Last time, we discussed the fundamental delivery schemes being used in many telco-based IPTV installations. To recap:
- By and large, telcos deliver broadband using some flavor of DSL.
- IP-based television being delivered over an xDSL infrastructure has implications for both the consumer and the provider. Specifically, the available bandwidth on DSL tends to be limited, forcing certain technology decisions throughout the video delivery infrastructure.
- Unlike the traditional cable model, only the channels being watched are “on the wire” at the subscriber’s premise at any one time.
- Compression technologies are offering better ability to deliver more content, at a higher resolution.
Regular use of Shilajit improves viagra 50 mg stamina, energy and strength in doing lovemaking act. Ginkgo biloba Apart from buy vardenafil levitra being used as yet. Research viagra lowest prices being carried out in 2007 suggests that in addition there may often be deterioration in the arteries and blood vessels carrying blood to male organ. In http://secretworldchronicle.com/2016/07/ep-8-24-rubicon-part-1/ levitra properien the event that you are no more sexually invigorated, the erection will go away.
I won’t go into the hairy details about all this, but some description of how only the channels being watched are on the wire would be helpful.
IPTV relies heavily on the concept of multicasting. Multicasting is a the middle-ground between the “one connection per subscriber, for any given channel” and “all subscribers get this channel”. Let me explain.
In a “unicast” model, each subscriber would indicate to the IPTV system that they are interested in watching a given channel. The system begins directing the video/audio content to that subscriber. Simple. Except that in this model, if another subscriber also wants to watch the same channel, another copy of that channel is put onto the wire, directed toward the second subscriber. The problem here is that the content is duplicated and more bandwidth is used. Regardless of whether or not each subscriber has their own dedicated DSL line to the provider, the back side of that is a single connection (to the DSLAM for example). If every subscriber served by a given DSLAM wanted to watch the same channel at the same time, the unicast model would require that the content be duplicated for each. This would consume an enourmous amount of bandwidth and would simply be impractical.
On the other end of the spectrum, each channel could be broadcast. In this model, it wouldn’t matter if one or 1000 subscribers wanted to watch a given channel, it would be at least be present at the serving DSLAM. Again this represents a huge network bandwidth penalty. So, what exactly is the “middle of the road”. Multicasting, to answer that question.
Multicasting (again in an abbreviated description) allows better use of network bandwidth by allowing only those who are interested in the same content to receive it. For DSL, this indicates that the DSLAM will be receiving only one copy of the video and audio, sending that content only to the subscribers that are currently watching that channel. If the DSLAM is serving 100 customers and only two are watching say, ESPN, then it will only receive a single copy of ESPN and will send that only to the customers watching it. If no customers are currently watching ESPN, the DSLAM has the option of “unsubscribing” from the channel, freeing up bandwidth for other channels. Note that there is a LOT going on here and also notice that much of this discussion centers around what’s happening at the DSLAM. Since the DSLAM provides individual connections and bandwidth to each subscriber, it’s a great point to control what data goes where, with respect to the subscribers it serves. Regardless, multicast allows the DSLAM (and other network elements) to limit bandwidth used to only that which is relevent at the time. Without it, the network would get congested very quickly.
So, back to IPTV at the subscriber premise…
Let’s say you have an 18 Megabit connection on your DSL modem. At first blush, that sounds pretty fast and so surely it move a lot of data. However, if you look at the bit rates consumed by video you start to very quickly wonder if it’s enough. Remember too that most of the time there’s more than video on the network. You have your Internet connection using that same 18 Megabits. That’s your Web browser, email client, game system, etc. Now, consider that most households have more than one TV and that very often more than one are in use at a time. Finally, remember that HDTV has become more and more popular and that HDTV takes much more bandwidth than standard definition television. Still feel like 18 Megabits is a lot? Lest you’re sticking to your guns thinking that’s a fat pipe, consider these facts:
- Using MPEG 2 encoding, getting a good quality standard definition TV channel consumes about 3.5-4 megabits per second.
- Using MPEG 2 encoding, getting a good quality 720p resolution high definition channel uses about 10 megabits per second.
So, if you’re watching ONE high definition TV channel at 720p resolution and one standard definition TV channel, you’re already using nearly 14 megabits per second! Since there’s some overhead associated with a DSL channel (for our purposes, use 2 megabits/sec), you’re pretty close to the limit of what can be delivered on your DSL link. Yeah, you still have about 2 Mbits/sec. for your Internet activities. What happens though if someone wants to watch a third channel or individual watching the standard definition channel decides to watch another high def channel? Well, adding the third SD channel puts us over our limit, and changing the SD channel to an HD channel ups the ante’ by 6 Mbps, also pushing us over the edge.
So, there’s a problem here wouldn’t you say? What’s the answer?
There are really only two answers to this dilemma:
- Increase the amount of available bandwidth. Simply put: get a faster DSL line. However, that’s not always possible and unless you get a pretty significant boost in bit-rate, the improvement to your situation may be marginal.
- Make better use of the bandwidth you have. This one makes sense for any situation, unless you somehow happen to have unlimited bandwidth.
To better use the bandwidth you have, you might take a few paths:
- Send the data for any given channel at a lower bit rate. This can save bandwidth, but results in poor quality video.
- Somehow increase the compression of a given video/audio stream.
The latter option is where H.264 (AVC) MPEG 4 encoding begins to shine. Notice that the bit rates I quoted for standard and high definition video were for MPEG 2 encoding. MPEG 2 is a standard for video encoding that’s been around for a pretty long time and is well understood and well established. It’s the basis for DVD video encoding, as well as for over-the-air digital TV (the move to all-digital television takes place on June 12, 2009). However, its compression rates aren’t so good for IPTV as we’ve demonstrated. When you consider the rapid move to HDTV, those compression rates aren’t really sufficient for DSL-delivered television, given the prevailing data rates.
H.264 achieves considerably higher compression rates for a given bit-rate, while maintaining good picture quality. It’s nominally about 2x as efficient in its compression, resulting in a video channel of equivalent quality as MPEG 2, at about half the bit rate. So, using the MPEG 4 compression rates, let’s do the same example as before:
- Using H.264 encoding, getting a good quality standard definition TV channel consumes about 1.75 – 2 megabits per second.
- Using H.264 encoding, getting a good quality 720p resolution high definition channel uses about 4-9 megabits per second.
You’ll notice that the numbers aren’t exactly half those of MPEG 2. Experience shows that some variations occur and these are “thumbnail” ranges. In any case, let’s consider the things you can do with that extra bandwidth, in a more summarized form:
1 SD channel (480i) = 2 Mbps
1 720p HD channel = 7 Mbps
1 SD channel (480i) = 2 Mbps
…still at 11 Mbps of an available 16 (remember we’re assuming 2 Mbps overhead of our 18 Mbps). We’ve already been able to add another SD channel and have . Let’s keep going…
1 HD channel (720p) = 7 Mbps
1 HD channel (720p) = 7 Mbps
Now, we can watch 2 HD channels and still have 2 Mbps for Internet or even another SD channel. Another thing to note here is that since most Internet activity is “best effort”, the visible effects on that kind of traffic can often be missed because the data being delivered are not time-sensitive.
All of this still demonstrates the squeeze we’re put in with the limits I’ve set for our example. However, the 18 Mbps rate isn’t some hyperbolic example put out to show the extremes. This is a common bit rate that can be delivered by the Telcos, using either ADSL2+ or VDSL2. Both kinds can reach higher bit rates, but that usually comes at the cost of either decreased reliability or the need to get the DSLAM closer to the customer. Generally, the further away from the DSLAM (or serving DSL line card), the lower the reliable bit rate. Again, there are rules of thumb and other factors that play into all this, but the fact remains that it’s quite hard for many Telcos to provide bit rates at or above our hypothetical 18 Mpbs. If you go with the 18 Mbps as a common bit rate, the examples show quite clearly the problems facing IPTV subscribers and providers alike.
Next time: What does this all mean?