Hi Mike, something like that, as a first approximation. But that's not the same as the spectrum occupied. You really can't extend that to the hub without specifying some architecture constraints.
If we go with your 4Mbps (average) number for VoD streams, 9 streams (or programs if you like) could be packed into a single 6 MHz channel using 256 QAM. {How did I get this 9, simplistically: the 256 QAM channel has an information bit rate of 38.8Mbps. 38.8Mbps divided by 4Mbps gives 9 plus a remainder. Fractional streams don't do any good so this remainder serves as headroom}
With 500 users on the node, 500 users divided by 9 users per (6MHz)channel gives 55.6 channels. So we round that number up, and it would take 56 whole channels (6 MHz wide), which occupies 56*6=336 MHz of spectrum on the cable
Now the hub would have to be WDM- or multiple-fiber-capable in order to support this level of capacity for any more than 1 node (actually it’s too much bandwidth for even a single node on 750MHz plant but lets ignore that and use your scenario). If there were 10 nodes served by this hub and you wanted this same 100% VoD usage scenario, one scenario would be that the hub has a dedicated fiber (or wavelength) coming in from the primary hub that contains 10 subchannels of 336MHz VoD spectrum. Each of these subchannels is somehow identified so that it associated to a particular node. The secondary hub would then strip out these 10 incoming 336MHz chunks of VoD spectrum and insert them separately into (for example) the range 550MHz to 896MHz (oops we’re beyond our 750MHz plant) on the output port for the node to which that particular chunk of 336MHz spectrum is destined for. There would also be another wavelength (or separate fiber) coming in to the secondary hub that contains the normal broadcast channels from say 50MHz to 550MHz. This would be equally split so each node gets a copy of it (because this is the broadcast portion), and once that is done it is combined with the 336MHz of VoD spectrum that is particular to a given node and the whole thing is sent to that particular node. In this way, each node would have 100%VoD capability and also get its legacy channels. If you also want data and digital TV channels, it's a different exercise but the same idea. But I can almost guarantee this is not the way they are doing it today since they aren’t expecting anywhere near this capacity level for VoD. This doesn’t even consider the upstream, which is a lot more stuff too.
Non-WDM or non-multiple-fiber example:
Assume each node serves 500 homes and there are 10 nodes served by the hub and there is 60MHz of upper spectrum dedicated to VoD (550 MHz to 610MHz for example). Then each node would have its own assigned 6 MHz channel (60 MHz of VoD spectrum gives 10 6-MHz channels, and we said 10 nodes so 1 channel per node). Node 1 gets the 6 MHz channel from 550-556MHz, Node 2 557-563MHz,…node 10 604-610MHz. All these 10 channels reach all the 10 nodes, but each node knows to look only at its assigned channel. Using the 4Mbps number again for a VoD stream, this allows 9 simultaneous users per node (or 90 per hub), and doesn’t require any of the spectrum shifting and stacking operations I mentioned in the hypothetical example. Adding those shifting/stacking operations is expensive since they require new equipment and/or fiber from the hub all the way back to the master headend.
So to summarize, in the 1st case we had a more advanced network with the ability to provide dedicated VoD spectrum to each node (sufficient to allow 100% simultaneous usage by all users on all nodes), overlaid with the legacy broadcast spectrum that is broadcast to all nodes. In the second case (and probably the more likely today) we have a basic power splitting hub that broadcasts the same spectrum to each node, but each node has a dedicated chunk of spectrum it is assigned for VoD, out of the total spectrum available for VoD. For this case only a small number of users per node can use VoD simultaneously.
There are some dynamic allocation bandaids that could be implemented in the second case to milk a little longer life out of it, but if VoD really becomes a “must have” then even these will be insufficient. Movie watching has a different dynamic than web surfing, and probably VoD is used primarily from 7pm to 11pm, with little use during the other parts of the day. I’d also guess that Saturday and Sunday nights have heavier demand than the weekdays. It would be hard to believe that only 9 homes out of 500 on a given node would want to watch a movie during primetime Saturday. But as I suggested before, they can artificially limit the demand by offering a lame selection of movies and not update them. |
