Currently, various embedded modems on the market can transmit data via the Public Switched Telephone Network (PSTN), including standard serial and parallel interfaces, synchronous and asynchronous communication data transmission formats, and support for multiple modem standard protocols—V.92, V.90, V.34, V.32bis, V.22bis, etc.—and error correction protocols such as V.44, V.42, and MNP, as well as data compression protocols such as V.42bis and MNP5. I. Communication Technology Communication refers to the transmission of data between computers or peripheral devices. Therefore, the "message" here is a type of information, consisting of numbers "1" and "0" that follow certain rules and reflect specific information—either a single data item or a batch of data. Data communication involves the transmission of data between two devices. Commonly used data communication methods include parallel communication and serial communication. When the distance is short and a high transmission rate is required, parallel communication is usually used. When the devices are far apart, data is often transmitted serially. (I) Parallel Communication and Serial Communication Parallel communication is relatively simple and can be divided into parallel communication with different bit widths, such as 8-bit parallel communication, 16-bit parallel communication, etc. In parallel data transmission, 8-bit parallel communication transmits 8 data bits simultaneously from one device to another. The transmitting device transmits the 8 data bits to the receiving device through 8 data lines. After receiving these data, the receiving device can use them directly without any modification. The characteristic of parallel communication is that each bit of data is transmitted or received simultaneously. Serial communication transmits data bit by bit, and therefore, under the same conditions, its transmission speed is slower than that of parallel communication. However, in practical applications, serial data transmission is often chosen because serial communication requires at most two wires to send or receive data, one for sending and the other for receiving. Depending on the different operating modes of serial communication, the transmitting and receiving lines can also be combined into one, becoming a transmit/receive multiplexed line (such as half-duplex). The hardware for implementing serial data transmission is economical and practical. (II) Serial Data Transmission In serial data transmission, only one bit of data is transmitted from the source to the destination at a time. Compared to parallel data transmission, which transmits several bits of data simultaneously, serial data transmission is slower. In serial data transmission, each bit is sent sequentially from the source to the destination. This requires synchronization between the data source and the data destination to distinguish between bits, characters, and messages. The data link control achieves 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. This information synchronizes the hardware clocks in the receiving and sending stations, ensuring that the signal sent from the source is correctly recognized by the destination. There are two data transmission modes in serial data communication: asynchronous serial data transmission and synchronous serial data transmission. Asynchronous Communication: The data format transmitted in asynchronous communication (also called a serial frame) consists of one start bit, 5, 6, 7, or 8 data bits, 1, 1.5, or 2 stop bits, and one parity bit. The start bit is conventionally set to 0, and the idle bit is conventionally set to 1. Asynchronous communication essentially means that A and B use independent clocks. Each data segment begins with a start bit and ends with a stop bit. The start bit triggers the synchronization clocks of both A and B. One bit in each asynchronous serial frame is strictly synchronized with each other, and the bit periods are the same. Asynchronous communication relies on start and stop bits to maintain communication synchronization, has lower hardware requirements, and is relatively simple and flexible to implement. Synchronous communication: The data format transmitted in synchronous communication (also called synchronous frames) consists of multiple data segments. Each frame has two (or one) synchronization characters as start bits to trigger the synchronization clock to begin sending or receiving data. Idle bits need to send synchronization characters. Synchronous communication relies on synchronization characters to maintain communication synchronization. No synchronization characters need to be inserted between data segments within a data group, resulting in no gaps and thus faster transmission speeds. However, it requires an accurate clock to achieve strict synchronization between the sender and receiver, placing higher demands on hardware. It is suitable for transmitting batches of data. II. Working Principle of Embedded Modems A modem consists of transmitting, receiving, control, interface, control panel, and power supply components. The data terminal equipment provides data to be transmitted in binary serial signal form. This data is converted to internal logic levels via the interface and sent to the transmitting section. The modulation circuit then modulates the data into the signal required by the line and transmits it to the line. The receiving section receives signals from the line, filters, demodulates, and converts them to digital signals before sending them to the digital terminal equipment. Telephone lines allow communication between parties thousands of kilometers apart because repeater amplification devices are installed at regular intervals to ensure clear voice. If a modem is configured on these devices, data transmission can occur wherever telephone service is available. The voice bandwidth of a typical telephone line is in the range of 300–3400 Hz; the signal frequency for transmitting digital signals must also be within this range. Common modulation methods include Frequency Shift Keying (FSK), Phase Shift Keying (PSK, DPSK), Amplitude Modulation (PAM, QAM), and Pulse Code Modulation (PCM). Modems typically have three operating modes: on-hook mode, call mode, and online mode. On-hook mode occurs when the telephone line is not connected; call mode occurs when both parties are talking on the phone; and online mode occurs when the modem is connected and data is being transmitted. After a modem is powered on, it typically enters on-hook mode first, then enters call mode after dialing a number, and finally enters online mode through the modem's "handshake" process. The connection between a modem and a computer involves the interface between a Data Circuit-terminating Equipment (DCE) and a Data Terminal Equipment (DTE). The interface between DCE and DTE is a crucial aspect of computer network usage. (I) DTE and DCE A DTE (Data Terminal Equipment) is a device with certain data processing capabilities and the ability to send and receive data. A DTE can be a computer, a terminal, or various I/O devices. Most data processing terminal devices have limited data transmission capabilities. If two DTE devices located far apart are directly connected, communication is often impossible. An intermediate device called a Data Circuit-terminal Equipment (DCE) must be added between the DTE and the transmission line. The DCE provides signal conversion and encoding functions between the DTE and the transmission line, and is responsible for establishing, maintaining, and releasing data link connections. A typical DCE is a modem connected to an analog telephone line. Digital devices communicate via a modem to access the telephone network by transmitting digital data using analog signals. (II) RS-232C Serial Port Embedded modems are usually connected to computers via RS-232C serial port signal lines. RS-232 allows a transmitting device to connect to a receiving device to transmit data; its original specification had a maximum transmission speed of 20Kbps, but in reality, current applications far exceed this speed range. RS-232 can be said to be a fairly simple communication standard. Without hardware flow control, it can achieve full-duplex transmission using at most three signal lines. RS-232C serial port signals are divided into three categories: transmit signals, handshake signals, and ground. 1. Transmit signals: refer to TXD (transmit data signal line) and RXD (receive data signal line). 2. Handshake signals: refer to six signals: RTS, CTS, DTR, DSR, DCD, and RI. Their functions are as follows: RTS (Request to transmit) is a handshake signal sent by the PC to the modem. CTS (Clear to transmit) is a handshake signal sent by the modem to the PC. DTR (Data Terminal Ready) is a communication signal sent from the PC to the MODEM. DSR (Data Ready) is a communication signal sent from the MODEM to the PC. It indicates the working status of the local MODEM. DCD (Transmission Detection) is a status signal sent from the MODEM to the PC. RI (Ring Indicator) is a status signal sent from the MODEM to the PC. 3. Ground signal (GND) provides the same potential reference point for the connected host and MODEM. III. Modulation and Protocol Standards In the field of communications, a protocol refers to a set of common technical rules or specifications that both communicating parties should follow. If these rules or specifications are accepted by a large number of users, they can be called a standard. The most basic function of a MODEM is modulation and demodulation, and a series of technical standards have been developed in recent years; in addition, most modern MODEM products also incorporate compression and error correction technologies to improve transmission speed. (I) Standard MODEM Protocol The basic function of a MODEM is to convert between binary digital signals provided by a computer and analog signals supported by the telephone network, enabling the computer to use the telephone network for long-distance data communication. The core of modulation and demodulation technology is how to increase the transmission speed of digital information in telephone channels with limited bandwidth (≤4kHz). This speed is often measured in bit rate, that is, bits per second (bps). The earliest modem, the Bell 103, was launched by AT&T in 1958. It used a simple frequency modulation technology, FSK (Frequency Shift Keying), and only provided a transmission speed of 300bps. CCITT issued a similar technical standard, V.21, based on the Bell 103. In the 1970s, AT&T's Bell 212 used a 4-DPSK technology that combined amplitude modulation and frequency modulation, achieving a transmission speed of 1200bps. A similar standard from CCITT was called V.22. Bell 103 (V.21) and Bell 212 (V.22) are rarely used now, but to ensure compatibility with earlier modems, many modems still integrate these two technologies as an option. In the mid-1980s, the CCITT V.22 bis standard was adopted by most modem manufacturers. Its 16-QAM (quadrature modulation with 12 phase angles and 4 amplitude phases) modulation technology achieved a transmission speed of 2400bps. Subsequently, CCITT issued the V.32 standard, which used 32-TCM (grid-coded modulation) technology and could achieve a speed of 9600bps. CCITT issued the V.32bis standard in 1991. V.32bis uses 128-TCM modulation technology, achieving a maximum transmission speed of 14400bps, and can downgrade to four speed ranges—12000bps, 9600bps, 7200bps, and 4800bps—depending on line quality. In 1993, CCITT launched the V.34 standard, which could achieve a maximum speed of 28800bps. This standard could also operate at multiple speed levels, such as 28.8K/26.4K/21.6K/19.2K/16.8K/14.4K/12K/9600/7200/4800bps. V.90 is a 56Kbps data transmission standard developed by ITU-T. V.90 enables modems to receive data at rates up to 56Kbps on the PSTN. V.90 connection technology uses a bidirectional channel: an uplink channel and a downlink channel. The downlink (receive) channel of the V.90 client modem can achieve a higher transmission speed of 56K. The V.90 standard can support an uplink rate of 33.6Kbps. (II) Protocol Standards for Compression and Error Correction Technologies In order to further improve the data transmission speed of modems, in addition to the continuous improvement of modulation and demodulation technologies mentioned above, data compression technology has also been introduced into modems in recent years. Error correction technology was introduced along with the adoption of compression technology. Microcom's error correction and compression protocol, often abbreviated as MNP (Microcom Network Protocol), consists of a series of independent error correction and compression protocols. MNP1–MNP4 and MNP10 are error correction protocols, while MNP5 and MNP7 are compression protocols. MNP has become the industry standard for compression and error correction technology. In 1988, the CCITT issued the V.42 error correction standard. V.42 includes MNP4 as an option. If one modem supports V.42 and the other supports MNP4, they can automatically negotiate and execute MNP4 error correction processing. Regarding data compression protocols, MNP5 and V.42bis are the most popular. V.42bis, issued by CCITT in 1989, is a more efficient compression protocol. Because V.42bis has an automatic testing function, it can automatically switch between compression mode and transparent mode (no compression) through online testing, making it more adaptable than MNP5. Compression and error correction technologies are closely related. If the V.42bis compression protocol is selected, the modem will automatically enable the V.42 error correction protocol; conversely, if MNP5 compression is used, MNP error correction will naturally be employed. IV. Conclusion Embedded modems possess all the functions of traditional modems, utilize telephone lines (PSTN) to solve data transmission problems, and are small in size, highly reliable, flexible, and convenient, making them ideal for communication between terminal devices with relatively low communication volumes. They have broad application prospects and a large market potential in areas such as power distribution automation, remote meter reading, tax-controlled POS machines, and bank-tax POS machines.