March 07, 2006
Making USB without wires work for consumers
There
are at least three flavors of wireless USB: Cable-Free USB,
Certified-Wireless USB and USB-to-TCP/IP converter. This article
evaluates each and concludes Cable-Free USB is best for ultrawideband
without wires.
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By
Calvin Harrison, Freescale Semiconductor
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The Universal Serial Bus (USB) is one of today's most useful and
prevalent connection methods. So many electronic components in the
market—from cameras to computers to cell phones—have USB-enabled
connections that hubs were created to allow even more devices to be
connected to a computer than the computer has physical connections. All
those connections are accompanied by plenty of wires, each representing
a limitation on the consumer.
There are several physical connector types for USB and since physical
connection must matched between devices, there's plenty of potential
for confusion. As USB devices become more visible in homes and offices,
it’s desirable to reduce the wiring to allow better device placement
and reduce clutter and complexity.
Wireless connections are the obvious solution. With so many
devices coming from different markets and a wide variety of companies,
there are different concepts for solving the problem of getting rid of
the USB cable.
There are three basic strategies for transferring USB signals wirelessly:
- The USB serial data stream is converted directly into
Ultra-Wideband (UWB) radio signals. The devices do not need to know if
the USB is transported through a cable or through a wireless signal.
- Use only the USB connector, but repackage the data so that the connector is really sending something else, usually TCP/IP.
- Redefine USB internally to electronic systems, and only preserve
compatibility at the applications layer of software. This is the least
backward-compatible solution.
There are several solutions for wireless USB on the market today, and emerging solutions that are expected in 2006 and beyond.
Strategy #1: Cable-Free USB
Cable-Free USB, a combination of Freescale Semiconductor’s Direct
Sequence Ultra-Wideband (DS-UWB) chipset and Icron Technologies’
ExtremeUSB—Wireless Edition technology, enables a true USB connection
wirelessly.
This technology was demonstrated in products at CES 2006. Cable-Free
USB is compatible with the USB 2.0 standard and fully
backward-compatible with previous USB standards. No driver changes or
software installation are required; the technology supports the
plug-and-play functionality defined by the USB Implementers Forum. With
Enhanced Host Controller Interface (EHCI) and Universal Host Controller
Interface (UHCI) support, no host controller changes are required.
These criteria make Cable-Free USB a very user-friendly solution.
Cable-Free USB supports major operating systems that support USB
(including Microsoft Windows 2000, Windows XP, MAC OS X, Solaris, Unix
and Linux) without modifications to drivers or the OS. It also supports
USB transfer types, including control, bulk, interrupt and isochronous.
Security is the same as that of wired USB; it is provided at the UWB
PHY/MAC layer using Advanced Encryption Standard (AES) 128-bit
encryption with RSA key exchange. The technology supports many
association models including factory matched pairs, one time push
button association and near-field communication (NFC). Cable-Free USB
provides an equivalent experience to wired USB, including support of up
to 127 connections.
DS-UWB radio technology allows very high data rates at very low
transmission power, with the added benefit of requiring very little
system power consumption. Freescale’s rule of thumb for power
consumption is one megabit of transmission rate for one milliWatt of
system consumption.
Cable-Free USB is a "true USB" interface to the
high-performance DS-UWB radio system which strictly adheres to the USB
2.0 interface. There are several advantages to strictly adhering to the
USB standard. A DS-UWB-enabled system can connect to a USB-enabled
system without any software updates required, because it is based on
the USB 2.0 standard.
A DS-UWB-enabled system can be placed on a mini-card form factor and
use USB connections directly, without modification to the host system.
This approach can help connect the more than 700 million USB devices in
the market—as they exist today, with no changes—and provides a solution
today for achieving USB wireless operation. Figure 1 illustrates some
wireless USB applications.
Click here for Figure 1
1. Connection scheme for Cable-Free USB enabled hub: How USB and UWB are combined.
Using USB in an RF environment introduces inherent difficulties of delay, latency and power management.
Originally, Icron’s ExtremeUSB technology was developed to
enable USB devices to be used in industrial and commercial environments
where operational requirements (typically the 5 meter limit) often
exceed those of the desktop for which USB was designed. In particular,
this technology overcomes the limitations imposed by the turnaround
timer (TT).
Removing this limitation enables the conventional RF techniques such as
error correction to be used. Operating at the USB protocol layer, the
ExtremeUSB stack is independent of the physical media used for data
transmission.
The TT parameter limits the time that any host or device may
take to respond to a request or to acknowledge data reception. The TT
was introduced into USB design to prevent an errant device from
consuming a disproportionate amount of time on the shared bus. It helps
maintain high bus occupancy by limiting the time that the bus sits idle
waiting for a response that may never arrive.
In wired USB, the TT budget is allocated to the various hubs, cables
and devices that constitute the worst-case (greatest delay) topology.
In wireless USB, there are additional factors that can push the
turnaround time beyond the allowed limits. Some radios take longer than
others to switch between RF transmit and receive modes. Restricted RF
bandwidth can force longer transmission times. Lost packets equate to
an infinite TT. Adding error correction, scrambling and encryption
functions increases latency and consumes additional bandwidth.
Another major difference between wired and wireless communications is
the much higher error rate at the physical layer that must be expected.
Compensating for this requires the use of error correction and
scrambling functions that exacerbate the TT issue mentioned earlier. In
the limiting case, when the error rate becomes very high,
re-transmission of an entire packet may be required.
Standard USB was not designed with these issues in mind and
does not accommodate them well, if at all. Cable-Free USB is designed
to enable standard USB hosts, hubs and devices to be used in high-delay
situations. In addition, any particular implementation can combine
support for speed and transfer-type variants as required. Just like
standard USB, devices with different speed and transfer type attributes
can be attached to and detached from the system at random. Cable-Free
USB recognizes each device automatically and provides the appropriate
protocol handling.
Click here for Figure 2
2. Icron’s ExtremeUSB Stack.
As shown in Figure 2, the zero-install Extreme USB does not affect
the systems that it connects to—no software installation is required.
The Cable-Free USB solution is engineered to be self-contained.
Strategy #2: Repackage the data and reuse the USB connector
Another strategy is to repackage the UWB signal through the USB
connectors. One solution that is being proposed in the industry does
this with a dongle and remote USB hub.
On the host side (typically a PC), the dongle is attached. Software
must be installed on the PC. This allows the PC to intercept the USB
communications at the software level, then reformulate and send it as a
TCP/IP signal through the USB connector, and then over the air. This,
in turn, is received by the hub, which is really a TCP/IP-to-USB
converter box.
This strategy provides for some backwards compatibility by
using the existing USB connectors, but it does require driver updates
and adding a TCP/IP-to-USB conversion layer.
A disadvantage of the TCP/IP solution is that isochronous
communications are not possible, which means that systems engineers
must build in extraordinary amounts of communications headroom—and that
means extra expense—to achieve trouble-free performance for audiovisual
systems.
While using existing USB connectors is very good for the market,
ultimately the best market solution should not require a bulk software
change at the driver level for thousands and thousands of device types
and countless millions of lines of code.
Another important limitation of this solution is that because
TCP/IP is being transported, this type of system can only be an interim
solution on the way to interoperating with other Certified Wireless USB
systems.
Products with this capability embedded, or even as dongles, will not
work with the emerging Wireless USB standard and create a second "new
flavor" of USB for consumers to juggle.
Strategy #3: Redefine USB for wireless
A third solution is to
redefine the USB specification for wireless and account for the goal of
allowing local interconnections of devices wirelessly. In essence, the
only portion of the original USB that would remain is the software
connections from the applications layer into the software driver layer;
everything else would change—including hardware, drivers and even
system architecture.
This type of system is USB almost in name only and in fact does go by a
new name: Certified Wireless USB. This solution aims to use a different
approach to DS-UWB, called Multiband Orthogonal Frequency Division
Multiplexing (MB-OFDM).
An advantage to this type of approach is that systems can be
designed for this specification and optimized for this solution. This
is true with almost every ground-up system design. But the implications
for an established USB ecosystem should not be underestimated.
So, what is a provider of systems that’s motivated by profit
supposed to do today? Rather than design for the future and wait for
sales to happen, most providers would be forced to have both a wired
USB solution and a wireless USB solution, with appropriate complexity
and solutions for the system to properly handle both. Or they might
take a wait-and-see approach to the market.
For example, for mini-cards to be implemented on a PC, wired
USB signals will be present since the specification allows both USB and
the PCI Express interconnect technology to be used.
Therefore, most systems would not eliminate the USB bus anyway. Even
though the USB connectors could be eliminated on PC systems that use
wireless, the USB bus would remain and the software developers would
still be required to write new software for this new wireless USB.
In other words, this method of implementing wireless USB is always an add-on to the system—not a substitute.
Finally, to underscore the significance of the software issue, it’s
worth noting that most embedded systems, such as the systems in
camcorders and phones, run operating systems other than Microsoft
Windows. Therefore, redefining software drivers for USB means that you
affect countless embedded software systems.
Obviously, as manufacturers evaluate their technology choices
to enable a wireless USB solution, there are many issues to consider.
Cable-Free USB enables these manufacturers to use the installed base of
USB devices, enables "closed" systems such as existing cell phones,
camcorders, printers, etc. to take advantage of wireless USB, and
supports a variety of operating systems—anything and everything that
USB devices support.
As Cable-Free USB technology becomes available to consumers throughout
2006, the demand for wireless freedom is expected to escalate. This may
make the most important issue for manufacturers to consider: consumer
happiness.
About the author
Calvin Harrison is manager of marketing and business development for Freescale Semiconductor’s UWB Operations.. He can be reached at calvin.harrison@freescale.com.
