Why Does DSL Require an Internet Modem?

by David Lipscomb

Digital Subscriber Line or DSL Internet services operate on existing phone lines. Signals from your DSL service provider are sent from a local hub, called a Digital Subscriber Line Access Multiplexer, or DSLAM. Decoding these modulated feeds requires a modem, which processes the data transmitted to and from your computer. DSL modems are commonly called "gateways," another term for a combination DSL modem and router. A DSL modem provides an always-on connection, differing significantly from dial-up services that also use copper phone wires. The service's high speeds and dedicated lines offer typically reliable service and quick Internet access.


A Digital Subscriber Line modem is a device normally supplied by your service provider and installed external to your computers. The modem connects to an incoming phone line; a router connects to the modem via a single Ethernet cable, which then connects to each computer in the same manner. Exceptions to this are when the router is wireless, in which case suitably equipped machines such as smartphones and tablets connect over a Wi-Fi link. Power for the modem is supplied by a conventional AC adapter.

Residential Gateways

Residential gateways function primarily as decoding and routing hardware, distributing Internet data among your network devices and back to the service provider. These units are combination routers and modems, allowing less hardware and simpler setup. Often supplied by your service provider, these units are normally wired and wireless, offering maximum network flexibility. Advanced residential gateways decode multiple signal types sent over phone lines, including Internet, telephony and digital television services. An example of this system configuration is AT&T's U-verse, which routes all digital services through a single gateway.

Service Connection Considerations

The speed of a DSL Internet connection is a function of the distance between the local DSLAM and your modem. The further you are away from this hub, the slower your maximum speeds will be. Generally, six miles is the maximum range. Many DSL providers install local splits or bridge taps to increase coverage to mitigate this. Your modem cannot do anything to improve this situation, acting solely as an intermediary between your ISP and your network.

Telephony Conflicts

DSL services often require filters at each phone jack, preventing Internet data sent from the modem to each computer from overriding lower-level telephone feeds. An alternate and costlier method of bypassing this is by running dedicated phone and Internet lines. In this case, the DSL modem continues to route Internet data, while phone conversations remain clear without a plastic dongle at each jack.

About the Author

David Lipscomb is a professional writer and public relations practitioner. Lipscomb brings more than a decade of experience in the consumer electronics and advertising industries. Lipscomb holds a degree in public relations from Webster University.

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