2.5 Low-Bandwidth Codecs and RTP Header Compression

This topic describes how low-bandwidth codecs and RTP header compression can reduce IP WAN bandwidth consumption.

The use of a low-bandwidth codec and cRTP reduces the bandwidth requirements of a call on a WAN link.

The figure illustrates the effect of using RTP header compression to conserve bandwidth on the IP WAN.

In the example, a voice packet for a call that has default settings (G.711 codec and a 20-ms packetization period) is being passed along a Frame Relay link. The frame has a total size of 206 B, comprising 6 B of Frame Relay header, 20 B of IP header, 8 B of UDP header, 12 B of RTP header, and 160 B of digitized voice. The packet rate is 50 pps, resulting in a bandwidth need of 82.4 kbps.

When you use cRTP and change the codec to G.729, the required bandwidth changes as follows: The frame now has a total size of 28 or 30 B per frame, comprising 6 B of Frame Relay header, 2 or 4 B of cRTP header (depending on whether the UDP checksum is preserved), and 20 B of digitized, compressed voice. The packet rate is still 50 pps (because the packetization period was not changed), resulting in bandwidth needs of 11.2 or 12 kbps.

Seven G.729 calls with cRTP enabled require less bandwidth than one G.711 call without cRTP (assuming that cRTP is used without preserving the UDP checksum).

Note

While the audio codec configuration affects the end-to-end path, cRTP only affects WAN links where cRTP is enabled. RTP header compression is configured on a per-link basis.

Codec Configuration in Cisco Unified Communications Manager

The codec that is used for a call is determined by the region configuration in Cisco Unified Communications Manager.

The codec that is used depends on the region configuration in Cisco Unified Communications Manager:

  • Each region is configured with a maximum audio bandwidth per call:
    1. Within the configured region
    2. Toward other specific regions (manually added)
    3. Toward all other regions (which have not been manually added)
  • Each region is assigned to a device pool.
  • A device pool is assigned to each device.
  • The codec that is actually used depends on the capabilities of the two devices and on the configured codec preference list.
  • If devices cannot agree on a common codec, a transcoding device is required.

Each region in Cisco Unified Communications Manager is configured with the maximum audio bandwidth requirements to be used per call:

  • Within the configured region
  • Toward a specific other region (manually configured)
  • Toward all other regions (not manually configured)

Regions are assigned to device pools (one region per device pool), and a device pool is assigned to each device. The codec that is actually used depends on the capabilities of the two devices and on the configured codec preference list. The assigned codec is the first codec of the applicable codec preference list that is supported by both devices and does not exceed the permitted bandwidth requirements. If devices cannot agree on a common codec, a transcoder is invoked.