Tutorials - ADSL > Understanding Contention

Tutorials & FAQs: ADSL: Understanding Contention

This Tutorial no longer represents the way that ADSL and ADSL2 products are delivered by BT, but the detail remains of interest

As people should be aware, the ADSL product line sold by PlusNet is a contended service of either 20:1 or 50:1. These are the only two contentions currently possible under the BT IPStream package, which PlusNet uses and as provided to them by BT.

DataStream ADSL providers have far more control over the connection including contentions of 1:1. DataStream does not allow for different speeds, instead, the provide has almost full control over the process used in Step 2 below.

People get very confused on what contention is and isn't, even those people that do know, can still be in the dark to many of the facts.

Cold hard fact:
The word Contention means to "To compete in order to win something".

To extend that, it means that when you are using your ADSL connection, you are competing with other people, in order to gain certain aspects of the service.

Be your account a 20:1 or 50:1, this means you are competing with either 19 or 49 other users/connections, in order to get the full 512Kb, 1Mb or 2Mb line speed of the product you are on.

That is only a basic concept of contention, or at least the contention that ISPs report. At this point, many will still be confused, or have a pretty good grasp. To understand it further, we have to look at exactly how your ADSL connection traverses the BT network, from your premises to your selected ISP.

This should give you a better understanding of what ISPs call ADSL contention, but it is likely to disturb others of you, when you understand how deep it actually goes.


Step 1 - Your phone line

Your line connects to the exchange DSLAM equipment, which is designed to handle your incoming ADSL signal. It then converts it into something more presentable and capable of traveling the longer distances of the BT network.

There is no contention at this point, as it is required that each line is connected to its very own port of the DSLAM. That is to say, contention here is 1:1 or one line into one port.


Step 2 - Linking your exchange to the BT POPs

This is the real start of the contention ballgame.

Your data, and that of other ADSL customers, are transferred onto the BT ATM network using backhaul links from the exchange.

The size of the exchange, the number of 20:1 and 50:1 customers (ie, is it mainly business or residential customers?) and the existing connections to the rest of the BT network, will all help determine what type and size of connection this is.

Many people assume that because 50 users are sharing bandwidth, that means there are 50 users connected to a single 512Kb back-haul (50:1 account type).

This is not true. If it were like this, then there is a very extreme chance of one single user using the full potential of the connection in one go, leaving others chewing dust and crawling.

Instead, BT provide 4Mb/s back-hauls as standard. As noted above, the size and type will depend on the exchange. A little calculation is needed here to show what happens. Let's base our example on a 512Kb/s 50:1 IPStream Home connection.

First, let's work the 4Mb into Kb:
4Mb/s = 4 * 1024 Kb/s = 4096Kb/s

Next, how many 512Kb/s ADSL lines should this supply:
4096 / 512 = 8

This means, that only 8 customers could be supplied at a contention ratio of 1:1, ie, no customer could affect another user's speeds.

Because this is a 50:1 product, the following applies
50:1 * 8 = 400
For those who are not sure, to perform maths using ratios, you calculate how many times the right
goes into the left. 50:1 is the same as 50/1. As such, the value is 50


That is to say that up to 400 customers will share that single 4Mb/s back-haul. The 50:1 advertised ratio is still maintained, only instead of working with a single block of 50 customers, this back-haul handles 8 lots of 50:1 (IE 8 lots of 50) customers.

There is a chance that a single customer can quickly fill the whole bandwidth pipe (as noted above), by using this method of increased pipe size, lighter users can use the bandwidth that the heavy user isn't. This gives people a much better and fair chance.

There is still a chance that a few customers could quickly fill and saturate the whole bandwidth; however, unlike 50 users on one 512Kb/s connection, they would all need to be using there connection to the max at the same time. Although there may be people like this on a single exchange, you are more likely to see them doing this at different times of the day.

At least, that is the reasoning behind why they supply back-hauls like this and it works very well in most parts.

That being said, contention is contention. ADSL is sold as a contended product and from time to time, or in the future, it may be a much larger problem.

BT try to upgrade in advance of full capacity, and in most cases, do not even fill a single pipe to the maximum levels.

The same applies to other 50:1 products and to 20:1, though BT do use separate back-hauls for 50:1 and 20:1 customers. This is mainly for quality control.

Look at it this way, to mix 20:1 accounts and 50:1 accounts would see the ratio decrease for 50:1 users, and increase for 20:1 users. As you can see, it keeps people happy (or tries to).

With the introduction of the new 1Mb 50:1 home product, many people are further confused if contention is going to kick in on them more. Provided BT are performing their calculations correctly, then contention will be no more of an issue.

It is possible for a 4Mb/s back-haul to handle a mixed number of customers and the different 50:1 account types. The following could be one example of how this is done. Note: This is unconfirmed. BT does not let slip on all that much detail.

200 users on 512Kb/s 50:1 would take up 1/2 (2Mb/s) of the back-haul
100 users on 1Mb/s 50:1 would take up 1/2 (2Mb/s) of the back-haul

That totals to 300 users, yet contention levels of 50:1 are still being maintained.

The problem with 1Mb ADSL is the increased chance of back-haul saturation. Where four 512Kb/s users use 1/4 of the 4Mb/s back-haul capacity, it only takes 2 1Mb/s to use the same.

As you can see, this has the potential for the effects of contention to kick in more quickly. This will depend on how people user their newer and faster connections.

Under a worst case scenario, customers should see speeds reduced to a certain level. Because products are sold with contention, ISPs and BT are under no obligation to fix things when speeds are under these levels.

That being said, this should never happen and if it were to, you would likely see a boycott of ADSL. See the end of this FAQ for a few hints on what to do when you have a connection under that which you expect it to perform.

Here is a list of each product type available under IPStream and the minimum you should ever see with contention:

  • 512Kb/s 50:1
    • 10.24Kb/s (1.28KB/s)

  • 1Mb/s 50:1
    • 20.48Kb/s (2.56KB/s)

  • 512Kb/s 20:1
    • 25.6Kb/s (3.2KB/s)

  • 1Mb/s 20:1
    • 51.2Kb/s (6.4KB/s)

  • 2Mb/s 20:1
    • 102.4Kb/s (12.8KB/s)


Whereas modem connections are contended on the ability to gain an initial connection, you can see here, ADSL is contended on the speed once connected.

These are worst cases though, so this should never happen.


Step 3 - POP to ISP handover

Once your traffic is on a back-haul to one of the POPs (Point Of Presence), it usually means that the contention will be maintained throughout the network, until it is time to be handed to the ISP.

This is another point where contention can be inflicted on your traffic; however, calculating contention levels here is near impossible without exact customers figures, something that ISPs do not like to give out.

An ISP may have multiple paths through which your connection may reach it, however, once your connection is established (authentication of your DSL connection and your IP address has been assigned to you), you are stuck with a path until you make a new connection to the ISP (though that may not guarantee a change).

PlusNet use the 155Mb/s type of connection from the BT network (The connection is known as BT Central 155Mb/s L2TP).

This allows up to 8000 connection sessions at any one time. As you can see, 155Mb/s divided between 8000 sessions provides 19.84Kb/s per session, meaning these connections are contended too.

If all connections are 512Kb/s, then that would mean a contention level of 25.81:1. However, these links handle all customer connections, so the higher speed 1Mb & 2Mb connections are un-accounted for here.

Without the exact figures of customers an ISP has on each product, it is impossible to tell what contention levels, if any at all, are kicking in. The only way an ISP can minimise this, is to provide an upgrade plan that will provide over-capacity.

As ISPs do not like to provide information on their customer levels, it is near impossible to tell if contention is a factor here. However, PlusNet provides a services utilisation graph to tell you how much of the capacity is in use for today and backdating one week.

PlusNet have between 4000 to 8000 customers per pipe. However, many customers through the use of PCI or USB modems, disconnect from the internet when not in use. As such, there is a average 3000 active customers connected at a time.

For those that wish to know, this makes the average contention of 9.68:1 on these pipes. This is much less than the possible figure above.


Step 4 - External connectivity

Once within the ISPs network, your data is yet to flow onto the Internet. This is again another point that contention can be seen.

However, ISP's would usually have a peering arrangement that would provide them with near limitless capacity making it almost a non-issue. You would have to find out these arrangements to know if this is a possible factor.

As proof of concept, to know if there is contention, you would see faster speeds from systems within your ISPs network, and slower from the Internet.

Slow speeds from the Internet are by no means proof, though. What happens with your data on the Internet is out of the control of your ISP.

I should note in the above, most of this is a worst case scenario, meaning that if you see problems, they should be no worse than this. If they are, you have a definite complaint, provided you can prove that your own equipment (ADSL modem, splitters and phones) and wiring are not to blame.

You may like to consult the "ADSL Troubleshooting" and "ADSL Wiring and filters FAQ" guides for help on diagnosing issues, prior to to submitting a fault to PlusNet

Most calculations above, have been done using a 512Kb/s service. Contentions and speed figures will vary depending on the number and type of connections.
Original Article by: acarr - Edited by: MauriceB