3.15 FC-AL Characteristics

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3.15 FC-AL Characteristics

There are three topologies supported by Fibre Channel: loop, point-to-point, and switched. FC-AL is the loop topology that will be discussed here. The Fibre Channel Arbitrated Loop (FC-AL) is a means by which to connect up to 126 devices in a serial loop configuration.

In the FC-AL, each port discovers when it has been attached to a loop. Addresses are assigned automatically on initialization. Access to the loop is arbitrated; there are no collisions and there is no single permanent loop master.

FC-AL permits fair access on a single arbitrated loop. Access fairness means every port wanting to initiate traffic will have the opportunity to own the loop and initiate traffic before any other port has the opportunity to own the loop for the second time.

3.15.1 Types of Loops FC-AL Private Loop

A private loop is enclosed and known only to itself. Figure 3-16 demonstrates a common configuration used with FC-AL for Fibre Channel Mass Storage. In this example, the processor node only has one Fibre Channel host bus adapter (HBA).

Figure 3-16. FC-AL Private Loop

The processor node is connected to the hub. Then the three devices ”in this case disk arrays ”are connected to the hub and the loop is formed . If the hub is not used, the connection with all three disk arrays cannot be made. A connection to only one, in point-to-point fashion, could be accomplished. Another option would be to install three HBAs into the processor node and connect each to a disk array separately.

The hub, then, provides an advantage in saving HBA slots in the processor node and allows multiple storage devices to be added to the loop. FC-AL Public Loop

A public loop, shown in Figure 3-17, requires a fabric and has at least one FL_Port connecting to a fabric. A public loop extends the reach of the loop topology by attaching the loop to a fabric. Public loops are a way to leverage the cost of one switched connection over many devices in a loop. Connecting a loop to a fabric is similar to connecting a local area network (LAN) to a wide area network (WAN). The fabric is usually represented by a cloud, as shown in Figure 3-18.

Figure 3-17. FC-AL Public Loop
Figure 3-18. FC-AL Fabric
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Storage Area Networks. Designing and Implementing a Mass Storage System
Storage Area Networks: Designing and Implementing a Mass Storage System
ISBN: 0130279595
EAN: 2147483647
Year: 2000
Pages: 88

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