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Wireless
mobility a key advantage
By Chip Elliott, Principal Engineer, BBN Technologies, Cambridge,
Mass. EE Times.
Feb
14, 2003
Ad hoc networks, the "next great thing" in the consumer Internet,
have hidden roots that run surprisingly deep into military R&D.
Although ad hoc networking was born for battlefields, its most far-reaching
effect will be in quite a different realm. It is the missing ingredient
for the wireless Internet.
Today
every advanced military is rolling out large-scale ad hoc networks
to carry a variety of tactical communications traffic. The new networks
carry all forms of traffic-voice, video, data-using robust, meshed
connectivity between wireless routers.
In
these networks, many or all nodes may be moving. For instance, the
great majority of routers may be mounted in vehicles driving at
60 mph. Some may be in helicopters traveling at much higher speeds
across a battlefield. The networks are thus very dynamic; links
between routers appear and disappear in a matter of seconds. Such
ad hoc networks are up and running in the United States, Canada
and England under their respective militaries.
The
basic technology needed for ad hoc networks is best explained by
network layers. At the physical layer, some form of packet-based
wireless transceivers is required. In principle, these may be based
on a wide variety of physical channels-RF or optical, or even acoustic
or magnetic in specialized circumstances. In practice, RF is generally
employed. In commercial settings, the IEEE 802.11 radio family is
a nearly universal favorite, running at 2.4 or 5 GHz, although some
companies employ a proprietary radio. Bluetooth and other low-power
radios are also used. Military systems generally call for advanced
radios with improved transmission security, lower probabilities
of intercept and detection, and anti-jam capabilities.
Specialized
ad hoc network protocols run above the physical layer and implement
a range of mechanisms peculiar to ad hoc networking. Neighbor discovery
allows a router to determine which other radio routers are currently
within range. Broadcast "beacon" transmissions are usually employed
to aid in neighbor discovery. Topology control determines which
of all potential links to neighbors should actually be used. In
systems with directional antennas, this mechanism may include beam-steering
decisions. Link characterization provides a means for determining
when a radio link is of sufficient quality to use for data traffic,
how good it is compared with other links (for example, how reliably
it can deliver packets) and when it should be removed from service.
Ad hoc routing determines where distant nodes are in the network,
to some degree of accuracy, and the hop-by-hop paths by which datagrams
may be sent to those distant nodes. Forwarding mechanisms actually
relay packets onward through the ad hoc network.
Finally,
a variety of Internet-level services must also be provided. These
include admission control, provision of Internet Protocol addresses
(for example, by DHCP or other means), mapping between IP addresses
and link-layer addresses, name servers, exit routers to other portions
of the Internet, network-management systems, public-key services
for secured networks and so forth.
Spurred
by today's military successes in ad hoc networking, and by this
technology's obvious potential outside the military, dozens of academic
research groups across the world have recently entered the field.
A hand-ful of commercial companies have also joined in, often with
technology originally developed for the Defense Advanced Research
Projects Agency (Darpa).
Academic
research has mainly focused on the routing aspect of ad hoc networks,
with two competing lines of approach-proactive routing and reactive
routing. Proactive techniques attempt to determine the location
of nodes in the network at all times, although perhaps not with
perfect fidelity, so that traffic can be readily forwarded to these
nodes when the need arises. Reactive routing, by contrast, attempts
to find a node's location only when traffic is being sent to that
node, usually employing some form of flood search to find the location.
A further distinction is that proactive routing schemes continually
reevaluate the paths by which traffic flows, to adjust them to the
changing circumstances, whereas reactive schemes generally stick
with an existing path until it can no longer be used, even if it
becomes far less than optimal as the network nodes move.
Missing
link
Ad
hoc networking's most important role is likely to be as the enabler
for the wireless Internet. RF spectrum is finite, but each wireless
Internet user needs as much of it as possible for high-speed connectivity.
This requirement leads inexorably toward a very large number of
basestations to minimize the number of users contending for a shared
basestation antenna. In turn, this leads to ad hoc networking, whose
self-organizing, self-healing properties minimize the planning and
maintenance needed for large radio networks.
Two
demands in the wireless Internet space are very clear: standards
and scalability. Both are ultimately network protocol issues. The
need for worldwide, open standards has been the path to all networking
success to date. But, remarkably, it hasn't yet been tried in the
wireless Internet. Here the Internet standards are essential, and
this means the standard set of Internet protocols, not a version
tailored for wireless users. The lower layers of the protocol stack
will almost certainly be IEEE 802.11 in one form or another.
A variety of new techniques is just emerging from the lab, all distinctly
useful for the commercial wireless Internet as well as the military.
Within the past year, Cesar Santivanez and Ram Ramanathan, both
of BBN, have provided the first theoretical understanding of exactly
how ad hoc routing protocols scale. This understanding led to the
development of "Hazy Sighted" routing-a novel, easy-to-implement
link state variant that proves that hierarchy is not mandatory for
scalability. In fact, it appears that Hazy Sighted routing scales
far better than previously known techniques. This is a fundamental
result with immense practical importance.
White
papers on our products and technologies are available upon request.
E-mail us at innovation@bluetronix.net
or call 440.247.3434.
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