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DSL

2.2.7 DSL
Digital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers. DSL service is considered broadband, as opposed to the baseband service for typical LANs. Broadband refers to a technique which uses multiple frequencies within the same physical medium to transmit data. The term xDSL covers a number of similar yet competing forms of DSL technologies:
  • Asymmetric DSL (ADSL)
  • Symmetric DSL (SDSL)
  • High Bit Rate DSL (HDSL)
  • ISDN (like) DSL (IDSL)
  • Consumer DSL (CDSL), also called DSL-lite or G.lite
DSL technology allows the service provider to offer high-speed network services to customers, utilizing installed local loop copper lines. DSL technology allows the local loop line to be used for normal telephone voice connection and an always-on connection for instant network connectivity. Multiple DSL subscriber lines are multiplexed into a single, high capacity link by the use of a DSL Access Multiplexer (DSLAM) at the provider location. DSLAMs incorporate TDM technology to aggregate many subscriber lines into a less cumbersome single medium, generally a T3/DS3 connection. Current DSL technologies are using sophisticated coding and modulation techniques to achieve data rates up to 8.192 Mbps.
The voice channel of a standard consumer telephone covers the frequency range of 330 Hz to 3.3 KHz. A frequency range, or window, of 4 KHz is regarded as the requirements for any voice transmission on the local loop. DSL technologies place upload (upstream) and download (downstream) data transmissions at frequencies above this 4 KHz window. This technique is what allows both voice and data transmissions to occur simultaneously on a DSL service.
The two basic types of DSL technologies are asymmetric (ADSL) and symmetric (SDSL). All forms of DSL service are categorized as ADSL or SDSL and there are several varieties of each type. Asymmetric service provides higher download or downstream bandwidth to the user than upload bandwidth. Symmetric service provides the same capacity in both directions.
Not all DSL technologies allow the use of a telephone. SDSL is called dry copper because it does not have a ring tone and does not offer telephone service on the same line. Therefore a separate line is required for the SDSL service.
The different varieties of DSL provide different bandwidths, with capabilities exceeding those of a T1 or E1 leased line. The transfer rates are dependent on the actual length of the local loop and the type and condition of its cabling. For satisfactory service, the loop must be less than 5.5 kilometers (3.5 miles). DSL availability is far from universal, and there are a wide variety of types, standards, and emerging standards. It is not a popular choice for enterprise computer departments to support home workers. Generally, a subscriber cannot choose to connect to the enterprise network directly, but must first connect to an Internet service provider (ISP). From here, an IP connection is made through the Internet to the enterprise. Thus, security risks are incurred. To address security concerns, DSL services provide capabilities for using Virtual Private Network (VPN) connections to a VPN server, which is typically located at the corporate site. 

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