Ad Hoc mobile networking is the uncharted
frontier of contemporary networking technology, and an research area
which at this time is being heavily funded in the US. In essence, Ad
Hoc networking is all about providing connectivity between mobile
nodes, which have no supporting connections to the fixed networking
In an Ad Hoc mobile network, every node in the network
carries its own router with it, and all nodes cooperate in carrying
The whole philosophy of the Ad Hoc networking model is a
radical departure from the highly structured, and frequently
hierarchical models employed for both local area and wide area
networking, currently in use.
In the established, fixed infrastructure model, the
routers and supporting functions such as name resolution are all
embedded within the networking infrastructure. Communication between a
pair of nodes requires that the nodes hand the traffic over to the
routers, which then forward the traffic over multiple router hops until
it arrives at the destination router, which then passes the traffic to
the recipient node.
In this model, with the exception of the odd host tasked
with acting as a router, mostly the routing function is performed by
the network, and the nodes are essentially clients of the "connectivity
service" provided by the networking infrastructure. The model is
hierarchical, insofar as traffic from small cells or subnets is
concentrated as it flows up into the network, which aggregates the
traffic associated with multiple virtual circuits or datagram
connections, and carries it across specific point to point
communications links to the geographical area within which the
destination (or source) nodes are situated.
This paradigm has generally served us well, and indeed
is the essence of the "catenet" model used in ARPANET and now the
Because the topology of the network is unchanging, or
changes at a very slow rate, mechanisms for name mapping, route
discovery, and route maintenance can be very lethargic in their
response times. Indeed, manual reconfiguration of the routing topology
is commonly employed.
An Ad Hoc mobile network is essentially incompatible
with this basic model, since it is highly time variant in topology.
At this point it is worth digressing into the practical,
application oriented, aspects of the Ad Hoc network, since the
technology promises many services which have hitherto been
The simplest Ad Hoc network can be envisaged as a
wireless radio network between a collection of vehicles, ships,
aircraft, or even people on foot, operating in a geographical area with
no networking infrastructure. Many examples of such scenarios come to
mind. A fleet of fishing vessels searching for schools of fish on the
high seas, a seismic survey team in a remote area, a disaster relief
operation, or aid operation, trying to function in an area which has
been stripped by a natural disaster of its communications
infrastructure, or if in the Third World, never had one in the first
place. Scientists on field outings, or indeed even a class of
school-children on an outing into a national park, all carrying laptops
or wearables. Cars and trucks on country highways or freeways, with
onboard Internet connectivity.
There are, of course, also a myriad of military
applications involving the networking of aircraft, helicopters, tanks,
ships, and even infantrymen with wearable computers.
The range of possible situations in which Ad Hoc
networking can be exploited is huge, and this is not an understatement
by any measure. A robust Ad Hoc networking scheme frees the individual
from the geographical constraints of the fixed network. In this respect
it is fundamentally different from established mobile networking, in
which mobile nodes are tied down by the need to remain within the
coverage of a wireless hub, connected to the fixed network
An Ad Hoc network, by its nature, provides mutual
connectivity between cooperating peer nodes. Nodes which cannot
directly communicate, are assisted by other nodes between which
connectivity exists, and which can connect to the end nodes which
intend to communicate. Therefore, every node in an Ad Hoc network must
have the capability to perform as a router if its peers require it to
Mutual connectivity does not imply the ability to access
the fixed infrastructure, and if connectivity to the fixed
infrastructure is required, then at least one node in the Ad Hoc
network must have the ability to connect to the fixed infrastructure
and carry traffic into and out of the Ad Hoc network.
Connectivity between nodes in an Ad Hoc network can be
provided by any channel which can carry traffic, and existing research
encompasses everything from short wave radio, VHF, UHF, omnidirectional
and directional microwave, and omnidirectional incoherent infrared or
directional infrared and visible band laser technology. Any scheme which
can carry a digital modulation without using a cable is a candidate for
Ad Hoc networking applications, indeed the ideal Ad Hoc network doesn't
really concern itself with the physical layer channel employed to carry
ones and zeroes between participating nodes. Moreover, with proper
protocol transparency, it need not be concerned with the type of
traffic it carries, other than with the issue of the amount of
bandwidth required for a given service.
We could envisage a situation in which you are
travelling in your company car down a country highway, and you receive
a "phone" or "video-phone" call from the office, which has hopped along
a series of other cars and trucks along the highway, from the vehicle
nearest to a fixed hub, in the nearest country town. The managing
director has decided to change your assignment and you have to spend
another three days in the middle of nowhere...
Bitten by a Taipan while bush-walking in North
Queensland, you make a call on your wearable computer/communicator,
which hops to a geologist's wearable, which hops to his 4WD, which in
turn hops across a couple of cars on the highway, several miles away,
and then hops to a fishing boat several miles off the coast, which
happens to have a satcom link via which the message is hopped to a
ground station in the vessel's home port, fifty kilometres away, from
where the message is routed to an emergency service dispatcher. Since
your wearable has a GPS receiver on it, the rescue helicopter is
scrambled and within twenty minutes you are enroute to a hospital.
What mature and robust Ad Hoc networking offers is
virtually universal connectivity, limited only by the link performance
and routing delays of the participating nodes, and their connectivity
to the established fixed network.
Too good to be true ? Perhaps, but certainly, as
fanciful as these examples may be, they are all well within the bounds
of today's technology, providing that suitable Ad Hoc routing protocols
exist and are implemented.
The technological challenges of Ad Hoc routing are very
much non-trivial !
The first "package" of problems derive from the
continuously varying topology, and potential throughput, of the Ad Hoc
Topology varies simply because some nodes will move in
and out of wireless link range of other nodes in the network, be it
through distance, or concealment behind terrain or other obstacles
which prevent transmission, such as inclement weather or rain which
soaks up microwaves and laser beams.
Network throughput will vary for two reasons. The
first, and obvious reason, is that the larger the number of hops your
traffic has to travel across to get to where it is going, the greater
the routing delays you incur, which cumulatively add up to increase the
latency of your link, and thus potential throughput, for a finite
buffer size in participating nodes. Since the network topology is
continuously changing, frequently in an unpredictable manner, the
number of hops between you and your destination node will also vary.
This has other implications we will discuss later.
The second reason why network throughput will vary is a
consequence of Shannon's information theory, since for a constant power
output and receiver sensitivity, as distance increases between two
wireless nodes, the signal/noise ratio declines and thus achievable link
bit rate drops. Therefore, as the signal weakens, the range of
potentially available services declines, or the bit error rate
increases. This variation of throughput with time is usually referred to
as "fading", as much as this term is used and abused in various niches
of communications theory.
At this time very few protocols for MAC layer
connections exist which can adaptively adjust their throughput to
accommodate variations in link performance. We are seeing the first
steps with the IEEE 802.11 wireless Ethernet, where link quality
degradation forces a reduction in link bit rate, albeit in large and
discrete chunks. We have yet to see a genuinely robust protocol which
dynamically "rubberbands" the bit rate through the channel to achieve a
desired balance of speed and bit error rate. In wireless networking,
where power and bandwidth come at a big premium, every snippet of
usable bit rate is valuable.
By far the biggest problem in current Ad Hoc networking
research is that of routing in a situation where the topology of the
network is changing continuously, somewhere within the network.
Static networks mostly use either Distance Vector
(DV) or Link State routing algorithms, neither of which are
spectacularly well suited to highly dynamic topologies.
Distance Vector, or Bellman-Ford schemes, such as those
used in the DARPA packet radio protocol, RIP, XNS or IPX, are based on
the idea of periodically broadcast tables of distances, typically in
hops, between a node and all possible destinations. A necessary
requirement is that the update rate is greater than the rate of
Link State schemes, such as those used in the OIS IS-IS
or Internet OSPF routing protocols, rely on the broadcast of complete
topology maps for the network.
In a highly dynamic wireless network, such protocols run
into a number of difficulties:
topologies may be highly redundant, with some nodes
being in the situation of being able to connect to a very large number
of neighbours, while others see very few neighbours.
bandwidth is scarce and cannot be wasted
battery power on portable equipment is a finite
resource which cannot be wasted
high rates of topology change require high update
Link State, and Distance Vector routing schemes fall
foul of these issues since they distribute a lot of routing
information, and with high rates of topology change this will eat into
bandwidth and thus battery power, moreso in highly redundant
topologies, where much of the information is effectively wasted.
Maintaining a current routing table on a node which does not
communicate much with its neighbours is a drain on critical resources
for no return.
From a perspective view, the routing problem really
decomposes into two problems. One is that of "route discovery", the
other is that of "route maintenance" whereby the validity of discovered
routing information is maintained.
Topologies in Ad Hoc networking are an issue within
themselves. In essence, nodes may move around with no clear geometrical
interrelationship, or may form clusters associated with groups of
individuals or vehicles moving around in relatively close mutual
proximity, or nodes may also form "linear topologies", when vehicles
travel down roads, railways, shipping lanes or air routes.
In a general sense, routing models are either based upon
a "Flat Topology" model, or a "Hierarchical Topology" model. In the
former, all nodes are peers, in the latter one node within a cluster
gathers traffic on behalf of "lesser" nodes in the cluster, and is
responsible for carrying this traffic in and out of the cluster. The
Hierarchical Topology is in many respects an offshoot of the static
networking model, and has generally not been a popular research area,
since it can result in less than optimal routing behaviour. Flat
Topologies are in most situations the best approach, since they can
provide for redundant paths in and out of cells, and should protocol
support exist, load balancing across multiple links.
Routing models can also be divided in yet another way:
"Proactive Routing" is any scheme which continuously
monitors the topology and maintains current routing tables regardless
of instantaneous demand. DV and LS schemes fall into this category.
While routing information is always available for a sender, the network
is being continuously flooded with routing management traffic, much of
which is unused.
"Reactive Routing" is any scheme where routing
information is gathered only on demand. In such schemes, a route is
discovered only when needed, and thus routing management traffic is
kept to its bare minimum. Reactive schemes have been most popular to
date, since they minimise the route management traffic overheads.
At this time research effort in Ad Hoc networking is
mostly confined to two large research projects, both centred in the US.
The IETF is sponsoring the MANET (Mobile Adhoc NETworking) project,
while the US DoD's DARPA is funding the GLOMO (GLObal MObility)
MANET is aimed primarily at extending the existing
Internet protocol suite to accommodate Ad Hoc routing functions, within
the context of IP connectivity. The physical channels are only of
peripheral interest, in how they constrain the behaviour of the network.
GLOMO has a much broader scope, and is aimed primarily
at developing the technology base for universal, transparent,
connectivity between military platforms, and troops, in the absence of
a fixed infrastructure. Therefore a large component of GLOMO research
effort delves into areas other than routing alone, such has wireless
link and antenna design, and the topology is not limited to flat
topology models alone. In this sense, Ad Hoc routing problems are only
a component of the GLOMO effort.
The complexity of the Ad Hoc routing problem is
reflected in the volume of research currently being conducted, no less
than six different schemes are being researched.
The Zone Routing Protocol (ZRP), proposed by Haas,
employs a proactive route discovery scheme within a local "zone"
immediate to a node, but uses a reactive scheme for connections outside
The Destination Sequenced Distance Vector (DSDV)
protocol, essentially a variant of the RFC 1058 RIP protocol, as
proposed by Perkins and Bhagwat, uses elements of the well established
RIP protocol, but adds sequence numbers to routing tables to eliminate
routing loops, and uses triggered updates to propagate topology changes
when these are discovered, in addition to RIP like periodic updates.
DSDV is loop free, and is designed to respond quickly when changes in
topology occur in between periodic update cycles. It is a proactive
routing protocol with some reactive features.
The Dynamic Source Routing (DSR) protocol, proposed by
Johnson, extends the source route model used in the current IP suite,
and is a purely reactive protocol. Every packet contains an ordered list
of intermediate routing nodes, every node maintains a route cache, and
if a route does not exist in the cache, a "route request" packet is
broadcast and propagated along until it hits the destination, or a node
which knows of the destination, upon which a reply packet is send to the
requesting node. Intermediate nodes add their address along the way,
and update their caches with eavesdropped routes. Routes are maintained
by watching for lost packets, upon which another route discovery must
be performed. The DSR model is an extension of the route discovery
scheme in the RFC 2002 mobile IP protocol, based in turn upon the
existing RFC 791 Loose Source Record & Routing protocol.
The Temporally Ordered Routing Algorithm (TORA),
proposed by Park and Corson, is another reactive route discovery
protocol, which uses a "link reversal" model in route discovery. A
route discovery query is broadcast and propagates out through the
network until it hits a destination or a node which knows a route to
the destination. A parameter, termed "height", which is a measure of
the responding node's distance to the sought destination node, is then
returned to the querying node. As the query response propagates back,
each intermediate node updates its TORA table with the route and
"height" to the destination node. The querying node then uses the
"height" to select the best route. TORA has an interesting property
that it frequently chooses the most convenient route, rather than the
shortest route, and in doing so attempts to minimise the routing
management traffic overhead.
The Ad hoc On Demand Distance Vector (AODV) protocol,
proposed by Perkins, blends elements of the DSR and DSDV protocols,
using the DSR reactive route discovery and maintenance models, in
combination with the sequence number and periodic update features of
the DSDV protocol.
An extensive simulation comparing DSDV, DSR, TORA and
AODV was conducted at CMU and results published late last year. The
conclusions from these simulations were by all means very interesting.
The simulation compared two important metrics, the ratio
of delivered packets to sent packets, and the routing management
Highly dynamic networks, where most nodes are
continuously moving, are the limiting situation for an Ad Hoc network
and by far the most demanding scenario to be handled.
Under these conditions, the DSR and AODV protocols
delivered 98% of packets, while the DSDV protocol foundered with a
70-75% delivery ratio. A 2% packet loss is respectable performance for
the environment in question, and if we compare the packet loss ratio,
then DSDV lost about 12-15 times as many packets in comparison with DSR
Routing overhead performance was also interesting, and
DSR won yet again. Interestingly, the DSDV protocol was the least
sensitive to the mobility of nodes, requiring an almost constant
overhead regardless of node mobility. In comparison, DSR incurred 1/2
the overhead of DSDV, while AODV incurred triple the overhead, and TORA
four times the overhead of DSDV. Between the best and worst, this is an
eightfold difference in overhead traffic.
Another interesting metric was an analysis of how
sensitive protocol performance was to increasing network activity,
measured by the number of active nodes.
Packet delivery performance was again best for AODV and
DSR, with a degradation of about 1% with a tripling of load. The same
tripling of load dragged TORA down to about 40% delivery ratio, making
it virtually dysfunctional.
Routing overhead traffic variations with load showed
that DSDV was the most robust, with a virtually constant overhead. DSR
and AODV displayed similar behaviour, with overhead growing roughly in
proportion to load. TORA performed poorly, with a tripling of load
resulting in 25 times the routing traffic overhead.
In terms of which protocols did best in choosing optimal
routes, DSR and DSDV did best.
It is perhaps a tribute to Occam, that ostensibly the
simplest of the protocols, DSR, appeared to deliver the best all round
performance. It also demonstrates a strong case for a reactive vs a
proactive route discovery scheme.
Other variations upon these schemes are also being
explored. The Location Aided Routing (LAR) protocol extends a DSR or
AODV protocol, by using GPS satellite navigation data to limit the
route discovery search to nodes which are known to be relevant.
In this context it is worth mentioning the work of
Imielinski, who is proposing the inclusion of GPS geographical location
data into IP addressing, to provide an alternate addressing scheme for
generic IP routing. As yet this idea does not appear to have been
incorporated into the MANET or GLOMO protocols.
Perhaps the best question to ask at this time, is "Quo
Vadis, Ad Hoc Networking ?", pun intended. Clearly the research effort
is beginning to converge and we can expect to see a final protocol
choice fairly soon. A good guess is that it will be a variant of
DSR/AODV, crossed with the source route based Mobile IP protocol.
In the longer term, the protocols devised for MANET may
be the solution to the growing unreliability of the fixed Internet
infrastructure. As its complexity continues to increase, the sheer
number of routing nodes against the failure rate or crash rate of a
typical routing node means that beyond some point, the rate of topology
changes through router crashes will get ahead of the topology update
rates accommodated by existing link state and distance vector algorithms
such as OSPF and RIP. Adoption of a protocol derived from the MANET
effort would defeat this problem.
Ad Hoc networking therefore promises much more than the