[Ignition]

Originally Posted by Russ D

BT's conditions for ADSL used to be that if you were within 3.5km of a BT exchange then you could get 1Mb broadband. Following recent work on their exchanges, they can now offer 2mb to those within 3.5km and 1mb to those within 6km which now means more people will be able to take up their service.

No, the limit for 2Mbit has always been approximately 3.5km, or 41dB of line attenuation at 300khz with no DSL CPE installed.

Originally Posted by Russ D

However another aspect to this is that BT can now guarantee 500k BB to anyone who lives near a recently-upgraded exchange (this means they must have a DSLAM device fitted which extends the range of their BB). Not sure what percentage of the population this now covers but around 1500 exchanges have been upgraded this way.

What? DSLAMs are DSL Aggregation Modules, the bit that goes in the exchange where all the incoming lines terminate. These do not extend ADSL range in any way, shape or form, they are an essential part of providing ADSL, they are the equivalent of the uBR on a cable network. BT still can't guarantee service to those livuing near the exchange, it's still dependent on the line synchronising properly. What they will do if service cannot be achieved is send an engineer to a customer's site to fit a noise filtering face plate onto the main telephone socket to try and get service to work.

What part of this is new technically? None - those face plates were used in the original ADSL installs in 2000. Policy wise it's a considerable shift, even if it is so that BT can get as many on DSL as possible for efficiency reasons.

Originally Posted by Russ D

The only thing is it's not ADSL - it's RAADSL which is Rate Adaptive ADSL. This means they will guarantee 500k down at the expense of the upload which will be anything from 64k to 256, depending on how much of the downstream you're using. It's not perfect but it means BB is available to more people.

Rate Adaptive DSL has been used for a while, ever since the limit for 512k ADSL was extended from 3.5km to 5.5km. It also doesn't actually care how much of the downstream you are using. ADSL is modulated using GDMT, this system involved use of 255 different discrete frequency bins. RF types will know that higher frequencies encounter more resistance and attenuation when transmitted over copper. DSL rate adapts when a line can't maintain the required downstream SNR, the DSLAM at the exchange removes a 'buffer zone' that sits between the upstream frequencies and the downstream ones, which means that you lose the higher upstream frequencies but gain some hopefully cleaner bins for downstream usage. Doesn't really care how much bandwidth you are using.

Oh and it's RADSL not RAADSL, Rate Adaptive Digital Subscriber Line.