Black Box 5000 series Reference Manual page 260

I F
NFORMATION RAMES
I Information
The (I) statistic indicates a transfer of a sequentially numbered frame containing an (I) information field.
To allow the sending of an Information frame a Receive Ready (RR) supervisory frame is sent by the remote
bridge/router requesting the connection.
S F
UPERVISORY RAMES
RR Receiver Ready
A Receive Ready (RR) supervisory frame is sent by the bridge/router in order to:
1) indicate that it is ready to receive an I frame;
2) acknowledge previously received I frames numbered up to and including N(R) - 1.
An RR frame may be used to indicate the clearance of a busy condition reported by the earlier transmission of an RNR
frame by that same bridge/router.
RNR Receiver Not Ready
The RNR statistic is generated by either remote bridge/router to indicate a busy condition. A busy condition essentially
indicates a temporary inability to accept incoming I frames. I frames numbered up to and including N(R) - 1 are
acknowledged.
I frame N(R) and any subsequent I frames received, are not acknowledged; the acceptance state of these
unacknowledged frames will be indicated in subsequent exchanges.
REJ REJect
The REJ supervisory frame is generated when a remote bridge/router requests transmission of I frames starting with
the frame numbered N(R). I frames numbered N(R) - 1 and below are acknowledged. Additional I frames (pending
initial transmission) may be transmitted following the retransmitted I frame(s).
Only one REJ exception condition for a given transfer direction may be established at any time. This REJ exception
condition is reset (cleared) upon the receipt of an I frame with an N(S) equal to the N(R) of the REJ frame. A REJ
frame may be used to indicate the clearance of a busy condition that was reported by the earlier transmission of an RNR
frame by that same bridge/router.
U F
NNUMBERED
SABM Set Asynchronous Balanced Mode
The SABM unnumbered command is generated to place the addressed bridge/router into an asynchronous balanced
mode information-transfer phase, where all command/response control fields will be one octet in length.
The receiving bridge/router confirms acceptance of the SABM by the transmission, at the first opportunity, of a UA
response.
Previously transmitted I frames that are unacknowledged when a SABM command is generated remain
unacknowledged. It is the responsibility of a higher level (e.g. TCP, XNS, LAT) to recover from the loss of the contents
(packets) of such I frames.
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RAMES
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