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Technical FAQ

Where is my BT Exchange located?

Some people may know immediatly when asked where their BT exchange is; others not. They have often been situated close to Post Office buildings (from the days when they used to be run by the post office).

Popular ways of finding out where the exchange could be:

However, a new ingenius method suggested by Steve D. on uk.telecom.broadband was to use the postoffice postcode checker here. Click the 'postcode finder' menu item, and enter in "British Telecom" under Premesis and your home town name. It will return a list of BT properties, one of which should be your local BT exchange. If you get no matches, try the names of nearby towns.

How does contention work?

As of March 2006, BT Wholesale has dropped the notion of up to 50:1 contention for Home products and 20:1 for the Office product ranges. It how states that Office products will be given a higher priority. This change in wording reflects the changes to the BT Wholesale network to cope with products like the Max services, where much larger chunks of bandwidth were used at the exchanges.

Contention is something that appears to be only talked about in the UK, but it is a major part of the Internet across the world. Without contention at the various interconnection points of the Internet it would be vastly more expensive to run and very few people would even have access to a 500Kbps service.

An urban myth has grown up that the 0.5Mbps service is shared between 50 users, the contention on BT IPStream 0.5Mbps and higher speed services is done very differently.

How is BT Wholesale Contention carried out

Their is no set contention figure given for the Home and Office products by BT Wholesale. As of 2006 BT Wholesale simply undertakes to ensure that Office connections are given a higher priority across the solely BT Wholesale controlled parts of the network. The BT Wholesale network is the section between the 5500 exchanges and one of 22 locations around the UK (these 22 locations are called Points of Presence or POPs), where the connection is handed onto the various Service Providers. At many exchanges and in the BT Wholesale controlled part of the network the contention is so low, that most of the visible contention is actually taking place in the ISP controlled segments.

BT Wholesale does monitor the bandwidth levels in the various pipes and can adjust them up and down to cope with increases in the usage level, though it may take sometime for action to be taken. It is also possible that there may be several pipes leaving an exchange, this can mean if you are seeing contention that not all other users on your exchange will be, and sometimes users on the same ISP and same service may actually be on a different pipe connecting to the ISP.

You may also see the pipe referred to as the backhaul, PVC or Virtual Path. BT at most exchanges has capacity installed for upto 155Mbps of traffic from ADSL users, and the smaller subtended exchange builds have around 30Mbps of bandwidth available. Which should be sufficient for most exchanges.

BT DataStream with In Span Handover
The 1Mbps and 2Mbps low price products, e.g. from Tiscali, Bulldog, Internet Central, Fast24 and others use a different system for contention. How this contention is controlled is down to the individual service provider. While a similar system is used to the BT IPStream product ranges, due to the costs the ISPs generally rent the minimum amount of bandwidth needed from BT Wholesale. Therefore if they have say two 2Mbps users and one 1Mbps user, they may still run the pipe at just 2Mbps, the result of this is that Datastream users often see higher levels of contention, the penalty for the lower price of Datastream products in general.

Contention controlled by the ISPs
The ISP still has a role to play in the contention arena, each ISP selling a BT IPStream based service rents a number of connections called BT Centrals between the BT Wholesale POPs and the ISPs own network. How many users are using each BT Central determines the contention ratio, the pipes come in various sizes, the most common been 34Mbps, 155Mbps and 622Mbps.

In the advertising by providers and on their own websites, some will make various claims about contention, it is difficult to compare these claims as there is no single method of arriving at the published figures, which can vary from 1:1, 30:1 to 50:1 contention. Such widely varying figures are possible because often a provider will talk about contention for a specific part of a network. Therefore the best guide is to see what actual customers of a service provider are saying about speeds of service.

How does a micro-filter work?

A micro-filter works by taking the combined telephone/ADSL signal and splitting it into two different signals.

The telephone plug on your micro-filter is designed to limit the frequencies your phone is able to use. Although you would never normally use the frequencies outside this range, actions such as picking up the receiver can generate frequencies outside normal voice range. Without the filter, this interference would corrupt data within the ADSL frequency band.

The second socket is the ADSL socket, this is essentially the raw signal, i.e. unfiltered, so an ADSL modem will see the high frequencies, but can just discard the normal voice frequencies.


PPPoA (PPP over ATM) and PPPoE (PPP over Ethernet) define how data is encapsulated over different types of network. At present (Summer 2003), all BTwholesale provided DSL lines use PPPoA. For a modem to work in the UK it must be G.DMT and PPPoA compliant. Visit our ADSL Hardware for a list of routers and modems, and where to buy them.

PPPoA refers to how data is transfered across BT's network. For more information see How ADSL Works.

My line has a DACS. Can I still obtain ADSL service?

If your line has a DACS (a device BT use to share a pair of copper wires between two phone lines), it must be removed from the line prior to ADSL activation.

The procedure to do this is to apply for your ADSL line as usual. When BTwholesale perform the various line tests, they should spot the DACS and arrange for the removal of the device. This may delay your order by a few days, but once removed it shouldn't stop you from obtaining ADSL so long as your line passes the necessary tests (see question above).

BT Wholesale may reject the order if their estimations of the costs of removing the DAC unit is higher than £900 (note: you will not be charged anything, the £900 figure is purely an internal BT cost). Also some service providers refuse orders from lines with a DAC unit, since they do not want the hassle these orders can generate, so it your order is rejected very quickly by an ISP you are advised to try with a different provider.

BT say I have fibre in my line?

DSL will only work over metallic wiring, i.e. copper and aluminium. If your phone line has any fibre optic cable between you and the exchange you currently won't be able to obtain DSL services. The fibre line is often referred to as TPON.

If BT installed the DSLAM at the point where the fibre optic cable was split out onto individual copper lines it would be possible for BT to provide DSL services. At present, BT are only deploying DSLAM's inside exchanges.

An upgrade programme is under way for TPON based customers, which involves running copper cabling to bypass the fibre. Phase 1 is well under way, with half the customers in that phase now able to order ADSL. Phase 2 is likely to complete in 2005.

The UK still has some lines provided over a TPON system, in some cases high demand has meant that previously installed copper overlays are now fully used, and a further phase of upgrades will be needed. Some areas where TPON is used have seen a total replacement of the TPON system.

Will an Ethernet modem/router lower my ping?

On the whole, replacing a USB modem with an Ethernet based device appears to lower pings by around 5 to 10ms. Some people see more improvement than others. The other advantage is that Ethernet routers remove the CPU load placed upon a computer by a USB modem (albeit fairly insignificant).

Reports suggest that PPTP mode may slightly increase ping times because the VPN presentation of the IP interface takes a little time to process.

What ping times should I expect?

This varies depending upon software and hardware. Ping times of around 15 to 30ms are normal. ADSL uses a form of data interleaving to help increase tolerance towards line noise. The data processing algorithm takes time and hence latency is increased.

If you have a Max product, there is a chance you may see your latency (ping) jump to a higher value if your line speeds are highly variable. This is a process called interleaving, and generally increases the ping times by 10 to 20ms. This extra delay is used to provider better error correction, and thus can stabilise a very intermittent line. Providers do have the option of disabling this feature, though you may not connect at a lower stable speed if the option is disabled.

What tests are performed when ADSL or an upgrade is ordered?

BT Wholesale will normally do some checks before activating an ADSL service, these are detailed below. If ordering a Max based product, the only check done is whether the service is available on your exchange, and that you've not had your line marked as RED, i.e. 'unsuitable for ADSL' due to a previous failed ADSL activation.

Edited to match the new limits that come into effect 6th September 2004

What is G.DMT 'wires-only'?

G.DMT is a ratified standard defining a common interface for ADSL modems. G.DMT 'wires-only' ADSL was launched as a product in January 2002 by BT Wholesale.

This allows users/ISP's to use DSL modems that they themselves have purchased, as oppossed to the Alcatel USB and Effecient 5861 that were previously supplied by BT on the IPStream500/S products.

Wires-only is also referred to as 'self-install' because there is no need for a BT engineer to visit at install time. Instead, a microfilter needs to be purchased and fitted to every phone socket of the line with ADSL to filter out the Data from the Voice frequencies.