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How Video On Demand Works (long and technical!)
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Old 08-01-2005, 15:17   #1
Tristan's Avatar
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How Video On Demand Works (long and technical!)

With the software required for VOD now being rolled out in Glasgow, it seems Digitalspy's rumours of a January launch on NTL are quite possibly accurate.

With this in mind, there's been some speculation about how VOD works, whether it will have the same facilities as Sky Plus, what sort of picture quality we can expect, etc etc. So I thought I should spend a little time explaining things

At the moment, the Front Row service operates using what's called "Near Video On Demand", or NVOD. What this means is that there are many video streams coming to your house, each showing the same movie with staggered start times. When you order a film, the software selects the appropriate stream and shows it on channel 0. (On NTL Bromley and Telewest, this has always been invisible to the end user. Langley users will remember having to tune to channels in the 200s for Front Row, prior to the CR3 update.) This is also the way Sky Box Office operates, and how the Sky Movies "multistart" function works (there is more digital bandwidth available on satellite than cable, and so it's relatively easy for Sky to use 10 or so different streams).

True Video On Demand works rather differently. Rather than receiving scores of video streams, and selecting the appropriate one, the box takes notice of just one stream. The difference is that this stream is for you, and only you. So you can pause, rewind, fast-forward and stop playback, all without affecting anyone else. How is this possible?

Well the first thing to point out is that, contrary to what some people have suggested, the box is not downloading any video content via the in-built cable modem. I don't even know whether this is possible, and it certainly wouldn't be very efficient. So forget scare stories about web-quality pictures, because it's completely untrue. Your box will be displaying a multiplexed DVB MPEG2 steam, the same as it does at the moment.

VOD works its magic by changing the source of the video. At the moment, all the steams are prepared at NTL's DTV HQ (try saying that when you're drunk) in Langley, from where they're forwarded on to your local "headend", and then on to your house. In this way, every house in the country receives the same video streams. VOD changes this by housing the programming on servers at your local headend, instead of centrally. This means that it becomes possible for your box to send "rewind" commands (for example) to the server, which will then start playing the stream backwards.

The eagle-eyed amongst you will have spotted a problem here however. Say there are 100 Front Row streams at the moment (I don't know the exact number, but it's a ballpark figure). If you replace this by local content at the same quality, you've still only got room for 100 streams. This isn't enough for the 10,000 subscribers who might be connected to a particular headend (I've no idea how accurate this figure is!) to each have their own stream. So how do you do it?

The answer is by using the "localness" of cable. For the greater part of its journey, all data travels around on fibre-optic connections. It's only in the last few hundred metres -- at the local cabinet level -- that this is transferred to coaxial (copper) cable and into your house. If you can then send different data to different street cabinets, then you can re-use the same frequencies on the coax cable in different areas. I believe this is already done with cable modem connections.

In this respect, it's possible to share the 100 "spaces" you've got between relatively few properties. How many you share it between will be a tradeoff. The lower the figure, the more equipment you need, and the more it costs to set up. The higher the figure, the greater the contention, and the greater the risk that there won't be a stream free when someone wants to use the service.

So that's how it works. But what will it do? Well, the first and most obvious application is to replace the NVOD Front Row system that's currently used. That's a no-brainer. But past that, you're really only limited by server disk space. A survey published last year suggested that VOD will offer a large library of old and not-so-old films, probably as a rival to Sky Movies. There are also things like classic programmes and sports. On the existing (DSL-based) VOD systems operating in the UK, Homechoice and KIT, the BBC have offered a large number of programmes -- so if you missed last week's Eastenders, you can watch it again on VOD. Channel 4 have a similar deal with Homechoice. As the first large-scale VOD deployment in the country, I suspect the On Demand Group will have been trying to make deals left, right and centre. We'll have to wait and see what they come up with though.

People have been wondering whether VOD will be cable's version of Sky Plus. It's true that some of the functionality doubles up, but really they're quite different beasts. For example, the only way you can see last week's Eastenders on Sky Plus is if your box recorded it in the first place. Likewise, pausing and rewinding films etc requires that these things are on your box's hard drive. Sky Plus also requires a new STB, whereas VOD will work on any cable box with a working return path. On the other hand, VOD will not allow subscribers to pause live TV, nor plan for future future viewing (though it's technically possible to do both these things on VOD, setting aside 80GB of disk space per subscriber is a very big ask). Both NTL and Telewest are said to be working on PVR devices, so it will be interesting to see how these work in relation to VOD.

Any questions, please ask. Hope this is of some use,


PS: Usual disclaimers apply. I don't work for NTL, Telewest, the On Demand Group, Sky, Homechoice, KIT, or anybody else for that matter. I have no qualifications in the subject. I could be completely, massively wrong.
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