TECHNICAL INFORMATION
In terms of how ADSL operates, ADSL modems create multiple channels by separating the available bandwidth of a telephone line into Frequency Division Multiplexing (FDM) or using a method called Echo Cancellation. The FDM works by assigning one band for upstream data and another for downstream data. The downstream path is then divided by Time Division Multiplexing, which means the total bandwidth is available only for a portion of the time, and then divided into one or more high speed channels. Echo Cancellation assigns the upstream band to overlap the downstream and separates the two.
The basic setup of an ADSL consists of an ADSL circuit that connects with an ADSL modem on each end of a twisted-pair telephone line. This creates three information channels. Firstly, a high speed downstream channel which is the transmission speed from the telephone network. Secondly, a medium speed duplex channel. Finally, a Plain Old Telephone Service (POTS) channel is established. The POTS channel is split off from the digital modem by filters which allow certain frequencies to pass while blocking others out. This guarantees uninterrupted service even if ADSL fails.
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