elstra) DSLAM DSL modem Splitter Splitter Figure 9.4 DSL system components and one at the CO (or RT if one is used) for upstream communications. A DSL modem converts the digital bits into analogue signals using a discrete multi-tone (DMT) modulator on the trans- mitter side and demodulates analogue signals into data bits on the receiver side. The CO serves a large number of DSL subscribers' CPE devices. Signals from CPE devices in the upstream are multiplexed together at the CO, and are forwarded to the internet. In the other direction, incoming signals from the internet need to be switched to their final destination. These functions are carried out by a device known as a DSL access multiplexer (DSLAM). DSLAM is in effect an edge router, which switches internet protocol (IP) packets onwards in both upstream and downstream. A typical DSL system is illustrated in Figure 9.4. Discrete Multi-tone The transmission spectrum in the ADSL standards is divided into many narrow-band subcarri- ers, and data transmission is carried out in parallel over these subcarriers. Each subcarrier is known as a tone, and the data transmission is called DMT. In this process, the input data are first converted from serial-to-parallel, and mapped into a number of symbols. Each symbol is then modulated onto a subcarrier which are then frequency division multiplexed together and transmitted as illustrated in Figure 9.5. Note that the subcarriers overlap and are orthogonal with respect to each other. The multi- plexing and demultiplexing of DMT signals is carried out using discrete Fourier transform (DFT) techniques. Here data are modulated in the frequency domain, and each data symbol [e.g. a 16-QAM symbol] is represented by the amplitude and phase of a frequency carrier, which is orthogonal to its neighbouring frequency carriers. Multiplexing of DMT signals and their transmission is shown in Figure 9.6. The receiver operation is the reverse of that of the transmitter and is shown in Figure 9.7. In ADSL, each subcarrier occupies a bandwidth 