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QoS Routing in Wireless Mesh Networks

Vinod Kone, Sudipto DasUniversity of California, Santa Barbara

ABSTRACTWireless Mesh Networks (WMNs) have been attracting bothacademia and the industry off late due to their inherent advan-tages of providing seamless broadband connectivity to com-munity users. Though there has been some recent research onrouting in these networks, a preliminary survery of literaturetells that not much has been done regarding providing Qualityof Service. This paper studies the performance of AQOR, aQoS aware routing protocol proposed primarily for MANETsand adopted to, but not adapted to, WMNs. We also propose anew protocol WMESH which addresses these drawbacks andis optimized for WMNs. Preliminary Qualnet simulation re-sults show the performance advantages of WMESH over otherrouting protocols.

1. INTRODUCTIONThe last couple of years have seen a huge growth

in the popularity of Wireless Mesh Networks (WMNs).Infrastructural Wireless Networks (like Access Points)provided unprecedented freedom of mobility to the Lap-top and PDA users by eliminating the need for wires. Butthe problem with such networks is that the very devicesthat provide “wireless” connectivity are themselves con-nected to the internet through wires. The Wireless Meshoffers a breakthrough approach that enables making theleap from localized HotSpots (provided by Infrastruc-ture Wireless Networks) to fully wireless HotZones withbuilding-wide or campus-wide coverage and even HotRegions that span an entire metropolitan area. On theother hand, Mobile Ad-hoc Networks (MANETs) [1]have also gained widespread popularity. This is becauseof the ease of deployment of such networks and the ex-tended connectivity they provide by means of multi-hoproutes.

We combine the features of both these networks todesign a Network Architecture which is a hybrid of thetwo discussed above. As shown in Fig. 1, our networkconsists of three types of nodes, Mesh Clients (which arethe end users), Mesh Routers (routers that communicatewith the Mesh Clients and other Mesh Routers) and In-ternet Gateway (Router that communicates with the MeshRouters within the network and the Internet)

The Novelty in our network is that we use Layer 3routing throughout the expanse of the network. Thishelps in having multi-hop routes and hence incorporat-ing the advantages of MANETs, viz. ease of deploymentand extended connectivity. In addition, the same rout-ing protocol runs on the Mesh Routers as well as theMesh Clients. The routing protocol modifies its behav-ior depending on the node on which it is being executed.

The Mesh Routers have two interfaces, operating on or-thogonal different channels, one for communicating withthe Mesh Clients and other for communicating with theother Mesh Routers. The Mesh Clients have only oneinterface. This network architecture allows communi-cation between nodes within the network, which is dif-ferent from what WMN does but similar to MANETs.Three types of routes are possible: (i) two communi-cating hosts are under the same Mesh Router, (ii) thecommunicating clients are located under different meshrouters and (iii) a Mesh Client communicates with ahost in the Internet. The protocol “intelligently” allowsrouting between all the possible routes. The WMESHrouting protocol provides Quality of Service (QoS) guar-antees from source to destination when communicatingend-points are within the network and from the client tothe internet gateway when the traffic is to the Internet.QoS metrics are Minimum Bandwidth (Bmin) and MaximumEnd-to-End Delay(Tmax). In cases where multiple routesare available, the protocol chooses the MOST Stableroute available. Another novel feature of our networkis the addressing scheme that has been used. These areexplained in further details in later sections.

Such networks have widespread applications becauseof its ease for deployment. In addition to all the fea-tures of WMNs, additional applications of our networkarchitecture is Video On Demand applications and Peer-to-Peer file sharing applications like Bit Torrents. Thisis because in our architecture two nodes under one MeshRouter can communicate with each other without exter-nal intervention and hence the requests that can be metlocally need not be sent outside the network.

In this paper, we study the performance of AQOR (Ad-hoc QoS On-demand Routing Protocol) [8] and it variantfor WMNs, known as WMR (Wireless Mesh Routing)[7], a QoS enabled routing protocol for wireless meshnetworks. We then optimize this protocol so that it canadapt to the WMN architecture and take advantage ofthe features of WMNs. Outcome is the WMESH routingprotocol that adapts to the WMN architecture.

The rest of the paper is organized as follows: Section2 gives some insights into the work carried out in thisfield, Section 3 explains the WMESH routing protocolin details, Section 4 provides simulation results in com-parison with AQOR and AODV [5, 6], while Section 5concludes the paper.

2. RELATED WORK AND MOTIVATIONWireless Mesh Networks is a “hot topic” in network

research [3,4] in recent years. But very little research has

Figure 1: Network Architecture

been done in designing a QoS based routing protocol forWMNs. One of the more popular solutions is AQOR andWMR. In this section we describe in brief how WMRworks. The WMR protocol is based on AQOR andincludes the following aspects:

Topology Discovery: This is the procedure that maintainsneighborhood information for each node by localinformation exchange. This is done by sending outperiodic beacons (HELLO packets).

Route Discovery: This algorithm uses the topology in-formation to obtain the route to destination. Theroutes are discovered on-demand. For internal traf-fic, the route is obtained from the source to thedestination while for external traffic the route isobtained to the nearest node that provides externalconnectivity. Again, Route Discovery consists oftwo phases, Route Exploration when the route isdiscovered, and Route Registration when with thereceipt of the first data packet all resources for thatflow are activated

Admission Control: This is the module that helps decidewhether a flow with the given QoS constraints willbe accepted or rejected. AQOR uses the followingmethodology for calculating the Bandwidth con-sumed at each node:

Bconsumed(I, j) = Buplink(I)(j) + Bdownlink(I)(j)(1)

Bavailable(I) = B −

X

J∈N(I)

Bself (J) (2)

Here, Bavailable(I) is the adaptive lower bound ofthe real available bandwidth, Bself (I) is the totalreserved bandwidth of all the existing flows at node

Figure 2: RREQs forwarded at each node in AQOR

I, Bconsumed(I, j) is an upper bound on the actualbandwidth consumed by a node along the path,Buplink(I)(j) and Bdownlink(I)(j) are the bandwidthconsumed by uplink and downlink neighbors re-spectively. By comparing Bavailable(I) and Bconsumed(I, j),each node can now decide whether to accept theflow or not.

Since AQOR and WMR are very identical protocols,in the rest of this paper we use the term AQOR for de-scribing the semantics of AQOR and WMR to avoidconfusion. With all these mechanisms in place, AQORseems to be a good routing protocol that guarantees effi-cient routing with QoS guarantees. But AQOR doesnottake advantage of the WMN network architecture wherethe routing is concerned. As a result, it causes unneces-sary control overhead as seen in Fig. 2. This is the dataobtained when 6000 data packets were transferred by 10flows in the network in a simulation of 600 seconds. Itcan be clearly seen that the number of Route Requests(RREQs) forwarded at each node is very high.

Since this is an on demand protocol, RREQs not onlyresult in high control overhead in the network, but it also

results in caching of packets at the source, that leads toincreased delays and jitter. From a closer analysis, itcan be inferred that the high overhead is because of thefollowing reasons:

• The Mesh Routers blindly forward the RREQswhich can be pruned, thereby reducing unneces-sary messages.

• AQOR makes route entries which it cannot everuse, like making route entries for neighbors. Thisresults in excess control overhead when these routestime out.

• AQOR never takes stability of the routes into ac-count. Hence unstable but shorter delay routes maybe chosen, resulting in frequent route breakages.

All these optimizations, applied to AQOR results inWMESH, our routing protocol for Wireless Mesh Net-works.

3. WMESH ROUTING PROTOCOLThe objective of WMESH routing protocol is to pro-

vide a QoS constrained route from source to destination.Specifically, the route selected by the protocol shouldprovide the minimum bandwidth Bmin requested by theapplication and the end to end latency shouldnot exceedTmax. In addition, we require the route to be the moststable among all possible candidate routes satisfying theabove constraints.

During the route discovery phase of the protocol, eachintermediate node uses an admission control scheme tocheck whether the flow can be accepted or not. If ac-cepted, a FLOW TABLE entry for that particular flowis created. We borrow the admission control schemeof AQOR which was explained in the previous section.Though a reservation is made for a certain flow, its onlyactivated when the node receives data packets. If a nodedoesnt receive data packets within 2Tmax, it means thatthe source has selected a different route and hence theFLOW TABLE entry is deleted. Admission control de-cision is also made when a node receives Route Replies(RREP) thus providing more consistency. For aidingadmission control, each node collects the bandwidth re-served at its one hop neighbors (piggybacked on HELLOpackets) and stores it in its NEIGHBOR TABLE.

WMESH routing is based on the AQOR scheme al-though there are several key differences between them.As explained earlier, the main drawback of AQOR isthat it doesnt not leverage the mesh network topology.In contrast, WMESH treats mesh routers and clients dif-ferently. This is described in detail in the next subsec-tion. Another key difference is that, AQOR selects theroute on which the first in-time RREP arrives, whereasWMESH selects a route which is more stable, althoughwe do include an option to default to the AQOR behav-ior. This essentially means that WMESH deals with 3metrics (bandwidth, delay and stability) whereas AQORdeals with the first 2 metrics.

3.1 Intelligent Routing

Intelligent routing deals with selective flooding of thecontrol packets. To put this into perspective, it can berecalled that the main application scenario envisaged forWMNs is video on demand where most of traffic is re-stricted within a mesh group. In such a case, broadcast-ing the control packets outside the mesh group doesntmake any sense. Moreover, it adds to the congestion ofnetwork which could be disruptive to other flows.

We propose two schemes to achieve the intelligentrouting. In the first scheme, we augment the IP headerof the packets with a Group Id (GID) header. All meshnodes belonging to a mesh group have the same GID,broadcasted to them by the mesh router in charge of thatgroup. A hash of the IP address of the mesh router it-self can be taken as the GID of the group. All the meshrouters broadcast the GIDs among themselves and main-tain a table which tells the next hop router for a particularGID. When a control packet broadcasted by the sourcereaches the mesh router, it first checks whether the des-tination node belongs to the same group or not. If yes,it only broadcasts the packet on the interface attachedto the group. Otherwise, it broacasts the packet on allits attached interfaces. The second scheme, is a coarseversion of the first scheme in the sense that there is noexchange of GIDs among the mesh routers. Instead, themesh router broadcasts the control packet into its owngroup if the subnet address of the destination belongsto the group’s subnet and broadcasts on all its interfacesotherwise. Here the assumtion is that, the subnet of dif-ferent mesh groups is unique. For our implementation,we adopted the second scheme for its simplicity andavoidance of extra control packets between routers.

3.2 Calculating Stability Of A RouteEach node in the network estimates the stability of

its one-hop neighbors. We define stability based onthe number of HELLO packets received by a node. Toexplain this more clearly, let us assume that nodes B

and C are the one hop neighbors of A. If in a par-ticular interval, A receives x HELLO packets from B

and y packets from C, and x < y then from the view-point of A, C is more stable than B. In addition,each node also takes into account the stability historyof its neighbor when estimating its stability. Formally,let every node calculate the number of HELLO pack-ets received from its neighbors periodically for everyWMESH STABILITY INTERVAL seconds. We defineCurrentStability as the percentage of HELLO packetsreceived with respect to the expected number of HELLOpackets in the last WMESH STABILITY INTERVAL.Let α be the weight factor for calculation of a neighbor’sstability. Stability of a neighbor is calculated as follows:

Stability = (α)CurrentStability+(1−α)Stability (3)

Every node maintains the Stability of its neighborsin its NEIGHBOR TABLE. To calculate, the stability ofthe route, control packets are piggybacked with stabilityinformation which are updated by intermediate nodesalong the route. To explain further, let source S needs tofind a route to destination D. During the route discoveryphase, S initializes Stability to 0 and piggybacks this

information on the RREQ packets. When a node I re-ceives a RREQ packet from previous hop P , it adds theStability of P to the stability field of the RREQ packet.Thus Reverse Route Stability (RRS) is the sum of sta-bilities of all the nodes along the path. Analogously, wecalculate the Forward Route Stability (RRS) by piggy-backing stability info on RREP packets and updating italong the route. To negate the effect of higher stabili-ties for longer routes, we take the hopcount of the routeinto account. Formally, RouteStability is calculated asfollows:

RouteStability =(FRS + RRS)

2 × HopCount(4)

3.3 Tackling Misbehaving NodesOne of the key differences between WMESH and

AQOR is the ability to tackle misbehaving nodes. AQORassumes the network to be a completely trusted environ-ment and hence may face problems in a real network con-sisting of malicious nodes. More specifically, we definea malcious node as one which doesnt route others packetsbut gets its packets routed to others. A malicious nodecan easily do this by not broadcasting HELLO packetsto its neighbors but receiving the neighbor’s broadcasts.Since the neighbors have no information about the mali-cious nodes, they select their routes via other neighbors.Moreover, when the malicious node wants to send pack-ets it simply uses the neighbors to route its packets.

To tackle this behavior, we propose a simple and intu-itive “Tit for Tat Rule” which is based on the stability ofa node. According to this rule, a node doesnot forwardthe packets of a neighbor if the neigbhor’s stability islower than a certain threshold. In the above scenario,since the malicious node suppresses its HELLO packets,its stability tends to 0 in the viewpoint of its neighbors.So, when the malicious node tries to send its own pack-ets, they are dropped by the neighbors. In effect, thestability metric imposes a co-operative environment inthe network.

3.4 QoS RecoveryWMESH detects QoS violations with the help of reser-

vation timeouts and FLOW TABLE entries. We identifytwo different QoS violations as follows. In the first case,an intermediate node receives a data packet but doesn’thave a corresponding FLOW TABLE entry for that flow.This essentially means that, the node has deleted the flowtable entry because of a reservation timeout. Hence, itsends a Route Error (RERR) packet back to the sourcewhich then re-initiates a route discovery procedure andreroutes the packets. A second case is where the des-tination detects, with the help of its FLOW TABLE,that data packets arriving at it are exceeding the Tmax

requested by the source. In this case, the destinationincrements its sequence number and broadcasts an un-solicited RREP back to the source. On receipt of thisRREP, the source immediately reroutes packets via thepath travelled by this RREP thus avoiding the lengthyrerouting process. This scheme is similar to QoS recov-ery scheme of AQOR.

Figure 3: Packet Delivery Ratio

4. EVALUATIONThe simulation of WMESH and AQOR was done in

Qualnet [2] simulator. For implementing AQOR, weadhered to the specifications given by the authors [8].The performance of the protocol is also compared withnon QoS aware AODV to show its effectiveness.

Our scenario consists of a total of 14 nodes (8 meshrouters and 6 mesh clients) which include two meshgroups and an internet gateway node. All the meshrouters have two radio interfaces operating on orthog-onal frequencies. One interface is used communicateamong mesh clients in the router’s mesh group, if any,and the other is used for communicating among othermesh routers. All the nodes are static and are placedin a topology of 1500m × 1500m. The protocol is im-plemented on top of 802.11b MAC protocol with a rawchannel bandwidth of 2 Mbps at each node.

In all our simulations, the application traffic was CBRsending 512 byte packets at an inter departure rate of1 sec i.e., requiring a bandwidth Bmin of 4 Kbps. Thenetwork was allowed to stabilize for the first 10 secs,and then the flows were introduced to the network. Eachflow was alive for a period of 600 secs and the entiresimulation was run for 15 minutes. The Tmax for eachflow was roughly decided based on the expected hopcount between the source and the destination.

For stability calcuation, HELLO packets were broad-casted every 1 sec and the WMESH STABILITY INTERVALwas set to 10 secs. Also, an equal weight was given forthe stability history of a node and its stability in the lastWMESH STABILITY INTERVAL (α = 0.5).

4.1 Scenario 1In this scenario, we evaluated the performance char-

acteristics of WMESH compared to AQOR and AODVin the presence of 10 flows in the network. The metricsused for comparison are packet delivery ratio and endto end latency of data packets. Packet delivery ratio isdefined as the ratio of total no.of packets received by adestination to the total no.of packets sent to the destina-tion. Packet Delivery Ratio(PDR) shows the effectivnessof the routing protocol.

From Fig. 3 we can observe that WMESH deliverspackets effectively irrespective of the type (inter group,intra group) of the flow. The PDR of WMESH is greaterthan 0.99 in all cases and performs as good as AODV

Figure 4: Average End to End Latency

Figure 5: Normalized Control Overhead

and AQOR, if not better. The difference is pronouncedat node 12 which is a flow between a mesh router and themesh client of a different group. Even though AODVand AQOR both lose packets, WMESH acheives bestperformance because of its intelligent routing. Sincethe control packet overhead in the mesh network of therouters is very less for WMESH, there is very less con-gestion and hence almost zero packet loss.

Fig. 4 plots the average end to end latency of thedifferent flows. It is worth noting that among the threedifferent protocols only WMESH satisfies the averageend to end latency for all the flows. In particular, Tmax

value for the flows whose destinations were 12 and 14was 18 ms. For both these flows WMESH gives anaverage latency of approx. 10ms whereas AQOR goesbeyond the acceptable limits. This shows that selectiverouting combined with stable path selections result inmuch better QoS behavior.

4.2 Scenario 2In this scenario, we test the performance of WMESH

with varying number of flows. Specificially, we simulatethe protocols in the above described network with 1,3,5,7and 10 flows and observed the behavior. The flows werea random mix of inter group flows and intra group flows.

We define Normalized Control Overhead as the ratio oftotal no.of RREQ packets forwarded at each node in thenetwork to the total number of data packets received bythe destinations in the network. This metric provides a

Figure 6: Average Packet Delivery Ratio

Figure 7: Average End to End Latency

measure of the amount of extra overhead required by thedifferent protocols for routing data packets. From Fig. 5it can be seen that WMESH performs significantly betterthan either AODV or AQOR. While the control overheadgoes as high as 50% for AODV and AQOR, WMESH hasconsistenly lower overhead of less than 10%. This resultsupports our claim that intelligent routing do reduce thecontrol overhead by a significant amount thus enablingbetter routing performance for data packets.

Figs.6 and 7 show the packet delivery ratio and endto end latency characteristics of WMESH with vary-ing amount of flows in the network. In both the casesWMESH outperforms the other two protocols, thus show-ing the validity of the optimization techniques proposedin this paper. Another observation from Fig.7 is thatWMESH adapts to the network efficiently than the othertwo contemporary protocols. As number of flows in-crease, WMESH succeeds in selecting better alternateroutes based on stability and thus producing less averageend to end delay and higher packet delivery ration.

5. CONCLUSIONIn this paper, we have developed WMESH, a QoS

aware routing protocol for Wireless Mesh Networks.In particular, the QoS metrics taken into account arebandwidth, end to end delay and stability of the routes.WMESH is based on AQOR routing protocol proposedfor MANETs but has significant differences with respectto routing and path selection. The stability metric pro-posed in this paper can be effectively used to tackle

misbehaving nodes in the network thus providing someamount security to the network. Preliminary simula-tion results in Qualnet simulator show that WMESHperforms significantly better than AQOR and AODV interms of packet delivery ratio, control overhead and endto end latency.

As future work, we plan to test WMESH in large scalemesh network and evaluate its performance characteris-tics. The adaptivity of our protocol in the presence ofmobile nodes is an aspect we are interested to look into.Finally, we would like to test the routing behavior ofWMESH in the presence of malicious nodes.

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