NRM, or Normal Response Mode, in High-Level Data Link Control (HDLC) is a specific type of data communication configuration. It defines how devices on a network interact with each other.
Understanding NRM
In HDLC, NRM operates in an unbalanced structure where communication is primarily controlled by a single device, referred to as the primary terminal. Secondary terminals communicate only upon the request of the primary terminal. Here’s a breakdown:
- Primary Terminal: This device manages the communication flow. It initiates data transfers and polls secondary terminals.
- Secondary Terminals: These devices only respond to the primary terminal. They do not initiate transmissions on their own.
- Polling Mechanism: The primary terminal must periodically poll each secondary terminal to see if they have any data to transmit.
Key Characteristics of NRM
| Feature | Description |
|---|---|
| Configuration | Unbalanced |
| Initiation | Only the primary terminal can start data transfer |
| Secondary Role | Secondary terminals respond only when polled by the primary terminal. |
| Data Flow | One-way flow initiated by the primary. Secondary terminals only send in response to the primary's requests. |
How NRM Works in Practice
- The primary terminal sends a poll request to a specific secondary terminal.
- The polled secondary terminal, if it has data, responds to the primary. Otherwise, it may send a negative acknowledgment or no response, depending on the protocol.
- The primary terminal receives the response and processes it.
- This cycle is repeated with other secondary terminals as needed.
Example Scenario
Imagine a system where a central computer (the primary terminal) manages several remote sensors (the secondary terminals).
- The central computer periodically polls each sensor.
- If a sensor has data (e.g., a temperature reading), it transmits the data upon being polled.
- If no new reading exists, the sensor might send a "no new data" response, or not respond at all.
Practical Insights
- Centralized Control: NRM provides centralized control over data flow, making it suitable for environments where a central master needs to manage subordinate devices.
- Simplicity: It's relatively straightforward to implement due to the clear hierarchy of primary and secondary devices.
- Potential Inefficiency: Polling can become inefficient if many secondary devices rarely have data to send. Constant polling can waste bandwidth.
- Deterministic Behavior: The polling approach ensures more deterministic access to the channel, beneficial in time-sensitive applications.
In summary, Normal Response Mode in HDLC provides a structured, polled communication method where a primary terminal initiates data transfers, and secondary terminals respond only upon request, making it a good fit for many centralized data collection and management scenarios.
