What are the three main responsibilities of the MAC sublayer in IEEE 802.3 Ethernet?

Building the frame structure, addressing it with source and destination MAC addresses, and adding the FCS trailer for error detection.

The MAC (Media Access Control) sublayer is responsible for data encapsulation — taking a network-layer packet and wrapping it so it can travel across one LAN link. Cisco's IEEE 802.3 encapsulation has three jobs:

1. Ethernet frame — defines the internal structure of the frame: where the address fields, the payload, and the trailer sit, and the size limits (64–1518 bytes). Without an agreed structure the receiver could not tell one field from the next.
2. Ethernet addressing — inserts a source and destination MAC address so the frame can be delivered NIC-to-NIC (network interface card) on the same LAN. The destination tells switches/NICs who should receive it; the source lets the receiver reply and lets switches learn.
3. Error detection — appends a Frame Check Sequence (FCS) trailer. The sender computes a CRC (Cyclic Redundancy Check) over the frame; the receiver recomputes it and, on mismatch, discards the frame. This catches bits flipped in transit but does not fix them — recovery is left to higher layers.

(Note the MAC sublayer's other big role, media access, is covered separately; these three are specifically the encapsulation duties.)

Go deeper:

• Medium access control (Wikipedia) — lists the MAC sublayer's core duties: frame delimiting, addressing, and error protection via the FCS.

• Ethernet frame structure (Electronics Notes) — how the structure, addressing, and FCS trailer fit together in the encapsulated frame.