What are the responsibilities of the MAC sublayer regarding media access?

The MAC sublayer defines the media standards (copper/fiber); legacy shared half-duplex media use CSMA/CD to arbitrate access, while modern full-duplex switched links need no CSMA/CD.

The IEEE 802.3 MAC sublayer includes specifications for different Ethernet communications standards over various types of media including copper and fiber.

Legacy Ethernet (bus topology or hubs):

• Shared, half-duplex medium
• Uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Modern Ethernet LANs (switches):

• Use full-duplex connections
• Switches operate in full-duplex, so CSMA/CD is not required
• Each port is its own collision domain

Go deeper:

• CSMA/CD (Wikipedia) — the contention method legacy half-duplex media use, and why full-duplex switched links no longer need it.

• Duplex — telecommunications (Wikipedia) — full- vs half-duplex, the distinction that decides whether contention can even happen.