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IP stack's a tight fit for sensor nets -- Secon keynoter urges rethink of wired IP net concepts to channel Internet traffic onto wireless sensor networks
(Electronic Engineering Times Via Acquire Media NewsEdge) Burlingame, Calif. - Wireless sensor networks can usher in an era of ubiquitous computing if they embrace and tailor the latest Internet Protocol software stack for their ultralow power requirements, said a leading researcher in the field.
The vast majority of tomorrow's expanded Internet traffic could come from sensor nets if the devices can squeeze IP into their architectures. "We've been at this a decade, and we are about halfway there," said David Culler, a professor of computer science at UC Berkeley who was involved in the development of TinyOS, software that helped pioneer the field.
But getting IP to work at this level is not easy, because some wireless sensor nets have a budget of perhaps an hour of battery life a year, Culler said in a keynote speech here last week at the IEEE Secon conference.
"You have to be asleep almost all the time, but IP networks are supposed to be always on," he said. "About 99.5 percent of the time you are off, which means you have to work very, very well in the remaining half percent of time."
Engineers need to throw out certain ideas from wired IP nets, such as making nodes remember complex routing tables and carefully acknowledging every transmission. "If I have one node in a thousand-node network that comes on the net to say 'hello' and everyone acknowledges that, we are all dead," he quipped.
Researchers have been experimenting with architectures such as TinyOS for a decade. Initially the thinking was that wired-networking concepts such as layered software stacks with an address for each node would not fit into the tiny processing and power budgets of wireless sensor nets.
"We were wrong," Culler said, showing a complete IPv6 stack suitable for existing sensor nodes. "It is doable with high reliability and low power."
Culler co-founded startup Arch Rock, which is already delivering IP-based wireless sensor nets. He is also co-chairman of an effort in the Internet Engineering Task Force to define a software layer that brings IPv6 routing to sensor nets using IEEE 802.15.4 radios. The IETF effort is based on the group's existing 6LoWPAN work.
While the work on IP sensor nets continues, many in the field are still divided over whether 802.15.4, Wi-Fi or other networks are the best means for transport. They are also split on the use of routing vs. mesh networking.
"There's a huge debate in the industry on this question," said Culler, adding that new routing and mesh protocols are still emerging. "No one has figured out the right way to do meshing," he said.
Secon hosted a full-day workshop on mesh networking focused on Wi-Fi. It included presentations from three companies shipping Wi-Fi mesh products for applications ranging from public safety to low-cost Web access.
Despite the activity, the IEEE 802.11s standard for Wi-Fi mesh networking is far from complete after extended work. Jorjeta Jetcheva, a systems architect at mesh specialist FireTide (Los Gatos, Calif.) and a .11s participant, gave several reasons for the delay, including unsolved problems in wireless security and difficulty defining routing concepts in a group that is, according to its charter, prevented from discussions of that network layer.
An Intel researcher said Wi-Fi meshes could open up new kinds of applications. For example, Time Warner is working with router maker FON Wireless Ltd. (Madrid) on ways to create Wi-Fi mesh-based neighborhood networks. Startups Mushroom Networks (San Diego) and WiBoost (Seattle) are also pursuing novel peer-to-peer apps using Wi-Fi meshes.
Most of the Secon conference focused on software issues. Culler noted significant progress in flash, controllers, radios and silicon-based sensors. But he said the still-fluid nature of software for sensor nets is to some extent hampering further progress in the underlying chips.
For example, the firmware abstractions that RF chip vendors create to simplify programming can actually prevent programmers from optimizing acknowledgement schemes. In addition, popular TI Chipcon 2420 radios can lose their configuration data if shut off, forcing engineers to copy channel information back into the parts when they are turned back on.
"There's a lot that could be done at the lower levels, and things are not even necessarily going in the right direction today," Culler said. "The hardware designers still don't have a stable software layer as a reference to design against, and we software developers haven't helped them much."
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