Tuesday, Jun 18th

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What is DSL?

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Networking questions are popular among telecom companies like Aricent and core networking houses like Cisco. Networks are all around us. Afterall, we live in a web :) But are we aware of the technology around us as well?


Well, most of us use DSL connections at home. DSL is the most common form of broadband internet in India. Not many people know what DSL actually is? Lets try and clear some heads. (Let me assume that you know what an analog and a digital signal means).

  1. Ok, so DSL stands for Digital Subscriber Line.
  2. Telephone service companies (like BSNL and MTNL) connect your homes to their telephone exchanges over copper wires that are wound around each other in a twisted pair. (The twist prevents interference among wires. A telephone exchange is where many such wires meet. Many such telephone exchanges are also connected to create the whole telephone network.)
  3. Now in old devices, voice used to travel on these wires as an analog signal. An input device such as a phone set takes an acoustic signal (your voice which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change).
  4. Now to transfer data (which is digital) on these (analog) lines (from your computer), you needed to have a MODEM installed. the modem would convert the digital data from your computer to analog signal and send it across the copper lines (modulation) and similarly convert the analog back to digital for the computer to understand (de-modulation).
  5. Analog transmission could only use a small portion of the available amount of information that could be transmitted over copper wires. Hence, the maximum amount of data that you could receive was about 56 Kbps.
  6. That was in the MODEM era. Then came DSL. DSL is a technology that assumes digital data need not be converted to analog form and back. Digital data is transmitted to your computer directly as digital data only.
  7. With DSL, the phone company installs a splitter which separates the digital data from analog. A part of the bandwidth is used for the analog phone call while the other part is used for digital transmission of data.Digital Subscriber Line
  8. What the splitter actually does is, it divides the bandwidth of the cable into a lower frequency band and a higher frequency band. The voice signals then travel in the lower band, while the data travels in the higher. To keep the voice band clear of bleeding signal noise, a small filter is commonly installed on all telephone lines in the house, blocking the higher frequencies.
  9. This means that you can use your telephone and computer on the same line and at the same time
  10. This is the basic fundamental behind DSL. A popular form of DSL is ADSL, which we normally use in our homes.  In DSL, you transmit equal information to and fro, therefor the bandwidth for upload and download remain same. In reality, however, we download more than we upload. So ADSL (short for Asymmetric DSL), does an unequal separation of bandwidth providing 6.1 Mbps (megabits per second) of data to be sent downstream and up to 640 Kbps upstream.

The DSLAM shown in the diagram here is the DSL Access Multiplexer. And as the name suggests, it multiplexes the various data connections (or requests) it receives from all the customers connected with the Central Office. The other part at the Central Exchange is the switch which takes care of the voice calls.

image source: wikipedia (under creative commons licence)

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