ADSL

ADSL technology is based on data-over-voice approach, in which the frequencies used for data transmission is above the POTS and/or ISDN basic access channel. The available bandwidth is divided up into upstream and downstream. The region between 4kHz and 40kHz is called the guard band which separates the POTS from ADSL frequencies.  There are two methods of creating the separate channels:

1. Frequency Division Multiplexing (FDM). FDM assigns one band for upstream and another band for downstream. The two channels are further divided by Time Division Multiplexing (TDM).

2. Echo Cancellation. The upstream and downstream channels overlap and are separated by using echo cancellation techniques, which are also used in V.32 and V.34 analog modems. Echo cancellation is necessary to minimise the reflection of signals that occurs in the common channel.

 

Figure 3. Separation of upstream and downstream. Source: [3]

The ADSL network unit at the central office will demultiplex the POTS and the low bit rate upstream signals, and multiplex the POTS with the high bit rate downstream signal using a DSLAM (Digital Subscriber Line Access Multiplexer). The ADSL modems at both ends of the system require demultiplexing of the ADSL signals and the POTS signals. A POTS Splitter carries out the demultiplexing. The POTS splitter first makes a duplicate of the incoming signal. One copy is sent through a low pass filter to obtain the POTS signals, which is sent to the analog telephone. The second copy is sent to the ADSL modem, which will remove the POTS signal to recover the ADSL signal. The diagram below illustrates this setup.

Figure 4. POTS Splitter