2.7 Multicast MOH from Branch Router Flash

This topic describes how multicast MOH from branch router flash can reduce IP WAN bandwidth consumption.

Multicast MOH from branch router flash is a feature for multisite deployments with centralized call processing:

  • It works only with multicast MOH.
  • It is based on the MOH capabilities of SRST.
  • Cisco IOS SRST gateway is configured for multicast MOH.
    1. The MOH stream is sent independent of SRST status (even if no failover scenario).
  • Cisco Unified Communications Manager and IP phone are not aware that SRST gateway is involved:
    1. The IP phone is configured to use Cisco Unified Communications Manager multicast MOH server.
    2. The stream of the Cisco Unified Communications Manager MOH server is prevented from being sent over the IP WAN.
    3. The SRST router locally generates the multicast MOH stream.
    4. The IP phone plays the received stream.

Multicast MOH from branch router flash is a feature for multisite deployments that use centralized call processing.

The feature works only with multicast MOH and is based on MOH capabilities of Cisco Unified SRST. The Cisco IOS SRST gateway is configured for multicast MOH and continuously sends an MOH stream, regardless of its SRST mode (standby or fallback mode).

In fact, neither Cisco Unified Communications Manager nor the remote IP phones are aware that the Cisco Unified SRST gateway is involved. To them, it appears as though a multicast MOH stream has been generated by the Cisco Unified Communications Manager MOH server and has been received by the remote IP phones.

Therefore, the remote IP phones are configured to use the centralized Cisco Unified Communications Manager MOH server as their MOH source. The Cisco Unified Communications Manager MOH server is configured for multicast MOH (mandatory). In addition, you can ensure that the actual MOH traffic that is generated by the Cisco Unified Communications Manager does not enter the IP WAN.

The Cisco Unified Communications Manager MOH server and the Cisco IOS SRST gateway that is located at the remote site must use the same multicast address and port number for their streams. The SRST gateway permanently generates a multicast MOH stream with an identical multicast IP address and port number. The IP phone simply listens to this stream because it appears to be coming from the Cisco Unified Communications Manager MOH server.

When you use multicast MOH from branch router flash, G.711 must be enabled between the Cisco Unified Communications Manager MOH server and the remote IP phones. This action is necessary because the branch SRST MOH feature supports only G.711. Therefore, the stream that is set up by Cisco Unified Communications Manager in the signaling messages must also be G.711. Because the packets are not sent across the WAN, configuring the high-bandwidth G.711 codec is not a problem as long as it is enabled only for MOH. All other audio streams (such as calls between phones) that are sent over the WAN should use the low-bandwidth G.729codec.

Multicast MOH from Branch Router Flash Example

The figure illustrates how multicast MOH from branch router flash works.

In the figure, the Cisco Unified Communications Manager MOH server is configured for multicast MOH and therefore sends multicast MOH packets to the network. The multicast MOH packets are prevented from reaching the WAN interface that interconnects to the two sites.

The Cisco Unified SRST gateway that is located at the remote site is configured to send identical multicast MOH packets to the network of the remote site.

IP phones at the main site receive the multicast MOH packets that are generated by the Cisco Unified Communications Manager MOH server. IP phones at the remote site receive the multicast MOH packets that are generated by the Cisco Unified SRST gateway.

MOH packets never cross the IP WAN.

Alternatives to Multicast MOH from Branch Router Flash

If multicast MOH from branch router flash cannot be used (for instance, because the branch router does not support the feature or does not have a Cisco Unified SRST feature license), you can consider these alternatives:

  • Use multicast MOH: When using multicast MOH over the IP WAN, you can significantly reduce the number of required MOH streams. Less bandwidth is required, compared to multiple unicast MOH streams. The IP network, however, must support multicast routing for the path from the MOH server to the remote IP phones.
  • Use G.729 for MOH to remote sites: If multicast MOH is also not an option (for instance, because multicast routing cannot be enabled in the network), you may still be able to reduce the bandwidth that is consumed by MOH. When you change the codec that is used for the MOH streams to G.729 and you potentially enablecRTP on the IP WAN, each individual MOH stream requires less bandwidth and hence reduces the load on the WAN link. The bandwidth savings are identical to the bandwidth savings that you achieve when using G.729 and cRTP for standard audio streams. To use G.729 for MOH streams, you must put the MOH server and the remote IP phones into different regions, and you must limit the audio codec between these two regions to 8 kbps.