Final answer:
The payload portion of an ATM cell is 48 bytes. ATM cells have a total size of 53 bytes including a 5-byte header. The asked question pertains specifically to the data-carrying component of the cell, which is the 48-byte payload.
Step-by-step explanation:
The payload portion of an ATM (Asynchronous Transfer Mode) cell is 48 bytes. An ATM cell is made up of two main parts: the payload which contains the actual data being transferred and a header which is used for routing and other types of network control information. The header is 5 bytes in size, bringing the total size of an ATM cell to 53 bytes. However, the question specifically asks about the payload portion, which is the 48 bytes dedicated to carrying user data.