In ADSL, the data throughput in the upstream direction (the direction to the service provider) is lower, hence the designation of asymmetric service. In symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal.

The main advantage of ADSL is that the connection appears to be always 'on', so there is no wait to get connected to Internet, it is much faster than ISDN (about 50 KB/sec). There are no call charges: the charges include an installation charge, then a fixed fee for a set period: month, quarter or year.

You must have a BT PSTN (standard) phone line connected to the place where the ADSL service will be used. If your telephone line is a 'Digital Access Carrier System' (DACS) or shared line, it can not be used for ADSL.
To check if you have a DACS line, follow the phone line out the window until it reaches a black and/or grey box. If the line is 'DACS', the grey box will usually have a DACS or DACS 2 label on it. If you find you are on one of those lines, arrange for BT to convert it to a single user line before installing ADSL.
Note that DACS boxes are sometimes in places where you can not check them yourself: on top of poles, under the ground or in BT's PCP's. Then, you will have to phone BT to find out if you have one.

However, there are disadvantages: A DSL connection works better when you are closer to the provider's central office. The connection is faster for receiving data than it is for sending data over the Internet. The service is not available everywhere. Slow internet speed is also often a result of a bad attenuation and the signal to noise ratio (SNR).
In situations where repeated disconnect and reconnections have lowered the speed of your line, allowing some time for a reset may improve the situation. A reset can be accomplished by turning off the router for just over half an hour (+30mins) to allow ADSL "MAX" (up to 8Mbps) lines, and sometimes faster 16-24Mbps ADSL2+ lines as well, to reset.

A standard telephone installation consists of a pair of copper wires that the phone company installs in your home. The copper wires have lots of room for carrying more than your phone conversations -- they are capable of handling a much greater bandwidth, or range of frequencies, than that demanded for voice. DSL exploits this "extra capacity" to carry information on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.

To understand DSL, there are a couple of important things to know about a normal telephone line One of the ways of making the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry.
Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz (cycles per second ). This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. The wires themselves have the potential to handle frequencies up to several million Hertz in most cases. The use of such a small portion of the wire's total bandwidth is historic -- remember that the telephone system has been in place, using a pair of copper wires to each home, for about a century.
By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analogue data can safely use much more of the telephone line's capacity. DSL does just that.

ADSL splits the 1.1 MHz maximum bandwidth of a copper wire connection into 4KHz channels and uses only the bottom 4KHz channel for normal voice and fax data. The other 256 available channels are used for parallel digital communication, with, for a typical home connection, one frequency band of 64 channels reserved for uplink data and a higher band of 128 channels reserved for downlink data. It should be clear that if the uplink rate is kept the same at 64 channels, the maximum downlink rate uses 192 4KHz channels, giving the 8Mbit/s maximum download rate for ADSL. At present, 2MbiVs is the maximum being made available.

In effect, ADSL takes a serial string of digital data and turns it into a parallel string, thus increasing data throughput. The encoding and decoding are done at the exchange and at the user site, as is the case for conventional modem dial-up.

The modulation used for ADSL is Discrete Multitone (DMT), now universally adopted as the standard. An earlier system, called Carrierless Amplitude Phase (CAP), could use all the band- width above 4KHz as a single transmission channel and had the advantage of being closely related to the Quadrature Amplitude Modulation (QAM) technique used by high-speed modems at rates above 9,600biVs. However, DMT offers more flexibility, although the costs are higher for silicon encoders and decoders.

At the user end, the ADSL modem or router collects high-frequency digital data and assembles it for transmission to the PC or network. At the exchange end, a Digital Subscriber Line Access Multiplexer connects the ADSL user to the wider Internet, combining multiple ADSL lines into a single data connection for the broader Asynchronous Transfer Mode (ATM) network behind.

Most homes and small business users are connected to an asymmetric DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit (most of the time).

Other types of DSL include:

• Very high bit-rate DSL (VDSL) - This is a fast connection, but works only over a short distance.
• Symmetric DSL (SDSL) - This connection, used mainly by small businesses, doesn't allow you to use the phone at the same time, but the speed of receiving and sending data is the same.
• Rate-adaptive DSL (RADSL) - This is a variation of ADSL, but the modem can adjust the speed of the connection depending on the length and quality of the line.