Service classes in Fibre Channel SANs

By Rick Cook

Fibre Channel offers several connection strategies, called classes of service, for connecting devices through a Fibre Channel fabric. ("Fabric" is Fibre Channel's term for the cross-point switch at the heart of a fabric topology Fibre Channel. This is not to be confused with the Fibre Channel Arbitrated Loop -- FC-AL -- topology, which may or may not use switches and doesn't use them in quite the same way.)

Class 1 service establishes a dedicated connection through the fabric before transferring data, much like a virtual private network. Class 2 and 3 services don't require a dedicated channel. In Class 2 service, the receiving device sends an acknowledgment message to tell the sending device the data has been received. Class 3 service has no acknowledgment.

There is also a hybrid service, called Intermix, that allows other devices to use the Class 1 channel's bandwidth when no information is being sent over the Class 1 connection.

Besides these basic services, the Fibre Channel specification lists three other classes of service: Classes 4, 5 and 6. Class 4 allocates fractional bandwidth of a channel to establish a virtual circuit between two devices. Thus a single channel may serve many virtual circuits. Class 5 is not fully defined. Class 6 is multi-cast (one-to-many) transmission through the fabric.

Additional resources:

• Brocade has a white paper on its Web site that deals with classes of service as they apply to SANs: http://www.brocade.com/products/technical_briefs/class.html.
• An excellent tutorial on Fibre Channel, including classes of service, is at the University of New Hampshire's Interoperability Lab Web site: http://www.iol.unh.edu/training/fc/fc_tutorial.html.
• More tips from SearchStorage are available at: http://www.searchstorage.com/searchStorage_Tips_Page/0,1800,,00.html.

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.