What is Digital Subscriber Line (xDSL)

Digital subscriber line technology is a method fixed line telecom providers use to transport data across the local loop copper wire at faster rates than can be achieved using voice band modems. This is accomplished by utilizing carrier frequencies much higher than those used in voice communications. This also creates the opportunity that over the same copper twisted pair wires there can be both a voice and data call taking place simultaneously. A simple frequency splitter fitted at the termination point separates the voice call from the data (internet) connection.

DSL is generally a short distance technique used over the last mile, but can stretch up to 10km but performance drops off sharply after only 2 km.

The most well none and provisioned form of DSL is ADSL (asymmetric data subscriber link) this is because it naturally fits the unequal upload/download requirements of standard home consumer internet traffic. ASDL is asymmetrical in that there is a much larger download capability (from the internet to the subscriber) that there is upload (from the subscriber to the internet). This makes ADSL a good fit for most common consumer’s internet requirements.

ADSL is full duplex which means it can transmit and receive over the same connection at the same time. It accomplishes this by using two different frequency bands. When transmitting data downstram the telco is sending traffic over a carrier frequency of 138 -1104KHz. When the user is transmitting upstream, sending requests to the internet it is sent by their on-premises ADSL modem at 26kHz. This wide separation ensures that the traffic streams do not conflict or cause cross interference.

ADSL has three transport modes the most common being ATM which itself has two sub protocols PPPoA and PPPoE you will often see these on the configuration of your ADSL modem. These are Telco transport protocols so they should never need to be changed on the modems configuration.

Alasdair Gilchrist

Alasdair is a technical writer with interest in business practice, operational strategy, start up philosophy and affordable technology. He lives in Nonthaburi, Thailand with his wife and daughter, and writes terrible novels as a hobby.

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