3. Fibre Channel

The Fibre Channel (FC) specification is another set of standards being developed by ANSI and is intended to provide high-speed, inexpensive data transfer between all levels of computers and/or storage devices and other peripherals. [21]

Early efforts started with the Fibre Channel Systems Initiative (FCSI), a joint program supported by HP, IBM, and Sun. The FCSI is a closed organization, focused on interoperability between the three members' products. In 1993, the Fibre Channel Association (FCA), now a consortium of over 80 organizations was formed. The FCA is an open organization, with the objective of providing a forum for system integrators, manufacturers, etc. who develop FC products, and to complement the activities of the ANSI X3T11 committee which is developing the FC standards. At the Fall '95 InterOp conference, FCSI announced that it had finished its charter and would dissolve, leaving the advancement of Fibre Channel technology to the FCA.

Another organization, the Fibre Channel Loop Community (FCLC) is a smaller group focused on the use of Fibre Channel Arbitrated Loop (FC-AL) technology, one of the three topologies the Fibre Channel supports. The primary users of FC-AL are peripheral storage manufacturers.

3.1 Fibre Channel Overview

Fibre Channel (FC) defines a high-speed protocol originally intended for superworkstations, large array storage media and high performance desktop applications. It has found application usage in applications originally built using the SCSI (Small Computer System Interface) standard. Fibre Channel supports multiple data rates up to 4 Gb/s, in switched and shared architectures, in connection-oriented and connection-less modes. The physical media of FC will support ATM.

Data communications may be categorized into two types; channels and networks. Channels are point-to-point links between communicating devices. Channels operate at hardware speeds, with minimal software overhead, and interconnect only a small number of devices over short distances [22]. Examples of channel data communications include both SCSI and HiPPI.

Networks, on the other hand, provide low to moderate speed connections. Networks interconnect many devices, some of which may be physically distributed over long distances. While networks have a higher software overhead, they are more flexible in supporting a variety of applications.

Basically, channels are hardware intensive while networks are software intensive. Channels are simple, provide higher performance and guaranteed delivery, whereas networks are more flexible at the cost of lower throughput. Fibre Channel combines the desirable attributes of each. [23]

3.1.1 Fibre Channel Architecture

Fibre Channel provides a high-speed serial link and supports higher level protocols such as SCSI, HiPPI, ATM, and IP. Figure FC-1 illustrates the Fibre Channel protocol architecture. There are five layers, FC0 - FC4, which are briefly covered below. The first three layers are specified in the FC-PH document. [21]

Figure FC-1.

Fibre Channel Layers

3.1.1.1 FC-0 Layer

FC-0 specifies the physical characteristics of the interface and media, including cables, connectors, transmitters/receivers etc. Currently, there are products available for the 133 Mbaud rate, up to the 1 Gbaud speed. The higher rates have been approved as standards, however no products are available yet. Copper media (coax and twisted pair) is also supported for limited distances.

3.1.1.2 FC-1 Layer

FC-1 defines the 8B/10B encoding/decoding scheme. FC transmits 10 bits for every 8 data bits. Therefore the actual data rate is approximately 25% less (i.e. 100 Mbps data rate with the 133 Mbps signaling rate). IBM, which gives a royalty-free license for its use, has patented the 8B/10B encoding scheme.

3.1.1.3 FC-2 Layer

FC-2 is the core of the Fibre Channel specifications and serves as the transport mechanism of Fibre Channel. FC-2 defines the framing structure for Fibre Channel frames, the signaling protocol, the flow control, as well as the service classes that Fibre Channel supports. To support all these functions, FC-2 defines several message formats: Ordered Set, Frame, Sequence, and Exchange. The Fibre Channel service classes are discussed in the next subsection.

3.1.1.4 FC-3 Layer

FC-3 defines common services necessary for the higher level capabilities. While the FC-2 layer deals with individual N_Ports, the FC-3 layer covers functions that span multiple N_Ports. Among the advanced features envisioned are:

3.1.1.5 FC-4 Layer

FC-4 defines the upper layer protocol mapping to the Fibre Channel services. The FC-4 layer is analogous to the ATM SSCS layer. Each mapping is called a "protocol profile" and provides guidelines for implementing a specific protocol over Fibre Channel. Several protocol profiles are defined including SCSI, IPI-3, HiPPI, and IEEE 802.2 LLC for legacy LAN data.

3.1.2 Fibre Channel Topologies

Fibre Channel supports three topologies, point-to-point, arbitrated loop, and switched. All three topologies are fully interoperable, so the topology is transparent to the attached devices. Figure FC-3 illustrates the three topologies.

Figure FC-3

Fibre Channel Topologies

In Fibre Channel parlance, the switch topology defines a Fabric that interconnects each end-station. Switch ports are called Fabric or F_Ports, while end-station ports are referred to as Node or N_Ports. Ports in the loop topology are generically called L_Ports. Loop ports are further distinguish by whether the device is a node or fabric port. (FL_Port/NL_Port)

Both point-to-point and loop topologies require all nodes communicate at the same data rate. However, with a fabric topology, dynamic rate conversion is supported, so that a 266 Mbps device could communicate with a 1.062 Gbps device.

3.1.3 Fibre Channel Classes

Presently, Fibre Channel supports three classes of services, and a fourth which is a combination of classes 1 and 2. (Additional classes are specified in FC-PH enhancement proposals) Much like the service classes supported by ATM, these different classes allow Fibre Channel to support a variety of communication needs. [23]

3.1.3.1 Class 1

The FC Class 1 service, analogous to ATM Class A, provides a circuit-emulation service for time-sensitive applications such as video teleconferencing. Class 1 service is designed for dedicated, non-bursty links between supercomputers. Class 1 traffic is circuit-switched.

3.1.3.2 Class 2

The FC Class 2 service provides guaranteed delivery for connectionless traffic. Class 2 traffic is switched on each frame rather than on a connection. An acknowledgment from the destination provides an end-to-end guarantee of delivery. There is no corresponding ATM class since ATM does not guarantee delivery.

3.1.3.3 Class 3

The FC Class 3 service offers a best-effort connectionless service. Class 3 is similar to Class 2, except that no guarantee is given for delivery. Class 3 service is closest to ATM Class Y service, which is also a non-guaranteed, connectionless service with flow control mechanisms.

3.1.3.4 Intermix Class

The FC Intermix class is a combination of Class 1 and Class 2 services. Intermix reserves the full bandwidth of a dedicated connection, but also allows connectionless traffic within the fabric to share the link while the Class 1 connection is idle.