CAP was the original technology used for DSL deployments, but the most-widely used method now is DMT. DMT modulation technique 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 for 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 500kbps and a downstream data rate of up to 8Mbps.

ADSL-Lite, another variant of ADSL standard, offers upstream speeds up to 512kbps and downstream speeds of 1.5Mbps. Further, ADSL has many variants like ADSL2, splitter-less ADSL2, ADSL2+ and ADSL++.

ADSL standards
ADSL versions, along with their respective upstream and downstream data rates, are listed in Table above. ADSL2 or G.DMT.bis is defined in ITU G.992.3 and is an improved version of ADSL, with data rates in downstream as 12Mbps and 3.5Mbps in upstream. Splitter-less ADSL2 or 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 the speed of ADSL2. The upstream speed is up to 3.5Mbps.

Thus, ADSL2+ doubles the frequency band of typical ADSL from 1.1MHz to 2.2MHz. 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 is the sum of individual speed of all provisioned ports to the end-user. It means that, if two lines with 24Mbps were bonded, the net result would be a speed of 48Mbps.

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 3.75MHz.

ADSL set-up
Basically, ADSL caters to connections between Internet service providers (ISPs) and subscribers. In order to provide ADSL connections, the existing telephone network is utilised and a network element called digital subscriber line access multiplexer (DSLAM) is installed at the central office/telephone exchange along with a modem placed at the subscriber’s home or workplace. The DSLAM works like a concentrator. It provides multiple DSL connections for access to the bandwidth available to the DSLAM from the Internet backbone. The connection to the subscriber is then given from DSLAM via copper lines.

There are two types of DSLAMs—remote DSLAM and CO DSLAM. The remote DSLAM sits in the digital loop carrier (DLC) system in the neighbourhood of the subscriber. On the other hand, CO DSLAM sits in the telco’s central office and has a large capacity. The ADSL modem performs the task of converting the ADSL physical-layer signal into a format that can be understood by a computer or any other equipment connected to it. Fig. 1 shows a typical ADSL set-up.

The frequency selector switch at the telephone exchange/central office is essentially a filter that filters out the incoming ADSL signal and diverts the voice traffic to the voice switch. The voice switch is further connected to the public switched telephone network (PSTN). The data traffic is directed towards the DSLAM, which is connected to the Internet backbone.

The pair from DSLAM is terminated into the main distribution frame (MDF), from where connections are provided to subscribers via twisted copper-pair lines. These twisted pair lines terminate at a DP box placed near the subscriber’s home or workplace. The connection from the DP box is first run to the splitter that sits in the customer’s premises. The splitter, which is basically a low-pass filter, removes the plain old telephone system (POTS) signal (voice signal, 300Hz-3400Hz) from the incoming ADSL signal. The high-frequency filtered ADSL signal is then given to an ADSL modem. Finally, the connection is taken out from the Ethernet port of ADSL modem and is terminated to the subscriber’s computer.

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