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Contention

Stef
Grafter
Posts: 247
Registered: 13-08-2007

Contention

Please help me clarify something: Am I right or wrong with the following two statements?

1/ If a BT exchange gets busier and busier (ie more people signup for ADSL) your connection will inevitably get slower but this is nothing to do with PN traffic shaping? You just have to wait for BT to put in more capacity or switch to a LLU supplier (if available in your area).

2/ Two users of the same BT exchange can get different performance depending on how they are connected to that exchange (i.e. across how many 'cards' they are connected to?)

Am I on the right track here?

Is there a good HTTP:// explaining this?

Thanks, Stefan
15 REPLIES
Stef
Grafter
Posts: 247
Registered: 13-08-2007

Re: Contention

Quote
1/ If a BT exchange gets busier and busier (ie more people signup for ADSL) your connection will inevitably get slower but this is nothing to do with PN traffic shaping? You just have to wait for BT to put in more capacity or switch to a LLU supplier (if available in your area).


Or move to a lower contention product?
N/A

Contention

basically i believe it is this

you are on a 50:1 connection mean you share the pipe with another 50 customers and once it hits 50 then that is the pipe full and you have to share the resources of that pipe with the other 50 users.

same can be said that the other end when you are at plus.net which is a 30:1
LiamM
Grafter
Posts: 5,636
Registered: 12-08-2007

Contention

They are the theoretical maximums yes, but in reality, contention is generally much less than that. For example, if your connection speed dropped in half, it would still only be 2:1.
Stef
Grafter
Posts: 247
Registered: 13-08-2007

Contention

Quote
basically i believe it is this

you are on a 50:1 connection mean you share the pipe with another 50 customers and once it hits 50 then that is the pipe full and you have to share the resources of that pipe with the other 50 users.

same can be said that the other end when you are at plus.net which is a 30:1


So the contention 'throttle' is a BT throttle and has nothing to do with PN traffic shaping?

(Presumably if you are sharing your 50:1 exchange card with 10 users you will get faster speeds than if you are sharing with 50 - a full card?)

Are these really MUX cards? i.e. 50 pairs of copper terminate on one physical card before hitting System X?

(Sorry my telecoms knowledge is about 25 years out of date!)
Stef
Grafter
Posts: 247
Registered: 13-08-2007

Contention

Quote
They are the theoretical maximums yes, but in reality, contention is generally much less than that. For example, if your connection speed dropped in half, it would still only be 2:1.


Ok, but with 50 users all doing downloads that would be 1/50th of the bandwith each i.e. 1/50th of 2MB/sec? (not allowing for compression)

This would mean that if 50 users on the same 'card' were all doing simultaneous downlaods this would bring performance to its knees - or worse - irrespective of PN traffic shaping?

It seems to me that this is why constant p2p downloading is corrosive of shared bandwith.

The 'solution' pay for lower contention products?

What contention ratio are PAYG accounts?

(Sorry, Just trying to understand!)
N/A

Contention

yes you see a theretical degregation if your pipe is full but it is never allowed or suppose to hit the point before it starts putting onto a new pipe.

say the pipe hada max of 1gb bandwidth and all 50 spaces taken up you devide 1024 by 50 and that basically what you got but then again not all users max there connections every second........and u should get more out of the pipe ie max adsl and adsl2+ do up to 24mb/s

so the less the contention ratio there is the more bandwidth for you.
N/A

Contention

to answer you question to martinfamily it would not be 1/50 of 2mb connection as the wiring going to exchange and to your house can handle gb of data i believe so you are actually sharing the value of the pipe so if the pipe has 500mb bandwidth you can basically say 500/50 although this is not truely true due to ppl not maxing connections and such like.

p2p doesnt degrade everyone connections but it does effect it it just means that the bandwidth on that pipe is maxed out and they need to route it to a new pipe and once everything is maxed bt have to get off their lazy a** and upgrade the exchange again to handle the bandwidth.

you also got to remember not everyone will be on the net at once.
N/A

Contention

"What contention ratio are PAYG accounts?"

30:1 - it says on Plus's products web page, under the price block.
N/A

Contention

50:1 bt end 30:1 at plusnets end
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

Contention

Hi,

At each BT exchange there will be one or more DSLAM's. Each of these DSLAM's will handle both IPStream Office (20:1) and IPStream Home (50:1) and will have several Virtual Paths (VP's) taking all the data out of the exchange.

Each VP will be larger than the maximum speed that is offered at the exchange, i.e. they are larger than 2Mbps, for example they may be 10Mbps.

With a 10Mbps VP you can have up 1000 512kbps customers sharing that capacity for it to be within the 50:1 design limit

i.e. 1000 x 512kbps / 50 = 10Mbps

With all the regrades going on the VP's are being enlarged as more people upgrade to 1 and 2Mbps and in preparation for the faster speeds.

But as you can easily see with a 10Mbps VP it doesn't take too many customers for all the bandwidth to be used up and people see the effects of contention. As each VP is seperate it is certainly possible for just one VP at an exchange to see contention problems at any given time.
Len
Grafter
Posts: 79
Registered: 31-07-2007

Contention

Dave , could you explain something for me?
If the contention is 50:1 at my local exchange and your network is contended at 30:1 how will I see any benefit of your lower contention ratio?

Surely if the bottleneck is at my local exchange any benefit of your networks 30:1 will be masked by the 50:1 of the local end?
My thinking ( perhaps wrongly! ) leads to the conclusion that my maximum download speed will be limited by the section of the network that has the highest contention ratio. ( All other thing being equal)

I have not experienced any slowdowns but would like to know if my thinking is indeed correct.

Len
Stef
Grafter
Posts: 247
Registered: 13-08-2007

Contention

Quote
Hi,

At each BT exchange there will be one or more DSLAM's. Each of these DSLAM's will handle both IPStream Office (20:1) and IPStream Home (50:1) and will have several Virtual Paths (VP's) taking all the data out of the exchange.

Each VP will be larger than the maximum speed that is offered at the exchange, i.e. they are larger than 2Mbps, for example they may be 10Mbps.

With a 10Mbps VP you can have up 1000 512kbps customers sharing that capacity for it to be within the 50:1 design limit

i.e. 1000 x 512kbps / 50 = 10Mbps

With all the regrades going on the VP's are being enlarged as more people upgrade to 1 and 2Mbps and in preparation for the faster speeds.

But as you can easily see with a 10Mbps VP it doesn't take too many customers for all the bandwidth to be used up and people see the effects of contention. As each VP is seperate it is certainly possible for just one VP at an exchange to see contention problems at any given time.


Thanks for this... very informative (so is the article I read over on ADSL guide too)
Ben_Brown
Grafter
Posts: 2,839
Registered: 13-06-2007

Contention

The Plusnet contention ratio is used to calculate your assured rate, which is the slowest speed you should ever experience.

Although the BT contention ratio can affect the connection, most of the time there is more VP capacity available than is needed, so this doesn't have much of an effect.
N/A

Contention

Ben,

So are you just saying that the PN contention is actually 30:1, and that BT is only nominally 50:1 but actually much better than 50:1? Or are they different types of contention?
As lenhickman, I had always assumed that it was the same type of contention, e.g. http://www.comsoc.org/ci/public/preview/roberts.html for example Wink , so limited more by the 50:1

Geoff