[ale] OT: DOCSIS Info

Matt Rutherford matthew.g.rutherford at gmail.com
Wed Jan 18 16:51:01 EST 2012


Interesting talk - thank you!

-Matt R


On Sun, Jan 15, 2012 at 7:40 AM, JD <jdp at algoloma.com> wrote:

> A Cisco DOCSIS guy, Biran Wilson, gives talks at the hacker-cons
> periodically.
> Find the OuterZ0ne videos if you're interested in more detailed, organized
> information. DOCSIS3 lets channels be mixed for digital cable video and
> data. It
> also allows channel bonding.
>
> See the Brian Wilson talk - Docsis Coolness
> 2010: http://www.archive.org/details/Outerz0ne2010Videos
>
> I had both residential and business comcast service through 2 different
> lines to
> the house.  It was weird calling for business ISP support during multi-hour
> outages using VoIP connected to the residential line not seeing any issues
> at all.
>
> I love it when my neighbors talk about how DSL doesn't share bandwidth so
> it is
> better. That means they aren't on my cable line.  The fact that DSL is
> usually
> 3x slower ALL-THE-TIME for the same price doesn't matter, it is still
> "better."
> ADSL2+ is changing the performance difference a little, but still has
> other issues.
>
> There is enough hate for telecom and cable companies to go around. They
> have
> most of us by the balls.
>
>
>
> On 01/14/2012 10:17 PM, Matt Rutherford wrote:
> > Trying not to run on to much but here is an overview of how DOCSIS does
> dynamic
> > bandwidth. I'll leave out the more detailed guts of it. An in-depth of
> this
> > subject is way too OT here.
> >
> > DOCSIS has a feature that allows for burst speed provisioning within a
> specified
> > range when there is bandwidth availability on the server. This can be
> applied to
> > a set amount of a specific transaction, ie: 5MB, 10MB, 20MB, etc. This
> provides
> > the nice bump in initial download/buffer speed on large videos (see
> Youtube,
> > Netflix, etc) and also is quite noticeable on speed test sites since
> they are
> > file-transfer speed based. Also looks great for marketing and
> advertising in
> > comparison to the speeds offered on DSL. This is all handled by the CMTS.
> >
> > Typically the burst speed will be set to the around the same level as
> your
> > provisioned speed, though this is not always true. IE: a 22mbps
> connection will
> > burst speeds up to 44mbps (down) for a pre-determined amount of upwards
> or
> > downwards transfer. Since cable speeds are non-synchronous, you may have
> a 5mbps
> > upstream with bursts up to 10mbps. This cannot be relied on however,
> hence the
> > careful use of 'up to (X)mbps' in advertising.
> >
> > Peak usage hours for residential areas for cable internet bandwidth run
> > 5pm-midnight as Kirsa said, so I'd add a +1 to the congestion theory.
> There are
> > other causes as well, but tech support should be checking for those
> symptoms
> > anytime you call in (SNR, bad RX/TX, and high levels of error correction
> to
> > packets from the modem).
> >
> > During heavy usage if there is saturation of the bandwidth availability
> to below
> > a specified threshold, protocols kick in to limit users to speeds lower
> than
> > their 'provisioned' default. This is done dynamically since each CMTS has
> > multiple RF cards separated into upstream and downstream. You may have a
> > non-saturated upstream but a saturated or congested downstream as far as
> > bandwidth goes - we rarely saw upstream problems except for military
> barracks,
> > college towns, and other edge cases. There are other layers of bandwidth
> > management and bottlenecks that impact speeds which can exist at the
> node or
> > network level, but outside of RF issues the most typical and noticeable
> change
> > in speeds comes from this type of congestion.
> >
> > -Matt R
> >
> >
> > On Sat, Jan 14, 2012 at 8:48 PM, Jim Kinney <jim.kinney at gmail.com
> > <mailto:jim.kinney at gmail.com>> wrote:
> >
> >     All ISPs know the speedtest sites. Comcast is known to privide
> priority routing.
> >
> >     On Jan 14, 2012 7:53 PM, "Drifter" <drifter at oppositelock.org
> >     <mailto:drifter at oppositelock.org>> wrote:
> >
> >         On Saturday, January 14, 2012 05:36:53 pm Kirsa wrote:
> >         > I would not be surprised if you experienced a similar speed
> issue next
> >         > Friday though... On almost any residential cable node Friday
> 5-12pm is
> >         > peak time and has the most utilization of the entire week. So
> that you
> >         > were down to 4mbit/s on Friday evening and 2mbit/s by Friday
> night
> >         > sounds suspiciously like congestion to me, despite the faulty
> >         > equipment.
> >         [Rest trimmed for brevity.]
> >
> >         I had the same speed issues at 9 am Saturday morning. Snail-slow
> download
> >         speeds continued all day Saturday until I replaced the defective
> hub.
> >         Then: BINGO! full speed returned.  In fact, a speed about 19:45
> Saturday
> >         (Speakeasy's test) returned the rather unbelievable download
> speed of
> >         40.59 mbs! The upload speed was only 4.16. Obviously some sort
> of burst
> >         speed. Does Comcast have some way to know when I access a speed
> test site
> >         and goose the speed for a few seconds?
> >
> >         Sean
> >         ______________
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