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Digital Subscriber Line DSL
Posted Date: 23 May 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: Ramkumar Member Level: Diamond
Rating:  Points: 2
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(DSL, or Digital Subscriber Loop, xDSL - see below) A family of digital telecommunications protocols designed to allow high speed data communication over the existing copper telephone lines between end-users and telephone companies.
When two conventional modems are connected through the telephone system (PSTN), it treats the communication the same as voice conversations. This has the advantage that there is no investment required from the telephone company (telco) but the disadvantage is that the bandwidth available for the communication is the same as that available for voice conversations, usually 64 kb/s (DS0) at most. The twisted-pair copper cables into individual homes or offices can usually carry significantly more than 64 kb/s but the telco needs to handle the signal as digital rather than analog.
There are many implementation of the basic scheme, differing in the communication protocol used and providing varying service levels. The throughput of the communication can be anything from about 128 kb/s to over 8 Mb/s, the communication can be either symmetric or asymmetric (i.e. the available bandwidth may or may not be the same upstream and downstream). Equipment prices and service fees also vary considerably.
The first technology based on DSL was ISDN, although ISDN is not often recognised as such nowadays. Since then a large number of other protocols have been developed, collectively referred to as xDSL, including HDSL, SDSL, ADSL, and VDSL. As yet none of these have reached very wide deployment but wider deployment is expected for 1998-1999.
High bit-rate Digital Subscriber Line:
(HDSL) A form of Digital Subscriber Line, providing T1 or E1 connections over two or three twisted-pair copper lines, respectively. Unlike most other forms of DSL HDSL is not a typical consumer service, it's used mostly to replace traditional T1/E1 connections, such as connecting PBXes to telco offices. The advantage of HDSL over the Alternate Mark Inversion line coding scheme traditionally used on T1/E1 lines is that it requires about an order of magnitude lower bandwidth to carry the same traffic.
Single-line Digital Subscriber Line:
(SDSL, or Single-pair High Speed Digital Subscriber Line, S-HDSL) A form of Digital Subscriber Line similar to HDSL but providing T1 or E1 connections over a single twisted-pair copper line.
Asymmetric Digital Subscriber Line:
(ADSL, or Asymmetric Digital Subscriber Loop) A form of Digital Subscriber Line in which the bandwidth available for downstream connection is significantly larger then for upstream. Although designed to minimise the effect of crosstalk between the upstream and downstream channels this setup is well suited for web browsing and client-server applications as well as for some emerging applications such as video on demand.
The data-rate of ADSL strongly depends on the length and quality of the line connecting the end-user to the telephone company. Typically the upstream data flow is between 16 and 640 kilobits per second while the downstream data flow is between 1.5 and 9 megabits per second. ADSL also provides a voice channel.
ADSL can carry digital data, analog voice, and broadcast MPEG2 video in a variety of implementations to meet customer needs.
Very high bit-rate Digital Subscriber Line:
(VDSL) A form of Digital Subscriber Line similar to ADSL but providing higher speeds at reduced lengths.
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