[asterisk-users] Audio Dropouts During Call

[Dave Platt]

>> A good Ethernet cable-pair tester can spot such things pretty quickly.
> 
> I disagree.
> 
> *Certainly*, incorrect pair terminations can cause the sort of problems 
> described, however I haven't yet come across a cable tester which can identify 
> that a cable correctly connected from end to end with wires {1..8} <-> {1..8} 
> is in fact not correctly connected in ethernet 1-2 3-6 4-5 7-8 pairs.
> 
> All cable testers I have so far encountered will check that all wires are 
> correctly connected end to end and not cross connected etc., but have no clue 
> whether the wire joining pin 3 at one end to pin 3 at the other end is twisted 
> together with the cable from pin 4 or pin 6.
> 
> I would be very interested to find such a tester, if you can point us at one.

Well, I did say a _good_ cable tester, and that means "expensive" :-)

I agree, the simple DC-continuity type of cable tester won't catch more
than a fraction of the problems.  These inexpensive testers ($50-$100
range, usually) can diagnose open wires, or cases where the two ends of
the cable are wired differently and the signals don't match up at all.
As you correctly point out, they're useless for "mixed pair" problems
since these don't show up at all on DC.  Electrical detection of such
problems has to be done in a high-frequency (signal or pulse) domain.

What you need is a tester which has either or both of two capabilities:

(1) Crosstalk measurement - excite one pair with a signal, and measure
    the other pairs/wires to see if some of the signal leaks across onto
    them.

(2) TDR - Time Domain Reflectometry.  Excite a pair with a known pulse,
    and measure the voltage across the pair over time.  This lets you
    measure the actual impedance of the pair all the way to the "far
    end".  A swapped-pair problem will show up pretty quickly as an
    incorrect (and varying) impedance on the "pair" you are trying to
    drive.  TDR can locate broken wires and sorts - not just the fact
    that they're there, but how far down the cable they are.  A good
    TDR setup can even let you "see" things like a place where the cable
    has been bent too sharply or pinched, and the twisted-pair wires
    have been smooshed out of their proper configuration.

As to specifics, a bit of Google-fu turns out the following
possibilities.  I haven't used any of them myself, but the data sheet
summaries of capabilities look promising.

The high-end Fluke cable-testers such as the CIQ-100, DSX-5000, and
DSX-8000 would probably work - these are described as being able to
locate crosstalk faults and impedance faults.

Another very interesting device is the PocketEthernet tester, which
sells for 199 Euros (less without VAT).  It analyzes the wiremap, has
TDR capability, and includes a bunch of higher-level diagnostics as
well (bit-error-rate, Ethernet link analysis and capabilities-
announcement, DHCP, ping tests, bit-error-rate testing, etc.).  It
uses a tablet or smartphone as its GUI (connected via BlueTooth) and
generates a pretty spiffy-looking report in PDF format.  I don't
know what its sensitivity would be to such problems on a short (e.g.
jumper) cable - that'd be a good question to ask the manufacturer.

For diagnosing signal integrity problems in a building's installed
Ethernet wiring, I'd want to have a device of this sort available.
The high-end devices can probably be rented.  The PocketEthernet is
cheap enough that I'd just buy one if I can to do any work of this
sort.

There's also another dirt-cheap method for diagnosing bad Ethernet
jumpers - substitution.  Buy a bunch of known-good CAT-6E (e.g.)
jumpers from a reliable vendor, and inspect them.  Mark them as
"This is a good one".  If you have a suspect connection, start
swapping cables one at a time - replace each jumper with one of
your known-good supply and re-test to see if the problem goes
away.  If it does, take the cable you removed from service and
_immediately_ cut off both ends right at the connector, and then
throw the whole thing into the trash, so that no one will _ever_
put it back in service.  Do not try to salvage, repair, or re-use
a bad jumper - life's too short to try to pinch pennies like that.
(If you don't do this, the old cable _will_ come back to haunt you
some day.)