[Asterisk-Users] Red Alarm

[Andrew Kohlsmith]

> An E1 can be a long way from the box with the right cable. However many
> people use the wrong cable. Using a LAN cable for an E1 often gives
> errors if the cable is more than just a few metres long. Although the
> plugs look the same, the twisted pairs should be grouped differently in
> an E1 cable, and it really makes a difference. If the drop cable is only
> a couple of metres long, a LAN cable is usually adequate. This is also
> true for T1s.

Actually that's not entirely true.

standard 568A/B wired cable does not split pairs for ethernet or DSX1 
wiring.  The problem is that DSX1 uses pins (1,2),(4,5) and ethernet (1,2),
(3,6)  (parenthesis show pairing).  DSX1 must have the (1,2) and (4,5) 
pairs swapped to match the TX to the RX at each end, whereas normal 
ethernet does not, as the switch is cross-wired.  Using an ethernet 
crossover cable does not help since it is swapping (1,2) and (3,6), not 
(1,2) and (4,5).  

The problem with using CAT5 for long telco runs is that the impedance is 
wrong at the line clock rate (~1MHz).  IIRC the impedance for telco is 
specified at 600 ohms @ 1MHz, whereas for CAT5 the impedance is actually 
specified at around 100MHz, where the ethernet line rate is.  You can get 
away with it so long as the impedance is right, but unless you've got the 
data sheets you're playing guessing games.

Regards,
Andrew