At the Data Link layer, which technique is typically used to detect errors in frames carried over a physical link?

Introduction:
Link-level reliability depends on quickly identifying corrupted frames so higher layers can recover efficiently. The canonical mechanism used by common Layer-2 protocols is an error-detecting code appended to each frame. This question asks you to select that mechanism.

Given Data / Assumptions:

• Focus: error detection at Layer 2 (not correction).
• Examples: Ethernet, HDLC, PPP, and related protocols.
• Physical channel impairments can flip bits; the receiver must detect this.

Concept / Approach:
A Cyclic Redundancy Check (CRC) generates a Frame Check Sequence (FCS). The sender computes an FCS over the frame; the receiver recomputes and compares. A mismatch indicates corruption, prompting discard and possible retransmission by an upper-layer protocol or link-level procedure, depending on the technology.

Step-by-Step Solution:

Verification / Alternative check:
Ethernet uses a 32-bit CRC; HDLC/PPP use CRC-16/CRC-32 variants, confirming CRC/FCS as the standard Layer-2 detection approach.

Why Other Options Are Wrong:

• Bit stuffing: prevents flag emulation within payloads; not detection.
• Hamming codes: provide correction; not the typical Layer-2 detection method in these protocols.
• Equalization: Physical-layer channel equalization, not a Layer-2 integrity check.
• None: false because CRC is correct.

Common Pitfalls:
Confusing framing and signal-processing terms with integrity checking; assuming that any error-related term equals Layer-2 detection.

Final Answer:
Cyclic Redundancy codes.