[LINK] Net neutrality and bandwidth caps.

Robin Whittle rw at firstpr.com.au
Wed May 6 20:43:45 AEST 2009

This is completely untrue:

>> Bandwidth costs
>> are almost zero, and given how things are set up, there is no way a  
>> single person or even a small number can max out the bandwidth of a  
>> cable loop.

HFC cable has limited downstream bandwidth which must be shared,
depending on how many 6 to 8MHz RF channels are devoted to cable
modems.  These are about 4 bits per Hz, if I remember correctly.

The upstream link is far worse, with upstream only between 30 and 45
or so, maybe a little higher.  I recall less than 1 bit per Hz there.

Since maybe as much as 50% of traffic is peer-to-peer:


with typically symmetrical upstream and downstream, it is clear that
this is not going to work out well on a cable modem system.

A cable system may server 1000 homes or so.  If 300 of them have
cable modems, operating at the same time - especially with P2P file
sharing programs which run 24 hours a day - it is easy to see that
this sort of usage does come up against hardware limits.

It would be possible to break the system into 10 with 100 homes each.
 That would help a lot, but it involves taking fibre deeper into the
access network, and more expense with opto-electronics there and in
the head-end.

Upstream is less efficient per Hz due to mixing of noise from all
sources, including the cumulative noise of the upstream amplifiers in
series, and multiple tributaries of these feeding the electro-optical
unit which converts the upstream frequencies to an RF modulated
optical signal.  Also, there is the need for multiple cable modems to
take it in turns to transmit, so even if half the frequency range (30
to 750MHz is the typical range) was used for upstream, there would
still be about 1/4 the upstream bandwidth compared to downstream.

For historical reasons, including compatibility with existing cable
TV system which I understand had FM channels (or is it to avoid cable
modems transmitting at FM frequencies, and due to the need for a
substantial guard band between upstream and downstream (analogue
filters in the amplifiers), HFC is bound to be suffering from limited
upstream capacity.  Also, I think both HFC and ADSL were designed
with the fantasy of customers being good little consumers, sucking
lots of "content".

I am not suggesting that there are no problems with bandwidth caps
etc.  Just that if there is going to be an argument against them, it
needs to be based on the realities of the HFC and DSL systems, and
the realities of the upstream costs.  The sentence I objected too
makes me think of a spoilt child insisting that everything they want
is free and should be made available to them without any fuss.

  - Robin

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