Which of the following is not related to data link layer?

Concept: The OSI model assigns framing, error control, and medium access control (including MAC addressing) to the Data Link Layer (Layer 2) — all single-hop responsibilities between two directly-connected nodes. Congestion control, by contrast, regulates the overall volume of traffic flowing across a network that spans multiple links and routers, so it is a Network Layer function (reinforced at the Transport Layer), not a Data Link Layer one.

Application: Checking each listed function against the data link layer's scope:

• Framing: splits the bit stream arriving from the network layer into discrete frames with header/trailer boundaries — a Data Link Layer function.

• Error control: detects and corrects bit errors introduced during transmission over a single link — a Data Link Layer function.

• MAC addressing: uses physical hardware addresses to deliver frames to the correct node sharing that link — a Data Link Layer function.

• Congestion control: manages traffic load across the entire network path, involving multiple routers and links — NOT a Data Link Layer function.

Note the distinction the question is testing: flow control (pacing transmission to match a receiver's capacity on a single link, e.g. via sliding-window protocols) IS performed at the Data Link Layer, but congestion control (preventing network-wide overload across many links and routers) is not — the two are often confused, but only flow control belongs to Layer 2.

Cross-check: Framing, error control, and MAC addressing are covered in every standard treatment of the Data Link Layer, while congestion-avoidance mechanisms (leaky bucket, token bucket, RED, TCP congestion control) belong to the Network/Transport Layer chapters. Of the four functions listed, congestion control is the one not related to the data link layer.