According to the OSI reference model, which layer is Layer 1 responsible for transmitting raw bits over a physical medium?

Introduction / Context:
The OSI model partitions network functions into seven layers, each with distinct responsibilities. Correctly mapping tasks to layers helps diagnose connectivity issues and properly design protocols and hardware interfaces.

Given Data / Assumptions:

• Layer numbering runs from 1 (bottom) to 7 (top).
• We focus on the lowest layer that deals with bit-level transmission.
• Standard OSI layer naming applies.

Concept / Approach:
Layer 1 is the Physical Layer. It defines electrical/optical signaling, voltage levels, timing, connectors, cable specs, and RF for wireless. It converts frames into raw bitstreams for transmission across the medium and back. Above it, the Data Link Layer (often called 'link layer') handles framing, MAC addressing, and error detection; the Network Layer (Layer 3) handles routing; Transport (Layer 4) handles end-to-end reliability/flow control.

Step-by-Step Solution:

Verification / Alternative check:
Examples: Ethernet PHY chips, RJ-45 pinouts, optical wavelengths, and Wi-Fi radio parameters are all Layer 1 topics, confirming the association.

Why Other Options Are Wrong:

• link layer: That is Layer 2 (Data Link), not Layer 1.
• network layer: Layer 3; handles routing.
• transport layer: Layer 4; handles segments and reliability.

Common Pitfalls:
Confusing the colloquial 'link layer' term with a generic concept and misplacing it at Layer 1, or assuming MAC addressing is physical—it is Data Link.

Final Answer:
physical layer