Which of the following is a MAC sublayer function?

• A. Adds Layer 2 control information to network protocol data.
• B. Places information in the frame that identifies which network layer protocol is being used for the frame.
• C. Enables IPv4 and IPv6 to utilize the same network interface and media.
• D. Integrates various physical technologies.

Answer: (A)

The MAC sublayer’s job is to wrap the network-layer data in a frame and add the Layer 2 control information that enables frame delivery on the local link. This means attaching the header and trailer that provide source and destination MAC addresses, a length or EtherType field to indicate the upper-layer protocol, and a frame check sequence for error detection. In short, the MAC layer encapsulates data with the necessary Layer 2 control data so frames can be properly transmitted over the local medium.

The idea of identifying the upper-layer protocol or multiplexing IPv4 and IPv6 over the same interface is handled by other mechanisms (the LLC/Frame Type aspects and higher-layer decisions), not the fundamental MAC sublayer function. Integrating various physical technologies is a job of the Physical layer.