The transmission protocols of a modem include Modulation Protocol, Error Control Protocol, Data Compression Protocol, and File Transfer Protocol.

1. Error Control Protocol

As the transmission rate of modems continues to increase, noise and abnormal changes in current on telephone lines can cause errors in data transmission. The problem that error control protocols need to solve is how to ensure the accuracy of data during high-speed transmission. There are currently two industry standards for error control protocols: MNP4 and V4.2. Among them, MNP (Microcom Network Protocols) is a transmission protocol developed by Microcom, including MNP1-MNP10. Due to commercial reasons, Microcom has currently only released MNP1-MNP5, among which MNP4 is one of the widely used error control protocols. V4.2 is an improved version of MNP4 developed by the International Telecommunication Union, which includes two control algorithms: MNP4 and LAP-M. Therefore, a modem using V4.2 protocol can establish an error free control connection with a modem that only supports MNP4 protocol, while the opposite is not possible. So when purchasing a modem, it is best to choose a modem that supports V4.2 protocol.
In addition, some cheap modem cards on the market do not have hard error correction function to reduce costs, but instead use software error correction methods. Everyone should pay attention to distinguishing when purchasing, and not be confused by words such as "with error correction function" on the packaging box.

2. Data compression protocol.

In order to increase data transmission volume and shorten transmission time, most modems currently compress data before transmission. Similar to error control protocols, there are also two industry standards for data compression protocols: MNP5 and V4.2bis. MNP5 adopts two compression algorithms, Rnu Length encoding and Huffman encoding, with a maximum compression ratio of 2:1. And V4.2bis adopts Lempel Ziv compression technology, with a maximum compression ratio of 4:1. That's why V4.2bis is faster than MNP5. It should be noted that the data compression protocol is based on the error control protocol, and MNP5 requires the support of MNP4, while V4.2bis also requires the support of V4.2. And although V4.2 includes MNP4, V4.2bis does not include MNP5.

3. File Transfer Protocol

File transfer is the main form of data exchange. In order to correctly identify and transfer files during file transfer, we need to establish a unified transfer protocol between two computers. This protocol includes content such as file identification, start and end times of transmission, error detection and correction. There are several common transmission protocols, including:

ASCII: This is the fastest transmission protocol, but it can only transmit text files.
Xmodem: This ancient transmission protocol has a slower speed, but due to the use of CRC error detection method, the transmission accuracy can reach up to 99.6%.
Ymodem: This is an improved version of Xmodem, which uses 1024 bit segment transmission and is faster than Xmodem.
Zmodem: Zmodem adopts streaming transmission method, with fast transmission speed, and also has functions such as automatic changing of segment size and breakpoint continuation, fast error detection, etc. This is currently the most popular file transfer protocol.