Chapter 16

       

                                                                    xDSL

       DSL   is a family of technologies that provide internet access by transmitting digital data over the wires of a local telephone network. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly installed DSL technology. DSL service is delivered simultaneously with wired telephone service on the same telephone line. This is possible because DSL uses higher frequency bands for data separated by filtering. On the customer premises, a DSL filter on each outlet removes the high frequency interference, to enable simultaneous use of the telephone and data.

       The data bit rate of consumer DSL services typically ranges from 256 kbit/s to 40 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. In ADSL, the data throughput in the upstream direction, (the direction to the service provider) is lower, hence the designation of asymmetric service. In Symmetric Digital Subscriber Line (SDSL) services, the downstream and upstream data rates are equal.

       A DSL circuit provides digital service. The underlying technology of transport across DSL facilities uses high-frequency sinusoidal carrier wave modulation, which is an analog signal transmission. A DSL circuit terminates at each end in a modem which modulates patterns of bits into certain high-frequency impulses for transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem retransmits, in digital form, to its interfaced equipment, such as a computer, router, switch, etc.

Benefits & Applications

Benefits

• High-speed data service

– DSL typically >10x faster than

56-kbps analog modem

• Always on connection

– No need to “dial-up”

• Uses existing copper wires

– Co-exists w/ POTS service

• Reasonably priced today and

getting cheaper

Applications

• High speed Internet access

• SOHO

• Multimedia, Long distance

learning, gaming

• Video on Demand

• VPN

• VoDSL

          Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call.[1] A splitter, or DSL filter, allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the telephone exchange (the last mile), typically less than 4 kilometres (2 mi),[2] but has been known to exceed 8 kilometres (5 mi) if the originally laid wire gauge allows for further distribution.

           IDSL is a system in which digital data is transmitted at 128 Kbps on a regular copper telephone line (twisted pair) from a user to a destination using digital (rather than analog or voice) transmission, bypassing the telephone company's central office equipment that handles analog signals. IDSL uses the Integrated Services Digital Network (Integrated Services Digital Network) Basic Rate Interface in ISDN transmission code.

           HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. HDSL can carry as much on a single wire of twisted-pair cable as can be carried on a T1 line (up to 1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in Europe over a somewhat longer range and is considered an alternative to a T1 or E1 connection.

         SDSL (Symmetric Digital Subscriber Line) is high-speed Internet access service with matching upstream and downstream data rates. That is, data can be sent to the Internet from the client machine or received from the Internet with equal bandwidth availability in both directions.

         RADSL is an implementation of ADSL that automatically adjusts the connection speed to adjust for the quality of the telephone line. As line conditions change, you can see the speeds changing in each direction during the transmission.

         SHDSL technology can transport data symmetrically at data rates from 192 Kbps to 2,320 Kbps. SHDSL utilizes a single copper wire pair, making it an affordable DSL option attractive to small businesses.This service delivers voice and data services based on highly innovative communication technologies and will thus be able to replace older communication technologies such as T1, E1, HDSL, HDSL2, SDSL, ISDN, and IDSL in the future.

          VDSL was developed to support exceptionally high-bandwidth applications such as High-Definition Television (HDTV). VDSL is not as widely deployed as other forms of DSL service. However, VDSL can achieve data rates up to approximately 51,840 Kbps, making it the fastest available form of DSL.

http://compnetworking.about.com/od/dsldigitalsubscriberline/l/bldef_vdsl.htm

http://www.wisegeek.com/what-is-sdsl.htm

http://www.xilinx.com/esp/wired/optical/collateral/DSL.pdf

http://en.wikipedia.org/wiki/Digital_subscriber_line