Storage Basics: Fibre Channel Page 2


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Fibre Channel Standards
There are five layers to the Fibre Channel standard. Each layer is responsible for a certain set of functions or capabilities. In a sense it's a little like the OSI model (except that the OSI model has seven layers, not five) in that each layer in the model is reliant on the layer directly above or below for performing certain functions. The layers are numbered FC-0 to FC-4 from bottom to top. The following is a very brief explanation of the standards and their function.

  • FC-0 - Physical Layer : This layer defines cabling, connectors and the signaling that controls the data. Performs a very similar function to the OSI physical layer.

  • FC-1 - Transmission Protocol Layer : This layer is responsible for things such as error detection, maintenance of links and data synchronization.

  • FC-2 - Framing and Signaling Protocol Layer : This layer is responsible for segmentation and reassembly of data packets that are sent and received by the device. Sequencing and flow control are also performed at this layer.

  • FC-3 - Common Services Layer : This layer provides services such as multi-casting and striping.

  • FC-4 - Upper Layer Protocol Mapping Layer : This layer provides the communication point between upper layer protocols (such as SCSI) and the lower FC layers. The FC-4 layer makes it possible for more than SCSI data to travel over a Fibre Channel link.

By conforming to the layer format, products and applications that perform at one layer can be automatically compatible with products and applications that reside at another layer.

Market Challenges
InfiniBand (IB) has emerged as a formidable contender to Fibre Channel technologies and their associated products. However, IB is a nascent technology, and though it offers increased I/O speeds, the products associated with it are immature. That being said, some companies such as Mellanox Technologies appear to be leading this connectivity paradigm that could significantly cut into the Fibre Channel market in the future.

SCSI 3 has not outlived its usefulness, though, and since it offers considerable cost savings over Fibre Channel, Fibre Channel is only appropriate if SCSI 3 bus storage technologies will not suffice. Today Fibre Channel drives are more expensive than SCSI drives, but as Fibre Channel drives decrease in price, it is expected that the SCSI drive market share to migrate to Fibre Channel drives.

In Part Two....
In the next part of this look at Fibre Channel, we look at Fibre channel implementation considerations such as topologies, switches and port types.

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