Thread: Dial Plans
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Unread 06-04-2007, 05:23 AM   #10
jenalcom
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I think there was also a clear difference in the design philosophy of the telephone systems between the US and the UK.

The UK system used a thing called a translator to take your area code and convert it to routing digits that sent it to the right destination - sometimes direct, sometimes through various other switches. Plus the full number "local" number was sent with the routing digits. If the route was full then tough, the call didn't go through.

The US system worked differently in that the originating switch would only send the area code through the system preferably direct to the final area code. If that route was full it would send the area code to another area code switch which would then try and send it to where it needed to go. So the request would get passed from area to area until finally it landed in the right area. At that time the destination switch would request the "local" part of the number. Quite a clever system really with lots of redundancy routing built in.

Before LD dialling in the UK you could access other (relatively close) areas by dialling the routing digits yourself. These usually began with a "9" and could be as many as 5 or 6 digits long to get to a further away exchange/switch. These routing digits were published for subscribers to use. There were a lot more that were not published that could be used if you were in the know. For instance it was possible to dial from the UK to Australia for the cost of a local call if you dialled the correct codes! 4 pence (approx 10 cents) for as long as you wanted! But then you could do that in the US as well with the help of a "blue (?) box".

The UK LD area codes were originally based on the alpha characters on the dial/keypad. Or at least the three and four digit ones were. The main centres had short codes - 01=London, 021=Birmingham, 031=Edinburgh, 041=Glasgow, 051=Liverpool, 061=Manchester. Luckily the six main area names fitted with the dial/key allocations! The other areas mainly used similar dialling conventions - Reading was 0734 (0RE4), Horsham was 0408 (0HO8), etc.

I never really got into the European area codes but I suspect that most of them were base on a "routing code" system where the short codes were major centres and the longer codes were fed from those centres.

All in all fascinating stuff.

Alan
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