[c-nsp] DS3 Y-Cable

[Bill Wichers]

> You're correct, this won't work.  On the receive side (from the loop
> towards the router), it may be OK if you're lucky but you'll have a fat
> impedance bump that will very likely cause errors.  On the receive side,
> a cheapo cable-TV two-way splitter would actually do the trick nicely if
> you convert 75-ohm BNC to F.  You'll have a little over 3dB loss but
> Cisco sometimes has issues with DS3 signals that are too hot so this
> could be a blessing.

Not exactly... Your typical TV splitter has a low-end frequency limit of
around 5 MHz. DS3 signals have frequency components down to around 3 MHz.
The result will be bad wave shape and voltage levels, and will probably
cause at least bit errors, but possibly framing errors and other things
too.

I don't think there is really any way to do this kind of "poor man's
protection" on a DS3 circuit -- you need some electronics in there
somewhere. And in reality, you still have the single points of failure of
the telco's DS3 gear, the cable run, probably at least one DSX panel
somewhere, etc... I even remember reading somewhere that SBC did a study
and found the #1 cause of CO failures was people snagging DS3 cable and
pulling the *cable* out of the BNC *connector* (the result was a new,
special "S" suffix DS3 cable with 2% more braid than "regular" 734/735
cable). All of this would imply that trying to find a way to link two
routers and CT3 interfaces to one CT3 circuit won't really accomplish very
much in terms of increased realibility, but will bring all kinds of
strange new problems to deal with.

It seems like this "Y-cable DS3 protection" question comes up on the list
every 6-12 months or so. Strange...

     -Bill


*****************************
Waveform Technology
Systems Engineer