Which sublayer adds Layer 2 control information to network protocol data?

• A. MAC sublayer
• B. LLC sublayer
• C. Physical layer
• D. Data Link layer (core)

Answer: (B)

The sublayer that adds Layer 2 control information to the data from higher layers is the LLC sublayer. In Ethernet, the Data Link layer is split into two parts: the LLC (Logical Link Control) and the MAC (Media Access Control). The LLC header provides control information and protocol identification (like DSAP/SSAP fields and a control field), which allows the data link layer to multiplex different network-layer protocols over the same link and to indicate how the frame should be processed by the receiving end. The MAC sublayer, on the other hand, handles access to the physical medium and uses hardware addressing, frame delimiting, and error detection, but it doesn’t insert the upper-layer protocol identification. The Physical layer is the actual transmission of raw bits, so it doesn’t deal with framing or protocol control. Data Link includes both sublayers, but the one responsible for the Layer 2 control information is the LLC sublayer.