Television online

And, since a huge amount of TV is still watched in real time, to make that part of the system more efficient, it can be delivered using a method known as multicast.

Multicast
On a normal TCP/IP network, information is sent from point to point, from your web browser to a server, for example, and then a stream of data is sent back again.

And if 20,000 people all decide to watch Eastenders as it’s broadcast, 20,000 copies of the same stream will be sent simultaneously, one to each user. It’s hardly efficient; and that’s where multicasting comes in.

There’s a range of internet addresses set aside for multicast use, which is a one-to-many connection, where a server can send out one stream and many people can connect to it, provided they’re on a network that supports multicasting. This means each bit of equipment between server and viewer needs to support it.

In the UK, the BBC has done trials with a few ISPs, but it’s not something you’ ll find as standard with a home broadband connection. And though it would potentially make online TV viewing more efficient, the problem of who’ll pay for upgrades once again raises its head.

Of course, video doesn’t have to be delivered at the same time to everyone ­ and many people think that as we become used to services such as Freeview Playback and Sky+, we’ll become more and more used to watching when we want to.

So IPTV also encompasses video-on-demand, usually using a protocol called RTSP, or Real Time Streaming Protocol, which allows a server to provide a list of available content that can be requested by a client. RTSP is already used by Apple’s Quicktime Player, Real Player and others, though in an IPTV network it’ s most likely to deliver MPEG2 or MPEG4/AVC, as the main broadcast digital TV streams do.

The final component of an IPTV system is the set-top box, which provides a programme guide, allowing users to select one of the live streams, or request a connection to the server holding video-on-demand content.

The network matters
That all sounds fairly straightforward ­ and in theory, it’s not too difficult. You can, for instance, set up your own in-house video-on-demand server, using tools such as VLC or Apple’s Quicktime Streaming Server, and then connect to those streams, using a web browser, a client such as VLC, Quicktime Player or even a media streamer, if it supports the RTSP protocol.

But for effective IPTV, the network is crucial, which is why many IPTV systems in use are based around their own network. Not only does creating a network for IPTV mean you can design it for multicast, if you want to provide live TV, but you can ensure there’s always enough bandwidth.

As anyone who’s tried to watch TV streamed over the internet knows, if things are congested, it can be a stop-start experience. That might be fine for catching up on something you missed, but not if it’s your only way of watching TV.

To provide quality TV via the internet, you need to be able to guarantee bandwidth. Although the current version of the Internet Protocol, IPv4, doesn’t have a built-in Quality of Service mechanism, you can protect certain functions, which is how ISPs can slow Bittorrent while not affecting web browsing.

It’s also how ISPs that offer TV make sure it works; with BT’s Vision, your file download speed can drop considerably when TV is being watched via the set-top box, while at its introduction the old Home Choice TV service ­ now owned by Tiscali ­ reserved a small amount of the broadband capacity for data, and kept the rest for TV.