Frequently Asked Questions on Fiber Channel

Questions

1. What is Fibre Channel?
2. How do I choose between Fibre Channel and SCSI technology?
3. What is a SAN?
4. Which Fibre Channel host adapter is right for my application?
5. How does the "physical connection" come into play?
6. What are the differences between Arbitrated Loop and Fabric?
7. Do I need a hub/switch port for each Fibre Channel node or can I just chain Fibre Channel nodes together?
8. When is it recommended to use a hub versus a switch?
9. What are the effects of powering off or disconnecting a node on a Fibre Channel Loop?
10. I've heard that I can use dual loops in Fibre Channel for redundancy, how does this work?
11. What types of management tools are required for SANs?
12. What are the rules for cabling Fibre Channel devices?
13. I've heard there are two types of laser devices in Fibre Channel, what are they?
14. Can different laser types be connected together?
15. Are there any hazards with the fiber optics?
16. What is a GBIC?
17. What is an MIA?
18. How can the exact model of the Fibre Channel host adapter be determined for a unit previously purchased?
19. Can I boot off of a Fibre Channel drive connected to the ATTO host Adapter?

Answers

Fibre Channel operates at up to 2.125 Gigabits per second. It is a data transfer interface technology that maps several common transport protocols including IP and SCSI, allowing it to merge high-speed I/O and networking functionality in a single connectivity technology. Fibre Channel is an open standard as defined by ANSI and OSI standards and operates over copper and fiber optic cabling at distances of up to 10 Kilometers.

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If shared storage is required then Fibre Channel is the technology of choice. If transfer speeds are more important than sharing storage, then SCSI is preferable.

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A SAN - Storage Area Network - is a configuration where servers and workstations are connected to storage in a flexible, scalable high performance, high capacity, managed environment. Fibre Channel products are the core building blocks for SANs.

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Data transfer speeds and type of physical connection are two factors that differentiate the adapters. If you need to achieve speeds of up to 200 MB per second, then you should consider the 2-Gigabit ExpressPCI Fibre Channel host adapters. If you require less than 100 MB per second, then you might want to consider the more economical line of 1-Gigabit host adapters.

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Fibre Channel specifications allow for both copper and optical connections. Although copper connections are more economical, they are typically limited to cable runs of less than 30 meters. Optical connections are a bit more expensive, but they can be used for connections of up to 500 meters and are less susceptible to EMI (Electro Magnetic Interference).

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Arbitrated Loop is a topology in which host computers and storage devices are connected together with hubs. Hubs may be cascaded to increase the number of loop participants (up to 126). The available Fibre Channel bandwidth of 100 MBPS is shared amongst all of the devices. If four computers were communicating with four separate storage devices on the loop, each connection would be able to sustain approximately 25 MBPS. Because of this sharing, devices must arbitrate for access to the loop before sending data. A Fabric requires one or more switches to interconnect host computers with storage devices. With a Fabric, the bandwidth is not shared. Each connection between two ports on the switch has a dedicated 100 MBPS.

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Fibre Channel nodes can be directly connected to one another in a point-to-point topology, but this is not very practical. Because each Fibre Channel node acts as a repeater for every other node on a Fibre Channel Loop, one down or disconnected node can take the entire loop down. It is highly recommended that each node be cabled through a hub or switch. Hubs and switches, with their ability to automatically bypass a down node, are an essential availability tool in many Fibre Channel networks. For disk subsystems and RAID subsystems connected to a arbitrated loop, it is strongly recommended that each device or node within the subsystem have a Port Bypass Circuit associated with it so that any node may be bypassed and allow for "Hot Swapping" of a device. Some people do choose to daisy chain drive enclosures together off of the same hub/switch port.

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Hubs are typically used in smaller SANs. You have to consider what applications will be run on the SAN, how much bandwidth will be required for each computer at any one point in time, and the cost of hubs versus switches. For most storage applications, Arbitrated Loop provides more than enough bandwidth for efficient performance. If four different communications were occurring simultaneously, the maximum available bandwidth for each transmission would be 25 MBPS. Each of these computers will also have to arbitrate for access to the loop. If your application depends upon time dependent delivery of data (such as digital video), arbitration could result in delays between sending frames of data. ATTO Technology has qualified our five port Fibre Center hub with a digital video solution requiring 15 MBPS throughput with up to four host computers running simultaneously.

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In FC-AL, because each Fibre Channel node acts as a repeater for all other nodes that it is connected to, one down node will bring the entire loop down. To circumvent this possibility, for FC-AL implementations, it is recommended that Port Bypass Circuits (PBCs) be used. The Port Bypass Circuit is basically an electronic switch that will allow a node to be bypassed and electronically removed from the loop. The PBC allows a device to be powered down and removed without interrupting traffic or data integrity on the loop. If Fibre Channel nodes are cabled through a hub or switch, the hub heals the loop in the event of node failure, bypassing the non-operational node. All major Fibre Channel vendors implement port bypass circuitry in their products.

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The only way to provide full redundancy in Fibre Channel systems is achieved by cabling two fully independent, redundant loops. Two servers, each with a host adapter, two hubs or switches, and two separate drive arrays would be required. In addition, fail-over software is needed to detect when one path goes down and trigger the switchover event. This cabling scheme provides two independent paths for data with fully redundant hardware. There are other ways to reduce downtime. Two Host Adapters in one server, each connected to separate hubs or switches, going to a dual loop storage array will offer some level of failover. With two host adapters in the same server, all of the connected storage will be detected twice. Fail-over software is again required to distinguish between the primary and secondary drive, detect when one path goes down and trigger the switchover event. The problem here is that there is still only one server. If that fails, the system is down. The other issue is with a dual loop array. Each loop feeds the same drive. So if the drive itself fails, the system is down. While the number of single point failures is greatly reduced, it is not a true redundant system.

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There are many software tools available today to manage different aspects of the SAN. The only one that is really considered mandatory in a multiple computer configuration is what ATTO calls Volume Management Software. With Networked Attached Storage, each server is connected to its own bank of storage. This storage can be shared with other workstations or servers over a LAN or WAN. It is each server's responsibility to manage access to its storage. With a SAN, any connected server or workstation has direct access to all of the available storage. There is no dedicated server available to manage the data. This generates a few basic concerns. Namely, what happens if more than one computer is accessing a stored file at the same time, and how does one computer know that a file has been updated, deleted or created by another computer? ATTO AccelWare software was designed to manage these potential data corrupting issues. It is an easy to use software tool that executes in the background on each and every connected computer. Each computer is assigned different access privileges for every storage volume detected. Only one system will have write access to a particular volume at any one point in time. All other systems can have read access, or no access at all. Privileges can be easily modified when desired. Accelware also acts to continuously update the meta data on each computer for all of the storage so that every system will always know exactly what data is out there.

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There are two types of cable medium that can be used with Fibre Channel; Copper or Fibre Optic. Copper cables can use either DB9 or HSSDC connectors and are good for distances up to 25 meters. An active cable is required for cable distances greater than 15 meters. Optical cables use SC or duplex SC connectors. There are two different types of optical cables allowed; multi mode and single mode. Multi mode should only be used with short wave laser transceivers while single mode cables are meant to be used with long wave laser devices. The fibers in multi mode cable come in either a 62.5 micron thickness, which is good for distances up to 175 meters, or a 50 micron thickness, good for 500 meters. Long wave lasers/single mode cables are good for distances up to 10 km.

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Both OFC (Optical Fibre Control) and Non-OFC lasers are currently specified for use in Fibre Channel products. OFC optics uses a high powered laser that is controlled with a handshake to protect users from eye damage. Non-OFC optics uses a lower powered laser that is safer to the eye. The MIAs that we are selling are OFC with the built in safety precautions.

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Both OFC (Optical Fibre Control) and Non-OFC lasers are currently specified for use in Fibre Channel products. These two types of optics are incompatible. The lack of the corresponding handshake in the Non-OFC optics prohibits their inter-operation. There are also Long wave length and short wave lasers. These two types of lasers do not intermix either. Be careful when selecting GLMs and cables to purchase compatible optical products. Color keying is being promoted to make different optics types visibly recognizable. Look for standardization around one or two type of lasers in Fibre Channel for interoperability and ease of use.

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Both OFC and Non-OFC optics offer protection in different forms. OFC optics uses a high powered laser and thus employs a hand-shake mechanism that turns the laser off when it is unplugged to protect users from eye damage. Non-OFC optics uses a low powered laser that is safer to the eye, eliminating the need for transmission protection. Examining any laser without knowing if it is transmitting is never recommended.

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GBIC stands for Gigabit Interface Converter. It is a removable Fibre Channel transceiver unit that plugs into a socket on a Fibre Channel device. Fibre Channel cables are then plugged into the GBIC. They provide a simple method to switch between the copper signals on the circuit boards of the Fibre Channel devices to an optical signal used to interconnect devices. GBICs are universal in that they can be swapped on many vendors' Fibre Channel devices. Optical GBICs come with two types of lasers, short wave (good for 500 meters) or long wave (good for 10 km). Copper GBICs are also available. They are nothing more than a signal pass through device.

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MIA stands for Media Interface Adapter. Their purpose is to convert a copper FC signal coming out of a DB9 connector of a Fibre Channel device to an optical signal. Right now, they are only available with short wave lasers. This means that cable distances are limited to 500 meters.

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There are a variety of ways to determine the model of the host adapter. There is a sticker on the Fibre Channel controller chip of each ExpressPCI host adapter that identifies the model. If the host adapter is installed in a MAC and is not visible, launch the ATTO Express ProTools application. Double click the ATTO Technology bus in the left-hand window. The model will be listed four lines down on the left. If the host adapter is installed in a PC and is not visible, boot the computer and hit Control-F when prompted to launch the ATTO configuration program. Select the host adapter configuration option. Select the following options: Adapter Menu - Configure Adapter Channels. The model will be listed at the top of the page.

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Yes. You can boot externally from a PC or a MAC. Detailed instructions are included in the installation manual and in a read me file included with the firmware/driver files.

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