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Asynchronous vs Synchronous Framing

All data links have to somehow provide byte-level synchronisation of the sender and receiver -- that is, to delineate byte boundaries in a stream of bits.
 
In an asynchronous system, such as is normally used in RS232-interfaces, each individual byte (or character) sent on a data link is prefixed with an extra start bit and has a stop bit appended. When no data is being sent, the line "idles" in a constant "logic 1" state. A "1-to-0" transition initiates the start bit and flags the start of a new byte.
 
Async timing diagram
In the alternative synchronous systems, a whole frame or block of data is sent as a single unit, with a synchronising preamble of a few bytes. This has much lower overhead, and is by far the most common in "leased-line" data links, see later. We saw an example of this in the Ethernet/802.3 frame format in the previous lecture.
 
Lecture 14: Point-to-Point Data Links Copyright © 2004 P.Scott, La Trobe University Bendigo.

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