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Configure IP Networks for Real-Time Voice Traffic
Configure RTP Header Compression
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Real-Time Transport Protocol (RTP) is used for carrying packetized audio traffic over an IP network.
RTP header compression compresses the IP/UDP/RTP header in an RTP data packet from 40 bytes
to approximately 2 to 4 bytes (most of the time), as shown in Figure 2-1.
This compression feature is beneficial if you are running Voice over IP over slow links. Enabling
compression on both ends of a low-bandwidth serial link can greatly reduce the network overhead if
there is a lot of RTP traffic on that slow link.
Typically, an RTP packet has a payload of approximately 20 to 160 bytes for audio applications that
use compressed payloads. RTP header compression is especially beneficial when the RTP payload
size is small (for example, compressed audio payloads between 20 and 50 bytes).
Figure 2-1 RTP Header Compression
Before RTP header compression:
20 bytes 8 bytes 12 bytes
IP
Header
UDP RTP Payload
After RTP header compression:
IP/UDP/RTP header
2 to 4 bytes
20 to 160 bytes
Payload
20 to 160 bytes
You should configure RTP header compression if the following conditions exist in your network:
• Slow links
• Need to save bandwidth
Note RTP header compression should not be used on links greater than 2 Mbps.
Perform the following tasks to configure RTP header compression for Voice over IP. The first task is
required; the second task is optional.
• Enable RTP Header Compression on a Serial Interface
• Change the Number of Header Compression Connections
Voice over IP for the Cisco 3600 Series Software Configuration Guide
12076