Extending SANs over MAN and WAN

SANs based on Fibre Channel are isolated installations. Because Fibre Channel is a switched, not routable, protocol, it cannot be routed over a wide area network or metropolitan area network. There are many good reasons to want to connect SANs over a long distance, data protection being chief among them. Copying blocks of data in near real time over a distance is a key component of data protection. Fibre Channel can reach to 2 kilometers, which is enough to get across a campus or over a river. However, doing so requires the laying or leasing of a dedicated fiber optic cable. That can be extremely costly. The solution is to find ways to interact with public networks available from telecommunications providers. That provides a balance between cost and function.

There are several ways to extend a SAN beyond its own cable limits by using public networks. The most popular is to change the transport by using the IP network to carry an FC frame. A protocol called FC-IP was developed to do this. The FC frame, including its payload, is encapsulated in an IP packet. The frame can now be routed over a public network. At its destination, the FC frame is stripped out and placed once again onto the Fibre Channel network. There are several SAN appliances that perform this function, as well as blades that integrate into Fiber Channel switches. A similar protocol called FC-BB sends FC frames over SONET.

Another way to get FC packets over a WAN or MAN is to crack open the frame; pull out the data; and place it in another type of packet, such as iSCSI. Converting to and from another type of protocol can be processor intensive and can lead to interesting address-mapping issues, but several products do this.

Finally, raw bits can be sent over an optical network. Optical switches convert electrical signals to wavelengths of light and send them out over a fiber optic cable. Several optical switches have blades available that convert physical Fibre Channel signals to optical and then send them over optical fiber. Flow control and other network functions have to be handled by the Fibre Channel switch. The optical network basically acts as very long, fat, fast cable.

Extending IP SANS

The problems of routing FC frames do not exist for IP SANs. IP SANs use a routable transport protocol, and IP switches already have interfaces for WANs and MANs. What remains to be seen is whether applications can tolerate the latency inherent in a WAN or MAN.