enfield. A single MNDX could provide service for 400 subscribers. 

Later in 1963, relief to Woking manual exchange was provided by Mayford which used two of the MNDX subscriber units and one Mobile Tandem Exchange (MTX) unit to provide 800 additional lines.

MTX's utilised some of the equipment within the MNDX, so could not be used in isolation. In the late Sixties, MTXs were produced at the Post Office Factory at Birmingham.

Later mobiles had smaller pneumatic tyres... Later models of the new mobiles had smaller pneumatic or solid tyres, suitable only for transportation by low-loader.

The Seventies
In the early Seventies the waiting lists for telephone service were growing due to the shortage of switching equipment in some areas. To improve provision, the Post Office placed an order (worth £3 million) with Plessey Telecommunications for 30-40 new mobile exchanges. The TXE2 electronic exchange was chosen to bring relief both to small towns and as a temporary replacement at exchanges undergoing refurbishment.

In 1972 the Post Office had approximately 250 conventional (Strowger) mobiles and 350 additional ones were on order, including 25 MXE2's of 1000 line capacity and 10 of 2000 lines.

The MXE2

The first mobile TXE2 designated MXE2, was operational in 1973 at Padgate, Lancashire to cater for growth in Warrington new town. Many MXE2s were equipped at Plessey's factory in Beeston, Nottingham. 

Key features were:

The first 2000 line MXE2, of contained within three trailers (2 line switching and 1 control trailer) was commissioned at High Wycombe in March 1974. A reduced battery standby time of 5 hours was possible.

Typical set-up time of the MXE2 on-site was approximately 6 weeks.

Logos & Serials

POST  OFFICE
ENGINEERING DEPT.
TELEPHONE MANAGER

GPO

POST  OFFICE
TELEPHONES
TELEPHONE MANAGER

Mobiles were finished in dark green and allocated sequential unit numbers in white lettering together with 'POST OFFICE TELEPHONES' or 'POST OFFICE ENGINEERING DEPT.' as well as, the superfluous 'TELEPHONE MANAGER' as many mobiles were used nationally and not assigned to particular Areas. The GPO logo complete with crown was also used for some time after the Post Office ceased to be a government department.

MNDX1 carried trailer serial number  T11892

Telecoms trailer serial numbers T205000-T207418 were used between 1965 and 1971. Thereafter, the 9 digit telecom vehicle identification numbers were used. Thus for MNDX No. 331 serial 71 830 0030 the details are as follows:

71

830

0030
Year of Delivery Mobile telephone exchange Unit serial number

1971 is the year of delivery (but not necessarily when the racks were fitted). The 830 refers to mobile telephone exchanges and the 0030 is the unit serial number. 

Between 1971 and 1981, serials 830 to 834 were also used to designate:

  • Mobile audio station trailers.
  • Mobile carrier station trailers.
  • Mobile radio station trailers.

The Eighties

The Eighties saw a steady increase in  the number and capacities of electronic exchanges and the introduction of digital technology, System X. The Strowger mobiles were becoming outdated and unnecessary as smaller more efficient switches were being developed. Strowger was being exported in containers and at home, Commsure was using the spare mobiles to house replacement PABX's. This was a decade of change as long established practices gave way to new methods of  working.


Containerisation

In the early Eighties, British Telecom's Teletrade produced complete Strowger exchanges in insulated, air conditioned standard 20 feet containers for export overseas. The basic unit was 400 lines capacity. Lettering was British Telecom blue on a yellow container.

Containerised 'reed switching systems' were also produced by Plessey for both the home and export markets.

Commsure

Commsure was devised by Phil Taylor in response to customer demand for an emergency (PABX) switch replacement service that could be used in the event of a fire or other major disaster at a business premises. This was necessary as the Stored Program Control PABX's in use during the Eighties took many months to order and to program to customer's requirements. The idea was adopted by British Telecom North East in 1984 and the first units were ready by February 1985.

Commsure units were:-

Additionally, each unit was fitted with a standby power supply and a separate operator suite with space for 2 operators and a manager/supervisor. A full set of alpha directories (on fiche) complete with a microfiche reader was also supplied as part of the package.

Once delivered to site and connected to the customer's internal wiring, the units were expected to be loaned for a period of  up to 6 months.

The TXD

The world's first transportable System X telephone exchange was delivered on 1st April 1987 to BT's East Anglia District as an expedient to cater for the rapid expansion of the Port of Felixstowe. Supplied by Plessey Major Systems Ltd of Liverpool, the RCU (Remote Concentrator Unit) with a capacity of  approx. 1500 lines, was housed in a standard 30 ft container which was positioned by a heavy lift 'Quinto' crane. The exchange keys were handed to EAD's District Manager, Colin Coleman by Plessey's Paul Leidecker.

The Nineties

The Nineties saw the end of the Analogue exchanges with the majority of  the Strowger and Electronic mobiles having been stripped-out for use as storage units, mobile shops and other purposes. Fully-equipped surviving units are rare, but examples can be seen at museums and preserved railways.

Network Mobiles

The digital network is perhaps more prone to a major failure as a result of human error such as a software problem (e.g. the 'Year 2000 bug') than the more resilient Strowger days. Even so, modern equipment is still vulnerable to fire or flood and there is still a need for mobile exchanges as we continue to rely upon the telephone service as a vital communication aid in today's global village.

To this end, the concept of mobiles still have a place as containerised digital node switches which have been produced to replace at short notice any trunk switch in the event of a serious failure.

Links to other mobiles


MNDX No.341
The Teradyne mobile MNDX No.341 being restored by Martin Loach.

MNDX No. 341 was a one of a batch of 200 mobile exchanges produced in 1972 for the GPO and was used in the South East Region. It was designed to be moved on low-loaders, hence the wheels are only to support the trailer and allow manoeuvring once on site.  




Two Thousand and Beyond

Having reached the new Millennium, mobile trailers could be used for conceivably any purpose ranging from a complete Personal Computer (PC) equipped office, Internet node, mainframe system, microwave up-link and broadcasting, as the various communications medias merge into a single data-stream.

 

Acknowledgements

Telephony by Atkinson; Chris Barlow; Mike Fletcher; Martin Loach; Peter Walker; Post Office Telecommunications Journals; Telephone Museum, Milton Keynes & Phil Goodwin; Post Office Vehicle Club; THG; Teletalk, staff newspaper of EAD.

References:

POEEJ Vol 31 October 1938 -Inauguration of the First Mobile Unit Automatic Exchange.
POEEJ Vol 32 April 1939 - A Mobile Automatic Telephone Exchange by R.W. Palmer and G.A.O. Abbott
POEEJ Vol 42 July 1949 - A 200-Line Mobile Automatic Exchange by E. Siddall and A.A. Page.

Compiled by Light-Straw © 1997-2007 [Mobile photos by Martin Loach]
Last update 5th July 2004