Plexus Institute Thursday Complexity Postsome new self-organizing technology
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Thursday Complexity Post
September 11, 2003
Wireless Networking Hits Boston Rooftops
This week's TCP features a piece that can be viewed on-line from MIT's
Technology Review. Special thanks go to Jeff Johnston for pointing out this
article.
The piece addresses some rather interesting new attempts to connect
people wirelessly with one another to an internet gateway through a collection
of rooftop antennae. The project, called Roofnet, is an unmanaged group of
computers running Linux and a Wi-Fi card, which self-organizes itself. It is a
self-configuring collection of computers that functions more efficiently for
data packets to move around, hopping from rooftop to rooftop. The overall
topology of the wireless network is constantly changing, but functions in a way
that allows a data packet to find its way from one node in the network to the
next, constantly re-evaluating old and new links, in more effective ways than
other types of networks.
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Networking from the Rooftop
MIT Technology Review Article
. "chimney connection" is part of MIT's Roofnet, a project to create a
self-organizing wireless network in which an amorphous, unmanaged collection of
cheap Linux computers equipped with Wi-Fi cards collaborate to efficiently
route data packets. Each computer and roof-mounted antenna at students'
apartments and MIT buildings is a node on the network and the arrangement in
which they are connected to each other-the topology of the network-is
constantly changing. "We want to understand how a whole bunch of computers
with short-range radios can self-configure a network, forming order out of
chaos," says computer science professor Robert Morris, who coordinates the
project.
- Networking from the Rooftop
by Erico Guizzo, Technology Review
August 29, 2003
http://www.technologyreview.com/articles/print_version/wo_guizzo082903.asp
A few weeks ago, MIT graduate student Shan Sinha canceled his broadband
Internet service. Now his Net connection comes through the chimney. From a
computer in the living room of his Cambridge, MA, apartment, a few blocks from
the MIT campus, a cable goes into the fireplace up to the roof, where it is
attached to an antenna. From there, data packets hop to another roof-mounted
antenna at a nearby student's apartment. That way, from roof to roof in
multiple hops, Sinha's data packets finally reach a gateway-a computer
connected to the fixed Internet-at MIT's computer science building. "We can't
use the fireplace," he says, "but that's the cost of free Internet."
Sinha's "chimney connection" is part of MIT's Roofnet, a project to
create a self-organizing wireless network in which an amorphous, unmanaged
collection of cheap Linux computers equipped with Wi-Fi cards collaborate to
efficiently route data packets. Each computer and roof-mounted antenna at
students' apartments and MIT buildings is a node on the network and the
arrangement in which they are connected to each other-the topology of the
network-is constantly changing. "We want to understand how a whole bunch of
computers with short-range radios can self-configure a network, forming order
out of chaos," says computer science professor Robert Morris, who coordinates
the project. The network has now more than 30 nodes in a 4-square kilometer
area surrounding the MIT campus. "We hope to reach a hundred nodes within a few
months," he says.
Research groups at universities such as Carnegie Mellon, Rice, UCLA, and
the University of Illinois at Urbana-Champaign, and at companies such as Nokia,
Intel, and Microsoft are developing similar systems. In each case, data packets
are routed through geographically dispersed and wirelessly connected nodes that
can be fixed in a building or moving with a user or vehicle. Applications of
these so-called multi-hop mesh networks include systems to connect people
carrying PDAs, tanks on a battlefield, or a large number of sensors in a
factory plant. And community mesh networks such as Roofnet, which are much
cheaper to deploy than DSL or cable hookups, are a promising way to overcome
the "last mile" barrier and bring high-speed Internet access to a large number
of people, especially those who live in rural areas or other places where the
infrastructure for wired broadband access is not available.
Community-owned wireless networks have appeared in several places in New
York, San Francisco, Seattle, London, and other cities. These networks usually
consist of a few interconnected base stations-known as wireless access
points-located in windows and rooftops providing Internet connectivity in
public spaces. The new generation of mesh networks such as Roofnet cover wider
areas and are much more dynamic in the way they route data. Their nodes are not
permanently connected; instead, they constantly revaluate the existing links
and form new ones. As a result, data follows much more tortuous paths to reach
the fixed Internet. And with tens or hundreds of nodes-some of them joining and
leaving the network in a random fashion and thus constantly changing its
topology-a difficult problem arises: how should data in these multi-hop
wireless nets be routed? What paths in this labyrinth of rooftop and window
antennas optimize the flow of packets?
Distance Matters
Most of the routing protocols now being proposed by mesh network
researchers borrow the shortest-path strategy used in the fixed Internet. These
protocols try to find the route with the fewest number of intermediate nodes
between sender and destination. For the wired Internet-with its nearly static
topology and reliable links-this scheme has been working pretty well: our
e-mails hop from router to router and reach the other side of the world in a
few seconds.
But it turns out that this shortest-path strategy might not be adequate
for sending packets through the air. In a wireless network, according to the
MIT group, distance matters: the longer the signal has to travel, the more it
will degrade. Moreover, the link quality between nodes varies unpredictably due
to such transient phenomena as trucks driving by, moisture in the air, or a
pigeon sitting on the antenna. The result is a considerable amount of packet
loss, transmission errors, and connections that simply appear and disappear
throughout the day. A routing protocol that minimizes the number of hops ends
up choosing longer distances for each hop-and therefore sending data over
low-quality wireless links.
The MIT group realized that new routing strategies were necessary when
they deployed an initial version of Roofnet last spring. They tried to
implement some of the proposed routing protocols discussed by the Internet
Engineering Task Force, the organization governing the Internet's technical
standards. But while these protocols work well in theory-and are generally
tested in computer simulations or small-scale, laboratory networks-they don't
take into account many unpredictable factors involved in radio communication.
The protocols usually assume, for example, that when one node can detect one
nearby, it can communicate well with its neighbor. But that turns out not
always to be true. The MIT researchers and other groups have found that many
times two nodes can "hear" each other by exchanging small probe packets, but
when they try to send real data, communication collapses due to inadequate
bandwidth. Morris and his group decided that the best way to develop robust
wireless routing protocols was to test them with a real network, real users,
and real traffic.
Other mesh network researchers say the MIT work represents an important
advance for debugging these routing schemes. "Their work is grounded in real
system building," says Victor Bahl, a senior researcher who leads the
networking group at Microsoft Research in Redmond, WA. "The insight that you
get out of building things is a lot more than you'll ever get if you just
simulate things." Demonstrating that such networking is viable in a real,
large-scale implementation, he says, is a crucial step toward attracting more
industry attention to the technology's potential.
Deploying such a network became possible because Wi-Fi technology has
gotten so cheap. A few years ago, Morris says, the price of the wireless cards
would have made the project prohibitively expensive. Each Roofnet node uses an
802.11b wireless networking card installed on a cheap PC running Linux and the
routing software. A coaxial cable connects the wireless card to an
omnidirectional antenna. The user then connects the PC to the Roofnet node. The
total cost of the equipment for each node is $685.
To deploy the network quickly, the MIT group distributes free
self-installation kits to students who want to participate in the project. For
these students, getting the Roofnet node running is part of the fun. "Our
antenna was put up by a friend of mine who does rock climbing," says graduate
student Roshan Baliga, who lives in a two-story building with no easy roof
access. "He scaled the side of the apartment to get to the roof, installed the
antenna, and then rappelled down."
MIT students are happy to participate in the project, especially because
they can save some money. "We compared a broadband cable connection to Roofnet
and couldn't tell the difference, so we cancelled the cable," says MIT senior
Walt Lin, who installed the antenna on his sloped roof.
The Road Ahead
With students surfing the Web, downloading music files, and working on
problem sets on remote servers, the network is running with real traffic. Now
Morris and the four graduate students working with him full time on the project
can test different routing strategies that better adapt to the hostile wireless
environment.
Their idea to cope with the unpredictable environmental disruptions is to
figure out not just whether two nodes can hear each other, but also measure how
well they can communicate. Instead of finding the shortest path between two
nodes, their protocols try to find the best path-the one in which data packets
won't get stuck or corrupted along the way. This requires a constant monitoring
of the links. Roughly once per second, each node sends out a small "hello"
broadcast packet. All the other nodes record whether they receive this probe,
keeping a history of the last 10 probes. So if, say, node A has sent out 10
probes and node B received 8 and node C received 4, then the routing software
knows that the path A-B is better than path A-C. Also, every 15 seconds, every
node sends a broadcast message that lists the nodes it knows how to reach-and
the link quality for each associated path. That way, all nodes have a complete,
continually updated, routing map of the entire network-and know the optimal
routes for reaching one another.
In building Roofnet, the MIT researchers found many things they didn't
expect. For example, the range of the 802.11b cards and antennas vary
considerably. "We're now skeptical about what manufacturers say," says John
Bicket, one of the grad students working on the project. "We found nodes that
couldn't talk across the street, but others could talk half a kilometer apart."
The cause might be local environmental conditions or even multiple reflections
of the same signal that cancel themselves out. Another surprising phenomenon is
the lack of symmetry in the link transmission quality: it is not uncommon for
node A to be able to send data to node B easily, while node B can't
reciprocate. Such anomalies complicate the development of routing schemes.
By debugging and fine-tuning their routing schemes, the MIT researchers
hope they will be able to use them in even more complicated systems. One such
situation would be when nodes are not static in rooftops, but moving at
different speeds in all directions-a scenario not far in the future, as more
and more people carry personal digital assistants and cars are beginning to be
equipped with computers. "It's a matter of tuning the protocol so that it can
handle mobility," says Sanjit Biswas, another student involved in the project.
Ultimately, Morris says the group plans to release the Roofnet routing
software as a freely dowloadable open source program. That means that anyone
with a computer and a Wi-Fi card would be able to install the routing software
and become a node in the network. Other people in other areas could also
download the software and create their own rooftop community networks.
Of course, many problems still need to be addressed. First, MIT can't
provide Internet access to non-MIT affiliates; the network would therefore
eventually have to find other gateways to the fixed Internet. But that raises
another complicated issue: most Internet service providers don't want their
users sharing their bandwidth. Also, the community networking technology needs
to guarantee a certain level of security and privacy. With users literally
sending their data through the air, via other people's nodes, some sort of
encryption will probably be necessary to avoid eavesdropping. It is also
necessary to guarantee a fair, balanced used of the system, to avoid that a
single user sucks all the bandwidth and clogs the network. Finally, the system
needs to be robust enough to resist some more pragmatic problems-such as when
snow forms in the antennas.
When the day comes, what will happen? Again, the MIT group wants to learn
by doing. "We'll see," says Morris.
*
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