6LoWPAN: Low-power IP connectivity

Until recently, extending IP out to wireless industrial networks was thought to be impractical, if not impossible. Vendors embraced proprietary protocols because they presumed that IP, which is memory- and bandwidth-intensive, couldn't be scaled down to operate on the microcontrollers and low-power wireless links used in these environments.

The release of the IETF 6LoWPAN draft standard for IPv6 communication over IEEE 802.15.4 redraws the landscape. 6LoWPAN's potential for low-power operation makes it attractive for use in everything from handhelds to instruments, and its built-in support for AES-128 encryption offers the basis for robust authentication and security.

IEEE 802.15.4, standardized in 2004, was designed to enable the development of compact, low-power, inexpensive embedded devices, such as sensors, that can run on batteries for one to five years. IEEE 802.15.4 carries information on radio transceivers at 2.4GHz -- roughly the same band as Wi-Fi but using about 1% of the power. Because this limits transmission range, collections of devices must work together to route information hop by hop over longer distances and around obstacles.

The charter of the IETF 6LoWPAN working group was to define how to carry IP-based communication over IEEE 802.15.4 links while conforming to open standards and assuring interoperability with other IP devices.

Doing this would eliminate the need for an array of complex gateways (one for each local 802.15.4 protocol), as well as application-specific adapters and gateway-specific security and management procedures. The utility of IP doesn't come for free, however: Addresses and headers are large, and data transfers may be too bulky to fit in a tiny 802.15.4 packet. The technical challenge addressed by the 6LoWPAN group was to devise a means of squeezing IP headers into small packets that would carry only the bare essentials.

Their answer was a "pay as you go" header-compression method that eliminates redundant or unnecessary network-level information in the IP header, which, on reception, derives that information from related fields in the link-level 802.15.4 header.

The simplest case -- one 802.15.4 device communicating with a nearby 802.15.4 device -- is handled very efficiently. The entire 40-byte IPv6 header is reduced to a Header Compression byte (HC1) and one byte of "hops left," because the source and destination IP addresses can be generated from the link-level 64-bit unique ID (EUID 64) or the 16-bit short address used in 802.15.4. The 8-byte User Datagram Protocol transport header is compressed to 4 bytes.

As the communication task becomes more complex, 6LoWPAN adapts accordingly. For communication with devices outside an embedded network, the larger IP address is included. When the amount of data exchanged is small enough to fit into a basic packet, it can be included with no overhead; for large transfers, a fragmentation header is added to keep track of how the message is broken into fragments. If a single 802.15.4 hop can get the packet to its destination, it can be transmitted with no overhead; multiple hops require the inclusion of a mesh-routing header.

The breakthrough of IETF 6LoWPAN is to achieve a very compact and efficient implementation of IP, removing the factors that previously gave rise to an array of ad hoc standards and proprietary protocols. This has particular value in the world of industrial protocols (such as BACNet, LonWorks, the Common Industrial Protocol and Supervisory Control and Data Acquisition), which originally were developed to provide interoperability over particular, industry-specific busses and links, from Controller Area Network Bus to AC power lines.

Developers of these protocols years ago created an IP option to allow operation over such "modern" technologies as Ethernet. The advent of 6LoWPAN lets these established protocols extend their IP option to new links, such as 802.15.4 -- and thus to interoperate automatically with emerging protocols designed specifically for 802.15.4 (such as ZigBee and SP100.11a). Now, for the first time low-power wireless devices of all kinds can join the IP family, taking their place alongside Wi-Fi, Ethernet and a host of other devices.

Culler is co-founder and CTO at Arch Rock Corp., and a professor of computer science at University of California, Berkeley. He can be reached at dculler@archrock.com.

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