|The Telex Service|
|| EXIT | Manual Telex | Auto Telex | Fleet Building | St Botolph's | Keybridge House | SecureStream 300 | Into the 21st Century ||
What was Telex?
Telex is a dial-up public switched teleprinter network for the transmission and delivery of printed messages between subscribers.
It evolved from the Telex Printergram service which was introduced on 15th August 1932. With the necessary additional signalling equipment*, this enabled a telephone subscriber to use a teleprinter no. 7B on his line. This could be used for intercommunication with another telephone subscriber with a teleprinter, or primarily for sending messages (printergrams) to the Central Telegraph Office.
* voice frequency convertors.
"In setting up the Telex Network, some allowance has been made for the Telegraph Automatic Switching System (TASS) to be absorbed into the Telex System should the decline in the Public Telegraph Service continue to the point where separate networks cannot be justified."
By 1945, it was decided that a dedicated telegraph network separate from the Telephone Service was needed, for the growing use of teleprinters.
In 1947 a manual exchange was established in London, together with a limited international service.
On 15th November 1954 the new (manually switched) public inland Telex Service opened. This was set up so that a group of (local) subscribers could share a pool of telegraph junctions to the nearest Telex manual board.
By 1955 the network served 2000 subscribers, and of these, 200 were served via automatic sub centres. Many of these sub centres were in shared accommodation with MCVF (Multi Channel Voice Frequency) terminal equipment. For simplicity and ease of maintenance, the switching equipment used only PO relays and type 2 uinselectors. Each sub centre was (usually) served by a single 4' 6" rack, 8' 6' high with a capacity for 20 subscribers and 10 junctions. Subscriber's lines terminated on a Station Line Circuit (SLC) which consisted of 3 relays, LS, IS and P.
Zone Exchanges (1955)
There were 6 Zone Telex exchanges with manual boards: Birmingham, Bristol, Glasgow, Leeds, Manchester, and London.
Two digit codes and three digit numbers produced five digit subscriber numbers. Places which shared the same Zone Exchanges are shown in bold type. The Sub - Centres (existing and planned) are listed underneath. Centres which were served by a manual board are also included.
In February 1956 the Post Office confirmed that the Telex Network was to be converted to automatic working and the pilot exchanges were chosen as Leeds and Shoreditch (London).
FIFTY Charging Areas were established, each of these being roughly the boundaries of the existing Telephone Managers' Offices:
The Automatic Telex Network was built around SIX fully interconnected Zone Exchanges (the same as the 1955 model) at:
Thus the first automatic telex exchange, serving 250 subscribers, began working in Leeds on 30th August 1958. It was officially opened by the assistant Postmaster General on 1st September 1958. Shoreditch in London, also opened in September 1958, had an initial capacity of 900 subscribers. A total of 21 exchanges were planned to be installed by 1961. By 1960 the full conversion to automatic working had been completed.
Fleet had a capacity for 12,000 subscriber lines and until 1970 was the only international telex exchange.
Telex Operating Instructions
St. Botolph's On Line
By about 1970 a second London Zone exchange was set up in St. Botolph's House, (138-139 Hounsditch) making a total of 7 fully interconnected automatic zone switching exchanges. All area exchanges, routed to their home zone and the London Zone exchanges, Fleet and St. Botolph's.
Zone Switching Centres & Boundaries as at 1975:
The hypothetical exchanges into Fleet Building were gradually replaced by physical exchanges up to about 1970, by which time all areas had physical exchanges.
In 1984 the first Stored Program Control (SPC) inland Telex exchange was opened in Sheffield. By 1989 there were eleven SPC exchanges, plus another being developed for Cambridge. The SPC exchanges were an 'off the shelf' solution from the Canadian Marconi Company, designated CMA 755 data exchanges.
By 1990, the CMA 755 exchanges were complemented by a single Plessey 8660 SPC exchange which had been developed for use in London.
The last Strowger Telex exchange closed in August 1992.
As digital exchanges were deployed the number of switching points was reduced.
SecureStream 300 (circa 1998)
The SecureStream 300 service (SS300) - formerly known as the CSDN (Circuit Switched Data Network) - offered very low speed (300 baud) high security private circuit functionality at distance independent flat top tariffs. The service was originally devised to improve the utilisation levels of BT's dedicated Telex network, as it used spare capacity on the platform, to provide a virtual private network for each customer. The network covered the whole of the UK and offered a high degree of built-in resilience through diversely routed dual path network configurations.
Into the 21st Century
Chris Drewe, author of 'BT's Telex Network: Past, Present - and Future?' writes:
"My Telex article in the April 1993 (Vol.12) issue of the IBTE journal brought the story into the 1990s. It was written to mark the closure of the last BT Strowger Telex exchange in the summer of 1992, but was held up awaiting a suitable slot for publication. Here's an update (as at May 2003) on what's happened since then:"
The Plessey (GPT) 8660 Fleet inland exchange and the Eltex international gateways have been closed, so the Canadian Marconi Company CMA-755 exchanges run the BT network exclusively. (The IBTE Journal for April 1989 has an article on this exchange design.) The CMC exchanges have been rationalised to fewer sites, but still retaining the UK geographic numbering scheme. And unlike Broadband, there's universal UK coverage! Customers can connect to exchanges by X.25 links - cheaper than renting a lot of lines for high traffic levels. Telex over IP is quite a hot topic now, too.
Global Telex Global Telex Service (GTS, i.e. outsourcing) has taken off in a modest way; BT currently serves 8 countries in the developing world. GTS is becoming quite competitive now, and several other network operators are pitching for business, particularly Swisscom. Some surprisingly large countries are abandoning Telex altogether, notably Poland.
SecureStream 300 Originally known as CSDN (Circuit Switched Data Network). In terms of numbers of lines, SecureStream 300 is quite a success, as there are nearly as many as Telex customers. Main users are several water companies for telemetry - flow rates, reservoir levels, etc. Another user is one of the big bank/building societies, for monitoring unexpected activity at branch buildings, such as out-of-hours opening and closing of doors. One of the courier companies uses it for vehicle tracking. They don't actually use SecureStream in their trucks, but it links radio terminal stations to their headquarters. The big advantage of Telex/SecureStream over telephone lines is having a definite `on-hook' line condition, so cut lines (or faults) will still generate an alarm call. And Finally?
Although Telex is widely dismissed as an outmoded relic of a bygone era, it still has the advantage over e-mail of communicating over real-time, end-to-end connections -- with the answerback facility confirming the connection to the called terminal and its identity, rather than launching messages into cyberspace and hoping that they arrive. This is a major consideration for communications such as time-sensitive financial information. Cable & Wireless also run a Telex service in the UK. The CMA-755 exchanges support subscriber and trunk lines at 50Bd ITA2 (5-unit Baudot code) and 300Bd ASCII, with inter-working available between the two types using X-ON/X-OFF handshaking on the ASCII side.
From 2004 BT no longer offered Telex services to new customers. In March 2008 BT discontinued its UK Telex service, Global Telex service, SecureStream 300 and all associated products.
Swiss Telex (in Switzerland) provides a global service and BT's Telex customers were able to transfer if they wished to continue service. BT and SwissTelex signed an agreement which allowed BT Telex subscribers in the UK to retain their telex identity (number(s) and answerback), with the international access code (51).
In the late 1970s a new colour was chosen by John Valentine for the Telex Division's marketing leaflets, it was said to match his suit and the colour was purple!
Chris Drewe writes... When the contract was placed with Canadian Marconi Company for their exchanges in the early 1980s, the question arose as to what colour the 19" racks should be. CMC offered any colour in the BS range; their usual offering was an attractive dark blue and light grey, but this was felt to be a change too far by British Telecom, then still part of the Post Office, and so it had to be good old Light Straw! Meanwhile, another part of BT wanted something distinctive for the new System X telephone exchanges, so they opted for an attractive dark blue and light grey.
Service Signals - Why Calls Failed
Some terms were derived from French words, e.g. occupe, absent and derange.
Called terminal busy (= telephone `engaged').
ABS ABSent "Controlled Not Ready"
Called terminal switched off.
DER out of orDER
"Uncontrolled Not Ready" Called terminal faulty OR Fault on line OR Line closed by exchange staff (= telephone TOS).
NP Not Permitted/No Path
Spare number or code dialled (= telephone NU) OR Excessive time between selection digits.
NA Not Allowed/No Access
Invalid use of facility code or feature.
NC No Circuits/Network Congestion
No free trunk circuits OR Exchange congested (= telephone EET). (Also used as catch-all if call fails for other reasons.)
NCH Number CHanged -- may be followed by new number.
Called line has new number, call may be automatically forwarded.
TZ - Time Zone - Equivalent of PSTN 'GRACE'.
IPOEE Printed Paper No. 215 The Automatic Telex Service.
IBTE Journals: April 1989 and April 1993.
|Design, images and text compiled by ©
Light-Straw. With thanks to John Lamble
and Chris Drewe. Page last updated 4th July 2013.
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