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ADSL (asymmetric digital subscriber line) modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Early proprietary ADSL modems used carrierless amplitude phase (CAP) modulation. All standardized asymmetric DSL variants, including ANSI T1.413 Issue 2, G.dmt, ADSL2, ADSL2+, VDSL2, and G.fast, use discrete multi-tone (DMT) modulation, also called (coded) orthogonal frequency-division multiplexing (OFDM or COFDM).
Standard twisted-pair telephone cable can, for short distances, carry signals with much higher frequencies than the cable's maximum frequency rating. ADSL broadband takes advantage of this capability. However, ADSL's performance gradually declines as the telephone cable's length increases. This limits ADSL broadband service to subscribers within a relatively short distance from the telephone exchange.
Cable modems use a range of radio frequencies originally intended to carry television signals. A single cable can carry radio and television signals at the same time as broadband internet service without interference. Multiple cable modems attached to a single cable can use the same frequency band by employing a low-level media access protocol to avoid conflicts. In the prevalent DOCSIS system, frequency-division duplexing (FDD) separates uplink and downlink signals. For a single-cable distribution system, the return signals from customers require bidirectional amplifiers or reverse path amplifiers that send specific customer frequency bands upstream to the cable plant amongst the downstream frequency bands.
Newer types of broadband modems are available, including satellite and power line modems.
Most consumers did not know about networking and routers when broadband became available. However, many people knew that a modem connected a computer to the Internet over a telephone line. To take advantage of consumers' familiarity with modems, companies called these devices broadband modems rather than using less familiar terms such as adapter, interface, transceiver, or bridge. In fact, broadband modems fit the definition of modem because they use complex waveforms to carry digital data. They use more advanced technology than dial-up modems: typically they can modulate and demodulate hundreds of channels simultaneously or use much wider channels than dial-up modems.
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