The three most popular classes of service for SANS in a Fibre Channel fabric are 1, 2 and 3. Class 1 is a virtual direct connection through the SAN between the transmitting and receiving devices, class 2 uses packets that acknowledge transmission (either after each packet or every N packets) and can follow multiple paths through the switch fabric. Class 3 also uses multiple paths but doesn't use the Fibre Channel acknowledgment. Detection and retransmission of corrupted information are handled by a higher-level protocol like IP rather than through Fibre Channel. All three classes are popular because they all serve different needs.
Class 1 itself is often used when a large amount of data must be transmitted over a long time, or when constant transfer latency is needed. (Constant latency is a big help in audio applications). With a lot of data, the setup and teardown times become a smaller part of the overhead and the sending and receiving devices can devote their full bandwidth to pumping through the data. However Class 1 has a number of drawbacks in other applications, so if you plan to use this class, you should know just what you're dealing with.
First, Class 1 circuits take time to set up and tear down. They can also waste bandwidth if the sender temporarily has nothing to transmit since the connection remains in effect until explicitly torn down. To deal with this, Fibre Channel has an Intermix option that allows interleaving Class 2 or 3 frames into a Class 1 connection. However this takes additional resources, must be supported by the switch(es) and managing the Intermix frames as they go through the system can be difficult.
Brocade Communications Systems has a white paper discussing classes of service and their effect on data transport on its web site at: www.brocade.com/SAN/white_papers/data_transport.html.
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.