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SDV FAQ

Discussion in 'TiVo Series3 HDTV DVRs' started by bdraw, Jul 3, 2007.

  1. Jul 15, 2007 #41 of 2401
    JimSpence

    JimSpence Just hangin'

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    Interesting topic. One question comes to mind. What happens if you request a channel and there isn't a free frequency from the "hub" you're attached to?
     
  2. Jul 15, 2007 #42 of 2401
    mfogarty5

    mfogarty5 New Member

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    Good question Jim.

    I have wondered this myself. I heard from a friend of mine here in Charlotte that one time he tried to watch an OnDemand channel, but couldn't because there wasn't any bandwidth available.

    I think that the cable companies will start with small SDV implementations so that if there are any bandwidth issues that very few people are affected, but after that initial implementation I think that SDV will be implemented quickly because the more channels that are switched the more valuable it is.

    ESPN Classic was recently removed from the analog tier(probably as part of the TWC agreement to get ESPNU and ESPN2 HD) here in Charlotte. My guess is that it was removed to make way for SDV because that one analog channel could support 6 simultaneous SDV channels.
     
  3. Jul 16, 2007 #43 of 2401
    sfhub

    sfhub Active Member

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    I've heard there is a spike in support calls.

    Some implementers are of the opinion you should never be consistently over 80% (or some percentage) of capacity allocated to SDV. Basically just like the phone company doesn't actually have switching capacity to handle every phone call (as evidenced by circuits being down during a natural disaster), but 99% of the time you don't get an all circuits are busy message.
     
  4. Jul 16, 2007 #44 of 2401
    BobCamp1

    BobCamp1 Active Member

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    I agree. I mean, who is watching PBS? Or that new UPN+WB channel? Or PAX? These would otherwise appear to be prime candidates for SDV, but it's probably less hassle to leave them alone for now. The cable companies don't want to press their luck by entering the gray areas of the law.
     
  5. Jul 16, 2007 #45 of 2401
    MichaelK

    MichaelK New Member

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    'zactly the bean counters figure out what they need to provide to get a bunch of nine's for reliability.

    I suspect it's not so tough to go bulletproof though once you go whole hog though. Figure 125 qam slices on a 750mhz system. Take out the slots for the “always on” locals and internet and voip and you still probably have like spots for 200 HD channels. (or 1,000+ SD).

    I think I’ve read a typical switched node has 150 houses. Lets assume 3 tv’s per house (made up WAG). 450 tv’s per node. IF I understand the ratings- during the most watched time of the day – primetime- when the most possible tv’s are on then 25% of the tv’s are tuned to the big 4 (which are always one). So that leaves 337 tv’s left to be serves by the remaining 200 HD channels. If I understand correctly, HBO, Disney, ESPN and a couple other ‘big gun’ cable channels peel off another 20% of cable households. So that’ gets you down to like 270 tv’s with 190+ possible pipe’s. No statistician but I’d feel pretty comfy that with the tv’s that are off and the fact that each node is likely serving a similar socioeconomic class (with similar tastes probably) that you can easily assure everyone the slice of bandwidth that they need.

    Again not a bean counter myself But I’d suspect you could easily make 20 channels fixed and then do 180 switched slots and provide an unlimited amount of channels to each node.

    And all that assumes 100% HD with SD viewers giving you like 6 times the headroom.
     
  6. Jul 16, 2007 #46 of 2401
    CharlesH

    CharlesH Member

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    Sacramento...
    What does it really cost for them to have a channel be switched? If it is effectively always in use, then when someone tunes to that channel, there will be a request from the set-top box to the SDV controller for its node, who will respond with the existing frequency assignment for that channel. Why bother trying to figure out which channels are in use at what time in which nodes; just switch everything and let the actual usage pattern sort it out.
     
  7. Jul 16, 2007 #47 of 2401
    bdraw

    bdraw Member

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    Using SDV for popular changes doesn't make any sense, there is a worse case and then there is unplausable.

    Here's a question, if you try to tune to a SDV channel on your S3, that someone else on the same node is currently watching, can you see it? I would suspect no, since the CableCARD doesn't tell the TiVo what QAM channel it is on. The reason I think you might though, is because I've been able to watch VOD and PPV with my PC QAM tuner before.
     
  8. Jul 16, 2007 #48 of 2401
    bdraw

    bdraw Member

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    They bother, cause it's the entire point, to save bandwidth. There was a post on the Official Moto blog about trying to figure out what channels to go SDV with, but it was pulled, here is the text.

    http://connectedhome2go.com/2007/07...rt-1-choosing-your-switched-digital-channels/
     
  9. Jul 16, 2007 #49 of 2401
    moyekj

    moyekj Well-Known Member

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    Mission...
    Currently with CableCards my S3 actually has 80 VOD channels assigned (VOD1->VOD80 and distributed over 8 256 QAMs). At any point in time I can channel up and down to find currently active channels. There is no discernable pattern to the assigned channels, they are spread out across the entire range. However, the only reason I can see them is the VOD channels are unencrypted. I would imagine that most channels on SDV would have encryption and therefore even if I knew which frequency (or frequency range) to check it wouldn't do me much good, not to mention the fact that it can skip to a different frequency at any moment anyway just as happens with VOD channels.
     
  10. Jul 16, 2007 #50 of 2401
    bdraw

    bdraw Member

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    Nope, sorry, if you can get it, I'll update the post.
     
  11. Jul 16, 2007 #51 of 2401
    sfhub

    sfhub Active Member

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    Doesn't switching everything and allowing the system to sort itself out at worse use the the *same* bandwidth (minus some small switching overhead) as having a mix of static/switched channels? They do need to make sure they have enough unused SDV capacity to handle peaks, so total SDV bandwidth usage should not be above some number, let's say 80% of total SDV bandwidth allocation.

    I'm not seeing why bandwidth would be the reason a cable company decided not to switch everything. Could someone explain this, since it appears multiple people have this opinion.

    On the other hand, I could understand keeping UDCP CableCARD users happy being a reason not to switch popular channels.

    In the TW Austin article in your first post, there were 2 examples given
    http://www.multichannel.com/article/CA6454447.html
    1) deploying a popular channel on SDV could end up eating up all your SDV bandwidth
    This seems just like a configuration issue. If you have room for 20 channels on SDV and decide to move a potentially popular channel to SDV, you should allocate room just freed up from the static channel into your SDV pool, so that it now has capacity for 21 channels. It seemed the TW Austin moved a popular channel to SDV but instead of expanding SDV capacity, they left it the same.

    2) popular PPV channels on SDV
    It seemed the problem they were describing was a problem with the SDV client updates not being distributed to all their customers causing a flood of support when the event occurred. This seems like a short term deployment issue, rather than a long term reason why all channels wouldn't be on SDV.

    Again, I'm *not* arguing to have all channels on SDV. I'm trying to understand why it makes absolutely no sense as some folks are asserting. I can see pros and cons of doing that, while others are saying there are only cons.
     
  12. Jul 16, 2007 #52 of 2401
    kjmcdonald

    kjmcdonald New Member

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    Another Reason for Locals to be SDV'd... Reducing bandwidth isn't the only benefit.
    One of the biggest vendors of SDV equipment is my ex-employer BigBand Networks Inc, they're also one of the leaders for doing what they call individually addressed advertising... This means that instead of sending differnet ads only to different neighborhoods (Optical Node) the cable companies would be able to sell and target advertising on a home by home basis, and according to demographics for each account. Cable Companies know that this is something that advertisers will pay much more money for, and so they will want this ability on the channels watched most often...(The locals) It's the SDV infrastructure that makes this possible.

    Extrapolating, BigBand would love it if the MSO's had only SDV and DOCSIS data traffic on thier networks.

    -Kyle
     
  13. Jul 16, 2007 #53 of 2401
    bdraw

    bdraw Member

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    Dividing up QAM channels for programming isn't always easy, different channels have different bandwidth requirements. A football game in 1080i requires more bits than a movie in 1080i. SDV isn't IPTV, each QAM channel can hold 38Mbps so if they put 3 channels on it, 2 that use 15Mbps and 1 SD that uses 4Mbps, they are wasting the other 4. Sure it might be possible to dynamically splice it up on the fly, with a sort of SUPER-statistical-multiplexing, but ultimately it adds a layer of complexity that just isn't there right now and while anything is possible, at this point no one is talking about trying to do that.

    At this point the cable companies want to figure out how to please more people most of the time. So with SDV, they can add lesser watched programming (like HD channels unfortunately) without dedicating 100% of the normal bandwidth required 100% of the time.
     
  14. Jul 16, 2007 #54 of 2401
    aymanme

    aymanme New Member

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    One of the reasons not to switch locals and other popular channels is that it defeats the purpose. These are amongst the most watched channels so they are always going to be consuming space in the SDV pool. The idea is to have the SDV pool able to accomodate all of the less popular, not "always on" channels (PPV, etc) are good candidates as are the premium tiers like foreign channels, etc.

    Second to answer the question about running out of space. It is possible that the SDV space is fully allocated and then the next person is just out of luck. However, the way it is supposed to work is that the headend is monitored and if the usage inches too far north of some high water mark, then they will either do a node split or reconfigure the channels for that group (i.e assign some to a single channel rather than 2 adjacent, bandwidth shape, etc.
     
  15. Jul 16, 2007 #55 of 2401
    HiDefGator

    HiDefGator New Member

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    You are assuming

    1. They know which channels would always be in use by someone in every neighborhood. Keep in mind each neighborhood may have different types of homeowners so the list would vary from node to node.

    2. The cost of addiitonal SDV channels is significant.

    I believe both of those asumptions are wrong. If they can put in 200 SDV channels for the same price as 175, then it makes more sense to switch everything and let it keep some channels stay locked in if need be. This strategy doesn't use any more bandwidth than not SDV'ing the channels you believe to be always on. And in most cases it will free up more bandwidth.
     
  16. Jul 16, 2007 #56 of 2401
    bdraw

    bdraw Member

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    If they really wanted to do this, then why would they lie to the media and tell us that they only want to use SDV for less popular channels?

    You're pretending they have one big pool for 200 channels, but that isn't how it's divided up. On a 900Mhz system, there are 150 38Mbps (QAM channels) not including overhead. They have to figure out which channels to put on each QAM channel, based on the bandwidth requirements of each channel, it isn't a simple one to one relationship, there's alot of planning required to efficiently use all the bandwidth. Why would they bother to engineer such a system when 80% of the channels would be sent 100% of the time, with no net gain?
     
  17. Jul 16, 2007 #57 of 2401
    Combat Medic

    Combat Medic No guts, no glory

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    How can there be that much planning? It isn't some guy with a slide rule trying to figure out how to get all of the channels to fit, its a computer program that does it all in less then a second.
     
  18. Jul 16, 2007 #58 of 2401
    sfhub

    sfhub Active Member

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    My question regarding this is how is this issue any different between static channels and SDV channels? Let's say NBC sometimes broadcasts movies and sometimes sports. In a static allocation, you need to allocate enough bandwidth for the worst case NBC sports broadcast, otherwise you won't have enough. Now when NBC broadcasts movies, that extra bandwidth is permanently lost to the system.

    BTW what you are describing is the classic fragmentation issue for hard disk filesystems.

    As a first approximation, they could completely avoid fragmentation simply by doing what many operators are doing today with their static systems, namely having separate pools for HD SDV and SD SDV. HD pools gets assigned 2 channels per QAM256. SD pools get assigned 8 channels per QAM256. Since each channel is given the same constant bandwidth, there is no fragmentation.
     
  19. Jul 16, 2007 #59 of 2401
    ah30k

    ah30k Active Member

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    Where did you see this statement that they only want to switch less popular channels?
     
  20. Jul 16, 2007 #60 of 2401
    HiDefGator

    HiDefGator New Member

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    Once again I believe your assumption is wrong. 80% of the channels are not always on in a given neighborhood. There is no channel that will "always" be on. It may only be not requested for a few minutes a day or even a few minutes a week. But during those few minuites they can not deliver it if it is switched.
     

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