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Abstract— Wireless Mesh Networks (WMN) are considered as
robust architecture to provide reliable connectivity to fixed and
mobile users. The redundant infrastructure provided by WMN
can be efficiently utilized using multipath routing. However, the
performance gain of multipath routing mainly depends on
efficient path selection protocols. The omni-directional wireless
medium, unstable link quality and mutual path interference are
some of those factors which greatly affect the performance of
multipath routing protocols. Routing metrics play a pivotal role
for the selection of reliable paths. This paper has investigated a
variety of existing single and multi-radio routing metrics to
analyze their limitations in finding multiple reliable paths around
interfering neighborhood. The paper thus proposes a Diversity
based Multipath Routing Metric (DMRM) which basis its
calculation on diverse wireless network conditions. The routing
metric not only captures interfering traffic load on the link but
also considers link loss ratio, inter and intra-flow interference,
multiple data rates of nodes and channel diversity during
multipath selection. DMRM integrates multiple routing
conditions to overcome the limitations of existing metrics. The
proposed metric is integrated with Diversity-based Multipath
Routing Protocol (DMRP). DMRP is designed to select optimal
path in terms of packet delivery ratio, lower level of interference,
fewer congested links and multipath load balancing to achieve
higher throughput and reduced end-to-end packet delay
. INTRODUCTION
The architecture of multihop Wireless Mesh Network
(WMN) extends the service coverage of internet to isolated
sites using fault tolerant, broadband mesh connections. The
mesh connectivity in the backbone WMN provides a redundant
infrastructure which can be used in locations where physical or
wired connections are not available. An important feature of
WMN is the availability of multi-radio, multi-channel mesh
routers which forwards its own as well as neighbor’s data
traffic to gateways. Their characteristics to dynamically selfheal
and self-organize, coupled with the ability to maintain
mesh connectivity with low set-up and maintenance cost made
them valuable in broad range of application domains [1].
Despite all these characteristics, many research challenges exist
which prevent multihop networks to achieve optimal
performance. The researchers [2][9] have identified that
designing efficient routing metrics and protocols exhibit a
significant impact in improving the performance of such
networks. In particular, the performance and dependability of
wireless multihop communications extensively depends on the
capability of these routing protocols to effectively distribute the
load on selected paths, considering the current and dynamic
network conditions [3-4].
A simplified scheme to deal with the intricacies of the
routing problems in wireless networks is to employ routing
protocols of wired network by assuming the two alike. This
assumption creates further problems when wireless link quality
variations, traffic load and channel diversity come into play. In
wired networks, routing load varies but link quality remains
same which is not the case of wireless networks where link
quality as well as load significantly varies even in static
wireless networks. Hence, shortest path routing paradigms
(such as Routing Information Protocol (RIP), Open Shortest
Path First (OSPF) and Boarder Gateway Protocol (BGP))
conceived for wired networks do not work well in wireless
domain. On the other hand, the majority of the routing
approaches designed for ad-hoc networks select a precomputed
path between sender and receiver pairs to forward
data packets using predetermined sequence of intermediate
nodes. The major consideration of ad-hoc routing protocols is
power consumption and link establishment. They concentrate
on route maintenance than route quality. Henceforth, these
approaches cannot be straightaway applied to wireless mesh
domain where route quality plays a vital role to achieve
optimized network performance.
Indeed, much of the research has been carried out to design
quality routing metrics for WMN; however, none of the metric
is sufficient for all kind of networks under all conditions. There
are certain and random changes in network’s environment such
as uncertain arrival of bursty data traffic, unpredictable
degradation of link quality, gradual rise of inter/intra-flow
interference and limited number of non-overlapping channels.
For these reasons, same routing metrics or protocols cannot be
applied under all conditions throughout network’s life time to
achieve best possible performance. The paper thus analyses
and identifies the limitations and appropriate use-cases of
existing single and multi-radio routing metrics of wireless
mesh network. The aim is to present a routing strategy which
not only works under dynamic network conditions to overcome
the existing limitations but also exploit the redundant mesh
infrastructure by creating multiple paths to destination. The
paper proposes Diversity based Routing Metric (DMRM)
which computes multiple paths considering interfering traffic load on the nodes, link loss ratio, inter and intra-flow
interference, multiple data rates of nodes and channel diversity.
The proposed metric is incorporated with Diversity based
Multipath Routing Protocol (DMRP) which selects optimal
paths in terms of improved packet delivery ratio and lower
level of interference, by avoiding congested links and
balancing load on multipath to achieve higher throughput and
reduced end-to-end packet delay.
The paper is organized in the following sections. Section 2
presents an analysis of existing single and multi-radio routing
metrics to provide the motivation for the design of a new
routing metric. Section 3 presents the proposed Diversity based
Multipath Routing Metric (DMRM) along with network and
interference model. In section 4, the design of Diversity based
Multipath Routing Protocol (DMRP) has been discussed. As a
final point, section 5 concludes the paper.
II. ROUTING METRICS
In WMN, the routing metrics have evolved over time to gain
added advantage of its multi-radio multichannel features. In the
earlier phase of development, a number of single radio routing
metrics had been adopted from MANETs, which formed the
basis of most efficient multi-radio routing metrics. In this
section, a brief overview of a number of existing single and
multi-radio routing metrics has been presented.
A. Single-radio Routing Metrics
Hop count is one of the most basic and earliest routing
metric. It selects routes with shortest distance to destination. In
wireless network scenarios, the performance of shortest paths
with long distance lower quality links is usually minimal
compared to longer, short distance linked paths [5]. This metric
is mostly useful in high mobility scenarios where the priority is
to establish paths for shorter time period. [6] Though hop count
metric provides highly stable paths, but it is not considered
useful in WMN scenarios, where the priority is to establish
high quality link-paths.
Expected Transmission Count ETX [7] is the earliest metric
for wireless networks which establishes paths based on link
and path quality measures. It calculates number of transmission
and retransmissions attempts of a link using active probing.
Equation 1represents the metric as follows
fr dd
ETX
*
1
(1)
where dr and df are the packet delivery ratios in reverse and
forward directions of a link respectfully. As compared to Hop
count metric, ETX gives better measurements in single radio
WMN, but lacks the ability to incorporate different link rates
and channel interference in multi-rate, multichannel
environments.
Expected Transmission Time ETT [8] metric extended ETX to
overcome its limitations. It incorporated links throughput in
addition to link quality in the metric measurements. The
representation of ETT metric is as follows