The WiFi modem sim card we often say is actually the abbreviation of Modulator (modulator) and Demodulator (demodulator), which is called WiFi modem sim card in Chinese.
1. How WiFi modem sim card work
Some people also call it "cat" affectionately according to the homonym of Modem. We know that the information in the computer is composed of "0" and "1" digital signals, but only analog electrical signals are transmitted on the telephone line. Therefore, when two computers want to transmit data through a telephone line, a device is required for digital-to-analog conversion. This digital-to-analog converter is the Modem we are going to discuss here. When the computer sends data, the WiFi modem sim card first converts the digital signal into a corresponding analog signal, a process called "modulation". Before the modulated signal is transmitted to another computer through the telephone carrier, the receiver's Modem is also responsible for restoring the analog signal to a digital signal that the computer can recognize. This process is called "demodulation". It is through such a digital-to-analog conversion process of "modulation" and "demodulation" that the long-distance communication between the two computers is realized.
2. WiFi modem sim card transmission mode
Modem was originally only used for data transmission. However, with the continuous growth of user demand and the fierce competition among manufacturers, more and more "two-in-one" and "three-in-one" Modem appear on the market. In addition to data transmission, these Modems also have fax and voice transmission functions.
2.1 Fax Modem
Faxing through Modem, in addition to saving the cost of a dedicated fax, has many advantages: you can directly fax the files in the computer to the other party's computer or fax machine without having to print the file first; Fax can be easily saved or edited; it can overcome the problem of the gradual disappearance of handwriting caused by the use of thermal paper in ordinary fax machines; due to the use of error correction technology in WiFi modem sim card, the fax quality is better than ordinary fax machines, especially for graphic faxes. It is so. Most of the current Fax Modem follow the V.29 and V.17 fax protocols. V.29 supports 9600bps fax rate, while V.17 can support 14400bps fax rate.
2.2 Voice Modem
The voice mode mainly provides the functions of phone recording and voicemail and full-duplex hands-free calling, which truly integrates the phone 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, U.s. Robotics, Intel and other companies in 1995. It is an extension of the existing V.42 error correction protocol. DSVD adopts Digi Talk's digital voice and data simultaneous interpretation technology, which enables Modem to make calls while transmitting data on ordinary telephone lines.
DSVD Modem reserves 8K bandwidth (some Modem reserves 8.5K bandwidth) for voice transmission, and the rest of the bandwidth is used for data transmission. The voice will be compressed before transmission, and then combined with the data to be transmitted, and transmitted to the other party through the telephone carrier. At the receiving end, the WiFi modem sim card first separates the voice and data, and then decompresses and converts the voice signal to digital/analog, thereby realizing the simultaneous interpretation of data/voice. DSVD Modem has broad application prospects in distance teaching, collaborative work, online games and so on. But at present, because the price of DSVD Modem is more expensive than ordinary Voice Modem, and to realize the data/voice simultaneous interpretation function, the other party needs to use DSVD Modem as well, which hinders the popularization of DSVD Modem to a certain extent.
2.3 WiFi modem sim card transmission rate
The transmission rate of the Modem refers to the amount of data that the WiFi modem sim card transmits per second. We usually say 14.4K, 28.8K, 33.6K, 56K, etc., referring to the transmission rate of Modem. The transfer rate is in bps (bits per second). Therefore, a 33.6K Modem can transmit 33600bit data per second. Since the current Modem compresses data during transmission, the data throughput of a 33.6K Modem can theoretically reach 115200bps, or even 230400bps.
The transmission rate of the 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 Modem's box or manual refer to the modulation protocol adopted by the WiFi modem sim card. 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 ITU (International Telecommunications Union), while V.fc is a 28800bps modulation protocol proposed by Rockwell, but it has not been widely supported.
The transmission rates we mentioned above are all obtained under ideal conditions. In actual use, the rate of Modem often cannot reach the nominal value.
3. The actual transfer rate mainly depends on the following factors:
3.1 The quality of the telephone line
Because the modulated signal is transmitted through the telephone line, if the quality of the telephone line is not good, the WiFi modem sim card will reduce the speed to ensure the accuracy. For this reason, when connecting the Modem, we should minimize the length of the connection, and cut off the excess connection, and do not stack it in a circle. In addition, it is best not to use an extension, and the connection should also avoid passing through sources of interference such as TV sets.
3.2 Is there enough bandwidth?
If a large number of people surf the Internet at the same time, it will cause congestion and blockage of the line, and the transmission rate of the Modem will naturally decrease accordingly. Therefore, whether the ISP can provide enough bandwidth is very critical. In addition, avoiding the Internet during busy hours is also a solution. Especially when downloading files, the download time during busy hours and non-busy hours can vary by several times.
3.3 The other party's Modem rate
The modulation protocols supported by WiFi modem sim card are backward compatible, and the actual connection rate depends on the party with the lower rate. 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.
3.4 Modem transmission protocol
Modem's transmission protocol includes modulation protocol (ModulaTIon Protocols), error control protocol (Error Control Protocols), data compression protocol (Data Compression Protocols) and file transfer protocol. We have discussed the modulation protocol earlier, and now we will focus on the remaining three transmission protocols.
3.5 Error Control Protocol
As the transmission rate of Modem continues to increase, the noise on the telephone line, the abnormal sudden change of current, etc., will cause errors in data transmission. What the error control protocol needs to solve is how to ensure the accuracy of data in high-speed transmission. There are two industry standards for the current error control protocol: MNP4 and V4.2. MNP (Microcom Network Protocols) is a transmission protocol developed by Microcom, including MNP1-MNP10. Due to commercial reasons, Microcom currently only announces MNP1-MNP5, among which MNP4 is one of the widely used error control protocols. V4.2 is an improved version of MNP4 formulated by the International Telecommunication Union, which includes two control algorithms, MNP4 and LAP-M. Therefore, a WiFi modem sim card that uses the V4.2 protocol can establish an error-free control connection with a Modem that only supports the MNP4 protocol, but not vice versa. So 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 a hard error correction function to reduce costs, but use a software error correction method. Everyone should pay attention to distinguish them when purchasing, and don't be confused by the words "with error correction function" on the box.
3.6 Data compression protocol.
In order to increase the amount of data transmission and shorten the transmission time, most modems will compress the data before transmission. Similar to error control protocols, there are also two industry standards for data compression protocols: MNP5 and V4.2bis. MNP5 adopts Rnu-Length encoding and Huffman encoding two compression algorithms, and the maximum compression ratio is 2:1. And V4.2bis uses Lempel-Ziv compression technology, the maximum compression ratio can reach 4:1. That's why V4.2bis is said to be 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. And, while V4.2 includes MNP4, V4.2bis does not include MNP5.
3.7 File Transfer Protocol
File transfers are the primary form of data exchange. When transferring files, we need to establish a unified transfer protocol between the two computers so that the files can be correctly identified and transferred. This agreement includes the identification of the file, the start and end time of transmission, the judgment and correction of errors and so on. Common transmission protocols are as follows:
3.7.1 ASCII: This is the fastest transfer protocol, but can only transfer text files.
3.7.2 Xmodem: This ancient transmission protocol is slow, but due to the CRC error detection method, the transmission accuracy can be as high as 99.6%.
3.7.3 Ymodem: This is an improved version of Xmodem, using 1024-bit segment transfer, faster than Xmodem.
3.7.4 Zmodem: Zmodem adopts the streaming (streaming) transmission mode, the transmission speed is fast, and it also has the functions of automatically changing the segment size, resuming the transmission from a breakpoint, and fast error detection. This is the most popular file transfer protocol out there.
In addition to the above, there are protocols such as Imodem, Jmodem, Bimodem, Kermit, Lynx, etc., which are omitted here because they are not supported by most manufacturers.
4. What is the difference between wireless network card and wifi
4.1 The meaning is different: the wireless network card is actually a terminal wireless network device, which needs to be used for Internet access through the wireless connection network under the wireless coverage of the wireless local area network; wifi is the trademark of the wifi alliance manufacturer as the brand certification of the product. A wireless LAN technology created from the IEEE 802.11 standard. A wireless card refers to a wireless WAN card that connects to a wireless WAN.
4.2 Different functions: The function and function of the wireless network card are equivalent to the wired modem, which is commonly known as "cat". It can connect to the Internet using a USIM or SIM card anywhere there is wireless phone coverage. The function and function of the wireless network card is like a WiFi modem sim card; the function of the wireless network wifi is actually to convert the wired network signal into a wireless signal, and use a wireless router to receive the relevant computers, mobile phones, tablets, etc. that support its technology.
