802.11s Wireless mesh standard gets boost from OLPC, open source

by John Cox

May 9, 2008 —

The proposed IEEE wireless
LAN mesh specification is already getting some traction, though still over
18 months from final ratification, thanks to early experimentation by the One
Laptop Per Child Foundation and a recently launched open source project.

This hands-on experience, fed back into the work of the IEEE 802.11s Task Group,
has already led to several changes in the draft standard, with other changes
being considered. When deployed, 802.11s will let different types and brands
of wireless devices find each other, interconnect securely, and forward traffic
on behalf of other mesh nodes, forging new paths automatically if nodes move
or wireless links fail. WLAN deployments will be cheaper and easier, say advocates.

PacketHop
earlier this month announced it will release in July the first commercial implementation
of the IEEE mesh draft .

The 802.11s standard promises to cut wireless networking loose from its Ethernet
cable moorings, creating what some call "opportunistic" networks.
"If you think about this underlying technology to form opportunistic connections,
it changes a lot of the assumptions of wireless LANs," says Robert Withrow,
adviser to the CTO Common Engineering Group, part of the Office of the Chief
Technology Office for Nortel Networks, which supports both the OLPC and the
open80211s
project, and was an early and active member of the 802.11s task group, which
launched in 2004.

"A piece of information might be coming from your [mesh] neighbor,"
Withrow says. "Well, who is that neighbor? Does he have a right to give
you information?" He also says that wireless networks are rapidly moving
from people-to-people connections, to people-to-machine and machine-to-machine
connections, which are vastly more numerous, dubbed by Nortel
as "hyperconnectivity." "It's difficult to do this in a hierarchical
way," he says.

The 802.11 standard does specify what's called an ad hoc mode, which lets,
for example, wireless laptops connect with each other. But this requires every
participating node to be connected directly to every other node. An 802.11s
mesh doesn't have this limitation. Proprietary mesh protocols abound, mostly
for outdoor wireless networks, though Ruckus Wireless and Aerohive recently
unveiled indoor mesh gear. But these protocols are for mesh infrastructures,
not clients, and because they're proprietary, they don't interoperate. (Compare
wireless mesh products with our online Buyer's Guide.)

OLPC embraces mesh

The OLPC began studying mesh benefits in 2006. Mesh
would let the group's inexpensive Linux-based
laptops create their own wireless network and communicate, without the need
for access points or intermediate servers, as well as share whatever wide-area
connection, such as a satellite link, that might be available for a village
school in Cambodia, Brazil, or Haiti.

Using the nascent 802.11s draft was a natural choice, says Michail Bletsas,
chief connectivity officer for OLPC. The 802.11s mesh works at Layer 2, so no
changes would have to be made to the TCP/IP network stack or other higher layer
applications.
One corollary benefit of that, he saw, was that the 802.11s code could be run
on the 802.11 network adapter module, with a system-on-a-chip having its own
memory and small CPU. That meant the OLPC laptop could suspend or shut down
its main CPU to save power, but trickle some juice to keep the radio alive as
a mesh node, forwarding traffic on behalf of other mesh participants.

"It only takes 800 milliwatts, compared to about 12 watts if it's implemented
on the laptop," Bletsas says. With some additional optimizations, OLPC
can cut the power needed to 400 milliwatts in some cases.

In late 2006, semiconductor maker Marvell hired Cozybit, an embedded wireless
consultancy, to implement and tune a very early version of the 802.11s mesh
protocol for OLPC. The code was running on OLPC laptops in early 2007..

The mesh 'gotchas' emerge

One issue that quickly emerged with operational code was that some of the most
common consumer-grade access points misinterpreted some packets it picked up
from 802.11s nodes, whether the access point connected to the mesh or not, says
Osama Aboul Magd, adviser on strategic standards, Office of the CTO, Nortel,
and actively involved in identifying this problem. The 802.11s nodes used the
802.11 frame format called wireless distribution system (WDS). Receiving a WDS
packet, the conventional access point started broadcasting to all clients, as
it normally would, creating in effect an unintentional denial-of-service attack,
Magd says.

The OLPC and Nortel proposed the Task Group consider tweaking the frame format
so 802.11s nodes and existing access points could work in harmony.

The OLPC team also saw an opportunity to leverage Layer 2 information from
the network for use by higher-layer applications. Opportunistic mesh networks
by definition lack a central directory. Instead, a mesh node uses a discovery
technique to identify its neighbors and chart data paths through the mesh. By
adding a bit more information to these exchanges, and making the information
accessible to higher-layer functions, mesh nodes can not only learn of their
neighbors, but about their neighbors, Bletsas says, identifying one as file
server, another as an Internet gateway. OLPC has started talking with the Task
Group about doing this in some standard way.

Taking mesh to open source

In September 2007, the developers at Cozybit, with sponsorship from OLPC and
Nortel, launched the open80211s
consortium, dedicated to creating an open source implementation and reference
design of the most recent 802.11s draft. The timing was right. "There was
a high-quality 802.11 stack donated to the Linux community," says Cozybit
President and CEO Javier Cardona. "We saw it would be relatively easy to
modify it, to support some of the features of the up-and-coming 11s standard."

So far, about half of the 802.11s draft is now in code, including dynamic path
selection, frame forwarding, and peer link management. Two missing pieces include
security and power conservation, Cardona says. Earlier this year, the source
code was accepted for testing by one branch of the Linux Wireless group, which
maintains the wireless subsystem for the Linux kernel, Cardona says. That's
a key first step toward eventually including the open 802.11s code as part of
the kernel.

At OLPC's behest, Cozybit is developing a driver and firmware, due out in early
June for beta testing, to let the OLPC laptop radio now run the open source
802.11s code.

Network World