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Broadband Access Networks IP router or bridge Head-end Cable modem Cable modem Cable modem Figure 8.2 Data over a cable TV network 8.4 ADSL Networks In the same way that cable modems leverage cable television networks to provide broadband access, so Digital Subscriber Line (DSL) technologies leverage existing telephone networks to provide broadband access. The most important of these technologies is Asymmetric Digital Subscriber Link (ADSL). ADSL is a member of a family of subscriber link technologies referred to as xDSL. It includes High-speed Digital Subscriber Line (HDSL), Very high-speed Digital Subscriber Line (VDSL) and others. The most commonly deployed is ADSL [ARM2000]. ADSL uses the existing PSTN twisted pair copper loop otherwise used for standard telephony. It operates in parallel with the existing telephone service but entirely independently of it. Broadband communications are transmitted over the copper loop at different frequencies to that used by standard telephony. With appropriate equipment it can be run in parallel but without affecting the existing telephony service. Figure 8.3 shows the main components of the ADSL architecture. At each end of the analog loop is an ADSL Transmission Unit (TU). It modulates the digital bitstream onto the local analog loop using frequencies above those used by telephony. At the local PSTN exchange the bitstream is demodulated by another ADSL TU and passed onto the parallel data network via a Digital Subscriber Line Access Module (DSLAM) which provides connectivity to the Internet. Unlike other networks, ADSL is not an end-toend networking technology. It merely provides the last hop to a customer site. At the exchange, communications over the ADSL link are separated from any other telephony communications and transferred through an entirely independent network. ADSL is, as its name suggests, highly asymmetric in its upstream and downstream data rates. Downstream rates are typically from 1.5 Mbps to 9 Mbps, whereas upstream rates are typically 16 kbps to 640 kbps. The actual speeds obtained depend on the distance and quality of the copper loop between the customer s residence and the local exchange. The international telecommunications union (ITU) specification for ADSL (G.992.1) specifies two quite distinct paths with quite different latency characteristics in each direction [ITU1999a]. The fast path minimises latency but at the possible expense of

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