Various DSL Technologies And How They Differ (Part 2)

7/7/2013 9:20:13 AM

Different xDSL technologies

The DSL technology is based on dis­crete multi-tone modulation technique and covers a number of similar yet competing forms of DSL (collectively termed as xDSL) including IDSL, HDSL, SHDSL, ADSL, RADSL, UDSL, Etherloop, VDSL and GDSL. xDSL is drawing significant attention from implementers and service providers because it promises to deliver high-bandwidth data rates to dispersed locations with relatively small changes to the existing telco infrastructure.

These technologies are differentiated by:

1.    Speed of transmission

2.    Maximum distance of signal transmission

3.    Variation in speed between up­stream and downstream

4.    Symmetric or asymmetric charac­ter of the connection

Currently, the primary focus in xDSL is on the development and deployment of ADSL and VDSL tech­nologies and architectures.

ISDN digital subscriber line (IDSL)

This is an integrated services digital network (ISDN)-based technol­ogy that provides data flow rates of 144 kbps, which is slightly higher than the dual-channel ISDN data rate of 128 kbps. The goodness of IDSL lies in its 'always-on' connectivity which trans­mits data via a data network rather than the carrier's voice network. Thus it gives freedom from overloading of voice switches by data users.

IDSL uses a 2B1Q (two-binary, one-quaternary) line code on a single copper pair line to transmit information through the ISDN 'U' interface. The ma­jor limitation of IDSL is that customers cannot access ISDN signaling or voice services. But if the requirement is Inter­net browsing at higher speed, IDSL is a better option than ISDN.

High-bit rate digital subscriber line (HDSL)

High-bit rate digital subscriber line (HDSL)

High-bit rate digital subscriber line (HDSL)

HDSL is a symmetric technology that provides the same amount of bandwidth for upstream and downstream traffic. It offers speed of 2.048 Mbps over two copper pairs with operating distances of 3.6 km to 4.6 km, and is ideal for connecting PBX systems, digital local loops, point of presence, Internet servers and campus- based networks.


HDSL-II, another tech­nology proposed within the American National Standard Institute (ANSI) and the European Telecommunica­tion Standard Institute (ETSI), offers the performance of HDSL but over a single pair.

HDSL was originally developed in USA, as a better technology for high-­speed local exchange carrier systems which carry high-speed data links and voice channels using T1 lines. T-carrier circuits operate at 1.544Mbps and were carried using alternate mark inversion (AMI) line code. Due to limited range of AMI, the line code B8ZS (bipolar 8-zero substitution) has been used. Later, 2B1Q line code was used, which allowed a 784kbps data rate over a single twisted­-pair cable and 1.544Mbps with two twisted-pair cables. But, the problem still continued due to the differences between the T1 (1.544Mbps) and E1 (2.048 Mbps) standards.

A new standard for HDSL has been developed using the carrier less amplitude phase modulation (CAP) line code, which reached the maximum bandwidth of 2.048 Mbps using two pairs of copper. HDSL can be used either at the T1 rate or the E1 rate. Mul­tiple of 64kbps channels inside the T1/ E1 frame can be used to provide slower speeds to customers but the line rate is still the full T1/El rate.

HDSL further gave birth to two new technologies, called HDSL2 and SDSL. HDSL2 offers the same data rate over a single pair of copper and can work up to longer distances over a low-quality or lower-gauge copper. On the other hand, single-line rate digital subscriber line (SDSL) is a multi-rate technology offering speeds ranging from 192 kbps to 2.3 Mbps using a single pair of copper.

Single-pair high-speed digital sub­scriber line (SHDSL)

Single-pair high­-speed DSL technology supports sym­metrical data rates. It is best suited for PBX, VPN, Web hosting and other data services that do not need the service guarantees of frame relay or the higher performance of a leased line. It cannot support voice service on the same pair as it takes over the entire bandwidth.

multiplexing services over SHDSL

The ITU-T recommendation G.991.2 defines the standards for SHDSL. With one pair of copper line, the SHDSL having multiple of 64kbps payload provides symmetrical download and upload data rates ranging from 192 kbps to 2.304 Mbps. Moreover, the SH­DSL provides symmetrical data rates from 384 kbps to 4.608 Mbps in 128kbps increments for two pair applications.

The distance covered is about 3 km and depends on the loop rate and noise conditions. One option to in­crease the coverage area is to decrease the data rates. Higher data rates can be achieved using two or four cop­per pairs, and one such extension of SHDSL provides data rates up to 5.696 Mbps.

The payload may be either unstruc­tured, T1, E1, multiple ISDN basic rate access (BRA), asynchronous transfer mode (ATM) cells or Ethernet packet transfer mode (PTM). In order to share the SHDSL bandwidth, a dual bearer mode can be used, which allows a combination of two types of payloads.

Asymmetric digital subscriber line (ADSL)

By studying different scenar­ios, it was realized that it was possible to transmit data more quickly from an exchange to a user. But when the user sent information to the exchange, it was more sensitive to the noise caused by electromagnetic disturbances (the nearer the subscriber to the exchange, the greater the concentration of cables, generating more crosstalk). So the idea was to use an asymmetric sys­tem, imposing a lower speed from the subscriber to the exchange. This idea gave birth to the asymmetric digital subscriber line technology, which was originally developed at Bellcore (now Telcordia Technologies) in 1988.

Asymmetric digital subscriber line (ADSL)

Asymmetric digital subscriber line (ADSL)

ADSL caters specifically to connec­tions between ISPs and customers. The Internet is used largely for download­ing files, HTML and graphical content. Processes like uploading files or other content to servers are limited to very few users. Hence the bandwidth re­quired for downstream data (from ISP to client) is more than that required for upstream data (from client to ISP).

This DSL-based technology enables transmission and reception of data at speeds higher than legacy copper media. The modulation technique used allows several bits to be represented by one transmission symbol.

In ADSL, bit rate allocation for a channel within the available band­width is not the same as for the other channels, and hence the term 'asym­metric.' In other words, the upstream bandwidth is smaller than the down­stream bandwidth. ADSL offers an upstream data rate of 500 kbps and a downstream data rate of up to 8 Mbps.

ADSL Lite, another variant of the ADSL standard, offers upstream speeds up to 500 kbps and down­stream speeds up to 1.5 Mbps. Further, ADSL has many variants like ADSL2, splitterless ADSL2, ADSL2+ and ADSL++.

ADSL2/G.DMT.bis is defined in ITU G.992.3 and is an improved ver­sion of ADSL with data rates of 12 Mbps in downstream and 3.5 Mbps in upstream. Splitterless ADSL2/G.lite.bis is defined in ITU G.992.4 and is capable of providing 1.536Mbps down­stream and 512kbps upstream.

ADSL2+ defined in ITU G.992.5 can provide up to 24Mbps theoretical downstream speed, which is double of the ADSL2 speed. The upstream speed is up to 3.5 Mbps. Thus ADSL2+ doubles the frequency band of typical ADSL from 1.1 MHz to 2.2 MHz. More importantly, ADSL2+ provides port bonding known as G.998.x or G.Bond. This is a very attractive feature of ADSL2+ in which the download and upload speeds are the sum of indi­vidual speed of all provisioned ports to the end user. It means if two lines with 24Mbps were bonded, the net result would be a speed of 48 Mbps.

ADSL++, another variant of ADSL, developed by Centillium Communica­tions, is capable of providing down­load speeds up to 50Mbps, and uses the frequency band up to 3.75 MHz.


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