This is the most popular version of the DSL technology used by many providers. It differs from the less common symmetric link (SDSL). The ADSL provides wider bandwidth for the transmission towards customer (i.e. downlink) than in the reverse direction (i.e. uplink). This results in faster download speed than upload.
The first version of ADSL standard provide up to 8 Mbps speed in downlink direction and up to 1 Mbps in uplink. This standard was approved in 1998 and it was called ANSI T1.413. It reserves 0 – 4 kHz band for voice, 26 – 138 kHz for upstream and 138 – 1100 kHz for downstream. However, it is used only in North America. More popular is ITU-T G.992.1 standard (also known as G.dmt) and it is commonly used throughout the world.
ADSL2 standards offers similar maximum throughputs to the ADSL, but thanks to implementation of more advanced coding techniques and higher modulation schemes, it is possible to achieve higher throughputs. Therefore, ADSL2 (described as ITU G.992.3 standard) provides up to 12 Mbps downstream data rate and up to 1.3 Mbps upstream data rate. The standard predicts also the implementation of ADSL2 which offers up to 3.5 Mbps (called as Annex J), but this increased uplink speed is at the expense of the bandwidth used by the POTS.
ADSL2+ (the ITU-T G.992.5 standard) doubles the downstream data peak rate, because it uses two times wider bandwidth. Therefore, it allows reaching up to 24 Mbps downstream and up to 1.4 Mbps (or 3.3 Mbps for Annex M) upstream speeds. However, in case of Annex M, the increased uplink speed comes at the expense of downlink throughput, because the uplink and the downlink frequencies are overlapped.