On the communication technology of embedded modem

On the communication technology of embedded modem

Abstract: due to the rapid development of multimedia technology, ordinary modems can no longer adapt to the current network data transmission, but when transmitting small data, embedded modems have been widely used. During communication, embedded modem products can realize data transmission like ordinary modems, provide parallel interfaces and serial interfaces, and provide synchronous and asynchronous data transmission formats. This paper mainly expounds the communication technology of embedded modem

Keywords: embedded modem; Data transmission; Protocol

nowadays, various embedded modems produced on the market can realize the data transmission function through the public switched network (PSTN), including standard serial and parallel interfaces, synchronous and asynchronous communication data transmission formats, support a variety of modem standard protocols V.90, V.34, v.32bis, V.22bis, etc., support error correction protocols such as v.42, MNP, and data compression protocols such as v.42bis, mnp5, etc

I. communication technology

communication refers to the data transmission between computers or peripheral devices. Therefore, the letter here is a kind of information, which is a data or a batch of data composed of numbers 1 and 0 with certain rules and reflecting the determined information. Data communication involves the transmission of data between two devices. The commonly used data communication methods include parallel communication and serial communication. When the distance is close and the transmission rate is required to be high, parallel communication is usually used. When the equipment is far away, the data is often transmitted in serial mode

(I) parallel communication and serial communication

parallel communication is relatively simple and can be divided into parallel communication with different bits (width), such as 8-bit parallel communication, 16 bit parallel communication, etc. In parallel data transmission, 8 data bits of 8-bit parallel communication are transmitted from one device to another at the same time, and the transmitting device transmits 8 data bits to the receiving device through 8 data lines. After receiving these data, the receiving equipment can be used directly without any change. The characteristic of parallel communication is that each data is transmitted or received at the same time

(II) serial data transmission mode

in serial data transmission, there is only one bit of data transmitted from the source to the destination each time. Compared with parallel data transmission, which transmits several bits of data at the same time, the transmission speed of serial data transmission is slower than that of parallel transmission. In serial data transmission, bits are sent from the source to the destination one by one, which requires synchronization between the data source and the data destination to distinguish bits, characters and messages. The data link will control the synchronization between the two stations. It requires that necessary additional information be added when bits, characters or messages are sent from one station to another. These information will synchronize the hardware clocks in the receiving station and the sending station, so as to ensure that the signals sent by the source are correctly recognized by the destination. There are two data transmission modes in serial data communication, namely asynchronous serial data transmission and synchronous serial data transmission

asynchronous communication: the data format transmitted by asynchronous communication (also known as serial frame) consists of 1 start bit, 5, 6, 7 or 8 data bits, 1, 1.5, 2 stop bits and 1 check bit

the start bit convention is 0, and the free bit convention is 1

the essence of asynchronous communication is that both sides of nail and B communication adopt independent clocks. Each data starts with the start bit and ends with the stop bit. The start bit triggers the synchronous clocks of a and B. One bit in each asynchronous serial frame is strictly synchronized with each other, and the bit period is the same

asynchronous communication relies on the start bit and stop bit to maintain communication synchronization. It has low hardware requirements and is relatively simple and flexible to implement

synchronous communication: the data format transmitted by synchronous communication (also known as synchronous serial frame) is composed of multiple data, and each frame has two (or one) synchronous characters as the starting bit to trigger the synchronous clock to start sending or receiving data. The idle bit needs to send synchronous characters

synchronous communication relies on synchronous characters to keep communication synchronized. There is no need to insert synchronization characters between the data in the data group, and there is no gap, so the transmission speed is fast, but an accurate clock is required to realize the strict synchronization between the sender and the receiver. The hardware requirements are high, and it is suitable for transmitting batch data

II. Working principle of embedded modem

modem is composed of sending, receiving, control, interface, control panel and power supply. The data terminal equipment provides the transmitted data in the form of binary serial signal, which is converted into internal logic level through the interface and sent to the transmission part, and then modulated into the signal required by the line through the modulation circuit and sent to the line. The receiving part receives the signal from the line, which is restored into a digital signal after filtering, inverse modulation and level conversion and sent to the digital terminal equipment

line enables both sides of the communication to talk to each other at a distance of thousands of kilometers, because relay amplification equipment is set at a certain distance to ensure clear voice. If modems are configured on these devices, data can be transmitted where they can be connected. Generally, the voice bandwidth of the line is in the range of 300 ~ 3400Hz. When it is used to transmit digital signals, the signal frequency must also be in this range. Common modulation methods include: frequency shift keying (FSK), phase shift keying (PSK, DPSK), amplitude modulation (PAM, QAM), pulse code modulation (PCM), etc

modem usually has three working modes: hang up mode, call mode and online mode. If the line is not connected, hang up; The way of communication is that both parties talk through each other; The modem has been connected, and the data transmission is online

after the modem is powered on, it usually enters the hang up mode first, dials the line through dialing, then enters the call mode, and finally enters the online mode through the handshake process of the modem

the connection between modem and computer is the interface problem between data circuit terminal equipment (DCE) and data terminal equipment (DTE). The interface between DCE and DTE is an important problem that the computer network makes the sample slip in the process of experiment

(I) DTE and DCE

DTE (dataterminalequipment) are devices with certain data processing capacity and the ability to send and receive data. DTE can be a computer or terminal, or various i/o devices. Most data processing terminal devices have limited data transmission capacity. If two DTE devices far away are directly connected, they often cannot communicate. An intermediate device called data circuit terminal equipment (DCE) must be added between DTE and transmission line. The role of DCE is to provide the function of signal transformation and coding between DTE and transmission line, and is responsible for establishing, maintaining and releasing the connection of data link. A typical DCE is a modem connected to an analog line. Digital devices connect to the network through modems to communicate, which uses analog signals to transmit digital data

(II) RS-232C serial port

embedded modems are usually connected to computers through RS-232C serial port signal line. RS-232 allows a sending device to connect to a receiving device to transmit data; The maximum transmission speed of its original specification is 20kbps, but in fact, the current application has long exceeded this speed range. RS-232 is a fairly simple communication standard. If hardware flow control is not used, full duplex transmission can be achieved by using only three signal lines at most

rs-232c serial port signals are divided into three categories: transmission signal, contact signal and ground wire

1. Transmission signal: refers to TXD (transmission data signal line) and RXD (reception data signal line)

2. Contact signal: refers to RTS, CTS, DTR, DSR, DCD and RI. Their functions are:

RTS (request transmission), which is the contact signal sent by PC to modem

cts (clear send) is the contact signal sent by the modem to the PC

dtr (data terminal ready) is the contact signal sent by PC to modem

dsr (data ready) is the contact signal sent by the modem to the PC. It indicates the working state of the local modem

dcd (transmission detection) is the status signal sent by the modem to the PC

ri (ringing indication) is the status signal sent by the modem to the PC

3. Ground wire signal (GND) provides the same potential reference point for the connected host and modem

III. modulation and protocol standards

in the field of communication, protocol refers to a set of common technical rules or specifications that communication parties should abide by. If this set of rules or regulations can ensure that 3D printing and additive manufacturing achieve the best mechanical performance and are accepted by more users, it can be called standard

the most basic function of modem is modulation and demodulation, and a series of technical standards have been developed in recent years; In addition, in order to improve the transmission speed of modem products, most of them also introduce compression and error correction technology

(I) standard modem protocol

the basic function of the modem is to convert the binary digital signal provided by the computer and the supported analog signal, so that the computer can use it for long-distance data communication. The core of modulation and demodulation technology is how to improve the transmission speed of digital information in the channel with limited bandwidth (4kHz). This speed is often measured by bit rate, that is, bitspersecond (abbreviated as BPS) transmitted per second

the bell103, the earliest modem, was launched by at T company in 1958. It uses a simple FM technology FSK (frequencyshiftkeying) to further expand the cooperation achievements, and only provides a transmission speed of 300BPS. CCITT issued a similar technical standard V.21 according to bell103

in the 1970s, at t's bell212 adopted 4 DPSK technology combining amplitude modulation and modulation to achieve a transmission speed of 1200BPS. A similar standard of CCITT is called v.22. Bell103 (V.21) and bell212 (v.22) are rarely used now, but in order to be compatible with early modems, many modems still integrate these two technologies into products as options

CCTV in the mid-1980s 22bis standard is adopted by most modem manufacturers. Its 16-QAM (quadrature modulation with 12 phase angles and 4 amplitude modulation phases) modulation technology achieves a transmission speed of 2400bps. Next, CCITT issued the V.32 standard that can realize 9600bps speed by using 32-tcm (grid code modulation) technology. CCITT issued the v.32bis standard in 1991. 5. 32bis adopts 128 TCM modulation technology, which can achieve a maximum transmission speed of 14400bps, and can work at four speeds of 12000bps, 9600bps, 7200bps and 4800bps according to the change of line quality. In 1993, CCITT launched the V.34 standard, which can achieve the maximum speed of 28800bps. This standard can also be 28.8k/26.4k/21.6k/19.2k/16.8k/14.4k/12k/9600/7200/4800bps