Contention Ratios

If the contention ratio of my connection is 50, I am sharing my Internet
link with 49 others.

Is that by hard-wiring, or software configurable?

Is it always the same 49? Or does it vary with the order of people
connecting?

Who decides who the bedfellows are? BT, or the ISP?

--
Joe Soap.

Death is nature's way of telling you to slow down.

> If the contention ratio of my connection is 50, I am sharing my Internet
> link with 49 others.

Up to 49 others

> Is that by hard-wiring, or software configurable?

It's configured by the local exchange. Neither your ISP or the telecoms
provider have any direct control.

> Is it always the same 49? Or does it vary with the order of people
> connecting?
>
> Who decides who the bedfellows are? BT, or the ISP?

Joe Soap wrote:
> If the contention ratio of my connection is 50, I am sharing my Internet
> link with 49 others.

You are sharing bandwidth, not a physical connection.
>
> Is that by hard-wiring, or software configurable?

Neither.
>
> Is it always the same 49? Or does it vary with the order of people
> connecting?

Neither.

It refers to the ratio of user available bandwidth (which is the download speed
on all of the lines connected to an exchange) to the available bandwidth between
the exchange and the rest of the network (the 'Backhaul')

It is more of a planning figure than a hard rule. If, as an example, BT have
100 customers on an exchange all with 5Mbit/sec lines, that's 500Mbit/sec of
bandwidth. At 50:1 contention, they will plan to make 1/50th of that bandwidth
available to the exchange, or 10Mbit/sec. BT add bandwidth in somewhat larger
chunks than 10Mbit (the BT centrals are 155Mbit I beleive) so if you are lucky
enough to be connected to an exchange with only 100 customers, you will never
see contention hit your throughput.

Bear in mind that BT are adding bandwidth to exchanges based on 'expected' use,
and not hard usage figures, so your exchange may be above or below its bandwidth
capacity.

The cost of providing 8Mbit of uncontended bandwidth per user would be
ridiculous (atleast with today's technology. It will change, one day)

Back to the example above, if two of those 5Mbit/sec users hammer their lines
flat out, they would saturate the 10Mb backhaul, and nobody else can get in,
right? Again, yes and no, because traffic shaping and quality of service built
into the DSLAM will even things out.

Also remember that not all users on an exchange have their packets sent to the
same BT central (we all use different ISP's, so my packets go a different route
to yours), and contention for bandwidth is likely to be more of an effect there.
An ISP which has thousands of customers, but has stripped costs to the bone,
probably is not providing enough core bandwidth, and all of their customers
suffer as a result. So it doesn't matter if your connection pegs at 8128kbps
because you practically live in the exchange, and nobody else in the city has
broadband, because the line will run at the rate of the slowest pipe in the
system (Think 'M25') (and you can probably guess who these ISP's are, eh?)


Different classes of service (50:1, 20:1, 1:1, etc.) are sent down different
virtual pipes with different bandwidth allocations on the backhaul, so that
people who have been sold a 1:1 service will always get the full bandwidth,
atleast as far as the BT central, etc.


There are probably a number of sites explaining all of this in far more detail,
with nice diagrams, etc. Google for things like 'contention', 'backhaul',
'dslam', etc.

Jim Howes wrote:
> Also remember that not all users on an exchange have their packets sent to the
> same BT central (we all use different ISP's, so my packets go a different route
> to yours), and contention for bandwidth is likely to be more of an effect there.

Hmm. I'm thinking that you're confusing two things. BT Centrals to the
ISPs are separate to the exchange backhaul.

> An ISP which has thousands of customers, but has stripped costs to the bone, probably is not providing enough core bandwidth, and all of their customers suffer as a result. So it doesn't matter if your connection pegs at 8128kbps because you practically live in the exchange, and nobody else in the city has broadband, because the line will run at the rate of the slowest pipe in the system (Think 'M25') (and you can probably guess who these ISP's are, eh?)

Well that's true but that will be down to the number of BT Centrals the
ISP has from the ATM network to the ISP's gateways.

This link http://www.adslguide.org.uk/qanda.as...technical#Q257 gives
a fairly decent discussion of contention and backhaul issues.

Geoff

Joe Soap wrote:
> If the contention ratio of my connection is 50, I am sharing my Internet
> link with 49 others.

no, that isn't how it works.

Firstly BT no longer quote contention rates, which in practice were
about half the stated level.

Secondly the contention ratio is an overselling factor bandwidth sold /
bandwidth available, so even if it were 50:1 this is more like to mean
that 250 of you are on a connection equivalent to 5 of you at full
capacity. Its a ratio, not a number of people on an anything.


Phil

On 13 Oct 2006 15:15:34 -0700, "PhilT" <(E-Mail Removed)> wrote:

>Firstly BT no longer quote contention rates, which in practice were
>about half the stated level.

They were NOWHERE NEAR half the stated level. You've obviously never
seen 25:1 contention because if you had you wouldn't forget it. I kid
you not.

Prior to max the only exchanges which hit that figure (or came close)
were some of the smaller ones which were activated under the old
system where you had to pre-register your interest. kitz suffered from
that if I remember correctly and had the backhaul figures explained to
her by someone fairly high up in wholesale - still on her site I
think? (www.kitz.co.uk) The maximum download speed she got was less
than her 56k modem IIRC. This was every single evening - peak time of
course.

I'm on an exchange where VoIP is impossible as even a 128kbps audio
stream will cause latency to increase to >500ms (my current record for
latency is 8700ms). It IS the exchange that is the problem as everyone
I have spoken to, regardless of ISP and whether they're on what used
to be 20:1 or 50:1 has the same problem. Its not max-dependent either
as it affects people who have been "maxed" and people who haven't. Is
it going to be "fixed"? No - its coming up for a year since it went
like this (coincided with new DSLAMs going in I think). I've taken
this as far as it is possible to take a complaint and got nowhere -
this is where the published contention levels may have helped, but now
of course BT can point at minimum speeds and say if it does that then
its fine. It isn't of course but why let the truth get in the way eh?

As soon as this exchange gets LLUd I'm off to whoever. I don't even
care if its Tiscali who unbundle it as ANYONE would be better than BT.
Before someone sticks their oar in - yes I do know that Tiscali are
crap but BT (here anyway) are worse.

ADSL was a hell of a lot better BEFORE BT dropped the published
contention levels. YMMV of course.
--
John Naismith

In response to what John Naismith <(E-Mail Removed)> posted in
news:(E-Mail Removed):

> but now
> of course BT can point at minimum speeds and say if it does that then
> its fine.

What prompted my query [partly, at least] was the fact that BT say a 1Mbs
connections ia regarded as 'normal' if the *actual* speed is in the range
200 - 1000kbps.

Who signs up knowingly for 1Mbps that might onkly be 200k?

--
Joe Soap.

Death is nature's way of telling you to slow down.

On Sat, 14 Oct 2006 08:56:24 UTC, Joe Soap
<Use-Author-Supplied-Address-Header@[127.1]> wrote:

> In response to what John Naismith <(E-Mail Removed)> posted in
> news:(E-Mail Removed):
>
> > but now
> > of course BT can point at minimum speeds and say if it does that then
> > its fine.
>
> What prompted my query [partly, at least] was the fact that BT say a 1Mbs
> connections ia regarded as 'normal' if the *actual* speed is in the range
> 200 - 1000kbps.
>
> Who signs up knowingly for 1Mbps that might onkly be 200k?

People do. After all, it's not a static thing; it will vary with day,
time, etc. depending on usage. It's not a fixed share.

--
[ 7'ism - a condition by which the sufferer experiences an inability
to give concise answers, express reasoned argument or opinion.
Usually accompanied by silly noises and gestures - incurable, early
euthanasia recommended. ]

In response to what Bob Eager <(E-Mail Removed)> posted in
news:176uZD2KcidF-pn2-(E-Mail Removed):

> On Sat, 14 Oct 2006 08:56:24 UTC, Joe Soap
> <Use-Author-Supplied-Address-Header@[127.1]> wrote:
>
>> In response to what John Naismith <(E-Mail Removed)> posted in
>> news:(E-Mail Removed):
>>
>> > but now
>> > of course BT can point at minimum speeds and say if it does that
>> > then its fine.
>>
>> What prompted my query [partly, at least] was the fact that BT say a
>> 1Mbs connections ia regarded as 'normal' if the *actual* speed is in
>> the range 200 - 1000kbps.
>>
>> Who signs up knowingly for 1Mbps that might onkly be 200k?
>
> People do. After all, it's not a static thing; it will vary with day,
> time, etc. depending on usage. It's not a fixed share.

But when I complained to the ISP about speed being consistently sub-300k,
they pointed to this BT link and effectively said 'tough, sod off'.

--
Joe Soap.

Death is nature's way of telling you to slow down.

On Sat, 14 Oct 2006 09:58:52 UTC, Joe Soap
<Use-Author-Supplied-Address-Header@[127.1]> wrote:

> >> What prompted my query [partly, at least] was the fact that BT say a
> >> 1Mbs connections ia regarded as 'normal' if the *actual* speed is in
> >> the range 200 - 1000kbps.
> >>
> >> Who signs up knowingly for 1Mbps that might onkly be 200k?
> >
> > People do. After all, it's not a static thing; it will vary with day,
> > time, etc. depending on usage. It's not a fixed share.
>
> But when I complained to the ISP about speed being consistently sub-300k,
> they pointed to this BT link and effectively said 'tough, sod off'.

That's (probably) a different problem, and it might be that your line
can't do more. A good ISP will investigate (or get BT to).

--
[ 7'ism - a condition by which the sufferer experiences an inability
to give concise answers, express reasoned argument or opinion.
Usually accompanied by silly noises and gestures - incurable, early
euthanasia recommended. ]