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Quality of Service guidelines
RTP header compression has to function with exactness or it will disrupt audio. If for any reason
the compression at one end of the WAN link and decompression at the other end do not
function properly, the result can be intermittent loss of audio or one-way audio. This has been
very difficult to quantify, but there is some anecdotal evidence that cRTP sometimes leads to
voice-quality issues. One production site in particular experienced intermittent one-way audio,
the cause of which was garbled RTP audio samples inserted by the cRTP device. When, for
experimentation purposes, RTP header compression was disabled, the audio problems went
away.
The test
This section details the results of a simple RTP header compression test that was conducted in
a laboratory environment. Although this test was conducted using Cisco routers, the expected
behavior is the same for any router that performs this function as specified in RFC 2508. This
test was performed in the laboratory configuration that is shown in
Figure 84: Equipment configuration for RTP header compression test
In
Figure 84: Equipment configuration for RTP header compression
NetIQ Chariot v4.0 was used to simulate IP Telephony calls between the two endpoints.
Chariot v4.0 accurately simulates the characteristics of various codecs, and uses a
40-byte IP/UDP/RTP header.
Sniffer Pro v3.50.02 was used to capture the sent and received packets.
The Cisco 3600 had IOS v12.1(2)T, and the Cisco 1600 had IOS v12.0(12).
The Frederick Engineering Parascope WAN probe was tapped into the V.35 serial link to
take bandwidth measurements.
This test was performed using PPP encapsulation on the WAN link.
A single call was placed between the Chariot endpoints using various codecs, all sending 20-ms
voice packets.
326 Avaya Application Solutions IP Telephony Deployment Guide
Figure
84.
test:
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