2.4 Options to Reduce WAN Bandwidth

This topic describes solutions to bandwidth limitations in multisite deployments.

You can conserve bandwidth in the IP WAN in these ways:

  • Use low-bandwidth codecs over the IP WAN.
    1. For devices requiring G.711, deploy transcoders.
    2. For remote conference participants, deploy mixed conference bridges or transcoders.
  • Use RTP header compression.
  • Deploy local annunciators or disable remote annunciators.
  • Deploy local conference bridges.
  • Deploy local MTPs (if required).
  • Deploy transcoders or mixed conference bridges (if needed).
  • Deploy local MOH server or use multicast MOH from branch router flash.
  • Limit the number of calls by using CAC.

You can conserve bandwidth on the IP WAN by using these methods:

  • Use low-bandwidth codecs: When you use low-bandwidth (compression) codecs, such as G.729, the required bandwidth for digitized voice is 8 kbps, compared to the 64 kbps that is required by G.711 (Layer 2 overhead not considered).
  • Use RTP header compression: When using RTPheader compression (cRTP), you can compress the IP, UDP, and RTP header to 2 or 4 B (depending on whether the UDP checksum is preserved), compared to the 40 B that are required by these headers if cRTP is not used. cRTP is enabled per link. It can be selectively used on a slow WAN link (in general, below 768 kbps) and does not need to be enabled end-to-end across all WAN links.
  • Deploy local annunciators or disable remote annunciators: If spoken announcements are not required, you can disable the use of annunciators for IP phones that do not have a local annunciator. Otherwise, you can deploy local annunciators. Cisco Unified Communications Manager supports annunciators that run only on Cisco Unified Communications Manager servers (provided by the Cisco IP Voice Media Streaming Application service). Therefore, you can implement local annunciators only if you deploy a local Cisco Unified Communications Manager cluster or if you are using clustering over the IP WAN.
  • Deploy local conference bridges: If you deploy local conference bridges, the IP WAN is not used if all conference members are at the same site as the conference bridge.
  • Deploy local MTPs: If MTPs are required, you can deploy them locally at each site to avoid the need to cross the IP WAN when using MTP services.
  • Deploy transcoders or mixed conference bridges: If low-bandwidth codecs are not supported by all endpoints, you can use transcoders so that low-bandwidth codecs can be used across the IP WAN. Then, have the voice stream transcoded to G.711. For conferences with local members using G.711 and remote members using low-bandwidth codecs, you can deploy mixed conference bridges (hardware only) that support members with various codecs.
  • Deploy local MOH servers or use multicast MOH from branch router flash: Deploying local MOH servers means that Cisco Unified Communications Manager servers must be present at each site. In centralized call-processing models, in which this requirement does not apply, it is recommended that you use multicast MOH from branch router flash. This approach eliminates the need of streaming MOH over the IP WAN. If this approach is not an option, you should use multicast MOH instead of unicast MOH to reduce the number of MOH streams that must traverse the IP WAN. Multicast routing should be enabled in the network in order for multicast MOH to correctly function.
  • Limit the number of voic