Terminology » Data Communication » DSL Modem vs. Cable Modem: Key Differences

DSL Modem vs. Cable Modem: Key Differences

dsl modem cable modem data rate modulation frequency spectrum

This page compares DSL modems and cable modems, highlighting their key differences. A DSL modem utilizes Digital Subscriber Line (DSL) interface technology.

It offers higher data rates compared to a standard telephone system. These modems are typically external to your PC and connect via Ethernet or USB.

DSL modems operate on frequencies from 25kHz to 1MHz, deliberately outside the voice frequency band to prevent interference. One variant, the ADSL modem, can achieve downstream data rates of up to 8Mbps, with upstream rates around 640kbps. Both QAM and DMT modulation techniques are used.

Synchronous DSL modems provide equal bandwidth for both upstream and downstream traffic.

Cable modems also deliver high data rates, exceeding those of standard telephone systems. However, they require an existing cable TV infrastructure to function. Cable modems commonly use 64QAM for downstream communication, achieving data rates of approximately 30Mbps, and QPSK modulation for upstream, resulting in rates around 10Mbps. The total bandwidth supported by a cable modem system is approximately 750MHz, divided into channels, each with a bandwidth of 6MHz.

The frequency spectrum in a cable system is typically divided as follows:

  • 7 channels are allocated in the 5 to 50MHz band for upstream communication.
  • 83 TV channels are allocated in the 50MHz to 550MHz band.
  • 33 channels are allocated in the 550 to 750MHz band.