Saturday, 4 June 2011

Telephony, the definitive work

One of the treasures of my youth was the Harold Hill public library, which I should really write about some time. For some unknowable reason, their collection included the two-volume definitive work on the UK telephone system, Atkinson's Telephony, published in 1949. I've always been fascinated by complex electro-mechanical devices, and I borrowed these books dozens of times. Of course that may be why I'm fascinated by them.

Telephones are run by computers now, but back then everything was done with relays and stepping switches. A telephone exchange was a vast hall filled with them. The noise was just unimaginable as the switches followed the subscribers' dial pulses (telephone users have always been called subscribers) then rocketted off on their own to look for the next free switch, later clattering back to their rest position at the end of a call.

The automatic telephone system was invented, improbably, by an undertaker called Strowger. The story has it that the telephone operator in his small town just happened to be the wife of his rival. In those pre-automation times, people would call the operator and say "connect me to the undertaker". You can guess who got the business. The telephone dial and the clever two-dimensional stepping switch that interpreted it were his invention, and the system was forever after called the "Strowger System".

Manual telephone exchanges were still very much present when I was a teenager. I had a friend who lived in the next town, Brentwood, which was one of the very last places to be converted to automatic operation, in the 1970s. Even manual exchanges involved some complex electro-mechanical gadgetry, though, to detect when the user picked up their phone, make it ring and so on. Atkinson deals at length with this, and the problems of interconnecting manual and automatic systems. Operators are always referred to as "she" - not sexist, in itself anyway, just a reflection of the fact that all telephone operators were women back then. And not outsourced to Bangalore either - every telephone exchange, even automatic ones, had a hall full of operator positions to handle long-distance calls and other odd cases.

About half of the first volume deals with the basics - cable design, network organisation, and of course the relay. A relay is just an electro-mechanical switch, a bunch of contacts operated by a coil when electricity is applied. They're still used extensively today - a car has dozens of them - but in the telephone system there were millions of them, implementing complex logic functions that today would be a few dozen lines of C code. The basic idea is simple enough but the realities of the telephone system called for great subtlety in their design. There were relays that were quick to engage but slow to release, and vice versa, relays specially designed to respond quickly to dial pulses - all done by varying the windings, the magnetic arrangements and the contacts. Their design is a lost art now.

The rest of the first volume deals with the circuits and arrangements for manual exchanges. The real excitement is in the second volume, the circuits for automatic exchanges. Just how do you build an electro-mechanical telephone exchange? The basic concept seems simple - the pulses generated by a telephone dial operate Strowger switches, until all digits are dialled and the final switch - called a final selector because it works very differently from all the others - has made a connection to the called number. But actually making it all work, dealing with all the real-life complications, was a phenomenal achievement. It's not surprising that the first ever computers - built to crack German codes during World War II - were designed by engineers from the Post Office Research Laboratory. They were the only people to have designed circuits of this level of logical complexity.

It's worth commenting on the difference in the scale of the phone system in those days. The book uses the city of Derby as an example. Its population then was over 100,000 people. The telephone exchange had a maximum capacity of 4000 lines, or one for every 25 people. In 1949, ordinary people never had their own phone. Businesses did, and a small minority of better off people would have a phone at home. For ordinary people working- and middle-class people, there were phone boxes. (The book includes a lengthy section on the classic British phone box design, including its weight - being made largely of cast iron, it weighed 13 cwt, or about 660 kg). My parents didn't have a phone at home until about 1965.

There are lengthy chapters on the design of small rural exchanges, partly I suppose because they were small enough that the book could describe them in complete detail - each of the hundreds of selectors and relays. There are complete circuit diagrams, and text to explain the function of every last relay contact and selector. It is the best technical design documentation I have ever seen, bar none.

The smallest of the rural exchanges, designed to occupy a small brick building somewhere on the outskirts of a village, had just 100 lines. It replaced an operator in the back of the post office working a manual plug-and-socket switchboard, disproportionately expensive and only available during the day time. At night, the doctor would be plugged straight through to the main circuit, and everyone else would just have to wait until morning. There were always telegrams for anything urgent that might arise. (My wife tells the story of her father using just such a rural manual exchange, fuming as he waited for the operator, grumbling "She's knitting, I just know it, she's finishing her row before she answers". And he was probably right).

Sadly, the book was never updated to discuss the nec plus ultra of electromechanical telephony, Subscriber Trunk Dialling (STD). When it was written, trunk calls - anything further than the next town - were handled manually. You would dial 0 for the operator, and say in your best BBC voice, "Derby 6543 please". If there was a circuit available you'd be connected. More likely, she would call you back later when a circuit became free.

The first STD installation was in 1958, and the first public long-distance call was made by the Queen, from Bristol. It came to my home town in 1964, I can still remember the presentation in the local Post Office. It made no practical difference to us, nor to most people - the furthest we ever called was my aunt in London. Although calls were automatic, they were still ferociously expensive.

The implementation of STD relied on a magnificent piece of equipment called a register/translator, or its "marketing" name of GRACE. Each unit was a box about two feet square, packed full of relays and miniature uniselectors. As the subscriber dialled, each digit was remembered on a mechanical switch. As soon as the first three digits had been collected - the STD code, or as we would now call it, the area code - it could start translating them into the actual routing code which would switch the call through the trunk network to its destination. Once that was done, the remainder of the digits would be replayed, one by one, from the switches that stored them. Sadly there seems to be no detailed technical description of these systems still available. I was told by someone who joined the Post Office in the seventies that the schematics, spread out, occupied the whole floor of a large office. It was truly a tour de force of the available technology at the time.

It was thanks to my curiosity about the design of trunk network that a friend and I became "phreaks" in the 1960s, long before we knew the term existed. But that is a story for another time. For now, back to reading Atkinson... there are a couple of details of the UAX14 exchange design that I'm not clear about...

1 comment:

Peter Long said...

John - Fabulous - I've seen some of these in museums (working no less) and they are indeed works of art.

Well done for tracking down the book.

Peter