What Is Asynchronous Transmission?

Asynchronous Transmission: Asynchronous transmission divides the bits into groups for transmission. The group can be 8 characters or 1 character or longer. The sender can send these groups of bits at any time, and the receiver never knows when they will arrive. A common example is the communication of a computer keyboard with a host. Press an alphabetic key, numeric key, or special character key to send an 8-bit ASCII code. The keyboard can send codes at any time, depending on the user's input speed, and the internal hardware must be able to receive a typed character at any time.

ATM can ideally implement various QoS, which can support both connected services and connectionless services. Is a model of broadband ISDN (B-ISDN) technology. ATM is a switching technology. When transmitting data, the digital data is first cut into multiple fixed-length packets, and then transmitted using optical fiber or DS1 / DS3. When you reach your destination, regroup. ATM network can integrate sound, video and data at the same time. Provide the best transmission environment for various types of information.

Chinese name: Asynchronous transfer
Foreign name: Asynchronous Transmission Mode
Unit: character

Start bit: 0
Transmission mode: Asynchronous transfer mode
Alias: Information element relay
Field: computer network

Basic concepts of asynchronous transmission

Asynchronous transmission is a way of data transmission. Because the data is generally transmitted serially one by one, for example, when transmitting a string of character information, each character code consists of 7 binary bits. But in a series of binary bits, which 7-bit count does each 7-bit count from? When transmitting asynchronously, before transmitting each data character, first send a binary bit called the start bit. When the receiving end receives this signal, it knows that the 7 binary bits sent in succession is a character data. After this, one or two binary bits are given, which is called the end bit. After the receiver receives the end bit, it indicates the end of a data character transmission. In this way, in asynchronous transmission, each character is synchronized separately, that is, each binary bit in the character is synchronized, but the length of the gap between characters is not fixed. ^[1]

Asynchronous transmission, the English name AsynchronousTransfer Mode, ATM, is a technical basis for implementing B-ISDN. It is a fast packet switching technology based on circuit switching and packet switching. The main features of ATM are connection-oriented; using small, fixed-length units (53 bytes); eliminating link error control and flow control, etc. These measures improve transmission efficiency. The outstanding advantage of ATM is that it can provide corresponding quality of service (QOS) for each virtual connection, which can effectively support video and audio multimedia transmission, including voice, video, and data. In addition, ATM can achieve smooth and seamless LAN and WAN connection. ^[2]

Asynchronous transmission is generally based on characters. Regardless of the length of the character code used,

Asynchronous transfer When the character code is preceded by a "start" signal, its length is specified as 1 symbol, and the polarity is "0", that is, the polarity of the empty number; after the character code, a "stop" signal is added after it. The length is 1 or 2 symbols, and the polarity is "1", which is the same as the polarity of the signal. The function of adding and stopping signals is to distinguish the "characters" of serial transmission, that is, to realize serial Synchronization of transmitting and receiving code groups or characters.

Asynchronous Transmission Communication Protocol

When using asynchronous serial port to transmit a character message, there are the following conventions on the data format: the idle bit, start bit, data bit, parity bit, and stop bit are specified.

The meaning of each of them is as follows:

Start bit: A logic "0" signal is sent first, which indicates the beginning of the transmitted character.

Data bit: Immediately after the start bit. The number of data bits can be 4, 5, 6, 7, 8, etc., forming a character. Usually use ASC code. The transmission starts from the lowest bit and is located by the clock.

Parity bit: After adding this bit to the data bit, the number of bits of "1" should be even (even) or odd (odd) to verify the correctness of data transmission.

Stop bit: It is the end of a character data. Can be 1-bit, 1.5-bit, 2-bit high.

Idle bit: It is in a logic "1" state, indicating that there is no data transmission on the current line.

Baud rate: It is a pointer to measure the data transfer rate. Represents the number of binary bits transmitted per second. For example, the data transmission rate is 120 characters / second, and each character is 10 bits, and the baud rate of the data transmission is 10 × 120 = 1200 bits / second = 1200 baud.

Note: Asynchronous communication is transmitted on a character basis. After receiving the start signal, the receiving device can receive it correctly as long as it can maintain synchronization with the sending device within the transmission time of one character. The arrival of the start bit of the next character re-calibrates the synchronization (depending on the detection of the start bit to achieve self-synchronization of the clock of the sender and receiver).

Asynchronous Transmission Transmission Mode

Asynchronous Transfer Mode (ATM), also called information element relay. Asynchronous transfer mode (ATM) consists of a set of protocols in ATM reference mode. ATM uses a connection-oriented switching method, which uses cells as a unit. Each cell is 53 bytes long. The header occupies 5 bytes. A standard (ITU) implementation of cell relay, which is a switching technology that uses fixed-length packets (elements). The reason why it is called asynchronous is because the recurrence of information elements containing information from a certain user is not periodic.

ATM is a connection-oriented technology. It is a new technology specially developed to support broadband integrated service networks. It has no connection with current circuit switching. When the sender wants to communicate with the receiver, it sends a control signal through the UNI that requires a connection to be established. After the receiving end receives the control signal through the network and agrees to establish a connection, a virtual line is established. Unlike synchronous transfer mode (STM), ATM uses asynchronous time division multiplexing (statistical multiplexing). Information from different information sources is queued in a buffer. The cells in the column are output one by one to the transmission line, forming an end-to-end information flow. ATM has the following characteristics: due to the high quality of the transmission line, there is no need to perform error control segment by segment. ATM needs to establish a virtual connection to reserve network resources before communication, and maintain this connection during a call, so ATM works in a connection-oriented manner. The main function of the letterhead is to identify the service itself and its logical destination, with limited functions. The length of the letterhead is small, the delay is small, and the real-time performance is good.

ATM can ideally implement various QoS, which can support both connected services and connectionless services. Is a model of broadband ISDN (B-ISDN) technology.

The propagation speed of ATM is from 25 megabits per second to 155 megabits per second.

Asynchronous transmission structure

The ATM reference mode is divided into three layers: ATM Adaptation Layer (AAL), ATM layer, and physical layer. AAL connects higher layer protocols to the ATM layer, which is mainly responsible for the upper layer to exchange ATM cells with the ATM layer. After receiving information from the upper layer, AAL splits the data into ATM cells. After receiving information from the ATM layer, AAL must reassemble the data to form a format that the upper layer can recognize. The above process is called segmentation and reassembly ( SAR). Different AALs are used to support different types of traffic or services used on ATM networks.

The ATM layer is mainly responsible for forwarding cells from the AAL to the physical layer for easy transmission and forwarding cells from the physical layer to the AAL for its use in the end system. The ATM layer can determine where incoming cells should be forwarded; reset the corresponding connection identifier and forward the cells to the next link, buffer the cells, and handle various traffic management functions, such as cell loss priority marking, congestion Annotation and universal flow control access. In addition, the ATM layer is responsible for monitoring the transmission rate and obeying the service agreement (traffic policy).

The physical layer of ATM defines bit timing and other characteristics, and encodes and decodes data into the appropriate radio or light wave form for transmission and reception on a specific physical medium. In addition, it also provides frame adaptation functions, including cell description, header error check (HEC) generation and processing, performance monitoring, and load rate matching for different transmission formats. The physical layer usually uses SONET, DS3, optical fiber, twisted pair, etc.

How asynchronous transmission works

The main advantage of asynchronous transmission mode is its ability to propagate sound, data, graphics, and video images at speeds of up to 2 Gigabits per second. It allows network managers to dynamically reorganize the LAN when workstations require changes. Currently, the principle of LAN segmentation is that a workstation is geographically close to its LAN server. ATM will allow network managers to establish a logical rather than a physical segmentation. An ATM switch will allow you to build a logical network that is completely independent of the physical structure of the network.

Asynchronous transmission mode provides any point-to-point connection between two peers, ensuring full network bandwidth between the two points-45 megabits per second or 155 megabits (two interface speeds specified in the draft standard). Because ATM is media-independent, it can operate over a range of speeds.

Potential issues with asynchronous transfers

A potential problem with asynchronous transmission is that the receiver does not know when the data will arrive. Before it detects the data and responds, the first bit has passed. It's like someone unexpectedly came up to speak to you from behind, and you didn't have time to react and missed the first few words. Therefore, each asynchronously transmitted message starts with a start bit, which informs the receiver that the data has arrived, which gives the receiver time to respond, receive, and buffer the data bits; at the end of the transmission, a stop bit indicates Termination of this transmission. By convention, an idle (no data transmission) line actually carries a signal representing a binary one. The start bit of asynchronous transmission makes the signal 0, and the other bits make the signal change with the transmitted data information. Finally, the stop bit changes the signal back to 1, and the signal remains on until the next start bit is reached. For example, the number "1" on the keyboard, according to the 8-bit extended ASCII encoding, will send "00110001". At the same time, you need to add a start bit before the 8-bit bit and a stop bit after it.

The realization of asynchronous transmission is relatively easy. Because "synchronous" information is added to each message, the timing drift does not produce a large accumulation, but it generates a lot of overhead. In the above example, two more bits are transmitted every eight bits, and the total transmission load increases by 25%. For low-speed devices with a small amount of data transmission, the problem is not great, but for those high-speed devices with a large amount of data transmission, the 25% load increase is quite serious. Therefore, asynchronous transmission is often used for low-speed devices.

Asynchronous transfer related differences

In the synchronous transmission mode, the clocks of the sender and receiver are unified, and the transmission between characters is synchronized without interval.

Asynchronous transmission does not require the clocks of the sender and receiver to be exactly the same, and the transmission between characters is asynchronous.

Differences in asynchronous transmission

1, asynchronous transmission is character-oriented transmission, while synchronous transmission is bit-oriented transmission.

2. The unit of asynchronous transmission is character and the unit of synchronous transmission is frame.

3. Asynchronous transmission uses the character start and stop codes to seize the opportunity of resynchronization, while synchronous transmission is to extract synchronous information from the data.

4, asynchronous transmission has lower requirements on timing, and synchronous transmission often coordinates timing through specific clock lines.

5. Asynchronous transmission is less efficient than synchronous transmission.

Simple description of asynchronous transmission

Synchronous transmission means that the function that is called does not return if the data is not acknowledged by the other party.

When receiving, if the other party does not send data, your thread waits until there is data and returns, you can continue to execute other instructions

Asynchronous transmission is that you call a function to send data and return immediately, you can continue to do other things,

When receiving, the other party has data, you will receive a message, or your related receiving function will be called.

Asynchronous transfer image description

Asynchronous transmission: you transmit, I'll do my thing, tell me when the transmission is over

Synchronous transmission: You transmit now, I want to see your transmission is complete before I do anything else

Asynchronous transmission error correction method

All transmission media are susceptible to interference and problems introduced by the medium itself, such as resistance and signal attenuation. External interference can be caused by background noise, atmospheric radiation, machinery or even malfunctioning equipment. The number of bits affected by interference increases with the increase in transmission rate, because more bits are involved in the time frame of the interference. To correct these problems, error detection and correction methods are used.

In parity, the number of 1s in each group must always be the same (regardless of odd or even) to indicate that a group of bits is transmitted without error. The character-by-character check is called VRC (Vertical Redundancy Check). Block-by-block checking is called LRC (Vertical Redundancy Check). Before the transmission can begin, the parity methods of the two systems must agree. Even parity (the number of 1s must be even), odd parity (the number of 1s must be odd), empty parity (the parity is always 0), and parity parity (the parity is always 1).

New modems provide advanced error detection and correction methods that are more practical and effective than those discussed above.

Asynchronous communication

Asynchronous communication refers to data transmission between two devices that are not synchronized with each other through a timing mechanism or other technology. In asynchronous communication, the time interval between two characters is not fixed, and the time interval between each character in a character is fixed. Basically, the sender can transmit data at any time, and the receiver must be ready to receive when the information arrives. In contrast, isochronous transmission is a precisely synchronized bit stream, where the beginning of a character is located by a timing mechanism.

In mainframe / terminal environments that make heavy use of asynchronous and synchronous transmission, asynchronous transmission is used to transfer characters from the terminal where the user presses the keys periodically. The receiving system knows to wait for the next key press, even if it takes more time. Instead, isochronous transmissions are used as data links between large systems that regularly transmit large amounts of information. The protocol is optimized to take advantage of slow links on public telephone systems, so extraneous bits are removed from the transmission and the clock is used to separate characters.

In asynchronous communication, a character is encoded as a bit string and consists of a start bit, a data bit, a parity, and a stop bit. This type of information that starts with a start bit and ends with a stop bit is called a frame. Parity bits are sometimes used for error detection and correction. The "start-stop" mode of the transmission means that for each new character, the transmission restarts from the beginning, eliminating any timing differences that may have occurred during the last transmission. When differences do occur, error detection and correction mechanisms can request retransmissions.

When transmitting a character, it starts with a low-level start bit, and then transmits data bits. The number of data bits is 5-8. When transmitting, the data is transmitted in the order of the lowest bit first and the highest bit after. The parity bit is used to check the correctness of the data transmission. It may or may not be specified by the program. The last stop bit transmitted is a high level. The stop bit can be 1, 1.5 or 2 bits. The idle bits between the end of the stop bit and the start of the next character should be filled with a high level 2 (as long as the next character is not sent, the idle bit is always on the line).

The typical frame format in asynchronous communication is: 1 start bit, 7 (or 8) data bits, 1 parity bit, 2 stop bits.

In asynchronous communication, each time a character is received, the receiver must resynchronize with the sender, so the synchronous clock signal at the receiver does not need to be strictly synchronized with the sender, as long as they can stay synchronized within the transmission time of a character That is, this means that the requirements for clock signal drift are much lower than the synchronous signal, and the hardware cost is much lower, but transmitting a character asynchronously requires about 20% additional information bits, so the transmission efficiency is relatively low. Asynchronous communication is simple, reliable, and easy to implement, so it is widely used in various microcomputer systems.

A channel is a single communication path between two communication devices and is created by a physical connection or multiplexing technology. A circuit is an actual physical connection that provides a communication channel. The dial telephone system provides circuitry for the communication channel between the two systems. A simplex circuit is a unidirectional transmission path that transmits signals in a single direction. A half-duplex circuit is a transmission path that provides transmission in both directions, but only one direction at a time. A full-duplex link is a bidirectional transmission path capable of bidirectional transmission on two circuits simultaneously.

Asynchronous Transmission Interface Standard

The connection for asynchronous communication is defined in the physical layer of the OSI (Open Systems Interconnection) reference model. This layer defines specifications related to connector types, pinouts, and electrical signals. Standards such as RS-232, RS-449, CCITT V.24, etc. define these interfaces for various requirements.

Various standards are defined to ensure that connected devices can communicate with each other. The EIA (Electronic Industry Association) has set standards for the transmission of asynchronous information between computer equipment over copper wires. The EIA RS-232-C standard is a serial physical interface standard. RS is the abbreviation of "Recommended Standard" in English, 232 is the identification number, and C is the number of modifications. The RS-232-C bus standard has 25 signal lines, including a main channel and an auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines can be used, such as a send line, a receive line, and a ground line. RS-232-C defines the physical connection, signal voltage and timing, error checking and other functions, as well as the serial transmission of bit streams over a single line. In contrast, parallel transmission involves sending multiple bits simultaneously on multiple lines of the same cable, similar to a multi-lane highway.

The data transmission rate specified by the RS-232-C standard is 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud.

The EIA RS-232-C standard supports short-distance transmission. For example, use it to connect a computer to a modem. If the cable becomes too long, the current will decrease and the receiver may not be able to read it. The recommended maximum length of an RS-232 cable is 50 feet and the maximum signal rate is 20kbps. To connect internal systems over longer distances, set up a LAN. To connect to systems outside your building, use a modem and telephone system or other services provided by local and long-distance carriers.