Synchronous communication:

1. When a byte of data is transmitted or received at constant time interval with uniform phase differences, it is known as synchronous communication.
2. Bits of data frame are sent at fixed maximum time interval.
3. ISO synchronous communication is a case when maximum time interval is varied.
4. Frames sent over LAN is an example for synchronous serial communication.
5. Characteristics:

• Frames maintain constant phase difference. No permission for transmission at random intervals. No hand shaking provided during communication interval.
• Clock is needed to transmit bits of all bytes.
• Two separate lines are used for data bits and clock.
• Clock information is modulated by modulating clock with stream of bits.
• There are synchronizing and signaling bits.

1. Encoding methods:

• Frequency modulated signals are encoded using Manchester coding.
• Mid frequency modulation is done using quadrature amplitude modulation and bi-phase coding.

Asynchronous and iso-synchronous communication:

1. Frame is transmitted or received at variable intervals of time. Example, voice data.
2. Characteristics:

• Frames need not maintain constant phase difference and can be sent at variable time intervals.
• Though the clock must tick at certain rate for transmit bits of single byte, it is always implicit to asynchronous data receiver.
• Transmitter does not transmit any clock information along with serial stream of bits in asynchronous communication.
• When a device sends data in serial communication frame, it has to be as per protocol.