[GRLUG] Testing Comcast vs AT&T

[Michael Mol]

On Sat, Jul 9, 2011 at 11:44 AM, Matthew Seeley <matthew at threadlight.com> wrote:
> This isn't strictly related, but I noticed that Comcast is now offering
> 105mb service in Grand Rapids. (Or at least, when I typed in my address into
> their website, I was offered the following
> --> http://www.threadlight.com/comcast_woah.png )
> Does anyone have any experience with this? I'm *highly* skeptical of this
> offering, since the last time I had comcast, they couldn't reliably send me
> 8mb down, much less 100+
> Not that it matters, there's no way I can pay that price for internet. But
> still very curious about it :)

It's the difference between DOCSIS 2 and DOCSIS 3.

First, DOCSIS divides a frequency range of 5MHz to 1GHz into ~6MHz
segments called channels. Channels can be shared among an arbitrary
number of users. A channel may be used for upstream traffic or
downstream traffic. A downstream channel can carry as much as 37Mb/s.
An upstream channel can carry as much as 32Mb/s.

A consumer's modem will be configured to search for usable-quality
channels from within a particular set. DOCSIS 2 modems support the
active use of one channel for upstream and one channel for downstream.
DOCSIS 3 allows modems to actively use an arbitrary number of
channels, and specifies that modems must be able to, at minimum,
*find* four channels up and four channels down.

The exact behavior of a modem is configurable by the ISP, who pushes
configuration files to the modem. An ISP may specify an arbitrary
number of channels that a modem is allowed to actively use at the same
time.

The Motorola SB6120, what I'm using, supports a maximum of four
channels up and four channels down. Take that four channels down,
multipy that by 37Mb/s each, and you get 148Mb/s. That's plenty of
margin to provide a 105Mb/s  service.

Take that 1GHz-5Mhz number and divide it by 6Mhz, and you get about 33
channels. As cable companies migrate to IP-based delivery of services
(you should see the considerations DOCSIS3 gives for multicast), more
and more of those channels will be dedicated for video and phone
services. That's 33 channels to divide between upstream and
downstream; you could easily get about 600Mb/s down, aggregate, and
still have room for upstream. (Of course, all of these channels are
shared between all of the customers attached to a particular HFC head
and attached hubs, so no one customer will get that entire 600Mb/s
downstream pie)

Everything I just said, I learned this week, from:
* The PDF I sent the list a link to a couple days ago discussing
migrating networks from DOCSIS 2 to DOCSIS 3
* My modem's specific technical specs
* Wikipedia, where I got the 5Mhz-1GHz number.

-- 
:wq

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