ADSL (Asymmetric Digital Subscriber Line) is a newly standardized transmission technology facilitating simultaneous use of normal telephone services, data transmission of 8Mbps in the downstream and 1Mbps in the upstream. The dominant form of ADSL technology is Discrete Multi Tone (DMT). another forms of ADSL enabling technology is Carrierless Amplitude Phase Modulation (CAP). In this article, DMT ADSL system will be described.

DMT occupies the bandwidth between 26KHz and 1.1MHz This is divided into discrete 4KHz sub-channels, each hertz of which is allocated 0-15 bits and QAM before transmission. This gives each sub-channel a maximum load of 64 Kbps. The bandwidth of the copper pair is split into three distinct sections:

• Normal Speech Telephony (POTS)

• Low bit rate ADSL Return Channel (For Upstream)

• High bit rate ADSL forward channel (For Downstream)

Precisely how much benefit you see will greatly depend on how far you are from the central office of the company providing the ADSL service. Being a distance-sensitive technology, one of the ADSL’s drawbacks is that its transmission speed decreases with distance because copper lines attenuate signals proportionally to length and signal frequency. This shows the importance of the Inter-Operability and line qualification tests adopted by almost all service providers.

ADSL technology is aiming at providing new services to subscribers. Keeping the limitation of the ADSL in mind, a subscriber, 3.5KM away from the central office, can expect to achieve downstream transmission rate of 1.5Mbps which will afford MPEG-1 motion video, high speed internet access, and interactive gaming.

In summary, despite of the number of the limitations that the ADSL has, it brings a number of advantages to the subscribers:

• You can use the Internet while talking on the phone and watching TV.

• The Internet speed is much higher than the regular modem

• The technology doesn’t need a new cabling system. It used the copper wire already available