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
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