![]() ![]() A primary station maintains a separate logical link with each secondary station.Ī combined station is a combination of a primary and secondary station. It can only send response frames when requested by the primary station. The secondary station's frames are called responses. It is only activated when requested by the primary station. It has no ability, or direct responsibility for controlling the link. The secondary station is under the control of the primary station. If the data link protocol being used is HDLC, and a primary station is present, a secondary station must also be present on the data link. It also takes care of error recovery at the data link level (layer 2 of the OSI model). Despite this important aspect of being on the link, the primary station is also responsible for the organization of data flow on the link. A primary issues commands and secondary issues responses. It has the responsibility of controlling all other stations on the link (usually secondary stations). Within a network using HDLC as its data link protocol, if a configuration is used in which there is a primary station, it is used as the controlling station on the link. HDLC specifies the following three types of stations for data link control: ![]() In this lesson we shall consider the following aspects of HDLC: ![]() Two subsets widely in use are Synchronous Data Link Control (SDLC) and Link Access Procedure-Balanced (LAP-B). ![]() Other benefits of HDLC are that the control information is always in the same position, and specific bit patterns used for control differ dramatically from those in representing data, which reduces the chance of errors. Since Internet provides retransmission at higher levels (i.e., TCP), most Internet applications use HDLC's unreliable delivery mode, Unnumbered Information. HDLC supports several modes of operation, including a simple sliding-window mode for reliable delivery. It has been so widely implemented because it supports both half-duplex and full-duplex communication lines, point-to-point (peer to peer) and multi-point networks, and switched or non-switched channels. It has found itself being used throughout the world. It specifies a packitization standard for serial links. It falls under the ISO standards ISO 3309 and ISO 4335. It was developed by the International Organization for Standardization (ISO). On completion, the student will be able to:Įxplain how High-Level Data Link Control (HDLC) worksĮxplain how data transparency is maintained in HDLC ![]()
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