IEEE 802.5 (Token Ring) media access control: What is the functional purpose of the circulating token in a token-passing network?

Introduction:
Token-passing is a deterministic media access control (MAC) method used in technologies such as IEEE 802.5 Token Ring and certain token bus systems. The core idea is to avoid collisions by granting explicit, rotating permission to transmit. Understanding what the token does—and what it does not do—clarifies how access is coordinated.

Given Data / Assumptions:

• A logical ring connects stations.
• A short control frame (the token) circulates around the ring.
• Only one station at a time may send user data.

Concept / Approach:
The token is a special frame that confers transmit rights. A station must possess the token before sending data; once finished or after a time limit, it releases a token so the next station may transmit. This controlled turn-taking prevents collisions and provides bounded access times. The token does not inherently encode routing between different rings, nor does it permanently assign priority to a particular node.

Step-by-Step Solution:
1) Identify that only the token holder may transmit.2) Note that access rotates; no permanent “highest priority” is implied by simply holding the token.3) Recognize the token is a control frame, not a routing packet.4) Select the option that describes transmit permission.

Verification / Alternative check:
Standards and textbooks define the token as the MAC permission mechanism. Priority schemes can exist, but the basic token does not equal permanent top priority.

Why Other Options Are Wrong:

• Higher throughput on twisted pair: Throughput depends on many factors; not guaranteed solely by token passing.
• Permanent highest priority: Token possession is temporary.
• Token carries routing information: Incorrect; it is not a routing frame.
• None of the above: invalid because the transmit-permission statement is correct.

Common Pitfalls:
Assuming token passing always outperforms CSMA/CD; performance depends on load and implementation details.

Final Answer:
Only the station holding the token is permitted to transmit frames