How does IPTV work?

IPTV protocolsWhen you stream a program, you're not downloading it like an ordinary file. Instead, you're downloading a bit of a file, playing it, and, while it's playing, simultaneously downloading the next part of the file ready to play in a moment or two. None of the file is stored for very long. Streaming works because your computer (the client) and the computer it's receiving data from (the server) have both agreed to do things like this. The Internet successfully links practically all the world's computers because they all agree to talk to one another in the same way using prearranged technical procedures called protocols. Instead of using the ordinary, standard, web-based protocols for downloading (technically, they go by the names HTTP and FTP), streaming involves using protocols adapted for simultaneous downloading and playing, such as RTP (Real-Time Protocol) and RTSP (Real-Time Streaming Protocol). Multicast streaming involves using IGMP (IP Group Management Protocol; you'll occasionally see books and web pages replacing the M with "Membership"), which allows one server to broadcast to members of a group of clients (effectively, lots of people all watching the same TV channel).

Managed networksMaking IPTV available over the public Internet is very different from delivering it over a private, managed network, which is what many IPTV providers will ultimately elect to do: by controlling the entire network, they can guarantee a level of quality and service. In practice, this means having a highly organized, hierarchical network with a national office known as a super head-end (SHE, where programs are stored and the entire service is coordinated) feeding into regional hubs called video hub offices (VHOs) that, in turn, service local distribution offices linked to set-top boxes in individual homes.

Viewing programsEveryone who has a computer and a broadband Internet connection can watch IPTV, but most of us don't want to watch television on a crude laptop screen. That's why the future of IPTV is likely to involve viewers buying set-top boxes (sometimes called STBs) that receive input from your Internet connection (either via an Ethernet cable or Wi-Fi), decode the signal, and display a picture on your high-definition, widescreen TV. STBs are effectively standalone computers programmed to do only one thing: receive packets of streamed video, decrypt them, convert them back to video files (MPEG2, MPEG4, or whatever format they were in originally), and then display them as high-quality TV pictures. Apple TV works broadly this way, using a set-top box to run simple apps on a slimmed-down operating system (tvOS), which manages the process of streaming video via the Internet.

A black dongle held in the palm of a hand.

Photo: A typical dongle. It turns your existing TV into a streaming device, connecting it to the Internet via Wi-Fi and HDMI.

As a simple, more compact, and much more discreet alternative to a set-top box, you can use what's called a dongle, which looks a bit like a USB flash-memory stick, but allows secure access to Internet TV programs. The dongle plugs into an HDMI (high-speed, high-definition digital video) socket on your TV and connects via Wi-Fi to the Internet to stream TV programs, movies, and music directly. Some dongles are entirely self-sufficient: Roku and Amazon Fire work this way without any help from a computer or mobile device. Google's Chromecast is a little bit different: generally, you get it going with your computer, tablet, or smartphone (which effectively becomes a remote control), after which it directly streams your movie or TV program from the Internet.

With traditional TV, programs are broadcast by being turned into radio waves and beamed through the air to a rooftop antenna on your home. The antenna converts the waves back into electrical signals and your TV set decodes them to make its sound and picture (satellite TV works the same way, except the signal bounces into space and back, while cable TV sends the signal directly into your home without radio waves). How is TV Channels & VODs in 4K & Ultra HD different?

Storing programsLive programs are streamed as they're produced, but prerecorded programs and movies need to be stored in such a way that they can be selected and streamed on demand. Some VOD services limit the number of programs they make available not because they're short of storage space but because that's one way to limit the overall bandwidth of their service and its impact on the Internet. (For example, if the BBC made available every program it's ever produced on its iPlayer, which is free to use, a significant proportion of the entire UK Internet bandwidth would be taken up streaming TV soap operas and sitcoms, potentially slowing down the network for every other kind of Net traffic.)

Preparing programsFirst, the TV program (either prerecorded or captured live with a video camera) has to be converted into a digital format that can be delivered as packets using the Internet protocol. Sometimes the original program will be in digital format already; sometimes it will be in the form of a standard, analog TV picture (known as SD format) that needs an extra bit of processing (analog-to-digital conversion) to turn it into digital format. With current limitations on bandwidth, videos also need to be compressed (made into smaller files) so they can stream smoothly without buffering (periodic delays caused as the receiver builds up incoming packets). In practice, this means programs are encoded in either MPEG2 or MPEG4 format (MPEG4 is a newer form of video compression that gives higher quality for a similar bandwidth and requires only half as much bandwidth for carrying an SD picture as MPEG2). Once that's done, advertisements have to be inserted, and the information has to be encrypted.