HDLC DataLink Layer
Chapter 6
63
Physical Circuit AB carries command and response frames from A to B.
Physical Circuit BA carries command and response frames from B and A.
A frame often carries both a command and a response. Therefore, the
transmitter is responsible for sending all frames. However, a transmitter
that sends a command must receive the response from that command.
Internal software must direct responses from receiver to the transmitter.
Figure 6.2 shows this implementation.
Figure 6.2
Software Implementation of Data Paths
Physical Circuit AB
XMTR
A
RCVR
B
RCVR
A
XMTR
B
Physical Circuit BA
Commands and Responses
Commands and Responses
Path 1
Path 3
Path 4
Responses
Path 2
Responses
In Figure 6.2, Transmitter A sends a command to Receiver B over Path 1,
which is Physical Circuit AB. Receiver B sends the response over Path 2
to Transmitter B, which relays it over Path 3 to Receiver A, which relays
it over Path 4 to Transmitter A. Path 2 is provided by the internal
software at Station B. Path 4 is provided by internal software at
Station A.
The two-way simultaneous operation is symmetrical; that is, anything that
we can say about commands from Transmitter A to Receiver B and the
response paths applies equally to commands from Transmitter B to
Receiver A and paths. There are actually two independent instances of
the protocol operating simultaneously.