Chapter 28. Synchronous Transmission
Message Frames
Synchronous Transmission sends packets of characters simultaneously. Each packet is preceded by a Start Frame. This frame is used to tell the receiving station that a new packet of characters is arriving, and also 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). For example, only 4 bytes (2 Start Framing Bytes and 2 Stop Framing bytes) are required to transmit up to 8K bytes. Extra bytes (e.g. the Start and Stop Frame) that are not part of the data are called overhead. Packet overhead consists of control information that is used to control the communication.
Efficiency example: An Ethernet frame has an overhead of 26 bytes, including the "Start and Stop Frames", and the maximum data size is 1500 bytes. What is the Ethernet frame's efficiency?
Clocking: Self & Manchester Encoding
Synchronous transmission is more difficult and expensive to implement than asynchronous transmission. It is used with these higher transfer rates of communication: Ethernet, ArcNet, Token Ring etc. Synchronous transmission is used in fast transfer rates (100 Kbps to 100 Mbps). In order to achieve the high data rates, Manchester Line Encoding is used.
In the Manchester Code, there is a transition at the middle of each bit period. The mid-bit transition serves as both a clocking mechanism and as data. A low to high transition represents a 1 and a high to low transition represents a 0.
Manchester Encoding has no DC component and there is always a transition available for synchronizing receive and transmit clocks. Because of the continuous presence of these transitions, Manchester Encoding is also called a self-clocking code.
It has the added benefit of requiring the least amount of bandwidth versus the other Line Codes (Unipolar, Polar, etc.). Manchester coding requires 2 frequencies: the base carrier and 2 x the carrier frequency. All other types of Line Coding require a range from 0 hertz to the maximum transfer rate frequency. In other words, Manchester Encoding requires a Narrow Bandwidth
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