I'm sure I've explained this before in another forum topic, but, I suppose it couldn't hurt. [
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ADSL is actually a very simple technology. It works by transmitting audio signals, represented by electrical currents, over copper wires, just like any telephone, fax machine or voice band modem. What makes ADSL so special is the way in which it makes use of the existing copper wire infrastructure.
Copper wires are not in themselves a digital transmission medium. Instead, electrical corrents with variable potential difference (Voltage), are transmitted over the wire. On both sides, the electrical currents activate electromagnets which push and pull against permanent magnets to produce a sound signal. This is exactly the same principle as that used in sound systems. Electrical currents from the amplifier activate electromagnetic fields in the speakers, which push and pull against the permanent magnet to generate vibration, which generates pressure waves in the air, sound.
Transmitting digital data through a technology initially intended for audio transmissions presents a unique challenge. Since digital data is reprisented as a series of ones and zeros, fax machines and voice band modems have to perform digital to analogue and analogue to digital conversions. Basically, the ones and zeros are reprisented by different sound frequencies, which can be transmitted over the copper wire loop.
With normal telephones, fax machines and voice band modems, the frequencies that pass through the line are usually quite limited. The human voice will normally only occur between 300Hz and 3KHz. For this reason, computer operated exchanges, linked to the back bone by fiber optic cable, which first emerged in the 1970s, were mostly designed to only retransmit frequencies in this band over their digital link.
Hence, the range of sound frequencies between 300Hz and 3KHz became known as the voice band. Because digital exchanges will normally only repeat voice band transmissions, traditional voice band modems are limited to using tones on sound frequencies that fall within this band. This leaves the modems with very little distinct tones from which to chose for signalling.
The Shannon limit is the theoretical limit for the amount of data that can be transmitted simultaneously on any audio or electromagnetic (radio) frequency band, in other words, the maximum band width for that band.
For the voice band, the Shannon limit is 64kbps. In actual fact, the fastest voice band modems at the time of this writing can only operate at 56kbps or below. This is because the equation used to calculate the Shannon limit does not take in to account the loss of bandwidth due to interference or attenuation.
Clearly, this makes the voice band unsuitable for data transmissions that require large bandwidth. In the past, the only solution to this was to build high-capacity parallel digital lines, which carry the data as electronic pulses instead of variable potential difference currents. Another solution is to use fiber optic cable with many optical fiber strands, also allowing a large number of parallel signals. Both of these technologies are expensive to implement, and are limited by distance and surrounding terrain.
A solution had to be found which could deliver broadband data transmissions over the existing copper cable infrastructure. Enter DSL, or digital subscriber line. Many kinds of DSL are implemented around the world. By far the most popular is ADSL. It is not as fast as the other implementations, but it is the most robust and can operate over the greatest distances.
Remember that the voice band operates only between 300MHz and 3KHz. The actual copper cable is not limited to these frequencies. In fact, if the quality of the cable is high enough and the distance is short enough, you could, theoretically, send any sound frequency over a copper cable.
The disadvantage of copper cables is that tones at very high frequencies tend to run in to interference and attenuation when operating over long distances. This still doesn't mean it's impossible.
ADSL uses DMT (Discreet Multi-Tone) modulation to create 256 individual tones, used as channels, in the audio spectrum between 20KHz and 1MHz. While it is possible to use signals at frequencies above 1MHz, it would rarely work over long distances. With a maximum frequency of 1MHz, a distance of 3km - 5km can be traversed over copper wire, with relatively little signal loss.
This enormous frequency band offers a lot of bandwidth. ADSL, being asynchronous, uses this bandwidth to create a virtual ethernet over ATM circuit with different upstream and downstream bandwidth. This is achieved by using a few of the low frequency channels for sending data, and the rest, for receiving data.
The Shannon limit for the upstream channels is 800kbps, and the limit for the downstream channels is a sturdy 8MBps. Obviously, interference and attenuation do play a role, so most modems will not operate above 640kbps upstream and 6MBps downstream. This leaves plenty of margin for error, which guarantees a stable connection.
Splitting the line is quite a simple procudure. At the client side, a small electromagnetic filter is used. The filter cuts out all frequencies above 3KHz, the top end of the voice band, to protect telephones, fax machines and voice band modems from interference caused by the ADSL modem transmitting on the higher frequencies on the same line.
At the exchange side, the DSLAM listens only for frequencies above 20KHz, thus, there is no chance of it picking up on stray frequencies from voice band transmissions on the lower frequency bands on the line. The DSLAM performs an analogue to digital conversion of the high frequency tones to produce data. This data can then be sent on to the backbone network through the optical fiber or other digital connections, connecting the DSLAM and the exchange to the backbone.
In this way, ADSL makes full use of the existing copper cable, which is otherwise mostly un-utilised by voice band traffic, to deliver broadband internet access, without the need for massive new infrastructure to the client side.
As a last side note, I should point out that Telkom's DSLAMs do not use the full version of the G.DMT standard, which defines frequencies and transmission rates for ADSL. Instead, Telkom make use of the G.Lite standard. G.Lite is a trimmed down version of G.DMT, which can operate over slightly longer distances, but uses less channels and can deliver significantly less bandwidth. The maximum rates with G.Lite is 512KBps upstream and 2MBps downstream.
Willie Viljoen
Web Developer
Adaptive Web Development
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