[geeks] U.K. telephone wiring RJ-45

[Jonathan C. Patschke]

On Sun, 20 Jun 2004, Geoffrey S. Mendelson wrote:

> Now here is the part I am at a loss. When I was last in the U.K. the
> hotel I stayed at (a service flat) had an ISDN connection. It did not
> have a POTS (analog) line. The ISDN line was terminated in an RJ-45 (8
> pin U.S.) plug.

Are you sure it's RJ45 and not RJ48?  I've seen a lot of RJ48 ISDN (PRI,
mainly), but never an RJ45 ISDN from the telco.  They might do things
differently on the other side of the pond.

> Does anyone have know if this was typical internet misinformation? Should
> it have read "... homes and hotels may be wired with ISDN lines which use
> RJ-45 connectors, but these lines are not compatible with analog telephones"?

RJ45 would not be compatible if the line is ISDN because the ISDN T/R
pair would stomp the POTS T/R pair.  If it's RJ48X, the center pair
would typically be POTS T/R and the leftmost pair would be ISDN T/R.
All of the PRIs I've seen came in RJ48C, though, in which case there are
two T/R pairs, but neither of them are on the center two pins.

> Or is it possible he will need an RJ-45 to RJ-11 adaptor to connect his
> analog modem to an analog phone line? If so, how would it be wired?

If it's RJ45, there is only one T/R pair, and if it's an ISDN jack, you
need not an adapter cable, but an ISDN terminal adapter that gives you a
POTS port on the D channel.  If it's RJ48X, and the jack does both POTS
and ISDN, there's a 50% chance of your phone plugging right in and a 50%
chance you need to use the leftmost pair as your T/R.  You can make this
with a crimper and a 8-position plug and a 4-position jack and a length
of phonecord.  If it's RJ48C, it's either the leftmost pair (1 and 2) or
the pins 3 and 5.

I'm sure a lot of what you got is misinformation because it seems like
everyone calls EVERY 8-position jack/plug "RJ45".  RJ45 is an 8-position
jack in a very specific configuration (pair 1 on 4 & 5, 2 on 3 & 6, 3 on
1 & 2, and 4 on 7 & 8) that is designed to be backwards compatible with
RJ11.

The best you can probably do is find out from BT what they tend to do
with their ISDN.  If they don't bring POTS along with it, you're stuck
needing a TA or the UK equivalent of an NT1.

-- 
Jonathan Patschke )"We're Germans and we use Unix.  That's a combination
Elgin, TX        (  of two demographic groups known to have no sense of
USA               ) humour whatsoever." - Hanno Mueller in de.c.o.u.p