56 kbit/s modem
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A 56k modem is a voiceband modem nominally capable of download speeds up to 56 kbit/s (56,000 bits per second). In the late 1990s, they were the most popular access method for personal Internet usage, but their use has declined in the developed world as broadband Internet technologies such as DSL and cable Internet access, along with mobile data services such as 4G, have been more widely adopted and made available. In other parts of the world, ad hoc wireless networks are being more widely deployed.
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.
Initially there were two rival 56k modem systems. One was K56flex from Rockwell and Lucent. The other was X2 from USRobotics. In February 1998 the International Telecommunication Union (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 (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 was a 56 kbit/s modem protocol developed by USRobotics. 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 is an ITU-T recommendation for a modem, allowing 56 kbit/s PCM download and 33.6 kbit/s analog-modulated 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 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing 48 kbit/s PCM upload, but at the expense of download rates. For example a 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s, due to echo on the telephone line. To avoid this problem, V.92 modems offer the +PIG=1 command to turn off the digital upstream and instead use a 33.6 kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or higher. (See November and October 2000 update at http://modemsite.com/56k/v92s.asp )
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 low. Some providers such as Netzero offer V.92 lines, but with features like "Modem on Hold" turned off.
- 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 so the user can take voice calls.
- V.44 compression
- V.44 compression replaces the existing V.42bis compression standards. It generally allows for between 10% and 50% better compression. In most situations the improvement is around 15%.
See also 
- ITU-T Recommendation V.92: Enhancements to Recommendation V.90
- ITU-T Recommendations: Series V
- Brent Townshend