Introduction to Data Communications
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33. Synchronous Transmission

Synchronous Transmission sends packets of characters at a time. Each packet is preceded by a Start Frame which is used to tell the receiving station that a new packet of characters is arriving and to synchronize the receiving station's internal clock. The packets also have End Frames to indicate the end of the packet. The packet can contain up to 64,000 bits depending on the protocol. Both Start and End Frames have a special bit sequence that the receiving station recognizes to indicate the start and end of a packet. The Start and End Frames may be only 2 bytes each.

Efficiency

Synchronous transmission is more efficient than asynchronous (character transmission) as little as only 4 bytes (2 Start Framing Bytes and 2 Stop Framing bytes) are required to transmit up to 8K bytes. Extra bytes, like the Start and Stop Frame, that are not part of the data are called overhead. Packet overhead consists of control information used to control the communication.

Efficiency example: An Ethernet frame has an overhead of 26 bytes including the "Start and Stop Frames", the maximum data size is 1500 bytes. What is the Ethernet frame's efficiency?

Introduction to Data Communications
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