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Routing protocols in wireless mesh networks: challenges
and design considerations
Sonia Waharte &Raouf Boutaba &Youssef Iraqi &Brent Ishibashi
Published online: 6 July 2006
#Springer Science + Business Media, LLC 2006
Abstract Wireless Mesh Networks (WMNs) are an emerging technology that could
revolutionize the way wireless network access is provided. The interconnection of
access points using wireless links exhibits great potential in addressing the Blast mile^
connectivity issue. To realize this vision, it is imperative to provide efficient resource
management. Resource management encompasses a number of different issues,
including routing. Although a profusion of routing mechanisms has been proposed for
other wireless networks, the unique characteristics of WMNs (e.g., wireless
backbone) suggest that WMNs demand a specific solution. To have a clear and
precise focus on future research in WMN routing, the characteristics of WMNs that
have a strong impact on routing must be identified. Then a set of criteria is defined
against which the existing routing protocols from ad hoc, sensor, and WMNs can be
evaluated and performance metrics identified. This will serve as the basis for deriving
the key design features for routing in wireless mesh networks. Thus, this paper will
help to guide and refocus future works in this area.
Keywords Wireless mesh networks .Routing
1 Introduction
Extending high-speed IP connectivity to the Blast mile^is an open and on-going
research problem with no satisfactory solution. A number of potential solutions
have been proposed, including full end-to-end optical networks and wireless access
networks. However, deploying these networks requires the installation of a large
amount of wire/fibre. The initial investment costs for deployment, and the difficulty
Multimed Tools Appl (2006) 29: 285–303
DOI 10.1007/s11042-006-0012-8
S. Waharte (*):R. Boutaba :Y. Iraqi :B. Ishibashi
School of Computer Science, University of Waterloo, Waterloo, Canada
e-mail: swaharte@bbcr.uwaterloo.ca
R. Boutaba
e-mail: rboutaba@bbcr.uwaterloo.ca
Y. Iraqi
e-mail: iraqi@bbcr.uwaterloo.ca
B. Ishibashi
e-mail: bkishiba@bbcr.uwaterloo.ca
of deployment in some environment settings (established urban areas, wilderness,
etc.), have prevented the widespread realization of such access networks.
Wireless Mesh Networks (WMNs), consisting of wireless access networks
interconnected by a wireless backbone, present an attractive alternative. Compared
to optical networks, WMNs have low investment overhead and can be rapidly
deployed. The wireless infrastructure is self-organizing, self-optimizing, and fault
tolerant. It can extend IP connectivity to regions otherwise unreachable by any single
access technology. Many companies, such as Nokia [34], Microsoft [29], Motorola
[31] and Intel [18], are actively promoting wireless mesh networks as a full IP so-
lution. Initial field tests [44,45,50] have demonstrated WMN’s tremendous potential
and market value. WMNs combine concepts from a diverse set of existing and
emerging wireless technologies, including cellular technologies, ad hoc networks, and
sensor networks. The application of research results from these areas could greatly
contribute to the development, implementation, and growth of wireless mesh
networks.
However, the lack of a clear understanding of wireless mesh network character-
istics and the absence of targeted resource management and service provisioning
mechanisms can jeopardize their successful development. Issues inherent to Wireless
Mesh Networks require new research innovations. Moreover, it is crucial to realize
that such mechanisms should cope with consumers’ increasing demands for QoS
guarantees.
Delivering on QoS guarantees requires a strong resource management frame-
work, starting with an effective routing protocol. The multi-hop wireless nature of a
WMN demands a different approach to routing from conventional wireless access
networks. It has much more in common with the ad hoc and sensor network fields.
However, the overall properties of the individual nodes and the overall network are
very different in many ways. Therefore, it is unclear exactly how applicable these
approaches are to a WMN.
This paper addresses the issue of routing in a WMN, by considering the specific
characteristics of a WMN. It explores existing solutions, and evaluates their suitability
to the wireless mesh environment. Based on this evaluation, the need for developing
new routing mechanisms, specifically tailored for the unique characteristics of WMNs
is assessed. A number of issues and considerations are identified and presented, in
order to guide future work and the development of a WMN routing protocol.
The remainder of the paper is organized as follows. Section 2provides a general
overview of wireless mesh networks and the associated resource management issues.
Section 3identifies the characteristics of wireless mesh networks. Routing issues are
discussed in Section 4. Section 5concludes this work.
2 Wireless mesh networks
2.1 Wireless mesh network: architectural view
1) What is a wireless mesh network?: Formally, a network topology can be ab-
stracted by a graph GðV;EÞwhere Vis the set of vertices representing the
network nodes, and Eis the set of edges representing the communication links
between the vertices. In wireless environments, a mesh network is referred to as
a connected graph such that for each i;j2V,i6¼ j, there exists a path (subset of
286 Multimed Tools Appl (2006) 29: 285–303
edges) connecting iand j. This can be further extended to k-connected graphs if
path redundancy is considered. However, this strict definition fails to consider
the different characteristics of the nodes and edges forming the network.
Industry has adopted different views on the concept of a mesh network. The
proposals differ most significantly in the following areas:
&Network components: The role of mobile nodes as part of the wireless
mesh network architecture differentiates current proposals. MIT Roofnet
[30] and Nortel Networks’ solutions [35] do not consider mobile nodes as
part of their network infrastructure (i.e., only access points and network
gateways are included). On the other hand, MeshNetworks architecture [28]
considers meshing between access points, as well as between mobile nodes.
&Degrees of mobility: Some early work in WMNs [46] drew parallels between
ad hoc networks and mesh networks. However, current works tend to dis-
criminate these two network environments by considering that mesh networks
are formed by a wireless backbone of non-energy constrained nodes with low
(or no) mobility [9] whereas in some wireless multi-hop networks, such as
MANETs, energy conservation and user mobility are the primary research
focus. This shift of research concerns leads to the questioning of the suitability
of applying existing ad hoc networking protocols to wireless mesh networks.
&Traffic pattern: Wireless mesh networks exhibit unique traffic patterns,
which partially resemble ad hoc networks’ and sensor networks’. Similar to
sensor networks, data traffic is mainly expected to flow between users
(sensor nodes) and the network gateway(s) (destination station or sink).
This constitutes the main differentiator between wireless mesh networks and
ad hoc networks in some literature, such as in [20]. However, in a WMN,
traffic can also flow between any pair of user nodes (as in ad hoc networks).
To form a common understanding on what a wireless mesh network is, the
following definition is presented, that is general enough to encompass most current
mesh network architectures:
Definition A wireless mesh network is a packet-switched network with a static
wireless backbone.
Therefore,
&The wireless backbone topology is fixed and does not have to cope with
access point mobility. Modifications to infrastructure can only result from
the addition/removal or failure of access points.
&Pure ad hoc networks are not considered as wireless mesh networks.
2) Our view of the wireless mesh network architecture: Contrary to [51], which
regards a mesh network as composed of only two different entities, the mobile
nodes and the access points, a more general view of a mesh network is adopted
(similar to [35]). The mesh network architecture is composed of three different
network elements: network gateways, access points and mobile nodes (figure 1).
&Network Gateway: This network element allows access to the wired
infrastructure, possibly the Internet or other local networks. More than one
gateway can be deployed in a wireless mesh network.
Multimed Tools Appl (2006) 29: 285–303 287
&Access Points (APs): Low cost, flexible, and easy to deploy, the APs form
the network backbone spanning over wide areas. They can be embedded with
enhanced capabilities (directional antennas, multiple antennas, multiple
interface cards, etc.). Users connect to the APs, using wireless or wired
means. The APs are assumed to be static, with a low failure probability, and
no power constraints. This mesh of APs serves as a relay between the mobile
terminals and the network gateways.
&Mobile Nodes
1
: They include a wide range of devices, like PDAs, laptops
or cell phones, with varying degrees of mobility. Mobile nodes can
significantly differ in terms of energy autonomy, computation and transmis-
sion capabilities. They communicate with the wired infrastructure by directly
contacting the network gateway (according to their position and transmis-
sion capabilities) or by using the APs as relays.
In a wireless mesh network, it is not necessary for all APs to have direct connection
to the network gateways. The APs may need to forward their traffic through other
APs in order to reach a gateway. Access to the gateway could be further extended if
we envision a mesh topology formed between the mobile nodes. The mobile nodes
may be highly mobile, as in the case of a dynamic network topology (ad hoc-like).
2.2 Differences with existing wireless network technologies
To understand the specificities and constraints of wireless mesh networks, it is
important to position this technology in the landscape of wireless communications.
Depending on the network coverage, four distinct groups of wireless network
technologies can be identified:
&WPAN (Wireless Personal Area Network): Developed as cable replacement
technology. The most widely accepted protocol is IEEE 802.15.1 [15] (standard-
ization of Bluetooth [3]).
1
We interchangeably use the terms of users, mobile nodes or mobile terminals to refer to this
specific network component. Mobile Nodes is a generic term used to refer to users who may not
necessarily be mobile (i.e., static wireless terminals).
Fig. 1 Example of wireless mesh network topology
288 Multimed Tools Appl (2006) 29: 285–303
&WLAN (Wireless Local Area Network): In home and office environments. In
infrastructure mode, access to the wired network is achieved through one-hop
wireless transmission. In ad hoc mode, users interconnect without the support of
any infrastructure. The most commonly accepted Standard is IEEE 802.11 [14].
&WMAN (Wireless Metropolitan Area Network): Intended for larger coverage
areas such as cities. Current technological advances render high-throughput
wireless connections feasible and offer transmission coverage greater than
WLANs’. WMANs standardization effort is undergoing with IEEE 802.16 [16].
&WWAN (Wireless Wide Area Network): For data transmission over large areas
such as cities or countries using satellite systems or cellular networks. Although
several satellite systems have been successfully launched (Iridium [19], Globalstar
[11], etc.), the low offered throughput (around 10 kbps) restricts their practical
use to voice applications. On the other hand, high throughput (up to 2 Mbps)
cellular networks are able to support a much broader range of applications.
Recently, Wireless Sensor Networks (WSNs) have gained significant importance.
WSNs consist of an interconnection of tiny nodes, whose function is to retrieve
specific information from the environment and to transmit the result of this sensing
operation to a remote destination station. As their coverage depends on the target
application (it can potentially be of the size of a WMAN or a WLAN), and given that
these networks are data-centric and not user-centric (in that the loss of a node in a
sensor network is less important than the information it was sensing), they have been
excluded from the above categorization. The architectural differences between these
network technologies are summarized in Table 1. The comparisons are performed
by considering only the parts of the networks involving wireless communications.
Wireless Mesh Networks can be seen as a combination of WMANs, WLANs and
to a certain extent, wireless sensor networks. Data transmission is performed
through multi-hop wireless communications and involves the mobile nodes, network
gateways and access points. The available bandwidth depends on the underlying
network technology, with data rates as high as 54 Mbps. The traffic mix may include
multimedia streams and the network is expected to support thousands of mobile
users. Wireless mesh networks share similarities with WLANs and WMANs in
terms of the fixed infrastructure, and therefore suffer from the same bandwidth
limitations and the need to handle user mobility.
2.3 The importance of resource management in wireless mesh networks
In spite of the proliferation of wireless transmission technologies in recent years,
wireless bandwidth remains limited compared to wired technologies (LANs, optical,
etc.). The impact of environmental conditions and interference on network
performance further exacerbates this problem. To meet users’ quality-of-service
expectations, efficient resource management remains a great challenge in wireless
networks.
In general, power control, mobility management, and admission control are
resource management problems common to all wireless networks. In addition,
cellular networks present the unique challenge of channel allocation whereas
routing is a prominent problem in ad hoc networks. As an amalgamation of multiple
wireless technologies, WMNs face a combination of these problems, as well as those
of network configuration and deployment (see figure 2).
Multimed Tools Appl (2006) 29: 285–303 289
Table 1 Comparison of wireless network architectures
WWAN WMAN WLAN WPAN WSN
Cellular net Satellite net Infrastructure Ad hoc
Transmission One-hop Multihop One-hop One-hop Multihop Multihop Multihop
Network Base stations Satellites Base stations Access points Mobile nodes Mobile nodes Static nodes
Entities Mobile nodes Mobile nodes Mobile nodes Mobile nodes Sink
Max. offered 2 Mbps 10 kbps 1.5 Mbps 54 Mbps 54 Mbps 100 kbps 100 kbps
Throughput
Traffic Multimedia Voice Multimedia Multimedia Multimedia Multimedia Statistics
Users Hundreds Hundreds Hundreds Dozens Hundreds Hundreds Thousands
Capacity (per cell) (per satellite) (per AP)
Trans. Range km 105km 50 km 250 m 250 m 10 m 10 m
Frequency GSM: 800 MHz Iridium: 2 GHz IEEE 802.16a: 2.4/5 GHz 2.4/5 GHz 2.4 GHz 2.4 GHz
Bands UMTS: 2 GHz 2–11 GHz
Limitations Fixed deployment
cost
Cost Fixed deployment Fixed deployment Energy Bandwidth Energy
Long-term
deployment
Bandwidth Bandwidth Processing
capabilities
Delay Transmission
capabilities
290 Multimed Tools Appl (2006) 29: 285–303
Resource management in wireless mesh networks encompasses three main areas:
&Network Configuration and Deployment: The specific construct of WMNs (i.e.,
fixed wireless backbone and mobile end devices) leads to unique requirements in
terms of scalability, fault tolerance, path redundancy, QoS assurance, and network
coverage. In order to avoid over- or under-dimensioning, resulting in either heavy
interference zones or blind spots, it is important to optimize the deployment of the
access points (as in traditional cellular networks). For enhanced network
performance, it is highly desirable to have channel diversity to prevent wireless
interference and support increased number of users. This is traditionally achieved
using channel allocation mechanisms. In WMNs, this problem must be extended
to multi-hop communication, by considering not only channel allocation between
access points and mobile nodes (as per traditional cellular networks), but also
between access points.
&Routing: Routing in WMN extends network connectivity to end users through
multi-hop relays including the access points and the network gateways. This
ultimately should be done while optimizing network resource utilization and
accommodating users’ QoS requirements. The shared medium characteristics and
varying link capacity are some of the crucial design constraints in WMN routing.
Unlike ad hoc routing, WMN routing involves primarily a fixed backbone
consisting of non-energy constrained nodes (i.e., access points and network
gateways), although mobile and energy-constrained wireless nodes (i.e., mobile
devices) may also be considered.
&Mobility Management and Admission Control: Seamless user connectivity can be
obtained through efficient handoff and location management mechanisms, and
appropriate admission control policies. In ad hoc networks, routing and mobility are
tightly coupled due to node motion, while in cellular networks, mobility management
relies heavily on the underlying infrastructure of base stations, mobile switching
centers, and location databases. Wireless mesh networks must reconcile both
aspects, while accounting for its multi-hop nature (significantly more communication
overhead compared to one-hop communication in cellular networks).
Fig. 2 Resource management challenges: an overview
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Of the three research areas outlined above, WMN routing may seem to have the
most existing, viable solutions, as it has much to benefit from multi-hop routing in ad
hoc networks, which has received tremendous research attention and led to many
proposed protocols [13]. However, applying these protocols to WMNs may not be
optimal. For example, in the MIT Roofnet project [30], a preliminary exploration
involved implementing DSDV (Highly Dynamic Destination-Sequenced Distance
Vector) [41], an ad hoc routing protocol, in wireless mesh networks. The volume of
data traffic severely interfering with the transmission of control packets caused slow
path convergence and sub-optimal path setting.
In order to devise better routing protocols for WMNs, we must first analyze the
characteristics of WMNs that can impact on the routing. The criteria and
performance metrics, against which existing routing protocols from ad hoc, sensor,
and WMNs can be evaluated, must also be identified. This can then serve as a basis
for deriving the key design features of efficient routing in wireless mesh networks.
3 Wireless mesh networks characteristics
3.1 From a general perspective
Wireless mesh networks are a unique combination of wireless technologies, exhibiting
characteristics of each component (ad hoc, cellular and sensor networks). While
describing these characteristics, the commonalities and differences between wireless
mesh networks and the aforementioned wireless technologies will be emphasized.
&Transmission medium. All communications in wireless environments have the
following constraints: limited available bandwidth, dynamic changes in link
capacity (due to interference, noise, etc.), and asymmetrical links (interference,
multipath, etc.). Real world implementations have revealed the limitations of
simulations due to the complexity of such environments [33], and have stressed
the need for the deployment of testbeds in order to assess the validity of the
proposed solutions. The impact of the network conditions becomes more critical
in multi-hop wireless networks such as ad hoc and mesh networks, as difficulties
in bounding transmission delay and packet loss makes supporting QoS-sensitive
applications very challenging.
&Network deployment. In cellular networks and infrastructure-based WLANs,
base stations (access points) are deployed in specific locations. In Mobile Ad hoc
Networks (MANETs), the network topology is dynamically changing as users
can be highly mobile although still actively participating in the network
operations through packet forwarding mechanisms. Wireless mesh networks,
being a hybrid technology, blend a fixed wireless backbone with an edge network
consisting of mobile users.
&Wireless technology. Whereas base stations in cellular and ad hoc networks are
primarily deployed with omni-directional antenna technologies, the fixed
backbone of WMNs seems to favor the use of directional antennas for increased
throughput. However, the impact of environmental conditions on the network
performance needs to be taken into consideration, otherwise the communication
can significantly deteriorate due to external phenomena such as wind or rain
(causing link failure from disorientation of the antenna).
292 Multimed Tools Appl (2006) 29: 285–303
&Network infrastructure to support user mobility. As in ad hoc and cellular
networks, users may be mobile. Therefore handoff and location management are
important concerns in wireless mesh networks as well. To address these issues,
distributed and centralized approaches can be considered. Distributed databases
can be deployed in the access points and network gateways to maintain users’
profile and manage users’ mobility. A centralized approach can also be used,
with one entity responsible for maintaining location information. Techniques can
be borrowed from cellular technologies and applied to wireless mesh networks,
but the communication costs, whereas of little importance in cellular networks
(mainly involve fixed part of the network), have adverse effect in bandwidth-
constrained wireless mesh networks.
3.2 From a routing perspective
Wireless Mesh Networks exhibit unique characteristics that differentiate them from
other wireless and wired technologies. Therefore, existing routing protocols must be
revisited in order to consider their adaptability to WMNs. The main differences
relating to routing (Table 2) are:
&Network topology. A fixed wireless backbone differentiates WMNs from other
network types. Therefore, similar to MANETs, communication is performed
through multi-hop wireless transmissions. Unlike MANETs, node mobility in the
backbone infrastructure is not frequent.
&Traffic pattern. In cellular networks and WLANs, data is exchanged between
users and access points. In MANETs, traffic can flow between any pair of nodes.
In WMNs, data transmission is primarily between the mobile nodes and the
network gateway (some similarities can therefore be drawn with sensor net-
works). Traffic between two nodes in the mesh, although less prominent, should
also be considered.
&Inter-path interference. WMNs differ from wired networks due to the possibility
of interference between disjoint paths. Communication on a wireless link (when
considering the use of omni-directional antennas) is open (air medium), whereas
wired networks confine their signal to a particular wire. Therefore, a communi-
cation between two nodes can have an effect on the transmissions of all neighboring
nodes, leading to the well-known problems of hidden and exposed terminals.
Table 2 Routing characteristics summary
Wired
networks
MANETS WSNs WMNs
Topology Static Mobile Static Static
Traffic Any pair
of nodes
Any pair
of nodes
Sensor
to sink
Mobile node to network
gateway (mainly)
Inter-paths interference No Yes Yes Yes
Link capacity Fixed Varying Varying Varying
Channel diversity NA No No Yes
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&Link capacity. WMNs differ from wired network as the link capacity can vary
over time due to the very nature of wireless communications that are sensitive to
surrounding interference. This problem is even more critical when multiple
technologies use the same frequency band (e.g., ISM band).
&Channel diversity. WMNs can benefit from the possibility of introducing
channel diversity in the routing process, which is not possible in other wireless
networks due to node mobility (MANETs) or energy constraints (WSNs). This
technique can significantly reduce inter-nodes interference and increase the
overall throughput.
4 Routing
Routing can be referred to as the process of determining the end-to-end path between
a source node and a destination node. Although security issues are also a concern in
routing mechanisms, solutions for satisfying users’ quality of service requirements
while optimizing network resource utilization is the primary focus [25]. Although this
has been thoroughly studied in conventional networks (wired infrastructure) [5]and
mobile ad hoc networks [13] for unicast and multicast communications, the constraints
inherent to wireless mesh environments call for new, better-adapted routing protocols.
4.1 Routing protocols: evaluation criteria and performance metrics
1) Criteria for Categorization: Routing protocols can be broadly distinguished
based on four criteria: routing philosophy, network organization, location
awareness and mobility management.
&Routing philosophy: Routing approaches can be viewed as proactive,
reactive, or hybrid. In proactive routing protocols, paths are established
regardless of the willingness of a node to transmit data. In reactive (on-
demand) routing protocols, routing processes are initiated upon requests. In
hybrid routing protocols, some of the nodes may implement a proactive
routing protocol and others a reactive routing protocol.
&Network organization: In a flat organization, all the nodes have the same
role in the routing process whereas in a hierarchical organization, some nodes
may have specialized functions. For example, in wireless sensor networks,
cluster-based routing protocols entail the elections of super nodes (cluster-
heads) responsible for data gathering operations.
&Location awareness: Routing protocols may or may not use localization
systems embedded in the network nodes to obtain location information.
&Mobility management: A WMN must manage the mobility of user nodes
throughout the network. As they move, user devices change their point of
attachment to the network, connecting to the access point with which they
have the strongest signal. Mobility raises several issues, similar to those known
in both wired and cellular networks. In MANETs, mobility management has
been integrated into the routing process in order to cope with highly mobile
nodes. In wired and cellular networks, routing and mobility management have
been defined separately although complementary mechanisms.
294 Multimed Tools Appl (2006) 29: 285–303
2) Performance Metrics: Depending on the network characteristics, the routing
protocols can focus on optimizing one or more performance metrics. The
following is a non-exhaustive list including the most commonly used metrics:
&Hop Count: Number of hops between the source and the destination.
&Expected Transmission Count (ETX): This metric is more specific to
wireless communications. It accounts for data loss due to medium access
contention and environmental hazards, and considers the number of
retransmissions needed to successfully transmit a packet over a link [7,8].
&Expected Transmission Time (ETT): This metric is an enhancement of ETX
as it further includes the bandwidth of the link in its computation [9]. This is of
particular interest when different network technologies are used (IEEE 802.11a
and IEEE 802.11b for instance) in order to favor channel diverse paths.
&Energy consumption: A node energy level can be considered as a routing
metric if some nodes are energy-constrained and their involvement in the
routing process can lead to path failure if they suffer from energy depletion.
This problem is particularly important in MANETs and WSNs.
&Path availability/reliability: This metric estimates the percentage of time a
path is available. Node mobility effect can be captured by this metric. It is
particularly important in MANETs.
In the remaining of this paper, our discussion will focus on wireless multi-hop
networks: mobile ad hoc networks (MANETs), wireless sensor networks (WSNs),
and wireless mesh networks (WMNs). The key routing protocols for multi-hop
wireless networks are first summarized. Then, they are categorized according to the
identified criteria. Finally, the unique characteristics of WMNs are used to discuss
why existing routing protocols may not be appropriate for WMNs.
4.2 Brief summary of routing protocols
An exhaustive listing of existing routing protocols for wireless multi-hop networks
is beyond the scope of this paper. Instead, as wireless ad hoc networks, wireless
sensor networks and wireless mesh networks have similar properties, our
discussion is restricted to the key routing protocols proposed for each of these,
with particular emphasis on those proposed for wireless mesh networks. These
protocols and their classification according to the criteria previously identified are
shown in Table 3.
1) Routing Protocols in MANETs: In MANETs, many routing protocols have
been proposed in the last decade, each attempting to address a few aspects of
these networks. [13] provides a comprehensive survey on the subject. Among the
proposed protocols, the more note-worthy ones are (chronologically sorted):
DSDV (Highly Dynamic Destination-Sequenced Distance Vector) [41], DSR
(Dynamic Source Routing) [10], TORA (Temporally Ordered Routing Algorithm)
[37], CGSR (Clusterhead-Gateway Switch Routing) [6], GeoCast (Geographic
Addressing and Routing) [32], ZRP (Zone Routing Protocol) [53], DREAM
(Distance Routing Effect Algorithm for Mobility) [2], LAR (Location-Aided Routing)
[23], OLSR (Optimized Link State Routing Protocol) [36], AODV (Ad Hoc On
Demand Distance Vector Routing) [1], HSR (Hierarchical State Routing) [38], FSR
Multimed Tools Appl (2006) 29: 285–303 295
(Fisheye State Routing) [39], TBRPF (Topology Broadcast Based on Reverse Path
Forwarding) [48], LANMAR (Landmark Ad Hoc Routing Protocol) [40], and
GPSR (Greedy Perimeter Stateless Routing) [22].
2) Routing Protocols in WSNs: In wireless sensor networks, the choice of a routing
protocol depends on the targeted application. The bulk of the research work have
focused on two main application domains: environment monitoring and target
detection. Environment monitoring applications favor a global network organi-
zation. The main contributions are LEACH (Low Energy Adaptive Clustering
Hierarchy) [12] and PEGASIS (Power-Efficient Gathering in Sensor Information
Systems) [26]. In turn, target detection applications rely on sporadic data retrieval
due to the random occurrence of the targeted event. TEEN (Threshold sensitive
Energy Efficient Sensor Network protocol) [27], TTDD (Two-Tier Data
Dissemination Model) [52], Random Walks [47] and Rumor Routing [4]are
widely known contributions in this area. Some other protocols focused more on
efficient information dissemination such as SPIN (Sensor Protocols for Informa-
Table 3 Routing protocols in wireless environments
Routing
protocols
Proactive On-
demand
Flat Hierarchical Location-
aware
Metrics Mobility
Ad hoc DSDV X X No Hops Yes
DSR X X No Hops Yes
TORA X X No Hops Yes
CGSR X X No via CH Yes
GeoCast X X Yes Hops Yes
ZRP X X X No Hops
(zone)
Yes
DREAM X X Yes Hops Yes
LAR X X Yes Hops Yes
OLSR X X No Hops Yes
AODV X X No Hops Yes
HSR X X No via CH Yes
FSR X X No Hops Yes
TBRPF X X No Hops Yes
LANMAR X X No Hops
(zone)
Group
GPSR X X Yes Distance Yes
WSN LEACH X X No Energy Yes
PEGASIS X X No Energy Yes
TEEN X X No Energy Yes
SPIN X X No Energy No
Directed
Diffusion
X X No Energy Yes
TTDD X X Yes Energy No
Random
Walk
X X No Energy No
Rumor
Routing
X X X No Energy Limited
WMN MSR X X X No Proprietary Yes
SrcRR X X No ETT Not
considered
296 Multimed Tools Appl (2006) 29: 285–303
tion via Negotiation) [24] and Directed Diffusion [17]. We refer the reader to [21]
for more details on these protocols.
3) Routing Protocols in WMNs: Only a few protocols have been developed
specifically for WMNs. Several approaches have been considered. MIT (SrcRR
[30]) and MeshNetworks (MeshNetworks Scalable Routing [28]) designed new
protocols tailored for WMNs. MeshNetworks Scalable Routing (MSR) is a
hybrid routing protocol, supposedly able to support highly mobile users and to
dynamically adapt to networks conditions. As the protocol is not in the public
domain, it is not possible to verify the company’s claims. SrcRR is a variation of
DSR using the expected transmission time as a metric instead of the number of
hops. In other words, the shortest paths are determined based on least packet
loss.
Other works have focused on enhancing existing routing protocols with new
routing metrics more appropriate for WMNs. Indeed, the fixed wireless backbone
allows a better estimation of the link quality through regular measurements. It is
also possible to introduce channel diversity in the network infrastructure so as to
reduce interference and increase overall throughput [9,43].
4) Comparisons and Observations:FromTable3, it can be seen that in
MANETs, the most favored research approach is proactive routing; in sensor
networks both proactive and reactive approaches are equally used; and in mesh
networks, routing approaches are mainly reactive or hybrid. The choice of a
routing technique is made based on the network characteristics with the
greatest impact on routing. These are:
&Network size: The choice of a routing protocol is highly dependent on the
network size and node density. For instance, if the network is large, flooding
should be avoided, whereas this solution is satisfactory when the number of
nodes is small.
&Node mobility: It is important to evaluate the users degree of mobility in
order to design protocols adapted to the frequency of handoffs and route
updates.
&Traffic patterns: Traffic characteristics and traffic type can have a major
impact on routing design and resource management. For instance, when the
network is exposed to heavy traffic volumes, it is necessary to include load
balancing techniques in the routing, in order to optimize network resource
utilization and avoid congestion.
Control overhead is another important design criterion. The number of control
packets generated by the routing mechanism impacts the data transmission and
offered throughput, which needs to be evaluated.
Although reactive routing protocols are able to address node mobility, the
significant overhead and delay pertinent to reactive protocols are not acceptable for
delay-sensitive applications in energy-constrained networks. In wireless sensor
networks, routing protocols have been developed in accordance with the supported
applications. If data is only sent sporadically (e.g., target detection applications),
proactive routing protocols may not be the best choice. On the other hand,
environmental monitoring applications require constant data retrieval and hence
justify the use of proactive routing protocols. In wireless mesh networks, the routing
strategy should also be selected based on these factors. First, environmental
Multimed Tools Appl (2006) 29: 285–303 297
conditions have a significant impact on data transmission. Implementing a proactive
routing protocol based on metrics such as ETT or ETX is difficult as the link
capacity fluctuates overtime and the convergence time can be significant when
the control packets have to compete with data traffic. However, other parameters
can be very helpful for making the routing decisions. For instance, access point
location is readily available and tends to remain static over long periods of time.
Implementing a flat or hierarchical routing protocol depends on the network
complexity and the nodes capabilities. For instance, hierarchical routing protocols
have been proposed in scenarios where some nodes embed localization systems and
can therefore serve as reference points. This approach is also popular in energy-
constrained wireless sensor networks. The same mechanism may also be leveraged
in mesh networks for mobility management.
The choice of performance metrics to be used is also influenced by the network
specifics. It has been shown [9] that the number of hops constitute the best routing
metric when mobility is involved. However, in wireless mesh networks, the presence
of a fixed backbone can significantly impact the routing design. By gathering
relevant information on the actual physical environment, such as interference level,
more informed resource management can be performed.
4.3 How to design a WMN routing protocol?
To capture the essence of what has been discussed so far, the following questions
must be posed to help guide the design of an efficient routing protocol suitable for
wireless mesh networks.
&Which performance metric(s) should be used? The nature of a WMN demands
that the chosen routes be efficient. However, it is not entirely clear what should
be optimized. As long as the degree of node mobility is not high, [9] has shown
the advantage of using the expected transmission time to account for link
capacity and loss rate in the routing decision. Conversely, when the degree of
node mobility is high, minimizing the hop count is still the most sensible
decision.
&What hardware technologies will be used? Technologies such as directional
antennae have been considered in ad hoc networks. However due to user
mobility they required complicated solutions. This option can be considered in
wireless mesh networks, depending on deployment scenarios and the feasibility
of line-of-sight communications. However, this will considerably change the
network’s properties, as link properties and network connectivity will be
impacted. This may demand a drastic re-thinking of routing approaches, as links
and interactions between links must be re-considered.
&Proactive or reactive routing protocol? Or hybrid? Even though the presence of
a fixed wireless backbone seems to favor a proactive routing protocol, real-world
experiments conducted as part of the MIT Roofnet project [30] have revealed
the impact of changing network conditions on the routing protocols. In some
cases, the number of updates could not be disseminated fast enough due to the
contention of control traffic with data traffic, leading to non-optimal routing
decisions. A hybrid routing protocol seems a more sound approach given that
the wireless backbone will not suffer from node outages at a nearly or the same
frequency as in MANETs or sensor networks.
298 Multimed Tools Appl (2006) 29: 285–303
&Link or path optimization? Considering the impact of the network environment
on the routing decision, it is not clear if it is preferable to find an optimal path or
use a local optimization strategy based on optimal links.
&Integrated Routing and Mobility Management? Current IP mobility is separate
from the underlying IP routing protocol, but uses it in order to tunnel packets to
their destination. However, micromobility protocols such as Cellular IP [49] and
Hawaii [42] have implemented custom routing functionality. Ad hoc protocols
take this even further by integrating all mobility mechanisms within the context
of the routing protocol. Handling this (ad hoc) level of mobility is not needed
when devising a routing protocol for WMNs. However, as user mobility is an
integral part of the network, the routing and mobility management must either
be integrated, or must interact effectively with each other.
5 Conclusion and future research
With the rise of user expectation of anywhere connectivity and quality of service
guarantees, new wireless technologies are sought after for their versatility, ease of
deployment, and low cost. Wireless mesh networks present a promising solution by
extending network coverage based on mixture of wireless technologies through
multi-hop communications. WMNs exhibit several prominent characteristics that
make them stand apart from traditional wired or wireless networks, and hence call
for new resource management techniques.
Routing in multi-hop wireless networks has always been a challenging research
avenue. Previous works in this area have focused on ad hoc networks. However, the
disparity between mesh and ad hoc networks is significant enough to question the
suitability of ad hoc routing protocols for mesh networks.
In this paper, the characteristics of wireless mesh networks have been discussed
and compared with the properties of other wireless networks. Existing routing
protocols have been categorized according to these properties. We argue that new
routing protocols specifically adapted for WMNs are needed. A set of design
questions have been raised, relating to WMN routing. These questions require further
investigations, and consideration in the development of protocols for WMNs.
We hope that this paper will help in shaping future research in this area by
providing a more concise view and problem definition, design requirements and
constraints, and suggestions for possible research directions.
Acknowledgments This research is partially supported by Nortel Networks, Communications and
Information Technology Ontario (CITO) and the Natural Sciences and Engineering Research
Council (NSERC) of Canada.
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Sonia Waharte is a Ph.D. Candidate in the School of Computer Science at the University of
Waterloo. She received her M.Sc. in Computer Engineering from the National Institute of
Telecommunications, Paris, France in 2002 and her M.Sc. in Computer Science, with high honors,
from the University Paris VI in 2002. Her research interests include the design and analysis of network
protocols for sensor networks and resource management in wireless mesh networks.
Dr. Raouf Boutaba is an Associate Professor in the School of Computer Science of the University of
Waterloo. Before that he was with the Department of Electrical and Computer Engineering of the
University of Toronto. Before joining academia, he founded and was the director of the
telecommunications and distributed systems division of the Computer Science Research Institute of
Montreal (CRIM). Dr. Boutaba conducts research in the areas of network and distributed systems
management and resource management in multimedia wired and wireless networks. He has published
more than 150 papers in refereed journals and conference proceedings. He is the recipient of the
Premier’s Research Excellence Award, the NORTEL Networks research excellence Award and several
Best Paper awards. He is a fellow of the faculty of mathematics of the University of Waterloo and a
distinguished lecturer of the IEEE Computer Society. Dr. Boutaba is the Chairman of the Working
Group on Networks and Distributed Systems of the International Federation for Information
Processing (IFIP), the Vice Chair of the IEEE Communications Society Technical Committee on
Information Infrastructure, and the Director of standards board of the IEEE Communications
Society. He is the founder and acting editor in Chief of the IEEE eTransactions on Network and
Service Management, on the advisory editorial board of the Journal of Network and Systems
Management, on the editorial board of the KIKS/IEEE Journal of Communications and Networks,
the editorial board of the Journal of Computer Networks and the Journal of Computer
Communications. He has also served as a guest editor of several special issues of IEEE Journal of
Selected Areas in Communications (JSAC), the Journal of Computer Networks, the Journal of
Computer Communications and the Journal of Network and System Management. He acted as the
program chair for several conferences including IFIP Networking, IEEE CCNC, IEEE/IFIP NOMS,
IFIP/IEEE MMNS, IEEE FIW, IEEE ACC and symposia in IEEE ICC.
302 Multimed Tools Appl (2006) 29: 285–303
Youssef Iraqi received the B.Sc. in Computer Engineering, with high honors, from Mohamed V
University, Morocco, in 1995. He received his M.S. and Ph.D. degrees in computer science from the
University of Montreal in 2000 and 2003. He is currently a research assistant professor at the School
of Computer Science at the University of Waterloo. From 1996 to 1998, he was a research assistant at
the Computer Science Research Institute of Montreal, Canada. His research interests include network
and distributed systems management, resource management in multimedia wired and wireless
networks, and peer-to-peer networking.
Brent Ishibashi received the B.Sc. degree from the University of Guelph (Canada) in 2000, and the M.
Math degree from the University of Waterloo (Canada) in 2004. He is currently working towards a
Ph.D. degree at the School of Computer Science of the University of Waterloo.His research has
focused on resource management in multihop wireless network environments, particularly ad hoc and
wireless mesh networks. Past work has involved the investigation of cross-layer interactions
particularly between the link and network layers. Currently his work focuses on the management of
wireless resources in the deployment and configuration of wireless mesh networks.
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