[Asterisk-Users] Red Alarm

[Steve Underwood]

Andrew Kohlsmith wrote:

>>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.  
>
I've no idea what you mean here, since your next statements shows just 
*how* they are split. :-\

>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 
>
Not usually these days. The box on the wall normally needs a striaght 
through cable to the card for E1s and T1s. That is why so many people 
plug in a LAN cable and find it almost works.

>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).
>
Well, at least a crossover cable doesn't fool people into thinking they 
got it right. :-)

>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 
>
T1s are always 100-110ohm, E1s are the same when on pairs, and 75ohm on 
coax. Only analogue pairs are terminated at 600ohm, and no line can 
actually be greater than 120*PI (about 377) ohms - that is the impedance 
of free space. Fudgy 600ohm stuff works at audio frequencies, but you 
have to treat the line properly as a transmission line as the frequency 
rises.

>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.
>
There is no guessing involved. The impedances are pairing are all 
standard. You need specs, not data speets.

Regards,
Steve