Transmission modes of MODEM
Modem was initially used only for data transmission. However, with the growing demand of users and the fierce competition among manufacturers, there are more and more "2-in-1" and "3-in-1" Modems on the market, which can not only transmit data, but also have the functions of fax and voice transmission.
1, fax mode (Fax Modem)
Fax through the Modem, in addition to saving the cost of a dedicated fax, there are many benefits: you can directly fax the computer documents to each other's computers or fax machines, without having to print out the document; you can easily save or edit the received faxes; you can overcome the common fax machine The problem of gradual fading of handwriting due to the use of thermal paper can be overcome. Due to the error correction technology used by Modem, the fax quality is better than that of ordinary fax machines, especially for faxing of graphics.
Most current Fax Modems follow the V.29 and V.17 fax protocols. Where V.29 supports 9600bps fax rate and V.17 can support 14400bps fax rate.
2, voice mode (Voice Modem)
The voice mode mainly provides the telephone recording message and full duplex (Full Duplex) hands-free call function, really make the telephone and computer into one. Here, a new voice transmission mode, DSVD (Digital Simultaneous Voice and Data), is introduced.
DSVD is a voice transmission standard proposed by Hayes, Rockwell, U.s. Robotics, Intel, and other companies in 1995, and is an expansion of the existing V.42 error correction protocol. DSVD uses Digi Talk's Digital Simultaneous Voice and Data technology to enable Modems to make calls while transmitting data over an ordinary phone line. DSVD Modems reserve 8K of bandwidth (some Modems reserve 8.5K of bandwidth) for voice transmission, while the rest of the bandwidth is used for data transmission. Voice is compressed before transmission and then combined with the data to be transmitted and sent over the telephone carrier to the other user. At the receiving end, the Modem separates the voice from the data, then decompresses the voice signal and converts it to digital/analog, thus realizing the simultaneous transmission of data/voice.DSVD Modem has a wide range of applications in distance learning, collaborative work, and online games. However, at present, the price of DSVD Modem is more expensive than ordinary Voice Modem, and in order to realize the function of data/voice simultaneous transmission, it is necessary for the other party to use DSVD Modem as well, thus hindering the popularity of DSVD Modem to a certain extent.
Transmission Protocols of MODEM
The transmission protocols of Modem include Modulation Protocols, Error Control Protocols, Data Compression Protocols, and File Transfer Protocols. protocols.
1, modulation protocol
The information in the computer is composed of "0" and "1" digital signals, while the telephone line can only be passed on the analog electrical signals. So, when two computers want to transmit data over the phone line, a device is needed to be responsible for the conversion of digital to analog. This digital-to-analog converter is Modem. computer in sending data, the first Modem to the digital signal is converted to the corresponding analog signal, this process is called "modulation". After modulation of the signal through the telephone carrier transmission to another computer before, but also by the receiver of the Modem is responsible for the analog signal is reduced to a digital signal that the computer can identify, this process is called "demodulation". It is through such a "modulation" and "demodulation" of the digital-to-analog conversion process, thus realizing the remote communication between the two computers.
2. Error Control Protocol
As the Modem's transmission rate continues to increase, noise on the phone line, abnormal changes in current, and so on, can cause errors in data transmission. Error control protocols address how to ensure data accuracy in high-speed transmission. Currently there are two industry standards for error control protocols: MNP4 and V4.2. MNP (Microcom Network Protocols) is a transmission protocol developed by Microcom, which includes MNP1-MNP10. Due to commercial reasons, Microcom currently only publishes MNP1-MNP5, of which MNP1-MNP5 are the most widely used. For commercial reasons, Microcom has only published MNP1-MNP5, of which MNP4 is one of the widely used error control protocols. V4.2, on the other hand, is an improved version of MNP4 developed by the International Telecommunication Union, which contains both MNP4 and LAP-M control algorithms. Therefore, a Modem using the V4.2 protocol can establish an error-free control connection with a Modem that only supports the MNP4 protocol, and vice versa. Therefore, when purchasing a Modem, it is better to choose the Modem that supports V4.2 protocol; in addition, some cheap Modem cards on the market do not have the hard error correction function to reduce the cost, but use the software error correction method. We should be careful to distinguish between the two when purchasing, and not be confused by the words "with error correction" on the box.
3, data compression protocol
In order to increase the amount of data transmission, shorten the transmission time, most of the current Modem in the transmission of data will be compressed first. Similar to error control protocols, there are two industry standards for data compression protocols: MNP5 and V4.2bis. MNP5 uses two compression algorithms, Rnu-Length coding and Huffman coding, and has a maximum compression ratio of 2:1, while V4.2bis uses Lempel-Ziv compression and has a maximum compression ratio of up to 4:1. V4.2bis is faster than MNP5. It is important to note that the data compression protocol is based on the error control protocol, MNP5 needs the support of MNP4, and V4.2bis needs the support of V4.2. And, although V4.2 includes MNP4, V4.2bis does not include MNP5.
4. File Transfer Protocol
File transfer is the main form of data exchange. In file transfer, in order to make the file can be recognized and transmitted correctly, we need to establish a unified transmission protocol between two computers. This protocol includes the identification of the file, the start and end of the transmission time, error determination and correction, and so on. There are several common transfer protocols:
A) ASCII. This is the fastest transfer protocol, but it can only transfer text files.
B)Xmodem. This ancient transfer protocol is slower, but due to the use of CRC error detection methods, the accuracy of the transmission can be as high as 99.6%.
C)Ymodem. This is an improved version of Xmodem, which uses 1024-bit zones for transmission and is faster than Xmodem.
D) Zmodem: Zmodem uses streaming transmission method, which is faster and also has features such as automatic change of zone size and breakpoints, fast error detection, etc. It is currently the most popular file transfer protocol. This is currently the most popular file transfer protocol.