Fibre Channel provides a bi-directional, physical or logical point-to-point, low latency connection between two devices at a time. Like other serial interfaces such as Serial Attached SCSI (SAS), in Fibre Channel data is transmitted over the physical medium in serial fashion as opposed to parallel methods, which are used in SCSI and ATA physical interfaces. Serial transmission enables much longer connection distances as compared to parallel transmission methods, because far fewer signal lines are required, thereby reducing noise created by multiple signal lines all switching at the same time (cross talk).
A primary use for Fibre Channel is in the transport of block oriented storage traffic in SAN (Storage Area Network) applications. There are also specialized upper layer Fibre Channel protocols that are used in Defense and Avionics applications, to transport video streams for heads-up displays, for example.
Fibre Channel is designed to work with either optical fiber physical medium or copper cables in many connector and cable type configurations. Copper cables are relatively inexpensive, but they are useable only for shorter distances, in the range of 10 meters maximum. In general the higher the transmission speed, the shorter the distance that can be reliably supported by copper cable technology. A variety of copper solutions are available, the most commonly used being copper SFP (Small Form-factor Pluggable) or more recently SFP+ copper cables. SFP and SFP+ optical cabling solutions are also available, supporting reliable transmission distances greater than 10 meters, up to about 10 kilometers.
Fibre Channel is a layered protocol, and is modeled loosely on the OSI model for networks. In the OSI model, and in the case of Fibre Channel, each layer provides specific services and makes the results available to the next layer. Figure 1 below compares the defined OSI layer to the defined Fibre Channel layers.
|OSI Model||Fibre Channel|
|7 - Application|| |
|6 - Presentation|| |
|5 - Session||FC-4 Protocol map|
|4 - Transport||FC-3 Services|
|3 - Network||FC-2 Framing|
|2 – Data Link||FC-1 Data Link|
|1 - Physical||FC-0 Physical|
Figure 1, OSI Model and Fibre Channel Network Layers
The layers in the table represent different functions and services that exist within the Fibre channel protocol definition. As with the other communication standards, protocol level analysis is often focused at the link layer (FC-2) and above.
Fibre Channel, like any network architecture, transports blocks of user or application related information called payloads. Before sending a payload over the physical link, additional Fibre Channel specific control bytes are added to both the start and the end of the payload data. The combination of the control bytes and the payload data is called a frame, which is the basic unit of information in Fibre Channel. A minimum of 60 bytes of overhead data surround each frame for the purposes of maintaining minimum separation between frames, mark the start and end of a frame, and to check for transmission errors. Within a frame the actual user data being transported can vary from 0 to a maximum of 2112 bytes. Fibre Channel transfers data through switched or direct point-to-point connections which work by creating temporary connections between the source and destination devices. These connections last only until the transfer is completed and can be temporarily preempted by higher priority transfer requests.
Connections are made on Fibre Channel systems through “interconnect components” such as switches, hubs, and bridges. The ability of Fibre Channel to use different interconnect devices makes it flexible and scalable depending on user needs. For small Fibre Channel networks, inexpensive hubs and loop-switches may be used for connecting devices in a topology called Fiber Channel Arbitrated Loop (FC- AL). As Fibre Channel networks get larger and network bandwidth demands increase, full matrix switching may be implemented. A fully switched Fibre Channel network is called a Fabric topology.
Fabric topology permits multiple alternative paths to be established between any two ports in the Fabric. Loop (FC-AL) topology, on the other hand, is like a string of Christmas tree lights where the path goes serially from one device to the next and finally back to the originating device. In this type of topology if one device or the path between any two devices fails, the entire string of devices lose their connection. Loop and Fabric topologies can be combined to provide both low cost connectivity and high performance.
Fibre Channel protocol is designed to support very low latency and high data transfer rates. The currently approved standard supporting up to 8.5Gb/s, is generally referred to as 8GFC. Server virtualization and storage virtualization are broad trends that are driving the need for higher bandwidth. The need for high bandwidth in the network infrastructure is just now beginning to drive the replacement of previous product generations of 1, 2, and 4GFC by 8GFC.
Fibre Channel is a good choice for any environment with many servers needing access to centralized storage, computer data centers for example. Because of this, Fiber Channel enjoys over 80% market share as the network interface used in external storage systems such as SAN environments.