The modem is composed of transmission, reception, control, interface, control panel and power supply. The data terminal equipment provides the transmitted data in the form of a binary serial signal, converts it to an internal logic level through the interface, and sends it to the transmitting section. The signal modulated by the modulation circuit to the line requires it to be sent to the line. The receiving part receives the signal from the line, and after filtering, inverse modulation and level conversion, it is restored to a digital signal and sent to the digital terminal equipment.
The telephone line can make the two parties communicate with each other thousands of kilometers away from each other, because the relay amplifier equipment is provided at a certain distance to ensure clear voice. If you configure Modem on these devices, you can transfer data where you can talk to the phone. The voice bandwidth of a general telephone line is in the range of 300 to 3400 Hz. To use it to transmit digital signals, the signal frequency must also be in this range. There are three commonly used modulation methods:
Frequency shift keying (FSK), phase shift keying (PSK), phase amplitude modulation (PAM).
There are also three modulation methods for digital-to-analog conversion: (1) Frequency shift keying (FSK). Use special audio range to distinguish between sending data and receiving data. For example, the dedicated frequency specified by the binary logic of FM ModemBell-103 to send and receive data is: send, signal logic 0, frequency 1070Hz,
Signal logic 1, frequency 1270Hz; receive, signal logic 0, frequency 20 25Hz, signal logic 1, frequency 2225Hz. (2) Phase shift keying (PSK),
High-speed Modem commonly uses four-phase system, eight-phase system, and four-phase system uses four different phases to represent the four binary numbers 00, 01, 10, 11
Such as phase modulation ModemBell-212A type. This technology can make 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 higher, and the eight-phase system is more complicated. (3) Phase amplitude modulation (PAM),
In order to increase the transmission rate as much as possible without increasing the modulation rate, a combination of phase modulation and amplitude modulation is used. It can use 16 different phase and amplitude levels to enable 1200bps Modem to transmit 19200bps data signals.
After the modem is powered on, it usually enters the on-hook mode, dials the line through the phone and enters the call mode, and finally through the Modem
The "handshake" process enters the online mode.
The connection between the modem and the computer is the interface problem between the data circuit communication equipment 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 always includes DCE and DTE, so it is important to determine a unified standard interface, especially for public data networks. The data terminal equipment DTE is a data source that generates digital signals or a data sink that receives digital signals, or a combination of the two.
Such as computer terminals, printers, fax machines, etc. is DTE. The equipment that connects the data terminal equipment DTE to the analog channel is called the data circuit communication equipment DCE, like Modem is DCE. The connection standard between DTE and DCE is CCITTV.10 / X.26, and EIARS-423-A
Compatible, it is a kind of semi-balanced electrical characteristic interface. Normal Modem is usually connected to the computer through the RS-232C serial port signal line. (RS is the abbreviation of Recommended Standard), which was recognized by international organizations in 1969. The definition of RS-232 includes electrical characteristics (such as voltage value), mechanical characteristics (such as connector shape) and functional characteristics (such as pin signal). It allows a sending device to be connected to a receiving device to transfer data; the maximum transmission speed of its original specification is
20Kbps, but in fact, the current application is far beyond this speed range. RS-232 can be said to be a fairly simple communication standard. If hardware flow control is not used, at least three signal lines are needed to achieve full-duplex transmission. The electrical characteristics of RS-232 belong to the unbalanced transmission mode, and the anti-interference ability is weak, so the transmission distance is short, about 15 meters.
RS-232C serial port signals are divided into three categories: transmission signals, contact signals and ground wires.
(1) Transmission signal: refers to TXD (transmit data signal line) and RXD (receive data signal line). The format of information transmitted via TXD and received by RXD is: A transmission unit (byte) is composed of a start bit, a data bit, a parity bit, and a stop bit.
(2) Contact signal: refers to the six signals of RTS, CTS, DTR, DSR, DCD and RI, and their respective functions are: RTS (request to transmit), which is sent by the PC
Contact signal from MODEM. High level indicates that the PC requests to transfer data CTS (clear to send) to the MODEM, which is the MODEM to the PC
Contact signal from the machine. A high level indicates that the MODEM responds to the RTS signal sent by the PC and is ready to send data to the remote MODEM.
DTR (Data Terminal Ready) is a communication signal sent from the PC to the MODEM. The high-power screen indicates that the PC is in the ready state and the local modem
A communication channel can be established with the remote modem. If it is a low-power screen, the MODEM is forced to terminate the communication DSR (data device ready), yes
The communication signal sent by MODEM to the PC. It indicates the working status of the local MODEM. A high level indicates that the MODEM is not in a 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 has received the carrier signal from the remote modem.
RI (Ring Indication) is a status signal sent by the MODEM to the PC. A high level indicates that the local MODEM has received the ringing signal from the remote MODEM.
(3) Ground signal (GND), which provides 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. The reason why its transmission rate can be higher than the limit rate of 33.6Kbps on traditional telephone lines is because it uses a modulation and demodulation technology that is completely different from 33.6K. The principle and use requirements of the workpiece are also different from the 33.6 K high-speed modem.
The telephone line can make the two parties communicate with each other thousands of kilometers away from each other, because the relay amplifier equipment is provided at a certain distance to ensure clear voice. If you configure Modem on these devices, you can transfer data where you can talk to the phone. The voice bandwidth of a general telephone line is in the range of 300 to 3400 Hz. To use it to transmit digital signals, the signal frequency must also be in this range. There are three commonly used modulation methods:
Frequency shift keying (FSK), phase shift keying (PSK), phase amplitude modulation (PAM).
There are also three modulation methods for digital-to-analog conversion: (1) Frequency shift keying (FSK). Use special audio range to distinguish between sending data and receiving data. For example, the dedicated frequency specified by the binary logic of FM ModemBell-103 to send and receive data is: send, signal logic 0, frequency 1070Hz,
Signal logic 1, frequency 1270Hz; receive, signal logic 0, frequency 20 25Hz, signal logic 1, frequency 2225Hz. (2) Phase shift keying (PSK),
High-speed Modem commonly uses four-phase system, eight-phase system, and four-phase system uses four different phases to represent the four binary numbers 00, 01, 10, 11
Such as phase modulation ModemBell-212A type. This technology can make 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 higher, and the eight-phase system is more complicated. (3) Phase amplitude modulation (PAM),
In order to increase the transmission rate as much as possible without increasing the modulation rate, a combination of phase modulation and amplitude modulation is used. It can use 16 different phase and amplitude levels to enable 1200bps Modem to transmit 19200bps data signals.
After the modem is powered on, it usually enters the on-hook mode, dials the line through the phone and enters the call mode, and finally through the Modem
The "handshake" process enters the online mode.
The connection between the modem and the computer is the interface problem between the data circuit communication equipment 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 always includes DCE and DTE, so it is important to determine a unified standard interface, especially for public data networks. The data terminal equipment DTE is a data source that generates digital signals or a data sink that receives digital signals, or a combination of the two.
Such as computer terminals, printers, fax machines, etc. is DTE. The equipment that connects the data terminal equipment DTE to the analog channel is called the data circuit communication equipment DCE, like Modem is DCE. The connection standard between DTE and DCE is CCITTV.10 / X.26, and EIARS-423-A
Compatible, it is a kind of semi-balanced electrical characteristic interface. Normal Modem is usually connected to the computer through the RS-232C serial port signal line. (RS is the abbreviation of Recommended Standard), which was recognized by international organizations in 1969. The definition of RS-232 includes electrical characteristics (such as voltage value), mechanical characteristics (such as connector shape) and functional characteristics (such as pin signal). It allows a sending device to be connected to a receiving device to transfer data; the maximum transmission speed of its original specification is
20Kbps, but in fact, the current application is far beyond this speed range. RS-232 can be said to be a fairly simple communication standard. If hardware flow control is not used, at least three signal lines are needed to achieve full-duplex transmission. The electrical characteristics of RS-232 belong to the unbalanced transmission mode, and the anti-interference ability is weak, so the transmission distance is short, about 15 meters.
RS-232C serial port signals are divided into three categories: transmission signals, contact signals and ground wires.
(1) Transmission signal: refers to TXD (transmit data signal line) and RXD (receive data signal line). The format of information transmitted via TXD and received by RXD is: A transmission unit (byte) is composed of a start bit, a data bit, a parity bit, and a stop bit.
(2) Contact signal: refers to the six signals of RTS, CTS, DTR, DSR, DCD and RI, and their respective functions are: RTS (request to transmit), which is sent by the PC
Contact signal from MODEM. High level indicates that the PC requests to transfer data CTS (clear to send) to the MODEM, which is the MODEM to the PC
Contact signal from the machine. A high level indicates that the MODEM responds to the RTS signal sent by the PC and is ready to send data to the remote MODEM.
DTR (Data Terminal Ready) is a communication signal sent from the PC to the MODEM. The high-power screen indicates that the PC is in the ready state and the local modem
A communication channel can be established with the remote modem. If it is a low-power screen, the MODEM is forced to terminate the communication DSR (data device ready), yes
The communication signal sent by MODEM to the PC. It indicates the working status of the local MODEM. A high level indicates that the MODEM is not in a 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 has received the carrier signal from the remote modem.
RI (Ring Indication) is a status signal sent by the MODEM to the PC. A high level indicates that the local MODEM has received the ringing signal from the remote MODEM.
(3) Ground signal (GND), which provides 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. The reason why its transmission rate can be higher than the limit rate of 33.6Kbps on traditional telephone lines is because it uses a modulation and demodulation technology that is completely different from 33.6K. The principle and use requirements of the workpiece are also different from the 33.6 K high-speed modem.
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