HDLC is the common data link control protocol. When HDLC begins to set up the data link, it can select the specific operation mode.
The link operation mode can be divided into three types: a primary node mode, a secondary node mode, and a combination node mode.
The nodes that perform control function are called primary nodes. Other nodes controlled by primary nodes are called secondary nodes. The node that can serve as both the primary and secondary node is a combination node.
A primary node is responsible for organizing data flow and correcting errors on data flow. Frames sent from primary nodes to secondary nodes are called command frames, and frames returning from secondary nodes to primary nodes are called response frames.
The link that connects multiple nodes uses the polling technique, which makes other nodes as primary nodes, while in the point-to-point (P2P) link, every node can be a primary node. When being connected to multiple links, a node may be a primary node for some links and may be a secondary node for other links.
The common operation modes in HDLC can be divided into the following three types.
Normal Response Mode (NRM) is an unbalanced data link operation mode. Sometimes it is called Unbalanced Normal Response Mode.
This operation mode uses the terminal-oriented P2P or point-to-multipoint links.
Under this operation mode, a primary node starts the transmission process; only after receiving one command frame from the primary node, a secondary node responds to the primary node by transmitting frames.
Response information consists of one or multiple frames. If the response information consists of multiple frames, it must indicate which frame is the last frame.
With the rights to poll the node, select and send command frames to a secondary node, a primary node is responsible for managing the whole link and controlling the timeout, retransmission, and all recovery operations.
The Asynchronous Response Mode (ARM) is also an unbalanced data link operation mode.
Different from that of NRM, the transmission process of ARM is started by a secondary node. The secondary node actively sends one frame or a group of frames to the primary node.
Under this operation mode, the secondary node controls the timeout and retransmission.
This mode is necessary for the multi-node link that adopts the polling mode.
Asynchronous Balanced Mode (ABM) is an operation mode that allows any node to start transmission.
To improve the link transmission efficiency, more packets should be transmitted on both ends. Under this operation mode, any node at any time can start the transmission and every node can serve as a primary or a secondary node. Every node supports a group of the same protocols. Any node can transmit and receive commands, make response, and correct errors.
