01-02-2013, 11:24 AM
IMPROVED NETWORK CONNECTIVITY IN MANETS
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Abstract:
The growth in wireless communication technologies has resulted in a considerable amount of
attention given to mobile adhoc networks. All mobile hosts in an adhoc network are embedded with
packet forwarding capabilities. It is decentralized and is independent of infrastructure. Since mobile
hosts in an adhoc network usually move freely, the topology of the network changes dynamically and
disconnection occurs frequently. These characteristics require the routing protocols to find an
alternative path towards the destination for data transfer. The existing on-demand routing protocols
does the alternative path establishment only after the disconnection of links in the existing path. The
data sent by the source during alternate path establishment period will be lost leading to incomplete
data transfer. The network traffic will therefore increase considerably. This problem can be overcome
by establishing an alternative path when the existing path is more likely to be broken, by sending a
warning message to the source indicating the likelihood of disconnection. In this paper an attempt has
been made to analyze a protocol that improves the network connectivity by preempting the alternative
path before the existing link gets failed by monitoring the signal strength and ‘age of the path’.
INTRODUCTION
In recent days, connectivity with mobility is in great demand. This has given boost to lot
of research in the field of adhoc networks. MANETs are self-organisable, infrastructure less,
wireless, peer-peer, multi hop networks. They adopt distributed control in providing connectivity
from the source to the destination. Therefore the mobile units themselves need to take the
responsibility of discovering its nearest neighbors who are ready to route data packets to the
destination. Co-operation among all the mobile nodes is very essential in such cases. [1, 2].Typical
applications of these networks are outdoor events such as conferences, concerts and festivals,
places with no network infrastructure, outdoor emergencies and natural disasters and military
operations. A typical adhoc network is as shown in figure 1, which depicts its dynamic topology.
PREVIOUS WORKS
Routing plays a vital role in MANETs. The dynamic nature of the network along with the
wireless communication poses many challenges on the routing protocols. Many proposed routing
protocols for adhoc networks operate in an on-demand fashion. Dynamic Source Routing (DSR) is
a routing protocol adapted for MANETs with dynamic topology due to frequent movement of the
nodes in the network [13]. DSR has two phases viz. route discovery and route maintenance.
Route Discovery
This is a process of determination of a suitable route to the destination. To perform route
discovery, the source node broadcasts a route request packet with a recorded source node
addresses. Each node that hears the route request forwards the request (if appropriate), adding its
own address to the recorded source route field in the packet. The route request packet propagates
hop-by-hop outwards from the source node until either the destination node is found or until
another node is found that can offer a route to the destination [13].
Route Maintenance
Conventional routing protocols integrate route discovery with route maintenance by
continuously sending periodic routing updates. If the status of a link or node changes, the periodic
updates will eventually reflect the change in all other nodes, presumably resulting in the
computation of new routes. If a node along the path of a packet detects an error, the node returns a
route error packet to the sender. The route error packet contains the addresses of the nodes at both
ends of the hop in error. When a route error packet is received or overheard, the ‘hop in error’ is
removed from any route caches and all routes which contain this hop must be truncated at that
point. Route maintenance can also be performed using end-to-end acknowledgments rather than
the hop-by-hop acknowledgments described above [13].
In DSR, an alternative path discovery is initiated only after the path disconnection due to a
link failure. This will result in loss of a number of data packets. Therefore there is a need to
modify the conventional DSR protocol.
PREEMPTIVE ROUTING
Conventionally, a change of path occurs in networks when: (i) a link along the path fails or
(ii) a shorter path is found. A link failure results in multiple retransmissions being required to
detect the failure and a new path has to be found and used as in on-demand routing. In MANETs,
as the network topology frequently changes, path disconnections occur and this proves to be very
costly.
As soon as a chance of link failure is identified, the preemptive routing invokes the routing
algorithm to discover an alternative path before the actual link failure occurs. Thus, the
connectivity of the network can be improved. This technique is similar to the soft handoffs in
mobile telephone networks. The route maintenance algorithm is as followsi) Detect the path
which is more likely to be broken (ii) Invoke routing algorithm to discover an alternate path. (iii)
Continue the transmission with the new path [14].
SIMULATION RESULTS
The following assumptions were made for carrying out simulation [15, 16].
· The discrete event network simulator NS-2 has been used for analysis and
comparison of the adhoc routing protocols.
· The mobility model uses the random waypoint model in a rectangular field.
· The mobile node movement is restricted to a square cell of 600 X 600m
containing 70 nodes.
· A 512-byte User Datagram Protocol (UDP) generated by a constant bit rate (CBR)
traffic source is used.
· Simulation lasts for 600 seconds.
A comparison of preemptive routing with the traditional DSR algorithm is discussed here.
CONCLUSIONS
In this paper, an attempt has been made to improve the data connectivity in the network by
preemptively selecting an alternate path as soon as the signal strength falls below the threshold
value and the age of the path exceeds the predefined threshold. An analysis of preemptive protocol
is made with the DSR algorithm.
The performance of the preemptive algorithm is dependent on the preemptive ratio α. It is
clear from the analysis that preemptive algorithm performs better than DSR. But the offered load
on the network is more in preemptive protocol as continuously the signal strength and the age of
the path has to be examined and warning messages are to be sent back immediately.
As a part of future work, this algorithm can be implemented over energy aware routing
protocols available in the literature.