Here is the breakdown (Multicast causes less Network Congestion and is easier on the MOH Server) :)
Unicast and Multicast MoH
Unified CM supports two types of MoH transport mechanisms:
Unicast MoH consists of streams sent directly from the MoH server to the endpoint requesting an MoH audio stream. A unicast MoH stream is a point-to-point, one-way audio Real-Time Transport Protocol (RTP) stream between the server and the endpoint device. Unicast MoH uses a separate source stream for each user or connection. As more endpoint devices go on hold via a user or network event, the number of MoH streams increases. Thus, if twenty devices are on hold, then twenty streams of RTP traffic are generated over the network between the server and these endpoint devices. These additional MoH streams can potentially have a negative effect on network throughput and bandwidth. However, unicast MoH can be extremely useful in those networks where multicast is not enabled or where devices are not capable of multicast, thereby still allowing an administrator to take advantage of the MoH feature.
Multicast MoH consists of streams sent from the MoH server to a multicast group IP address that endpoints requesting an MoH audio stream can join as needed. A multicast MoH stream is a point-to-multipoint, one-way audio RTP stream between the MoH server and the multicast group IP address. Multicast music on hold conserves system resources and bandwidth because it enables multiple users to use the same audio source stream to provide music on hold. Thus, if twenty devices are on hold, then potentially only a single stream of RTP traffic is generated over the network. For this reason, multicast is an extremely attractive technology for the deployment of a service such as MoH because it greatly reduces the CPU impact on the source device and also greatly reduces the bandwidth consumption for delivery over common paths. However, multicast MoH can be problematic in situations where a network is not enabled for multicast or where the endpoint devices are not capable of handling multicast.
For information about IP multicast network design, refer to the IP Multicast SRND document, available online at
In some cases, administrators might want to configure a single Unified CM cluster to handle both unicast and multicast MoH streams. This configuration might be necessary because the telephony network contains devices or endpoint that do not support multicast or because some portions of the network are not enabled for multicast.
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