[asterisk-users] Audio Dropouts During Call

Dave Platt dplatt at radagast.org
Tue Apr 3 17:20:29 CDT 2018

> I looked at your network diagram. Try checking the configuration of the
> Ethernet ports on the firewall and the Asterisk box. Make sure they are
> set to auto-negotiate and not set to a fixed speed and fixed duplex.
> I have found in the past that if one end of a link is expecting auto-
> negotiation (as the switches probably are) and the other end is expecting
> a fixed configuration, things can degrade to half-duplex trying to talk
> to full-duplex, resulting in lots of collisions and packet loss when there
> is any kind of significant traffic.
> Your description would be consistent with the firewall introducing lots of
> LAN collisions when busy, in the central gigabit switch, even if the VoIP
> traffic isn't passing through the firewall.

Also, check the wiring.  Check each individual RJ-45 jumper, *and* the
in-house wiring, with a proper tester that can verify that the
individual pairs are hooked up correctly.

I've seen all kinds of hell occur, in situations where somebody used
telco-type RJ-45 connecting cables, in place of proper Ethernet
connecting cables.

The problem is this:  in a telco RJ-45 cable (such as was/is often used
for proprietary telephone systems) the individual wires are either not
in twisted pairs, or are twisted-pairs in a 1-2 3-4 5-6 7-8 arrangement.
These work fine for analog connections.  They're latent-death-on-wheels
for Ethernet.

Ethernet only works well if you connect the pairs as a 1-2, 3-6, 4-5,
7-8 arrangement, because this is how the signals are sent electrically.
Using the correct connections ensures that the signals on each pair are
"balanced" electrically - that is, the two wires in each twisted pair
are carrying equal-but-opposite currents for the two sides of an
individual signal.  This minimizes electrical coupling between pairs,
and thus minimizes crosstalk.

If you use a telco-style cable (these are often black, and flat), or if
you use what looks like an Ethernet cable but which had its wires
"punched down" to the connector in the wrong pairing, things go very
badly indeed.  One twisted pair might be carrying one TX signal and one
RX signal.  This pretty much *guarantees* terrible cross-talk between
the two.

The symptoms of this can be as was related... the connection appears to
work OK under light load, when there's usually traffic flowing in only
one direction at a time.  However, when you put a bidirectional load on
the connection, the signals going from A to B and from B to A cross-talk
with one another, leading to a very high rate of corrupted/dropped
packets on the network.

This will often show up in the end device's Ethernet packet statistics,
if you can get to them... look for a high rate of dropped or "bad"
packets, FCS (frame sequence check) errors, etc.

I've seen a fair number of cheap "Ethernet" cables that had been
manufactured wrong.  You should see a color pairing such as


indicates - pins 4 and 5 are a pair (blue, and white-and-blue), and the
next-outer pins are also a pair (orange, and white-with-orange).

If you see a pattern such as "white-with-green, green, white-with-blue,
blue, white-with-orange, orange, white-with-brown, brown" where there
are four color-matched pairs of wires next to one another, you've got a
bad cable.

The same error can occur when building wiring is "punched down" to the
RJ-45 jacks.

A good Ethernet cable-pair tester can spot such things pretty quickly.

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