Some Modem Standards

Although there have been many standards in modem technology over the years, here I will concentrate on a few, namely V.22, V.32, V.32bis, V34/+ and V.90. By its very nature, this area of computing is full of jargon, so although I will try and explain some of these terms in the glossary, be warned before continuing.


V.22

V.22 allows full duplex communications at 600 baud in each direction. It does this by splitting the available bandwidth into two seperate regions (using FDM), one for upstream communication and the other for downstream communication. Although the standard local loop has a frequency passband of about 3000Hz, only 2400Hz (1200 * 2) of this is used to allow for a safety margin on either side. Note: The send and receive bands are both 1200Hz wide (and could theoretically send 1200 baud), but modulation doubles the bandwidth requirements of the signal, so only 600 baud gets sent.

V.32

V.32 uses echo-cancellation to allow the modem to use the entire passband (except for the edges) for communication in both directions. This uses QAM to allow each signal element to hold one of sixteen values – i.e. 4 bits per signal element and thus allows 9600bps full duplex.

V.32bis

This is similar to V.32, except that it uses 6 bits per signal element instead of 4 to allow for a speed of 14,400bps full duplex

V.34 and V.34+

These are adaptive protocols and use line probing to adapt their speed to the quality of the telephone line that is currently being used. There are a variety of techniques to do this including changing the number of bits per signal element. On average, V.34+ can get 9.8 bits per signal element for a speed of 33,600bps, while V.34 can get a speed of 28,800bps.

V.90

This evolved from two opposing standards (X.2 from US Robotics, and K56flex from Rockwell) and was designed for use over the Internet, as opposed to the previous standards, which are simply designed to connect any two computers over the telephone system.
V.90 assumes that the remote host is digitally connected to the telephone network and transmits the digital signal directly, rather than converting it into an analogue signal first. However, this only applies downstream, where you can (theoretically) get a data rate of 56Kbps. However, for upload, you still have to go through the analogue-digital-analogue-digital process, so you can only get an upload speed of 33,600bps.


This has been a very brief, whistle-stop tour across the world of modem standards. If you want to find out more, there are many sources of information on the web. Some of these can be found below

Links

Link Hardware Central
Link ITU standards

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