Over the years, modem standards have tended to develop in a rather haphazard way. As well as defining the speed at which a modem may operate they determine how, exactly, a modem compresses data and performs its error control. The CCITT (Comite Consultatif International Telegraphique et Telephonique) and the ITU (International Telecommunications Union) ratify the V dot standards that are most often talked about.
V.22bis, V.32 and V.32bis were early standards specifying speeds of 2.4 Kbit/s, 9.6 Kbit/s and 14.4 Kbit/s respectively.
The V.34 standard was introduced towards the end of 1994, supporting 28.8 Kbit/s, and is now considered the minimum acceptable standard. V.34 modems are able to drop their speed to communicate with slower modems and interrogate the line, adjusting their speed up or down according to the prevailing line conditions.
In 1996 the V.34 standard was upgraded to V.34+, which allows for data transfer speeds of up to 33.6 Kbit/s, is backwards compatible with all previous standards, and adapts to line conditions to eke out the greatest usable amount of bandwidth.
The table below shows uncompressed data throughput rates for the various modem types. Data compression can increase throughput by a factor of 2 or 3. However, because graphic images on web pages are already compressed, the real multiplier for web browsing generally works out to around 1.5 to 2x the listed rates. Two figures are shown for V.90 modems because of the wide variation in connect speeds.
Other important V dot standards include V.17 which allows connection to Group III fax machines, which are ordinary standalone fax machines, V.42 which is a worldwide error correction standard designed to cope with garbled data caused by interference on phone lines, and V.42bis which is a data compression protocol. In late-1999 Hughes Network Systems proposed a new link-layer compression standard as a potential replacement to V42bis. The algorithm was subsequently reviewed by the American and International communication standards bodies, and adopted as a new compression standard called V.44. The new standard offers a higher compression ratio than V.42bis, resulting in data throughput improvements, typically in the 20% to 60% range.
The MNP (Microm Networking Protocol) standards go from MNP Class 1 to MNP Class 10. They do not stand alone, but operate in conjunction with other modem standards. MNP 1 is half-duplex. MNP Classes 2 to 4 deal with error control and can transmit data error-free by resending blocks of data that become corrupted in transmission. MNP Classes 5 to 10 address various modem operating parameters. MPN Class 5 is an advanced data compression protocol which can compress data by a factor of two, effectively doubling the speed of data transfer. MNP Class 10 is Microcom’s proprietary error-control protocol. It provides a set of adverse channel enhancements which help modems cope with bad phone connections by making multiple attempts to make a connection, and adjust both the size of the data packets and the speed of the transfer according to the condition of the line. The most common MNP protocols are numbers 2 to 5, with 10 also often included.
LAPM (Link Access Protocol for Modems), one of the two protocols specified by V.42 used for detection and correction of errors on a communications link between two modems, has largely superseded MNP. V.42bis is an algorithm used by modems to compress data by a theoretical ratio of 8:1. In the real world, however, a ratio of 2.5:1 is typical. MNP 4 error correction and MNP 5 compression are used as fallbacks if a remote modem doesn’t support LAPM or V.42bis.
The Hayes AT Command Set was developed by Hayes, the modem manufacturer, and is now a universal standard. Each command line must start with the two-character attention code AT (or at). The command set is simply a series of instructions for automatically dialling numbers, controlling the telephone connection and telling the computer what it is doing.
FTPs (file transfer protocols) were developed to help prevent errors when transferring files before standards were introduced. Zmodem is still widely used for file transfer over the serial port. If the received data doesn’t match the information used to check the quality of data, the system notifies the sender that an error has occurred and asks for a retransmission. This is the protocol used to download a file to a computer from another computer on the Internet.
BABT (British Approvals Boards of Telecommunications) is an important standard, since modems that are not BABT approved are not legal for use in Britain.