A look at arbitrated loop

July 1, 2002

Last month, I wrote about Fibre Channel technology, which establishes direct connections through the use of either a point-to-point, loop or fabric-based interconnection scheme.

Originally designed as a point-to-point networking technology using fiber-optic media, the development of Fibre Channel started in 1988 and became a standard in 1994. Unlike Ethernet topologies that require devices to share a common path, Fibre Channel topologies establish direct paths between devices allowing performance only limited by the speed of the connected devices. In addition to SCSI, Fibre-Channel can provide transport for a number of protocols including IP, ATM and 802.2 (Ethernet).

A, X and Y represent subsets of the drives in each system. FC-AL allows the administrator to isolate any drive from either controller (C or B). FC-AL can also maintain communications with any drive in the event of a failure.

Fibre-Channel can use one of three topologies: point-to-point, cross-point and arbitrated loop. The point-to-point topology, while the least flexible solution because it is limited to the connection of only two nodes, is useful for connecting a single server to single disk array. Using a series of interconnected intelligent switches, the cross-point topology provides the highest level of performance and scalability. Similar to the way modern telephone switching centers can route a call anywhere in the world based on a unique telephone number, a switching core called the fabric provides direct connections between the ports of two attached devices through one or a series of cascaded switches. Adding switches to a cross-point Fibre Channel network increases its performance because each switch provides additional paths.

Fibre Channel Arbitrated Loop or FC-AL has garnered the most interest and acceptance within the industry, primarily due to added support of copper-based media and multi-drop rings containing several devices.


The FC-AL network can be set up in two possible configurations: daisy-chained (loop) or through a hub. In the daisy-chain configuration, the transmit port of one device is connected to the receive port of the next device in the chain. Due to the shared nature of loop topologies, only one device may send data at any time. Access to the loop is determined by winning an arbitration to become the loop master. Configuring FC-AL in a loop topology presents several limits to the performance of the network because performance is affected by the number of devices attached to the loop. The ability to make changes to the network is limited because the addition or removal of new devices or segments within a loop will bring down the entire loop.

The most common topology used in the deployment of FC-AL uses a hub that forms a physical star interconnection. If a loop or non-responding device appears, the FC-AL hub will bypass the failure, thus maintaining the operational integrity of the network. The simplest hubs provide only a means to bypass ports not attached to the loop, while the more advanced devices feature a wide range of management options that can automatically sense, correct and report problems to the network administrator, as well as providing tools to monitor and configure devices remotely.

Figure 1. A comparison of Fibre Channel with other established data storage technologies.

Fibre Channel was originally developed as a means to separate network servers and disk-drive arrays without sacrificing performance. Storage specific interfaces, such a the Serial Storage Architecture (SSA) and the newer implementations of the SCSI standard, are still used extensively; however, Fiber Channel and especially FC-AL are faster, allow greater distance between devices and permit the ability to hot-swap devices without network interruptions.

How fast is FC-AL?

Inside a PC is a myriad of ribbon cables used to connect the disk drives to the PC's disk drive controller. The majority of PCs use a disk drive interface based on the IDE or SCSI standard. Ribbon cables are required because these interfaces transfer several lines of data over the individual wires simultaneously or in parallel. On the surface, it may seem that this would present the fastest method to move complex data, but for parallel transmission to work properly, all of the signals at the far end of the cable must arrive at the same time. Because the characteristics of each wire in that cable may be different, signals can pass at different speeds, particularly if that ribbon cable is pinched or damaged. To make sure all of the data arrives properly, the controller waits to receive the data for each line, thus requiring it to operate slower by default. In contrast, interfaces such as FC-AL and SSA are based on a serial data communications method where data is transferred over a single media, such as a pair of copper wires or fiber-optic media. The speed is limited only by the type of media used.

Presently FC-AL supports a maximum data rate of 106.4MB/s per port. It is possible to assign a second port to each connection that allows for that data rate in both directions simultaneously (full duplex), thus increasing data throughput to more than 200MB/s. In comparison, the fastest data transfer speed offered in current or future versions of the SCSI standard is 160MB/s. SSA operates at a maximum throughput of about 40MB/s.

As a practical matter, the capable throughput of either interface is far in excess of what the highest performing disk drives currently available can achieve.

Cable distance limitations

Transferring data (or RF for that matter) over copper wire can present a challenge, particularly as the bandwidth and frequency of the signal increases. Factors such as signal losses and cross talk between wiring limit the cables ability to carry signals over long distances.

FC-AL can operate over copper wire connections of as far as 30m; however, distances of as far as 10km can be achieved with fiber-optic cabling. The maximum distance recommended for current versions of SCSI is 25m and 680m for SSA using fiber-optic cabling.

In FC-AL networks, any number of drives can be added and removed provided the maximum number of devices does not exceed 128. It is possible to cascade multiple FC-AL loops that could support as many as 16 million devices. Figure 1 compares several common formats.

FC-AL clearly makes sense for those instances where there is a need to provide fast and reliable access to external arrays of disk drives as is typical for digital broadcast storage/playback systems.

McNamara, BE Radio's consultant on computer technology, is president of Applied Wireless Inc., New Market, MD.

The Networks articles have been approved by the SBE Certification Committee as study material that may assist your preparation for the SBE CBNT exam.