What are the different types of modem?
The whole modem industry is reliant on standards - without standards a modem from manufacturer A would not communicate with one from manufacturer B and the number of modems sold would be very small. Manufacturers work together in the ITU (International Telecommunications Union) which is the global standards authority for telecommunications. Some of the ITU Standards for the most common modems are outlined below:
|V.21||300||Frequency Shift Keying (FSK)|
|V.22||1200||Phase Shift Keying (PSK)|
|V.22bis||2400||Quadrature Amplitude Modulation (QAM)|
|V.32||9600||Trellis-Coded Quadrature Amplitude Modulation (QAM)|
|V.32bis||14400||Trellis-Coded Quadrature Amplitude Modulation (QAM)|
|V.34||28800||Trellis-Coded Quadrature Amplitude Modulation (QAM) plus other advanced techniques|
Most of today's modems are compatible with not just the very latest standards but with most of those that have gone before. This ensures that users can make connections to almost any modem that was ever made even if one modem is twenty years old and has to operate at only 300 bit/s.
Modems operate over a wide range of bit rates. Until the mid 1990s most modems operated between 300bps (V.21) and 9,600bps (V.32). Low bit rates were associated with the switched telephone network where some lines were very poor and signal impairments reduced the data rate to 2,400 bps or below. The higher rates of 4,800bps and 9,600bps were generally found on privately leased lines that were owned by telephone companies offering a higher grade of service.
The growth of the Internet provided a mass market for high-speed modems. Improved modulation techniques and better signal-processing technology has resulted in the production of modems of a higher quality. By the mid 90s, low cost modems operated at 14.4K (V.32bis) or 28.8K (V.34). By 1998, modems capable of operating at 56K baud (V.90,V.92) over conventional telephone lines became available for the price of a 1200bps modem only a decade earlier.
V.90, V.92 - 56K Modems
The 56K modem provides a fast connection using your existing telephone line. However, the nature of 56K technology is a little different than existing analog modems. It depends upon a half-digital connection to achieve its higher speeds.
We have learnt that a modem modulates outgoing data from digital to analog and demodulates incoming data from analog to digital. However, above a certain threshold the signal-to-noise ratio of any medium becomes too low to reliably transfer data. The analog phone line limits the speed of data transmission because of the inherent noise it contributes.
Todays telephone network is increasingly digital, in particular the portion of the phone connection between the phone company and the ISP. Digital lines have less noise and a higher ceiling. Because of this, phone companies have created techniques that take advantage of the digital portion of the phone network to achieve higher speeds than were not possible with purely analog lines.
This does mean that to take advantage of 56K technology, you will need to have an ISP that has digital phone lines to the public switched telephone network (PSTN). If the ISP offers 56K, they have the digital lines. Also, for a 56K modem to work, you can only have one analog-to-digital conversion between your home or office and the ISP. If you have more than one, you cannot use 56K and your connection will be limited to V.34 speeds (a maximum of 28.8 or 33.6K). The best way to test this is to borrow a 56K modem and dial into your local ISP from your home or office.
A disadvantage of a 56K connection is that it is asymmetric, which means it is faster for downloading than for uploading. When downloading, data is changing from a digital signal to an analog signal. This type of conversion is perfectly accurate so dowloading at speeds up to 56K is possible. However, when uploading, data is changing from analog to digital. Analog-to-digital conversions are imperfect, so noise is introduced into the signal. This is a minor disadvantage and does not prevent most people from using this technology.
Cable Modems - The Future
A cable modem is a device for providing data over a cable TV network. They are becoming popular for home use as they do not connect to your phone line, but connect to the same coaxial cable that you connect your television to. You connect the cable modem to the TV outlet for your cable TV and the cable TV operator connects a Cable Modem Termination System (CMTS) in his end (the 'head-End'). This allows faster direct access to the Internet via your cable company.
For a good glossary:
To find your modem manufacturer so that you can browse their technical information, find drivers or get other support:
To buy a modem at the most competitive price:
To visit the site of the current most popular modem manufacturer:
References for this web page:
A Brief Introduction To Modem Technology - Cable Modems
Designed by U0113650 April 2003