The working principle of modem

The Modem we often talk about is actually the abbreviation of modulator and demodulator, which is called modem in Chinese. Some people affectionately call it "cat" according to the homonym of Modem. As we know, the information in the computer is a digital signal composed of "0" and "1", but only analog electrical signals can be transmitted on the telephone line. Therefore, when two computers want to transmit data through telephone lines, a device is needed to convert the numbers into analog. This digital-to-analog converter is the modem we are going to discuss here. When the computer sends data, the modem first converts the digital signal into the corresponding analog signal, which is called "modulation". Before the modulation signal is transmitted to another computer by telephone carrier, the modem of the receiver is also responsible for restoring the analog signal to a digital signal that can be recognized by the computer. This process is called "demodulation". It is through such a digital-to-analog conversion process of "modulation" and "demodulation" that the remote communication between two computers is realized.

I. Type of modem

Generally speaking, according to the form and installation mode of modems, they can be roughly divided into the following four categories:

1, external modem

The external modem is located outside the chassis and connected to the host through the serial communication port. This modem is convenient, dexterous and easy to install, and the flashing indicator light is convenient to monitor the working state of the modem. However, external modems need to use additional power supplies and cables.

2. The built-in modem

When installing the built-in modem, the chassis needs to be disassembled, and the interrupt and COM port need to be set, so the installation is complicated. This modem takes up the expansion slot on the motherboard, but it does not need additional power and cables, and its price is cheaper than that of an external modem.

3.PCMCIA plug-in modem

Plug-in modems are mainly used in notebook computers, and their size is relatively thin. With a mobile phone, you can easily realize mobile office.

4. Rack modem

Rack modem is equivalent to concentrating a group of modems in a box or shell, which is powered by a unified power supply. Rack modems are mainly used in the central computer rooms of Internet/Intranet, telecommunication bureau, campus network, financial institutions and other networks.

In addition to the above four common modems, there are ISDN modems and a modem called Cable Modem, and there is also an ADSL modem. Cable Modem uses CATV cable to transmit signals, which not only has the function of modulation and demodulation, but also integrates routers, hubs and bridges. The theoretical transmission speed can reach more than 10Mbps. Through the cable modem, each user has an independent IP address, which is equivalent to having a private line. At present, Shenzhen Cable TV Tianwei Network Company has launched this kind of Internet access service based on cable TV network, with an access rate of 2Mbps- 10Mbps!

2. Modem transmission mode

Modems were originally used only for data transmission. However, with the increasing demand of users and fierce competition among manufacturers, there are more and more "two-in-one" and "three-in-one" modems in the market. These modems can not only transmit data, but also have fax and voice transmission functions.

1, fax mode (fax modem)

In addition to saving the cost of a special fax machine, faxing through a modem has many advantages: you can fax files from your computer directly to the other computer or fax machine without printing them first; You can save or edit the received fax conveniently; It can overcome the problem that the handwriting gradually fades due to the use of thermal paper in ordinary fax machines; Because the Modem adopts error correction technology, the fax quality is better than that of ordinary fax machines, especially graphic fax machines. At present, fax modem mostly follows V.29 and V. 17 fax protocols. Among them, V.29 supports the fax rate of 9600bps, and V. 17 supports the fax rate of 14400bps.

2. Voice mode (voice modem)

Voice mode mainly provides telephone message recording and full-duplex hands-free call, which truly realizes the integration of telephone and computer. Here, we mainly discuss a new voice transmission mode-DSVD (digital simulated voice and data). DSVD is a voice transmission standard proposed by Hayes, Rockwell, U.s.Robotics, Intel and other companies in 1995, which is an extension of the existing V.42 error correction protocol. DSVD adopts Digi Talk's digital voice and data synchronous transmission technology, so that the modem can make phone calls while transmitting data on ordinary telephone lines.

DSVD modems reserve 8K bandwidth (some modems reserve 8.5K bandwidth) for voice transmission and the rest for data transmission. Voice will be compressed before transmission, and then combined with the data to be transmitted, and transmitted to the other user through telephone carrier. At the receiving end, the modem first separates the voice from the data, and then decompresses the voice signal and converts it into digital/analog, thus realizing the simultaneous transmission of data/voice. DSVD modem has a wide application prospect in distance education, collaborative work and online games. But at present, DSVD Modem is more expensive than ordinary voice Modem, and the other party needs to use DSVD Modem to realize the function of data/voice transmission at the same time, which hinders the popularization of DSVD Modem to some extent.

3. The transmission rate of the modem

The transmission rate of modem refers to the amount of data transmitted by modem per second. We usually say 14.4K, 28.8K, 33.6K, 56K, etc. A that refers to the transmission rate of the modem. The transmission rate is in bps. Therefore, the 33.6K modem can transmit 33,600 bits of data per second. Because the current modem compresses data during transmission, the data throughput of 33.6K modem can theoretically reach 1 15200bps, or even 230400bps.

The transmission rate of modem is actually determined by the modulation protocol supported by modem. We usually see V.32, V.32bis, V.34, V.34+, V.fc and so on. In the package or specification of a modem, it refers to the modulation protocol adopted by the modem. Among them, V.32 is an asynchronous/synchronous full-duplex standard protocol with 4800/9600bps; V.32bis is an enhanced version of v.32, which supports the transmission rate of 14400bps. 34 is the standard protocol for synchronizing 28800bps full duplex; And V.34+ is a synchronous full-duplex 33600bps standard protocol. All the above standards are formulated by ITU (International Telecommunication Union), while V.fc is a 28800bps modulation protocol proposed by Rockwell, but it has not been widely supported.

When it comes to the transmission rate of Modem, we can't help but mention the 56K Modem that is currently being heated up. In fact, the standard of 56K has been put forward for many years, but because there have been two incompatible standards, K56flex led by Rockwell and X2 led by U.S.Robotics, the popularization of 56K Modem has been delayed. Fortunately, in February this year, with the efforts of the International Telecommunication Union, the standard of 56K was finally unified to ITU V9.0, and many modem manufacturers also introduced upgrade measures, and modems that really supported V9.0 blossomed everywhere. 56K is expected to become the mainstream of the market in one to two years. By the way, because many domestic ISPs don't provide 56K access service at present, you'd better consult your service provider before buying a 56K modem to avoid wasting.

The transmission rates mentioned above are all obtained under ideal conditions. But in practice, the speed of Modem often can't reach the nominal value. The actual transmission rate mainly depends on the following factors:

1, telephone line quality

Because the modulation signal is transmitted through the telephone line, if the quality of the telephone line is not good, the modem will slow down to ensure accuracy. To this end, when we connect modems, we should try to reduce the length of the connection, cut off redundant connections, and don't pile up in a circle. In addition, it is best not to use an extension, and the connection should avoid passing through interference sources such as TV sets.

2. Is the bandwidth enough?

If there are many people surfing the Internet at the same time, the lines will be crowded and blocked, and the transmission rate of Modem will naturally decrease. Therefore, it is important for ISP to provide enough bandwidth. In addition, avoiding surfing the internet during busy hours is also a solution. Especially when downloading files, the time spent downloading during busy and non-busy hours will be several times different.

3, the other end of the modem rate

The modulation protocols supported by Modem are backward compatible, and the actual connection rate depends on the lower rate party. Therefore, if the other party's modem is 14.4K, even if you use a 56K modem, you can only connect at the rate of14,400bps.

4. The transmission protocol of the modem

The transmission protocols of modem include modulation protocol, error control protocol, data compression protocol and file transfer protocol. We have discussed modulation protocols before, and now we will focus on the other three transmission protocols.

1, error control protocol

With the continuous improvement of modem transmission rate, the noise on the telephone line and the abnormal mutation of current will cause data transmission errors. The problem of error control protocol is how to ensure the accuracy of data in high-speed transmission. At present, there are two industrial standards for error control protocols: MNP4 and V4.2 Among them, MNP(Microcom Network Protocol) is a transmission protocol formulated by Microcom Company, including MNP1-MNP10. For commercial reasons, Microcom only published MNP 1-MNP5, among which MNP4 is one of the widely used error control protocols. V4.2 is an improved version of MNP4 formulated by ITU, which includes two control algorithms, MNP4 and LAP-M. Therefore, a modem using V4.2 protocol can establish error-free control connection with a modem that only supports MNP4 protocol, and vice versa. So when we buy a modem, we'd better choose a modem that supports V4.2 protocol.

In addition, in order to reduce the cost, some cheap Modem cards on the market do not have hard error correction function, but adopt software error correction method. Everyone should pay attention to discrimination when buying, and don't be confused by the words "with error correction function" on the box.

2. Data compression protocol.

In order to increase the data transmission capacity and shorten the transmission time, most modems will compress the data first when transmitting. Similar to the error control protocol, the data compression protocol has two industrial standards: MNP5 and V4.2bis MNP5 adopt two compression algorithms, Rnu-Length coding and Huffman coding, and the maximum compression ratio is 2: 1. V4.2bis adopts Lempel-Ziff compression technology, and the maximum compression ratio can reach 4: 1. This is why 4.2bis is faster than MNP5. It should be noted that the data compression protocol is based on the error control protocol, MNP5 needs the support of MNP4, and V4.2bis also needs the support of V4.2. Moreover, although V4.2 contains MNP4, V4.2bis does not contain MNP5.

3. File Transfer Protocol

File transfer is the main form of data exchange. When transmitting files, we need to establish a unified transmission protocol between two computers in order to correctly identify and transmit files. This protocol includes file identification, start and end time of transmission, error judgment and correction, etc. Common transport protocols are as follows:

ASCII: This is the fastest transmission protocol, but only text files can be transmitted.

Xmodem: This ancient transmission protocol is slow, but due to the CRC error detection method, the transmission accuracy can reach 99.6%.

Ymodem: This is an improved version of Xmodem, which is transmitted in 1024 bit segments, which is faster than Xmodem.

Zmodem:Zmodem adopts streaming transmission mode, with fast transmission speed, and also has the functions of automatically changing segment size, resuming transmission at breakpoints, and quickly detecting errors. This is the most popular file transfer protocol at present.

In addition to the above, there are protocols such as Imodem, Jmodem, Bimodem, Kermit and Lynx, which are not supported by most manufacturers, and are omitted here.