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Logical Link Control Overview
Type 2 Operations
Type 2 operations have the following characteristics:
•
The LLC and its peer must establish a data link layer,
virtual circuit/connection prior to any exchange of data.
•
The source and destination are peer LLCs in an asynchronous, balanced data
link connection.
•
The source and destination LLCs control traffic by means of a numbering
scheme for the sequential transfer of PDUs. The PDUs for each
virtual circuit/connection have independent sequence-numbering schemes.
•
The destination LLC acknowledges data PDUs that the source LLC sends by
informing the source LLC of the next sequence number expected.
LLC Functionality
The LLC sublayer, which can support multiple logical links concurrently:
•
Initiates and terminates control signal interchange with the XID, TEST,
SABME, and DISC PDUs.
•
Organizes data flow with the U, I, and UA PDUs. The level of organization
differs between type 1 and type 2 operations.
•
Interprets command PDUs it receives, and generates appropriate response
PDUs, which differ between type 1 and type 2 operations and LLC1 and
LLC2 service.
•
Manages error control and recovery with the REJ, RR, RNR, and FRMR
PDUs.
The LLC protocols generate and interpret command packets or frames called
protocol data units (PDUs), which Table 1-1 describes. Table 1-1 lists type 1 and
type 2 command PDUs and their counterpart response PDUs.
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