ransmissions. Recent ADSL systems are known as very-high-bit-rate DSL (VDSL) systems, which utilise a wider bandwidth and provide corresponding higher transmission rates. DSL technologies are standardised under the auspices of the ITU-T. G.991 standard specifies ADSL transceiver operation including modulation, channel coding, system bandwidth and trans- mission power levels. Other G99x series standards specify call set-up procedures (G.994), test- ing (G.996) and physical layer management (G.997). In particular G.992 defines how ADSL systems share the copper wire bandwidth with voice in a frequency division multiplexing (FDM) manner. The 0-4 kHz frequency band is reserved for voice, and 25-1104 kHz for upstream and downstream transmissions as illustrated in Figure 9.3. A guard band is allocated between 4 kHz and 25 kHz to minimise mutual interference between voice and data transmissions. Digital Subscriber Line System Architecture The DSL system structure is built with a star topology centred at an operator's exchange, also known as the central office (CO). A CO is connected to a customer premises equipment (CPE) device using twisted pair copper wires used hitherto for telephony services. In some configura- tions a CO is connected to a remote terminal (RT) with an optical fibre link and an RT and a CPE device are connected using twisted pair copper wires. Because the same medium is used for both DSL and telephony, a device is needed to ensure the two services do not interfere with each other's operation. This is made possible through a 'splitter', a passive device consisting of a low-pass filter and a high-pass filter. The low-pass filter passes the voice signal up to 4 kHz and suppresses all frequency energy above it. The high-pass filter suppresses all frequency energy below 25 kHz, and passes all frequencies above it as used for DSL. Two splitters are needed for each subscriber, one at the customer's premises for downstream communications Fixed Broadband Communications Systems IP Network Central office (e.g. T