xDigital Subscriber line (xDSL)

xDSL belongs to DSL family, the lower case x in xDSL stands for the many variation including the fallowing.

• Asymmetrical digital subscriber line (ADSL)

ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services.

ADSL is a modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under current technology.

Data rates for ADSL, based on installed wiring at varying gauges.

NOTE: The speeds and distances shown here are the theoretical limits based on good copper.

• Digital subscriber line (IDSL)

                DSL refers to a pair of modems that are installed on the local loop to facilitate higher speeds for data transmission.

                The IDSL technique is an all digital operating at two channels of 64 Kbps for voice or non voice operation and a 16 Kbps data channel for signaling, control, and data packets.

                A  DSL is used to deliver I SDN services. As the deployment of IDSL was speeding up on the local loop, the providers developed a new twist, called "always on, ISDN" mimicking a leased set of channels that are always connected.

The IDSL line connection enables 128 Kbps in total simultaneously.

• High bit−rate Digital Subscriber Line (HDSL)

HDSL was the first DSL to use higher frequency spectrum of copper, twisted pair cable.  HDSL was developed in the US, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier system, and also to carry high-speed corporate data links and voice channels, using T1 lines.

• Consumer Digital Subscriber Line (CDSL)

                CDSL is a model of DSL developed for the consumer that does not who is not looking for symmetrical high−speed communication. With other forms of DSL (such as ADSL and RADSL), splitters are used on the line to separate the voice and the data communications. CDSL does not use, nor need, a splitter on the line. Speeds of up to 1 Mbps in the download direction and 160 Kbps in the upward direction are provided. It is expected that the speeds and DSL will meet the needs of the average consumer for some time to come.

• Single High Speed DSL (SHDSL)

                SHDSL is a data communications technology that enables faster data transmission over copper telephone line than a conventional voice modem can provide. Compare to ADSL, SHDSL employs TC-PAM modulation and frequencies that include those used by analog plain old telephone service to provide equal transmit and receive data rates. SHDSL features symmetrical data rates in both the upstream and downstream directions, From 192 kbit/s to 2,312 kbit/s of payload in 8 kbit/s increments for one pair and 384 kbit/s to 4,624 kbit/s in 16 kbit/s increments for two pairs of wires.

• Rate−adaptive digital subscriber line (RADSL)

                 RADSL is a variation of asymmetric digital subscriber line technology. In RADSL the DSL modem adjust the upstream bandwidth to create a wider frequency band for the downstream traffic. Using this technique the line is more tolerant of errors caused by noise and signal loss.

• Very high−bit rate digital subscriber line (VDSL)

                VDSL was introduced to achieve the higher speeds. If, in fact, speeds of up to 50 Mbps are demanded, then the distance limitations of the local cable plant will be a factor. In order to achieve the speeds, you can expect that a fiber feed will be used to deliver VDSL.

DSL speeds and operations using current methods

xDSL Coding Techniques

• Discreet Multitone - DMT uses multiple narrowband carriers, all transmitting simultaneously in a parallel transmission mode. Each of these carriers carries a portion of the information being transmitted. These multiple discrete bands, or, in the world of frequency division multiplexing, sub-channels, are modulated independently of each other using a carrier frequency located in the center of the frequency being used. These carriers are then processed in parallel form. 
• Carrier-less Amplitude Phase Modulation CAP - CAP is closely aligned to QAM. QAM as a technique is widely understood in the industry and well deployed in our older modems. Both CAP and QAM are a single−carrier signal technique. The data rate is divided into two and modulated onto two different orthogonal carriers before being combined and transmitted. The main difference between CAP and QAM is in the way they are implemented. QAM generates two signals with a sine/cosine mixer and combines them onto the analog domain. CAP, on the other hand, generates its two orthogonal signals and executes them digitally. Using two digital transversal bandpass filters with equal amplitude characteristics and a p/2 difference in phase response, the signals are combined and fed into a digital−to−analog converter. Then the data is transmitted. The advantage of CAP over QAM is that CAP is done in silicon, which is more efficient and less expensive.

A design of an ADSL model and its model components

The intent of the model is to show the infrastructure of the network from the customer premises to the network provider.

Source:

http://en.wikipedia.org

Broadband Telecommunication Handbook 2nd edition by: Regis J. "BUD" Bates