Videography - September 1997
The Many Flavors of Fibre
by Sheldon Liebman
Over the past few years, we've written a number of stories about Fibre Channel. In those stories, we've covered products, manufacturers and users, but we've never really covered the technology itself. This month, we'll be looking at how Fibre Channel technology can be used to create a variety of networks.
Unlike many other networking protocols, Fibre Channel can actually be used for three different network topologies. One of the reasons this is possible is that the Fibre Channel standard supports both computers and storage devices as nodes on a network. SSA, which has been covered in this magazine over the past few years, is another type of network that can be used with both storage devices and computers. More recently, some companies have adapted SCSI so that it can also be used in this manner.
What's Your Point?
Until recently, the most popular way to connect fast disks or disk arrays to a computer was through SCSI. In its various forms, SCSI allows multiple peripherals, including disks, to be connected to a single computer workstation. Most people wouldn't refer to a SCSI disk array connected to their computer as a "network," but the Fibre Channel standard allows a similar connection to occur. This is called Point-to-Point networking and is being used to replace SCSI in many installations.
Fibre Channel's Point-to-Point configuration is the easiest way to start creating a Fibre Channel network. It is also the least flexible. To create this type of network, you need a computer, a network adapter card, driver software and a disk or disk array with a Fibre Channel controller.
Because there are only two nodes on the network, the software requirements for the device driver are minimal. On the flip side, because it is designed to work with only a computer and a storage device, there is no scalability with this configuration. With the proper connections and a more intelligent driver, however, this can be converted into another type of Fibre Channel network.
Given that SCSI seems to work okay for most people, why would one want to replace a SCSI array with a Fibre Channel network? The answers are speed and distance. With Fibre Channel, you can have at least 30 meters between devices, which is significantly farther than is possible with SCSI. Using long wavelength optical connections, this distance can stretch to 10 Kilometers and beyond. On the speed side, a lot depends on the drives and controllers being used. However, typical Fibre Channel connections operate anywhere from 30 MegaBytes per second (MBps) up to 70-80 MBps. Even with the latest versions of SCSI, 30 MBps is pushing the limits.
It's interesting to note that most of the Fibre Channel compatible disk arrays being sold today actually use SCSI disks. This is because native Fibre Channel disks have only recently become available. By using an array of SCSI drives with a single Fibre Channel controller, manufacturers like Ciprico and Storage Concepts have been able to deliver Fibre Channel compatible disk arrays for 2-3 years. One of the largest providers of this technology has actually been Sun Microsystems, who have delivered over 50,000 of these SCSI/Fibre Channel disk subsystems during the past three years.
Getting In the Loop
As companies grow, they eventually need to connect to multiple computers and/or multiple disk arrays. If the only way to use Fibre Channel was Point-to-Point, another network could be installed with shared access to the Fibre Channel array. By using a Fast Ethernet connection, for example, the information on the disk array might be accessed at approximately 10 MBps. To make it easy to connect multiple computers and arrays while staying within a Fibre Channel environment, Fibre Channel Arbitrated Loop was developed.
Fibre Channel Arbitrated Loop requires more intelligence at the driver level and the installation of a hub that is connected to all the nodes. As the network grows, more or larger hubs can be added so that all of the devices are connected together in a single system. The advantage of Fibre Channel Arbitrated Loop, or FC-AL, is that it allows a Fibre Channel network to be configured at a lower cost than if a full blown switch-based fabric is created.
The disadvantage of FC-AL, however, is that there is a limit to just how large you'll want to make the network. Since all of the devices share the network bandwidth, performance degrades after a certain number of devices are connected. If the total bandwidth available is 100 MBps, which is reasonable in a FC-AL environment, the first three or four devices that you add may actually increase the total performance of the network.
Using the Point-to-Point network for comparison, we mentioned that the performance might be only 30 MBps between the computer and storage device. Adding a second computer in an FC-AL configuration may allow both computers to communicate with the storage at 30 MBps for a total of 60 MBps of total performance, still less than the 100 MBps maximum of the standard. Adding another computer brings the total to 90 MBps, which may still be doable.
Beyond this point, however, adding devices brings the performance of the individual connections beyond the total bandwidth of the system. If ten devices are trying to share the same 100 MBps pipe, for example, the average performance will probably only be 10 MBps, which offers little benefit over a Fast Ethernet setup. Because of this, companies like Prisa Networks often recommend that FC-AL networks max out at 4-6 devices per loop.
More devices can be connected by creating multiple loops based on workflow requirements and connecting those loops together. When information needs to pass between the loops, however, the performance may degrade.
I'd Rather Switch Than
For large networks, the best method of sharing information involves using one or more switches to connect your devices. This configuration is also supported through Fibre Channel and represents the original intention of the standard.
The difference between a switched network and one that uses hubs is a concept that confuses many people and will be explained here. The short answer is that a switch has intelligence and communicates with every device connected to it. This results in a significant performance increase, although at a much higher price.
A hub is not an intelligent device. Basically, it connects all of the devices to a single pipe and lets them determine on their own how to move data between them. Data is moved in packets, with each packet having a source device and an end device. If two devices want to move data at the same time, the one that requests it first gets it first. The second device has to wait until the pipe is available.
All things being equal, one packet may go to device A and then another to device B, so that they share the pipe equally. Or, there may be a priority system set up depending upon what information is being requested. A video file, for example, may somehow have priority over a data file, resulting in a long wait for the device requesting data versus a device requesting video. However it happens, a user has little influence.
A switch, though, is an intelligent system. It communicates with all the devices on the network and understands what they want and how they want it. A switch acts as the backbone or spine of a network, connecting devices that want to share information in a way that eliminates competition. In effect, a switch makes every connection a Point-to-Point connection so that the maximum bandwidth possible between two devices is achieved. In the Fibre Channel standard, a switch knows where to find every asset in the network and can control them from a single location.
The switch in a Fibre Channel network operates in a similar manner to the switch in a telephone system. All phone lines are connected to a switch and all of the switches have a way to connect to each other. When you dial a call, the switches ensure that only two devices end up being connected. When this process fails, you hear multiple conversations on a single line or you lose the call you've made. When it works, however, many connections can exist simultaneously without interfering with each other.
Let's Mix it Up
By definition, a Point-to-Point connection can only exist between two devices. However, Loops and Switches can be connected together to gain maximum results for minimum costs. The latest generation of Fibre Channel switches now contain a "Fabric to Loop" feature, or FL. This means that you can take a hub and plug it into a switch. Because there is only limited information available from the loop, there are bandwidth limitations, but it does provide a much cleaner way to scale up an environment.
With FL, multiple devices end up connecting to a single port in the switch, so the switch can't create individual connections for each device. The entire loop, however, can have fast access to another device plugged into the switch. In order for this to work properly, there needs to be some intelligence built into the adapters used in the loop. Prisa Networks, for example, has done the work necessary to connect to either a switch or a loop.
Finally, there is the issue of connecting multiple switches together to create a large network. This also requires some caution. If you only use switches from one manufacturer, it is a pretty safe bet that the network will operate smoothly. Once you try to integrate across multiple manufacturers, however, the switches need to be able to talk to one another to ensure smooth communication across the network. Since many of these products are new, it's important to look into this before trying to connect these switches together.
FC Stands for Flexible
Moving video quickly requires a network that has been designed to accomplish that task. Fibre Channel has evolved into a strong solution for our market. The choices in connectivity make it easy to move into Fibre Channel slowly and expand as necessary. The raw speed of the hardware provides a pipe that can support compressed and uncompressed video streams. And the companies that support the standard, including Tektronix and HP, have significant investments in the video world. Over the next few years, it will be interesting to watch see how video professionals configure their networks using this evolving standard.
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