It is estimated that there are 500 million telephone lines enmeshing our planet. Obviously communication

systems designers ignore it at their own risk. It remains the prime vehicle for any new communication system

technologists as POTS (plain old telephone services)

modem. However, breaking rules remains an enjoyable activity and communication engineers are no

capacity theorem, but out of a realisation that the ubiquitous telephone line has a bandwidth of nearly 1 MHz.

This is a huge expansion in available bandwidth in comparison to the 4 KHz voice bandwidth utilised by the

voice band modems. Of course, this also meant that the exchange makers should handle the tsunami of

multi-mega bit data rates instead of the gentle streams of 56K that they were used to. Once this was

accepted, DSL’s future was secured.

So what can DSL do ? In plain terms, you can connect to the internet at rates of 8 Mbps or more, while you

can use your telephone simultaneously. It is ‘personal broadband’ at your disposal, over the old faithful

telephone line. Also, it is often ‘biased’ in your favour ie; you have a bigger bandwidth (and hence better bit

rate) to download (the “downstream” direction) than the telephone exchange has for you to upload

(“upstream” direction)

To go back a bit in time, the digitisation of the subscriber line started with ISDN. It offered bit rates upto 144

Kbps. This took real voice off the line, digitising and making it just another of the bitstreams commuting over

the line. Next was HDSL (High data rate DSL) which endeavoured to replace the T1/E1 lines with twisted

pairs. Bit rates upto 2 Mbps was possible. Ignoring a plethora of other interim DSL technologies, we come to

the dominant DSL technology of today - ADSL (Asymmetric DSL). Initially conceived as a video-on-demand

system, it evolved into a method that delivers 8 Mbps downstream, and 640 Kbps upstream, and can coexist

with existing POTS facilities. The DSL technology of the future is VDSL (Very high data rate DSL) which can

squeeze through an incredible 52 Mbps over twisted pairs - admittedly over much shorter distances than

DSLs.

How it works

Currently accepted ADSL standards use discrete multitone (DMT) modulation scheme to transmit data.

‘Multi-tone’ means that there are many carriers (usually 256), each of which individually undergoes QAM

(quadrature amplitude modulation). This means that there are sine and cosine versions of the carrier wave at

each carrier frequency, whose amplitudes are determined by the modulating bit pattern. The signal on the

line is thus a sum of a number of QAM modulated sine waves. In contrast, voice band modems transmit

data by modulating a single carrier , which is usually a 2.4 KHz sine wave. POTS compatibility is achieved by

keeping clear of the POTS band - 0 to 4 KHz, as well as by having an additional guard band - from 4 KHz to

30 Khz. The ADSL band extends from 30 KHz to around 1 MHz. This band is split into several ‘tones’, each

of which is modulated and occupies 4 KHz. Thus one could say that the DSL modem comprises of 256 ‘voice

modems’ all running in parallel. Approximately one-eighth of these bands are set apart for upstream data

transmission and the rest for downstream data. This bigger downstream bandwidth gives the ‘asymmetry’ to

ADSL. This goes well with web-based applications where the subscriber is more often downloading data off

the web, rather than sending data into the web.