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Table 76: Test call (20ms-packets) results
compression. Note that these are rough measurements.
Table 76: Test call (20ms-packets) results
Codec
G.711
(64 kbps)
G.729A
(8 kbps)
G.723.1
(5.3 kbps)
G.723.1
(6.3 kbps)
For each codec, there was an attempt to verify that the audio packets were received intact. This
was done by spot checking the audio packets before and after compression, using two Sniffer
protocol analyzers. For every codec except G.711, the RTP header and payload were identical
before and after compression. With G.711, however, the received packets had the PADDING
flag set in the RTP header, although the flag was not set when the packets were transmitted.
The PADDING flag indicates the presence of padding octets at the end of the RTP payload,
which cannot be true for G.711.
Configuration
To configure RTP header compression on a Cisco router:
1. Specify the number of RTP connections that can be compressed (cache allocation). In
interface configuration mode, the command is ip rtp compression-connections
<number>, where
The default for <number> is 32, and each call requires two connections.
The configurable range is 3 to 256 for PPP and HDLC using IOS v11.3 and later.
The configurable range is 3 to 1000 for PPP and HDLC using IOS v12.0(7)T and later.
For Frame Relay, the value is fixed at 256.
Payload
Packets
bytes
per
per packet
second
160
50
20
50
20
33
24
33
shows the results with and without RTP header
Avg WAN BW
consumption (kbps)
without
with
compressi
compressi
on
on
84
68.5
27.5
13
18
9
19
10
%
reduction
~18%
~53%
~50%
~47%
Issue 3.4.1 June 2005
RTP
327
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