QUESTION: If ATM is asynchronous by definition, why is synchronization even mentioned in the same sentence?
ANSWER: The term "Asynchronous Transfer Mode" applies to layer 2 of the OSI 7-layer model (the "data link" layer), whereas the term "synchronous network" applies to layer 1, the physical layer. Layers 2, 3, etc. always require a physical layer which for ATM is typically SONET or SDH; thus the "asynchronous" ATM system is often associated with a "synchronous" layer 1. In addition, if the ATM network offers circuit emulation service (CES), also referred to as constant bit-rate (CBR), then synchronous operation (that is, traceability to a primary reference source) is required to support the preferred timing transport mechanism, SRTS.
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