This month we are going to look at how Fibre Channel and Ethernet can be used together in the broadcast facility. Sometimes this discussion is cast as Fibre Channel vs. Ethernet, as if there were a competition between the two technologies. In reality, as both technologies have matured, the industry has adopted both in the areas where they make the most sense.
Ethernet is wildly popular. There are millions, if not billions, of devices in the world that use Ethernet, and because of this, the technology is quite inexpensive. Fibre Channel is not as widely deployed, but it has received a lot of attention in the area of storage networking — and for good reason. Fibre Channel is very fast, and it has been optimized to move large amounts of data, something that broadcasters can take advantage of.
Fibre Channel features
Fibre Channel started out as a way to move data between CPUs and storage systems without the overhead associated with Ethernet, and without the cable distance and device limitations associated with HIPPI and SCSI.
One of the keys to moving large files on a network is to move the data in large blocks. While the individual packet payload size is 2048 bytes, Fibre Channel permits the implementer to string a large number of payload packets together into a sequence (as large as 4GB) for delivery. Fibre Channel recognizes sequences at the hardware level, which means that large sequences can be delivered to a device without requiring a lot of processing power to read and interpret header information on the packets.
One other key feature of Fibre Channel is that because of the design of its lower levels, applications are assured that bandwidth is available when it is needed. This is not always the case with Ethernet networks, and it is one of the keys to the low overhead and high transfer capabilities of Fibre Channel.
For many years, storage devices were an integral part of the server itself. Servers were connected to disk drives using SCSI interfaces. But there were problems. Cables could not be longer than 8ft in practical implementations. And the total number of devices connected to the SCSI buss was limited to six once the controller was put in place. Fibre Channel resolves these problems while allowing manufacturers to continue to use SCSI software commands. It replaces the limited SCSI physical layer with a new architecture. Manufacturers treat it as a powerful cable extender.
What is Fibre Channel?
Fibre Channel is a two-way (duplex) communication channel that can be used to interconnect a wide variety of computers and storage subsystems. Each computer or storage subsystem is a node that has both a transmitter and receiver (these are combined in a device called a transceiver). Data is transmitted and received across a Fibre Channel link, which can be a copper wire (up to 25m), a short-wave optical fiber (up to 500m) or a long-wave optical fiber (up to 10km).
There are three main classes of service available with Fibre Channel. Class 1 is a dedicated connection for point-to-point operations. Class 2 provides a connectionless operation that requires a confirmation being sent back from the receiving node. Class 3 is a connectionless service that requires no confirmation; this is the class typically used for storage subsystems.
Fibre Channel frame structure
Data is sent across the Fibre Channel fabric as payload contained in frames. The 2K payload is surrounded by a header and footer, which help direct the frame through the network and correct errors that may have occurred in transmission. (See Figure 1.)
After the start of frame, there is a frame header. The header contains information about where the frame came from, where it is going and other information, which helps the frame to be correctly organized at the receiving end. Then comes the payload — the data to be transferred across the network. After the payload, there is a 4-byte CRC error check and finally a 4-byte end-of-frame marker.
Fibre Channel layer structure
Fibre Channel is designed in a layered structure. These layers are defined as FC-0 through FC-4, and much like the ISO layer model, they specify different functional components of the overall Fibre Channel technology. (See Figure 2.)
FC-0 defines the physical link used to connect the components. This includes physical measurements of connectors and fibers along with electrical parameters.
FC-1 defines the way data is encoded and decoded (commonly called the transmission protocol). This includes not only how the data is encoded and decoded but also how errors are handled.
FC-2, the signaling protocol layer, serves as the transport mechanism of Fibre Channel. FC-2 defines the framing rules for the data being transferred, the different mechanisms for controlling the three service classes and the means of managing the sequence of a data transfer.
The FC-3 level provides advanced features. This includes combining multiple ports to aggregate bandwidth, the ability for more than one port on a device to respond to the same address and multicasting.
FC-4 defines the application interfaces that can be used over Fibre Channel. While a number of interfaces are listed, the predominant ones are SCSI and IP.
Fibre Channel fabric can be configured in a number of different ways depending on the requirements and performance required across the network. Point-to-point is the simplest and least expensive topology to implement. It is also quite self-explanatory. In an equipment pair, the Fibre Channel gigabit linking modules (GLMs) are connected back-to-back. No hubs or other control devices are needed. Costs are low, the installation is simple, the bandwidth on the network is well-defined, and control and interoperability issues are limited, so resolving technical issues is a breeze.
The next step in topology is the Fibre Channel arbitrated loop (FC-AL). FC-AL has several advantages. In small configurations, it is simple, and for that reason, it is easy to troubleshoot. It is also expandable, with up to 126 devices per loop.
Single-loop FC-AL does have some problems, though. First, it is prone to failure. Because it is a single loop, a break anywhere in this loop crashes the entire network.
Second, in a single-loop configuration, Fibre Channel does not support simultaneous communications. This can seriously limit bandwidth on the network.
Broadcasters will find that most vendors employ a dual loop configuration. The dual-loop FC-AL eliminates the single loop failure mechanism. If one of the loops fails, the other assumes the load. A dual loop FC-AL also allows simultaneous communications between devices, greatly increasing the bandwidth available. While the cost of dual loop topology may be greater, for most applications, the security and performance increases are worth the increased costs.
The third common topology is switched fabric, which works by connecting full-bandwidth pipes between two devices that wish to communicate. This allows many devices to communicate at the same time, it increases the effective bandwidth available for each device dramatically, and it provides fault tolerance in large networks.
Fibre Channel and Ethernet in application
It is quite common to employ both Fibre Channel and Ethernet in a broadcast application. In the bottom of Figure 3, Fibre Channel is used to connect multiple processors to shared storage devices. At the top of the illustration, Ethernet is used to create a LAN, which is used to connect a number of workstations and servers together. This allows the processors to benefit from the fast, block-oriented technology of Fibre Channel when accessing storage, and it allows the workstations to take advantage of the ubiquitous nature of Ethernet.
Fibre Channel is one of the best ways to move large amounts of data between servers and storage devices. Its ability to put together a large number of packets into a single sequence for delivery is one of its key strengths. Ethernet is fast, widely deployed and well supported on the desktop. By combining these two, broadcasters can make the most of both technologies.
Brad Gilmer is president of Gilmer & Associates, executive director of the AAF Association and executive director of the Video Services Forum.
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