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By Henry Baltazar  |  Posted 2005-02-21 Print this article Print

Just as file services can be extended over a WAN, so, too, can SANs—using storage protocols such as iSCSI, FCIP (Fibre Channel over IP) and Fibre Channel over SONET (Synchronous Optical Network).

Fibre Channel is a storage networking protocol, but its distance limitation is 6.2 miles. However, some regulatory guidelines are recommending that IT managers keep a minimum of 100 miles between sites and are requiring IT managers to demonstrate the ability to rapidly restore business services after a disaster.

One way to bridge this gap is to use FCIP, a solid choice for companies that are trying to create direct connections between two geographically separated Fibre Channel SANs. FCIPs biggest benefit is that it is transparent to the hardware sitting on Fibre Channel SANs. (Devices have no idea their data is being sent over a WAN.)

FCIP is limited to point-to-point connections (connecting one SAN to another), but it works well and is fairly mature. FCIP takes Fibre Channel data and encapsulates it in IP so that the data can be sent over WAN links. In a basic FCIP implementation, FCIP gateways are placed at both the central and remote site and are positioned between the Fibre Channel SAN and IP network. The FCIP gateways establish a tunnel between themselves across the WAN to connect the sites .

FCIP gateways act as bridges between the networks, encapsulating Fibre Channel data on the way out and stripping off the IP headers of received FCIP traffic before forwarding them to the Fibre Channel SAN.

Another IP-based method of extending SANs is iSCSI, a newer protocol and probably the best choice for companies on strict budgets.

iSCSI is a TCP protocol that encapsulates SCSI commands. Unlike FCIP, iSCSI enables direct connections between hosts and targets. While FCIP is clearly a complementary protocol to Fibre Channel, iSCSI can be used as a replacement for Fibre Channel in environments that dont need high performance. (iSCSI is considerably slower than Fibre Channel.)

iSCSI is an IP-based protocol, so it does not require the addition of expensive Fibre Channel host bus adapters and switches to create a SAN.

Storage vendors LightSand Communications Inc., McData and Cisco also sell channel extender products, another means of extending SANs over WAN links. Unlike the IP solutions previously mentioned, these products maintain the Fibre Channel protocol through the WAN links. They also deliver much better performance than one would normally see in an IP solution.

To compensate for the latency on long-distance links, these gateway devices are loaded with credit buffers—large memory pools that provide a steady flow of information from SAN to SAN.

Channel extender products are the best choice for high-performance, synchronous replication of data. Channel extenders can use a variety of links, including SONET, dark fiber and DWDM (Dense Wavelength Division Multiplexing).

Senior Analyst Henry Baltazar can be reached at

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