ADSL (Asymmetric Digital Subscriber Line)

ADSL is the acronym for Asymmetric Digital Subscriber Line. ADSL is a loose set of protocols that allows for high speed Internet access over normal copper telephone lines or what is more commonly known as POTS (Plain Old Telephone Service). This is possible because the signals are sent digitally instead of through analog waves. ADSL is known as asymmetric because the download and upload speeds are not symmetrical with download speeds being averagely faster than upload speeds. Upstream data speeds are lower because requests for web pages normally do not require a lot of bandwidth.

ADSL provides Internet access that is constantly on unlike dial up phone access. That way, your computer remains connected to the Internet when it is powered on unless the cable is manually disconnected. ADSL allows for the simultaneous use of normal telephone services (voice), Integrated Service Digital Network (ISDN) and high speed data transmission such as video. ADSL provides much higher bandwidth than traditional dial up connections. Speeds can range from 512Kbps to 9 Mbps.

How ADSL works

ADSL makes use of your existing telephone line. It splits the line into two distinct channels, one for data connection and the other for voice. The ADSL signal requires 2 special modems. One is used at your end while the other is used in the telephone exchange. Each modem is geared to a different frequency. Your telephone line is prepared for ADSL by opening up the copper pair to high frequencies and then routing the digital data in these frequencies to a Digital Subscriber Line Access Multiplexer (DSLAM) which converts this signal to ATM packets. The signal is then sent back to the servers and the server then forwards the request and assigns an IP address to the client.

ADSL modulation exists in two schemes. One is Carrierless Amplitude (CAP) and the other is Discrete Multi-Tone (DMT). These modulation schemes allow data transmission over high frequencies thus increasing Internet access speed. The two schemes are used for both upstream and downstream data transmission. However, most modern installations are based on the DMT modulation scheme. The idea behind DMT is to split the bandwidth into a large number of sub channels and allocate data so that the throughput of every single sub channel is maximized.

Advantages of ADSL

1. Unlike traditional dial up connection where one session could only be used by one user, ADSL allows several users to share one account meaning the cost can be split between multiple users.
2. ADSL uses standard telephone lines for digital transmission thus setting the analog transmission signals apart from the digital. This allows normal usage of the telephone facility even as you are browsing the internet.
3. Allows for high speed Internet access without the cost of ISDN.
4. ADSL 2 and ADSL 2+ have the advantage that they offer several improvements to ADSL. Some of these improvements include faster data transmission of up to 20 Mbps, dynamic data rate adaptation, standby power saver, better resistance to noise etc.
5. ADSL transfers data digitally and digital data has a higher noise tolerance than analog data.