[GRLUG] Testing Comcast vs AT&T

[Bob Kline]

http://en.wikipedia.org/wiki/DOCSIS

See the URL above for lots of info,
and data in tabular form.

   -- Bob


On Sat, Jul 9, 2011 at 3:33 PM, Michael Mol <mikemol at gmail.com> wrote:

> 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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