A modem converts digital signals into specific frequencies for travel over telephone or cable television lines. The modem has evolved since the 1970s, with ADSL and SDSL offering faster speeds. Cable modems work on the same principle as DSL, while satellite modems are less common and more expensive. Internet providers offer modems, but subscribers can also choose to use their own.
A modem, short for modulator-demodulator, is an electronic device that converts digital signals from a computer into specific frequencies for travel over telephone or cable television lines. At the destination, the receiving modem demodulates the frequencies into digital data. Computers use modems to communicate with each other over a network.
The modem has greatly evolved since the 1970s when the 300 baud modem was used to connect computers to bulletin board systems (BBS). Each bit, digitally represented by 1 or 0, was transmitted as a specific tone in this original version. The receiving modem responded with its own dedicated frequencies so the two could “talk at the same time.” The technical term for this type of modem is asynchronous.
While the 300 baud modem could transmit about 30 characters per second, fast enough for text-based BBSes, people were soon sharing programs and graphics. This required faster modems, and the modem went through many incarnations in quick succession over the next three decades. In 1998 the standard dial-up version reached its transmission limit at 56 kilobits per second. Although many tens of times faster than the 300 baud modem, much greater speeds can be achieved with an ADSL (Asymmetric Digital Subscriber Line) modem.
The asymmetric designation simply means that the modem is faster at downloading (receiving data) than uploading (sending data). The ADSL modem has significant and immediate advantages over the dial-up version. While it uses a standard phone line like its dial-up version, it doesn’t block the line, making it possible to use the phone while browsing the Internet. ADSL service is an “always on” connection, unlike a dial-up modem which cannot be left connected indefinitely. Finally, ADSL is much faster than dial-up.
ADSL can accommodate a telephone conversation due to the bandwidth available on the dedicated copper wire, ranging from households and businesses to the telephone company (Telco). It is possible to parallel a multi-lane highway with several lanes open for additional traffic. The ADSL modem uses the additional lanes (higher frequencies) without interfering with existing voice traffic. ADSL requires ADSL service through an Internet Service Provider (ISP) and does not work with a dial-up account.
Some companies require the same bandwidth upstream and downstream. This means that they have to upload large files or programs to the Internet, as well as download large files. For equal bandwidth in both directions, you would need a Symmetric DSL (SDSL) account and an SDSL modem. SDSL uses voice traffic lanes to expand bandwidth, so a conversation is not possible when connected via SDSL as well. For this reason, most companies also use a dedicated telephone line.
Cable modems require the service of a cable TV provider to provide Internet connectivity. Cable access works on the same principle as DSL, in that digital data is transferred over cables using frequencies which are re-transduced into digital data. The frequencies used for data traffic do not interfere with existing TV traffic.
A less common version is a satellite modem, or satellite modem, which converts digital data into radio waves to communicate with a satellite dish. This not only comes with a built-in latency factor in most cases, but the service is more expensive than more conventional types of Internet connectivity. A satellite modem may be an option, however, for businesses or hobbyists in rural areas that don’t yet have DSL or cable service offerings.
While most DSL and cable providers offer a modem with Internet service, the subscriber can also choose to use their own. Often the Internet provider makes available a list of compatible modems, although these lists are rarely exhaustive. Online manuals and tech specs will usually also list the Internet service providers supported by the modem.
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