Synchronous? Asynchronous?

Answer-:

1. Asynchronous transfer
Typically, an asynchronous transfer is a transmission unit, with each character appended with a 1-bit starting bit and a 1-bit stop bit to mark the start and end of a character and to synchronize the data transfer in this implementation. The so-called asynchronous transfer refers to the time interval between a character and a character (beginning with the end of a character to the next character) that is variable, and does not need to strictly limit their time relationship. the starting bit corresponds to the binary value 0, which is expressed as a low level and occupies a 1-bit width. The stop bit corresponds to a binary value of 1, expressed as a high level, occupying a bit width. A character occupies the 5~8 bit, depending on the character set used by the data. For example, the Telegraph code character is 5 bits, the ASCII character is 7 bits, the Chinese character code is 8 bits. Additionally, a 1-bit parity bit is attached, and you can choose odd or even parity to implement simple error control for the character. In addition to the same data format (the number of bits of the character, the number of bits in the stop bit, the check digit and the check mode, etc.), the transmitting end and the receiving end should adopt the same transmission rate. Typical rates are: 9 b/S, 19.2kb/s, 56kb/s and so on.
Asynchronous transmission, also known as the end-to-end asynchronous communication, has the advantage of being simple and reliable and suitable for character-oriented and low-speed asynchronous communication situations. For example, the communication between a computer and a modem is this way. Its disadvantage is that the communication overhead, each transmission of a character to be additional two or three bits, communication efficiency is low. For example, when using a modem to surf the Internet, the general sense of speed is very slow, in addition to low transmission rate, and communication overhead, low communication efficiency is closely related.
2. Synchronous transmission
Typically, a synchronous transfer is a data block as a transmission unit. The head and tail of each block are appended with a special character or sequence of bits, marking the beginning and end of a block of data, and typically attaching a checksum sequence (such as a 16-bit or 32-bit CRC checksum) to control the data block in error. The so-called synchronous transmission refers to the time interval between the data block and the data block is fixed, and must strictly stipulate their time relationship .

Answer two:

Synchronization means that after the sender sends out the data, the receiver sends back the response before sending the next packet to the communication mode.
Asynchronous means that after the sender sends out the data, the receiver sends back the response, and then the next packet is communicated.

Synchronous? Asynchronous?