Level: Introductory Peter Seebach (crankyuser@seebs.plethora.net), Author, Freelance
26 Apr 2005 The USB specification may be an example of that hybrid de jure or de facto standard, one that clearly earned wide acceptance through its technical merit. Learn the history of the USB standard and some of its benefits to users and vendors, as well as where it missed the boat.
The USB standard is practically ubiquitous -- everything from laptops to
desktops to servers (to even embedded multimedia devices) has USB ports.
Some USB peripherals can even take other USB peripherals: a USB printer
may be able to connect to a USB camera. The initial standard saw
widespread adoption, and a newer, higher-speed specification (2.0) is now
fairly broadly used, although some devices or systems haven't yet been
upgraded.
USB is an interesting technology in that it specifies mechanical
connectors, electrical properties, and a higher-level protocol. Many
standards specify only one or two of these options. USB is a complete
standard. In fact, standardization in controller chips means that an
operating system with USB drivers can have any standard USB expansion card
on board and any standard USB device attached to the card -- and expect
the device to work!
From merely existing as a half-supported set of patches on Windows® 95 to
blossoming into a stable component of most major operating systems,
software support for USB has grown dramatically over the last eight years
or so.
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USB at a glance
USB 1.0 came about in November 1995. Introduced in September 1998, the
USB 1.1 standard (the first widely used one) was intended to become a de
facto standard; it was developed by major players that included DEC, IBM®,
Intel®, Microsoft®, and Compaq. USB peripherals can include everything from
modems to joysticks. Windows 95 Rev. B, Windows 98, Windows 2000, Windows
Me, and Mac OS 8 and higher support USB; version 2.0 is supported in Linux™
kernel 2.52 and up and in XP.
USB uses a tiered star topology, meaning that USB devices called
hubs can serve as connection ports for other USB devices. Only one
device needs to be actually plugged into the host PC. USB supports
stand-alone or embedded hubs, and a single USB port can support up to 127
devices.
USB 1.1 had two data rates: 12 Mbps for high-speed throughput devices
(disk drives) and 1.5 Mbps for lower bandwidth devices (joysticks). USB
2.0 (Hi-Speed USB 2.0), introduced in 2002, increased the peripheral-to-PC
speed to 480 Mbps. Besides offering extremely high-speed throughput to
external devices, it also enhances the performance of Web applications
that may need multiple high-speed devices to run simultaneously. A recent
spec, USG OTG 1.0a (USB On-The-Go), has been developed targeted at
portable devices -- it offers them a small connector to fit the mobile
form factor, low-power features, and most importantly, the limited ability
to communicate with other USB peripherals without a PC host.
A USB peripheral simply plugs into a port and works without installing a
card or powering the computer down (USB devices are hot-swappable).
Although a separate power supply is usually not necessary for a USB
device, it must be attached upstream to either the host PC or a powered
hub. In USB, configuration occurs automatically. Host controllers manage
driver software, bandwidth, and appropriate electrical power. USB
controllers also have improved the performance of game control devices
(joysticks, and so on) by eliminating the loss of calibration ("drift") due to
cable length and CPU overhead -- USB effectively reduces CPU overhead.
There are four types of USB connectors: Type A, Type B, Mini B, and Mini
A.
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Interface Darwinism
The history leading to the USB specification follows a, if not easily
predictable then at least, plausible evolutionary path.
The pre-USB landscape
Before USB, plugs were, well, plugs. A typical PC-compatible computer
would have one or two RS-232 serial ports (DB9) capable of about 115,200
Kbps. It would have a single parallel port capable of fairly high speeds
(roughly 300 Kbps or maybe 1.3 or 2.7 Mbps depending on which of the two
extensions to the specification you were using). It might have PS/2
keyboard and mouse ports or possibly an AT keyboard port and no mouse port,
or a special plug-in card for the mouse. If you had a good sound card, it
might provide a 13-pin joystick port, which might or might not support a
Y-cable to support a second joystick, and which probably supported a
special cable to give you MIDI in and out cables. Expansion cards might
give you more serial or parallel ports or possibly SCSI.
This was not good. You might have a scanner with SCSI and parallel
interfaces, a mouse that supported PS/2 and serial, a printer with serial
and parallel, a modem with only serial, a MIDI keyboard, a joystick -- by
this point, adding a single device could also demand that you reload
drivers for three other devices since you were going to have to shuffle
them around. Many devices came in two or three models that supported
different ports.
The parallel port, being the only cheap high-speed port in "port" (so to
speak), was horrendously abused. Any device connecting to a serial or
parallel port would require a custom driver and, by the way, did I mention
that there was no reliable way to figure out which custom driver that was
if you lost the CD?
Serial devices required you to configure the device and the computer for
the same baud rate, handshake, parity, data bits, and stop bits.
(Handshaking might be either hardware or software; hardware handshaking
had at least two incompatible protocols.)
This situation was awful for everyone. Peripheral vendors had to
navigate a twisty maze of alternative versions, and adding new peripherals was
difficult at best.... This was the epitome of the "good ole days" where
the whole scheme was bad.
Then, one day the sun rose just a bit brighter for users of peripherals
(which meant, everyone with a computer). The first USB specification, USB
1.0, showed up in November 1995 (the same year Apple adopted the IEEE 1394
Firewire standard). A few Windows systems at the time had USB ports.
The only USB devices I saw that early were an Intel-branded Webcam and
some very specialized front panel devices for use with the "Infinia" line
of Toshiba computers. (I still have one of these devices, but alas, no
drivers for it.)
Back then, USB drivers were fairly specialized patches each of which only
ran atop a specific version of Windows; different versions of Windows had
different USB drivers. The USB specification and hardware were almost
entirely irrelevant to most users. The USB 1.1 specification, which
improved things quite a bit, came out in September 1998. The 1998
specification was the one adopted in Windows 98.
The adoption problem
USB, even after support for it was available in Windows, faced an adoption
problem. Standard adoption is largely driven by network effects; the
utility of a standard-compatible device comes from its ability to
interoperate with other things compatible with the same standard. A
standard is only useful to you if there are compatible devices and
if there are a lot of them.
This creates a Catch-22 situation for adoption. If users lack USB ports
or drivers, those users cannot buy a USB device. For vendors, that limits
the market for USB devices and makes it more reasonable to develop
peripherals for other ports (such as the once-ubiquitous serial and
parallel ports, or the SCSI port if you also wanted to tap the Mac market).
Even if users would prefer a USB device, they would still be more willing
to accept a non-USB device since it can be connected to their computer.
Even if USB is a better, more desirable piece of technology, it may not be
more marketable than the alternatives! The number of people who would buy
a USB Webcam might be smaller than the number who would buy a serial
Webcam -- and almost all of them could be persuaded to buy a serial one
instead.
Enter the iMac.
The original "bondi blue" iMac was the first computer to offer USB ports
without offering "legacy" ports. That's right -- no serial ports, no ADB.
This changes the network effects. Before the iMac showed up, there were
many millions of PC users who had no USB ports and perhaps a couple of
million who had a USB port and also legacy ports. The biggest market in
1998 was in serial and parallel ports (or joystick ports, PS/2 ports,
and so on) -- there was no reason to target the USB market. That would just
restrict your audience.
The iMac presented a ready-made market of users who chose the Mac line
for its graphics capability. In turn, the iMac offered a captive audience
of users who would buy a USB peripheral but would not buy any other kind
of peripheral. These users provided a market for USB peripherals that
wasn't facing competition from other port choices. The result was a flood
of USB devices in white-and-blue plastic. This was a crucial turning
point that created a reason (tied to a proven system choice) to prefer USB
to non-USB ports.
Once adoption was foist onto this substantial segment of users, the
technical merits of the technology won out easily. USB's technical
superiority (for most peripherals) to the conglomeration of a half-dozen
different port types was unambiguous.
Survival of the fittest
After adoption was a proven benefit, USB simply annihilated a lot of its
competition. Joystick ports died. Serial and parallel ports suffered a
loss of popularity, although the plain old 16550 UART and other RS-232
serial devices still enjoy substantial popularity as console ports for
embedded systems and servers (see Resources). The PS/2 keyboard and mouse
ports have survived unchanged for most PCs, although a few systems don't
offer full support for them. My laptop has no PS/2 ports; its "port
replicator" does.
Firewire and Bluetooth are the serious, live competition to USB.
Bluetooth keyboards and mice are currently much more expensive than
wireless devices using other specs. If their price drops, they may offer a
real alternative. Firewire has been very successful in media and mass
storage where the higher performance (compared to USB 1.1) earned it early
dominance. Its weakness is the lack of detailed protocol specification.
No one would bother trying to make a Firewire keyboard.
Strengths and weaknesses
Now to look at some of the benefits and areas that are lacking in the USB
specification.
Strength: Specification flexibility and thoroughness
One of the interesting traits of USB is that it offers standardization of
the mechanical form of ports, their electrical qualities, and the protocol
used for them. This bears repeating because many
specifications don't go nearly this far. The RS-232 standard specifies
signaling, but not necessarily every common physical port design. The
parallel port standard defines signaling and physical structures, but
still doesn't say much about what data will pass over the port.
Specifications normally run the range from complete-but-specialized to
incomplete-but-generic. USB offers some fairly complete, specified
instruction, such as the mass storage spec or the human interface device
spec, but also leaves room for developer expansion and development. If you have a USB mass storage driver, any standard-compliant USB device
(whether it's a card reader, a disk, or a CD burner) will work with it.
When you have the human interface device driver, most keyboards, mice, and
joysticks will work without further intervention.
On the other hand, if your device is totally unlike any other, the spec
provides for sending packets back and forth, offering a flexible protocol
layer.
In some cases, a device will use only part of the USB spec. Nintendo
Gameboy SP is rechargeable -- and a third party sells a USB charging cable
for it. This uses the physical spec and power, but doesn't even show up
as a device. The protocol is ignored entirely. This is common enough
that some car and airplane AC adapters now offer a "dumb" USB port which
just provides power for rechargeable devices.
On the other side of the same coin, not all USB devices have recognizable
cables. My laptop has a built-in Bluetooth controller which is actually
attached over USB. It uses the USB protocol and probably standard USB
electrical signaling, but not the USB physical port specification.
(Another connector on the same laptop is probably USB, but the only device
I know of for it, a Webcam, never interested me.)
Weakness: Physical connectors
The physical connectors are one of the greatest weaknesses in the USB
specification. Originally, the USB spec provided a simple pairing: A
ports on computers, B ports on peripherals, with each cable having one end
of each type. This is not too bad of a scheme. However, the A port
design itself is not friendly from a usability perspective. It's too easy
to plug things in upside down. Labeling of which side is "up" is not
entirely consistent, so since ports are often on the back of a machine and
out of view, really the design ought to be distinguishable by feel.
Around the time of USB 2.0, a couple of new connectors started showing
up, one of which is much smaller than the original ones. That's nice, but
it introduces a potential incompatibility: now not all cables work with
all devices. (Some don't seem to be actually related to the standard. My
digital camera uses a port which I've never seen on anything else -- it is
the same as an older camera from the same vendor. This requires a
non-standard cable. For shame!)
One thing that appears to be underspecified is the clearance to be found
around a port. Many devices have no cable, but are too wide or thick to
allow another device to be plugged into an adjacent port. Manufacturers
clearly know about this -- many such devices come with a six-inch
extension cable to allow the device to be plugged in. It's still
annoying.
As for a more subtle "defect," I have some USB 2.0 devices whose plug is
inset so that not all cables will fit in.
Connectors are probably the weakest point of the USB spec since they are
harder to use than they should be. Some people complain about USB cables
coming unplugged under slight tension. A cable with a clear orientation
that a user could check by feel would have been a wonderful,
user-centered-design idea. It's one of the small details that Firewire
got right.
Strength: Revisiting the cable
A quick review of the cabling reminds me to point out the following: The
self-contained cable handling both signals and power was a genius of an
idea since it provided for a large number of USB devices which can run
entirely off the system-supplied power.
Strength: Documentation
Documentation is one of the best aspects of USB. Detailed documentation
on the specification is easily and freely available. This played a
substantial role in encouraging hardware developers to use the standard
protocols defined by the USB standard instead of trying to "innovate" in
ways that leave software developers scrambling to support undocumented
hardware.
This had the additional effect of providing consistently good open source
support for USB devices. Particular credit goes to Lennart Augustsson's
work on the NetBSD USB implementation which is quite frankly awe-inspiring
(see Resources). The NetBSD implementation was
unusually complete and stable back when USB devices were still fairly
experimental to many users. The net result is a huge base of code under
flexible and open licensing terms (providing a way to sanity-check
implementation questions) and also a lot of practical information about
device quirks.
The fact that the USB protocol includes specific documentation for
devices such as mass storage devices, keyboards and mice, and even a
standard printer device protocol (designed to support parallel/USB
adapters) made it possible for software developers to produce drivers that
would support a broad range of devices and for hardware developers to
produce devices that would work with a broad range of software.
It is hard to overstate the importance of clear, comprehensive
documentation. Aftermarket attempts to do the same thing for SCSI
interfaces and for scanners never achieved the kind of broad support and
reliable interoperability that USB's formal specification attained.
Alternatives and extensions
The USB 2.0 specification introduced higher-speed options at 480 Mbps (a
number which looks carefully calculated to exceed the performance of that
of the IEEE 1394 Firewire at 400 Mbps). The labeling of the speed of
devices has become very confusing, with recommendations that 12 Mbps
devices be labeled "USB 2.0" and 480 Mbps devices be labeled "Hi-Speed"
without a reference to the 2.0 spec! (According to the literature, 12 Mbps
devices should fall under both the 1.1 and 2.0 versions.) It's a good
thing to improve performance, but the confusing terminology hurts users.
A more curious thing is the proposal for wireless USB. This may sound
crazy, but a lot of the USB specification's value is in the clearly
written and well-documented protocol that allows driver writers to easily
support a variety of devices. This is a good idea for an extension, silly
though it might seem at first.
Learning the lessons
Advertising for the early USB didn't match up with true user experiences
very well. Enthusiastic proclamations that USB would support 127 devices
quickly faded as users learned that a seven-port hub was probably the
practical limit. Beyond that, bandwidth concerns and protocol overhead
made life too difficult.
The 12 Mbps bandwidth, while high compared with other existing protocols,
was not fast enough to be shared among too many devices. Before USB 2.0
came along, some vendors provided PCI cards with four separate USB 1.1
controllers, each with a single port, to allow users to get 48 Mbps of
total bandwidth, thereby repairing the utopian ideal by using a mundane
practice. Even on newer systems, multiple USB controllers seem to provide
improved performance and reduced latency. This matters more to users than
the theoretical option of connecting more than a hundred peripherals on a
single bus.
Once again, the cable design was poor. If nothing else, users would
benefit from a cable design which makes touch-and-feel connections easier.
The self-contained cable handling both signals and power was simply
brilliant. For the large number of USB devices which can run entirely off
the supplied power, USB's handling of this is masterful. It would be nice
if vendors were better about disclosing power requirements though. In
practice, 90% of devices "just work" without additional hassle.
The side effect of a market in USB charging devices is probably
unintentional, but absolutely fascinating.
The USB 2.0 spec helped address overall bandwidth problems, but
exacerbated confusion about performance by adopting an essentially
contradictory description for device speeds. It also introduced a new
connector type (the Mini-B for PDAs and cell phones and the like), a
reasonable connector that probably will cause no usability ripple at all
in the user's experience.
The USB spec has to be considered a rousing success, included in such
areas as:
- Self-powered devices
- Convenient expansion
- Reliable hot-plugging
- Excellent documentation
- Free implementations
- Comprehensive device specifications
- Low-cost parts
The experiences of everyone involved with the technology, from hardware
vendors to software developers to garden variety users, has been
immeasurably improved. Eight years ago, it seemed normal to explain to new
computer users why you needed to have six different port types. Now, it
seems ludicrous to even consider saying something like that.
The USB standard is a shining example of a standard earning wide
acceptance through clear technical merit.
Resources
About the author | 
| | Peter Seebach is a standards enthusiast who has been using USB devices for years. He has actually needed to use the USB keyboard spec in his own code. |
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