Each end point of a connection is called a station. A logical connection between two stations is called a link. Frames are the basic unit of communication on a link and are used for control and to carry data. A frame is either a command or a response. Data may be exchanged between stations using acknowledged or unacknowledged data transfer. A link has two non-operational modes : disconnected mode and initialization mode. Only unacknowledged data transfer is allowed in a non-operational mode, using unnumbered information frames (UI frames). Disconnected mode is the default state for a link. Initialization mode is used in an application dependant manner for station initialization and configuration. Most link layer protocols do not use initialization mode.
Two operational modes defined in ISO4335 are supported by the HDLC API : normal response mode (NRM) and asynchronous balanced mode (ABM). NRM is used in SNA SDLC applications. NRM uses a primary station to control the link and one or more secondary stations. A primary station sends only commands and a secondary station sends only responses. ABM is used for ITU standards such as LAPB and LAPD. ABM uses combined stations which operate as peers and can send both commands and responses.
A mode setting command is a frame that controls the link mode. An operational mode (NRM or ABM) is established using the SNRM (set normal response mode) and SABM (set asynchronous balanced mode) commands. Disconnected mode is set with the DISC (disconnect) command. Initialization mode is set with the SIM (set initialization mode) command.
A link in an operational mode allows acknowledged data transfer using information frames (I-frames). I-frames contain sequence numbers to ensure in order delivery and require acknowledgement by the peer station. Procedures exist to detect and recover from lost frames, increasing reliability.
Checkpoint detection occurs when a receiving station acknowledges all frames up to a known reference point. Unacknowledged frames sent before the reference point are resent. The acknowledgement contains the sequence number of the last correctly received in order frame. This prevents acknowledgement of correctly received frames sent after the lost frame (out of sequence), requiring retransmission of the lost frame and all frames sent after the lost frame.
The reject option (REJ) allows a receiving station to immediately request retransmission of frames starting with a specified sequence number. This improves efficiency by not waiting for an opportunity to perform checkpoint recovery, reducing the number of frames sent after the lost frame which must be resent. Most, but not all link layer protocols support the reject option.
The selective reject (SREJ) option allows a receiving station to immediately request retransmission of a specific lost frame, without requiring retransmission of frames sent subsequent to the lost frame. The multi-selective reject option allows multiple lost frames to be identified with a single SREJ frame. Not all link layer protocols support the selective and multi-selective reject options.
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