2. Mobile Adhoc Networks(MANETS
• A MANET consists of a number of mobile devices that come
together to form a network as needed, without any support
from any existing Internet infrastructure or any other kind
of fixed stations.
• MANETs are basically peer-to-peer, multihop wireless
3. networks in which information packets are transmitted in a
store-and-forward manner from a source to an arbitrary
destination, via intermediate nodes
• As nodes move, the connectivity may change based on
relative locations of other nodes.
• The resulting change in the network topology known at the
local level must be passed on to the other nodes so that old
topology information can be updated.
Characteristics of MANETs
4. • Salient characteristics of ad hoc networks are as follows [13.1]: 1. Dynamic
topologies: Nodes are free to move arbitrarily; thus, the network topology
may change randomly and unpredictably and primarily consists of
bidirectional links. In some cases, where the transmission power of two nodes
is different, a unidirectional link may exist.
2. Bandwidth-constrained and variable capacity links: Wireless links continue
to have significantly lower capacity than infrastructured networks. In addition,
the realized throughput of wireless communications—after accounting for the
effects of multiple access, fading, noise, interference conditions, and so on— is
often much less than a radio’s maximum transmission rate. One effect of
relatively low to moderate link capacities is that congestion is typically the norm
rather than the exception (i.e., aggregate application demand could likely
approach or exceed network capacity frequently). As a MANET is often simply
an extension of the fixed network infrastructure, mobile ad hoc users would
demand similar services.
3. Energy-constrained operation: Some or all of the MSs in a MANET may
rely on batteries or other exhaustible means for their energy. For these
nodes, the most important system design optimization criteria may be
energy conservation.
4. Limited physical security: MANETs are generally more prone to physical
security threats than wireline networks. The increased possibility of
eavesdropping, spoofing, and denial of service (DoS) attacks should be carefully
5. considered. To reduce security threats, many existing link security techniques are
often applied within wireless networks. As a side benefit, the decentralized nature
of MANET control provides additional robustness against the single points of
failure of centralized approaches. In addition, some envisioned networks (e.g.,
mobile military networks or highway networks) may be very large (e.g., tens or
hundreds of nodes per routing area). Scalability is a serious concern in MANETs.
Routing
• Routing in a MANET depends on many factors,
including modeling of the topology, selection of
routers, initiation of a route request, and specific
underlying characteristics that could serve as heuristics
in finding the path efficiently.
• The low resource availability in MANETs necessitates
efficient resource utilization; hence the motivation for
optimal routing.
• Also, the highly dynamic nature of these networks
6. places severe restrictions on any routing protocol
specifically designed for them. A network configuration
is also called a network topology.
• There are three major goals when selecting a routing protocol: 1.
Provide the maximum possible reliability by selecting alternative
routes if a node connectivity fails.
2. Route network traffic through the path with least cost by minimizing
the actual length between the source and destination through use of
the lowest number of intermediate nodes.
3. Give the nodes the best possible response time and throughput.
This is especially important for interactive sessions between user
applications.
In a MANET, each node is expected to serve as a router, and each
router is
indistinguishable from another in the sense that all routers execute the
same routing algorithm to compute routing paths through the entire
network.
7. Need for Routing
• MANET routing typically has the following goals:
1. Route computation must be distributed, because centralized routing in a
dynamic network is impossible, even for fairly small networks. 2. Route
computation should not involve maintenance of a global state, or even
significant amounts of volatile nonlocal state. In particular, link state routing is
not feasible due to the enormous state propagation overhead when the network
topology changes.
3. As few nodes as possible must be involved in route computation and state
propagation, as this involves monitoring and updating at least some states in
the network. On the other hand, every host must have quick access to the
routes on demand.
4. Each node must care only about the routes to its destination and must not be
involved in frequent topology updates for those portions of the network that
have no traffic.
5. Stale routes must be either avoided or detected and eliminated
quickly.
6. Broadcasts must be avoided as much as possible, because
8. broadcasts can be time consuming for MANETs [13.1]. The simpler
function of multicasting is observed to be even more complex than
uncontrolled broadcasting [13.1].
7. If the topology stabilizes, then routes must converge to the optimal
routes.
8. It is desirable to have a backup route when the primary route has
become stale and is to be recomputed.
• One of the major challenges in designing a routing protocol [13.3]
for MANETs stems from the fact that, on the one hand, a node
needs to know at least the reachability information to its neighbors
for determining a packet route; on the otherhand, in a MANET, the
network topology can change very frequently.
Routing Classification
• Existing routing protocols can be classified either as proactive or reactive . • Proactive
protocols attempt to evaluate continuously the routes within the network, so that
when a packet needs to be forwarded, the route is already known and can be
immediately used.
9. • The family of distance vector protocols is an example of a proactive scheme.
Reactive protocols, on the other hand, invoke a route determination procedure
only on demand.
• Thus, when a route is needed, some sort of global search procedure is initiated. •
The family of classical flooding algorithms belongs to the reactive group. Examples of
reactive (also called on-demand) ad hoc network routing protocols include ad hoc
on-demand distance vector (AODV) and temporally ordered routing algorithm (TORA)
The advantage of the proactive schemes is that whenever a route is needed, there is
negligible delay in determining the route. In reactive protocols, because route
information may not be available at the time a datagram is received, the delay to
determine a route can be significant.
Ad Hoc On-Demand Distance Vector
Routing
• Ad hoc on-demand distance vector (AODV)
routing is built over the DSDV algorithm • AODV is a
significant improvement over DSDV. AODV is a
pure on-demand route acquisition algorithm. The
10. nodes that are not on a particular path do not
maintain routing information, nor do they
participate in the routing table exchanges. • As a
result, the number of broadcasts required to
create the routes on demand via AODV is
minimized rather than doing broadcasts to
maintain complete route information in DSDV.
• When a source needs to send a message to a destination and does not
have a valid route to the latter, the source initiates a route discovery
process.
• Source sends a route request (RREQ) packet to all its neighbors, the latter
forward the request to all their neighbors, and so on, until either the
destination or an intermediate node with a “fresh enough” route to the
destination is reached.Figure illustrates the propagation of the broadcast
RREQs across the network.
• Each node has a unique sequence number and a broadcast ID, which is
incremented each time the node initiates a RREQ.
11. • The broadcast ID, together with the node’s IP address, uniquely identifies
every RREQ. The initiator node includes in the RREQ the following: • Its own
sequence number
• The broadcast ID
• The most recent sequence number the initiator has for the destination
• Intermediate nodes reply only if they have a route to the
destination with a sequence number greater than or at
least equal to that contained in the RREQ.
12. • To optimize the route performance, intermediate nodes
record the address of the neighbor from which they receive
the first copy of the broadcast packet.
• This establishes the best reverse path. All subsequently
received copies of the RREQ are discarded.
• Once the RREQ reaches the destination or an intermediate
node with a fresh enough route to the destination, the
intermediate/destination node sends a unicast route-reply
(RREP) message back to the neighbor from which it
received the first copy of the RREQ
• As the RREP travels back on the reverse path, the
nodes on this path set up their forward route
entries to point to the node from which the RREP
has just been received.
• These forward route entries indicate the active
13. forward route.
• The RREP continues traveling back along the
reverse path till it reaches the initiator of the
route discovery. Thus, AODV can support only the
use of symmetric links.
• A route timer is associated with each route entry. This
timer triggers the deletion of the route entry if it is not
used within the specified lifetime.
• When a source node moves, it can reinitiate the route
discovery procedure to find new routes to the destination. •
If the nodes along the route move, their upstream neighbors
(nodes justbefore them enroute from source to destination)
notice the movement and propagate a link failure notification
to their own active upstream neighbors, and so on until the
source node is reached.
14. • A link failure notification is essentially a ROUTE ERROR with
infinite metric.
• The source node can now choose to reinitiate the route-discovery
procedure if a route to that destination is still desired. • Another
protocol followed in route maintenance is the use of hello messages,
periodic local broadcasts by a node to inform other nodes in its
neighborhood of its presence.
• Hello messages ensure local connectivity. Nodes listen for
retransmission of data packets to make certain that the next hop is
still within reach.
• If such a retransmission is not heard, a variety of techniques may be
used for recouping the path.
• One such method is the reception of hello messages to determine
whether the next hop is within the communication range. • The
hello messages may also list other nodes from which a node has
heard, thereby relaying more information about network
connectivity.
