2.3.7 DSL

2.3.7 DSL
DSL, Digital Subscriber Line, uses existing copper phone lines. DSL is available only in certain areas, and you must be within a short distance of a switching station. Speeds can vary based upon type of DSL but are typically around 9Mbps (the theoretical maximum is 52Mbps). DSL is typically less expensive than even ISDN in terms of hardware, setup, and service costs, yet the need to be within a few miles of a switching station is a big deterrent.
There are several types of DSL to choose from, and not all types are available in all markets. The types available include:
• Asymmetric DSL (ADSL)—uses existing copper phone lines
• High-bit-rate DSL (HDSL)—requires two wire pairs
• ISDN DSL (IDSL)—uses existing ISDN facilities
• Rate Adaptive DSL (RADSL)—adjusts the speed based on signal quality
• Symmetric DSL (SDSL)—a version of HDSL using a single pair of wires (and providing slower rates)
• Very-high-bit-rate DSL (VDSL)—transmits over short distances; the connection rate increases as the distance decreases
To illustrate the performance possible with the different types and the way it varies, Table 2.2 shows the transmission rates and distances for various DSL implementations.