The xDSL Family

DSL technologies are sometimes referred to as last-mile technologies because they are used only for connections from between a telephone switching station and its subscriber, not between switching stations. The following diagram illustrates how ADSL modems can be incorporated into the existing networks.

 

Figure 2. xDSL in the End-to-End Network            Source: [1]

The major advantage of ADSL is that the technology was designed to operate on ordinary copper telephone lines already installed in most commercial and residential areas. xDSL modems transmit their data in the frequency region above POTS (i.e. above 4kHz), so that the transmission of voice and data are multiplexed. Therefore, if a fault occurs in the xDSL transmission, POTS (the telephone system) remains unaffected.

The digital transmission of information in xDSL systems is full duplex, meaning that information can be transmitted simultaneously in both upstream and downstream directions. The transmission may be symmetric, where the upstream and downstream bit rates are equal, or asymmetric, where the bit rate differs in upstream and downstream.

ADSL is just one of many in the xDSL family, which includes R-ADSL, HDSL, SDSL, and VDSL. These technologies vary in speed, distance limitation, and symmetry of upstream and downstream traffic. There is a compromise between speed and distance limitation, where, the higher the bit rate, the shorter the distance that a signal may be transmitted.