MacEase

FIREWIRE, USB, AND SCSI:
DETERMINING THE INTERFACE THAT'S BEST FOR YOU

If you've recently purchased a new Mac, then you already have a FireWire-equipped Mac; if you own an older Mac that doesn't include a FireWire port, you may be able to add FireWire to it by installing a FireWire PCI card. In this article, I'll discuss the pros and cons of using FireWire with your computer, describe how FireWire compares to USB and SCSI, and try to dispel some common myths/misinformation about FireWire.

FireWire is designed to allow you to connect various types of hardware (i.e., hard drives, scanners, video equipment, CD-RW drives, etc.) to your Mac; in the case of drives, this includes either internal or external devices. To appreciate Apple's FireWire technology, you need to understand the alternatives to it that are available.

THE SCSI INTERFACE

For many years, the SCSI (Small Computer Systems Interface) has been the standard interface used for connecting devices to your Mac. The benefits of using this interface standard include its having been widely used for many years, so most types of hardware are available with a built-in SCSI port. Also, over the years the SCSI interface has evolved, and it now is available in several configurations including ultra high speed up to 160 MB/sec (megabytes per second).

The downside to using SCSI-based devices include that this interface can be highly temperamental (i.e., devices refuse to mount, disk errors occur, strange system problems occur, etc.), no more than 7-15 devices can be connected together in a single SCSI chain, you must assign each device a distinct SCSI ID number before adding it to your SCSI chain, the last device in the chain must be terminated, and all the SCSI devices in the chain must be turned off (along with your computer) before a SCSI device may be added to or removed from the SCSI chain (i.e., SCSI doesn't support plug and play/hot swapping of devices). Additionally, you must be careful to turn on all your external SCSI devices (and, when appropriate, allow them to spin up to speed) before turning your computer on, and you must remember to shutdown your computer before turning these devices off. Also, you need to use thick, relatively expensive, and heavily shielded cables when connecting SCSI devices to each other and to your Mac.

Basically, the SCSI interface offers both a large selection of devices to choose from and is capable of high speed performance, but it can be a real pain to work with.

THE USB INTERFACE

Several years ago, Apple decided to add a USB (Universal Serial Bus) port to the Mac. The benefits of using this interface include your not having to assign a unique ID number to each of your USB devices (this is done for you by your computer), there is no need to terminate the last device in the USB chain, each USB port can support up to 127 devices, and the USB interface supports hot swapping (USB devices may be added and removed while your computer is turned on).

You ask, "Is there a downside to using USB-based devices?" Of course there is. ;-) While in theory USB devices should support hot swapping, after connecting some USB devices to your Mac, they won't work until you restart your computer. Also, some USB hard drives can't be used as a startup volume, and some USB devices won't work properly unless they are connected directly to a USB port on your computer. (In theory, you should be able to plug a USB device into a multi-port USB hub which expands the number of USB ports that are available to you.) Perhaps the biggest downside is speed or, more specifically, the lack thereof. The current USB interface supports a maximum speed of 12 Mbps (megabits per second), and this is only a small fraction of the speed that is supported by both the SCSI and FireWire interfaces.

Also, each device that is on a USB chain shares the bandwidth (data carrying capacity) for the port on the Mac that it's connected to. Consequently, when several USB devices are attempting to simultaneously transfer data, they must share the already limited capacity of the USB interface (this can dramatically slow down these devices and your Mac). For this reason, in the real world, it's not likely most users would want to have many USB devices connected to these low-capacity USB ports.

Basically, most USB hardware like printers, scanners, and storage devices are convenient to use, but they are relatively slow when compared to their SCSI and FireWire counterparts.

To make things a little more interesting (maybe I should say confusing), there is a USB 2.0 interface that may be appearing very soon, and it will support a data transfer rate of up to 480 Mbps. However, even though in theory it may be faster than FireWire, it has yet to prove itself, Apple hasn't yet agreed to support it, and there presently are virtually no devices that work with it (some USB 2.0 compatible devices are scheduled to ship this Spring).

SPECIAL NOTE: You should be careful when comparing performance specifications between these interfaces. While both SCSI and FireWire performance are often stated in MB/sec (megabytes per second), USB performance is commonly stated in Mbps (megabits per second).

Since there are 8 bits (a bit is the smallest unit of information that your computer works with) per byte, the USB rating of 12 Mbps translates to only 1.5 MB/sec, so you can see USB performance is far below that of either SCSI (up to 160 MB/sec) or FireWire (up to 50 MB/sec).

Also, while "port" and "bus" are often used interchangeably, the port is the place on your computer where you connect a cable to the bus (the "roadway" your data travels on in your computer). Bandwidth indicates the information carrying capacity of this "roadway." The interface (SCSI, FireWire and USB) determines the "rules of the road" (the electronic specifications that control how your data is handled).

THE FIREWIRE INTERFACE

With this background information, you now have some perspective for weighing the pros and cons of using Apple's FireWire technology. The pros are that FireWire is capable of supporting a data transfer rate of 50 MB/sec (more about this later), supports hot swapping of devices, supports a chain of up to 63 devices per port, does not require you to assign an ID to your FireWire devices, and FireWire devices do not need to be terminated.

Another nice feature of FireWire (USB and SCSI don't do this) is that it reserves 3.5 MB/sec of bandwidth for many types of video equipment that's attached to your Mac. Since, like USB and SCSI, your FireWire devices share the available bandwidth, this reserved bandwidth guarantees your video performance won't deteriorate if other devices are also running on your FireWire chain -- very cool!*

You are probably saying to yourself, "Hmmm...seems like FireWire includes the best features of both the SCSI and the USB interfaces." In theory, this appears to be the case; in practice, there are some caveats (myths) to be aware of.

FireWire is a relatively new technology, and there are still some bugs to be worked out (Apple is making good progress with this, though). For example, as with USB, not all external FireWire drives are able to boot your Mac, and not all external FireWire devices are fully plug and play. The biggest myth is about speed; there is a huge gap between the theoretical performance of most FireWire devices and the actual performance you will experience when using these devices (SCSI devices also will not always be able to perform at the theoretical maximum SCSI limit).

Storage devices (i.e., various types of drives) often contain a "bridge" chip, and the function of this chip is to provide communication between a non-native FireWire device and your computer's FireWire bus (port). This chip is required because most storage devices are not yet native FireWire devices, so without this "bridge" your computer wouldn't be able to communicate with these devices. The "weakest link" theory applies here: the slowest point in the chain of communication between your computer and your FireWire device determines the fastest speed at which this communication can take place.

The bridge chips used in today's FireWire storage devices usually support a maximum data throughput in the range of 12-16 MB/sec. While this is much faster than even the theoretical maximum the current USB interface can handle, high speed SCSI devices are able to support a much higher data throughput rate. However, a faster bridge chip capable of supporting about 40 MB/sec is just becoming available, so it should soon be possible for storage devices to realize much of the potential that is offered by FireWire.

CONSUMER ALERT

While you see storage devices claiming they support FireWire's 50 MB/sec capability, you most likely will never approach this level of performance with your system until the new bridge chips are in use. Further, most consumer hard drives and other storage devices -- regardless of the interface being used -- aren't even capable of reading and writing data at 50 MB/sec (probably 20-30 MB is a more realistic number for the better hard drives), so I believe it's misleading for companies not to supply/advertise the "real" limitations/peformance capabilities of their equipment (of course, this also applies to SCSI devices).

I expect that most of today's consumer scanners, video equipment, etc. also aren't capable of generating data at a rate that takes full advantage of FireWire's potential, though it would be difficult to tell this from their advertisements.

THE BOTTOM LINE 

If and when the USB 2.0 interface becomes available for Mac users, it may provide some serious competition for FireWire. (Apple is currently working on a new FireWire spec that may boost FireWire's performance to 800, 1600, and even 3200 Mbps.) At the moment, FireWire is dramatically faster than USB and provides similar ease-of-use benefits.

If speed is your most important criteria when deciding whether to use either SCSI or FireWire storage devices, some versions of SCSI potentially provide real-world performance that exceeds that of most FireWire storage devices, though at a cost of sacrificing the ease of use that is provided by FireWire. Speaking of cost, SCSI devices also frequently cost more than both USB and FireWire devices (I believe this is at least partially due to the SCSI licensing fee).

Since SCSI has been around for much longer than FireWire, there still are more SCSI-equipped than FireWire-equipped devices available, but the availability of devices that include a FireWire port is growing rapidly. Also, using the bridge chip enables companies to use FireWire technology with inexpensive IDE drives, and this already has resulted in some very attractive performance/price ratios for FireWire hard drives.

In my office, I use SCSI, USB, and FireWire devices, and my clear favorite is FireWire! I have both a VST full-height 30 GB external hard drive and a VST 10 GB Ultra Thin USB/FireWire combo drive. I use VST's excellent formatting software and have found these drives to provide a wonderful combination of performance and ease of use, though there are less expensive FireWire hard drives available from other vendors.

Unless you need the very highest performance that is available, I believe that of the three interfaces, you are likely to find FireWire provides the best combination of performance, ease of use, and price!

BTW, if you have a Mac (or Mac clone) that isn't equipped with FireWire but does include PCI (Peripheral Component Interconnect) slots, you can install a relatively inexpensive FireWire PCI card that will add FireWire ports to your Mac (be sure to also check on the minimum hardware and system requirements for the card).

There are also some reasonably priced FireWire/USB combo PCI cards on the market that you might want to consider (since most Macs have no more than 3 unused PCI slots, a combo PCI card includes two different types of ports while using only one of these valuable slots). If you want to add a SCSI port to your Mac, SCSI and SCSI combo cards are also available.

If you have a PowerBook that doesn't include either a USB or FireWire port, it may be equipped with a CardBus slot that will accept a FireWire or USB card (if your PowerBook has this slot, your manual should include information about it).

SOME FINAL NOTES

Please note that this article does not attempt to cover all aspects of using FireWire, USB, and SCSI devices with your Mac. I hope you will find it has clarified the major differences between the three interfaces and has assisted you in deciding which technology best fits your needs! 

In the near future, I'll be posting a follow-up article  that includes tips on using both USB and FireWire devices and that discusses FireWire and USB hubs and cables.

*Apple supplied the information regarding the 3.5 MB/sec of reserved bandwidth that is allocated to many FireWire video devices, but thanks to Mike Teener (a lead engineer on the original FireWire development team) I can provide additional info on this subject.

Apparently, the information supplied by Apple is an oversimplification of how FireWire handles video devices. In fact, FireWire is even better at handling these devices than the information supplied by Apple indicates.

Usually, FireWire video devices only reserve bandwidth when they need it, and the amount of bandwidth reserved will vary depending upon the particular needs of the device that's being used. Additionally, the protocol used by FireWire provides more efficient communication between your FireWire devices and your computer than is possible to achieve when using SCSI devices with the SCSI interface. This can result in better real-world performance when using FireWire devices (not just video devices) instead of SCSI devices even when the SCSI device is connected to a SCSI interface that has a greater bandwidth than FireWire.

© 2001 by Steve Becker. All rights reserved.