What Is a Null Modem?

Modem is an abbreviation of modulator and demodulator. It is a kind of computer hardware ^[1] , which can translate the digital signals of the computer into analog signals that can be transmitted along ordinary telephone lines, and these analog signals can be used by the other end of the line. The other modem received it and translated it into a computer-readable language. This simple process completes the communication between the two computers.

The modem is a Modulator (

Modem was originally developed as a semi-automatic ground air defense system in the 1950s (

Modem was originally only used for data transmission. However, as user demand continues to grow and

Modem

Modem installation process can be divided into

Modem

its

Although all ROCKWELL chips are used, some "cats" love to drop the line, and some "cats" cannot reach the nominal performance. The differences in the performance of the modems are caused by a variety of reasons. The first is the chip used by the modem, and the second is the choice of materials and circuit design. Generally, the circuit of the modem uses the public board design recommended by the chip manufacturer, and the difference in material selection is large. Modems produced by some small factories use second-hand components or low-quality components, which cause the Modem to become unstable due to heat and other reasons after long-term work. In order to maintain their credibility, well-known manufacturers generally use high-reliability components, which exceed the performance and stability of small manufacturers. Because of this, ROCKWELL's external modem is also used. The most expensive is more than 800 yuan, and the cheapest is only tens of yuan.

There are many foreign Modem brands. The more well-known ones are Heath, USR, Diamond, and American tanks. The USR's big black cat is considered to be the best in the minds of many enthusiasts. Although it uses TI chips, it is second to none in terms of performance and stability, and its built-in speaker volume can be freely adjusted, making it a favorite for staying up of

Foreign modems are not necessarily better than domestic ones. Many people like to buy modems from foreign brands. In fact, most of those modems are

Modem communication protocol

Communication protocols are often referred to as "data transmission standards." The general 56Kbps data transmission standard is the V.90 protocol specified by the ITU, which allows modems to achieve 56Kps data transmission rates on standard telephone switching networks. Modem protocols are loaded into the BIOS, so a limited upgrade can be achieved by refreshing the contents of the BIOS.

Modem error correction compression protocol

Modem dial mode During network communication, data is sent in the form of data packets. Due to problems such as signal attenuation and poor line quality, or interference, data packets are often lost or damaged during transmission. The role of the error correction protocol is to detect whether there is an error in the received data packet. Once an error is found, the error correction protocol will try to regain the correct data packet or try to repair the damaged data packet through an algorithm. Common error correction protocols are V.42 and MNP series. V.42 is an error correction protocol introduced by ITU-T (International Telecommunication Union). Its function is that once the data packet sent by the sender is lost, the receiver can immediately request the other party to resend the data packet. MNP is a series of protocols proposed by Microsoft Corporation. It is divided into MNP1--MNP10 for a total of 10 levels. The higher the level, the stronger the function and the backward compatibility. The role of MNP is that once V.42 fails to complete the application for an error packet Resend the task and it will try to correct it. Both of these error correction protocols are generally supported by Modems. The V.42 protocol additionally bears the task of data compression.

AT Modem AT command

(AT Commands)

Invented by Hayes, a modem command language that has become the de facto standard and is used by all modem manufacturers. Each command starts with the letter "AT", hence its name. AT followed by letters and numbers indicate specific functions. For example, "ATDT" is a dial command. Other commands include "initialize modem", "control speaker volume", "specify the number of rings for modem startup response", "select error correction format" Wait, the AT commands of different brands of modems are not exactly the same, please read the MODEM user manual carefully in order to use the AT commands correctly.

Modem baud rate

(Baud Rate)

The rate of the analog line signal, also called the modulation rate, is measured by the number of oscillations per second of the waveform. If the data is not compressed, the baud rate is equal to the number of data bits transmitted per second. If the data is compressed, then the number of data bits transmitted per second is usually greater than the modulation rate, so the exchange of baud and bits / second will occasionally occur. error.

DCE Modem DCE

Acronym for "Data Communication Equipment". DCE provides functions for establishing, maintaining, and terminating connections. A modem is a type of DCE.

DTE Modem DTE

Acronym for "Data Terminal Equipment". DTE provides or receives data. A computer connected to a modem is a type of DTE.

Modem other

Line rate

Also known as DCE rate, the unit is bit / s (bps). Refers to the transmission rate of data on a telephone line (or dedicated line) connecting two modems. Common rates are 56000bps, 334bps, 28800bps, and so on.

Port rate

Also called DTE rate or maximum throughput. Refers to the computer's serial port to modem transmission rate. Since modems today almost support this rate of V.42bis and MNP5 compression standards (both compression ratio is 4: 1), this rate is generally much higher than the line rate.

Dedicated Line / Dial

Dedicated line refers to two ordinary passive (or active) wires. There is no dial tone when dialing on a dedicated line, so special hardware support is required. Dial-up lines are ordinary telephone lines that dial through the telephone system. Common modems support dial-up lines, not necessarily dedicated lines.

Remote Setup

It means that after the local modem is connected to the remote modem, the remote user can set the parameters of the local modem.

Data bits and flow control

When the modem transmits data, each group of data transmitted must contain corresponding control data in the data packet. Different communication environments have different data bit and end bit standards. Flow control is used to coordinate the data stream transmission between the Modem and the computer. It can prevent data loss caused by the mismatch in communication processing speed between the computer and the Modem. There are two types of flow control: hardware flow control (RTS / CTS) and software flow control (XON / XOFF).

Data / Voice Simultaneous Interpretation (SVD)

The so-called data / voice simultaneous interpretation means that while the MODEM is performing data communication, it can also use ordinary telephones to talk. According to different implementation methods, there are two types of data / voice simultaneous interpretation: analog data / voice simultaneous interpretation (ASVD: Analog Simultaneous Voice and Data) and digital data / voice simultaneous interpretation (DSVD: Digital Simultaneous Voice and Data).