Can someone explain contention?

When a broadband service says they have a contention ratio of say 50:1
what is it that the 50 customers are sharing? If all 50 customers are on
the ISP's 512kbps service they can't be sharing a single 512kbps
connection as you would notice slow down all the time. I'm guessing it
is a much larger "pipe?"

tia

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On Wed, 28 Sep 2005 21:46:45 GMT, "Cullen Skink"
<(E-Mail Removed)> wrote:

>When a broadband service says they have a contention ratio of say 50:1
>what is it that the 50 customers are sharing? If all 50 customers are on
>the ISP's 512kbps service they can't be sharing a single 512kbps
>connection as you would notice slow down all the time. I'm guessing it
>is a much larger "pipe?"

forget "50 customers"

50:1 is a ratio. Lets call it 25600 : 512 instead. Or 450:9, etc.

Contention is defined as the amount of bandwidth sold to customers
divided by the amount available at the point in question.

So if 400 people with 1M lines are contended 50:1 there will be a
connection capacity of 400 * 1 / 50 = 8 Mbits/s.

Also note that in BT ADSL the ratio is "up to" 50:1 and generally held
to be about 40% of that level in practice.

Phil
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Cullen Skink wrote:
> When a broadband service says they have a contention ratio of say 50:1
> what is it that the 50 customers are sharing? If all 50 customers are on
> the ISP's 512kbps service they can't be sharing a single 512kbps
> connection as you would notice slow down all the time. I'm guessing it
> is a much larger "pipe?"
>

Nope, right first time. It relies on all 50 customers not requiring
bandwidth simultaneously but if they did the connection speed would slow
to 10kbps for each of them until demand started to drop again.

--
Tony

"I did make a mistake once - I thought I'd made a mistake but I hadn't"
Anon

On Wed, 28 Sep 2005 22:59:33 +0100, Phil Thompson
<(E-Mail Removed)> wrote:

>On Wed, 28 Sep 2005 21:46:45 GMT, "Cullen Skink"
><(E-Mail Removed)> wrote:
>
>>When a broadband service says they have a contention ratio of say 50:1
>>what is it that the 50 customers are sharing? If all 50 customers are on
>>the ISP's 512kbps service they can't be sharing a single 512kbps
>>connection as you would notice slow down all the time. I'm guessing it
>>is a much larger "pipe?"
>
>forget "50 customers"
>
>50:1 is a ratio. Lets call it 25600 : 512 instead. Or 450:9, etc.
>
>Contention is defined as the amount of bandwidth sold to customers
>divided by the amount available at the point in question.
>
>So if 400 people with 1M lines are contended 50:1 there will be a
>connection capacity of 400 * 1 / 50 = 8 Mbits/s.
>
>Also note that in BT ADSL the ratio is "up to" 50:1 and generally held
>to be about 40% of that level in practice.

I presume by that you mean in the second mile. (BT DSLAM to IP core)

If a reselling ISP has too many users* on a BT Central, I guess that
can become (or more likely, is) the determining factor. Correct?


*Around 8000 or more
--
Mark

No, the critical factor is the amount of traffic on your network, which
depends on the types of user and the amounts of bandwidth they are
consuming. For example, if you had a business park and a housing
estate sharing the same contended ethernet ring on a Cable TV network,
you'd expect the (small number of) business users each to be consuming
lots of bandwidth between 9-12 and 1-5, with little week-end usage, and
then the (large number of) consumers each to be consuming smaller
amounts of bandwidth from 4-10 and at week-ends.

Nick

On Thu, 29 Sep 2005 12:43:35 +0100, Mark <(E-Mail Removed)>
wrote:

>I presume by that you mean in the second mile. (BT DSLAM to IP core)

DSLAM to ATM "cloud" is the stated produce contention as far as BT are
concerend.

>If a reselling ISP has too many users* on a BT Central, I guess that
>can become (or more likely, is) the determining factor. Correct?

"reselling" is a misnomer I suspect, but yes if the contention is
greater on the BT Central then it will be the limiting factor not the
exchange backhaul contention. Hence Plusnet's "contention on the
Plusnet network" quoted in addition to the "BT contention".

In practice we have seen both exchange contention "Red VPs" and ISPs
with congestion problems (Demon Home)

With Centrals at £240/month per Mbit/s I suspect some Central
contentions are running well over 50:1 to make it pay. Doesn't matter
if the user demands are modest.

Phil
--
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http://bbs.adslguide.org.uk/postlist...&Board=tiscali

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Tony Raven wrote:
> Cullen Skink wrote:
>> When a broadband service says they have a contention ratio of say
>> 50:1 what is it that the 50 customers are sharing? If all 50
>> customers are on the ISP's 512kbps service they can't be sharing a
>> single 512kbps connection as you would notice slow down all the
>> time. I'm guessing it is a much larger "pipe?"
>>
>
> Nope, right first time. It relies on all 50 customers not requiring
> bandwidth simultaneously but if they did the connection speed would
> slow to 10kbps for each of them until demand started to drop again.

Surely not. Even with 4 people using it at the same time you would
notice a big drop off in performance and that doesn't seem to happen. If
one person was connected 24/7 doing downloads how could anyone else use
it? I think I'm missing something. :-)

On Thu, 29 Sep 2005 22:54:41 GMT, "Cullen Skink"
<(E-Mail Removed)> wrote:

>Surely not. Even with 4 people using it at the same time you would
>notice a big drop off in performance and that doesn't seem to happen. If
>one person was connected 24/7 doing downloads how could anyone else use
>it? I think I'm missing something

the 50 people thing is the error. The pipe might be 10 Mbits/s so it
needs 20 512k users at maximum download to fill it. At 50:1
contention there will be *up to* 1000 users able to use it, so 20
assholes can make life painful for a lot of people.

The rather feeble 2M backhauls used by datastream operators in the
early days were often completely utilised by a single 2M user so
nobody ever say their rated speed unless they happened to be the only
one using it.

Phil
--
Tiscali - dialup speeds at Broadband prices, see
http://bbs.adslguide.org.uk/postlist...&Board=tiscali

AOL - the unlimited ISP of choice for heavy downloaders.

Phil Thompson wrote:
> On Thu, 29 Sep 2005 22:54:41 GMT, "Cullen Skink"
> <(E-Mail Removed)> wrote:
>
>> Surely not. Even with 4 people using it at the same time you would
>> notice a big drop off in performance and that doesn't seem to
>> happen. If one person was connected 24/7 doing downloads how could
>> anyone else use it? I think I'm missing something
>
> the 50 people thing is the error. The pipe might be 10 Mbits/s so it
> needs 20 512k users at maximum download to fill it. At 50:1
> contention there will be *up to* 1000 users able to use it, so 20
> assholes can make life painful for a lot of people.

Thanks, the fog is lifting, I think. :-) I'm guessing they try to keep
the "up to" 1000 users much lower than that then as at that rate only 10%
online at any time would swamp it and 10% seems a lowish figure.

> The rather feeble 2M backhauls used by datastream operators in the
> early days were often completely utilised by a single 2M user so
> nobody ever say their rated speed unless they happened to be the only
> one using it.

Was thinking back to that as well. When BB was first being introduced
there was talk of it slowing down with only a few users online at a time
which is what got me thinking about it as it doesn't "appear" to be the
case now.

Is the contention ratio per exchange or is it configurable across the
network? I'm on datastream which probably makes a difference.

On Fri, 30 Sep 2005 09:36:14 GMT, "Cullen Skink"
<(E-Mail Removed)> wrote:

>Thanks, the fog is lifting, I think. :-) I'm guessing they try to keep
>the "up to" 1000 users much lower than that then as at that rate only 10%
>online at any time would swamp it and 10% seems a lowish figure.

its not 10% online its 10% going flat out, which is different. Its
built for web surfing and collecting email, which aren't flat out
applications.

>
>Is the contention ratio per exchange or is it configurable across the
>network? I'm on datastream which probably makes a difference.
>

contention ratio is strictly per virtual path, there may be several of
them handling IPstream. Business services should be on a different VP
as they have different contention (20:1 vs 50:1). For datastream there
is likely to be one VP and its likely to be much smaller due to small
number of users of one ISP on the exchange.

Phil
--
Tiscali - dialup speeds at Broadband prices, see
http://bbs.adslguide.org.uk/postlist...&Board=tiscali

AOL - the unlimited ISP of choice for heavy downloaders.