Synchronous communication vs. asynchronous communication, what is the main difference between synchr

Correct answer: d

Synchronous communication uses a unified clock, and each component sends or receives information in a fixed bus transmission cycle, and the end of one bus transmission cycle starts the next bus transmission cycle. It is suitable for short bus lengths and close access times of individual components, so it has a high transmission rate. ABC is the right reason, so choose D.

To put it simply, synchronous communication is a kind of bit synchronous communication technology, which requires the sender and receiver to have synchronous clock signals of the same frequency and same phase.

Only a specific synchronization character is appended to the front of the transmitted message to synchronize the sender and receiver, and then the transmission and reception are transmitted and received bit by bit under the control of the synchronization clock.

In contrast to synchronous communication, when sending characters, the time slot between the sent characters can be arbitrary. However, the receiver must always be ready to receive (if the receiver host does not have a power supply, then it makes no sense for the transmitter to send characters, because the receiver cannot receive it at all). The sender can start sending characters at any time, so it is necessary to add flags at the beginning and end of each character, that is, to add the start and stop bits, so that the receiver can receive each character correctly.

The advantage of asynchronous communication is that the communication equipment is simple and inexpensive, but the transmission efficiency is lower (because the start and stop bits account for a large proportion of the overhead).

The elimination method is adopted: a is not right, synchronous communication has nothing to do with the length of the line;

b, c is right; It follows that d is not right;

Therefore, as multiple choices, it is the B and C pairs.

Synchronous communication and asynchronous communication mainly have the difference between implementation conditions, efficiency, complexity and application.

1. Differences in implementation conditions.

1. When synchronous communication is used for data transmission, the sending and receiving parties are required to maintain complete synchronization, so the receiving and sending equipment must use the same clock.

2. Asynchronous communication does not require the receiver clock and the sender clock to be synchronized, and the sender can send the next byte after sending a byte at any long interval.

Second, the difference in efficiency.

1. Synchronous communication can achieve high-speed and large-capacity data transmission. High efficiency.

2. The utilization rate of asynchronous communication channels is low (because the overhead of the start bit and stop bit accounts for a large proportion), so the efficiency is relatively low.

3. The difference in complexity.

1. Synchronous communication requires that the clock and the receiving clock are strictly synchronized, and the hardware is complex, and the allowable error of the clocks on both sides is small.

2. Asynchronous communication is simple, and the clocks of both sides can allow a certain error.

4. Differences in the applicable circumstances.

1. Synchronous communication can be used for point-to-multipoint.

2. Asynchronous communication is only applicable to point-to-point.

1. Asynchronous communication is character-oriented communication, while synchronous communication is bit-oriented communication.

2. The unit of asynchronous communication is character, while the unit of synchronous communication is zhen.

3. Asynchronous communication seizes the opportunity of resynchronization through the start and stop codes of the start and end of the character, and synchronous communication extracts synchronous information from the data.

4. Asynchronous communication has low requirements for timing, and synchronous communication often coordinates timing through specific clock lines.

5. Asynchronous communication is less efficient than synchronous communication.

The efficiency of synchronous communication is high, while the efficiency of asynchronous communication is low.

Synchronous communication is more complex, and the allowable error of the clocks of both sides is small; Asynchronous communication is simple, and the two-party clock can tolerate a certain error.

Synchronous communication can be used for point-to-multipoint, and asynchronous communication is only available for point-to-point. Synchronous communication is different from asynchronous communication.

1. Synchronous communication requires that the clock frequency of the receiver and the clock frequency of the transmitter be the same, and the transmitter sends a continuous bit stream; Asynchronous communication does not require the receiver clock to be synchronized with the sender clock, and the sender can send the next byte after sending a byte at any long interval.

2. The synchronous communication efficiency is high, and the asynchronous communication efficiency is low.

3. The synchronous communication is more complicated, and the allowable error of the clocks of both sides is small; Asynchronous communication is simple, and the two-party clock can tolerate a certain error.

4. Synchronous communication can be used for point-to-multipoint, and asynchronous communication is only suitable for point-to-point.

The format of the data used for synchronous communication.

1. Single synchronization format, which will send a frame of data and use only one synchronization character. When the receiver receives and recognizes a full synchronization character, it receives data continuously. After the data is finished in one frame, the CRC verification is performed.

Synchronisation Characters Data CRC1 CRC2;

2. Double synchronization word format, at this time, use two synchronization characters for synchronization.

Sync Character 1 Sync Character 2 Data CRC1 CRC2;

3. The data format specified in the Synchronous Data Link Control (SDC) regulations.

Identifier 01111110 Address 8-bit Data CRC1 CRC2 Identifier 01111110;

4. It is a data format used in an external synchronization mode. In this case, the data sent in one frame does not contain synchronization characters. The sync signal is added to the serial interface via a dedicated control line.

When the SYNC arrives, indicating that the data part is started, the interface continuously receives the data and CRC checksum.

Data field CRC1 CRC2;

5. Data format specified in Advanced Data Link Control (HDLC) regulations. They are both used for synchronous communication.

Identifiers 01111110 Address 8 bits Control 8 bits Data CRC1 CRC2 Identifier 01111110;

CRC (Cyclic Redundancy Checks) stands for Cyclic Redundancy Checks. It is used to check whether there is an error in the transmission process, and is one of the important means to ensure the reliability of transmission.

Serial asynchronous communication (such as RS232 communication) is a common communication method between a host and an external hardware device. It can be transmitted in both directions. For example, the data received by the satellite signal receiving version is transmitted to the computer for processing, mainly using serial asynchronous communication processing.

In asynchronous communication, there are two important indicators in asynchronous communication: character frame format and baud rate. Data is usually transmitted in character or byte frames.

Character frames are sent frame by frame by the sender and are received by the receiving device frame by frame through the transmission line. The sender and receiver can control the sending and receiving of data by their own clocks, which are independent of each other and out of sync with each other.

The receiver detects the low-level logic transmitted from the transmission line"0"(i.e., the start bit of the character frame), the sender is sure that the transmitter has started to send data, and whenever the receiver receives the stop bit in the character frame, it knows that the character in a frame has been sent.

Typical frame formats in asynchronous communication are: 1 start bit, 7 bits (or 8 bits) data bits, 1 parity bit, and 2 stop bits.

1. Different definitions.

1. Asynchronous communication is character-oriented communication.

Synchronous communication is a kind of bit synchronous communication technology, which requires that both the sender and receiver have the synchronous clock signal of the same frequency and the same phase, and only need to attach a specific synchronization character to the front of the transmitted message, so that the sender and receiver establish synchronization, and then send and receive bit by bit under the control of the synchronous clock.

2. Synchronous communication is bit-oriented communication.

Asynchronous communication means that the time interval between two characters (8 bits) in the communication is not fixed, while the time interval between each bit within a character is fixed.

Second, the way is different.

1. Asynchronous communication seizes the opportunity of resynchronization through the start and stop codes of the start and end of the character.

2. Synchronous communication is to extract synchronous information from data.

Third, the requirements are different.

1. Asynchronous communication has low requirements for timing.

2. Synchronous communication often coordinates timing through specific clock lines.