Synchronous & Asynchronous transmission:
Synchronous is any type of communication in which the parties communicating are "live" or present in the same space and time. A chat room where both parties must be at their computer, connected to the Internet, and using software to communicate in the chat room protocols is a synchronous method of communication. E-mail is an example of an asynchronous mode of communication where one party can send a note to another person and the recipient need not be online to receive the e-mail. Synchronous mode of transmissions are illustrated in figure 3.11
The two ends of a link are synchronized, by carrying the transmitter’s clock information along with data. Bytes are transmitted continuously, if there are gaps then inserts idle bytes as padding
· This reduces overhead bits
· It overcomes the two main deficiencies of the asynchronous method, that of inefficiency and lack of error detection.
· For correct operation the receiver must start to sample the line at the correct instant
· Used in high speed transmission example: HDLC
Asynchronous refers to processes that proceed independently of each other until one process needs to "interrupt" the other process with a request. Using the client- server model, the server handles many asynchronous requests from its many clients. The client is often able to proceed with other work or must wait on the service requested from the server.
Asynchronous mode of transmissions is illustrated in figure 3.12. Here a Start and Stop signal is necessary before and after the character. Start signal is of same length as information bit. Stop signal is usually 1, 1.5 or 2 times the length of the information signal
· The character is self contained & Transmitter and receiver need not be synchronized
· Transmitting and receiving clocks are independent of each other
· Overhead of start and stop bits
· False recognition of these bits due to noise on the channel
· If channel is reliable, then suitable for high speed else low speed transmission
· Most common use is in the ASCII terminals