Digital Subscriber Line DSL

Digital subscriber line is a digital WAN technology that brings high-speed digital networking to homes and businesses over POTS. DSL has many types, including high-speed DSL (HDSL), very high bit-rate DSL (VDSL), and asymmetric DSL (ADSL). ADSL is the most common application because the uplink and downlink bandwidths are not symmetrical, meaning they are not of the same link speed.

ADSL provides high-speed data connections using the same copper phone lines that are used in POTS.

Although ADSL promises high bandwidth, the upstream is slower than the downstream data rate. Typically, the upstream transfer rate ranges from 64 kbps to 256 kbps, whereas downstream rates approach 1.544 Mbps.

Microsoft Windows 9x/NT/2000/XP can support ADSL connections with the installation of a DSL modem/adapter. When an ADSL network adapter is installed in Windows 2000, the adapter appears as either an Ethernet or dial-up interface. If ADSL appears as an Ethernet interface, the connection operates just as it would with an Ethernet connection, but if it appears as a dial-up interface, ADSL uses ATM.

When a customer is too far (beyond 18,000 feet) from an LEC central office (CO), DSL or single-line digital subscriber line (SDSL) offerings are usually deployed. IDSL is essentially the DSL "flavor" of ISDN-BRI, offering 144 kbps of bandwidth. Table 21-2 details the available bandwidth as determined by the distance from the CO.

Table 21-2. SDSL Distance/Bandwidth Chart

Distance (Feet)

Distance (Meters)

Data Rate

22,700

6,920

160 kbps

20,000

6,097

208 kbps

19,000

5,793

320 kbps

17,900

5,457

416 kbps

14,900

4,543

784 kbps

12,500

3,811

1.04 Mbps

9,500

2,896

1.568 Mbps

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