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High Data-Rate Digital Subscriber Line (HDSL)
HDSL is simply a better way of transmitting T1/E1 over copper
wires, using less bandwidth without repeaters. It uses more
advanced modulation techniques to transmit 1.544 Mbps over lines
up to 12,000 feet long.
Single-Line Digital Subscriber Line (SDSL)
SDSL is a single-line version of HDSL, transmitting T1/E1
signals over a single twisted pair, and able to operate over the
plain old telephone service (POTS) so that a single line can
support POTS and T-1/E-1 at the same time. It fits the market for
residence connection which must often work over a single
telephone line. However, SDSL will not reach much beyond 10,000
feet. At the same distance, ADSL reaches rates above 6 Mbps.
Asymmetric Digital Subscriber Line (ADSL)
ADSL is intended to complete the connection with the customer's
premise. It transmits two separate data streams with much more
bandwidth devoted to the downstream leg to the customer than
returning. It is effective because symmetric signals in many
pairs within a cable (as occurs in cables coming out of the
central office) significantly limit the data rate and possible
line length.
ADSL succeeds because it takes advantage of the fact that most of its target applications (video-on-demand, home shopping, Internet access, remote LAN access, multimedia, and PC services) function perfectly well with a relatively low upstream data rate. MPEG movies require 1.5 or 3.0 Mbps down stream but need only between 16 kbps and 64 kbps upstream. The protocols controlling Internet or LAN access require somewhat higher upstream rates but in most cases can get by with a 10 to 1 ratio of downstream to upstream bandwidth.
