At each end of the phone line or LAN, the computer needs to keep track of a few items. What is the number that should be assigned to the next I-frame transmitted, what was the number of the last I-frame received, and has it been acknowledged yet? There are time limits to detect lost messages, and a counter to managed the window of unacknowledged frames. The 802.2 standard calls this a "connection component." In SNA, it is a Link Station.
The Link Station controls the flow of data between two network nodes. Successive I-frames may belong to the same session, or they may belong to different programs or terminals. When an I-frame is acknowledged on the LAN or SDLC line, this does not mean that the data in it is correct or has been processed. The Link Station dumps incoming data into buffers and queues them up for later processing.
A SNA link station is the hardware or software within a node that enables the node to attach to, and provide control over, a link connection. It exchanges information and control signals with its partner link station in the adjacent node. Link stations use data link control protocols to transmit data over a link connection. A link connection is the physical medium over which data is transmitted. Examples of transmission media include telephone wires, microwave beams, fiber-optic cables, and satellite circuits. Multiple links between the same two nodes are referred to as parallel links.
A transmission group consists of 1 or more links between 2 nodes. Multilink TG is a TG consists of 2 or more parallel links. One advantage of multilink TG is that it preserve a session when link fails which parallel TG does not.
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