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The difference between SCSI and FC protocols

What is the main difference between a SCSI protocol and an FC protocol?

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Two major protocols are used in Fibre Channel SANs The Fibre Channel protocol (used by the hardware to communicate) and the SCSI protocol (used by software applications to talk to disks).

The SCSI protocol (small computer system interface) is used by operating systems for input/output operations to disk drives. Data is sent to from the host operating system to the disk drives in large chunks called "blocks" of data, normally in parallel over a physical interconnect of high-density 68-wire copper cables. Because SCSI is transmitted in parallel, each bit must arrive at the end of the cable at the same time. Due to signal strength and "jitter", this limited the maximum distance a disk drive could be from the host to under 20 meters. This protocol lies on top of the Fibre Channel protocol enabling SAN-attached server applications to talk to their disks.

FC (Fibre Channel) is just the underlying transport layer that SANs use to transmit data. This is the language used by the HBAs, hubs, switches and storage controllers in a SAN to talk to each other. The Fibre Channel protocol is a low-level language meaning that it's just used as a language between the actual hardware, not the applications running on it.

Actually, two protocols make up the Fibre Channel protocol. Fibre Channel Arbitrated Loop or FC-AL works with hubs and Fibre Channel Switched or FC-SW works with switches. Fibre Channel is the building block of the SAN highway. It's like the road of the highway where other protocols can run on top of it just as different cars and trucks run on top of an actual highway. In other words, if Fibre Channel is the road, then SCSI is the truck that moves the data cargo down the road.

The operating systems still use SCSI to communicate with the disk drives in a SAN as Fibre Channel SANs layer the SCSI protocol on top of the FC protocol. FC can run on copper cables or optical cables. Using optical cables, the SCSI protocol is serialized (the bits are converted from parallel to serial, one bit at a time) and transmitted as light pulses across the optical cable. Your data can now run at the speed of light and you are no longer limited to the shorter distances of SCSI cables. (Disks in an FC fabric can be located up to 100 thousand meters from the host! 100Km).

Chris

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This was first published in March 2003

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