IPTV Multicasting Explained
Internet Protocol (IP) multicast is a bandwidth-conserving mechanism for reducing data network traffic by simultaneously delivering a single stream of information to thousands of recipients. Multicasting is fundamental to the implementation of IPTV. This is how it works.
In diagram 1, none of the network switches have IGMP snooping or querying turned on and so the network is not multicast enabled. The backbone switch has all the streaming traffic coming into it from the MPEG IP Encoders. If we assume each stream is 4 Mbps the backbone switch will carry 4 Mbps x 3 streams = 12 Mbps of streaming traffic.
As the switches don’t have IGMP turned on, the streaming traffic will flood the entire network regardless of whether the user requests a particular stream or not.
In diagram 2, all switches have IGMP snooping turned on and the backbone switch has IGMP query turned on. Again, the backbone switch has a total of 12 Mbps of streaming traffic coming into it from the MPEG IP Encoders.
Switch 1 will only have 8 Mbps of traffic reaching it as a result of the two users who have requested the same red stream (4 Mbps) and a third user who has selected the green stream (4 Mbps). This is the distinct advantage of multicasting - the bandwidth used is per stream and not per user.
Switch 2 will only have 4 Mbps of traffic reaching it since only the blue stream has been requested by a user. There will be no streaming traffic on the port where there is no request for a stream.
Switch 3 will have no streaming traffic since none of the users connected to the switch have requested any streams.
In calculating the bandwidth requirement for IPTV, it should be assumed that all streams will be present on the backbone but, at most, only one stream will be present on a user port.