Data Link Layer and Higher
Asynchronous communication at the data link layer or higher protocol layers is known as statistical multiplexing or packet mode communication, for example asynchronous transfer mode (ATM). In this case the asynchronously transferred blocks are called data packets, for example ATM cells. The opposite is circuit switched communication, which provides constant bit rate, for example ISDN and SONET/SDH.
The packets may be encapsulated in a data frame, with a frame synchronization bit sequence indicating the start of the frame, and sometimes also a bit synchronization bit sequence, typically 01010101, for identification of the bit transition times. Note that at the physical layer, this is considered as synchronous serial communication. Examples of packet mode data link protocols that can be/are transferred using synchronous serial communication are the HDLC, Ethernet, PPP and USB protocols.
Read more about this topic: Asynchronous Communication
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