HDLC Normal Response Mode Basics | Generated by AI
Question: In HDLC, the operation mode where the transmission process is initiated by the primary station, and a secondary station can only transmit information to the primary station as a response after receiving a command frame from the primary station is called what?
A. Normal Response Mode (NRM) B. Asynchronous Response Mode (ARM) C. Asynchronous Balanced Mode (ABM) D. Synchronous Balanced Mode
Your answer: B Correct answer: A
Answer:
The correct answer is A. 正常响应方式 (Normal Response Mode, NRM).
Explanation
HDLC defines three main operational modes:
- Normal Response Mode (NRM)
This is a master-slave (unbalanced) configuration.
- Only the primary station can initiate transmission.
- The secondary station is not allowed to transmit anything until it receives a command frame (usually with the P/F bit set to poll) from the primary station.
- After being polled, the secondary may send one or more response frames (with the last frame having the F bit set).
- The primary station is fully responsible for link management, polling, timeouts, retransmissions, and error recovery. → This exactly matches the description in the question.
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Asynchronous Response Mode (ARM) This is still unbalanced (primary + secondary), but the secondary station can send frames at any time without waiting for permission from the primary (i.e., it can initiate transmission asynchronously). → This is not the mode described in the question.
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Asynchronous Balanced Mode (ABM) This is a balanced configuration (both sides are combined stations). Either station can initiate transmission at any time without needing permission from the other side. → Clearly different from the described behavior.
- Synchronous Balanced Mode This term is not a standard HDLC mode (it does not exist in the official HDLC standard).
Therefore, the scenario described — transmission started only by the primary station, and secondary station can only respond after receiving a command — is the definition of Normal Response Mode (NRM).
Your mistake: You chose B (Asynchronous Response Mode), which is the opposite case — the secondary is allowed to initiate transmission without being polled first.
References:
- HDLC protocol specification (ISO 13239 / ITU-T Recommendation X.25 LAPB documentation)
- Most computer networking textbooks covering Data Link Layer protocols (e.g., Tanenbaum, Kurose & Ross, Forouzan)