Abstract

In order to examine various data link control (DLC) procedures, one must first have a basic understanding of the overall network. The network depicted will be one implementing the IBM Systems Network Architecture (SNA). The fundamental communication system within SNA is referred to as the Transmission Subsystem. The Transmission Subsystem includes three types of elements. The first element, Data Link Control (DLC), transfers packets (or SNA Basic Transmission Units) across noisy transmission mediums. The protocol used here is that of SDLC (Synchronous Data Link Control). The second element, referred to as Path Control (PC), routes packets either to an end user in the node or to the proper DLC element for transmission back through the network. Path Control also blocks incoming messages and deblocks outgoing messages (i.e., SNA segmenting). Transmission Control (TC) helps manage SNA session (i.e., connection) initiation and termination, provides sequence number manipulation, controls pacing, and performs many other functions on behalf of the end user. Pacing is a means of controlling the rate at which TC sends and receives normal data flow requests.

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