Physical Layer
In asynchronous serial communication at the physical protocol layer, the data blocks are code words of a certain word length, for example octets (bytes) or ASCII characters, delimited by start bits and stop bits. A variable length space can be inserted between the code words. No bit synchronization signal is required. This is sometimes called character oriented communication. Examples are the RS-232C serial standard, and MNP2 and V.2 modems and older.
Read more about this topic: Asynchronous Communication
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