The modem consists of the transmit, receive, control, interface, control panel and power supply. The data terminal device provides the transmitted data in the form of a binary serial signal, which is converted into an internal logic level by the interface and sent to the transmitting portion, and is modulated by the modulation circuit to transmit the signal required by the line to the line. The receiving part receives the signal from the line, and after filtering, inverse modulation, level conversion, and then restored to a digital signal and sent to the digital terminal device. Nanning Ousibang Information Technology Co., Ltd. , https://www.ousibangvape.com
The telephone line allows the two parties to communicate with each other at thousands of kilometers apart, because relay amplification equipment is provided at regular intervals to ensure clear voice. If you configure the Modem on these devices, you can transfer data from the phone. The voice bandwidth of a general telephone line is in the range of 300 to 3400 Hz, and it is used to transmit digital signals, and the signal frequency must also be in this range. There are three commonly used modulation methods: frequency shift keying (FSK), phase shift keying (PSK), and phase amplitude modulation (PAM).
There are also three modulation methods for digital-to-analog conversion: (1) Frequency Shift Keying (FSK). Use a special audio range to distinguish between sending data and receiving data. For example, the frequency modulation ModemBell-103 type binary logic for transmitting and receiving data is specified: transmission, signal logic 0, frequency 1070Hz, signal logic 1, frequency 1270Hz; reception, signal logic 0, frequency 20 25Hz, signal logic 1, The frequency is 2225 Hz. (2) Phase shift keying (PSK), high-speed Modem is commonly used in four-phase system, eight-phase system, and four-phase system uses four different phases to represent four binary numbers 00, 01, 10, and 11, such as phase modulation. ModemBell-212A type. This technology can make the 300bps Modem transmit 600bps information, so it is meaningful to increase the signal transmission rate without increasing the line modulation rate, but the control is complicated, the cost is high, and the eight-phase system is more complicated. (3) Phase amplitude modulation (PAM), in order to maximize the transmission rate without increasing the modulation rate, a combination of phase modulation and amplitude modulation is employed. It can use 16 different phase and amplitude levels to enable a 1200 bps modem to transmit a 19200 bps data signal.
After the modem is powered on, it usually enters the on-hook mode, dials the line through the telephone, enters the call mode, and finally enters the online mode through the "handshake" process of the Modem.
The connection between the modem and the computer is an interface problem between the data circuit communication device DCE (Data Circutterminating Equipment) and the data terminal equipment DTE (Data Terminal Equipment). The interface between DCE and DTE is an important issue in the use of computer networks. Any communication station must always include DCE and DTE, so it is important to define a unified standard interface, especially for public data networks. The data terminal equipment DTE is a data source for generating a digital signal or a data sink for receiving a digital signal, or a combination of the two, such as a computer terminal, a printer, a facsimile machine, etc., which is a DTE. The device that connects the data terminal equipment DTE to the analog channel is called the data circuit communication device DCE, like Modem is DCE. The connection standard between DTE and DCE is CCITTV.10/X.26, which is compatible with EIARS-423-A. It is a semi-balanced electrical characteristic interface. Normal Modem is usually through RS-232C serial port signal line and computer. connection. (RS is the abbreviation of RecommendationStandard), which was recognized by international organizations in 1969. The definition of RS-232 includes electrical characteristics (such as voltage values), mechanical characteristics (such as joint shape), and functional characteristics (such as pin signals). It allows a sending device to connect to a receiving device to transfer data; its original specification has a maximum transfer speed of 20Kbps, but in fact, today's applications are far beyond this speed range. RS-232 can be said to be a fairly simple communication standard. If you do not use hardware flow control, you can use only three signal lines to achieve full-duplex transmission. The electrical characteristics of RS-232 are unbalanced transmission methods, and the anti-interference ability is weak, so the transmission distance is short, about 15 meters.
The RS-232C serial port signals are divided into three categories: transmit signals, contact signals, and ground lines.
(1) Transmission signal: Refers to TXD (transmit data signal line) and RXD (receive data signal line). The format of the information received via TXD transmission and RXD reception is: One transmission unit (byte) consists of a start bit, a data bit, a parity bit, and a stop bit.
(2) Contact signal: refers to six signals of RTS, CTS, DTR, DSR, DCD and RI. The respective functions are: RTS (request transmission), which is the contact signal sent by the PC to the MODEM. A high level indicates that the PC requests to transmit data CTS (clear transmission) to the MODEM, which is a contact signal sent by the MODEM to the PC. A high level indicates that the MODEM responds to the RTS signal from the PC and is ready to send data to the remote MODEM.
DTR (data terminal ready) is the contact signal sent by the PC to the MODEM. The high power screen indicates that the PC is in the ready state, and a communication channel can be established between the local MODEM and the remote MODEM. If it is a low screen, the MODEM is forced to terminate the communication DSR (data device ready), which is the contact signal sent by the MODEM to the PC. It indicates the working state of the local MODEM, and the high level indicates that the MODEM is not in the test call state, and can establish a channel with the remote MODEM. DCD (Transmission Detection) is a status signal sent by the MODEM to the PC. A high level indicates that the local DCE receives the carrier signal from the remote MODEM.
RI (ringing indication) is the status signal sent by the MODEM to the PC. The high level indicates that the local MODEM receives the ringing signal from the remote MODEM.
(3) Ground signal (GND), providing the same potential reference point for the connected PC and MODEM. The 56K high-speed Modem is a dial-up high-speed modem that was only launched in 1997. Its transmission rate can be higher than the 33.6Kbps limit rate on the traditional telephone line because it uses a modulation and demodulation technology that is completely different from 33.6K. The principle of the workpiece and the requirements for use are also different from those of the 33.6 K high-speed Modem.