Wireless modem is a kind of computer hardware, it can translate computer digital signal into the analog signal transmitted over regular telephone lines, and the analog signal can be line another modem at the other end of the receiving, and translated into computer language can understand. This simple process completes the communication between two computers.
Basic principle: the average person's voice is 300-3400 hz frequency range, for the voice signal transmission in the ordinary telephone system, the online road assign it a certain bandwidth, international standard take 4 KHZ for a road (the band width of the occupied. In the transmission process: speech signal to a 300-3400 hz frequency input, the sender's telephone transform the voice signal into analog signals, the analog signal through a frequency division multiplexer to change, can transmit multi-channel analog signal at the same time on line, later when they arrive in the receiving end after a XiePin process to restore it to its original frequency range of the analog signal, again by the receiving party telephone converts analog signals into sound signal.
The information in the computer is composed of "0" and "1" to form a digital signal, while the transmission on the telephone line can only be analog electrical signal. Do not take any measures to use analog channel to transmit digital signals inevitably there will be a big mistake (distortion), therefore, in the ordinary telephone network to transmit data, it must to digital signal conversion to telephone network within the required when the original design of the audio spectrum (that is, 300 hz - 3400 hz).
Modulation is to control the carrier wave waveform with the baseband pulse, forming a signal suitable for the transmission of the line.
The demodulation is that when the modulated signal arrives at the receiving end, the analog signal that has been transformed by the modulator will remove the carrier and return to the original baseband digital signal.
A modem can also convert an audio signal into a higher frequency signal and convert a higher frequency signal into an audio signal. The other purpose of modulation is to facilitate line reuse so as to improve line utilization.
Based on the three main parameters of carrier signal, modulation can be divided into three kinds: amplitude modulation, frequency modulation and phase modulation.
Modem was originally used for data transmission only. However, with the increasing demand of users and fierce competition among manufacturers, there are more and more "two in one" and "three in one" Modem in the market. In addition to data transmission, these modems have facsimile and voice transmission functions.
1. Fax Modem
Through fax Modem, in addition to save the cost of a dedicated fax, benefits and there are many: can fax the documents in computer directly to the other side of the computer or fax machines, without having to print out the document first. It can be conveniently saved or edited with the fax received. It can overcome the problem that ordinary fax machine is gradually fading because of the use of heat sensitive paper; Because Modem USES the correct technique, the fax quality is better than the normal fax machine, especially for the graphic fax. The Fax Modem mostly follows the Fax protocol of v. 29 and v. 17. V.29 supports 9600bps fax rate, while v. 17 can support the fax rate of 14400bps.
2. Voice modems
The voice mode mainly provides voice mail message and full-duplex hands-free call function, which really makes the telephone and computer integrated. Here, it is mainly a new mode of phonetic transmission - DSVD (Digital Simultaneous Voice and Data). The DSVD is by Hayes, Rockwell,
In 1995, a speech transmission standard proposed by u.s.-based companies such as U.S.R. obotics and Intel is an expansion of the existing v. 42 error correction protocol. DSVD USES Digi Talk's digital voice and data transfer technology to enable Modem to communicate on the same side of the normal phone line as data transmission.
DSVD Modem retains 8K bandwidth (and some Modem reserves 8.5K of bandwidth) for voice transmission, while the rest of the bandwidth is used for data transmission. The voice is compressed before transmission and then combined with the data that needs to be transmitted to the user through the phone carrier. At the receiving end, the Modem separates the speech from the data, then decompresses the speech signal and the number/mode conversion, thus the data/speech transmission of the data is realized. DSVD Modem has a wide application prospect in remote teaching, collaborative work and online games. Since DSVD Modem is more expensive than ordinary Voice Modem, and it is necessary to realize the data/Voice synthetic function, DSVD Modem is also needed, which in some way hinders the popularization of DSVD Modem.