There are two main types of digital data transmission: serial and parallel. Serial transfer transmits data one bit at a time through a single line. Parallel transfer transmits more than one bit of data simultaneously using separate lines. Serial transmission can be either asynchronous
or synchronous. In asynchronous transmission, each byte is identified with special start and stop bits. This has become the standard for personal computers. Synchronous transmission requires all the data to be sent at the same rate. It is faster and more efficient than asynchronous transmission, and is used on larger computer systems.
The direction of data flow is either simplex, half duplex or full duplex.
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Simplex mode allows transmission in one direction only, from the sender to the receiver. It is limited and is not widely used in telecommunications.
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Half duplex mode allows transmission in both directions but not at the same time. This means the sender and the receiver take turns.
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Full duplex mode allows transmission in both directions at the same time.