"Martin Underwood" wrote in message
...
"John Shelley" wrote in message
news
Martin Underwood wrote:
snip


Yeah, silly question, on reflection! I presume the equipment has to accept
a
fixed number of digits (previously seven, now eight) and identify the
first
four (previously three) as the district and the remaining four as the
subscriber number. If the stream of digits begins with a 0, an alternative
algorithm identifies from the digits that follow how many are the exchange
(eg "20" signifies London, "1344" signifies Bracknell). I can see that if
you only dial the final four digits, they could be confused with 0
signifying "what follows is an exchange" or 1 signifying special numbers
like emergency (112), directory enquiries (118xxx) etc.

Actually the local exchange simply routes all numbers that start with a 0 to
the associated trunk exchange (properly called a DMSU, for Digital Main
Switching Unit).

The DMSU does geographic mapping, routes the call to the relevant DMSU on
the other end which in turn routes it to the relevant local exchange.

If it is a non-geographic number (07*, 08*, 09*) the DMSU routes it to a
special platform that does really clever lookups. That is how a call to a
call centre number at 3am is answerd in India, at 3pm in, say, Sunderland
and at 11pm in the USA.

snip

digit exchange codes are actually located in the same building. Here in
Harrow the exchange building housed both the 8427 and 8863 exchanges and
probably others as well. With the arrival of electronic exchanges the
physical space needed for an exchange was vastly reduced so adding extra
switching capacity within a building that was built to house a
mechanical
exchange isn't a problem. The extra exchange numbers are also needed
for
the non BT operators.

Ah, so new suscribers in an area potentially get a brand new district
number
that's unrelated to that of all the other subscribers in that area? Yes, I
suppose that's one way of solving the problem. Do all subscribers in one
area get one new code and all those in another area get different code:
can
you still say "xxxx [a new code] is Harrow, alongside yyyy [the existing
code]" or is the code-to-location mapping lost?

The association of a single code with a geographic area disappeared decades
ago. Harrow has the 8424, 8427, 8861 and 8863 codes. In the predigital
days I believe these were normally co-located in the same exchange building.

Nowadays the exchange equipment is orders of magnitudes smaller, so in some
case your 'local' exchange is actually located in an exchange building in a
neighbouring area, along with half a dozen other 'local' exchanges.

Your 'phone line will physically be connected to a some sort of device
locally. However this could be a consolidation device that takes all of
those lines on to a neighbouring exchange building. Alternatively it could
just take *some* of those lines to a neighbouring exchange building if there
are logistcal reasons. For example the one room in the building still being
used for exchanges only has room for three and a half sets of lines - don't
forget that at some stage 9,999 lines have to be connected up to each local
exchange.

To answer your specific question, I believe that anyone in Harrow will get
one of the above codes *if one is available*. If not they will get one form
one the exchange in the exchange building where their 'phone line ends up.

regards

Stephen