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HDLC DataLink Layer
Chapter 6
624
Figure 6.6 shows a receive not ready (RNR) response to the initial
message transmission because the message sink is full. After the message
sink is no longer full, a retransmission of the message causes a receive
ready (RR) response.
Figure 6.6
Message Transfer with Message Sink Full
Full
1,RR4
3 OK
Packet
1,D3,7
Packet
SOURCE
XMTR
LINK
RCVR
SINK
1,RNR3
Not Full
3,RR3
1,D3,7
Not Full
Figure 6.7 shows a reject (REJ) response to a data frame because it’s out
of sequence. The frame is out of sequence because the receiver had to
discard the previous data frame. After the transmitter sends the frame
with the proper send sequence number, the receiver sends an RR response.
Figure 6.7
Message Transfer with Data Frame Out of Sequence
1,RR5
4 OK
Packet
1,D3,???
Packet
SOURCE
XMTR
LINK
RCVR
SINK
1,RR4
1,D4,7
Not Full
Packet
1,REJ3
1,D3,7
1,D4,7
3 OK
Packet
Packet
Not Full