HDLC (High-Level Data Link Control) employs three primary communication modes to establish and manage data links. These modes dictate the relationship between stations on the link and the flow of information. According to the provided reference, the three modes are:
- NRM (Normal Response Mode)
- ABM (Asynchronous Balanced Mode)
- ARM (Asynchronous Response Mode)
Here's a more detailed look at each mode:
HDLC Communication Modes Explained
| Mode | Description | Station Roles | Typical Applications |
|---|---|---|---|
| NRM (Normal Response Mode) | One primary station controls communication with one or more secondary stations. Secondary stations can only transmit when given permission by the primary station. | One primary station, one or more secondary stations | Polling environments, historically common in mainframe computer communication with terminals. The primary station polls the secondary stations to see if they have data to transmit. |
| ABM (Asynchronous Balanced Mode) | Each station has equal responsibility and can initiate communication without permission from another station. It's a peer-to-peer communication. | Two combined stations (each can act as both primary and secondary) | X.25 networks, modern data communication scenarios where devices need to communicate directly without a master-slave relationship. This mode is often used in packet switching networks and provides a more efficient communication method compared to NRM due to the reduced overhead of polling. |
| ARM (Asynchronous Response Mode) | A primary station initiates communication, but the secondary station can respond without explicit permission after the initial setup. Similar to NRM, but offers more flexibility. | One primary station, one or more secondary stations | Rarely used in modern networks. ARM provided a compromise between NRM and ABM, allowing a secondary station to respond without polling after an initial connection was established. However, ABM's flexibility and efficiency have made it the preferred choice in most applications. |
Frame Types in HDLC
The provided reference also mentions the format of the control field within HDLC frames, which is crucial for understanding how the modes operate:
- I-frame (Information frame): Used to transfer user data. If the first bit of the control field is
0, the frame is an I-frame. - S-frame (Supervisory frame): Used for control purposes such as acknowledging received frames or requesting retransmission. If the first two bits of the control field are
10, the frame is an S-frame.
These frame types facilitate the reliable transmission of data and control signals within the chosen communication mode.
