What protocol does modem use?

The earliest modem can be traced back to the one launched by at&t in 1958. Data phone number , Used to connect a terminal device to a remote host. . The technical standard of this modem is called bell103, which uses Simple Frequency Modulation Technology ( Frequency Shift Keying Only BPS of transmission speed is provided. CCITT issued a similar technical standard V.21 according to bell103.
Since then, the development of modulation and demodulation technology has slowed down, and the second influential modem standard was not found until 1970s: at&t's bell212. Use bell212 4-dpsk technology combined with Amplitude Modulation and adjustment To achieve the transfer speed of ipvbps. A similar standard of CCITT is V.22.
Currently, bell103 (V.21) and bell212 (V.22) are rarely used, but to be compatible with earlier modem versions, Many modem systems still integrate these two technologies into the product as an option.
In the middle of 1980s, With the popularization of microcomputer and the launch of modem chip, the modulation and demodulation technology entered a period of rapid development. First, CCITT V.22 bis (bis stands for "second" in French) is adopted by most modem manufacturers. 16-qam (12 phase angles and orthogonal modulation of four amplitude modulation phases ) Modulation technology achieves the transmission speed of bps. Next, CCITT adopted 32-tcm (Grille encoding Modulation ) Technology to achieve the BPS speed V.32 standard. Although some manufacturers, such as Hayes, telebit, and Motorola, launched a 9600 bps modem product based on their own patented technology for the purpose of Technical Monopoly, however, the V.32 standards were eventually unified. Since then, CCITT has strengthened the research on modem standards and promulgated v.32bis standards in 1991. V.32bis uses 128-tcm modulation technology to achieve a transmission speed of up to 14400bps, and can return to four speed ranges, including 12000 BPS, 9600bps, BPS, and 4800bps, based on line quality changes. In 1993, CCITT introduced the v. Fast trial standard, which was subsequently revised to the formal standard. V.34 can achieve the highest speed of 28800bps, which is also the highest line speed that modem can achieve so far. This standard can also be used for speed reduction of 28.8 K/26.4 K/21.6 K/19.2 K/16.8 K/12 k/14.4/9600/7200/4800bps.
In addition to the above mainstream modulation and demodulation standards, there are also some protocol standards applicable to American 4-wire phones and specialized fax machines, such as v.26bis, v.27bis, V.29, V.33, etc., which are not described here. The main modulation and demodulation standards and their technical features are collected in table 1.
Iii. protocol standards for compression and Error Correction
In order to further improve the data transmission speed of modem, in addition to the above improvements to the modulation and demodulation technology, data compression technology has also been introduced in recent years. The error correction technology is introduced with the adoption of the compression technology. . After compression and decompression are added to the modem, it becomes sensitive to the line transmission error code. A small transmission error in the line may cause a long string of data errors during decompression.
Microcom is a pioneer in the use of error correction and compression technologies.Microcom error correction and compression protocols are often abbreviated as MNP (microcom network protocol), which consists of a series of independent error correction and compression protocols. Mnp1 ~ Mnp4 and mnp10 are error correction Protocols, while mnp5 and mnp7 are compression protocols. Most modem manufacturers purchase MNP patented technology licenses and implement them in their modem products. Therefore, MNP has become an industrial standard for compression and Error Correction Technology.
Mnp1 and mnp2 adopt character-oriented error correction methods. due to their poor error correction capability and low transmission efficiency, mnp1 and mnp2 are rarely used. Mnp3 is a full-duplex Error Correction protocol for bits. It converts the asynchronous format data sent by modem to the synchronous format frame, and adds the CRC check error code, then, the synchronous frame error checking and re-sending error correction methods are used for processing. While mnp4 adds the adaptive frame length feature on the basis of mnp3, which further improves the transmission efficiency, and thus becomes a widely used modem Error Correction protocol. Mnp10 is a protocol with stronger Error Correction capabilities. It applies to noise environments like cellular telephone networks, but generally modem does not support this protocol.
In 1988, CCITT began to intervene in modem error correction and issued v.42 Error Correction standards. V.42 uses the error correction technology known as the LAP-M (Link Access Procedure for modem), in fact is the X.25 network link protocol LAP-B into modem, to achieve similar error correction. V.42 also uses mnp4 as an option. If one of the two modem supports v.42 and the other supports mnp4, the two can automatically perform troubleshooting of mnp4 through negotiation.
Mnp5 and v.42bis are the most popular data compression protocols. Mnp5 uses two types of Compression Algorithm , One is the Huffman code ), The other is Run-length code ). For common ASCII plain text files, mnp5 can obtain a 2: 1 compression ratio, that is, after compression, the amount of data transmitted is only half of the original data. Mnp7 has taken another step on the basis of mnp5, The increase in the occurrence frequency of "character pairs" is carried out with the compression ratio increased.
V.42bis, promulgated by CCITT in 1989, is a more effective compression protocol. V.42bis used Adaptive Dictionary algorithm Lempel-Ziv , For a typical ASCII text can reach a compression ratio, and the real-time performance is better than that of mnp5 .
If the transmitted file has been compressed (such as Arc, LHA, and PKZIP), it is best not to use the MNP compression protocol in the modem. In this case, the MNP compression of modem does not produce the actual compression effect, but takes a certain amount of processing time. However, v.42bis has an automatic test function and can be automatically switched between the compression mode and the transparent mode (without compression processing) through online tests. Therefore, v.42bis has better adaptability than mnp5. If possible, we recommend that you use the v.42bis protocol as much as possible.
It is worth noting that the compression technology is closely related to the error correction technology. If v.42bis compression protocol is selected, the modem will automatically enable v.42 Error Correction protocol; If mnp5 compression is used, MNP is used for error correction. Table 2 lists the main features of the preceding compression and Error Correction protocols.