Adsl

466 views

Published on

Published in: Business, Technology
  • Be the first to comment

  • Be the first to like this

Adsl

  1. 1. Adsl(Asymmetric Digital Subscriber Line)<br />49871018<br />王婉貞<br />
  2. 2. Asymmetric Digital Subscriber Line (ADSL) is one form of the Digital Subscriber Line technology, a data communications technology that enables faster data transmission over coppertelephone lines than a conventional voicebandmodem can provide. It does this by utilizing frequencies that are not used by a voice telephone call.[1] A splitter - or microfilter - 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 central office, 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 farther distribution. In 2005, the ability to transmit copper ADSL/DSL services over a fiber optic cable became possible by utilizing the RLH ADSL/DSL fiber optic link, providing distances from one point to the opposite end of the system of more than 30 miles.<br />
  3. 3. Operation<br />Currently, most ADSL communication is full-duplex. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD), or time-division duplex (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user.<br />
  4. 4. A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband devices are connected to the line. The DSL signal is now present on all telephone wiring in the building, causing attenuation and echo. A way to circumvent this is to go back to the original model, and install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer and may not work on some household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.<br />
  5. 5. DSL signals may be degraded by older telephone lines, surge protectors, poorly designed microfilters, radio frequency interference, electrical noise, and by long telephone extension cords. Telephone extension cords are typically made with small-gauge multi-strand copper conductors which do not maintain a noise-reducing pair twist. Such cable is more susceptible to electromagnetic interference and has more attenuation than solid twisted-pair copper wires typically wired to telephone jacks. These effects are especially significant where the customer's phone line is more than 4 km from the DSLAM in the telephone exchange, which causes the signal levels to be lower relative to any local noise and attenuation. This will have the effect of reducing speeds or causing connection failures.<br />
  6. 6. ADSL standards<br />
  7. 7. THE END <br />

×