Introduction to Data Communications
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33a. Clocking: Self & Manchester Encoding

Synchronous transmission is more difficult and expensive to implement than asynchronous transmission. It is used with all 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 a clocking mechanism and also 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 compared to 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

Introduction to Data Communications
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