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Asynchronous vs Synchronous Framing

All data links have to provide byte-level synchronisation of the sender and receiver.
 
In an asynchronous system, such as is normally used with RS232-interface modems, each character (or byte) sent on a data link is prefixed with an extra start bit and has a stop bit appended, thus:
Async timing diagram
In synchronous systems, a whole block or frame 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.
 
Lecture 14: Point-to-Point Data Links Copyright © 2002 P.Scott, La Trobe University Bendigo.

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