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Cisco IP Telephony Troubleshooting Guide for Cisco CallManager Release 3.0(1)
levels (start with 50% of the maximum audio level). Most of the time, the problems will occur
when attaching to the PSTN by way of a digital or analog gateway. Cisco IP Phone users may
complain that they hear their own voice being reflected back to them. Now, although the true
source of the problem is almost always at the far end, it is nearly always impossible to change
anything in the PSTN. So the first step is to determine which gateway is being used. If a digital
gateway is in use, then it may be possible to add additional padding in the transmit direction
(towards the PSTN), in the hopes that the lower signal strength will yield less reflected energy.
Additionally, you can adjust the receive level so that any reflected audio is reduced even further.
It is very important to remember to make small adjustments at a time. Too much attenuation of
the signal will make the audio impossible to hear on both sides. Alternatively, you can contact
the carrier and request to have the lines checked. On a typical T1/PRI circuit in North America,
the input signal should be –15 dB. If the signal level is much higher (-5 dB, for example), then
echo will be the likely result.
A log should be kept of all calls that experience echo. The time of the problem, the source phone
number, and the number called should all be recorded. Gateways have a fixed time of 16 ms of
echo cancellation. If the delay in the reflected audio is longer than this, the echo chancellor will
be unable to work properly. This should not be an issue for local calls, and long distance calls
should have external echo chancellors built into the network at the Central Office. This is one of
the reasons why it is important to note the external phone number of a call that experiences echo.
Check Your Loads
Gateway and phone loads should be verified. Check CCO (Cisco Connection Online at
www.cisco.com) for the latest software loads, new patches, or release notes relating to the
problem.
One-Way Audio or No Audio
One-way audio occurs when one person cannot hear another person during a call. This can be
caused by an improperly configured Cisco IOS Gateway, a firewall, or a routing or default
gateway problem, among other things.
There are a number of causes for one-way audio or no audio during a call. The most common
cause is an improperly configured device. For instance, Cisco CallManager handles the call setup
for a Cisco IP Phone. The actual audio stream occurs between the two Cisco IP Phones (or
between the Cisco IP Phone and a gateway). So it is entirely possible that the Cisco CallManager
is able to signal to a destination phone (making it ring) when the phone originating the call does
not have an IP route to the destination phone. A common cause for this is when the default
gateway in the phone is improperly configured manually or on the DHCP server.
If a call consistently has one-way audio, take a PC that is on the same subnet as the phone and
has the same default gateway and try to ping the destination Cisco IP Phone. Then take a PC that
is on the same subnet as the destination phone (with the same default gateway as the destination
phone) and ping the source phone. Both of those tests should work. Other things can affect the
audio traffic include a firewall or packet filter (such as access lists on a router) that may be
blocking the audio in one or both directions. If the one-way audio occurs only through a voice-
© 2000 Cisco Systems, Inc.
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