56 kbit/s modem

56k modems are voiceband modems nominally 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, their use declining as broadband technologies such as DSL gain wider availability.

peed

The effective throughput is 6.65 kilo"bytes" per second, since approximately 5% of the total throughput is used for V.42 framing (a variant of HDLC framing). However, 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 the system being dialed into has a digital connection to the telephone system, such as DS0 service. By the time 56k modems came out, most of the telephone system beyond the local loop was already digital, so it was sensible to design a protocol to take 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.

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 dialup modems, resulting in a maximum of 53.3 kbit/s. However, 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, the dialup modem remains common, since high speed rural Internet connections are often scarce.

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.

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. Fact|date=January 2008 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 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

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 low. Some providers such as Netzero offer V.92 lines, but with features like "Modem on Hold" turned off.

;Quick ConnectThis 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%.

See also

* ITU-T V-Series Recommendations
* Dial Up
* List of device bandwidths

References

Notes

External links

* [http://www.itu.int/rec/T-REC-V.92/en ITU-T Recommendation V.92: Enhancements to Recommendation V.90]
* [http://www.itu.int/rec/T-REC-V/en ITU-T Recommendations: Series V]
* [http://www.v92.com/ V92.com]
* [http://web.mit.edu/invent/iow/townshend.html Brent Townshend]