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Computer Communications xxx (2008) xxx–xxx

MRP: Wireless mesh networks routing protocol

Jangeun Jun, Mihail L. Sichitiu *

Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695-7911, USA

Abstract

Wireless Mesh Networks (WMNs) are one of the few commonly implemented types of mobile ad-hoc networks (MANETs); severalcompanies offer WMNs for broadband Internet access and for extending the coverage of wireless local area networks. Several particu-larities differentiate WMNs from MANETs. First, in WMNs, most of the traffic originates or terminates at the gateways (nodes con-nected to the wired infrastructure/Internet). Second, in most applications, WMN nodes tend to be neatly differentiated as eitherstationary nodes (providing connectivity and coverage) or mobile nodes (utilizing the coverage afforded by the stationary nodes). Whilegeneral MANET routing protocols can be used in WMNs, it is expected that a protocol that takes the particularities of WMNs intoaccount will outperform the general protocol. In this paper, we propose such a routing protocol and evaluate its performance via sim-ulations. Results show that, for WMNs, the proposed routing protocol outperforms general purpose MANET protocols in terms of rout-ing overhead, packet delivery ratio, network throughput, end-to-end delay, and average hop-count.� 2008 Elsevier B.V. All rights reserved.

Keywords: Wireless mesh networks; Routing protocol; Tree formation

1. Introduction

Wireless Mesh Networks (WMNs) [1–3] are a relativelynew wireless multihop technology that has much in com-mon with the mobile ad hoc networks (MANETs). In itsgeneral form (Fig. 1), a WMN is a set of wireless nodes thatcan communicate with each other, forwarding each other’spackets. Like in MANETs, each node is both a host and awireless router. Clients can connect to the WMN routersusing common networking interfaces (e.g., Ethernet,802.11, Bluetooth) or, in some cases, a PCI or a PCMCIAbus (i.e., the WMN router is embedded in a network card).In most proposed applications, the WMN provides connec-tivity to an infrastructure network, typically connected tothe Internet. We will call the nodes providing Internet con-nectivity gateways.

There are four types of links presented in Fig. 1. Exceptfor the intra-mesh links (that have to be wireless), all otherlinks can be either wireless or wired. The same or different

0140-3664/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.comcom.2008.01.038

* Corresponding author. Tel.: +1 919 515 7348.E-mail addresses: [email protected] (J. Jun), mlsichit@ncsu.

edu (M.L. Sichitiu).

Please cite this article in press as: J. Jun, M.L. Sichitiu, MRP: Wireldoi:10.1016/j.comcom.2008.01.038

technologies can be used for the four link types. The linkchoice usually represents a tradeoff between the cost/com-plexity and the performance of the WMN. Standardizationefforts are initiated by IEEE (802.11s, 802.15.5, 802.16a).

Several companies [4–18] developed WMN products fordifferent applications. One of the most popular WMNapplications is providing broadband Internet access[9,10,12,14,15,17,18]. In this scenario, WMN routers areinstalled on the roofs of the clients and/or light poles inthe coverage area of the WMN. Mobile clients may roamwhile being handed over from one wireless router toanother (some products may be able to handle highwayspeeds [10]). The main advantage of WMNs in comparisonto traditional broadband Internet access technologies(cable-modem and xDSL) is the dramatically reduced ini-tial investment and deployment time. The main advantagein comparison to fixed wireless metropolitan area networks(WMANs) (e.g., IEEE 802.16) is the market coverage [19](especially in areas with significant obstructions – trees,high-rise buildings) and reliability (multiple availableroutes can avoid failed nodes and poor links). Further-more, some implementations allow for mobile user access,while the current IEEE WMAN standard only allows

ess mesh networks routing protocol, Comput. Commun. (2008),

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Fig. 1. A wireless mesh network connecting several stationary and mobile clients to the Internet.

2 J. Jun, M.L. Sichitiu / Computer Communications xxx (2008) xxx–xxx

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stationary users (although work for mobility extensions isunderway).

A second major class of WMN products targets thebooming wireless local area network (WLAN) market(mainly IEEE 802.11) [4,7,8,6,12,16]. An important draw-back of WLAN technology in multi-access point (AP)deployments is the requirement to separately provide wirednetwork connectivity to each AP, offsetting some of theadvantages of a wireless network. WMNs in this categorysolve the problem by placing the APs in range of each otherand allowing them to forward each other’s packets to andfrom a common gateway (another AP connected to the dis-tribution system). The main drawback of these deploy-ments is the reduced bandwidth available to the users(this can become a major problem in scenarios with manyactive users [20]). Some companies are counteracting thereduction in bandwidth by using multiple radios on differ-ent channels and directional antennas [4,8].

Other companies found niche applications for WMNs.Firetide [5] uses WMNs to provide network connectivity.Basically, the WMN cloud behaves like one big Ethernetswitch that can be used to connect all devices plugged in anyof the wireless routers (including, for example, 802.11 APs).

Routing is a fundamental characteristic of WMNs. Therouting protocol’s strengths and weaknesses are reflecteddirectly in the WMN’s characteristics. Several advantagesof WMNs over competing technologies are directly enabledby the routing protocol:

� Reliability: The routing protocol should be able toreroute fast around failed nodes and broken links; uponthe failure of a gateway, it should be able to redistributethe orphaned clients among neighboring gateways. Forthis property, fast reconfiguration and support of multi-ple gateways is essential.� Mobile user connectivity: To ensure seamless mobile user

connectivity, the routing protocol should enable fasthand-offs.


� Scalability/efficiency: If the routing protocol has a highoverhead, it will be impossible to scale the WMN to alarge number of nodes.� QoS: In addition to support from the medium access

control (MAC) layer and the forwarding engine, select-ing the ‘‘best” routes for different traffic classes is anessential ingredient for QoS support.

Taxonomically, WMNs are a particular type of mobilead hoc network (MANET) [21,1]. WMNs share the samemultihop characteristics and mobility-related issues asMANETs. However, there are also significant differencesbetween WMNs and general MANETs:

� Gateways: Most WMNs are designed to provide connec-tivity to a distribution system (usually connected to theInternet). Therefore, they have specialized nodes (thegateways) that provide connectivity to the distributionsystem.� Traffic pattern: In WMNs, most of the traffic is expected

to flow between the clients and the Internet (via the gate-ways). In general MANETs, the common assumption isthat any node is equally likely to be the source or thedestination of a traffic flow.� Mobility: In most WMNs, nodes belong to two distinct

categories: either stationary (e.g., on lamp poles, roof-tops, etc.) or mobile, capable of roaming in the coveragearea provided by the stationary nodes. In MANETs, it isoften assumed that all nodes have homogeneous mobil-ity characteristics.

In this paper, we propose a new mesh routing protocol(MRP), specifically geared toward WMNs. There have beenmany routing algorithms proposed for MANETs [21–35].However, the characteristics and requirements of WMNsare considerably different than those of general MANETssuch that a new routing protocol can significantly outper-form the general MANET routing protocols. The situation


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is similar to the case of wireless sensor networks, where dif-ferences from general MANETs prompted the developmentof specialized MAC and routing protocols [36].

MANET protocols rely on variations of a floodingmechanism for route discovery and recovery. Some of themattempt to reduce the flooding overhead by utilizing loca-tion information [32–35], limiting the flooding diameter[27,37–39], or electing dedicated relay nodes [40]. However,none of them completely eliminates flooding. In contrast,our route discovery and recovery schemes do not requireflooding. The main difference between the proposed andexisting routing protocols is the absence of flooding.

The proposed routing protocol only maintains routingtrees between clients and the gateways of the WMN. Thismirrors the flow of data in the network and eliminatesthe overhead associated with maintaining direct routesbetween the clients. Furthermore, we show that, by prefer-ring stable routes, the backbone of stationary nodes pres-ent in WMNs can be effectively used to improve theperformance of the routing protocol. Finally, we introducea novel link failure detection scheme that takes intoaccount the inherently unreliable nature of wireless links.

The remainder of the paper is organized as follows. Sec-tion 2 provides a brief overview of the related work. In Sec-tion 3, two versions of the proposed protocol are described,compared, contrasted, and merged into a third hybrid ver-sion. The performance of the three versions of the pro-posed protocol is evaluated via simulations in Section 4.Section 5 concludes the paper.

2. Related work

There are hundreds of proposed routing protocols.Many of them have been standardized by IETF and havebeen in use for many years. Some of those protocols haveproven themselves in the Internet and are expected to con-tinue to perform well for many years to come [41–45].

In the ad-hoc networking arena, several classes of rout-ing protocols have been proposed and carefully analyzed[37,22,23]. The first class of proposed protocols was derivedfrom existing ‘‘table-driven” protocols in the Internet (e.g.,[25]) optimizing many of the aspects that reduced the effi-ciency of the existing Internet routing protocols inMANET environments.

In contrast to table-driven routing protocols, on-demandrouting protocols (e.g., AODV [28,46] and DSR [29]) weredesigned specifically for ad hoc networks with frequent dis-connections (due to topology changes). They often outper-form the table-driven routing protocols in scenarios withlarge networks with relatively few active connections.

In a class of their own, the geographical routing algo-rithms (e.g., LAR [32], DREAM [33], and ZRP [47]) takeadvantage of node location information (that can be rela-tively inexpensive to distribute [34]) to reduce routing over-head and improve the performance of the protocol.Geocasting [48,49] takes a rather unique view to address-ing, as it considers any node within a given geographical


region as a valid destination. Several specialized applica-tions can greatly benefit from geocasting.

It is well-known [37] that current routing protocols scalepoorly with the number of nodes, number of flows andincrease in mobility. Several protocols were designed forscalability [33,40,38,50–52] and show significantly betterperformance in large ad hoc networks.

The WMN companies are using a variety of routing pro-tocols to satisfy their needs. Some are proprietary and heldsecret (e.g., [10]), while others use well-known ad hoc rout-ing protocols (e.g., Firetide [5] uses Topology Broadcastbased on Reverse-Path Forwarding (TBRPF) [50]). Othercompanies rely on the IEEE 802.1 spanning tree protocolfor routing at layer 2 (e.g., MeshDynamics [8]).

It has long been recognized that many ad hoc networkswould benefit from a connection to a fixed infrastructure,and several solutions have been developed to support thistype of connectivity. In particular, the SURAN [53] andWINGS [54] projects solved many of the problems associ-ated with these networks (MAC layer, channel access, inte-gration with existing Internet protocols, mobilitymanagement, transport layer efficiency, etc.).

In [55], the authors propose extensions to existing routingprotocols (AODV [28], DSR [29] and SOAR [56]) such thatthey optimize access to a set of nodes called netmarks (simi-lar to what we call gateways). They evaluate the performanceof the extended SOAR routing protocol (a link state routingprotocol) and show via simulations that it outperforms bothDSR and AODV. We do not propose to extend a link stateprotocol, but rather a new design specifically optimized forWMNs. Furthermore, our proposed protocol takes advan-tage of the fixed nodes in WMNs (by selecting stable routes).

In [57], the authors address routing issues in a hierarchi-cal, two-level MANET. They propose a hybrid schemeusing existing protocols in both hierarchies (DSDV [25]at the lower level, i.e., in the clusters connected to the gate-ways, and AODV in the backbone). They also propose anextension of AODV: Hybrid-AODV (H-AODV). Bothapproaches are shown to outperform classical AODV interms of packet delivery ratio and delay for large networks.Our work is different in that it does not consider mobilebackbones, and it does not use or extend existing protocols.

The LUNAR [58] system provides a simple routing pro-tocol for small MANETs (limited to three hops). The pro-tocol is predictable as well as easy to implement andanalyze. It features auto-configuration and gateway rout-ing, thus, making it a complete set of protocols. Its goalsand functionality are, however, very different from thoseof the protocol proposed in this paper.

The K-hop routing protocol (KRP) [39,59] is designedto provide service in an ad hoc network with nodes at mostK hops away from a gateway. An extension of AODV (theflooding mechanism in AODV is limited to K-hops) is usedto discover routes to the gateways. The gateways them-selves publish and update a bulletin board of reachablenodes ensuring network-wide dissemination of informa-tion. The approach allows for multiple routes to multiple



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gateways. Our approach is significantly different from KRPin that we use neither flooding not a bulletin for route dis-covery and maintenance.

Landmark Routing (LANMAR) [51,52], is an extensionof the Fisheye State Routing (FSR) [38] that takes advantageof logical groups that tend to move together (e.g., soldiers ina platoon). Our approach is significantly different, as it is nota link state approach, and it does not take advantage ofgroup mobility (unrealistic for general WMNs).

In [60], a gateway discovery scheme was proposed as apart of an architecture connecting generic MANETs andIPv6 networks. The scheme in [60] relies on flooding forroute discovery, while the routing protocol proposed in thispaper does not. Furthermore, the return paths (i.e., fromthe gateway to nodes in the WMN) are not established, aMANET routing protocol being assumed for this purpose.

AODV-ST [61] is a hybrid routing protocol designed forWMNs. It uses proactive tree formations by using a beac-oning protocol initiated at the gateways to form spanningtrees rooted at the gateway. For intra-mesh routing, it usesAODV to discover routes that bypass the gateways. Similarto our work, the main assumption is that the common-casetraffic is to and from the gateway, and thus, uses the pro-actively constructed trees. In contrast to our work, thecommon-case protocol (that forms the routing trees) is pro-active, periodically flooding the network (AODV, used forintra-mesh routing, also floods RREQ messages). Our pro-tocol does not use flooding.

ROMER [62] is a credit-based, opportunistic forward-ing mechanism capable of using multiple routes at shorttime-scales and building long-term reliable routes at a costof some redundancy. It is shown that for WMNs with sig-nificant variability in link quality, ROMER can signifi-cantly improve the packet delivery ratio of the network.Similarly, results presented in [63] demonstrate thatmulti-path routing can improve the reliability of WMNseven when the paths used are not disjoint.

Several articles considered routing-related issues forWMNs; in [64,65], routing is considered in conjunctionwith channel assignment for multichannel WMNs basedon commodity IEEE 802.11 compatible hardware. Severalarticles [66–68] show that non-conventional metrics mayresult in an increased capacity of WMNs.

In MANETs, existing routing protocols employ flood-ing to discover and maintain routes between arbitrary pairsof nodes. In WMNs, most of the traffic is assumed to flowto and from the gateways connected to the Internet. Withthis assumption, we designed a protocol that does not useflooding either for route discovery or for recovery. Further-more, we introduce a novel link failure detection procedurethat is shown to effectively resolve the problem of misiden-tifying temporary link errors as permanent link breaks.

3. Protocol description

In the design of the protocol, we decided to optimizethe common case. In WMNs, the most common traffic


flows are to and from the Internet (downloads from theInternet are by far the most common case, but TCPacknowledgments form streams in the opposite direction).Thus, we decided that any node in a WMN will onlyknow how to reach one gateway and is, in general, reach-able only from a gateway. Any small amount of client-to-client traffic can be routed through the common parent ofthe clients (potentially the gateway). In essence, the routesto and from the gateway form a tree rooted at thegateway.

In this version of the protocol, every client chooses a sin-gle gateway to connect to the Internet. If the node moves orif this gateway fails, the node may choose a different gate-way. We, thus, assumed a network topology similar to theone shown in Fig. 2. The gateways are connected usingwired or wireless links to a single super-gateway that is fur-ther connected to the Internet. All traffic is funneledthrough the gateways to the super-gateway and, eventually,to the Internet. Mobile users are free to roam as long asthey are in the coverage area of the network. Small deploy-ments may use a single gateway and forego the use of asuper-gateway.

One of the design requirements we observed was thepossibility to implement the protocol in the user space. Inother words, the protocol does not have access to the for-warding engine other than through setting the routingtable. We chose to transport the MRP messages usingUDP packets for reasons of performance (TCP’s overheadand delay would be too high). The choice requires a loss-tolerant protocol design. All three versions of the protocolpresented in this paper interface with the kernel onlythrough the routing table changing calls and by intercept-ing ICMP packet delivery failure messages.

3.1. MRP on-demand

When designing the protocol, we faced the well-knownproactive vs. on-demand question commonly encounteredin MANETs [22]. The first version of the protocol is purelyon-demand: when a node is joining the network, it will askthe closest gateway or neighboring user nodes for a route(we will shortly elaborate on the criteria used to choose agateway). Thus, the first step for a node that wishes to jointhe network is to broadcast locally a route discovery(RDIS) message. In contrast to existing MANET protocols(e.g., AODV and DSR), the RDIS packets in MRP are notflooded through the network, and are only received by theone-hop neighbors of the source. Initially, the joining nodeis in disconnected state.

We take advantage of the fact that all connected neigh-bors already know a route to the gateway and the signifi-cant routing metrics of those routes. Thus, all of thenodes receiving the RDIS message (one-hop neighbors ofthe joining node) will reply with a route advertisement(RADV) packet with the metrics of their current routes.The very first user node to join the network will receive thisRADV packet only from the neighboring gateway(s).


Fig. 2. General WMN topology.


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The RADV packets are unicasted after a small randomdelay to avoid collisions. Without this delay, severalRADV packets (triggered by the same RDIS) from (poten-tially hidden from each other) neighbors of the joiningnode, will collide at the joining node. If there are no con-nected neighboring nodes, or all of the advertisements arelost, the joining node will periodically broadcast the RDISmessage until it receives an advertisement. The new node,joining the network will store all RADV packets (waitfor a time chosen slightly longer than the longest randomdelay used to generate RADVs). Once the joining nodereceives all the RADVs, it will select one or more upstreamroutes as a function of the node’s requirements and theoffered routes.

The current design of the protocol is open to a variety ofmetrics that can be used for differentiating routes:

� Hop-count: The basic and common metric in routingprotocols.� Route stability: This metric allows a joining node (given

the choice) to select routes that include stationary nodes(those routes are presumably more stable than thoseincluding mobile nodes).� Minimum delay: Important for delay sensitive applica-

tions (e.g., VoIP, telnet, etc).� Maximum bandwidth: Important for bandwidth inten-

sive applications (e.g., ftp, peer-to-peer).� Minimum packet loss: Important for loss sensitive

applications.

In contrast to other papers, we do not propose any newmetrics or methods for computing the metrics above. Com-puting many of them is an area of active research [66,68];we simply include the possibility that they can be used inmaking routing decisions.

In the simulations, we included the case where routeswith high stability (i.e., employing stationary nodes) arepreferred to routes with minimum hop-counts. In Section


4, preferring stability to hop-counts, is shown to reducerouting overhead, increase the data throughput, and reducethe energy consumption of the mobile nodes at the expenseof longer routes.

Routing overhead and packet loss is reduced as thenumber of route breakage is minimized. In many applica-tions of wireless mesh networks, the mobile nodes have lim-ited energy resources (unless the nodes are mounted onvehicles); thus, power consumption becomes an importantissue. If routes through stationary (and powered) nodes arepreferred, packet forwarding of the mobile nodes is mini-mized. Other energy efficiency aspects of MRP are dis-cussed in Section 3.3.

Once the joining node selects a route (based on theRADVs it received), the node practically has a routetoward the Internet (as all of the neighbor nodes haveroutes toward the Internet). The node can start to senddata immediately toward the Internet. We call this thehalf-connected state (as the node has a route to the Internet,but none of the nodes in the Internet can reach it). In thesecond phase, the joining node registers with the gateway.The registration has the main function of providing areverse path from the Internet to the joining node. Wecould have simply used the reverse path of the packetsgoing toward the Internet to provide the return path, butthat would imply that the routing protocol can monitorthe forwarded data packets (a requirement we eliminatedfor portability, as explained above).

The registration process proceeds in two steps. In thefirst step, a registration request (RREG) is unicasted tothe gateway (it goes through the MRP layer at every inter-mediate hop). As the RREG travels toward the gateway,intermediate nodes set up an entry in their routing tablesenabling the return path (from the Internet to the joiningnode).

The existence of the return path assumes bidirectionallinks, but as pointed out in the literature [69], unidirec-tional links automatically exclude many of the common



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MAC protocols (e.g., MACA, 802.11, 802.16, and certainlyall of those that require acknowledgments). Upon receiptof the RREG packet, the gateway sends a registrationacknowledgment (RACK) directly to the joining node,and it also forwards the RREG message to the super-gate-way (such that the super-gateway will know to which gate-way to forward the packets for the joining node). If any ofthe RREG or RACK messages are lost (which is detectedby a time out mechanism), the joining node reverts to dis-connected state and re-initiates the route acquisition pro-cess (as very likely the route it initially chose just becamedisconnected).

Upon receipt of a registration acknowledgment, thejoining node enters the fully-connected state in which itcan send and receive data to and from the Internet.

Fig. 3 depicts the sequence of messages discussed above,from route discovery to registration acknowledgment.None of the four messages used to establish the routes(RDIS, RADV, RREG, and RACK) are flooded into thenetwork. This is in clear contrast to other MANET routingprotocols where the overhead/node ratio increases with the

Fig. 3. MRP route establish

Fig. 4. Finite state mac


size of the network. Thus, we expect that the overhead ofMRP will be significantly lower than that of existingMANET protocols, especially for large networks.

For wireless networks, the failure to forward a packet tothe next hop may represent a route failure, but, more oftenthan not, it might be simply the effect of interference or atemporary fading effect. Therefore, over-reacting to apacket loss (i.e., entering the disconnected state and re-ini-tiating the route discovery process) may be detrimental tothe protocol’s performance. Hence, upon suspecting a linkloss, instead of entering the disconnected state, the nodeenters a temporary state named verify-link state.

While in verify-link state, the node probes the availabil-ity of the suspected link by using special route-check pack-ets (RCHKs) that are unicasted to the next hop. If a replyis received (we reused RADV for the reply), the node againbecomes fully-connected; otherwise, it enters the discon-nected state. If a node loses connection to the gateway,all of its children will lose connection to the gateway. Evenif it discovers a new route to the gateway, the children willnot be able to use this new route to receive data from the

ment message sequence.

hine for MRP-OD.



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gateway (the reverse routes will be broken). Hence, when anode loses connection to the gateway, it will send a routeerror (RERR) message to all of its children. Each of thosechildren will enter the disconnected state and re-initiate theroute discovery process. A detailed analysis on the effect ofthe verify-link state on MRP’s performance is presentedlater in this section.

Fig. 4 depicts the finite state machine for the MRP pro-tocol we just described. Since, in some ways, it resemblesthe on-demand protocols of MANET (route discovery ini-tiated by the joining node and route errors initiated bypacket loss), we will call this version MRP on-demand

(MRP-OD).Due to the tree structure of the routing graph (both to

and from the gateways), and the use of hop-counts, theprotocol is guaranteed to be loop free (it will be loop freeas long as the routing metric is strictly positive).

3.2. MRP beacon mode

MRP-OD is fully functional; but, when a user node doesnot send any packets, it cannot detect that its route is nolonger valid. Any packets from the gateway will be unableto reach the disconnected node.

A second version of MRP uses beacons to advertiseroutes: each node in fully-connected state (including thegateway) periodically sends beacons advertising theavailable routes. Each beacon acts as a gratuitous routeadvertisement (we used the same RADV packets as forMRP-OD). We will call this version MRP beacon mode

(MRP-B). Notice that the MRP beacons are different fromthe 802.11 beacons in that they occur at different timeintervals, are originated and consumed by the MRP ratherthan the MAC layer, and contain routing informationinstead of MAC layer management information.

In MRP-B, a node that wishes to join the network doesnot have to send an RDIS. Instead, it simply listens to theneighboring nodes slightly more than the beacon periodand collects beacons (route advertisements). The routeselection and registration procedures are identical toMRP-OD (the same states are used).

For detecting route disconnections, in addition to relyingon packet forwarding failure, MRP-B utilizes the beaconmessages originally intended for route setup. In MRP-B,each node monitors beacons sent from their parents. If a pre-defined number of beacons are missing, the node will enterthe disconnected state and re-initiate the route discoveryprocess. Also, as in the case of MRP-OD, if there is a packetforwarding failure, the node will enter the verify-link stateand either become fully connected (if the RCHK is acknowl-edged) or will disconnect (if no reply is received). If linkdetection based only on beacons is employed, a real linkbreakage will be detected only after a considerable delay(several beacon periods).

The state diagram of MRP-B is practically identical toMRP-OD except for changing the triggers in some of thetransitions, adding a periodic beaconing while in fully-con-


nected state and a direct transition (triggered by beaconloss detection) from the fully-connected state to the discon-nected state.

There is a trade-off between the beacon interval and theperformance of the protocol: the more often a beacon issent, the faster a node will be able to join the networkand detect a route disconnection. However, it also increasesthe overall overhead of the routing protocol. Depending onthe required performance, a reasonable range of beaconperiods can be from tens of milliseconds to a few seconds.

We expect the MRP-B to exhibit better delay perfor-mance than MRP-OD because MRP-B is able to detect aroute disconnection sooner than MRP-OD. This is trueespecially when traffic load is low and as a result, detectingforward failure does not promptly reflect a route change.On the other hand, MRP-B will have a higher overhead(due to the beacon messages).

3.3. Hybrid MRP

The two MRP versions we discussed are not mutuallyexclusive. They can be easily combined into a hybrid pro-tocol (MRP-H). We simply use the same states as inMRP-OD and MRP-B and combine the state transitionsfrom the two versions.

In MRP-H, the joining node broadcasts route discover-ies (RDISs) and waits for route advertisements (RADVs)for a time equal to the minimum between the random delayof MRP-OD and the beacon period of MRP-B. Thereceived RADVs include those generated in response tothe RDISs, as well as the beacons. The joining node thenselects the route and registers with the gateway. A routeerror can be discovered by either missing beacons or packetforwarding errors. The state diagram of MRP-H is similarto the one corresponding to MRP-OD (Fig. 4), except thatthere are multiple triggers for the state transitions.

We expect MRP-H to perform better than either MRP-B or MRP-OD as MRP-H detects link failures faster thaneither of the two versions and is able to discover betterroutes faster: the pool of available routes at the momentof the route selection will be larger due to the redundancyoffered by the beacons and triggered answers. (This featureonly makes a difference if some of the RDIS or RADVpackets are lost due to noise/interference.)

Since none of the three versions of the protocol usesflooding to establish or repair routes, all three should scalewell to large networks.

With respect to energy consumption, a mobile user withlimited energy resources can minimize the energy expendedfor routing by employing MRP-OD (where no periodicbeacons are employed). The rest of the network may runthe more energy-expensive MRP-H or MRP-B. Thus, formobile nodes, MRP can minimize both the energyexpended for routing, as well as for forwarding (by selectingroutes that avoid the mobile nodes).

If the nodes in a large mesh network are powered upsimultaneously (e.g., after a power outage), the first tier



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of nodes (with direct connectivity to the gateways) will dis-cover routes first, then, the second tier (nodes two hopsaway from the gateway), and the process will continuefrom the center to the edges of the network.

The design of MRP is based on the assumption thatmost data flows are established between client nodes andthe gateway. Routes between two arbitrary client nodesmay be sub-optimal. In particular, two client nodes (unlessthey are parents of one another), will always communicatethrough their common parent (which can be several hopsaway from either of them, and possibly the gateway); how-ever, client-to-client communication is assumed to be infre-quent in WMNs (for most applications).

In a multi-gateway situation, the protocol ensures thatthe network nodes choose a single gateway to be their routeto the Internet (the choice is dictated by the routing met-ric). Thus, the network self-decomposes in a collection ofdisjoint ‘‘cells” centered at the gateways. If a gateway fails,the first tier nodes (the direct neighbors of the gateway) willdetect the route failure, and propagate the error to all theirchildren. All nodes in the mesh ‘‘cell” belonging to thefailed gateway will suddenly become orphans. The nodesat the periphery (as new nodes in disconnected mode inMRP) will discover alternate routes (higher cost than theinitial route) to other nearby gateways. Eventually allnodes in the failed ‘‘cell” will be distributed among neigh-boring gateways.

3.4. Verify-link state

The verify-link state is designed to reduce the uncer-tainty introduced by wireless links and to avoid unneces-sary disconnections and route re-establishments. Wedefine a false disconnection as the case where an MRP nodeerroneously enters a disconnected state due to temporarilydisabled links (e.g., caused by fading or interference). Wedefine a true disconnection as the case where a node entersa disconnected state due to permanent link breakage (e.g.,caused by mobility or node failure).

False disconnections and consequent route re-establish-ment may cause significant performance degradation. Fre-quent re-routing will also reduce the available bandwidthfor end users due to increased routing overhead. Therefore,one of the design goals of MRP is to suppress false discon-nections and to identify and promptly respond to true dis-connections. To this end, we introduced the verify-linkstate whose state transition and packet exchange (i.e.,RCHK and RADV) were detailed in Section 3.1.

One of the key parameters in the verify-link state mech-anism is the verify-link-failure (VLF)-timeout value, whichcontrols how long a node should stay in verify-link statebefore it transitions to disconnected state (assuming thatno reply was received). The choice of a VLF-timeout valueis a trade-off: a large VLF-timeout will reduce false discon-nections but causes delay in true disconnection situations.On the other hand, a small VLF-timeout will ensureprompt handover in a true disconnection but increases


the number of false disconnections. To study the efficiencyof the verify-link state and to identify a working range ofVLF-timeout values, we investigated the behavior ofMRP through simulation experiments over a wide rangeof VLF-timeout values.

Since traffic load intensity and mobility intensity areexpected to affect false disconnections and true disconnec-tions, respectively, we tested VLF-timeout values for differ-ent traffic and mobility scenarios. Three performancemetrics were used to measure how correctly and efficientlyroutes are maintained when different VLF-timeout valuesare used with the forward failure detection: routing over-head, packet delivery ratio (PDR), and end-to-end delay.The metrics are defined as follows:

Routing Overhead �PN

n¼1

PP csn

p¼1bn;p

T sim � Nðbps=nodeÞ; ð1Þ

PDR �PN

n¼1P drnPN

n¼1P dsn

ð%Þ; ð2Þ

Delay �PN

n¼1

PP drn

p¼1Dn;pPN

n¼1P drn

ðms=packetÞ; ð3Þ

where:Nis the total number of nodes,P cs

n is the total number of broadcast/unicast routingcontrol packets sent (either generated or for-warded) by the nth node (including beacon pack-ets),

bn;p is the number of bits in the pth packet received bythe nth node,

T sim is the total simulation time,P dr

n is the total number of unicast data packets re-ceived by the nth node,

P dsn is the total number of unicast data packets sent by

the nth node, andDn;p is the end-to-end delay experienced by the pth

packet received by the nth node.

We use Poisson traffic with data packets serving as probes.Every user node has two flows: one toward the gateway andthe other one from the gateway. Packet size is 1500 bytes. Thetraffic intensity is varied by changing the average packetinterval ranging from 1.5 to 4.0 s (in this case, the numberof mobile nodes was fixed at 16). Mobility intensity is con-trolled by the number of mobile nodes from 0 to 49 (in thiscase, the packet interval was fixed to 3.0 s). Randomly-deployed mobile nodes move according to the random way-point mobility model [70] with zero pause time, a maximumspeed of 10 m/s, and a minimum speed of 1 m/s (to avoid thedrawbacks highlighted in [71]). The mobile nodes moveinside a rectangular area with a single gateway in the centerand 144 uniformly deployed fixed nodes. Total simulationtime is 400 s. The size of the rectangular area is determinedby the number of fixed nodes to ensure constant node density(approximately 25 nodes per km2).

Fig. 5 shows the performance of MRP as a function ofVLF-timeout under different traffic intensity. Only the



ARTICLE IN PRESS

result for the MRP-H is presented to avoid cluttering. TheVLF-timeout was increased from 6 ms to 12 s. The resultsfor the first (0 s) and the last VLF-timeout value (400 s) arenot shown to scale. They are included to provide insight onthe MRP’s behavior for extreme values. The VLF-timeoutof 0 s represents the case where the verify-link state is com-pletely disabled; in this case, one forwarding failure is inter-preted as a route failure and, thus, it triggers a routerediscovery. The VLF-timeout of 400 s represents the casewhere packet forward failures are always ignored through-out the simulation time.

For small VLF-timeout values, the curves show lowPDR and high end-to-end delay due to false disconnec-tions. A false disconnection will trigger a route rediscoverythat implies the exchange of several control packets (RDIS,RADV, RREG, and RACK). If the falsely disconnectednode has children nodes, RERR packets are generatedwhich leads to a cascade of false disconnections and routere-establishments.

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Packet Interval = 4.0 sPacket Interval = 3.5 sPacket Interval = 3.0 sPacket Interval = 2.5 sPacket Interval = 2.0 sPacket Interval = 1.5 s

102

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1

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VLF T

End

toen

d D

elay

[s]

a

c

Fig. 5. The effect of verify-link state as a function of VLF-timeout and differeand, (c) end-to-end delay.


As the VLF-timeout is increased, the overall perfor-mance is improved due to the reduced probability of falsedisconnections. However, if VLF-timeout is too large, truedisconnections (from mobility) are temporarily ignored,leading to loss of packets for the mobile nodes.

Fig. 6 shows the performance of MRP-H as a functionof VLF-timeout and mobility intensity. When the numberof mobile nodes is increased, the loss from ignored true dis-connection increases.

Considering the results in Figs. 5 and 6, the VLF-timeout should be at least 1 s (as for lower values, thefalse disconnections considerably lower the PDR inFig. 5(b)). Fig. 6(b) shows that the PDR decreases stea-dily as the VLF increases over 1 s, as true disconnectionsare identified too late. In our simulations, we chose aVLF-timeout of 3 s. The performance could likely beimproved if the value of the VLF-timeout is determinedadaptively; but, we do not explore that option in thispaper.

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]


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[s]


a b

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Fig. 6. The effect of verify-link state as a function of VLF-timeout and mobility intensity: (a) routing overhead, (b) packet delivery ratio and (c) end-to-enddelay.


ARTICLE IN PRESS

4. Performance evaluation

In this section, we present the results of the simula-tions which we performed using QualNet [72] to evaluatethe performance of MRP. QualNet is the commercialversion of GloMoSim[73] (which uses Parsec [74], a par-allel discrete-event simulator). We chose QualNet becauseit provides accurate models (high level of detail) at eachlayer, including the physical and MAC layers. Resultsindicate that the accuracy of the models has a significantimpact on the simulation results for wirelessnetworks[75,76].

4.1. Performance metrics

We compared the performance of MRP with other rout-ing protocols in terms of five metrics: routing overhead,packet delivery ratio (PDR), throughput, end-to-end delay,and average hop-count.


We employ Internet traffic as well as Poisson traffic inthe performance evaluation; we use network throughputas a performance metric for Internet traffic and PDR as aperformance metric for Poisson traffic. In the Internet traf-fic scenarios, the applications use mostly TCP as a trans-port layer, and the PDR does not capture the effect ofpacket loss on the TCP throughput.

The routing overhead, PDR, and the end-to-end delayare defined in Section 3.4. We define the network through-put and hop-count as:

Network Throughput�PN

n¼1

PP drn

p¼1bn;p

T simðbpsÞ; ð4Þ

Average Hop�Count�PN

n¼1

PP drn

p¼1hn;pPN

n¼1P drn

ðhope per packetÞ; ð5Þ

where N, P drn , bn;p, T sim, n and p are defined in Section 3.4,

and hn;p is the number of hops traversed by the pth packetof the nth node.



ARTICLE IN PRESS

We believe that the chosen metrics can be traded againsteach other (e.g., higher overhead for lower delays, lowerthroughput for lower delays, etc.), and a fair comparisonhas to consider all of the metrics.

4.2. Simulation setup

For the physical and MAC layers, we used the defaultIEEE 802.11b as physical and MAC models (in RTS/CTS mode). The error rate in Qualnet (version 3.8) gradu-ally decreases from approximately 0% at 270 m to almost100% at 300 m. The physical layer uses a two-ray path-loss,constant shadowing model without fading.

For the higher layers, we used the standard TCP/IP pro-tocol stack, and we implemented MRP as described in Sec-tion 3. For MRP-B, we chose a beacon interval of 1 s. ForMRP-OD and MRP-H, we chose the small random inter-val for sending route requests of 2 s. For MRP-H, we chosethe beacon interval of 2 s (we shall see that the performanceis comparable with MRP-B with 1 s beacon interval).

In order to assess the contribution of preferring stabilityto hop counts to the overall performance of the protocol,we simulate MRP-H with and without route stability andwe labeled the corresponding results as MRP-H andMRP-S, respectively.

Simulations are run for two different traffic types: Pois-son (Section 4.3) and Internet traffic (Section 4.4). For thePoisson traffic, user data packets serve as probes, and theperformance metrics indicate how correctly and efficientlyroutes are maintained by different protocols. On the otherhand, the Internet traffic model is used to evaluate the per-formance of the protocol in a more realistic environment.The Internet traffic is created by mixing several popularInternet applications based on real Internet traffic measure-ment. The traffic composition trend captured from the realInternet traffic [77] was accurately reproduced in our simu-lation using multiple application protocol models providedby QualNet [72]: HTTP, TELNET, FTP, DNS, and VBRmodels. Traffic composition ratio was reproduced at alldetail levels: flow, packet, and byte.

Simulations were performed to evaluate the influence ofthe following parameters:

� traffic load,� network size,� degree of mobility,� perturbation, and� intra-mesh traffic ratio,

where the perturbation represents the degree of random-ness in geographical node distribution, and the intra-meshtraffic ratio indicates the percentage of the user traffic flowswhose both ends are user nodes (as opposed to the typicalmesh network case where one end is a gateway).

To avoid the large number of graphs that would result,if we performed every experiment for every possible setting,we chose a base case and varied one parameter at a time.


For each parameter variation, we present the four metricsdefined in Section 4.1, for all of the protocols under consid-eration. For the base case, we used the following scenario:

� The Poisson traffic has one incoming and one outgoingflow for every user with a mean inter-arrival rate of onepacket every 1.5 s.� The Internet traffic has (on average) one flow for every

user (in the proportion reported in [77], i.e., 75% HTTP,5% FTP, etc.).� The stationary network consists of 144 nodes uniformly

distributed in a rectangular area of 2.4 � 2.4 km2.� There are 36 mobile nodes deployed in random loca-

tions. They move according to the random waypointmobility model [70] with zero pause time, a maximumspeed of 10 m/s, and a minimum speed of 1 m/s.� There is a single gateway at the center of the network.� The stationary nodes are deployed with zero perturba-

tion (i.e., they are deployed in a grid of size 250 m).� There is no intra-mesh traffic.� Total simulation time is 400 s.

To evaluate the performance of MRP, we chose six well-known routing protocols representative of different classesof routing protocols: AODV [28,46], DSR [29], LANMAR[51,52], OLSR [40], RIP2 [45], and ZRP [47]. AODV andOLSR are standardized by the MANET workgroup ofIETF. AODV and DSR are widely known on-demand

(reactive) MANET routing protocols. LANMAR isdesigned for routing in large scale ad-hoc networks withgroup mobility. In QualNet implementation, LANMARuses Fisheye State Routing (FSR) protocol for local scoperouting. OLSR is based on the Inria implementation [78],and it is an optimized link-state, table-driven protocoldesigned for MANETs. RIP2 is a table-driven distance-vec-

tor protocol originally designed for wired Internet. TheRIP2 model in QualNet follows the Cisco implementation.ZRP is a hybrid ad-hoc routing protocol where proactiveand reactive approaches are employed for intra- andinter-zone routing, respectively.

For each scenario, we simulated the network 30 timeswith different random seeds (resulting in different initialplacements and mobility for the mobile nodes and in differ-ent inter-arrival times of the offered load). In every graph,we present the average of the 30 experiments. To avoidcluttering the figures, confidence intervals are notpresented.

4.3. Poisson traffic

In this section, simulation results for Poisson traffic areexamined for five different scenarios where traffic load, net-work size, mobility, perturbation, and intra-mesh trafficratio are varied from the base case. Except for the scenarioof varied intra-mesh traffic, all nodes have packet streamsflowing both to and from the gateway. The same trafficload is applied in both directions.



ARTICLE IN PRESS

4.3.1. Traffic load

We increase the load of the Poisson traffic by reducingthe mean packet inter-arrival time from 1.5 to 0.67 s (i.e.,from 0.67 to 1.5 packets per second).

Fig. 7 depicts the simulation results for the overhead,PDR, delay, and hop-count, respectively. In each graph,curves for nine routing protocols (including four versionsof MRP) are plotted as the offered load increases.

Fig. 7(a) shows that the routing overhead of AODVrises as traffic load is increased. The main reason behindthis increase is the corresponding increase in lost packets(that triggers the route discovery process). DSR showslower overhead; however, the overhead of source routeheader is not included in the overhead calculation. In con-trast, MRP and all the proactive protocols show overheadimmunity to the traffic load. All versions of MRP andLANMAR form the group of the lowest overhead of all

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Traffic Load [pps]

End

toen

d D

elay

[ms]


a

c

Fig. 7. The performance of the routing protocols for Poisson traffic (a) routinghop-count as a function of traffic load.


considered protocols. MRP-H has a very low overhead,due to the larger beacon period (twice as large as MRP-B).

Fig. 7(b) shows that, as the offered traffic load intensi-fies, the PDR of AODV, DSR, OLSR, and ZRP drops fas-ter than MRP. Although MRP-S shows smaller PDR thanthe other versions of MRP, it still outperforms the otherprotocols. The drop in PDR is due to the packet loss inthe queues, as well as lost routes due to the routing proto-cols’ attempt to restore failed routes (or what are consid-ered to be failed routes due to packet drops). RIP2 showsthe lowest PDR for the highest traffic load, indicating theunsuitability of wireline-oriented protocol for ad-hoc ormesh networks.

The delay increase in almost all of the protocols shownin Fig. 7(c) is due to the larger queuing delays resultingfrom the increase in offered load. MRP-OD has a higherdelay than MRP-B and MRP-H, as it discovers broken

0.7 0.8 0.9 1 1.1 1.2 1.3 1.40

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b

d

overhead, (b) packet delivery ratio, (c) end-to-end delay, and (d) average



ARTICLE IN PRESS

routes later than the other two. The low delays of RIP2 andLANMAR have to be considered with the very low PDR inFig. 7(b): the delay statistics consider only the few packetsthat reach their destinations.

Fig. 7(d) shows that the hop-count of each packet isalmost insensitive to an increase in traffic load. Two on-demand protocols (AODV and MRP-OD) show a slightincrease in the hop-count. The hop-count result of DSRis not included because measuring the TTL field of IP pack-ets is not possible in QualNet due to its source routingimplementation.

As expected, among the versions of MRP, MRP-H exhib-its the best overall performance as it combines strengths fromboth MRP-B and MRP-OD. MRP-S shows smaller delaysand hop-count at the expense of a decrease in PDR.

4.3.2. Network size

In this scenario, the number of fixed nodes is increasedwhile keeping the network density constant (i.e., increasing

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rhea

d [b

ps/n

ode]


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2500

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3500


End

toen

d D

elay

[ms]


a

c

Fig. 8. The performance of the routing protocols for Poisson traffic (a) routinghop-count as a function of network size.


the network diameter). The four graphs for overhead,PDR, delay, and hop-count are shown in Fig. 8, as a func-tion of the network size.

In Fig. 8(a), MRP shows almost constant overhead,while all of the other protocols show the overhead increas-ing with the network size. This is expected because forMRP, the overhead for each node does not increase withthe total number of nodes (see Section 3). As a result,MRP also maintains the highest PDR and the lowest delay.This supports the claim that, for WMNs, MRP scales bet-ter than the existing protocols.

Fig. 8(b) shows that the PDR of all routing protocolsdecreases with the increase in network size. This is expectedas the network capacity most likely decreases with theincrease in network size. For large networks (i.e., over100 fixed nodes), broken links due to mobility trigger routerediscoveries, which incur high overhead with flooding.Simulation results for varied mobility will be presentedand discussed in detail in the following section.

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ARTICLE IN PRESS

Fig. 8(c) shows high delay for DSR and ZRP comparedto other protocols. All versions of MRP (and especiallyMRP-S) show good delay performance for scaled networksize.

The hop-count shown in Fig. 8(d) directly reflects theincrease in network size. The versions of MRP, AODV,and OLSR show hop-counts close to the ideal hop-count.ZRP shows longer than ideal routes indicating suboptimalroutes, while LANMAR and RIP2 only route successfullyto/from nodes closer to the gateway, and thus show shorterhop-counts.

4.3.3. Number of mobile nodes

In this scenario, we evaluate the performance of MRPand other protocols when the number of mobile nodesincreases from 9 to 49. The results are shown in Fig. 9.

As in the cases of traffic load and network size, Fig. 9(a)shows that MRP shows a low and steady overhead as thenumber of mobile nodes is increased. In Fig. 9(b), MRP

10 15 20 25 30 35 40 45

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ode]


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toen

d D

elay

[ms]


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Fig. 9. The performance of the routing protocols for Poisson traffic (a) routinghop-count as a function of mobility.


shows higher PDR than other protocols as the mobilityintensifies. RIP2 shows a sharp drop in PDR as it wasdesigned for stationary networks with sporadic topologychanges.

The results supports the claim that, for WMNs, MRPsupports mobility more efficiently than existing protocols.

4.3.4. Perturbation

The performance of MRP for increased randomness inthe deployment of the stationary nodes is shown inFig. 10. For this scenario, the stationary nodes are per-turbed uniformly from their ideal grid positions. Thedegree of perturbation indicates by how much a node candeviate from its ideal position. A perturbation of 100%indicates that a node can be moved by up to one grid sizein any direction.

As the perturbation increases, the network becomes lessregular and hot spots are created. In Fig. 10(a), DSR andAODV show increased overhead, while all versions of

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ARTICLE IN PRESS

MRP remain low and almost constant. OLSR and ZRPshow a decrease in overhead; in OLSR, the multipointrelays (MPRs) benefit from non-uniform node distribution.Likewise, in ZRP, as perturbation increases, fewer routingzones are required to cover the user nodes, and thus, lessinter-zone, on-demand routing is needed.

Fig. 10(b) and (c) show decreased PDR and increaseddelays as the network becomes less regular. MRP showsbetter PDR and delay performance than other protocols.Among the versions of MRP, MRP-H shows the highestPDR while MRP-S shows the lowest hop-count and delay.

Fig. 10(d) shows that the hop-count for MRP increasesslightly as the network becomes less regular; this is due tothe route selection algorithm that favors stability to lowerhop-counts.

4.3.5. Intra-mesh traffic

In this scenario, the intra-mesh traffic ratio is increasedfrom zero to 0.8. The ratio is calculated as the number of

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Fig. 10. The performance of the routing protocols for Poisson traffic (a) routinhop-count as a function of the perturbation in stationary nodes grid deploym


intra-mesh traffic flows over the total number of user trafficflows. For example, the ratio of 0.8 indicates that 80% ofthe user traffic flows are both originated and terminatedwithin the mesh network (i.e., neither end of a traffic flowis a gateway.) The higher the intra-mesh traffic ratio, thestronger the characteristics of ad-hoc networks rather thanthose of mesh networks. The results are presented inFig. 11.

In Fig. 11(a), on-demand protocols such as AODV,DSR, and ZRP (ZRP has both on-demand and table-dri-ven components) show a small increase in overhead asintra-mesh traffic ratio is increased. These protocols haveto discover routes for destinations located farther awayfrom the source due to increased intra-mesh traffic.Table-driven protocols show almost constant overhead.

Fig. 11(b) shows increases in PDR for AODV, DSR,and ZRP; while for MRP, the PDR decreases as theintra-mesh traffic ratio increases. This result is expectedas AODV, DSR, and ZRP are designed for generic ad-

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ARTICLE IN PRESS

hoc networks where all of the user nodes are assumed to beuniformly likely to send data to any destination in the net-work. In contrast, routes in MRP are always routedthrough the common parent of the source and the destina-tion (often the gateway); thus, the routes are sub-optimal.The extent of this effect is shown in Fig. 11(d).

4.4. Internet traffic

In this section, simulation results for Internet traffic areexamined for five different scenarios where traffic load, net-work size, mobility, perturbation, and intra-mesh trafficratio are varied from the base case. The purpose of usingInternet traffic is to evaluate the performance of MRPand other protocols in a more realistic environment. TheInternet traffic model is created by mixing several popularInternet applications based on real Internet traffic measure-ments as depicted in Section 4.2.

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Fig. 11. The performance of the routing protocols for Poisson traffic (a) routinhop-count as a function of the intra-mesh traffic ratio.


4.4.1. Traffic load

In this scenario, we vary the relative traffic load of Inter-net traffic from 0.2 to 1.4. The relative traffic load is con-trolled by changing the number of active users whilemaintaining the ratio among different applications. For atraffic load of 1.0, every node is likely to have one activeapplication. If the load is greater than 1.0, some nodesare likely to have more than one active application.

Fig. 12(a) shows that the routing overhead of on-demand protocols (AODV, DSR, and ZRP) increases withthe traffic load. This result and its cause is similar to thecase of Poisson traffic: more packets tend to trigger moreroute discoveries for those protocols. In contrast, MRPand all of the table-driven protocols show overhead immu-nity to the traffic load.

As the offered traffic increases, the network throughputapproaches network capacity due to the feedback loop ofTCP that adjusts the transmission rate to the available

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MRP ODMRP BMRP SAODVLANMAROLSRRIP2ZRP

a b

c d

Fig. 12. The performance of the routing protocols for Internet traffic (a) routing overhead, (b) network throughput, (c) end-to-end delay, and (d) averagehop-count as a function of traffic load.


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bandwidth. Fig. 12(b) shows that AODV, DSR, and OLSRhave lower saturation throughput than MRP. The lowerthroughput is due to the packet loss in the queues andfailed routes due to packet drops. MRP-OD has lowerthroughput than MRP-H and MRP-B as it discovers bro-ken routes later than the other two.

In Fig. 12(c), the delay increase for all protocols is dueto the larger queuing delays resulting from increasedoffered load. DSR shows higher delay than other protocols.The low delay of LANMAR has to be considered in con-junction with its very low throughput (the delay statisticsonly consider the few packets delivered to their destina-tions). MRP-S has a higher delay than other versions ofMRP due to the more frequent disconnections.

4.4.2. Network sizeWe increase the network size in the same manner as in

the Poisson traffic case, i.e., increase the number of fixed


nodes while keeping the density constant. The results areshown in Fig. 13.

Fig. 13(a) shows that the overhead of MRP stays almostconstant, while that of other protocols (especially OLSR)rises as the network size increases. As in the case of Poissontraffic, this result is expected in MRP as the overhead foreach node does not increase with the total number ofnodes.

Fig. 13(b) shows that MRP-H and MRP-B maintain thehighest throughput. While packet loss simply reduces thePDR in Poisson traffic where UDP is used as its transportlayer, the majority of the applications in Internet traffic relyon TCP. In response to packet loss and route rediscoveries,TCP congestion control reduces its transmission rate, thusdecreasing the throughput.

In Fig. 13(c), it is shown that the delay increases forall of the routing protocols as the network diameterincreases.


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The increase in hop-count of MRP in Fig. 13(d) is cor-related to the increase in the network diameter. AODV hasa lower hop-count than MRP, as AODV often choosesshorter (but possibly less reliable) routes, while MRP pre-fers longer routes through stationary nodes.

4.4.3. Number of mobile nodesThe number of mobile nodes is increased in the same

manner as in the Poisson case. Fig. 14(a) shows thatMRP produces a low and almost constant overhead asthe number of mobile nodes is increased.

In Fig. 14(b), MRP-H and MRP-B show the highest net-work throughput as the mobility intensifies. MRP-ODshows relatively low throughput because MRP-OD relieson packet loss to detect a broken link; multiple lost packetsdrastically affect the TCP’s congestion control mechanism(that times-out multiple times before a route is rediscov-


ered). Table-driven protocols such as RIP2, ZRP (ZRP istable-driven for intra-zone routing), and OLSR showdecrease in the network throughput (see Fig. 15).

In Fig. 14(c), MRP shows higher delay than AODV.This result should be considered with AODV’s lowerthroughput and shorter hop-count in Fig. 14(b) and (d),which indicates that AODV chooses shorter routes, butloses more packets than MRP due to less reliable routes.DSR shows higher delay than other protocols.

4.4.4. Perturbation

The deployment of the stationary nodes is perturbed inthe same manner as in the Poisson case. The perturbation isincreased from zero to 80%.

In Fig. 15(a), MRP shows almost constant overhead.Interestingly, for Internet traffic, the control overhead ofAODV and DSR does not increase as in the case of Poisson


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traffic. As perturbation is increased, the overhead of OLSRdecreases for the same reason as for the Poisson case (Sec-tion 4.3.4) (see Fig. 10).

Figs. 15(b) and (c) show decrease in the networkthroughput and increase in delay for all protocols as gridis perturbed. MRP shows better throughput and reason-able delay performance compared to other protocols.Among three versions of MRP, MRP-H performs best.

Fig. 15(d) shows that the hop-count of MRP slightlyincreases, while that of all others decreases. Again, this isthe effect of the route selection procedure that prefers sta-ble routes to lower hop-counts.

4.4.5. Intra-mesh traffic ratio

The intra-mesh traffic ratio is varied in the same manneras in the Poisson case. Intra-mesh traffic ratio is increasedfrom zero to 0.8. The results are shown in Fig. 16.


In Fig. 16(a), on-demand protocols such as AODV,DSR, and ZRP (ZRP uses on-demand approach forinter-zone routing) show an increased overhead becausethese protocols have to perform route discovery for desti-nations located farther away. As expected, MRP andtable-driven protocols show almost constant overhead.

Fig. 16(b) shows increased network throughput forAODV, DSR, OLSR, and ZRP, while MRP shows adecrease. This result is expected because the former proto-cols are designed for generic ad-hoc networks with destina-tions at arbitrary locations in the network. In contrast,routes in MRP always involve the common parent of thesource and the destination. The resulting suboptimal routesare also reflected in the delay and hop-count performanceshown in Fig. 16(c) and (d), respectively.

The simulation results show that MRP performs well inWMNs that have the most user traffic to and from the gate-


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way (and intra-mesh traffic is a small percentage of theoverall traffic).

In this section, we showed that for WMNs, in mostcases, MRP outperforms the other protocols for the con-sidered metrics. MRP-H shows less overhead, higherPDR (for the Poisson traffic model), higher networkthroughput (for the Internet traffic model), and lowerdelay. MRP shows slightly higher hop-count than someof the other protocols as its route selection algorithm pre-fers stabler rather than smaller hop-count routes. MRPconsistently performed better than others when traffic load,network size, mobility, and perturbation were increased.As MRP is optimized for WMNs, when intra-mesh trafficratio is increased, MRP’s advantage withers. Among theMRP versions, MRP-H showed the best performance, fol-lowed closely by MRP-B (although with twice the overheadof MRP-H). As expected, MRP-S consistently showedslightly improved delay and hop count at the expense of


a decrease in PDR and a slight increase in overhead.MRP-OD showed relatively poor performance comparedto the other versions.

The reduction in overhead and the use of stable routesare the two main sources of performance increase thatare enabled by assumptions only valid for WMNs. Theassumption that most flows are from and to the gateway,enables MRP to form and maintain routes only to the gate-way (thus, reducing the overhead). The assumption of afixed backbone enabled MRP to leverage the stable linksof the backbone at a cost of a slight increase in the pathlength.

5. Conclusion

Wireless mesh networks are becoming increasingly pop-ular as they have significant advantages over competingtechnologies. In this paper, we introduce a new routing


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protocol specifically designed for those networks. Thedesign of the proposed routing protocol takes advantageof the particularities of WMNs, only maintaining routing

trees to and from the gateways. Simulation results showthat, for WMNs, in most situations, the proposed protocoloutperforms existing Internet and MANET routingprotocols.

The proposed protocol is very simple, and, conse-quently easy to implement. It is suitable for wirelessmesh networks used for Internet access as it requires agateway for its correct operations. For the same reason,it is unsuitable for networks without a centralized struc-ture (e.g., car-to-car networks or pure MANETs). Theprotocol performs best when used in most commonWMN scenarios where most of the traffic flows arebetween the centralized gateway and the network nodes(as opposed to between peer nodes in the network).However, when the intra-mesh traffic dominates the total


traffic a traditional (or a MANET) routing protocol maybe a better choice.

We believe that the performance of the proposed proto-col can be further improved if multiple routes are used(e.g., incoming and outgoing flows to the same or differentgateways). We will explore this option in future work.

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