7.3.1 Digital Subscriber Line (DSL)

Digital Subscriber Line (DSL) is a relatively new, medium bandwidth, remote access service running over standard telephone wiring. It is currently targeted at SOHO applications for Internet access; access to the service provider's central office is enabled using a DSL modem on the customer's premises. Upstream bandwidth is available at speeds of between 16 Kbps and 1.5 Mbps. Downstream speeds may be as high as 32 Mbps. Service offering at present is patchy; many parts of the United States still do not have access to DSL, and the service is just beginning to be deployed across Europe at the time of writing. DSL is not a single service; it comprises a number of services designed to cope with different kinds of applications, as follows (most of these acronyms were originally defined by Bellcore):

• IDSL ISDN DSL

• ADSL Asymmetric DSL

• HDSL High Bit Rate DSL

• VDSL Very High Bit Rate DSL (sometimes referred to as BDSL)

• SDSL Single-Line DSL (or symmetric DSL)

• CDSL Consumer DSL

• RDSL Rate-Adaptive DSL

Because of the variety of flavors on offer, DSL is often referred to as xDSL. At present the two main services of interest being deployed are ADSL and HDSL. It is expected that some of the other services may disappear and new ones emerge as the service and technology matures.