56 Kbit/s Modem - Speed

Speed

The 56 kbit/s theoretical speed is only possible when the system being dialled into has a digital connection to the telephone system, such as DS0 service. By the time 56k modems came into use, most of the telephone system beyond the local loop was already digital, so the new 56k protocols took advantage of this.

If both calling party and called party have an analog connection, the voiceband signal will be converted from analog to digital and then back to analog. Each conversion adds noise, and there will be too much noise from the second conversion for 56k to work. The modem's negotiation processes will fall back to the less demanding 33.6 kbit/s mode. Other local loop conditions, such as certain antiquated pair gain systems, may have similar results.

In 8-N-1 connections (1 start bit, 8 data bits, No parity bit, 1 stop bit), which were typical before LAPM became widespread, the actual throughput is a maximum of 5.6 kilobytes per second, since ten bits are transmitted for every 8-bit byte, although effective throughput can be increased as high as 32 kbytes/s using internal compression, or 100 kbytes/s using ISP-side compression.

Effective throughput will vary, depending on framing protocols, noise, data compression, and other factors. See V.42 and Comparison of synchronous and asynchronous signalling article for more detail.

The upload speed is 33.6 kbit/s if an analog voiceband modem is used (V.90), or 48.0 kbit/s using a digital modem (V.92). Due to the design of public telecommunications networks, higher speed dialup modems are unlikely to ever appear. Also, depending on the quality of the line conditions, the user may not be able to reach this maximum speed. While faster communications such as DSL and cable modems became widely available to urban consumers in the early 2000s in the United States, dial-up Internet access remains common, since high speed rural Internet connections are often scarce and because people may still use it to send faxes.