How a modem works

The demodulator is an important component in the modulated DC amplifier circuit. It restores the amplified AC voltage to a DC voltage, and its magnitude and polarity must correspond to the amplitude and phase of the AC voltage. The figure below is the principle circuit of the demodulator. RL is the load, and C is the filter capacitor, which is used to smooth the output DC voltage. The demodulation switch K has the same frequency as the input AC signal, Ui. When Ui is positive, the switch is turned on, and the output is equal to the input voltage UO. After the smoothing effect of capacitor C, a smooth DC voltage UO should be obtained. On the contrary, if Ui is negative, the switch is turned on, and when Ui is positive, the switch is turned off, then the output end will obtain a negative pulsating DC voltage UO.

The modem we often talk about is actually the abbreviation of Modulator and Demodulator, which is called modem in Chinese. Some people also call it "cat" affectionately based on the homonym of Modem. We know that the information in the computer is composed of digital signals composed of "0" and "1", while the information transmitted on the telephone line can only be analog electrical signals. Therefore, when two computers want to transmit data through the telephone line, a device is needed to be responsible for the digital-to-analog conversion. 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. This process is called "modulation". Before the modulated signal is transmitted to another computer through the telephone carrier, it must also be restored to a digital signal that can be recognized by the computer through the receiving Modem. This process is called "demodulation". It is through such a digital-to-analog conversion process of "modulation" and "demodulation" that remote communication between two computers is realized.

1. Types of Modems

Generally speaking, according to the form and installation method of Modem, it can be roughly divided into the following four categories:

1. External Modem

An external modem is placed outside the chassis and connected to the host through a serial communication port. This type of modem is convenient and easy to install, and the flashing indicator light makes it easy to monitor the working status of the modem. However, an external modem requires an additional power supply and cable.

2. Built-in Modem

The installation of built-in modem requires disassembling the computer case and setting the interrupt and COM port, which is more complicated. This type of modem occupies the expansion slot on the motherboard, but does not require additional power supply and cable, and is cheaper than external modem.

3. PCMCIA card-type modem

Card-type modems are mainly used in notebook computers. They are small and compact. When used with mobile phones, they can be used for mobile office work.

4. Rack-mounted Modem

Rack-mounted modems are equivalent to a group of modems concentrated in a box or shell, and powered by a unified power supply. Rack-mounted modems are mainly used in the central computer room of Internet/Intranet, telecommunications bureaus, campus networks, financial institutions and other networks.

In addition to the above four common modems, there are now ISDN modems and a modem called Cable Modem, and there is also an ADSL modem. Cable Modem uses cable TV to transmit signals. It not only has modulation and demodulation functions, but also integrates routers, hubs, and bridges. The theoretical transmission speed can reach more than 10Mbps. When surfing the Internet through Cable Modem, each user has an independent IP address, which is equivalent to having a personal dedicated line. At present, Shenzhen Cable TV Station Tianwei Network Company has launched this 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 continuous growth of user demand and fierce competition among manufacturers, more and more "two-in-one" and "three-in-one" modems are now on the market. In addition to data transmission, these modems also have fax and voice transmission functions.

1. Fax Modem

Faxing via a modem not only saves the cost of a dedicated fax machine, but also has many other benefits: you can directly fax files in your computer to the other party's computer or fax machine without having to print the files out first; you can easily save or edit received faxes; you can overcome the problem of gradual fading of words in ordinary fax machines due to the use of thermal paper; because the modem uses error correction technology, the fax quality is better than that of ordinary fax machines, especially for faxing graphics. Most of the current fax modems follow the V.29 and V.17 fax protocols. V.29 supports a fax rate of 9600bps, while V.17 supports a fax rate of 14400bps.

2. Voice Modem

The voice mode mainly provides the functions of telephone recording and full-duplex hands-free calling, which truly integrates the telephone and the computer. Here, we mainly discuss a new voice transmission mode - DSVD (Digital Simultaneous Voice and Data). DSVD is a voice transmission standard proposed by Hayes, Rockwell, UsRobotics, Intel and other companies in 1995. It is an expansion of the existing V.42 error correction protocol. DSVD uses Digi Talk's digital voice and data simultaneous transmission technology to enable the modem to transmit data and make calls on ordinary telephone lines at the same time.

DSVD Modem reserves 8K bandwidth (some modems reserve 8.5K bandwidth) for voice transmission, and the rest of the bandwidth is used 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 the telephone carrier. At the receiving end, the modem first separates voice from data, and then decompresses and converts the voice signal to digital/analog, thereby realizing data/voice simultaneous transmission. DSVD Modem has broad application prospects in remote teaching, collaborative work, online games, etc. However, at present, the price of DSVD Modem is more expensive than that of ordinary Voice Modem, and to realize the data/voice simultaneous transmission function, the other party needs to use DSVD Modem, which to some extent hinders the popularization of DSVD Modem.

3. Modem transmission rate

The transmission rate of a modem refers to the amount of data that the modem transmits per second. The 14.4K, 28.8K, 33.6K, 56K, etc. that we usually talk about refer to the transmission rate of the modem. The transmission rate is measured in bps (bits per second). Therefore, a 33.6K modem can transmit 33600 bits of data per second. Since current modems compress data during transmission, the data throughput of a 33.6K modem can theoretically reach 115200bps, or even 230400bps.

The transmission rate of a modem is actually determined by the modulation protocol supported by the modem. The V.32, V.32bis, V.34, V.34+, V.fc, etc. that we usually see on the packaging box or manual of the modem refer to the modulation protocol used by the modem. Among them, V.32 is an asynchronous/synchronous 4800/9600bps full-duplex standard protocol; V.32bis is an enhanced version of V.32, supporting a transmission rate of 14400bps; V.34 is a synchronous 28800bps full-duplex standard protocol; and V.34+ is a synchronous full-duplex 33600bps standard protocol. The above standards are all formulated by the ITU (International Telecommunication Union), and V.fc is a 28800bps modulation protocol proposed by Rockwell, but it has not been widely supported.

When talking about the transmission rate of modems, we cannot fail to mention the 56K modem that is being hyped nowadays. In fact, the 56K standard has been proposed for many years, but due to the long-standing existence of two incompatible standards, K56flex led by Rockwell and X2 led by USRobotics, the 56K modem has not been popularized. Fortunately, in February this year, with the efforts of the International Telecommunication Union, the 56K standard was finally unified as ITU V9.0. Many modem manufacturers have also introduced upgrade measures, and modems that truly support V9.0 have also blossomed everywhere. 56K is expected to become the mainstream of the market within one to two years. Here, by the way, since many ISPs in China do not provide 56K access services, it is best to ask your service provider before purchasing a 56K modem to avoid waste.
The transmission rates we mentioned above are all obtained under ideal conditions. In actual use, the rate of the modem often cannot reach the nominal value. The actual transmission rate mainly depends on the following factors:

1. The quality of the telephone line

Because the modulated signal is transmitted via the telephone line, if the telephone line quality is poor, the modem will reduce the rate to ensure accuracy. For this reason, when connecting the modem, we should try to shorten the length of the connection, cut off the excess connection, and never pile it up in a circle. In addition, it is best not to use an extension, and the connection should also avoid passing over interference sources such as TVs.

2. Is there enough bandwidth?

If there are many people surfing the Internet at the same time, the line will be crowded and blocked, and the transmission rate of the modem will naturally decrease. Therefore, it is critical whether the ISP can 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 hours and non-busy hours will differ by several times.

3. The other party’s modem speed

The modulation protocols supported by the 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 a rate of 14400bps.

4. Modem transmission protocol

Modem transmission protocols include modulation protocols, error control protocols, data compression protocols, and file transfer protocols. We have already discussed the modulation protocols, and now we will focus on the other three transmission protocols.

1. Error Control Protocol

As the transmission rate of modems continues to increase, noise on telephone lines, abnormal current changes, etc. will cause errors in data transmission. The error control protocol needs to solve how to ensure the accuracy of data in high-speed transmission. There are currently two industrial 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 only released MNP1-MNP5, among which MNP4 is one of the most 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 the V4.2 protocol can establish an error-free control connection with a modem that only supports the MNP4 protocol, but not vice versa. Therefore, when we buy a modem, it is best to choose a modem that supports the V4.2 protocol.

In addition, some cheap modem cards on the market do not have hard error correction function to reduce costs, but use software error correction. Please be careful when purchasing and do not be confused by words such as "with error correction function" on the packaging box.

2. Data Compression Protocol

In order to increase the amount of data transmission and shorten the transmission time, most modems now compress the data before transmission. Similar to the error control protocol, there are two industrial standards for data compression protocol: MNP5 and V4.2bis. MNP5 uses two compression algorithms, Rnu-Length encoding and Huffman encoding, with a maximum compression ratio of 2:1. V4.2bis uses Lempel-Ziv compression technology, with a maximum compression ratio of 4:1. This is why V4.2bis 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. In addition, although V4.2 includes MNP4, V4.2bis does not include MNP5.

3. File Transfer Protocol

File transfer is the main form of data exchange. When transferring files, in order to correctly identify and transfer files, we need to establish a unified transfer protocol between two computers. This protocol includes file identification, start and end time of transfer, error judgment and correction, etc. Common transfer protocols are as follows:
ASCII: This is the fastest transfer protocol, but it can only transfer text files.

Xmodem: This old transmission protocol is slow, but due to the use of CRC error detection method, the transmission accuracy can be as high as 99.6%.
Ymodem: This is an improved version of Xmodem, using 1024-bit segment transmission, which is faster than Xmodem.
Zmodem: Zmodem uses streaming transmission method, with faster transmission speed, and also has functions such as automatic change of segment size, breakpoint resumption, and fast error detection. This is the most popular file transfer protocol.

In addition to the above, there are also Imodem, Jmodem, Bimodem, Kermit, Lynx and other protocols. Since they are not supported by most manufacturers, they will be omitted here.