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RTP
Issue 6 January 2008 329
It is important to understand that all 20-ms G.729 audio packets, regardless of the vendor, are
constructed like this. Not only is the structure of the packet the same, but the method of
encoding and decoding the audio itself is also the same. This sameness is what allows an
Avaya IP Telephone to communicate directly with a Cisco IP Telephone, or any other IP
Telephone, when using matching codecs. The packets from the application perspective are
identical.
Network perspective
RTP header compression is a mechanism that routers use to reduce the 40 bytes of protocol
overhead to approximately 2 to 4 bytes. Cisco routers use this mechanism, as does the Avaya
X330WAN router, which is a module for the P330 chassis. RTP header compression can
drastically reduce the IP Telephony bandwidth consumption on a WAN link when using 20-ms
G.729 audio. When the combined 40-byte header is reduced to 4 bytes, the total IP packet size
is reduced by 60% (from 60 bytes to 24 bytes). This equates to reducing the total IP Telephony
WAN bandwidth consumption by roughly half, and it applies to all 20-ms G.729 audio packets,
regardless of the vendor.
Recommendations for RTP header compression
Enterprises that deploy routers that are capable of this feature might be able to benefit from it.
However, Cisco recommends caution in using RTP header compression on its routers because
it can significantly tax the processor if the compression is done in software. Depending on the
processor load before compression, enabling RTP header compression can significantly slow
down the router, or cause the router to stop completely. For best results, use a hardware/IOS/
interface module combination that permits the compression to be done in hardware.
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.
Table 56: Anatomy of 20-ms G.729 audio packet
IP header UDP header RTP header 20 ms of G.729 audio
20 B 8 B 12 B 20 B