[Asterisk-Users] Re: Shielding of T1/E1 cables WAS RE: Pinoutsfor T1/E1 crossover

[Andrew Kohlsmith]

On Monday 24 April 2006 16:13, Rich Adamson wrote:
> A 'real' T1 cable would use a twisted pair for pins 1 & 2 and another
> twisted pair for 4 & 5.  Looks like a typical cat5 straight-through
> cable uses twisted pairs straight across pins 1 through 8.

Nope.  A cable wired for ethernet and a cable wired for T1 are wired 
*identically*:

Pair 1: Pins 4&5
Pair 2: Pins 1&2
Pair 3: Pins 3&6
Pair 4: Pins 7&8

Where the difference lies in what pairs are used.  T1 uses pairs 1 and 2, 
while standard 10/100 ethernet uses pairs 2 and 3.  This is why a T1 
crossover and an ethernet crossover are not the same cable, but a T1 patch 
and ethernet patch are.

> So, if my eyes are worth a damn (which they probably aren't), a cat5
> cable uses one lead from a twisted pair for pin 4 (on the T1) and lead
> from another twisted pair for pin 5. Not cool.

Nope it's cool, the pair *is* pins 4&5, not 3&4.  I don't have a good solid 
answer for this, other than perhaps one of tradition.  In the olden days of 
ethernet (aethernet?) it was standard practice to run the phone and computer 
networks on the same cable.  The phone would use pair 1 (the center pins) and 
ethernet used pairs 2 and 3.  No problems.

> If that's correct, then a cat5 cable should not be used for a T1 run of
> any significant length as the transmit leads (pins 4 & 5) would be
> highly susceptible to induced noise.

See above.  If the pair was being split then yes, but it's not.  Any properly 
made ethernet patch cable will have a pair on pins 4&5.

-A.