Kitz Forum

Broadband Related => Broadband Technology => Topic started by: tickmike on September 28, 2013, 11:02:39 AM

Title: Contention ratios ?.
Post by: tickmike on September 28, 2013, 11:02:39 AM
I asked my ISP (Eclipse) about if there is any difference on the 'Contention ratios' on a Business or Home package ?, part of there answer is below ..! .
Is this correct what they say ?.

"Contention ratios are no longer used when referring to exchanges. The reason for this is that the installation of Fibre backhauls means that Contention is no longer a relevant issue.

With regards to your second query: your speed will be affected by the exchange you are connected to, but again Contention ratios will not factor into your connection."
Title: Re: Contention ratios ?.
Post by: GigabitEthernet on October 05, 2013, 02:37:34 PM
Contention could be an issue. I am pretty sure business customers get higher priority traffic during business hours, meaning that if the exchange is congested, their traffic will be faster.

I don't think you're likely to notice it though.
Title: Re: Contention ratios ?.
Post by: Black Sheep on October 06, 2013, 08:53:09 AM
Kitz is the expert on this, and I'm sure she'll be along to clarify the situation shortly ? :)

Just my own thoughts, as I've no idea really, but isn't 'contention' down to the amount of backhaul the ISP rents from BTw, versus, the amount of EU's with that same ISP who are online ??
I don't know where I've got that info from, but am sure I was told that the ISP's rent 'x' amount of backhaul bandwidth, after applying some kind of diversity factor ?? IE: If the ISP has an EU base of 1000, then diversity factor dictates that only 700 will probably be on-line at any one time. Therefore, the backhaul will be based around the 700 figure.

As I say, I can't remember for the life in me where I've heard or been told that, but it kinda makes sense in my head.  :)
Kitz, please enlighten us.  ;D
Title: Re: Contention ratios ?.
Post by: sheddyian on October 06, 2013, 08:56:46 AM
In the early days of broadband, I can remember some ISPs quoting contention ratios of 25:1 (25 users sharing one what?) and business / premium products being offered with a lower contention rate.

Since it's a long time since I've seen anything like this being quoted, I assume it's no longer relevant?

Title: Re: Contention ratios ?.
Post by: roseway on October 06, 2013, 10:03:25 AM
The Kitz page on the subject is here:

Contention isn't used now in any formal sense, but congestion can still exist in any part of the network.
Title: Re: Contention ratios ?.
Post by: Chrysalis on October 15, 2013, 12:29:08 PM
the backhauls will be contended, they have to be to be cost effective, plus to comply with ofcom regulations however contended != congestion, the 2 are different things.

eg. on my home lan if I have 5 pcs connected to my router at once, I am contending the WAN connection across those 5 pc's but if I only use one at a time there is no fighting for the bandwidth.

On adsl when they advertised 50:1 the actual contention was much lower then that, likewise office was typically contended lower than 20:1, when adsl max launched contention ratios did increase, and in some cases to what I consider abusive levels, eg. it became known plusnet for a while contended over 200:1 within their BT central pipes.  So one reason it stopped been disclosed was some isp's were exceeding 50:1 so they could sell higher speed adsl services at the same price as the old lower speed adsl services. but also the fact now days contention ratios are very dynamic, isp's instead try to just not have 'visible' contention, so the actual contention ratio will change as they see fit.
Title: Re: Contention ratios ?.
Post by: kitz on October 15, 2013, 01:36:01 PM
Sorry only just seen this - still have catching up to do :/

Contention ratios aren't in use any more.   They went out when BTw brought in CBC (Capacity Based Charging) for the ISP centrals and it was basically down to the ISP how many users they placed on their pipes.  Some ISPs managed it much more efficiently than others eg: Zen - v - Virgin dsl. 

Slightly later when maxdsl came in then you cant put an official contention ratio on ISP/BT bandwidth, because different users connect at so many different sync speed making it impossible to do the old type maths of say 50 @ 512kbps.

Instead now its majorly up to the ISPs to monitor how much bandwidth all of their users are using on their pipes.  BT provide graphing abilities and once a pipe bandwidth approaches 98% full then the pipe can start to run hot and you may see some packet loss.  Time for the ISP to order more bandwidth.  When it reaches 100% capacity EU speeds drop for all users.

Bandwidth is expensive for the ISPs so they try deter overuse by customers and its why you now get restrictions such as traffic shaping or caps.  Prior to CBC these were unheard of.

Business users pay more, and the ISPs can deal with this in a few ways
1) Higher download caps
2) Less traffic shaping
3) Have pipes reserved purely for their business accounts which have more spare capacity than the home users.  This spare capacity means that business users would be less likely to see congestion than their home users.

Congestion can still be an issue at an exchange level, sometimes BT may be caught out and certain VPs may run 'hot' until BTw get around to lighting more capacity. (This is what dark fibre was all about).  Hot VPs are/were more likely to occur on 20CN exchanges as their VPs were smaller and based around the line cards that people shared.
21CN is more efficient in that the MSAN has large 'super' VPs rather than split into chunks like with the DSLAMs.

BS is right there are figures for the ISP - its called session limits and BTw police these.  It used to be a maximum of 25,600 EUs for a 622Mb central.  The ISP estimates that at anyone time say only 70% of their users will actually have their routers switched on, and take this into account when planning how much bandwidth to buy for their centrals.   Problems occur though when an ISP has a large customer base who keep routers on 24/7 and their may be times when some users cant connect. When CBC first came in, it wasnt unusual to have to try a couple of times to connect, but these days its much rarer as the ISPs now have a better idea of their customer base habits.

Theres more info here about the  BT Centrals (   Be aware though that Centrals are gradually being replaced and within the next year or so WBC/WBMC ( will mean that all BTw ISPs will be expected to purchase MSILs or dedicated host links.

Theres more info ~ What is a MSIL? ( together with the 3 options an ISP has of purchasing bandwidth & host links.  There is also currently a 4th option (IPSc) but that will be also gradually be wound down after BTw stop supplying new centrals in 2014. Lifetime of a central is about 2 yrs, so centrals/IPStream/Datastream/IPSc will likely be totally gone by 2016 - if not sooner. I think all but the smallest ISPs are already moving over... the larger ones already have.

And of course things are different again for LLU. 

Title: Re: Contention ratios ?.
Post by: Chrysalis on October 15, 2013, 04:12:07 PM

Kitz before CBC came in the price premium used to be on the speed of the adsl port.  eg. a 512kbit adsl port was much cheaper than a 2mbit adsl port.  So adsl products were fixed speed, if a line couldnt handle that speed it had no service.  So based on that contention was easier to manage but also the fact back then BT backhauls (the bt central pipes) were much cheaper, isps and ofcom however decided this was holding back broadband in the uk so things changed,

So in came adsl max, the ports cost the same as old fixed speed 512kbit ports, meaning burst speed now had no premium attached  but to protect BT's revenue the price to the isp's was shifted to BT central capacity, these pipes shot up in price.  The only way isp's could make the numbers work was to increase the contention on these pipes.  This is how retail prices were kept down for the new upto 8mbit services.  So previously congestion was almost unheard of then isp's started struggling.  Apparently they didnt expect people to use the internet more when they had faster speeds :D we heard that how many times? :) 

The reason things have got better now is BT's 21CN network.  After they built this BT started phasing out the old 20CN legacy network.  The BT backhaul on 21CN which is WBC, WMBC and IPSC is significantly cheaper per mbit than the old BT centrals were. So capacity is cheaper, meaning contention levels are lower, congestion is less and of course what we now seeing as well isp's dropping traffic management policies.  LLU is still cheapest to run but WBC is now closer to it than older 20CN backhaul.

The isp's I know of using LLU backhaul are zen. sky and talktalk.  Zen dont use it for LLU adsl but are using it for FTTC services and its how they have managed to start selling unlimited FTTC.

VM were planning on selling FTTC services using their own backhaul as well, but after the liberty global takeover they done a huge U turn.