Control - Unnumbered Information (LLC Type 1)

a

If LLC is used connectionless (CLLS - connectionless link services), the control field is 1 bytes in length (U-format LLC).

The first two bits (value: 11) indicate unnumbered information (unnumbered: without sequence numbers).

The P/F bit (poll/final) indicates the flow direction.

The M bits (modifier function bits) indicate the subfunction of the UI-LPDU:

MM-MMM = 00-000 = UI - Unnumbered Information

MM-MMM = 11-101 = XID - Exchange Identification

MM-MMM = 00-111 = TEST - Test

MM-MMM = 11-110 = SABME - Set Asychronous Balanced Mode Extended

MM-MMM = 00-110 = UA - Unnumbered Acknowledgment

MM-MMM = 11-000 = DM - Disconnected Mode

MM-MMM = 00-010 = DISC - Disconnect

MM-MMM = 10-001 = FRMR - Frame Reject

LLC Type 1 - U-Format - DISC

DISK-LPDUs (Disconnect LLC PDU) are used by LLC stations to stop an asynchronous balanced mode operation (that had been started by SABME-LPDU); the remote link station is indicated to change to asynchronous disconnected mode. The remote link station has to respond with an unnumbered acknowledgment (UA-LPDU response) if it is in asynchronous balanced mode, or with a DM-LPDU response if it is in asynchronous disconnected mode.

LLC Type 1 - U-Format - DM

DM-LPDUs (Disconnected Mode LLC PDU) are used by LLC stations to notify that they are logically disconnected.

LLC Type 1 - U-Format - FRMR

FRMR-LPDUs (Frame Reject LLC PDU) are used by LLC stations to indicate that an incoming LPDU couldn't be received properly.

LLC Type 1 - U-Format - SABME

SABME-LPDUs (Set Asynchronous Balanced Mode Extended LLC PDU) are used by LLC stations to negotiate data exchange in extended asynchronous balanced mode. The remote link station has to reply to a SABME command with a UA-LPDU (acknowledgment) or DM-LPDU (reject).

LLC Type 1 - U-Format - TEST

TEST-LPDUs (Test LLC PDU) are used by LLC stations to send test characters within the information field which have to be repeated unchanged by the destination station.

LLC Type 1 - U-Format - UA

UA-LPDUs (Unnumbered Acknowledgment LLC PDU) (response) are used by LLC stations to acknowledge SABME-LPDUs or DISC-LPDUs.

LLC Type 1 - U-Format - XID

XID-LPDUs (Exchange Identifier LLC PDU) are used by LLC stations to convey data flow control parameters.