DOCSIS 3.0 shared throughput....

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Will DOCSIS 3.0 mean every user can watch a different IPTV channel

For IPTV to work every TV viewer (1-3 per household?) needs to have a
dedicated 1-2Mbps (SDTV) or 7-10Mbps (HDTV) connection.

I'm just looking at the data througput of ADSL2+, and it seems more
suitable for IPTV, but I'm sure the cable companies aren't sitting
still. I've seen DOCSIS 3.0 touted as allowing up to 160Mbps download
speeds, but if IPTV takes off and that cable connection is shared
between 150 people, then during primetime it might run out of steam.

Anyone know?

Re: 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.
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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.

Re: DOCSIS 3.0 shared throughput....

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!

Re: DOCSIS 3.0 shared throughput....

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

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

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).


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