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Data Flow Specifications
The FDDI standard specifies that data travels on the primary ring in a
counterclockwise direction, and that data traveling on the secondary ring rotates
clockwise. (Refer to Figure 3-1.) As you attach the hub to the ring and network
devices to the hub, be aware of the data flow direction.
Table 2-3 shows the four supported port types and the direction of data flow
through each port. Figure 2-9 illustrates the various port types and the data flow
direction allowed by the FDDI standard.
Table 2-3. FDDI Port Types
Port
Data Flow Direction
A
Primary In/Secondary Out
(PI/SO)
B
Primary Out/Secondary In
(PO/SI)
M
Master (PO/PI)
S
Slave (PI/PO)
Table 2-3 and Figure 2-9 show that the primary ring (solid line) enters the A
port as Primary In (PI) and exits as Primary Out (PO) through port B. The
secondary ring (dashed line) enters through port B as Secondary In (SI) and
exits through port A as Secondary Out (SO).
For both master (M) and slave (S) ports, the primary ring enters the port as PI
and exits as PO. The secondary ring never connects to these two ports.
Installing the Hub
Use
Connects dual attachment
devices to the trunk ring
Connects dual attachment
devices to the trunk ring
Connects single attachment
devices to the hub port; uses
the primary ring only
Connects single attachment
devices to the hub; uses the
rimary ring only
2-23
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