At the local exchanges the TV video streams are forwarded to the end users after the users have been successfully authenticated. The local exchange also deals with any channel changes and also provides the hardware and software required to provide VoD (Video on Demand) services and also any local advertising. The video streams that arrive at the local exchange from the headend are multicast streams, with a single multicast IP Address identifying each individual TV channel stream. As local DSL loop will have restricted bandwidth to each user, normally only a single multicast TV channel is forwarded to each user via the DSLAM (DSL Access Multiplexer), bearing in mind that bandwidth reduces according to the length of the cable from local exchange to individual user.
A protocol called IGMP (Internet Group Message Protocol) is used between the local user STB and the exchange equipment. Anytime a user selects a channel on their remote control, an IGMP message is sent from the STB to the exchange equipment indicating a requirement for a particular multicast stream. Authorization for the stream is verified and that user is added to the list of members for that particular channel and the relevant stream is forwarded over the DSL line. Between 1 and 4 individual streams are forwarded to the consumer STB depending on bandwidth available on the DSL local loop.
Multicasting and use of the IGMP protocol is really only appropriate for broadcast TV channels, known as Linear TV, when it comes to Video on Demand or Catch-Up TV where users demand that video content is delivered at a time convenient for them, other procedures and protocols are required. At the local exchange a VoD server can deliver specific Unicast IP streams containing the relevant content to each individual user. The most common protocol used to effect this is RTSP (Real Time Streaming Protocol). The local user has a video recorder or DVD recorder control over the content stream, enabling play, pause stop and rewind facilities, and it is RTSP that allows this control.
SDTV channels consume between 1 and 2Mbps of bandwidth on the local loop depending on the compression protocol used, and in fact if MPEG-2 is used then around 3.5Mbps is needed for a single channel. HDTV will consume up to 8Mbps utilizing H.264 or Windows Media formats, with MPEG-2 consuming up to 20Mbps of bandwidth. Due to the fact that users are often watching one channel while recording another, it is important that the DSL line can support the forwarding of multiple channel streams to the STB.