Synchronous and Asynchronous Transmission:

Another method of setting of terminals denotes synchronous or asynchronous transmission. Many terminals can only communicate in one or the other mode. Asynchronous transmission is also known as "start-stop" transmission. This means that every transmitted character (or group of characters) is defined by a special character at the beginning and end denoting 'beginning' and tend' of that character.. Thus, the receiving terminal does not have to operate "in phase" with each other and transmission is interpreted or decoded at the receiving terminal exactly as it was encoded by the sender In other words, in synchronous communication data is sent from one machine to another in a continuous stream, the sending and receiving equipment must be in step with each other throughout the transmission. This type of transmission is used for high speed data transmission, generally between two mainframes. 

Asynchronous form of communication is used by most information systems attached to the public telephone network.