ADSL - How it works
Written by Harry Fairhead   
Wednesday, 28 September 2011
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ADSL - How it works
The future

You may think of ASDL as yesterday's comms. But far from being phased out, this is a technology we are going to have to rely on for some time to come.


Being able to communicate at reasonably high data rates has revolutionized the sort of application we can build. While it is easy to think that it is mobile communication - WiFi and 3G - that has brought about the biggest change it is the way we use the copper wires originally only intended as low quality voice connections as data pipes.

Phone lines have always been an obvious way to send data from one place to another. However in the early days the speeds were very low - typically 300bps. The reason for this was simply that the phone lines were never designed to carry a wide range of frequencies. In fact theory proved that around 30Kbps was the fastest you could possibly get data though on such limited lines and yet here we are with 100Kbps and we didn't need to dig up the road? How does this work?

The local loop

While most things about computing and communication have been upgraded many times, the one thing that has remained mostly untouched is the phone connection inside your building.

The piece of wire that connects your building to the telephone network – the local loop – is still just two pieces of thin copper wire twisted together. This is fine for voice telephone communications but this same technology is a problem when it comes to data communication using a modem – modulator-demodulator. This converts the digital zero/one signal into audio tones which can travel along the voice circuits in the same way as a voice call.

How much data you can squeeze through a wire, or any communication channel in fact, is governed by two things – the range of frequencies it will carry, its bandwidth and the amount of noise present. Claude Shannon proved in 1948, by inventing information theory, that if you knew the bandwidth and the noise you could work out a theoretical upper limit on the number of bits per second a channel could carry. For a typical twisted pair phone line this maximum is around 30Kbps.

Early attempts to reach this theoretical maximum made use of an odd fact - after the local loop the phone system is digital. V90/92 modem can actually work at 56Kbps by connecting directly to the AtoD and DtoA converters at the exchange. This is faster than the theoretical maximum allowed for a purely analogue modem and it does seem to represent the fastest that can be achieved without installing new hardware at the exchange.

The key to getting the speed even higher on the local loop does depend on changing the hardware at the exchange. The limitation on using modems on standard phone lines is mostly due to the way that the local loop is used. As a voice call only uses up to 4KHz the local loop is limited to delivering just this narrow range and 4KHz represents the highest frequency used by any stages further up the line.

The wires that go back to the exchange are capable of being used at a much higher frequency than is normally utilised in a standard phone call. All we have to do is to change the equipment at the exchange and possibly remove any loading coils on the line which where used to improve the voice quality and the maximum frequency you can transmit rises to 1MHz or more.

The quality of the line goes down with the distance to the exchange and many other factors but, if the wire is less than 5.5Km or 3.5 miles long, it should manage to carry frequencies up to 1.1Mhz – which should be compared to the 4 to 8KHz range that voice telephony needs. However the maximum frequency that a line can work at depends on its length. The closer you are to the exchange the higher the data rate you can expect.

If the cable connecting you to the exchange is more than 3.5miles/5.5Km long it can’t be used for ADSL. This is the theoretical limit for the most common ADSL 512Kbps speed and 3.5Km is the limiting distance for a premium 1 and 2Mbps ADSL connection. More modern services such as ADSL2 or ADSL2+ can boost speeds to 12Mbps or more depending on distance.

ADSL can deliver anything a minimum of 512Kbps download speed and 256Kbps upload speeds and you can use a phone via the same line at the same time. Exactly how does this work?


To make use of this increased frequency range your phone line has to be connected to some different electronics at the exchange. This what ADSL – Asymmetric Digital Subscriber Line – or broadband is all about.



ADSL uses lots of small channels.


The way that ADSL works is another miracle of modern technology. Its complicated and sophisticated and yet it if it’s working properly you shouldn't notice it’s even there. The ADSL modem or router that your PC connects to works hard to get the maximum amount of data down the wires using a method called Discrete Multi-Tone DMT. It divides the frequency range available into 256 (the actual number can vary) separate 4KHz channels. Each of the channels is used independently and the ADSL modem constantly searches for the best channels to use. By monitoring and shifting which channels are used the ADSL modem gets the maximum amount of data through the line no matter what its noise or other problems it is subject to. Some of the lower frequency channels are so good that they can be used bi-directionally, The key idea in ADSL is to use everything to get the maximum data rate while keeping things reliable.

Where does the voice go?

Answer: the bottom 4KHz channel is filtered out and used for voice communications and special low-pass filters are fitted to phone sockets that have handsets connected. If you have an ADSL socket and want to use it as a standard phone line then you might have to use a “micro-filter” to remove the data channels which otherwise make an audible high pitched clicking but most sockets have the filter built in.

The remaining 256 channels are used for digital communication. Typically the best set of 64 is used for uplink data and the best 128 are used for downlink data. This means that the upload speed is different from the download speed and this is the why it’s called Asymmetric DSL. It could be arranged so that the speeds were the same in both directions, this is called Symmetric or SDSL, but in practice it is assumed that you will want to download data faster then you will want to upload it. So important is download performance that ADSL will drop the upload speed before it starts to reduce the download speed if the line quality is bad.

You can find ADSL with a range of different up/down speeds but the speed that you actually get depends on the quality of the line which in turn mostly depends on your distance from the exchange. Currently there are three versions of the technology (ignoring minor variations and obsolete standards) and their maximum data rates can be seen in the table below.



Max Downstream rate

Max Upstream rate

ADSL 12.0 Mbit/s 1.3 Mbit/s
ADSL2 12.0 Mbit/s 3.5 Mbit/s
ADSL2+ 24.0 Mbit/s 3.3 Mbit/s


What really makes ADSL different from simple schemes to make use of the increased bandwidth is that it constantly monitors the performance of the channels available to it and changes which are used for upstream and downstream data. If a particular channel becomes unreliable due to noise then it is avoided. In each case the coding used for a particular channel also takes account of how noisy it is. Noisy channels use more bits for error correction and so reduce their useful throughput. The ADSL hardware basically "learns" the characteristics of the connection so as to optimize the data throughput.


Last Updated ( Saturday, 10 May 2014 )