DSL Modem - How IT Works - How Data Is Transmitted

How Data Is Transmitted

The device at the local switching center which communicates with the DSL modem is called a Digital Subscriber Line Access Multiplexer (DSLAM), which is connected directly to the Internet. The local switching center must be equipped with these devices to offer DSL service.

With ADSL, the modem and the DSLAM communicate by a protocol called discrete multitone modulation (DMT), which is a form of frequency division multiplexing. The modem only uses frequencies above 8 kHz, to avoid interfering with normal phone service. The bandwidth of the line between 8 kHz and about 1 MHz is divided into 247 separate channels, each 4 kHz wide. A separate carrier signal carries information on each channel. Thus the system acts like 247 separate modems operating simultaneously. The bits of the incoming digital data are split up and sent in parallel over the channels. Each data stream is sent using an error-correcting code to allow minor bit errors due to noise to be corrected at the receiving end. Most of the channels are unidirectional, carrying download data from the DSLAM to the modem, but some on the low freqency end are bidirectional, to carry the smaller quantity of upload traffic. The modem constantly monitors the transmission quality on each channel, and if it is too impaired it will shift the signal to other channels. The modem is constantly shifting data between channels searching for the best transmission rates. Thus interference or poor quality lines will generally not interrupt transmission, but only cause the data rate of the modem to degrade.

For example, when downloading a web page, the packets of web page data travel over optical fiber Internet lines directly to the DSLAM at the neighborhood telephone exchange. At the DSLAM they are split into as many as 247 parallel data streams. Each is modulated onto a separate carrier signal and sent through a separate frequency channel over the subscriber's telephone line to the DSL modem. The modem demodulates the carrier, extracting the data stream from each carrier signal, performs error correction, puts the data together again in the proper order, and sends it to the computer over the Ethernet line.

Read more about this topic:  DSL Modem, How It Works

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