ADSL

The pinnacle of ADSL technology is ADSL2+ Annex M.

ADSL2+ is a rate adaptive service with a download speed <24Mb although some providers refer to it as <20Mb or <16Mb so as to minimise disappointment when the headline speed is not achieved. The standard form of ADSL2+ is Annex A (although that qualifier is seldom added) with an upload speed of up to 1.25Mb. The Annex M variant doubles the upload speed to 2.5Mb by shifting a small amount of frequency from the download channel.

Where ADSL2+ is not available the fallback is ADSL Max. This is a rate adaptive technology with a download speed <8Mb. The consumer grade (although often mis-sold to businesses) Max service has an upload speed <442Kb. The business grade Max Premium service has an upload speed <832Kb and a lower contention ratio.

All ADSL technology runs over copper wiring. This degrades with distance from the exchange so the closer you are, the better your speed.

The advent of LLU a few years ago has significantly changed the landscape. Each ISP can, for a fee, place their own DSLAM or more recently MSAN within a BT Exchange and provide their own backhaul network to the Internet. This allows the ISP to offer a unique range of both business and consumer services and has a lower cost structure than IPstream for those ISP’s targeting the very low cost, “free” or multi-play consumer services. With about 5500 BT Exchanges in the UK, around a third now offer some form of LLU service. Not surprisingly these are clustered around high density urban communities rather than low density rural. This leads to the feast or famine scenario where a town centre Exchange may have half a dozen LLU operators while the one down the road has none.