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To resolve the simulated incompatibility, remove the md5secret token from sip.conf
and attempt to place a call with the X-Lite softphone again.
Troubleshooting Quality-of-Service Issues
Not all troubleshooting situations involve signaling. As you may have read in prior
chapters, noise problems and dropouts stemming from jitter and packet loss are
cause for troubleshooting. Fortunately, the standards for sound transmission—RTP
and codecs—are simple compared to call signaling, so troubleshooting sound issues
is generally easier.
That’s not to say all sound-related issues are a snap to dismiss. Sound issues are
affected by many disciplines of networking: switching, routing, QoS, and packetiza-
tion. The way congestion on a routed link shows up is in dropouts, or in busy sig-
nals if you’re using a QoS solution. On a switched link, like Ethernet, busy signals
may occur long before dropouts are perceptible. So symptoms are different on differ-
ent links.
As with signaling, good diagnostic logging is valuable for troubleshooting. Packet
analysis is less useful when examining sound quality problems. One can certainly
identify jitter using packet capture, but it’s just easier to identify jitter across IP links
using tools like traceroute and pathping, which succinctly show the variances in
round-trip latency on every hop of a route. If you need to dig deeper, capture tools
like Ethereal and its commercial equivalents can assist you in isolating jitter at the
RTP level.
Commercial packet analysis tools
If firewalls or other filters block ICMP or TCP traffic along the call path you’re trou-
bleshooting, then you may use a packet capture tool with advanced VoIP capabili-
ties, like Network General’s ubiquitous Sniffer or WildPacket’s EtherPeek VX. Both
permit analysis of signaling and media packets, readily understand the structure of
RTP, can identify jitter, and can reveal sources of congestion on your network.
When the going gets tough, call in the end users
The human ear is a great instrument for troubleshooting. You can carry an IP phone
from one server closet to the next, listening to calls to the same destination from dif-
ferent points along the call path. This process of elimination will help you narrow
down a group of potential congestion points until you’ve identified the one that’s to
blame. On large, expansive networks, where this can’t be done by a single person,
you could enlist help from other people within the organization. Even end users are
pretty reliable when it comes to assisting with this type of troubleshooting. Ask them
to make a call that you expect to traverse the call path you’re looking at, and then
ask them how it sounds or if they get a busy signal. Both can be indicators of conges-
tion, though a busy signal will happen only in networks with QoS.

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