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Comcast: unexplained bandwidth caps


By: Scott Bradner


Comcast is in the news again.  Over the last few months it seems like a new Comcast related story was breaking every few weeks -- all of them quite bad news for Comcast.  The PR people over there sure must be busy.  Now Comcast is in the news again for a number of different reasons, one of which has received far more publicity than the others. 


A few days ago Comcast let it be known that they were testing a mechanism that "slowed down" the traffic of heavy users of its high-speed Internet service.  ( Around the same time Comcast settled with the Florida Attorney General who had charged Comcast with not properly disclosing information, for example, customer data usage caps.  Neither of these stories received much press coverage.  In the middle of these stores came one about Comcast that did receive a lot of coverage -- Comcast announced that it was going to put a usage cap of 250 GB/mo on all residential users.  ( ) Repeated violations of the cap could get a customer disconnected for a year.  After a bunch of fervor it turned out that Comcast had been doing this all along but this was the first time it was actually willing to tell anyone what the limit was (this willingness just might be related to the Florida case). 


Comcast has not actually said just why they have a usage cap, at least not anyplace I've seen.  For example, their FAQ on the limits ( does not include a "why" question and their announcement of the feature does not say why they are doing it.  (   Comcast has implied that it has something to do with fighting congestion and most of the press coverage seems to assume that is the goal.  But, as I've written about before, usage caps or usage-based fees do not, and cannot, have anything to do with fighting congestion.  (Broadband pricing: solutions that are orthogonal to any real problem


The "slowing" mechanism Comcast is testing is directly related to fighting the effects of congestion.  According to published reports, Comcast is not actually slowing traffic (as the headlines would have you believe), instead, in times of congestion they are temporally setting a lower priority on traffic from customers which have been judged to be receiving or sending too much traffic in the proceeding few minutes. This will slow traffic if the congestion persists because some of the lower priority traffic will likely be dropped when the router buffers overflow and will have to be retransmitted, which takes longer.


If a usage cap is unrelated to fighting congestion then we must look for other reasons for Comcast maintaining such a cap.  It is possible that the Comcast powers that be are not technically cluefull enough to understand that a cap does not help reduce congestion but Comcast has some very smart people working for it that I'm sure understand the technology so the powers would have to be ignoring their own people, which I hope is not the case.


It's far more likely that the cap is in place to make sure that the Comcast network cannot be relied upon by Comcast competitors for high quality video delivery.  Comcast's current cap of 250 GB/mo, which they could change any time they wanted to, works out to a few hours per day of HD video, not very much.  A couple of people in a household could easily exceed that on a regular basis, at least until Comcast pulled the plug.


It sure looks to me like Comcast is engaging in anticompetitive behavior.  Considering the current FCC bias against cable companies (see FCC: regulating through 3D glasses - I'm more than a bit surprised that the FCC has not started to nose around.  Maybe the rumors about AT&T and other telephone companies are considering usage-based fees is keeping the FCC at bay.


disclaimer:  Harvard uses usage-based fees for some things like food but, as far as I know, most of the rest of the student fees are flat rate.  In any case, the above represents my own views, not those of the university.