Up to now, Internet streams have been distributed using 'unicast', which consists of each user receiving their own individual stream. The bandwidth required for Internet streams by broadcasters and ISPs can be calculated as follows:

Unicast bandwidth = stream bandwidth  x  number of users


Unicast therefore scales very badly when there are a high number of users, because the amount of bandwidth to deliver even a standard-definition TV channel to broadcast-sized audiences would be unfeasibly large. Multicast solves unicast's bandwidth problem for live Internet streams by only requiring that a single stream of each channel be carried on an Internet link instead of potentially thousands or tens of thousands — so the percentage bandwidth saving compared to unicast is 99% once there are 100 users (1 stream instead of 100 streams), and the percentage bandwidth saving continues to increase the more users there are.

Multicast therefore allows broadcasters and ISPs to carry high bandwidth broadcast streams to an unlimited number of users, which makes it possible to deliver live HDTV channels, high quality digital radio streams and SDTV channels. Multicast is the key technology that will be used to deliver live IPTV channels to users, and five out of the big six UK broadband providers all have plans or are already providing IPTV services to their subscribers.