QUESTION: How many bytes does the physical layer of ATM consist of?
ANSWER: Strictly speaking, ATM is a protocol that rides above the physical layer. The physical layer, or "Layer 1" comprises the actual transmitted bits. If there is some framing involved, such as DS1 framing for carrying a payload of 1.536 Mbps in a 1.544 Mbps bit-stream, the "Physical Layer" (or "Layer 1") is often considered to be the payload, or 1.536 Mbps in the example mentioned.
ATM is structured as "cells" comprising 53 octets (bytes). Thus, continuing with the example of framed DS1, one ATM cell may span more that one DS1 frame [each DS1 frame is comprised of 24 octets in 125 microsec (8 kHz frame-rate), corresponding to 1.536 Mbps]. Of the 53 octets, 5 octets are used for "overhead" and include information related to addressing and control. The remaining 48 octets are considered the payload.
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