[e2e] Skype and congestion collapse.

[Jon Crowcroft]

You're dead right about cable modem nets - i hadnt thought of them partly because i am 
a parochial brit and we haev a lot more dsl than cable...and tv has gone a different way for digital than i
expected

the project i was citing was called Higherview - funded by BT at UCL - 
http://www.cs.ucl.ac.uk/research/higherview/publications.htm
anna watson's PhD was i think the key work...the link to that there seems to work...though some of the others havnt
aged so well..

In missive <d7be02757083ca813a7611bdb778a4b2 at extremenetworks.com>, RJ Atkinson typed:

 >>
 >>On Mar 5, 2005, at 05:32, Jon Crowcroft wrote:
 >>> if we all took 576kbps DSL lines and sent 24*7 MPEG2, we would (today)
 >>> cause trouble - but in a few years time, who's to say what the minimum 
 >>> entry point is?
 >>
 >>Datum:
 >>	With DOCSIS cable modems, which are pretty widely used today throughout
 >>Europe, North America, Australia, Japan, and other parts of Asia, a 
 >>given
 >>cable modem subscriber has a single upstream frequency available.  On 
 >>that
 >>shared upstream frequency, the standard permits either QPSK or QAM 
 >>modulation.
 >>In practice, European cable plant will permit QAM to work yielding 
 >>rather
 >>more usable bandwidth.  Outside Europe, cable plant rarely supports QAM
 >>(from an RF S/N perspective), so one is stuck with QPSK.  This means 
 >>that
 >>the shared upstream can typically get (outside Europe) about 1.5 Mbps.
 >>It is unlikely that the cable plant would be reworked to support QPSK,
 >>because that is relatively expensive.  It is practical, however, to 
 >>space
 >>(georgraphically) division multiplex upstreams so that fewer customers
 >>are sharing a given upstream.
 >>
 >>	Even so, I would not be optimistic that a cable modem end user would 
 >>ever
 >>really be able to use 512 Kbps (or more) of upstream capacity (except in
 >>the much smaller geographic area, nearly all fibre, cable plant in 
 >>selected
 >>parts of Europe where QAM could be used upstream).
 >>
 >>[Typically, the DOCSIS upstream is operating in the lower "roll off" 
 >>region
 >>of the RF spectrum (e.g. ~27 MHz), where the CATV RF transmission gear 
 >>is
 >>becoming marginal.  This is done because there is (today) more revenue 
 >>from
 >>carrying an additional TV channel than from giving that same bandwidth 
 >>to
 >>upstream cable modem use.  The economics could change at some point,
 >>though right now that seems unlikely.]
 >>
 >>With DOCSIS, the shared downstream is significantly less of an issue,
 >>because RF S/N ratio is much better downstream and because the cable
 >>operator normally allocates one TV channel (not in the roll off region)
 >>for that purpose.
 >>
 >>
 >>> you know what: users do NOT like variable quality -if you  aregoing to 
 >>> support a given rate, dont go ABOVE it if you are later going to have 
 >>> to go back down
 >>> to that rate  0 if you have a lower rate, only support that. this is 
 >>> critical
 >>> for audio (but less so for video) - so all the work on fancy codecs 
 >>> and user/channel/codec adaption we all did 2 decades back for 10 years 
 >>> - you know,
 >>> was all misguided.
 >>
 >>Very interesting result, IMHO.  Thanks for describing both test regime
 >>and results.  Is there a suggested bibliographic citation or two to 
 >>read ?
 >>
 >>> so if you look at the Book we wrote on all this stuff (Internetworking 
 >>> Multimedia, Handley/Crowcroft/Wakeman -
 >>> morgan kauffman pubs), we were wrong (though all the stuff on rtp and 
 >>> sip and realtime on IP and multicast there is
 >>> is still pretty up-to-date:)
 >>
 >>Sounds like time for a 2nd Edition. :-)
 >>
 >>Cheers,
 >>
 >>Ran
 >>rja at extremenetworks.com
 >>

 cheers

   jon