Re: [Hampshire] Broadband Issues

Hugo Mills <hugo@???> wrote:
> Only in this case, it isn't. It's showing the
> maximum protocol speed, not the (likely) data
> rate between the modem and the exchange,
> which is generally the limiting factor.

Is that a feature of the DG834? The routers I've
worked with all display the current sync rate.

That aside, the router will be reporting the ATM
rate - the IP-layer bandwidth will a little less.
At an ATM data rate of ~8Mb/s, the IP rate is
around 7.5Mb/s.

>> The actual measured rate is the rate between
>> the client PC and a test server. To get to that
>> server, the data must pass through the ISP's
>> network. That gives the ISP all sorts of
>> control over how fast data is transferred.
>> Alternatively, the maximum speed is dictated by
>> the maximum speed that the ISP can support
>> internally.
> Not really. The maximum speed of the ISP's data
> network is likely to be many order of magnitude
> faster than the maximum speed of the line
> between house and exchange, assuming low
> contention(*).

Yes, the ISP's network is likely to be much faster
than the DSL connection. However, you're sharing
it with lots of other people.

The first limit is the exchange contention -
typically 50:1 for IPStream; your 8Mb/s has been
sold fifty times over by BT. Some services - like
LLU - are available at 20:1 contention, and I
believe 21CN is available uncontended.

At the other end of the carrier's network, there's
the BT Central or LLU equivalent. These are fixed
bandwidth - 155Mb/s or 622Mb/s for BT, typically
1Gb/s for LLU - and are shared by several thousand
DSL connections. These need to be manually load-
balanced to ensure they're not saturated, as each
DSL connection is assigned to a single central.

_Then_ you get to the ISP's network. End-user
L2TP traffic has to be switched and terminated,
then the IP traffic routed to peers or Tier-1
transit. I expect most ISPs have 1Gb/s backbones
now, but some of the smaller ones may well have
100Mb/s links. Only the larger networks will be
running at 10Gb/s, and then only in the core.

The peering and transit feeds are a further
limitation. LINX and LONAP - the two Internet
Exchanges in London - offer 1Gb/s links, but many
companies only have 100Mb/s connections. Tier-1
transit links may also be contended, as the big
boys make you pay for your bandwidth.

You're then onto the wider Internet, where
anything goes.


Vic's point is valid - traffic shaping is
possible, and is being done. It's an attractive
way for ISPs to control their bandwidth costs, and
the margins are narrow in this business.

Nick.

--
Nick Chalk ................. once a Radio Designer
Confidence is failing to understand the problem.