According to the X.25 architecture, how many distinct protocol layers are specified at the serial interface gateway between user equipment and the packet-switched network?

Introduction:
X.25 defined a complete suite for connecting Data Terminal Equipment (DTE) to a packet-switched network via a serial interface. Its layering mirrors the lower portion of the OSI model and clarifies responsibilities at the network edge.

Given Data / Assumptions:

• We are concerned with the DTE–DCE interface (user equipment to network).
• Physical, link, and network functions are required.
• Higher-layer transport/session logic is outside X.25’s lower-layer scope.

Concept / Approach:
X.25 specifies three layers: (1) Physical layer (e.g., X.21, V.24/V.28) provides electrical/connector characteristics; (2) Data link layer uses LAPB for reliable framing and error control; (3) Packet layer provides virtual circuits and network addressing/flow control. Together they deliver reliable packet service to higher layers.

Step-by-Step Solution:
1) Identify physical signaling needs → Physical layer.2) Ensure reliable frame transport → Data link via LAPB.3) Provide virtual circuit services → Packet layer (level 3) of X.25.

Verification / Alternative check:
Host stacks historically implemented these three levels to reach PDNs, with higher-layer protocols (transport/session) riding above the packet layer.

Why Other Options Are Wrong:

• 2 or 4 or 6: do not match the standardized three-layer decomposition in X.25 at the interface.
• None of the above: invalid because three is correct.

Common Pitfalls:
Confusing X.25’s three-layer model with the full seven-layer OSI model or with TCP/IP layering.

Final Answer:
3