Specifications

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Cisco 1751 Router Software Configuration Guide
OL-1070-01
Configure RTP Header Compression
Real-Time Transport Protocol (RTP) is used for carrying audio traffic in packets 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 1.
This compression feature is beneficial if you are running VoIP 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 1 RTP Header Compression
You should configure RTP header compression if the following conditions describe your network:
• Links slower than 2 Mbps
• Need to save bandwidth
Note Do not use RTP header compression on links greater than 2 Mbps.
Perform the following tasks to configure RTP header compression for VoIP. 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
Before RTP header compression:
20 bytes 8 bytes
20 to 160 bytes
12 bytes
IP
Header
UDP
RTP Payload
After RTP header compression:
2 to 4 bytes
20 to 160 bytes
IP/UDP/RTP header
Payload
12076