A survey on routing algorithms and routing metrics for wireless mesh networks

World Applied Sciences Journal 30 (7): 870-886, 2014
ISSN 1818-4952
© IDOSI Publications, 2014
DOI: 10.5829/idosi.wasj.2014.30.07.1465
Correspoding Author: Mohammad Siraj, PSATRI, College of Engineering, King Saud University, P.O. Box: 800, Riyadh 11421
870
A Survey on Routing Algorithms and
Routing Metrics for Wireless Mesh Networks
Mohammad Siraj
PSATRI, College of Engineering, King Saud University, P.O. Box: 800, Riyadh 11421
Submitted: Nov 30, 2013; Accepted: Feb 10, 2014; Published: Feb 23, 2014
Abstract: Wireless Mesh Network (WMN) is an emerging wireless technology for various futuristic
applications like video on demand, healthcare systems, disaster and emergency, modern intelligent transport
systems, public security systems, broadband internet for home and campus networking. Routing is an important
factor for forwarding a data packet from source node to destination node.Wireless routing is totally different
than routing in a wired network. Routing metrics are a crucial part of routing protocols affecting the performance
of wireless mesh networks (WMN). When the routing protocols are implemented, the routing metrics are
allocated to various paths. Their job is to compute the best routing path. They are assimilated in routing
protocols to enhance WMNs efficiency in terms of reliability, latency, throughput, error rate and cost. This
paper presents a survey of important routing protocols and routing metrics for WMNs.
Key words: Interference Routing protocol Routing metric Wireless Mesh Network
INTRODUCTION Generally, the mesh routers are static. Figure 1 shows an
architecture of WMN.
IEEE 802.11 Multi Hop Wireless Mesh Network Routing metricsare very essential for calculation of
(WMN) [1, 2] is a promising technology, seen as a the best quality path. It does so by capturing an
promising potential for Internet Service Providers (ISPs) accurately good quality link. Due to the presence of static
and other end-users to deliver a consistent wireless nodes and common wireless medium in WMNs designing
broadband service access and a robust network at a a good routing metric is challenging.To study the impact
rational cost. They are now being seen as last few miles of a good routing metric design is a challenge for
connectivity. Wireless networks differ from wired researchers. To understand these challenges the current
networks. They can organize and configure themselves routing metrics which have been presented for WMN
dynamically. i.e., establishment and maintenance of the protocols are analyzed and a comparison of their
network is done automatically by the mesh nodes. This prominent features have been done for their advantages
automatic establishment and maintenance has numerous and drawbacks.
benefits for the end-users. Service coverage becomes The remainder of this paper is organized as follows.
dependable and network is robust. Deployment and In section 2 important design requirements of an efficient
maintenance of WMN is easy and economical. Mesh mesh routing protocol are discussed. In section 3, the
networks comprises of mesh clients and mesh routers. review of important routing protocols for WMNs have
The end users can use WMN to access the internet by been discussed. Section 4 discusses the design
connecting to any of the routers.The client could be a requirements and characteristics of routing metrics.
notebook, PDAs, desktop PCs or a mobile phone whereas Section 5 describes a review of important routing metrics
mesh routers are a part of backbone. They differ from for WMNs followed by a conclusion.
mobile ad-hoc network (MANET), as the problems in
MANETs are caused by the absence of infrastructure and Requirements of an Efficient Mesh Routing Protocols:
nodes mobility [3]. In WMNs connectivity between the An efficient mesh routing protocol should have the
mesh clients and mesh routers are through multiple hops. following features.

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Fig. 1: Wireless Mesh Architecture [4]
Distributed: The protocol should be a distributed, as network size. This requires that control overhead should
centralized routing involves higher control in order to be minimized and routing protocol should be able to adapt
have improved reliability. As node mobility is very to the network size. In [5, 6] researchers showed that
minimal, therefore, mesh networks prefer the decentralized scalability declines with the large number of users.
routing protocol. Gupta and Kumar [7] computed per node capacity. The
Adaptable to Topology Changes: The algorithm should would be able to scale or not [8-10]. The network
adapt to minimal changes in the topology caused by coverage area results in an increase of load. In [11-13],
node's mobility. This is a substitute to the uniformly showed how to keep the deterioration to a minimum level.
distributed traffic within the network and needs The effect of physical layer on scalability was shown in
maintenance of the routing paths of all the nodes [14]. It was seen thatdirectional antenna increases
continuously. performance and scalability [15-16]. Other researchers
Loop-Free: This is a fundamental requirement of any WMN.
routing protocol to avoid unnecessary wastage of
bandwidth. In the mesh network, due to node mobility, Quality of Service (QoS): In WMNs, the quality of
loops may be formed during the route establishment. service (QoS) features requires a great amount of
So a routing protocol should eliminate such loops. A challenge due to resource limitations and node movement.
routing protocol should be loop-free in order to avoid Therefore, it is essential that QoS metrics are incorporated
wastage of bandwidth. in WMN routing protocols for route discovery and
Security: If the security doesn’t exist, then the routing is on end to end delay, bandwidth, Packet delivery ratio
protocol issusceptible to different types of attacks such and energy and mechanism overheads. Hence, it is
as spoofing and redirect messages. To prevent such mandatory for WMNs to have a proficient routing and
vulnerabilities, security schemes are required. QoS mechanism for supporting various applications.
Cryptographic mechanisms are utilized to solve security The routing protocol should ensure the required level of
issues and prevent such attacks. quality of service and that too should also be in real time
Scalability: Scalability is the protocol capability to have focused their work to study and improve QoS in
maintain its performance with an increase of users and WMN.
topology of the network decides whether the network
[17-20] have also studied the effects on scalability on
maintenance to support end-to-end QoS. QoS main focus
to support current traffic. Some of the researchers [21-28]

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Routing Protocols: The routing protocols can be number of hops for every destination. To avoid loops,
categorized into proactive routing, reactive routing and DSDV uses sequence numbers. The routing updates are
hybrid routing protocols. Some of the important protocols either event driven or time driven. Each node transmits
in these categories have been discussed below. routing table updates and its routing information to its
Proactive Routing Protocols: Proactive routing of updates are possible. The first one is full dump and an
broadcasts periodic HELLO messages, like traditional incremental update. In the full dump, the complete routing
routing in the internet, in order to determine the global information is carried and may require the number of
view of the network topology, which is useful when route Network Protocol Data Unit (NPDU). In contrast, only
establishments are needed. However, established routes, entries of available destinations with recent changes
which are cached in each node might never be used. from the routing table are sent in the incremental update.
This leads to wastage of network bandwidth, especially in When the nodes are relatively static, incremental update
high node density. In addition, in proactive routing avoids extra traffic compared to the full dump update.
protocols, there is a trade-off between the freshness of However, the full dump update is more efficient in the
cached routes and the frequency of message broadcasts. network with high speed mobile nodes. In both update
Frequent broadcast messages are useful in order for the mechanisms, the route update packet is sent with a
packet carrier node to calculate efficient routes to the unique sequence number with the routing information.
specified destination. However, this is at the expense of During the selection, the path having the greatest
high bandwidth consumption, which grants the channel sequence number is selected as the current path. If two
for broadcast traffic. On the other hand, this type of paths have the same sequence number, the shortest path
routing is suitable for real time applications (delay is selected.
sensitive services) since the route between a pair of
sources and destinations is created beforehand. In other Optimized Link State Routing Protocol (OLSR):
words, the source does not need to flood route discovery Optimized Link State Routing Protocol is built over the
requests on demand as the route is established in the link state algorithm. In this protocol, information about the
background. In spite of the low end-to-end latency of link state of every node is broadcasted to every other
packet forwarding, the recovery of unused cached routes node of the network. In OLSR, all nodes keep a track of
wastes massive bandwidth, especially in high mobile the information of their two hop neighbors. HELLO
environments. Few key examples of proactive routing messages are used by OLSR to get the information about
protocols are Destination-Sequenced Distance Vector the links. Multi Point Relays (MPR) is a vital feature of the
Routing (DSDV) [29], Fisheye State Routing (FSR) [30], OLSR protocol to minimize broadcasting. Each node
Optimized Link State Routing (OLSR) [31], Source Tree selects a set of MPR among its one hop bidirectional link
Adaptive Routing (STAR) protocol [32], HEAT Protocol neighbors to all other nodes those are two hops away.
[33], Wireless Routing Protocol (WRP) [34], Mobile Mesh This set can change with timeand is specified by the
Routing Protocol (MMRP) [35], Linked Cluster selecting nodes in their HELLO messages.
Architecture (LCA) [36], Hierarchical State Routing Whenever a node transmits a message, all of its
protocol (HSR) [37], Topology Dissemination based on neighbors receive the message. Only that MPR which
Reverse-Path Forwarding routing protocol (TBRPF) [38], receives the message for the first time, transmits the
Direction Forward Routing (DFR) [39] and Distributed message. Due to this, the overhead due to flooding is
Bellman-Ford Routing Protocol (DBF) [40]. In this paper minimized.
only a few important proactive protocols are discussed. Two types of control messages are used by OLSR.
DestinationSequencedDistanceVectorRoutingProtocol messages are periodically sent. The TC messages can be
(DSDV): Destination Sequenced Distance Vector Routing forwarded only by MPR hosts.
Protocol is proactive unicast routing protocol. DSDV is The major advantage of OLSR over other proactive
based on traditional Bellman Ford algorithm. In DSDV, protocols is that it broadcasts its link state information
every node maintains a routing table. Each entry in the rather than routing tables and messages can be delivered
routing table keeps information of all likely node in any order due to the sequence number. This protocol
destinations in the network. It also keeps a record of the is good for large and dense networks.
adjacent neighbor nodes periodically. In DSDV, two types
They are HELLO and TOPOLOGY CONTROL (TC). TC

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Fig. 2: MRP route establishment message sequence [9]
Mesh Networking Routing Protocol (MRP): Mesh Fish-Eye State Routing Protocol (FSR): In [30], the
Networking Routing Protocol (MRP) is a proactive authors developed an efficient link state algorithm that
protocol. In this protocol, to connect to the internet every maintains the global knowledge of the network topology
client chooses a gateway. In the eventuality of the node at each node and disseminates the local information to
moving away or breaking down of the gateway node a the direct neighbor nodes instead of the whole network.
different gateway is selected. The entire traffic movement In FSR protocol, the updates of link state information vary
to the internet is through the gateway. One of the with the distance.
versions of Mesh Protocol is MRP On-Demand. Towards the destination. Figure 3 shows the basic
MRP on-demand (MRP-O) [41] is a purely on-demand operation of FSR. That is, every node defines a boundary
protocol. This protocol uses the messages like Route around itself. The inner boundary is formed by the closer
Discovery Message (RDIS),Route Advertisement nodes and they receive the link state information with the
(RADV), Registration Request (RREG), Route Check highest frequency, whereas the further nodes broadcast
Packets (RCHK) and Registration Acknowledgment the update with lower frequency. Thus, the FSR protocol
(RACK). The node which is intended to join the network exchanges the link state information frequently with the
will send RDIS to its neighboring user nodes to find the vicinity nodes and with lower frequency for the further
way to the closest gateway. Here only the source node nodes. In this way, the nodes can get up-to-date link state
one-hop neighbors gets the message. The nodes information about the nearby neighbor nodes.
receiving the RDIS message respond by sending a RADV Apparently, there is a trade-off between the reduction of
packet containing the information about their current overhead and the staleness of the link state information,
routes metrics. All the neighboring nodes will send the leading to suboptimal route selection.
RADV packets with some random delay to avoid
collisions. If a new node joins the network, then all RADV Reactive Routing Protocol: The basic operation of
packets will be stored by it. Once all the RADV’s have reactive routing protocols [42-44] is route discovery from
been received, it will choose one or more upstream route source node to destination node and works in reverse to
to perform routing. Figure 2 showsthe MRP route the on demand Topology-based routing. This routing
establishment message sequence. The metrics used for solution establishes a route when a node makes a request
this protocol are hop-count, route stability, minimum to transmit packets to another node in the network.
delay, maximum bandwidth and minimum packet loss. At this time, the node re-broadcasts the requested route
The metrics used for this protocol are hop-count, route establishment to find the intended destination. When the
stability, minimum delay, maximum bandwidth and destination receives the query (or the en-route nodes
minimum packet loss. know the path to it), it responds to the source for route

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Fig. 3: FSR protocol basic operation are forwarded hop-by-hop. Unlike other on-demand
establishment between source and destination. Whenever for neighbor detection, or link status detection in DSR.
the source node wants to transmit data packets towards Therefore, DSR operates truly on demand to minimize the
the destination, it floods the network with route request routing overhead. The route maintenance mechanism is
packets. The destination sends a route reply message, initiated when a node cannot deliver packet to its
after which the source sends the data packet to the next-hop node as shown in Figure 6. This node then
destination. The advantage of this approach is that it generates route error messages towards the source node
does not maintain unused routes and reduces bandwidth to find the most viable route as shown in Figure 7. Hence,
overhead in the network. the broken link is removed from the route cache of the
Some of the important examples of reactive protocol source node.
are Ad Hoc on Demand Distance Vector (AODV)
protocol, Temporarily Ordered Routing Algorithm (TORA) Ad-HocOn-DemandDistanceVectorRoutingAlgorithm
protocol [45], Dynamic Source Routing (DSR) protocol (AODV): AODV is a reactive unicast routing protocol.
[46], ABR (Associativity Based Routing) protocol [47], AODV protocol doesn’t deploy flooding. AODV does not
Link Quality Source Routing Algorithm (LQSR) protocol store routing information of all the nodes, instead it just
[48], Dynamic MANET on Demand (DYMO) [49], keeps information about the nodes falling on the active
Lightweight Mobile Routing protocol (LMR) [50], route.
Load-Balancing Curveball Routing (LBCR) [51], Scalable In AODV when a source node has data packets and
Location Update-Based Routing Protocol (ScrRR ) [52] intends to communicate with another node, it initiates the
and Interference-Aware Load-Balancing Routing (IALBR) route discovery process in the network. As the source
[53]. has no suitable route for the destination, it broadcasts the
Dynamic Source Routing Algorithm (DSR): Dynamic RREQ packet contains the address of the source node,
Source Routing Algorithm (DSR) is the source based address of the destination node and broadcast id.
routing protocol where the source records the sequence Broadcast id is an identifier that contains the most-current
of intermediate nodes in a data packet which is sent to the sequence number of the source and the destination node.
destination. The basic operation of the protocol consists Each RREQ begins with the least Time to Live (TTL)
of two phases, namely the path discovery and path value. The TTL value increments by 1 if the destination
maintenance process. In DSR, a path discovery phase is node is not found.HELLO messages, are used to notify
begun when the source node without a valid path intends adjacent neighbor nodes.
to transmit a data packet to the destination node. DSR Routing tables keep records for a specific period.
applies the source routing strategy to broadcast a route A cache is maintained by each node. The cache keeps the
request message. This includes source id, destination id, entries of the received RREQs. The RREQ with the
a route record with an empty list of addresses of all greatest sequence numbers is accepted and others are
intermediate nodes and a unique request id towards the rejected. The cache also saves the return path. RREP
destination node. On receiving route request, an message is generated and is transmitted back to the
intermediate node caches the route record. Figure 4 shows source node provided the sequence number of the
the construction of route cache during node discovery. destination node is equal to or larger. This can be seen
A route reply message may be replied if the destination from Figure 9.
node is arrived. The destination node cache stores the
route record. Then it uses the cached path in the route
record for the propagation of route reply back to the
source node as shown in Figure 5. Otherwise, if this node
is not in the route cache of route request, it appends its
address to the route record and broadcasts the route
request messages. To avoid the overhead, DSR optionally
defines the unique request id for each message in the
route discovery mechanism. In addition, these messages
driven algorithms, there are no proactive periodic probes
Route request (RREQ) message as shown in Figure 8.

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Fig. 4: Constructing route cache during node discovery
Fig. 5: Route Reply
Fig. 6: Route Error between Node 5 and Node 8
Fig. 7: Route Error sent from N5 to N1
Link Quality Source Routing Algorithm (LQSR): Link LQSR allocates comparative weights to the links
Quality Source Routing Algorithm was designed by between the wireless meh nodes after the nodes have
Microsoft Research Group for their Mesh Connectivity been detected. Besides allocating weights, for every
Layer (MCL). Through LQSR, the computers are probable link the bandwidth, loss and the channel are
connected to form a mesh network using WiMax or Wi-Fi. calculated. All the nodes are conveyed this information.
The LQSR protocol is built on Dynamic Source Routing Depending on this information, LQSR uses Weighted
(DSR). Cumulative Expected Transmission Time (WCETT)

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Fig. 8: Broadcasting RREQ Packets
Fig. 9: Route of the RREP to the Source
routing metric to find the best routing path from source to Generally, TORA offers several routes from source to
the destination node. The path between two nodes may destination. TORA functions can be broadly classified
be one hop or more depending whether intermediate into three types, which are a) 1) route creation, 2)
nodes exist or not between the source and the destination maintaining route and 3) removal of route. It uses height
node. The route is amended accordingly if the optimal metric during route creation and maintenance to form a
path between source and destination is modified. Directed Acrylic Graph. (DAG) fixed at destination.
This amendment is done without interruption of the link Links are allocated on the basis of relative height of the
among the nodes. The LQSR protocol does not require neighboring nodes. Timing is very crucial in TORA as
very little administration like DSR protocol to function the height metric relies on link failure logical time.
automatically. When TORA wishes to delete the routes, it broadcasts a
In LQSR, after a node gets a RREQ packet, a link clear (CLR) packet flooding the entire network to delete
quality metric is appended for the link over which the the invalidated paths.
packet is received. When the source node gets a RREP The messages move in a downward flow that is a
packet, it contains information about link quality and from a higher height node to a lower height node.
node. To get the information about link state, adjacent Discovery and updating of routes is through Query (QRY)
nodes are broadcasted HELLO messages by LQSR. and Update (UPD) packets. The QRY packet circulates
These messages are utilized to find out the quality of the over the network until it finds a node which is either the
link over which the message was got. destination node or has route information. A UPD packet
containing the node’s height is then broadcasted.
Temporally-Ordered Routing Algorithm- (TORA): On getting the UPD packet each node sets its height
Temporally Ordered Routing Algorithm (TORA) is an on larger than the height mentioned in the UPD message.
demand routing protocol. It is a distributed routing After setting its height higher, this node broadcasts its
protocol. It is designed to reduce the communication own UPD packet resulting in many routes.
overhead involved in adjusting to the changes which
occur whenever there is a change in network topology. Hybrid Routing Protocol: Hybrid routing protocols
TORA's control messages are generally restricted to a combines the advantages of both reactive and proactive
very minor group of nodes. It ensures loop free routes. routing protocols for performance and scalability.

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It provides a mechanism such that it implements proactive Zone Routing Protocol (ZRP): ZRP falls in the category
routing for the nearby and frequently used routes. The of a hybrid protocol having reactive and proactive routing
reactive routing technique is used for far away nodes and characteristics. It creates routing zones based on the
seldom used for data relay. They minimize proactive neighbor nodes hopping distance. There are two zones
routing protocol control overhead and reduces the delay created by it. One is the inside zone also called Intra Zone
in reactive routing protocol during the route discovery where the packets are sent to the peripheral nodes from
process. Examples of hybrid routing protocols are Hazy- the originating node and the other zone which is outside
Sighted Link State Routing (HSLS) protocol [54], Zone zone also called Inter Zone from where the packet is
Routing Protocol (ZRP) [55], Hybrid Ad-Hoc Routing delivered to the sink node. This is shown in Figure 10
Protocol (HARP), Hybrid Routing Protocol for Large Scale below. The zones are formed from the origin node at two
Ad-Hoc Networks with Mobile Backbones (HRPLS) [56] hops away.
and Zone-based Hierarchical Link State routing (ZHLS) ZRP is created by two routing protocols, which are
[57]. In this section only a few important hybrid protocols Intra-Zone Routing Protocol (IARP) [57], a proactive
are discussed. routing protocol used for intra zone and a reactive routing
Hazy-Sighted Link State Routing Algorithm (HSLS): used for Inter Zone, respectively. The path from origin to
Hazy-Sighted Link State Routing Protocol (HSLS) is one sink node is made by IARP. Generally, all the current
of the most efficient hybrid routing protocol for mesh proactive routing algorithms can be used as the IARP for
networks. Cowing Foundation is involved in the ZRP. For the inter zone paths, it uses IERP to find the
development of HSLS. Researchers at BBN Technologies route. The originating node generates a route request to
created the HSLS routing protocol. The network is not peripheral nodes. Peripheral nodes check their zone for
flooded by HSLS. HSLS features are built on reactive, the sink node. If the destination node is not a part of this
proactive and suboptimal routing methodology. By zone, the peripheral node appends its own address to the
restricting link state updates in time and space domain, route request packet and forwards the packet to its
the updates are combined and sent once thereby saving peripheral nodes. If the sink node is a member of its zone,
transmission capacity. it generates a route reply packet on the reverse path back
In HSLS, two proactive algorithms are integrated. to the source. The originating node saves this path and
The algorithms are Near-Sighted Link-State Routing sends the route reply to the sink nodes.
(NSLSR) and Discretized Link-State Routing (DLSR).
NSLSR restricts the number of hops for the Design Requirements and Characteristics of Routing
communication of routing information. DLSR restricts the Metrics: There are four key requirements of routing
number of times a routing information may be metrics for ensuring a good performance. First of all, there
communicated. The reactive routing is needed due to a should be route stability to guarantee that the network is
failed attempt in the usage of a nearby link. This causes stable. It means that the routing metrics should not
the expiry of the next timer, maybe to recover the change the route frequently. Secondly, WMN
information in finding a substitute route. The reactive characteristics can be captured by the metric for ensuring
routing takes place when there is a failure to use a good performance is on the paths having least weights.
neighboring link which causes the expiry of the next timer. Thirdly, for computing, the routing metrics should have
It then tries to find another route. proficient algorithms with polynomial complexity to
The main aim is to measure the global network compute paths having the least weight. Lastly, the routing
wastage. It includes transmitting route updates of metrics should precisely capture quality links and help in
inefficient transmission routes. Researchers used calculating efficient paths while ensuring that routing
arithmetical optimization technique to compute how much protocols do not create forwarding loops. For creating a
time link state updates take to transmit and what is the routing metric some important components required are
coverage of nodes link state updating. Whenever there is number of hops, channel diversity and link quality. In a
a broken connection, a local routing cache update is WMN, a good routing metric should address the
required. This is the reactive portion of the algorithm. following criteria.
HSLS provides good scalability properties and creates
good routes in real time. The data transfer and routing Intra-Flow Interference: Intra-flow interference takes
information are distributed, so it provides good reliability place due to the interference of adjacent nodes on the
and performance. same routing path. They compete against each other for
protocol called Inter-Zone Routing Protocol (IERP) [57],

P
O
S
Originating Node
Sink Node
Intra Zone
Inter Zone
Peripheral Node
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Fig. 10: An example of ZRP Protocol
Fig. 11: Intra-flow interference [58]
utilizing the channel. This intra-flow interference causes Figure 12 shows inter-flow interference because of the
throughput to degrade severely due to the consumption two paths namely 1 4 3 and 5 6 7. A good inter-flow
of the flow bandwidth across every node on the same aware metric should assign a low weight 1 2 3 then to
routing path. The Hop-Count of the flow increases with 1 4 3 as path 1 2 3 has less inter-flow interference.
an increase in the end to end delay. This causes
congestion. For example, in Figure 11, it is shown that the Load Balancing: It is defined as the capability of a routing
path 1 2 3 causes intra-flow interference because of the metric to balance the traffic load so that overloading of
reuse of channel 1 on the flow 1 2 and from 2 3. So the gateways in wireless mesh networks can be avoided and
path 1 4 3 does not have intra-flow interference as two the network resources are used fairly. WMNs are different
different channels are assigned between 1 4 and 4 3. than other wired and wireless networks. The environment
It can be said, that 1 4 3 is a better path in comparison to of the shared channel, presence of stationary mesh node,
1 2 3. So a good intra-flow aware interference metric traffic routing pattern from the user to gateway node and
should assign 1 4 3 a lower weight than 1 2 3. In other the usage of multi radios distinguishes them from other
words, a good routing metric reduces inter-flow networks. This unique characteristics and unbalanced
interference by selection of different channels for load in WMN causes load balancing problem. Load
neighboring nodes coming on the same path. balancing problem becomes an important issue in WMN
Inter-Flow Interference: Inter-flow interference occurs the traffic in WMNs is directed towards the Internet
due to the other flows operational on one particular gateway. This may result in an increase in traffic load on
channel and nodes are contending for that channel. some path which are connected to the internet gateway.
This is caused by the multiple flows between different Due to the unbalanced load, channel overloading,
routing paths as shown in Figure 12. This consumes the gateway overloading and center loading may occur.
bandwidth of the nodes which are on this route. Channel overloading occurs when some of the channels
Furthermore, it competes for bandwidth occupation with are overloaded as compared to other channels. Center
the neighboring nodes. In comparison to intra-flow loading occurs when the nodes at the centre of the
interference, inter-flow interference is harder to control network topology are used more during the traffic flow
due to the involvement of multiple flows and routes. as they fall on the shortest path. The traffic is oriented
as the volume of the network traffic is very high. Most of

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Fig. 12: Inter-Flow interference [59]
towards the gateways as the traffic is routed through Routing Metrics: It is a recognized fact that routing
them. Due to the heavy traffic volume, congestion may metrics are a crucial part affecting the performance of
occur. The focusing of traffic on gateway causes a drop wireless mesh networks (WMN). When the routing
in the packets due to the overflow in the buffer. This protocols are implemented, the routing metrics are
dropping of packets is unfavorable for the network to be allocated to various paths. Their job is to compute the
efficient. This is called gateway overloading. best routing path. They are assimilated in routing
Thus, there is a critical need to avoid congestion due protocols to enhance WMNs efficiency in terms of
to the unbalanced load. This can be done by load reliability, latency, throughput, error rate and cost.
balancing at different gateways and on the paths leading Routing metric is very essential for calculation of the
to the gateway. Congested links should be avoided by best quality path. It does so by capturing an
segregating congested links from the new paths to have accurately good quality link. To study the impact of a
load balancing. Load balancing and interference are good routing metric design, is a challenge for researchers.
related. So to solve the problem of unbalanced load, To understand these challenges the current routing
interference problem should also be examined. metrics which have been presented for WMN
Isotonicity: It is defined as the characteristic of the prominent features have been done for their advantages
routing metric by which it ensures that the order relation and drawbacks.
between the weights of any two paths is conserved if
both are preceded by a third path which is common to the Hop-Count Metric: The most common metric used in the
two paths. For example, if W (p) is the weight which is the multi-hop routing protocols is Hop-Count metric. It is
function along the path p and W (q) is the weight which used in protocols like DSDV, DSR and AODV. Hop-Count
is the function of path q, isotonicity can be defined as: A metric finds route having the minimum number of hops.
routing metric is isotonic if the quadruplet (A, , W, ) is Hop-Count can outclass other metrics, which are
isotonic and if W (p) W (q) implies W(r p) W(r q) dependent on load in high agility situations. This shows
as shown in Figure 2.10. its agility. The metric is very stable and also has the
Isotonicity is a very crucial requirement of a routing isotonicity characteristic. As a result, least weight paths
metric for the efficiency of routing protocol. can be discovered proficiently. The weakness of this
Besides these characteristics, it is essential for a metric is that it does not address interference, channel
metric to find routes with the maximum throughput diversity, varying load of the link and capacity of the load.
constantly. They should have a low packet delivery ratio These are the factors that are experienced by the links.
during the transmission of the packet. It treats all the links identical. It finds path having poor
The next section will discuss the current routing throughput and high packet loss ratio. This is because the
metrics deployed by WMN routing protocol for their links which are slower, take a lot of time to transmit
strengths and drawbacks. packets.
protocols are analyzed and a comparison of their

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=
∑
1
*
ETX
M Mf r
=
*
S
ETT ETX
B
=
World Appl. Sci. J., 30 (7): 870-886, 2014
880
W(p) W(q) W(r p) W(r q)
Fig. 13: Isotonicity
Expected Transmission Count (ETX) Metric: The ETX computation of loop free routes and least weight as it is
metric [60] was presented to address the problems faced isotonic. The weakness of this metric is that it is not agile
by Hop-Count metric. The ETX is the number of and is meant for single channel only. It does not consider
transmissions required to successfully deliver a packet links interference and only captures a link loss ratio
over a wireless link at the MAC layer. The ETX of a path thereby compromising link quality affects the link data
is the summation of ETX of every link over the path. In rate transmission. It does not calculate traffic loss rate
mathematical terms, it can be written: precisely as it doesn’t capture variation in rates of
P= 1-(1-P ) (1-P ) (5.1) network, because it does not consider varying link loadf r
where, minimize intra-flow interference as it is not able to
P = Probability of unsuccessful transmission of packet differentiate among diverse channel routes and same
in a link from node a to node b. channel routes.
P = Probability of path loss in forward direction.f
P = Probability of path loss in reverse direction. Expected Transmission Time (ETT) Metric: ETT metricr
The expected number of transmissions to account the bandwidth of different links. ETT is the time
successfully deliver a packet in one hop can be taken to communicate a packet successfully to the MAC
represented by. layer.
(5.2) (5.4)
The ETX metric for a single link is measured in terms S = Packet size (average) and
of forward and reverse delivery ratio. B = Bandwidth of the current link.
(5.3) ETT metric is got by summation of all the ETT values
where, The advantage of ETT is, that it is isotonic and
M = Forward delivery ratio is (1-P ) increases the performance of the whole network byf f
M = Reverse delivery ratio is (1-P ) increasing the path throughput by determining linkr r
ETX measures the packet loss rate. Every one is unable to avoid the routing through the nodes and links
second, probe packets are sent to all neighboring nodes. which are severely loaded. It does not reduce inter- flow
On getting the probe packet, the neighboring node sums and intra-flow interference as it has not been designed for
the number of packets received. Based on this multi radio networks.
information, every ten seconds packet loss rate is
computed. It finds paths, which have a high throughput Weighted Cumulative Expected Transmission Time
with the minimum hops because a long path will have less (WCETT) Metric: WCETT [61] was presented toenhance
throughput because of intra-flow interference. It indirectly the ETT metric in the multi radio mesh networks by taking
handles inter-flow interference. It allows proficient into account the diversity of the channels.
transmission. It leads to an unbalanced load of the
and routes through heavily loaded nodes. ETX doesn’t
[61] was an enhancement over ETX as it took into
where,
of the different links on the path.
capability. It contains all the same drawbacks of ETX. ETT

(1 ) * *WCETT ETT MaxXp j=− +∑
1
n
X ETT j kj i
hops on channel j
= ≤ ≤∑
1
( )
* min( )
MIC p IRU CSCL i
N ETT
i p i p
= +
∈ ∈
∑ ∑
*
S
ETT ETXjk jk
Bjk
=
B j
B jk
jk
=
1
N
RC B gj j jl jl
k
= −
=
∑
World Appl. Sci. J., 30 (7): 870-886, 2014
881
The WCETT metric of a path p is defined as follows: (5.7)
(5.5)
where, ETT value
X = Summation of links ETT values which are on It consists of components of MIC, IRU (Interferencej
channel j in a system having orthogonal channels. Aware Resource Usage) and CSC (Channel Switching
(5.6)
is a tunable parameter between 0 1 which controls CSC = w2 if Ch Ch , 0 w1 < w2
the preferences over the path length versus channel
diversity. N is the group of neighbors’ nodes, which interferes
WCETT is a weighted average of two components. with communications on link I. CH denotes channel
The first term is usually the summation of the individual allocated for i node communication and i-1 denotes the
link ETTs while the second term is the summation of the earlier hop of i node on the path p.
ETTs of every link of a given channel. This adds channel MIC addresses both types of interference. Breaking
diversity to the routing metric causing low intra-flow into imaginary nodes through a least weight algorithm
interference. Using WCETT, multi radio wireless mesh like Dijkstra's algorithm [64], it can be made isotonic.
network's performance is enhanced in comparison to ETX, This metric is based upon the assumption that whatever
ETT and Hop-Count metrics. WCETT metric is not links are positioned in the interference region for a
isotonic and due to this, it can’t be used with link state specific link adds an equal amount of interference. It
routing protocols. Secondly, the inter-flow interference calculates total interference on a particular link through
effects are not explicitly taken into account by WCETT. the placement of interference creating nodes irrespective
As a result of this, sometimes paths are created, which of participating in transmission concurrently or not.
have high levels of interference. WCETT is very effective Intra-flow interference is captured in two successive links
in selecting paths having channel diversity as it takes only by the CSC which is the second component.
intra-flow interference into consideration. It retains the
gains of ETT metric barring isotonicity.Multi-radio Load Aware Expected Transmission Time (LAETT
wireless mesh network performance is enhanced after Metric): There are two main aims of LAETT [65]. The aim
using WCETT in comparison to Hop-Count, ETX and is to create a path for fulfilling the flow bandwidth demand
ETT metrics. It maintains a balance between delay and and to keep a space for the future needs. It is a
throughput. combination of load estimation and features of wireless
WCETT does not consider the locality information of access. It comprises of an implementation of ETT metric.
the links instead, it considers those links which are
operational on that channel. It assumes that there is an (5.8)
interference by the links which are operational on the
same channel which may lead to congested paths. It is not
isotonic as it can be seen by the presence of the second ETX = Expected transmission count on the link (j, k)
term. As it does not have isotonicity, it is not easy to use S = Size of the packet
for link state routing protocols. This metric has the same B = Bit rate
limitations as ETT/ETX metric as it does not estimate
actual link presence. Also it does not take into account
inter-flow interference effect. As a result, routes with high
interference may be established.
Metric of Interference and Channel Switching (MIC):
MIC metric [62-63] is an isotonic metric and designed to
consider inter and intra-flow interference effects besides
providing load balancing. For a path p
N = Total nodes and min (ETT) = Network’s least
Cost).
IRU = ETT * N .L L L
CSC = w1 if Ch = CHi i–1 i
i i–1 i
L
i
th
th
jk
jk
Where B is the j node communication ratej
th
= Link Quality Factorjk
= 1 for a very good linkjk
Remaining capacity (RC) for every node is.i

*
2
S
LAETT ETXjk jk RC RCj k
jk
=
+
( )i
EETT ETTk i
link IS k
=
∈
∑
( )
( ) , ( ) ( ) ( )
( )
IL Qjk
AIL Q N Q N Q N Qjk L j k
N QNL L
= = ∪∑
1
min( ) * min( ), ( ) 0
1
min( ), ( ) 0
ETT AIL N Ql
ETT N Ql
= ≠
= =
( ) (1 )* * max1
1
n
iAware p iAWARE Xi k l k
i
=− + ≤ ≤
=
∑
ETTkiAWAREk
IRk
=
World Appl. Sci. J., 30 (7): 870-886, 2014
882
f is the rate of transmission of current flow Nj that travels MTI metric can be defined by the following equation.jl
across j node. The flow cost on the leftover bandwidthth
is weighted by factor . A superior communication MTI (Q) = ETT (Q) * AIL (Q), N (Q) 0jl
results in a lower usage of bandwidth in comparison to MTI (Q) = ETT (Q), N (Q) = 0
inferior communication.
LAETT is defined by: when communicating amongst nodes j and kjk
(5.9)
The equation 5.9 consists of two parts. The later part
captures the leftover bandwidth present on either sides of Il (Q) = Neighbors interfering load
the node. If there are two paths having identical aggregate N (Q) = Interfering nodes set of neighbors j and k.
ETX weight, LAETT gives importance to the path having
the greatest leftover bandwidth. LAETT advantage is that “Scaling factor is applied to MTI metric for
it is isotonic and load aware. It utilizes shortest weighted balancing the difference in magnitude of two components
routing path for balancing the network load. It also (MTI and CSC). can be represented as:
captures traffic load and quality of links. The weakness of
this metric is that it does not consider intra-flow
interference and is not considered as it does not consider
inter-flow interference.
Exclusive Expected Transmission Time (EETT): EETT Min (AIL) ”= load average and
[66] is an innovative routing metric which is interference Min (ETT) = least ETT
aware. It finds multichannel routes having minimum
interference to have a high throughput. Multi-channel The estimation of interfering nodes' load is a key
paths are given better valuation by it. The k link EETT is issue in the deployment.th
given by the following equation: ILA takes into account disadvantages of prevailing
(5.10) successfully discovers lower congested and a low level
The path’s weight is the addition of EETTs of packet loss ratio. Inter-flow interference is calculated by
complete links falling on that route. EEET has all the ILA. The drawback of this metric is that in two successive
benefits of ETT as it has been built over ETT. It is also links only, the second component CSC can capture
isotonic. It efficiently takes into account intra-flow intra-flow interference.
interference directly and considers inter-flow interference
indirectly. The drawback of this is that it does not Interference Aware Routing Metric (iAWARE):
consider load variation. iAWARE metric [68] can be said as the first metric which
Interference Load Aware (ILA): ILAmetric [67] consists WMN. The iAWARE metric can be illustrated as.
of two components: Channel Switching Cost (CSC) and
Metric of channel interference (MTI). (5.11)
CSC component can be defined by the following
equation.
CSC = w1 if Ch = Ch value is.j j–1 j
CSCj = w2 if Ch Ch , 0 w1 < w2j–1 j
CH (j) represents channel assigned for node the
transmission and j-1 represents the previous hop of the Ir =Interference ratio among two nodes a and b for a
node j along path p. link k.
j jk jk l
j ij l
AIL = Neighbors average load which may interferejk
using channel Q.
jk
L
metrics like WCETT, ETX, ETT and Hop-Count. It
interference path which has high throughput and a low
considers both inter-flow and intra-flow interference in
X is identical to WCETT. For a link k,the iAWAREk
k

min( ( ), ( )
( )
( )
( )
IR IR a IR bk k k
SINR alIR aj
SNR al
=
=
World Appl. Sci. J., 30 (7): 870-886, 2014
883
Table 1: Comparison of the existing metrics.
Routing Metrics Inter Intra Link Loss Ratio Over Heads Load Balancing Multi Channel Isoto Nocity Link Data Rate Multi /Single Radio
HOP NO NO NO NO NO NO YES NO SINGLE
ETX NO NO YES NO NO NO YES NO SINGLE
ETT NO NO YES NO NO NO YES YES SINGLE
WCETT NO YES YES NO NO YES NO YES MULTI
MIC YES NO YES YES NO YES YES YES MULTI
ILA YES YES YES YES NO YES NO YES MULTI
iAWARE YES YES YES NO NO YES NO YES MULTI
EETT YES YES YES NO NO YES YES YES MULTI
LAETT NO NO YES NO YES NO YES YES MULTI
CONCLUSION
SNR (a) = for link l node a’s signal to noise ratio protocols and routing metrics proposed for wireless meshi
iAWARE can capture varying transmission rates, the basis of geographical, hierarchical and multi-path
intra and inter-flow interference and varying link loss ratio routing butin this survey only the broadly classified
effects. This metric preserves all the features of WCETT routing protocols for wireless mesh networks have
barring the mechanism of countering inter-flow been considered. Also an attempt to present a
interference calculation. Average interference produced comprehensive review of popular routing metrics has
from neighboring nodes is directly measured. SINR been done.
implementation is a big step forward for minimizing
inter-flow interference routing in comparison with metrics ACKNOWLEDGEMENT
ETX and WCETT. The weakness of iAware is that it does
not possess isotonicity. If a link has higher IR in The researcher wish to thank the Deanship ofj
comparison to ETT , iAWARE value is lower as a result a Scientific Research, College of Engineering, King Saudj
lower ETT path but having greater interference is chosen. University for supporting this research.
The biggest weakness of iAWARE is that it allocates
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