Don't forget to use multicast

As most people deploy VOIP and IP Telephony, they are primarily concerned about making sure that the system they choose will work well with their network and their existing voice infrastructure. This is easy to understand, as QoS and PBX integration are two of the biggest gotchas. One technology that is often overlooked, however, is multicast.

Multicast, of course, is not necessary for the majority of VOIP calls because these calls are "point-to-point", i.e. there is only a single source and single destination, so there's no point in multicast. You might guess that conference calls could use multicast, but actually these have to use special bridging software or hardware that sets up individual calls between all the users and then combines the signals from each call, rather than having all participants multicast their voices to all other participants and then requiring their handsets to mix all the streams together.

So it's easy to see why multicast gets overlooked. But it does have an important application in modern IP Telephony networks, and that is for Music On Hold or (MOH). Typically, you will have an MOH server, which sends a one-way stream of music to everyone on hold. Users on hold do not need to manipulate this stream, or synchronize (that is, it's ok if a user gets put on hold during the middle of a song; you don't expect to start a new song for each user), or send any voice back to it.

Thus, MOH is a perfect candidate or multicast.

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Unfortunately, as I mentioned, it's often overlooked as it is not terribly important. By default, most MOH servers are configured to unicast a stream of music to each user, but most MOH servers also support multicast, where only a single stream is sent, and all users on hold simply join the multicast group. Fortunately, most MOH servers are capable of sending a large number of streams across a LAN, usually enough to handle all the people on hold at a given time, but you can still save yourself some bandwidth, particularly on WANs by implementing multicast.

Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.

This was first published in August 2003

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