North American Network Operators Group

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RE: Comcast blocking p2p uploads

  • From: Frank Bulk
  • Date: Fri Oct 19 22:34:14 2007


1) Correct/
2) DSL and fiber have limitations, too.  The modulation and spectrum width
can vary, but most MSOs have their forward configured with a maximum of
around 38 Mbps (256-QAM, 6 MHz wide) and the return in the 9 Mbps range
(64-QAM, 3.2 MHz wide).  Charts here:
With DOCSIS 2.0 there is the capability of using a wide band for return and
increasing the modulation, and with DOCSIS 3.0, through channel bonding,
higher downstream rates.
3) It's recommended that there are no more than 250 cable modem users per
upstream; with a 1:4 DS:US configuration, that would be 1000 per downstream.
Of course, MSOs can mix things up a bit by allocating more than one DS
and/or US channel on the same plant, and therefore support many
more/different users.
4) Not sure where you got 45 Mbps for return traffic.  But yes, a few
Slingboxes can fill the upstream.

PacketCable 1.1, a widely deployed standard, allows MSOs to assign their
voice higher priority over regular data traffic.  There are over 2M
customers of "digital telephony" today.


-----Original Message-----
From: owner-nanog@xxxxxxxxx [mailto:owner-nanog@xxxxxxxxx] On Behalf Of Sean
Sent: Friday, October 19, 2007 6:17 PM
To: Nanog
Subject: Re: Comcast blocking p2p uploads

Martin Hannigan wrote:

> O&M, etc. We already know that the givens are that it's generally
> socially unacceptable to filter, but without Comcast's motivation
> being know, it's hard to speculate as to the "why" they did it. Let's
> not.

It's not at all hard to imagine WHY.  In fact, it's almost a given.

1) Comcast is an MSO.  As such, their access (last mile) is over a coax or a
2) HFC has limitations on bandwidth.  The frequencies that most MSOs use for
data give it somewhere around a DS3's worth of return traffic.  The forward
traffic (to the customer) is greater.
3) The HFC plant almost always includes at least a few thousand customers
leg.  These customers have to share the same return bandwidth.
4) With only 45 meg of return traffic, and a few thousand customers, it is
pretty easy to see how a few high capacity customers could have a negative
impact on the rest of the customers.

In addition to this, you have other applications, such as voip, that rides
same infrastructure.  In many places there is no real ability to tag the
traffic with a higher class service, so it has to contend just like everyone

You can add to this that in some markets, the only real bandwidth is via
multiple T1 or DS3 due to it being more remote.  You ever wonder why some
have cable modem but not DSL?  That's usually because the telcos can't get
bandwidth there.  Right or not, many MSOs will turn up markets on a handful
T1 circuits until they can get a DS3 or greater installed.

As to the SPECIFIC reason why Comcast is deploying the Sandvine instead of
another architecture, or using another method of rate limiting...  Well, I
probably comment on that as well, but I'm uncertain that my friends and
associates at the MSOs and hardware vendors would look kindly on that.
Since I
no longer work for a MSO, I really no longer have any insight.

It's just a way that an MSO might manage their network in order to make 90%
more of their customers happy while reducing the need to deploy more
hardware to
split the plants.