Wireless USB

Wired users have long benefited from the common plug-and-play high-speed bus provided by USB 1.0 and 2.0. But while enterprises and consumers have moved to wireless architectures, USB has remained tethered--until now.

Two groups are attempting to design the wireless USB standard. Vendor Freescale is touting its proprietary Cable-Free USB. The USB Implementers Forum's Certified Wireless USB, on the other hand, is an open alternative with backing from industry heavyweights.

Enabling Technology

UWB (ultrawideband) radio technology makes wireless USB possible. In contrast with wireless technologies such as Wi-Fi, which offers maximum speeds of 54 Mbps, UWB uses roughly 25 times the amount of spectrum and offers a much larger "pipe" that tops out at 480 Mbps. By spreading the bitstream over a large bandwidth using low-power pulses, UWB can limit interference and co-exist with other technologies using the same spectrum. In addition, UWB's power consumption is about 10 times more efficient than Wi-Fi, so it improves battery life.Originally developed for military applications, such as surveillance and radar, UWB now can be used license-free by consumers and enterprises thanks to a 2002 FCC ruling, which included stringent power output constraints and limited the technology to the 3.1-GHz to 10.6-GHz spectrum. As a result, UWB's range is limited to approximately 10 to 15 meters.

The attempt to standardize UWB technology for high-speed WPAN (wireless personal area network) use began in 2002 under the IEEE's 802.15.3a working group. After consolidating 23 physical-layer specifications into two proposals, the group unanimously voted to withdraw the project in 2006, citing irreconcilable conflicts between the WiMedia Alliance's MB-OFDM (Multiband Orthogonal Frequency Division Multiplexing) and the UWB Forum's DS-UWB (Direct Sequence UWB) specs. WiMedia's platform was recognized in December 2005 by Ecma International as standards ECMA-368 and ECMA-369, which were approved by the ISO earlier this month.

Since the breakup, the WiMedia Alliance has focused on promoting and certifying interoperability of UWB technology based on MB-OFDM. In the alliance's most recent certification event, six silicon vendors-- Alereon, Realtek Semiconductor, Staccato Communications, Tzero Technologies, WiQuest Communications and Wisair--passed PHY-level interoperability tests. The next step is certification of entire WiMedia platforms, including both PHY and MAC layers, slated for early 2007.

Taking notice of the technology's potential, the USB Implementers Forum has selected WiMedia's radio platform as a key component of its Certified Wireless USB 1.0 standard. In addition, the Bluetooth SIG has selected WiMedia's version of UWB for its next high-speed Bluetooth revision. In contrast, the UWB Forum, which focuses on DS-UWB development, has faced troubles: Its key member, Freescale Semiconductor, withdrew from the forum to focus on bringing its version of wireless USB, dubbed Cable-Free USB, to consumers.

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Two Choices

Certified Wireless USB and Cable-Free USB have striking differences and are not interoperable. Certified Wireless derives from the USB Implementers Forum, which also created the popular USB standards; Cable-Free USB is based on Icron's proprietary ExtremeUSB technology, running on top of Freescale's UWB chipset. Cable-Free is designed around the USB 2.0 specification, whereas Certified Wireless USB is a new protocol designed with wireless in mind. Certified Wireless also touts higher speeds, but its spec requires new drivers, which may slow adoption; Cable-Free uses USB 2.0 drivers.

There are also important architectural differences. Cable-Free aims to replace a single USB cable in a point-to-point approach using a UWB-enabled USB hub and dongle that connects to existing wired USB connectors. This leads to a simpler security model since the hub and dongle are sold as a matched pair, preventing unauthorized connections. In contrast, Certified Wireless USB uses a hub-and-spoke model in which one host adapter, installed on a PC, communicates with as many as 127 Certified Wireless USB devices in the area. To prevent unauthorized communications, Certified Wireless USB supports two initial association models to pair a device with a host.

Cable-Free's advantage should be simplicity and time to market, but its edge is slipping. In contrast to Freescale's proprietary solution, Certified Wireless USB has strong industry backing from Intel, Microsoft, NEC and a range of silicon providers. However, at the time of this writing, neither Cable-Free nor Certified Wireless USB products have shipped; many factors, like a more forward-thinking solution and widespread industry endorsement, will likely tip the scales in Certified Wireless USB's favor.

Given that there are 2 billion wired USB devices in existence, it will take time to move them to wireless USB. The first wave of wireless USB devices will appear as a USB dongle and hub combination, which will let existing wired USB devices go wireless. Both Belkin and Gefen recently announced wireless USB dongle-hub products based on a reference design by Wisair, but these devices come with a hefty price tag (about $200), limiting widespread adoption until production volumes can bring prices down. n

Jameson Blandford is the lab director at the Center For Emerging Network Technologies at Syracuse University. Write to him at [email protected].