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A digital subscriber line (DSL) is a digital telephone line. That is, both sound and data travel over the wires as digital bits. Voices get encoded into binary digits, travel to the destination, and are then decoded back into sounds, but there is no need to encode and decode digital data into sounds and then back again. DSL is sometimes described as asymmetric (ADSL), because it is based on the observation that almost all network interactions are as follows:
That is, it takes very little outgoing bandwidth to send a user's command (e.g. mouse click on a URL in a browser). By contrast, a great deal of bandwidth is needed to retrieve the response to that command (e.g., webpage), because it is much larger by comparison to the user command. DSL divides up the digital phone line's bandwidth accordingly, providing an effective bandwidth of 576 Kbps for outgoing (or upstream) packets and a maximum bandwidth of 6.144 Mbps for incoming (or downstream) packets. A DSL modem accomplishes this by modulating the higher frequencies on a phone line that are unused by the line's normal voice-carrier wave. More precisely, it divides those frequencies into 286 different channels, 255 for "downstream" traffic, 31 for "upstream" traffic, and 2 for control signals. This is like having a traditional modem that, instead of having a single modulator and a single demodulator, has 31 modulators, and 255 demodulators, all working in parallel. Even more remarkable is the fact that ADSL keeps all of this "data" traffic on frequencies distinct from those used to carry the line's "voice" traffic, so that using a DSL modem, a person can talk on the phone and access the Internet at the same time, using the same telephone line! In most markets, the cost of DSL is slightly higher than the cost of a second phone line, but most users report that the difference in speed more than justifies the price difference. A strong argument can be made that DSL represents the most cost-effective home-access solution available today. |
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If you encounter technical errors, contact rit@calvin.edu. |
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