Stag Lane ISCs
Stag Lane ISCs
Stag Lane was planned and commissioned within only 3 years when it was realised that the equipment destined for installation in Mondial House (which was still being built in 1971) would not be ready to cope with the rapid growth and demand for international calls. Stag Lane, Edgware was the site of a former airfield, hence the ISCs were named after the aircraft industry and the flying pioneers of the Thirties.

Photo by Unit Manager Bob Outtram: Mollison ARM20 circa 1989 shortly after closure (notice the stacked up ladders). 
| EXIT | Site History | International Rescue | Misc | The End | References |
| The Switches | Mollison | De Havilland |

Site History
With RAF Hendon close by, the area is steeped in the history of the early aviators...

Learn about Hendon Aerodrome

Rather less well known is the smaller airfield and factories from which the De Havilland aircraft were built and from which the flying pioneers of the 1930s were trained...

Learn about Stag Lane Aerodrome

International Rescue

In the early 1970s, international call traffic was growing at nearly 20% per annum and to meet the shortfall in both buildings and equipment, an expedient site was urgently needed. By June 1972 a lease had been obtained on the site of a former airfield at Stag Lane...

Hunts Ends at Stag Lane
John Burt, Senior Executive Engineer of the project, recalls how his team came to choose Stag Lane as a major ISC (International Switching Centre).
The Switches

Stag Lane consisted of two International Gateways, a Plessey 5005T crossbar (TXK2) called DeHavilland and the Ericsson ARM20 crossbar (TXK5) called Mollison, there were also models of both at the training centre.

Mollison TXK5
The Mollison Units
The Ericsson ARM20 crossbar (TXK5), which was installed at Stag Lane, was named Mollison, there were also models of both at the training centre. This ARM 20 was the largest in the world and consisted of four separate switches...

Mollison International Maintenance Centre (IMC) Photo: 'Youngleavers'
Project Manager: John Burt (Senior Executive Engineer) and later Colin Moore.

BIS date October 1974

The Mollison ISC equipment:

  • Was ordered as a result of international competitive tendering.
  • Was fitted with the ARM20 switch manufactured by L.M. Ericsson of Sweden.
  • Used the ARM201/202 Crossbar system.
  • Was designated TXK5 by the Post Office.
  • Occupied a floor space of approximately 5000 square metres.
  • Mostly terminated circuits using CCITT R2 signalling, which was new to the UK.
Between 1971 to 1975 Peter Walker, an Executive Engineer at PO Telecoms Headquarters, was working on national and international crossbar exchanges and international signalling systems. This included the type approval of software on the GEC Mark 1C processor used at London’s Sector Switching Centres, system appraisal of the Ericsson ARM crossbar exchange for use at the Mollison International Switching Centre, and the introduction of the R2 signalling system into the international network. He also developed a model for predicting post-dialling delay in international networks.

By 1989, digital switches were taking over and as Director of Planning BT International, Peter was responsible for the closure of the Crossbar units, but he recalls with a sense of achievement and some modesty...

"Mollison TXK5 was introduced in 1974/5 at Stag Lane, Edgware. This used the LM Ericsson ARM201/202 crossbar system which had separate incoming and outgoing units (the latter switched 10 wires!). It was a simple unit designed to deliver no frills IDD capacity, so couldn't handle transit, operator assistance etc. This unit did 14 years service. Each unit had a capacity of 4000e. At 8000e, Mollison ISC was the world's largest crossbar exchange and largest ISC at the time."

De Havilland TXK2

Project Manager: Pat Hastings [This is being verified]

  • Installation of the exchange began in February 1973.
  • BIS date September 1975
DeHavilland was installed in the second of the two large single storey buildings and used Plessey 5005T switching kit which had been destined for Mondial House.

It was 'fully featured' meaning it could handle all types of international calls:

  • Directly dialled by subscribers
  • Operator controlled calls
  • Transit calls * (those routed via other countries)
*That was traffic coming in from other countries, being switched in the UK and destined for other countries. Of course, all ISCs could cope with traffic switched via other countries.

"DeHavilland TXK2 was introduced in 1975, also at Stag Lane. It had a capacity of 2500e and was fully featured. DeHavilland was fitted with TXK2 equipment, the ATE 5005T manufactured by Plessey."

Learn more about De Havilland

The Stag Lane complex also had...

  • A common Repeater Station terminating approximately 17,000 international circuits.
  • A regional training school.
  • All manually controlled calls were routed via ICCs in Glasgow, Leicester or London.
  • Access to London Director Area via PCM systems.
  • Links to HF networks.
Stag Lane, like many of the other ISCs required commitments from several sections of the Post Office Telecommunications:
  • ET7.1 David Ball did the submarine cable planning.
  • ET7.2.1 Arthur Watson planned the routes.
  • ET7.2.2 Malcolm Lewis looked at various systems issues - and was also responsible for the design of the strowger technology at Kelvin IDQ (the first one).
  • ET7.3 looked after the computer controlled systems used for telex and telegraph switching.
Stag Lane ISCs closed in late 1989 and the site was sold soon afterwards.

Mr R.W. Button:
Was Deputy Controller of the Project and Works Division in International and Maritime Telecommunications Region, and was involved with the Stag Lane project from the outset. As Stag Lane became operational, he was appointed to head the new Network Control Division of the Post Office's External Telecommunications Executive.


POEEJ July 1977 The TXK2 Switching System at Wood Street and De Havilland International Gateway Exchanges by D.C. Modi and K.W. Young.
POEEJ October 1977 The TXK5 Switching System at Mollison and Thames International Gateway Exchanges by P.J.Walker and D.R. Ballinger.
POTJ Spring 1974 Hunt ends at Stag Lane by R.W. Button.
POEEJ October 1976 A New Maintenance Aid for International Exchanges by N.V. West and D.J. Sylvester.

With thanks to: John Burt, Bob Outtram, Peter Walker, and Youngleavers.
Design, images and text compiled by © Light-Straw. Page last updated June 2015 revision. Checked May 2021.

All logos and trade marks are the property of their respective owners and are used on the Light Straw site(s) for review only. Students and researchers are recommended to make their own independent enquiries as to the accuracy of the information contained therein.