Ad hoc network protocols are how the format and order of
messages exchanged between nodes as well as actions taken on the sending and
receiving messages. The major challenges that a routing protocol designed ad
hoc wireless networks faces are mobility of nodes, resource constraints,
error-prone channel state, and hidden and exposed terminal problems.
Mobility: the network topology in an ad hoc network is
highly dynamic due to the movement of nodes; hence an on-going session suffers
frequent path breaks. Disruption occurs either due to the movement of the
intermediate nodes in the path or due to the movement of end nodes. Such
situations do not arise because of reliable links in wired networks where all
the nodes are stationary. Even though the wired network protocols find
alternate routes during path breaks, their convergence is very slow. Therefore,
wired network routing protocols cannot be used in ad hoc wireless networks
where the mobility of nodes results in frequently changing network topologies.
Routing protocols for ad hoc wireless networks must be able to perform
efficient and effective mobility management.
Bandwidth constraint: abundant bandwidth is available in
wired networks due to the advent of fiber optics and due to the exploitation of
wavelength division multiplexing (WDM) technologies. But in a wireless network,
the radio band is limited, and hence the data rates it can offer are much less
than what a wired network can offer. This requires that the routing protocols
use the bandwidth optimally by keeping the overhead as low as possible. The
limited bandwidth availability also imposes a constraint on routing protocols
in maintaining consistent topological information. Due to the frequent changes
in topology, maintaining consistent topological information at all the nodes
involves more control overhead which, in turn, results in more bandwidth
wastage. As efficient routing protocols in wired networks require the complete
topology information, they may not be suitable for routing in the ad hoc wireless
networking environment.
Error-prone shared broadcast radio channel: the broadcast
nature of the radio channel poses a unique challenge in ad hoc networks. The
wireless links have time-varying characteristics in terms of link capacity and
link-error probability. This requires that the ad hoc network routing protocol
interacts with the MAC layer to find alternate routes through better-quality
links. Also, transmissions in ad hoc wireless networks result in collisions of
data and control packets. This is attributed to the hidden terminal problem
[5]. Therefore, it is required that ad hoc wireless network routing protocols
find paths with less congestion.
Hidden and exposed terminal problems: the hidden terminal
problem refers to the collision of packets at receiving node due to the
simultaneous transmission of those nodes that are not within the direct
transmission range of the sender, but are within the transmission range of the
receiver, collision occurs when both nodes transmit packets at the same time without
knowing about the transmission of each other.
Solutions for this problem include medium access collision avoidance
(MACA) [6], medium access collision avoidance for wireless (MACAW) [7], floor
acquisition multiple access (FAMA) [8], and dual busy tone multiple access
(DBTMA) [9].
Resource constrains: two essential and limited resources
that form the major constraint for the nodes in an ad hoc network are battery
life and processing power. Devices used in ad hoc networks in most cases
require portability, and hence they also have size and weight constraints along
with the restrictions on the power source. Increasing the battery power and
processing ability makes the node bulky and less portable. Thus ad hoc network
routing protocols must optimally manage these resources.
Due to the issues in an ad hoc network environment discussed
so far, wired network routing protocols cannot be used in ad hoc networks.
Hence, ad hoc networks require specialized routing protocols that address the
challenges described above. A routing protocol for ad hoc networks should have
the following characteristics:
1. It must
be fully distributed, as centralized routing involves high control overhead and
hence is not scalable. Distributed routing is more fault-tolerant than centralized
routing, which involves the risk of single point of failure.
2. It must
be adaptive to frequent topology changes caused by the mobility of nodes.
3. Route
computation and maintenance must involve a minimum number of nodes. Each node
in the network must have quick access to routers, that is, minimum connection
setup time is desired.
4. It must
be localized, as global state maintenance involves a huge state propagation
control overhead.
5. It must
be loop-free and free from stale routes.
6. The
number of packet collisions must be kept to a minimum by limiting the number of
broadcasts made by each node. The transmissions should be reliable to reduce
message loss and to prevent the occurrence of stale routes.
7. It must
converge to optimal routes ones the network topology becomes stable. The
convergence must be quick.
8. It must
optimally use scarce resources such as bandwidth, computing power, memory, and
battery power.
9. Every
node in the network should try to store information regarding the stable local
topology only. Frequent changes in local topology, and changes in the topology
of parts of the network with which the node does not have any traffic
correspondence, must not in any way affect the node, that is, changes in remote
parts of the network must not cause updates in topology information maintained
by the node.
10. It should
be able to provide a certain level of quality of service (QoS) as demanded by
the applications, and should also offer support for time-sensitive
traffic.
Routing protocols for ad hoc networks can be classified into
several types based on different criteria. Routing information update mechanism
is one of criteria. Ad hoc network routing protocols can be classified into
three major types: proactive or table-driven routing protocols, reactive or
on-demand routing protocols, and hybrid routing protocols.
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