DOCSIS 3.0 shared throughput....

One feature of DOCSIS 3.0 is that downstream QAMs can be combined easily. Right now, using QAM 256, a downstream carrier is limited to about 36Mbps (raw wire speed). This can be shared by up to 1800 households (depending on the size of the ARP table in the CMTS), but most systems have a much lower number of homes per downstream, some as few as 500.

Future cable designs will allow for 1024 QAM. This will raise downstream throughput to about 70Mbps, depending on the symbol rate. The reason the cable companies aren't doing this now is because it requires much higher carrier-to-noise levels than can be achieved with current designs for consistant delivery. This increased data rate, along with multiplexing several QAMs together and pushing fiber optics deeper into the network, will allow cable companies to keep ahead of the ADSL guys, as long as the management wants to. Not sure about FTTH, but they have to get a better backup power solution before I'd sign up for lifeline service with them.

Now, all this downstream bandwith will be matched with better high data rate modulation for the upstream as well, although I don't think we'll see symmetrical upstream/downstream bandwidth for several more years, if only becuase of current system design and difficulty in keeping noise out of the upstream (house to headend) path. Currently, cable systems have up to 800MHz of bandwidth going into your house, but only about 35MHz of noisy bandwidth leaving it. Once analog television goes away, cable systems will be able to redesign, changing to mid-split (200MHz) or high-split (400MHz) amplifiers, greatly increasing the RF bandwidth available. I really doubt we'll see that for a long time.

we're still going to pull more information than we produce (and the vast majority of people never produce any information). This means that, for most of us, there isn't much demand for 100Mbps upstream speeds, but 100Mbps+ downstream from a subscriber modem will be almost necessary as we move from "broadcast" television to IPTV and switched digital systems.

Slightly off topic, but related to this, is the Comcast all digital tv project. Over the next few months we'll start to see Comcast rolling out all digital to the major markets. This will require a set top box or cablecard for every television (something that's going to happen anyway with HDTV). The Motorola DCT 700 will be used for the most part, as it is inexpensive and doesn't take up much shelf space (about the size of a cablemodem).

One of the reasons for this is marketing and matching the DirectTV and Dish network claims of an "all digital signal." Like just making something digital makes it better, but hey, that's what we're all told now, even though phone audio quality is much worse than I remember in the 80's.

More importantly, though, is that Comcast has begun to realize that the future of mass comunications is on demand, and is starting to put the pieces in place for a massive rollout of all on demand, switched video. Analog television is a big old steam engine getting in the way of making the network much more efficiently and it has to go. Much of the new stuff (not even getting into the phone stuff) is going to run on a DOCSIS platform, and so the available bandwidth for modems and DOCSIS has to be increased. This also allows for easer speed increases for traditional high speed internet as well, but will require DOCSIS 3.0 modems to be deployed.

Eric provided a great discussion of the topics. I want to add one more detail.

Consider the current cable system. While a single DOCSIS 2.0/1.x channel is about 38mbps and a "four channel" DOCSIS 3.0 channel can be ~160mbps, this is not the total bandwidth available on the coax. As you've seen from Eric's post, the coax provides about 750MHz of downstream spectrum into the home. With 256 QAM and 6Mhz channels, this means there is roughly 4.7 gbps (that's gigabits-per-second) available if all the spectrum is used for digital transmission.

It's up to the cable company to decide how to partition that 4.7gbps channel among users, applications and services. For example, VOD may be delivered to some houses in your neighborhood using channel 105 while a bunch of other houses are channel 67.

Node splits, fiber deep technology, switched video, digital only video, and

1024 QAM will only increase the effective bandwidth available to each subscriber.

DSL cannot compete with cable for digital video delivery (IP or otherwise), that's why phone companies like Verzion are very busy installing passive fiber networks to the home (PON FTTH).

-Gary

Thanks Eric & Gary, I haven't posted on Newsgroups for a while and it was great to get such informative answers.

I live in Australia and we have one phone company owning all the copper nationwide, though there is a separate cable company in many cities with far, far fewer subscribers (like 5% uptake?). I've heard so much about ADSL2+ and future technologies that I guess I was hoping there was a future also for the cable company... competition is good!

So thanks for letting me know there is!