同步与异步传输（Synchronous and asynchronous transmission）.doc

同步与异步传输（Synchronous and asynchronous transmission） (asynchronous) synchronous and asynchronous communication Asynchronous and synchronous communication Asynchronous communication is a very common mode of communication. Asynchronous communications when sending characters, the time interval between the characters sent can be arbitrary. Of course, the receiver must always be ready to receive (if the hosts power is not added, the sending character at the sending end is meaningless because the receiver cannot accept it at all). The sender can send characters start at any time, so must sign in place to start each character and ending with, namely with start and stop bits, so that the receiver can correctly receive down each character. The advantage of asynchronous communication is that the communication equipment is simple and cheap, but the transmission efficiency is low (because the ratio of the bit start and stop bits is larger). Asynchronous communications can also be units that send frames. The receiver must be ready to receive frames at any time. This is where the header of the frame must have some special bit combinations that allow the receiver to find the beginning of a frame. This is also called frame bounding. The frame delimiter also contains the end position of the determined frame. There are two ways. One is to have a special bit combination at the end of the frame to mark the end of the frame. Or a frame length field is provided in the frame header. Note that in asynchronous transmission frame, is not to say that the sender must be with start and stop bits sent again, but that for each character in the frame, the sender can send a frame at any time, and the time interval between frames can be arbitrary the. All bits in a frame are sent continuously. The sender does not have to coordinate with the receiver before sending a frame (without bit synchronization first). The time at which each character begins to transmit can be any t0 0110110 start bit end bit t.