3.1 Fibre Channel Layers

Fibre Channel is a standards-based networking architecture. The standards provide definitions for physical-layer attributes, transport controls, and upper-level support of TCP/IP, SCSI-3, High Performance Parallel Interface (HiPPI), and other protocols. Fibre Channel is a gigabit transport, with current implementations at 1Gbps and 2Gbps. The governing body for Fibre Channel standards is the NCITS/ANSI T11X3 committee. Resources for the various Fibre Channel documents are listed in Appendix A.

Fibre Channel standards define a multilayered architecture for moving data across the network. As listed in Table 3-1, these layers are numbered from FC-0 to FC-4. The top layer, FC-4, establishes the interface between Fibre Channel and upper-level applications. The serial SCSI protocol, for example, must associate Fibre Channel devices with storage resources that can be accessed by the operating system. For host bus adapters, this function is typically fulfilled by the device driver provided by the vendor. FC-3 is under development and may include facilities for data encryption and compression. The FC-2 layer defines how blocks of data handed down by the upper-level application will be segmented into sequences of frames for hand-off to the transport layers. This layer also includes various classes of services and flow control mechanisms. The lower two layers FC-1 and FC-0 focus on the actual transport of data across the network. FC-1 provides facilities for encoding and decoding data for shipment at gigabit speeds and defines the command structure for accessing the media. FC-0 establishes standards for various media types, allowable lengths, and signaling.

Table 3-1. Fibre Channel Layers

Layer Number	Function	Content
FC-4	Upper-layer protocol interface	For example, SCSI-3, IP
FC-3	Common services	Under construction
FC-2	Data delivery	Framing, flow control, service class
FC-1	Ordered sets/byte encoding	8b/10b encoding, link controls
FC-0	Physical interface	Optical/electrical, cable plant

Collectively, the Fibre Channel layers fall within the lower levels of the OSI model and can be considered a link layer network. Fibre Channel assumes a single unpartitioned network and homogeneous address space for the entire network fabric. Although theoretically this address space can be quite large (15.5 million addresses in a single switched fabric), in practice Fibre Channel fabrics commonly support tens of devices in a single SAN or, in some data center applications, a few hundred devices.

Fibre Channel's layered architecture is implemented on three transport topologies: point-to-point, arbitrated loop, and switched fabric. Point-to-point is a dedicated connection between two devices only, typically a server and a disk. Arbitrated loop is a shared medium, similar to Token Ring or Fiber Distributed Data Interface (FDDI), and employs a special superset of commands to control access to the medium by multiple devices. A Fibre Channel fabric is one or more switches providing higher-level services and switched bandwidth of 100MBps or 200MBps (200MBps and 400MBps full duplex) per port. Each of these topologies is discussed in detail in Chapter 4.