EXFO FTB-800 Series for FTB-1 - Manual (Page 312)

Test Results
302 FTB-700G/800 Series
ISDN Logger

I (Information) command is used to transfer, across a data link
connection, sequentially numbered frames containing information
fields provided by Layer 3.
Used while multiple frame operation is in effect on point-to-point
data link.

UI (Unnumbered Information) command is used to send
information.

DISC (Disconnect) command is used to terminate the multiple
frame operation.

DM (Disconnect Mode) response is used to report that the data link
layer is in a state such that multiple frame operation cannot be
performed.

FRMR (Frame Reject) response is used to report an error condition
that is not recoverable by a retransmission.

XID (Exchange Identification) command/response is used for
connection management to exchange information.

RNR (Receive Not Ready) command/response is supervisory
frame used by a data link layer entity to indicate a busy condition
or to ask the status of its peer data link layer entity.

REJ (Reject) command/response supervisory frame is used by a
data link layer entity to request retransmission or to ask the status
of its peer data link layer entity.

RR (Receive ready) supervisory frame is used by a data link layer
entity to indicate it is ready to receive an I frame. Besides this, it is
used to acknowledge previously received I frames numbered up to
and including N(R)-1 and clear a busy condition that was indicated
by the earlier transmission of an RNR frame by the same data.
In addition to indicating the status of a data link layer entity, the RR
command with the P bit set to 1 may be used by the data link layer
entity to ask for the status of its peer data link layer entity.