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What's the difference: Fibre Channel Fabric vs. Arbitrated Loop

By Rick Cook

Fibre Channel for SANs comes mostly in two flavors: Fibre Channel Arbitrated Loop (FC-AL) and Fibre Channel Fabric. The two differ considerably in their topology and capabilities. (A third topology, point-to-point, isn't as commonly used for SANs.)

FC-AL is the most common and least expensive form of Fibre Channel. It links up to 127 ports in a network sharing the media (either Cat 5, unshielded twisted-pair copper or optical fiber). When a device has data to put on the channel, it requests the use of the media by sending an arbitration signal. If more than one device attempts to use the channel at the same time, the system uses the arbitration signal to decide which device gets the use of the channel. The device with control of the loop then sends an 'open' signal to the destination device and starts sending data. The connection is essentially point-to-point with all the devices between the source and destination of the loop simply repeating the data to pass it on.

Although the network's topology is usually in a circle, the devices may also be connected through hubs for reliability and ease of management. A hub or concentrator makes cabling easier and can detect and bypass a bad device or segment of broken fiber so it won't bring down the whole network.

Fibre Channel fabric is much simpler than FC-AL, but also more expensive. It relies on one or more central switches to establish direct, point-to-point connections between the source and destination devices. With fabric, many devices can be active at the same time and the medium is not shared (each pair has its own virtual circuit, established by the switch).

The complete Fibre Channel standard is quite complex. However, a good, basic tutorial is available from the Interoperability Lab at the University of New Hampshire (see additional resources section below)

Additional resources:

About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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