56k modem
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56k modems are voiceband modems capable of download speeds up to 56 kbit/s (56,000 bits per second). At the beginning of the 21st Century, most personal computers contained one, though their use is declining as broadband technologies such as DSL gain wider availability.
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[edit] Speed
The effective throughput is 6.65 kilobytes per second, since aproximately 5% of the throughput is used for V.42 framing (a variant of HDLC framing). This framing protocol is much more efficient than asyncronous serial framing which wastes 25% of the bits for start bit and stop bit, and works for any kind of data including compressed data. Using on-the-fly V.42 compression, these modems can achieve higher rates of 32 kilobytes per second. (See the Comparison of synchronous and asynchronous signalling article for more detail.) Note that when the modem is connected through an asyncronous serial link to the host computer, the host serial link speed needs to be set at least 25% higher than modem speed in order to support the modem throughput (typically the host speed is set to 115,200 bit/s or higher), which the operating system normally does automatically.
The 56 kbit/s speed is only possible when connecting to a service that has DS0 or other digital connection to the phone network, and just one digital-to-analog connection (to the home). If two analog connections exist, then the 56 kbit/s digital modem can not be used, and the built-in 33.6 kbit/s analog modem will be used instead. 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.
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. Due to crosstalk considerations, the Federal Communications Commission (FCC) in the United States and Canadian Radio-television and Telecommunications Commission (CRTC) in Canada have placed a power output restriction on dial-up modems, resulting in a "speed limit" of 53.3 kbit/s.
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.
[edit] History
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 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.
[edit] 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.
[edit] 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.[citation needed] The incompatibilities experienced between variant Lucent and Rockwell chipsets in K56flex were also avoided as there was only one X2 standard.
[edit] V.90
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.
[edit] V.92
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 option 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 https://www.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 minimal. 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.
- 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%.
[edit] See also
[edit] References
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
[edit] Notes
[edit] External links
| Telephone network modem standards |  |
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ITU V-Series | V.92 | K56flex | X2 | MNP | Hayes command set |
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