Keywords: ADSL, SDSL, CAP, DMT, transceiver, DSLAM
Abstract: A DSL connection manages to squeeze more information through a standard phone line -- and lets you make regular telephone calls even when you're online. A DSL modem on your phone line splits your single phone line into two frequencies - one for data and one forvoice. This allows you to make and receive phone calls without interrupting the Internet connection. Over any given link, the maximum DSL speed is determined by the distance between the customer site and the Central Office.DSL operates at speeds from 144 Kbps to 1.54 Mbps, even faster, up to 6.0 Mbps--so customers can choose the rate that meets their specific needs.
Astandard telephone installation consists of a pair of copper wires that the phone company installs in your home. The copper wires have lots of room for carrying more than your phone conversations -- they are capable of handling a much greater bandwidth, or range of frequencies, than that demanded for voice.(which is in a frequency range of 0 to 3 400 Hertz). DSL exploits this "extra capacity" to carryinformation on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.
There are two basic categories namely: SDSL – Symmetric DSL and ADSL - Asymmetric DSL. SDSL ensures the same transfer rate in both directions, that is in: “upstream”(from the client to the Internet) and in “downstream”(from the Internet to the client) ADSL ensures in „downstream” a large bandwidth channel, situated at high frequencies and a more narrow band channel in „upstream” situated at low frequencies. This division of the frequency bands has two reasons: the information quantity transmitted in „downstream” is larger and it is reduced the crosstalk, which is higher at high frequencies.
Precisely how muchbenefit you see from ADSL will greatly depend on how far you are from the central office of the company providing the ADSL service. ADSL is a distance-sensitive technology: As the connection's length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 5,460 meters, though for speed and quality of service reasons many ADSL providers place a lowerlimit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future.
You might wonder -- if distance is a limitation for DSL, why is it not also a limitation for voice telephone calls? The answer lies in smallamplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL.
Splitting the Signal
The CAP System
There are two competing and incompatible standards for ADSL:carrierless amplitude/phase (CAP) system, which is an earlier and more easily implemented standard and discrete multitone, or DMT. Most of the ADSL equipment installed today uses DMT.
CAP operates by dividing the signals on the telephone line into three distinct bands (channels) widely separated, which minimizes the possibility of interference between the channels on one line, or between thesignals on different lines.
The DMT System
DMT also divides signals into separate channels, but doesn't use two fairly broad channels for upstream and downstream data. Instead, DMT divides the data into 247 separate channels, each 4 kHz wide.
One way to think about it is to imagine that the phone company divides your copper line into 247 different 4-KHz lines and then attaches a modem to...