The Truth About USB Speed- Dan
Knight
August 30, 1999
Until May 1998, the Mac world was pretty much oblivious
regarding the Universal Serial Bus (USB) found on many
Windows computers. When Steve Jobs announced the iMac,
he told the world it would have no ADB ports for the mouse
and keyboard; no Mac serial ports for printers, modems, and
LocalTalk; and no SCSI port for connecting external
drives. Where the first Mac had two serial ports, the iMac would
have two Universal Serial Bus (USB) ports. Apple was going USB, and we didn't know what to
think. Although Microsoft, Intel, and their cohorts had created
USB and were seeing it built into many Wintel computers, the
simple fact was that USB was a bust. Nobody seemed to
have any printers, modems, scanners, disk drives, game
controllers, mice, keyboards, joysticks, or other devices
that actually plugged into USB. Steve Jobs threw down the gauntlet. If they wanted to
sell to the iMac market, they had three months to create USB
devices. The USB SpecificationUSB is like ADB on steroids. Both were designed as
inexpensive extensible buses, although ADB is excruciatingly
slow compared to USB. Still, ADB could support up to seven
devices, if they didn't require too much power. There were
even ADB modems, although the were only 2400 bps. Like ADB, USB supplies power to devices, although some
devices need more power than it provides. But USB goes
beyond ADB: it can manage 127 devices, allows powered hubs,
and lets you plug and unplug peripherals without shutting
down the computer. Then there's speed. ADB was 10 kbps. Apple's serial ports
supported 230.4 kbps for LocalTalk. But USB has a 12,000
kbps (12 Mbps) bandwidth. That compares favorably with almost all serial protocols,
including 10 Mbps ethernet. The Real WorldThe promise is 12 Mbps throughput, but does anything
actually deliver it? Thus far, the answer appears to be no. In an article about Macworld
Expo last year, Henry Norr wrote, "...the sustained
transfer rate for the Mac (USB) version [of the SparQ
drive] will be 0.7 MB/sec. That's even slower than the
parallel port version, which has a sustained rate of 0.9
MB/sec and a burst rate of 2 MB/sec." Converted to megabits per second, that put the USB SparQ
drive at 5.6 Mbps -- about half the rated bandwidth of USB.
But then again, that's the manufacturer's specification, not
a benchmark result. Things have scarcely improved since then. A member of the Retro-Talk email list reported 31 MB/min.
backing up to the USB Aiwa tape drive. That's just 4.2 Mbps.
(Update: See Donovan Watts' review of the Aiwa
TD-UM8000 USB tape drive, which shows a 10% higher
throughput of 4.7 Mbps.) A review
on The iMac.com of the McPower case for turning an IDE
drive into an external USB drive reported a throughput of
0.64 MBps (megabytes per second) or 5.1 Mbps. Iomega rates their USB Zip with a sustained transfer rate
of 1.2 MBps or 9.6 Mbps, but no review reports nearly
that throughput. A benchmark on Microtech's site shows
USB performance of 163 KBps (1.3 Mbps) for the Iomega USB
Zip drive and 198 KBps (1.6 Mbps) for Microtech's drive. A review of the Imation SuperDisk 2x shows sustained
rates of 0.7 MBps (5.6 Mpbs), among the best speeds yet seen
on USB. This also explains why reviews of USB CD-R warn against
burning CDs at more than 2x speed. Going to 4x is 600 Kbps
-- more bandwidth than may be available on the Universal
Serial Bus! (The CD-Recordable
FAQ says, "USB recorders work fine at 4x when connected
directly to the computer. You may need to reduce speed to 2x
if you use a hub." If anyone can verify this on the Mac,
please let me
know.*) - * One reader has written to confirm this. Using a
Power Mac G3 and Sony Spressa USB CD-RW drive, he can
burn reliably at 2X, but every attempt at 4X fails --
whether connected to a hub or directly to the computer.
CharisMac (who makes Discribe) says it is a bandwidth
problem with USB.
[12/1/99]
In reviewing the Que! CD-Rewritable, MacCentral
noted, "Data and audio CDs can be recorded from your Mac's
internal CD-ROM or DVD-ROM drive to the QUE! drive at 1x or
2x speeds only. This is due to the slow nature of the USB
interface." Other reviews, linked on Que's
Press Room page, note the same thing -- you can't record
CD-R or CD-RW reliably at 4x. Note that this is a limitation of USB, not of the Que!
drive in particular. And you'll probably never see a USB DVD player. USB is
only providing about 4-6 Mbps, but DVD needs more than that,
over 10 Mbps. ConclusionDifferent benchmarks will yield slightly different
numbers, but it's quite clear that the promised 12 Mbps
performance of USB remains an unfilled promise. No real
world test shows even half that throughput, whether the item
being tested is a hard drive, Zip drive, SuperDisk drive, or
tape drive. In the real world, consider USB a 5-6 Mbps standard for
now. Perhaps future drivers will improve throughput (Apple
has version
1.3 in prerelease), but for moving large quantities of
data, USB is slow. The original SCSI protocol was a lot faster. The
Macintosh SE could support throughput of 5.2 Mbps, about the
same as USB provides today. The Mac II, introduced at the
same time as the SE, supported throughput to 11 Mbps -- but
that was really possible. And since then, various SCSI standards and
implementations have become even faster. There has been some talk of USB2, an even faster USB
protocol that would be backward compatible with older USB
hardware, but I don't think that's a good idea. Why not? First, because USB doesn't seem to provide
promised bandwidth. Why should we expect a faster version to
do any better? Second, because Apple has a better idea. FireWire currently
supports bandwidths of 100, 200, and 400 Mbps -- eight to 32
times faster than USB's theoretical maximum. The Blue G3
already has FireWire ports; rumors are the next iMac will,
too. Instead of revising USB, which is adequate for many
things, the computer industry should stick with USB as a low
speed bus for input devices, floppy drives, etc., but
promote FireWire for hard drives, removable media drives,
tape drives, and other devices that can outpace USB. Now that we know USB's limitations, we should learn to
live with them -- and move forward with FireWire as our high
speed bus.  Further Reading- How fast is
fast?, Dan Knight, Online Tech Journal, 7/19/99.
Compares theoretical bandwidth of USB, SCSI, FireWire,
and more.
- USB
users may face bandwidth problems, InfoWorld,
6/8/98.
- USB
compared to other ports, MacSpeedZone
- SCSI throughput, Mac
Online Tech Journal. Historical overview of SCSI,
including real world and theoretical throughput
rates.
- Introduction to
FireWire, Online Tech Journal.
- FireWire a form
of SCSI, Alex Timbol, Online Tech Journal.
Explains how FireWire grew out of the SCSI standard.
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