Introduction to Data Communications
37c. Classes of LLC
 

37c. Classes of LLC

There are 2 Classes of Logical Link Control defined:

- Class I : Type 1 operation only (connectionless)

- Class II: Both Type 1 (connectionless) and Type 2 (connection-oriented) operation allowed.

37d. LLC PDU Control Field Formats

There are 3 LLC PDU Control field formats:

a) Un-numbered (U-Format PDU)
b) Information Transfer (I-Format PDU)
c) Supervisory (S-Format PDU)

Un-numbered (U-Format PDU)

The last 2 bits are set to 1, to indicate U-Format Control Field.

M - Modifier bits, they are set depending on the mode of operation: Command, Response or Data

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

The Un-numbered LLC Control field is used mainly in Type 1 (connectionless) operation. The PDUs are not numbered, they are sent out and hopefully arrive at their destination. U-Format PDUs can be commands, responses and data. There are only 8 bits in a U-Format LLC PDU. In the U-Format (Unnumbered), there are 8 commands & responses:

UI - Unnumbered information (here's some data - hope it arrives)

DISC - Disconnect (we're done, shut her down)

SABME  - Set Asynchronous Balanced Mode Extended (start now)

XID - Exchange IDs (Here's who I am, who are you?)

TEST - Test the link (Here's a test, send me back a test)

UA - Unnumbered Acknowledgement (Yes, I'm still here)

DM - Disconnect Mode (I'm disconnecting)

FRMR - Frame Reject (Bad frame - reject)