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 FC-AL is a means by which to connect two or more (up to 126) devices in a serial loop configuration. This solution is considered low cost because it does not use hubs and switches in small to medium configurations. It also provides capability to connect to larger loops and fabrics where hubs and switches are used.
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.
3.1.1 Types of Loops
18.104.22.168 FC-AL Private Loop
A private loop is enclosed and known only to itself. Figure 3-1 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 3-1. 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 then 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. Currently, this is the type of loop configuration Hewlett-Packard supports.
22.214.171.124 FC-AL Public Loop
A Public Loop, shown in Figure 3-2, 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 3-3.
Figure 3-2. FC-AL Public Loop
Figure 3-3. FC-AL Fabric
Hewlett-Packard does not currently support public loops or fabrics.