A new standard for HDSL has been developed using the carrierless 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/E1 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.
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 realised 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.
ADSL caters specifially to connections between ISPs and customers. The Internet is used largely for downloading fies, HTML and graphical content. Processes like uploading filesor 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 band-width 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 define in ITU G.992.4 and is capable of providing 1.536Mbps down-stream 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.