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Adigital subscriber line (DSL)modem is a device used to connect acomputer orrouter to atelephone line which provides thedigital subscriber line (DSL) service for connection to theInternet, which is often calledDSL broadband. The modem connects to a single computer or router, through an Ethernet port, USB port, or is installed in a computerPCI slot.
The more commonDSL router is a standalone device that combines the function of a DSL modem and arouter, and can connect multiple computers through multipleEthernet ports or an integralwireless access point. Also called aresidential gateway, a DSL router usually manages the connection and sharing of the DSL service in a home or small office network.
Different DSL routers and modems support different DSL technology variants:VDSL,SDSL, andADSL.
A DSL router consists of a box with anRJ11 jack to connect to a standard subscriber telephone line. It has several RJ45 jacks forEthernet cables to connect it to computers or printers, creating a local network. It usually also has aUSB jack which can be used to connect to computers via aUSB cable, to allow connection to computers without an Ethernet port. Awireless DSL router also has antennas to allow it to act as awireless access point, so computers can connect to it forming awireless network. Power is usually supplied by a cord from awall wart transformer.
It usually has a series of LED status lights which show the status of parts of the DSL communications link:
Many routers provide an internal web page to the local network for device configuration and status reporting. Most DSL routers are designed to be installed by the customer for which a CD or DVD containing an installation program is supplied. The program may also activate the DSL service. Upon powering the router it may take several minutes for the local network and DSL link to initialize, usually indicated by the status lights turning green. There are also PCI DSL modems, which plug into an available PCI card slot on a computer.
Thepublic switched telephone network, the network ofswitching centers, trunk lines,amplifiers and switches which transmitstelephone calls from one phone to another, is designed to carryvoice frequency signals, and is therefore limited to abandwidth of 3.4 kHz. Before DSL,voice-band modems transmitted information through the telephone network withaudio frequencies within that bandwidth, which limited them to a data rate of about 56 kbit/s. However, the copper wires that connect telephones with the localswitching center (telephone exchange), called thesubscriber loop, are actually able to carry a much wider band of frequencies, up to severalmegahertz.[2] This capacity is unused in normal phone service. DSL uses these higher frequencies to send digital data between the DSL modem and the local switching center, without interfering with normal telephone service. At the local switching center the data is transferred directly between the customer's phone line and internet lines, so DSL signals do not travel through the telephone network itself. It is not necessary to dial a telephone number to initiate a connection; the DSL connection is "on" whenever the modem is on.
The device at the local switching center which communicates with the DSL modem is called aDigital Subscriber Line Access Multiplexer (DSLAM), which is connected directly to the Internet.[2] 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 calleddiscrete multitone modulation (DMT), which is a form offrequency division multiplexing.[2] 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.[2] A separatecarrier signal carries information in each channel. Thus the system acts like 247 separate modems operating simultaneously. Thebits of the incoming digital data are split up and sent in parallel over the channels. Each data stream is sent using anerror-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 frequency 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.[2] 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 overoptical fiber Internet lines from theserver computer directly to the DSLAM at the neighborhoodtelephone exchange. At the DSLAM they are split into as many as 247 parallel data streams. Each ismodulated onto a separate carrier signal and sent through a separate frequency channel over the subscriber's telephone line to the DSL modem. The modemdemodulates the carrier, extracting the data stream from each carrier signal, performserror correction, puts the data together again in the proper order, and sends it to the computer over theEthernet line, or for a wireless (Wi-Fi) network by radio signals.
Most consumer DSL lines use one of several varieties ofAsymmetric DSL (ADSL).[2] The "asymmetric" means that more of the bandwidth of the line is dedicated todownstream (download) data thanupstream (upload) data, so, download rates are faster than upload rates, because most users download much larger quantities of data than they upload. Because the telephone lines were never designed to carry such high frequency signals, DSL is distance-sensitive. The farther away from the switching center the modem is, the longer the telephone wires, the weaker the signal, and the lower the data rate that the modem can achieve. Users in cities, close to switching centers, may have access to higher rate service, up to 24 Mbit/s.[2] The distance limit for ADSL is 18 000 feet (5.5 km or 3.4 miles).[2] However, other devices installed in telephone lines by the phone company, such asloading coils andbridge taps, block the signal, and may disqualify a given phone line from DSL service.
To prevent the DSL signal from entering the phone lines into telephones, answering machines, faxes and other devices where it could cause interference, DSL modems come withlow pass filters which must be plugged into the phone lines going to all voiceband devices on the same line. The filter blocks all frequencies above 4 kHz, so it blocks the DSL signal while allowing voice frequency signals through. A filter must not be inserted in the phone line going to the DSL modem, because it would block the communication between the modem and switching center.
A DSL modem modulates high-frequency tones for transmission to adigital subscriber line access multiplexer (DSLAM), and receives and demodulates them from the DSLAM. It serves fundamentally the same purpose as thevoice-band modem that was a mainstay in the late 20th century, but differs from it in important ways.
Most of these differences are of little interest to consumers, except the greater speed of DSL and the ability to use the telephone even when the computer is online.
As technology advances, functions that are provided by multiplechips can be integrated onto one chip. Higher levels of integration have benefited DSL just as they benefited other computer hardware. A DSL modem requires the following for its operation; exactly what is on the circuit card and how it is arranged can change as technology improves:
Apart from connecting to a DSL service, many modems offer additional integrated features, forming aresidential gateway:
Media related toADSL modems at Wikimedia Commons