RTP
Issue 3.4.1 June 2005
325
Application perspective
Table 75: Anatomy of 20-ms G.729 audio packet
shows the anatomy of a 20-ms G.729 audio
packet, which is recommended for use across limited bandwidth WAN links. Notice that
two-thirds of the packet is consumed by overhead (IP, UDP, and RTP), and only one-third is
used by the actual audio.
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.
Table 75: 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
Summary of Contents for Application Solutions
Page 1: ...Avaya Application Solutions IP Telephony Deployment Guide 555 245 600 Issue 3 4 1 June 2005 ...
Page 20: ...About This Book 20 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 21: ...Issue 3 4 1 June 2005 21 Section 1 Avaya Application Solutions product guide ...
Page 22: ...22 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 106: ...Call processing 106 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 124: ...Avaya LAN switching products 124 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 139: ...Issue 3 4 1 June 2005 139 Section 2 Deploying IP Telephony ...
Page 140: ...140 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 186: ...Traffic engineering 186 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 204: ...Security 204 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 228: ...Avaya Integrated Management 228 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 274: ...Reliability and Recovery 274 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 275: ...Issue 3 4 1 June 2005 275 Section 3 Getting the IP network ready for telephony ...
Page 276: ...276 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 356: ...Network recovery 356 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 366: ...Network assessment offer 366 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 367: ...Issue 3 4 1 June 2005 367 Appendixes ...
Page 368: ...Appendixes 368 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 394: ...Access list 394 Avaya Application Solutions IP Telephony Deployment Guide ...
Page 414: ...DHCP TFTP 414 Avaya Application Solutions IP Telephony Deployment Guide ...