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How Video On Demand Works (long and technical!)
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Old 08-01-2005, 15:17   #1
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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 invisable 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 steams, and selecting the appropriate one, the box takes notice of just one stream. The difference is that this steam 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 transfered 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 beleive 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 subscibers 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,

Tristan

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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Old 08-01-2005, 15:29   #2
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Re: How Video On Demand Works (long and technical!)

Quote:
Originally Posted by Tristan
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.
Very very accurate - Ntl's chief executive, Simon Duffy told us, VOD was launching in January.

http://www.cableforum.co.uk/article/...ary-vod-launch

But can I just say if what you say is right on how it all works - this is one hell of a post and I would just like to personally thank you Tristan for taking the time to post it up. Thread stuck.
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Old 08-01-2005, 15:30   #3
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Re: How Video On Demand Works (long and technical!)

Quote:
Originally Posted by Mick
But can I just say if what you say is right on how it all works - this is one hell of a post and I would just like to personally thank you Tristan for taking the time to post it up.
Hear! Hear!
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Old 08-01-2005, 15:37   #4
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Re: How Video On Demand Works (long and technical!)

I'll second that - nice explanation, Tristan - just a shame I've got to spread the love before repping you again!
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Old 08-01-2005, 16:18   #5
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Re: How Video On Demand Works (long and technical!)

Quote:
Originally Posted by Mick
Very very accurate - Ntl's chief executive, Simon Duffy told us, VOD was launching in January.

http://www.cableforum.co.uk/article/...ary-vod-launch

But can I just say if what you say is right on how it all works - this is one hell of a post and I would just like to personally thank you Tristan for taking the time to post it up. Thread stuck.
Can someone please green rep Tristan for me
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Old 08-01-2005, 16:48   #6
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Re: How Video On Demand Works (long and technical!)

done
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Old 08-01-2005, 17:19   #7
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Re: How Video On Demand Works (long and technical!)

repped as well, but you say it is getting deployed in glasgow, does this mean we will all get a wee booklet again to explain what is happening?

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Old 08-01-2005, 17:27   #8
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Re: How Video On Demand Works (long and technical!)

Excellent post.... pos rep given
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Old 09-01-2005, 12:51   #9
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Re: How Video On Demand Works (long and technical!)

Thanks Tristan
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Old 09-01-2005, 14:01   #10
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Re: How Video On Demand Works (long and technical!)

Another excellent post Tristan. Your contributions to the DTV part of these forums in particular are immense and are greatly appreciated. Just don't go leaving like proppin
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Old 09-01-2005, 16:23   #11
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Re: How Video On Demand Works (long and technical!)

Quote:
Originally Posted by Tristan
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 transfered 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 beleive this is already done with cable modem connections.

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

Tristan

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.
Thats a very good description Tristan, I don't work for ntl either but would just like to add that the VOD kit feeding the Nodal cabinets will be located in the current "Hubsites" The VOD streams will probably use DWDM technology to combine with the existing Analogue/Digital Tv and cable modem downstream. The current TV and cable modem services are fed to an optical transmitter (DFB) to feed generally 4 nodes in a ring of 8, the output of the DFB in the original architecture fed a 4 way optical splitter, the DWDM diplexer to add in the VOD service would be located between the DFB and optical splitter. It is likely they will follow a similar route as they did with cable modems, ie: use the existing architecture and further split down nodal areas as and when demand for the service increased. (The 4 way optical split could become a 2 way, and double up on DFB's for existing services etc or use a 2 way split to feed a 4 way with 2 DWDM's) The optical receiver at the node location has a photodiode that converts the RF modulated optical signal back to RF, because this is a wideband device it has the capability of accepting the different wavelenghts and demodulating them to their original frequencies.

Sorry if thats a bit boring, I thought there may be some who were interested in how that part of the network works. Most people appreciate what goes on in the headend and at local distribution level, but theres not usually much mentioned about the bits in between.
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Old 11-01-2005, 22:09   #12
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Re: How Video On Demand Works (long and technical!)

Quote:
Originally Posted by Escapee
Thats a very good description Tristan, I don't work for ntl either but would just like to add that the VOD kit feeding the Nodal cabinets will be located in the current "Hubsites" The VOD streams will probably use DWDM technology to combine with the existing Analogue/Digital Tv and cable modem downstream. The current TV and cable modem services are fed to an optical transmitter (DFB) to feed generally 4 nodes in a ring of 8, the output of the DFB in the original architecture fed a 4 way optical splitter, the DWDM diplexer to add in the VOD service would be located between the DFB and optical splitter. It is likely they will follow a similar route as they did with cable modems, ie: use the existing architecture and further split down nodal areas as and when demand for the service increased. (The 4 way optical split could become a 2 way, and double up on DFB's for existing services etc or use a 2 way split to feed a 4 way with 2 DWDM's) The optical receiver at the node location has a photodiode that converts the RF modulated optical signal back to RF, because this is a wideband device it has the capability of accepting the different wavelenghts and demodulating them to their original frequencies.

You're just showing off now. Hands up who got that. Is it even English?!
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Old 11-01-2005, 22:30   #13
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Re: How Video On Demand Works (long and technical!)

It's all greek to me!
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Old 11-01-2005, 22:36   #14
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Re: How Video On Demand Works (long and technical!)

Very good post.
Thanks. It has taught me things i didnt know before and was a good read.
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Old 11-01-2005, 23:30   #15
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Re: How Video On Demand Works (long and technical!)

Having read Tristans description of how VOD works it got me thinking. Would it be possible to supply the normal digital channels using the same method as VOD but without the pause, rewind, etc. that VOD has? That is just transmitting the live pictures from the selected channel to your TV. At the moment all 200 or so channels are sent to every STB taking up a hell of a lot of bandwidth. If only the selected channel was sent to your STB from the headend or hubsite there could be plenty of bandwidth available for other uses.
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