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From: tjrob@ihlpl.att.com (Thomas J Roberts)
Newsgroups: comp.dcom.telecom
Subject: Re: A Few ISDN Questions
Message-ID: <4612@accuvax.nwu.edu>
Date: 1 Mar 90 15:47:20 GMT
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 From article <4459@accuvax.nwu.edu>, by casbs@csli.stanford.edu (Lynn Gale):
 
> Is it known at this point in time what medium ISDN will run on?  In
> particular, what number of wire pairs are necessary and do they need
> to be shielded or unshielded?  (Thinking about wiring decisions with
> the future in mind...)

ISDN is considerably more complicated than traditional analog phone
service. This is reflected in its wiring.

The following describes wiring for the ISDN Basic Rate Interface,
which carries 2 Bearer (64 kb/s) and one Delta (16 kb/s signaling)
channels (i.e. it can carry 2 simultaneous voice or data calls and up
to 15 packet data calls aggregating 16 kilobits per second).  This is
the ISDN equivalent to the analog loop.

Between the telco office and the subscriber's location, the U
interface carries bidirectional digital data on 1 (unshielded) twisted
pair.  If no loading coils are present, existing wires can be used.
Most Central Office switches can support the U interface up to 20,000
 - 25,000 cable feet without repeaters (various equipments can be used
to extend this, I don't know the details).

Within the subscriber's building, the NT1 unit converts the 1-pair U
interface into the (unshielded) 2-pair S interface, which carries
unidirectional digital data on each pair. Most NT1s can support up to
about 1000 cable feet. Note that because the S interface has very
little protection (e.g. lightning), it is unsuitable to use between
buildings.

Unlike traditional analog service, ISDN cannot provide power to the
station set (or the NT1) over the same wiring as the signaling and
data. Most AT&T installations use 4-pair wiring within the building,
with 2 pairs carrying the S interface signaling, 1 pair carrying power
to the station set, and 1 pair is spare. In a typical (large office
building) installation, on each floor a wiring closet is used to hold
the NT1s and battery-supported power supplies for the NT1s and the
station sets on the floor.

The ISDN set uses a larger modular plug than traditional analog sets
(it needs 4 pairs, not just 2 pairs). If you wire the building using 4
pairs between the wiring closet(s) and the station set locations, and
use the larger modular jacks, you can still plug in the smaller analog
plugs, using the middle two pairs (that is, the 2-pair plug will
physically connect to the 4-pair jack, but will only connect to 2 of
the pairs). When (if) you convert to ISDN, you will then only need to
rewire the wiring closet(s), not the individual station wiring.  I
don't know part numbers for the various connectors.


Tom Roberts
AT&T Bell Laboratories
att!ihlpl!tjrob