猫的工作原理（How does a cat work）

1、猫的工作原理（How does a cat work）MODEM reviewWhat we often call Modem is actually the abbreviation of Modulator (modulator) and Demodulator (demodulator), and Chinese is called modem. It was also called the cat, affectionately called Modem. We know that the information in the computer is composed of 0 and

2、 1 to form a digital signal, while the telephone line can only be analog signals. Thus, when two computers want to transmit data over a telephone line, a device is needed for the conversion of digital signals. This digital to analog converter is the Modem were going to talk about here. When the comp

3、uter transmits data, it converts the digital signals into corresponding analog signals by Modem. This process is called modulation. The modulated signal is transmitted through the telephone carrier to another computer before, also through the receiver Modem is responsible for the analog signal into

4、digital signal can be recognized by computer, this process we call demodulation. It is through such a modulation and demodulation of the digital to analog conversion process, so as to achieve the remote communication between two computers.I. category of ModemGenerally speaking, according to the shap

5、e and installation methods of Modem, they can be roughly divided into the following four categories:1, external ModemThe external Modem is placed outside the cabinet and connected with the host through serial communication port. This Modem is convenient, smart, easy to install, flashing indicator li

6、ght, easy to monitor the working status of Modem. But external Modem requires the use of additional power and cable.2, built-in ModemThe built-in Modem needs to be disassembled when installing, and the interrupt and the COM port are set up, and the installation is tedious. The Modem takes up the slo

7、t on the motherboard, but requires no extra power and cable, and the price is cheaper than the external Modem.3, PCMCIA plug-in ModemPlug type Modem is mainly used for notebook computer, small volume. With mobile phones, mobile office can be easily implemented.4, rack type ModemRack type Modem is eq

8、uivalent to a set of Modem concentrated in a box or shell, and powered by a unified power supply. Rack type Modem is mainly used in the central computer room of Internet/Intranet, telecommunication bureau, campus network, financial institution and so on.In addition to the four common Modem, there ar

9、e now ISDN modems and a modem called Cable Modem, and a ADSL modem. Cable Modem uses the cable of cable to carry on the signal transmission, not only has the modulation and demodulation function, but also gathers the router, the hub, the bridge device in a body, the theory transmission speed may be

10、more than 10Mbps. Through the Cable Modem Internet access, each user has an independent IP address, equivalent to owning a personal line. At present, the Shenzhen cable television Print-Rite network company has introduced this kind of Internet access service based on the cable television network, th

11、e access speed is 2Mbps-10Mbps!Two. Modem transport modeModem was originally used only for data transmission. However, with the continuous growth of user demand and fierce competition among vendors, more and more two in one and three in one Modem have appeared in the market. In addition to data tran

12、smission, these Modem also have fax and voice transmission capabilities.1, fax mode (Fax Modem)Fax by Modem, in addition, a dedicated fax fee, there are many benefits: can directly to the computer file in the computer or fax fax to each other, there is no need to print out the file; the received fax

13、 can be conveniently stored or editing; fax machine can be overcome because the use of thermal paper and handwriting fading problem caused by; because Modem uses error correction technology, fax quality than ordinary fax machine, especially for the graphic fax is even more so. Most of the current Fa

14、x Modem follows the V.29 and V.17 fax protocols. Where V.29 supports the 9600bps fax rate, V.17 supports the fax rate of 14400bps.2, voice mode (Voice Modem)Voice mode mainly provides telephone recording messages and full duplex hands-free call function, which really makes the phone and the computer

15、 integrated. Here, we mainly discuss a new voice transfer mode -DSVD (Digital Simultaneous Voice and Data). DSVD is a speech transmission standard proposed by companies such as Hayes, Rockwell, U.s.Robotics and Intel in 1995. It is an extension of the existing V.42 error correction protocol. DSVD th

16、rough digital voice and data transmission technology using Digi Talk, the Modem in the ordinary telephone line side data transmission side of the call.DSVD Modem preserves the bandwidth of 8K (and some Modem preserves 8.5K bandwidth) for voice transmission,The rest of the bandwidth is used for data

17、transmission. The speech is compressed before being transmitted, then integrated with the data to be transmitted, and transmitted to each other via the telephone carrier. At the receiving end, Modem put the voice and data are separated, then the speech signal is extracted and a / D conversion, so as

18、 to realize the data / voice transmission. DSVD Modem has a wide range of applications in distance education, collaborative work, online games and so on. But at present, due to the DSVD Modem price is more expensive than ordinary Voice Modem, and to realize the data / voice transmission function, ne

19、ed each other to use DSVD Modem, which hinders the popularity of DSVD Modem to a certain extent.Three. Modem transmission rateThe Modem transfer rate refers to the amount of data that Modem transmits per second. We usually say 14.4K, 28.8K, 33.6K, 56K, and so on, that is, the Modem transmission rate

20、. The transmission rate is BPS (bit / sec). Therefore, a 33.6K Modem can transmit 33600bit data per second. Since the current Modem data is compressed during transmission, the data throughput of 33.6K Modem can theoretically reach 115200bps or even 230400bps.The transmission rate of Modem is actuall

21、y 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 Modem package or manual, which refers to the modulation protocol adopted by Modem. Where V.32 is a non synchronous / synchronous full duplex 4800/9600bps standard protocol; V.

22、32bis is an enhanced version of V.32, the transmission rate of 14400bps is V.34 support; synchronous full duplex 28800bps standard protocol; V.34+ for synchronous full duplex 33600bps standard protocol. The above standards are made by the ITU (International Communications Union), while V.fc is the 2

23、8800bps modulation protocol proposed by Rockwell, but it has not been widely supported.Mention the transmission rate of Modem, you can not help but mention the current hot 56K Modem. In fact, 56K standards have been proposed for many years, but due to the long existence of Rockwell led by K56flex an

24、d U.S.Robotics led by X2, the two incompatible standards, making 56K Modem delay popularity. Fortunately, in February this year, the International Telecommunication Unions efforts, the 56K standard for ITU V9.0 finally unified, many Modem manufacturers also have introduced measures to upgrade, but t

25、he real support of V9.0 Modem also has blossom everywhere. 56K is expected to become the mainstream of the market in one to two years. Here, by the way, because, at present, many domestic ISP does not provide 56K access services, so before buying 56K Modem, it is best to ask your service providers t

26、o find out, so as not to waste.The transmission rates mentioned above are all in ideal condition. In actual use, the rate of Modem often can not reach the nominal value. The actual transmission rate depends mainly on the following factors:1, the quality of the telephone lineBecause the modulated sig

27、nal is transmitted over the telephone line, if the phone line is of poor quality, the Modem will slow down to ensure accuracy. For this reason, when we connect to the Modem, we should try to minimize the length of the line, and the extra wires to be cut. Do not pile up in a circle. In addition, it i

28、s better not to use an extension, the connection should also avoid interference in the television and other sources.2, is there sufficient bandwidth?If the number of people accessing the Internet at the same time is very large, it will cause congestion and congestion of the line, and the transmissio

29、n rate of Modem will naturally decline. Therefore, whether ISP can provide sufficient bandwidth is critical. Also, avoiding the Internet during busy hours is also a solution. Especially when downloading files, it will take several times as much time to download between busy and non busy hours.3, the

30、 other side of the Modem speedThe modulation protocol supported by Modem is backward compatible, and the actual connection rate depends on the lower rate side. Therefore, if the other partys Modem is 14.4K, even if you are using 56Ks Modem, you can only connect at the rate of 14400bps.Four. Modem tr

31、ansport protocolModem transport protocols include modulation protocol (Modulation Protocols), error control protocol (Error Control Protocols), data compression protocol (Data Compression Protocols) and file transfer protocol. The modulation protocol we discussed earlier, now focuses on the remainin

32、g three transport protocols.1, error control protocolWith the increasing transmission rate of Modem, the noise on the telephone line and the abrupt change of the current will cause data transmission error.What the error control protocol should solve is how to ensure the accuracy of data in high spee

33、d transmission. The current error control protocol has two industrial standards: MNP4 and V4.2. Among them, MNP (Microcom Network Protocols) is a transport protocol developed by Microcom company, including MNP1-MNP10. For commercial reasons, Microcom currently only announces MNP1-MNP5, where MNP4 is

34、 one of the widely used error control protocols. The V4.2 is an improved version of MNP4 developed by the International Telecommunication Union. It contains two kinds of control algorithms, MNP4 and LAP-M. Thus, a Modem that uses the V4.2 protocol can establish a no error control connection with a M

35、odem that supports only the MNP4 protocol, and vice versa. So when we buy Modem, wed better choose the Modem that supports the V4.2 protocol.In addition, some inexpensive Modem cards on the market do not have the ability to correct errors in order to reduce costs, but use software correction methods

36、. We should pay attention to distinguish between the purchase, not for the box with error correction function and other words confused.2, data compression protocol.In order to improve the data transmission and shorten the transmission time, most of the Modem will compress the data at the time of tra

37、nsmission. Similar to the error control protocol, there are two industrial standards for data compression protocols: MNP5 and V4.2bis. MNP5 employs two compression algorithms, Rnu-Length encoding and Huffman encoding, with a maximum compression ratio of 2:1. And V4.2bis uses Lempel-Ziv compression technology, the maximum compression ratio of up to 4:1. Thats why V4.2bis is faster than MNP5. It should be noted that data compression protocols are based on error control protocols, and that MNP5 requires MNP4 support, and