[c-nsp] T1 Cables

[Jay Hennigan]

Janet Plato wrote:
> DS1 cabling does not use the normal USOC pairing, so you forgo
> common mode rejection of noise.  

Sure it does, USOC RJ48.  This is also the standard EIA pairing, same as 
just about everything else.  Pairs 1 and 3 of 568A or 1 and 2 of 568B. 
Regular "ethernet" jumpers are also EIA 568, so the pairing is correct, 
although one of the active pairs for DS1 is different from 10-base-T or 
100-base-T.

There are several different USOC pairing schemes for 8-pin plug/jack 
combinations.  USOC RJ48 *IS* the DS1 standard.  USOC RJ45 (which is 
what the physical jack/plug format is often erroneously called) is 
something completely different, no it's not ethernet, but the pairing is 
still compatible.  USOC RJ61 is VERY rare, pairing NOT compatible.  USOC 
RJ31X also not compatible, used for alarm jacks.

http://www.hotlinkdata.com/wiring_diagram.htm

> When a signal pin and that
> signal's ground share a common path with normal spacing, for
> example in a single twisted pair, they pick up the same amount of
> noise from the environment.  The noise is common to both wires,
> and since the receiver is expecting to see the signal expressed as
> a difference in voltage between signal pin and ground pin, not
> an absolute voltage difference, the noise that is common to both
> wires is rejected.

But T-1 is a balanced differential signal, so there is no signal pin and 
ground but instead two signal pins per direction of transmission. 
Regular "ethernet whips" indeed have the correct wires in twisted pairs 
for T-1, pins 1 and 2 are a pair and pins 4 and 5 are a pair.

> Having said that, I've run T1 short distances in a normal data
> center environment on some really crappy wire without problems.
> I suspect you either have a problem not related to your cabling,
> or enough noise that you really need the common-mode noise
> rejection afforded by correct pairing.

The pairing of generic 8-pin UTP data cables is correct for T-1.  I 
agree that the original poster's issue is not with using conventional 
"ethernet whip" jumpers for DS1.  He may indeed have a layer one 
problem, but EIA 568A or 568B jumpers (as long as both ends are the 
same) aren't it.

> I do not have a source of such cables, I've not used such things
> in a long time.

You may be thinking of the old-school individually shielded twisted pair 
22-gauge T-1 cable.  This was a special low-capacitance cable with two 
pairs, insulated with a large diameter foam plastic dielectric, each 
pair individually shielded.  This cable was designed to carry T-1 
signals for thousands of feet without repeaters or CSU/DSU equipment.

Occasionally I've seen a telco use this stuff as a ten-foot jumper from 
a protector terminating a half-mile or so of 600-pair copper to a 
smartjack on the backboard.  I guess this gave them a warm fuzzy feeling 
that at least the last ten feet was cabled better than the first 
half-mile.

This cable pre-dates the use of 8-pin modular plugs for T-1 
terminations.  It was sometimes used with D-sub 15 pin plugs and jacks. 
  It is very difficult to terminate it correctly into an 8-pin modular 
plug because the insulation is too thick.  I haven't seen it used in a 
new installation in a very long time.


--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
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