Low End Mac's Online Tech Journal

USB: Okay for the Low End

Apple has adopted the Universal Serial Bus (USB) for the iMac, replacing the SCSI, serial, and ADB ports Apple has used for over a decade.

At first glance, USB has a lot going for it. (Visit usb.org for details.)

• plug and play simplicity, even with power on
• 12Mbps bandwidth (faster than regular ethernet)
• supports up to 127 devices
• works with hubs or daisy chains
• provides power to peripherals

And the list of USB products is extensive: keyboards, mice, joysticks, digital cameras, scanners, telephones, video cameras, speakers, displays, and soon even disk drives.

Many Mac users already know USB is slow compared with SCSI - 12Mbps is less than one-third the bandwidth of SCSI-1, and today's Macs use SCSI-2 and SCSI-3, even faster protocols.

"USB users may face bandwidth problems" in InfoWorld (8 June 1998, p. 14) notes some potential problems with USB.

For instance, every USB device has to share the same bandwidth. What happens when your USB speakers and USB video camera both demand the bulk of your bandwidth?

That depends on which was plugged in first!

According to the article, you'll either get good video and poor sound or poor video and good sound. (A good reason not to use USB for speakers!) USB is plug and play, but you'll need to know the correct order for plugging things in.

Also, the USB specification calls for ports to provide power, but the amount varies. A passive hub, as in the iMac keyboard, need only supply 100 milliamps to each connected device, while a powered hub (such as the ports in the iMac) must supply up to 500 milliamps.

If a device needs more power than is available, you won't know it until you discover the device isn't mounted. And at least on the Windows side, the OS won't tell you a device is connected but unable to mount. (Let's hope Apple gets that right!)

What's the alternative?

We can hope that Apple will offer FireWire as an option on the iMac and a standard feature on future Power Books and Power Macs. A plug and play protocol for the power user, benefits of FireWire over USB include:

• 8 to 32 times faster (100-400Mbps)
• provides up to 60W of power to peripherals
• can connect computers as well as devices
• faster than SCSI

While USB will be fine for modems, mice, keyboards, and joysticks, serious users will want the greater bandwidth of FireWire for external drives, live video, scanners, and other high throughput tasks.

Real World Throughput (added July 9, 1998)

In a column on Macworld Expo, Henry Norr notes a 0.7MB/sec. sustained throughput for the USB SparQ drive. This translates to 5.6Mbps or about half the potential bandwidth of USB. It is also comparable to the maximum throughput over 10Mbps ethernet.

Put in perspective, the best network backup speeds I see over a switched 10Base-T network with Retrospect and a DDS-2 tape drive is 42.8 MB/min. Assuming SparQ performance is typical for USB drives, 0.7MB/sec. equals 42MB/min. Not blazingly fast, but neither is it terribly slow.

Backing up 1GB of data at this speed would take about 25 minutes, and a bit less if the backup software compresses data before writing it.

This means USB is not just viable for squirting little files to a floppy, Zip, SparQ, or SuperDisk, but also acceptable for backup. SparQ media, at 3 for US$100, are also reasonably sized and priced for this, making them a very real alternative to the more expensive digital tape drives.

Conclusion

Apple has generated some real excitement for USB, even though it was invented on the Wintel side of the road. Apple is leveraging the expertise of those who have developed USB devices for the Windows market with the fact that USB has not even come close to replacing serial and parallel devices in the Wintel world.

With an expected 400,000 iMacs sold by the end of September, Apple may create the largest single market for USB devices - especially since iMac owners won't have options such as ADB, serial, or parallel ports.

Further reading

• USB compared with other ports, Mac Speed Zone
• FireWire: connection of the (near) future, MacTimes