Class 2 Service and SANs
Rick Cook

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 are acknowledged after 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.

For reliability, you can choose Class 2 service, which doesn't maintain a fixed connection between sending and receiving devices. Each frame is sent separately and its destination address is examined by the switch(es) it goes through. Class 2 includes flow control between the connecting devices and at link level between the fabric and communicating devices. The class also supports messaging to indicate if a frame is rejected by either the fabric or the destination device.

The flow control and error-handling offered by Class 2 offer high data reliability and integrity. Class 2 service also makes very efficient use of the SAN's resources since frames can be multiplexed and sent by the most efficient route. It also allows speed matching between the sending and receiving

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device. Given buffers in the switches the current 1-Gbps device ports on today's SANs will be able to communicate with the 2 and 4 Gbps device ports on future Fibre Channel SANs.

Class 2's main drawback is relatively high overhead compared to Class 3 service. Although frames can be acknowledged in groups, the acknowledgment process still takes time. And because frames may follow different paths through the network they don't necessarily arrive in order. That can mean delays and extra work reassembling the frame sequence on the receiving end. Some vendors deal with the out-of-order problem by making sure all the frames follow the same path.

Brocade Communications Systems has a white paper discussing classes of service and their effect on data transport on its web site at:

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.

This was first published in November 2000

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