Digital Subscriber Line (DSL)

DSL is a technology that allows high speed data transmission over normal telephone lines at low cost. There is minimal load on the CO and the ISPs can provide multimedia services using the much higher speeds than offered by normal modem connections. There are many flavours of DSL, the higher the frequency the greater the effect of distance and crosstalk.

Traditional copper wire technologies used for digital data and voice transmission such as T1 and E1 are low-speed technologies, however higher speed technologies are now required. So Digital Subscriber Line Technologies have been developed to enable not only higher access required down to the user, but also using existing infrastructure. The ANSI T1E1.4 working group are responsible for overseeing the development of the various DSL technologies. The European Technical Standards Institute (ETSI) also contribute from the European point of view.

At each end a splitter is used to separate out the voice traffic from the high speed data traffic. The voice traffic is diverted to the voice switch and the data traffic is fed into a xDSL modem.

The technology is based on RFC 1577 (IP over ATM) that runs on the link to the main network and on the link to the remote location where ATM could run to the desktop or conversion to Ethernet could be made. The main links could also be RFC 1483 Frame Relay (multiple PVCs) or PPP over ATM (limited to one PVC).

There are many different forms of digital subscriber line, the most common form is Asymmetric DSL (ADSL). This is the form that is most commonly used in the domestic environment. DSL means the modem despite the fact that line is one of the words in the acronym, so two DSLs are required to make a circuit. The DSL was used, in Basic Rate ISDN, to take the copper Twisted Pair wire and multiplex two B-channels and a D-channel. The most common wire guage is 24 AWG.

With ISDN ISL (IDSL) Basic Rate ISDN is terminated on a dedicated box at the CO and can run up to 18,000ft. Frame Relay can run over this and the data rate can be up to 144 Kbps (64 + 64 + 16).

High data rate Digital Subscriber Line (HDSL) can be seen as a replacement for T1 and E1. HDSL can run up to 12,000 ft and uses two pairs for T1, and three pairs for E1. HDSL uses a more efficient encoding technique than the AMI Encoding used in E1 and T1 and gives data rates of between 384 Kbps and 2 Mbps.

Single line Digital Subscriber Line (SDSL) is a single pair version of HDSL. The advantage of this is that the premises that has only a single telephone line can still take advantage of high data speeds (768kbps). There is a distance limitation however, one pair of wires can run up to 9,000ft with the same performance in both directions i.e. Symmetric.

Very high data rate Digital Subscriber Line (VDSL) is being developed as as much simpler technology than ADSL. It is by nature based asymmetric and operates at speeds many times faster than ADSL such as the following:

Upstream rates fall within a probable range from 1.6 Mbps to 2.3 Mbps. VDSL still provides error correction and may also in the future integrate with ATM

This document will concentrate mainly on ADSL as this technology is currently the most common.

ADSL uses one pair of wires at distances up to 18,000ft. Asymmetric refers to the fact that speeds of up to 8 Mbps can be attained downstream (from the CO to the client) whilst up to 640 Kbps is the upstream speed. The ADSL modem in the CO is called the ADSL Transmission Unit - Central (ATU-C) whilst the ADSL modem at the remote end is called the ADSL Transmission Unit - Remote (ATU-R). At the CO there is the ADSL Transmission Unit - Central (ATU-C) sometimes called the DSL ATM Multiplexer (DSLAM) which houses many ADSL connections.

The ADSL modems provide three channels; a high speed channel that operates one-way down to the client; a duplex channel for client interaction and a POTS channel for the telephone. The POTS channel is separated from the other two channels via the use of filters. ADSL has been designed to carry video, IP and ATM and th downstream speeds have various operational distance limitations depending on a number of factors such as the wire gauge:

The main reason why have copper pairs are twisted, is because when Transmission is symmetric there is a much greater chance of crosstalk and this limits the transmission distances when many copper pairs are bundled together. Fortunately, domestic Internet traffic is very much asymmetric e.g. download of high bandwidth multimedia files compared with low-bandwidth HTTP Requests. There are a number of download speeds available for ADSL:

Upstream speeds can vary from 16 Kbps to 640 Kbps.

ADSL has to cope with error correction for video signals and it has to distinguish what hard data signals. This is because ADSL has capabilities to apply it error correction when the year signals are detected. The error correction can add up to 20 milliseconds of Delay, this delay would not be acceptable for data traffic. ADSL is not the only used the circuit switch traffic, but also packet switch traffic and may also be used for ATM traffic in the future.

Because video was envisaged to be used extensively over ADSL, forward error correction operating on a symbol by symbol basis, has been built in to the standard, this adds a 20ms delay. This error correction is there to cope with impulse noise that occurs and therefore degrades the video quality.

The traditional copper twisted pair telephone line can operate at a bandwidth of up to about 1.5MHz. ADSL has been designed to work within this. Over a number of kilometres, a 1MHz signal can be attenuated by up to 90dB. ADSL technology uses analogue filters, digital signal processing, amplifiers and complex algorithms to maintain the integrity of the signal.

ADSL uses either one of two modulation types, Carrierless Amplitude (CAP) (also known as Globespan) and the ANSI standard Discrete Multitone (DMT).

When ADSL uses Frequency Division Multiplexing (FDM) to create the multiple channels, it splits off the POTS channel into a separate 4KHz channel which is situated at the low end of the 1.5MHz frequency spectrum of the carrier (often called the DC end). This is because voice only uses 3.4Khz of bandwidth. See the following diagram:

Then it assigns a channel to the duplex multimedia stream that operates at between 16-832Kbps, this band sits between 25 and 160KHz. Finally the high-speed downstream multimedia stream that operates between 1.5Mbps and 6.1Mbps is placed at 240KHz to 1.5MHz. Each of the multimedia channels are Time Division Multiplexed (TDM) into further lower speed channels. The upstream channel has 32 x 4KHz sub-channels and the downstream channel has 247 x 4KHz channels. The common method of splitting up the bandwidth into sub-channels is called Discrete Multitone (DMT) and is designed such that if problems occur at a particular frequency such as interference or bridge taps on the wire path, then only one or two sub-channels become affected, data can still be carried on the unaffected sub-channels.

When ADSL uses Echo Cancellation to create multiple channels, the multimedia channels bandwidths overlap and the two are separated by echo cancellation. This does not happen with the POTS channel which is separated in the normal way using a low pass filter that was tuned to filtering the frequencies below 4KHz.

Rate Adaptive DSL (RADSL) is an extension to ADSL that uses a technique to adapt the data rate to the conditions of the line.

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