56k modems are voiceband modems capable of download speeds upto 56 kbit/s (5.6 kbyte/s) when connecting to a service that has ISDN or other digital connection to the phone network. The upload speed is limited to 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. Due to crosstalk considerations, the FCC in the United States and CRTC in Canada have placed a loudness limit on dial-up modems, resulting in a "speed limit" of 53 kbps.

56k modems can only work at 56K when there is only a single pulse-code modulation step in the path between the modem and the digital equipment at the provider. When this is not the case or when two 56k modems are used to communicate with each other they will generally fall back to 33.6 kbit/s V.32bis. The 56 kbit/s transmission exploits the fact that most telephone exchanges are interconnected with digital lines and so can use more efficient transmission techniques over twisted-pair lines.

It is important to realize that all of these speeds are theoretical maximums of the technology, the reality of line conditions means that real life speeds achieved are usually somewhere between 40 kbit/s and 50 kbit/s. While faster communications such as DSL and Cable modems became widely available to urban consumers around the turn of the century, the dialup modem remains common, since high speed rural Internet connections are often scarce or too expensive except for business users.

Initially there were two rival 56k modem systems. One was K56flex from Rockwell and Lucent. The other was X2 from US Robotics. In February 1998 the ITU drafted a 56 kbit/s standard called V.90 which was carefully designed to allow both types of modem to be converted to it by a firmware upgrade. This was formally approved during September 1998.

K56flex

K56flex (a combination of Rockwell's K56Plus and Lucent's V.Flex2 proposals) was a proprietary modem chipset from Rockwell and Lucent that gave users the possibility of receiving data on ordinary phone lines at 56 kbit/s as opposed to the previous maximum of 33.6 kbit/s.

K56flex is a combination of two competitors' efforts at 56K technology. Lucent developed the K56 protocol, while Rockwell developed the 56flex protocol. This occasionally led to incompatibilities between Lucent and Rockwell chipsets as their implementation of K56flex differed.

X2

X2 was a 56 kbit/s modem protocol developed by U.S. Robotics.

The protocol used V.34+ to upload at 33.6 kbit/s, and downloaded under PCM at 56 kbit/s.

X2 was found to consistently have a lower top end speed, yet overall performance was faster than K56flex. The incompatibilities experienced between variant Lucent and Rockwell chipsets in K56flex were also avoided as there was only one X2 standard.

V.90

V.90 is an ITU-T recommendation for a modem, allowing 56 kbit/s download and 33.6 kbit/s upload. It replaced two vendor standards (K56flex and X2) and was designed to allow modems from both prior standards to be flash upgraded to support it. It was developed between March 1998 and February 1999. It is also known as V.Last as it was anticipated to be the last standard for modems operating near the channel capacity of POTS lines to be developed. A follow-on standard, V.92, was developed later in 1999 to replace V.90.

V.92

V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing 56 kbit/s download and 48 kbit/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56k modems only used PCM for downstream data.

V.92 was first presented in August 1999. It was intended to succeed the V.90 standards. Like earlier protocol improvements, V.92 was ineffective unless implemented at both ends of the connection. Unlike those, this enhancement was introduced at a time when, due to the spread of broadband Internet access, dial-up service was declining rather than growing, so Internet service providers were buying few new modems, and uptake was minimal.

Quick Connect

This reduces negotiation times to around 10 seconds instead of over 20 seconds. Quick connect works by training the client modem on the first call; analog and digital characteristics are stored in a local profile and then retrieved for future connections.

"Modem on Hold" (MOH)

This allows the connection to be temporarily severed and then reconnected, reducing the possibility of dropped connections. This is particularly useful for lines that have call waiting.

PCM upstream

Pulse Code Modulation or PCM allows higher rate digital transmissions over the traditional phone lines. PCM Upstream provides a digital connection for upstream data, reducing latency and allowing for a maximum upload speed of about 48 kbit/s. Previously this speed was only achieved in the downstream side in standards such as V.90.

V.44 compression

V.44 compression replaces the existing V.42bis compression standards. It generally allows for between 10 and 120% better compression. In most situations the improvement is around 15%.

See also

• ITU-T V-Series Recommendations
• dial up
• List of device bandwidths

External links

• ITU-T Recommendation V.92: Enhancements to Recommendation V.90
• ITU-T Recommendations: Series V
• V92.com
• Brent Townshend