A Novel Routing Algorithm Based on Chord forHybrid Wireless Ad-hoc Networks

Jing Cao1 State Key Laboratory of Virtual Reality

Technology and Systems, Beijing 100083, China2 School of Computer Science, Beihang University,

Beijing 100083, ChinaEmail: caojing@vrlab.buaa.edu.cn

Wei Wu1 State Key Laboratory of Virtual Reality

Technology and Systems, Beijing 100083, China2 School of Computer Science, Beihang University,

Beijing 100083, ChinaEmail: caojing@vrlab.buaa.edu.cn

Abstract—The scale of network and bandwidth of link havebeen limited by current routing technologies in hybrid wirelessAd-hoc networks, so we present a new routing algorithm. Inthe routing algorithm, the routing information is treated as aresource, and it can be stored and searched in a distributed waywith Chord. The routing of data packet prefers to forward byinfrastructure. In comparison with OLSR, the algorithm can notonly improve the network throughput by average 25%, but alsosave the delay of End-to-End by average 30%. And with theincreasing of network scale, the benefit will be more significant.

I. INTRODUCTION

Wireless Ad-hoc network is a kind of new network tech-nology based on the wireless communication. It has manycharacteristics, such as self-organization, wireless multi-hop,mobile, fast-built and etc. The ad-hoc network is a multi-hop autonomic system which is composed by some mobilenodes with wireless transceivers. Its nodes all have routingfunction. It is independent of the network infrastructure, easyto be built and extended, mobile, and strongly living. So ithas expansive application foreground in some situations whichneed build communication network quickly or is not easy toconstruct the network infrastructure beforehand, such as mili-tary affairs, incidents, scientific reviews in field, and personalcommunications. Hybrid Ad-hoc Network[2] is a multi-hopcommunication network abutting upon network infrastructure.It locates between wire-network and wireless ad-hoc network.

The routing technology is one of the main research ques-tions in Wireless Ad-hoc Network field. At the present time,routing protocols according to trigger time can be dividedinto table-driving routing and on-demand routing, includingAODV[3] , OLSR[4], DSR[5], WRP[6] and etc. Wireless Ad-hoc Network technology is limited by the amount of routingnodes, link bandwidth, and other factors, and many applica-tions are in the form of Hybrid Wireless Network. So we canmake use of the high-bandwidth and stable communicationlink of the network infrastructure to achieve routing. Literature[2] brought forward a routing technology, in which a mobilenode connects indirectly to the base station node throughthe wireless multi-hop relays, and the nodes of the networkinfrastructure participate in the hybrid network communicationas possible. Literature [7] conducted a model analysis of Hy-

brid Ad-hoc Network’s communication capability. Accordingto the different numbers of infrastructure nodes, it listed outthe upper limits for data throughput that each wireless nodecould achieve. Ivan and other persons studied Hybrid Ad-hoc Network communication model consisting of the ad-hocnetwork and some fixed access points[8]. They divided thenodes of Hybrid Ad-hoc Network into conventional nodesand special nodes which attached with some special features,such as the tiny sensor nodes and the communication nodesin the wireless sensor network, the wireless mobile nodes andthe fixed AP nodes in the wireless local network. They alsodiscussed the improved algorithm for broadcasting and routingprotocols in a pure wireless ad-hoc network.

However, the nodes are ad-hoc and mobile, and the networktopology is changing dynamically. The nodes accessing tothe network infrastructure need to know the routes to anymobile nodes, which ensure the communication with anynodes through the network infrastructure. So the costs forrouting information storage, maintenance and updating willsurge with the increasing of network scale and the amountof mobile nodes. In addition, the movement of a mobile nodemakes it can sometimes access to the network infrastructure orsometimes not. The current routing protocol can not supportsuch a node in change so as to make full use of the networkinfrastructure. To resolve such two problems, this paper in-troduces the distributed resources positioning technology ofthe structured P2P network into the routing technology inHybrid Ad-hoc Network. It will improve the performanceof communication and extend the scale of network by usingrouting calculation of the network infrastructure to support thewireless nodes. Resource Location is the basic key technologyof P2P network for determining the location of informationresources and getting the search strategy accessing to theinformation resources. Based on the difference of topologicalstructure and information transmitting models, it has followinglocation mechanism: the resource location technology basedon the structure searching [9,10] and the resource locationtechnology based on flood[11,12]. In this paper the method ofdistributed store and search the routing resource using Chordstructure[13] is a kind of resource location technology.

Using the routing algorithm in this paper, the mobile nodes

The 8th IFIP Annual Mediterranean Ad Hoc Networking Workshop 2009

978-1-4244-4661-2/09/$25.00 ©2009 IEEE 48

Fig. 1. Structure and Roles in Hybrid Network

can switch roles according to their own states. The distributedstorage and search of routing resources with Chord will avoidthe sharp increasing cost because of the nodes moving. Itwill also increase the throughput and reduce the end-to-endtransmission delay.

II. BASIC IDEA

A. Topological Structure and Related Definitions of HybridAd-hoc Network

The hybrid ad-hoc network studied in this paper is shownin Figure 1. Every mobile network node has been given only afixed IP address as a mark, and the mobile node’s IP address isnot changed with its location and the structure of the networktopology changing. There are two roles types about nodes.One type is ”the wireless mobile node” (Mobile Node: mobilenode or MN for short), another is ”the associate node accessto the network infrastructure” (Associate Node: associate nodeor AN for short).

MN node is an ad-hoc communication node in the hybridad-hoc network. There is only one network interface in it. Itcan communicate with the surrounding nodes through wirelesslinks only. AN node has not only the network interface tocommunicate with the surrounding ad-hoc nodes, but also theinterface to the network infrastructure. It can access to thenetwork infrastructure, and is the node joining the wirelesscommunication nodes and the network infrastructure. Allwireless network nodes can change their roles according toaccess the network infrastructure or not. And at any time theymay choose associate nodes in the network to communicateor move to the position where they can access to the net-work infrastructure. It means they can switch their own rolesaccording to different needs.

When the mobile node’s location changed, the associatenodes connecting to it will change too. So the associationswitches. In this paper, Associate Information (AI) is definedto describe this change. As Expression (1) shows below:

AI = {< destinationIP, associateIP > |desMobileIP ∈ MN, associateIP ∈ AN} (1)

If the entire network infrastructure is regarded as a partlytransparent one-hop link for the wireless network, the nexthop address through the infrastructure to the destination nodeis the associate node corresponded to the destination node.Therefore the associated information is corresponding to theresources needed to be maintained in P2P Resource Locationprocess. Any nodes in the network must maintain the routingresources to other nodes, namely Table Item (TI). Its definitionis shown in Expression (2):

TI =< destinationIP,NextHop, Metric > (2)

In it, the destionationIP, NextHop and Metric mean respec-tively destination address, the next hop address and the priorityof the next hop address.

Firstly the mobile node in the hybrid ad-hoc network hasto establish association to the associate nodes. Secondly thenode will search the route to the destination node throughthe network infrastructure. The next-hop node through theinfrastructure to the destination node is the associate nodecorresponding to the destination node. So the table item TIcan be further described as Expression (3):

TI =< destinationIP, associateIP >= AI (3)

B. Routing Resources Distributed Storage

When the node moves, the change of associate informationwill cause the network routing changed. Such changes needinformation mechanism to achieve. The general operation is tomake every node in the network to maintain the routing tableitems to all other nodes when the IP addresses of the nodes arestable. The exchange of routing information can be achievedby broadcast. However, with the expansion of network scalethis method of maintaining and exchanging information is toomuch on the cost. This paper introduces a way that maintainall the associate nodes with a Chord ring, and the routingtable items TI will be abstract as a kind of routing resourcein the P2P network. So calculating the route to destinationIPis the process of looking for the associate node’s address(associatedIP) whose keyword is destinationIP. Every node inChord must maintain a Finger table structure, which is definedas follow: every associate node maintains M items in Chordwhich accommodates 2M associate nodes at most. The item iis consisted of Starti and Successor (Starti), such as Expression(4) and (7) below:

Starti = (CurrentNodeID + 2i−1)mod2M (i ∈ [1,M ])(4)

AssociateNodeSet = {node|node ∈ WirelessNodeSet

&node ∈ LanNodeSet}(5)

SuccNodeSet(key) = {node|node ∈ AssociateNodeSet

&Hash(node) ≥ Hash(key)}(6)

49

Fig. 2. Example of Associate Nodes and Corresponding Chord

Successor(key) = {node|(node, node′) ∈ SuccNodeSet

&Hash(node) ≥ Hash(node′)}(7)

In Expression (4), Starti is the ID value of the associate nodewhose distance is 2i-1 from the current node CurrentNodeIDwhen the mode is 2M.

In Expression (5), the set of associate nodes is defined asthe nodes who can both communicate with the mobile nodes(AssociateNodeSet) and access to the network infrastructure(LanNodeSet).

In Expression (6), the set of subsequent nodes whosekeyword key is defined as the associate nodes whose hashvalues are not less than Hash(key) in the set of associate nodes.

In Expression (7), Successor(key) is the first access nodewhose id is not less than the value key in the subsequent nodesset of the node whose keyword is key. This is also called afinger value of the node, marked with finger[key]. The secondelement in Finger item is Successor(Starti). Figure 2 shows anexample of Chord structure when M=6 and the node’s fingertable that ID is 8. Among it, the solid line arrow is pointing tothe node’s position in Chord corresponding to the value Startiof every item in the finger table. The dotted curve arrow ispointing to the subsequent nodes of the current node, namelythe nodes corresponding to the item Successor(Starti). It isworth noting that the value Successor(Starti) must be pointingto a real node in Chord, although the value Starti perhaps not.

In addition, each node in Chord needs to maintain theinformation of its former and subsequent nodes. As shownin 2, any node’s former node is the first node anti-clockwiseand the subsequent node is the first node clockwise. The fingervalue of the subsequent node is equal to Finger[1] in the fingertable.

In this paper we used MD5 algorithm to calculate the IPaddress Hash value for the cable communication interfaceof each associate node, in order to calculate the unique ID

Fig. 3. Node Structure of Chord on Resources Distributed Storage

value of the associate node in Chord. At the same time thedestination node’s IP address Hash value is also calculated inthe table item, in order to confirm the node which stores thetable item in the whole Chord. To ensure that every relevantnode storing the routing tables is relatively balanced, in thispaper we used the method that the routing table items storedby all relevant nodes are distributed throughout the network.The routing table items maintained by each associate node areparts of the overall distributed routing table. It has been storedall the routing information resources among the current, theformer and the subsequent node.

When the node becomes an associate node or an ordinarymobile node, the structure content of Chord, namely thecontent of the Finger table, is changed correspondingly. Sothe routing problem is abstract as the problem to look up therouting information resources in the whole network. It mustbe pointed out that the time-complexity on searching in Chordis only O(log n), and it won’t increase dramatically followingthe network scale’s increasing. This is the main reason whywe chose Chord ring for storage in this paper.

C. Routing Algorithm

According to the roles of nodes and the distributed storagestructures of the routing table items, the routing data packetcan be divided into four types:

1 The source node and the destination node are all ANnodes;

2 The source node is MN node, and the destination node isAN node;

3 The source node is AN node, and the destination node isMN node;

4 The source node and the destination node are all MNnodes.

The first type is the core of this routing algorithm. Thealgorithm flow is shown in Table 1:

In the routing process described in Type 1), in order notto change the configuration of the network infrastructure andthe routing table structure, we carried out the method that thedata are forwarded by the application layer’s tunnels in thenetwork infrastructure. When the data packet is imported tothe network infrastructure, the associate node of the sourcenode packages it, and then delivers it to the associate node ofthe destination node. When the data packet is reported fromthe network infrastructure, the associate node which received

50

Algorithm 1: FindNextHopInput : src, dest.Output: NextHop.//On a source AN node searching the next hop address todestination AN nodeNextHop ←− FALSEfor i ∈ [0,M ] do

// M is the number of items stored by the AN node ifSuccussor(src + 2i−1) = Hash(dest) then

NextHop ←− Succussor(src + 2i−1)break

returnNextHop

Algorithm 2: Routing Algorithm of Source Node andDestination Node Both AN NodesInput : src, dest.Output: NextHop.// Access the copy of an IP Packet and get thedestionationIP; // Get the IP address of CurrentNode andassign it to CurrentNodeIP;DestinationID ←− HASH(destionationIP )CurrentNodeID ←− HASH(CurrentNodeIP )if FindNextHop(CurrentNodeID, DestionationID)then

Transfer the packet to the nextHop;return;

else// Search recursively the destination AN node in theChord ring of the entire network fornodeID ∈ SuccNodeSet(CurrentNodeID) do

if FindNextHop(nodeID, destinationID)then

Return nexthop to the CurrentNode, Transferthe packet to the nextHopreturn

elseCurrentNodeID ←− nodeID

data releases the data packet and delivers it to the next-hopnode of the wireless network according to the wireless networkrouting table maintained itself.

Type 2) and Type 3) are an expansion of Type 1). Theirrouting processes are divided two parts, from MN to AN andfrom AN to AN. It is the association AI that from MN to AN,and it is described in Type 1) that from AN to AN.

Analogously, Type 4) is a moreexpansion of Type 2) or Type 3), but it still exists that the

data are forwarded from the source MN to the destinationMN through the wireless link. So choosing the forward pathis the problem must be faced to. As shown in Figure 4, thenode MN1 sends some data to the node MN4. The forwardpaths thought about to choose are a)MN1-¿AN1-¿the networkinfrastructure -¿AN3-¿MN4 and b)MN1-¿MN2-¿AN2-¿MN3-

Fig. 4. Forward Path Selected

¿MN4. Considering such a case, we adopted the greedyalgorithm. The forward path is chose according to the amountof hops to the associate nodes and the destination nodes. Thepath is chose preferentially, in which the hops in the wirelessnetwork are the least before the data imported to the networkinfrastructure. The data will be sent to the associate node toforward through the network infrastructure if the hops to theassociate node are few. If the hops are same to the associatenode and to the destination node, or the hops to the associatenode are more, the data will be forwarded through the wirelessnetwork. Such a choice policy won’t ensure it is the best pathfrom the source node to the destination, but it is the path inwhich the cost will be lower.

III. EXPERIMENT EVALUATE

We used the simulated experiments to analysis the time coston the routing search and the bandwidth cost of this algorithm.And the network throughput, the end-to-end translation delay,and other performance parameters are contrasted with OLSR.The experiments validated the purpose when the network scaleincreased in a network scene combining the physical andthe emulate parts. The emulate mode was to startup manyrouting protocol courses to simulate many network nodes,and the associate nodes are organized with Chord. The wholeexperiment finished by 10 18 computers, including 4 notebookcomputers with the wireless network-card supporting 802.11g.The mutual messages were produced timely by the timers inthe experiment process, and they simulated the communicateevents in the real network.

A. Routing Cost

Figure 5(1) is the statistics about the average routing searchtime. There were 50 400 routing protocol procedures. Everyprocedure simulated an associate node in the network. Thesimulated associate nodes stored 10 times routing informationthan the associate nodes, and timely they could advance therouting search requests on occasion. The procedures recordedthe routing search time cost every time and made statistics.From the figure we could find out that for a certain node thetime used to search the routing source information increasedgradually with the amount of the nodes increased. But the timeincreasing scope reduced. This is consistent with the time-complexity O(logN) of searching resource on Chord basically.

51

(a)

(b)

Fig. 5. Analysis and Contrast on Routing Cost

Figure 5(2) shows the statistics result about the costs on thenetwork structure maintenance, the routing search, the cableinterface transmitting and the wireless interface transmitting,when 30-240 associate nodes generated randomly 100 timesthan the number of nodes for routing and updating requestsin 200 seconds. As the network maintenance expenses andthe wireless network interface costs are regularly sent to thefixed-size messages, the expenses has nothing to do with thenumber of the nodes and is a constant. It didn’t increase withthe number of the nodes increasing, so the cost on routingquery and updating impacted the network expansion. When thenumber of the associate nodes increased from 30 to eight timesmore than before, each node’s cost on query and updating onlyincreased by 60

B. Comparative Analysis between routing algorithm andOLSR

This comparative analysis about this routing algorithm andOLSR focuses on throughput and latency two aspects. Theexperiments are consisted of five experiments, including dataforwarding on 1 to 4 hops with OLSR and data forwardingthrough the network infrastructure with the routing algorithmdescribed in this paper (2 A2M, meaning two associate nodesand two mobile nodes). The results are shown in Figure 6.

As shown in Figure 6(1), with OLSR the fluctuation extentof the end-to-end node’s throughput increased with the numberof hops increasing when there were different numbers ofhops between nodes. But it wasn’t affected basically withthe algorithm described in this paper. The reason is thatany interference is likely to cause the network link-off andmessage packets losing, if the network topology becomes

(a)

(b)

Fig. 6. Throughput Changes on Different End-to-End Links

unstable with the number of hops increasing. So the end-to-end data throughput fluctuates in different time. It is shown inFigure 6(2) that throughput deviates from its expected value ofstandard deviation statistics. The routing algorithm describedin this paper can make the data to forward through the networkinfrastructure from a wireless communication node to another.So the throughput won’t be reduced like in the wireless multi-hops network because of messages sending and receivingat the turn. Experimental results show that if the networkinfrastructure exists and only a data stream is transmitting,the throughput using the algorithm described in this paperwill arrive at 75 percent of the wireless one-hop transmissionthroughput. It is more than 80 percent of the throughput whenthere are multi-hop routing (more than two hops). At the sametime, because of the effective use of the cable link in thisalgorithm, its end-to-end stability is better than OLSR. Thestandard deviation of throughput over time is also the smallest.

The contentions of sharing channels and channel conflictswill increase with the hops increasing, so the number ofwireless hops is the main effect factor of network throughput inthis routing algorithm. Comparatively OLSR is more impactedby the scale of message. The network throughput with thisalgorithm will decrease with the wireless hops increasing.As shown in Figure 7(1), the throughput with this routingalgorithm is between the throughput in one or two hops linkswith OLSR. It is because that forwarding data by the cablenetwork improves the network throughput and reduces theoccupier on the wireless link. The transmission delay usingthe routing algorithm is more than it in the wireless ad-hoctwo-hop link, and a little less than it in the wireless ad-hocthree-hop link.

IV. CONCLUSIONS

In the view of current hybrid ad-hoc network routingtechnologies constrained by the scale of the amount of nodes,

52

(a) Network Throughput

(b) Transfer Delay

Fig. 7. Performance Compare between OLSR and Chord-based RoutingAlgorithm

the link bandwidth and other factors, we present a routingalgorithm in the hybrid ad-hoc network based on Chord.It prefers to forward the data packet through the networkinfrastructure. In this paper, the experiments evaluated therouting cost, the average time to search route and otherperformance parameters. And for the wireless communicationover two hops, the network throughput with the chord-basedrouting algorithm is 25% improvement and the end-to-endcommunication delay is 30% lower than with OLSR. Thefuture work of our research is to study traversing in the hybridad-hoc private network and switching the associate nodes’relations in the link layer.

ACKNOWLEDGMENT

This work was supported in part by the China 863 Programunder Grant 2006AA01Z259. Jing Cao is with Beijing Uni-versity of Aeronautics & Astronautics, Beijing 100191 China(e-mail: caojing.china@ gmail.com). Wei Wu is with BeijingUniversity of Aeronautics & Astronautics, Beijing, 100191China. He is now with the School of Computer Science (e-mail: wuwei@vrlab.buaa.edu.cn).

REFERENCES

[1] Ramanathan Ram and REDI Jason, A Brief Overview of ad hoc net-works: challenges and directions, IEEE Communications Magazine, 50thAnniversory Commemorative Issus, May 2002

[2] Salem N.B, Buttyn Levente and Hubaux Jean-Pierre, Node Cooperationin Hybrid Ad hoc Networks. IEEE Transactions on Mobile Computing[C],2006: 365-376.

[3] Perkins C.E, Royer E.M and Das S.R, Ad hoc On-Demand DistanceVector (AODV) Routing,http://www.ietf.org/rfc/rfc3561.txt, 2003

[4] Jacquet, Muhlethaler P, and Qayyum, A Optimized Link State RoutingProtocol, http://www.ietf.org/rfc/rfc3626.txt, 2003.

[5] Johnson David B, Maltz David A and Hu Yih-Chun, The Dy-namic Source Routing Protocol for Mobile Ad hoc Network(DSR).http://www.ietf.org/internet-drafts/draft-ietf- manet-dsr-10.txt, 2003.

[6] Murthy S, Garcia-Luna, Aceves J J. An Effient Routing Protocol forWireless Networks. ACM Mobile Networks and Applications Journal,1996:1(2):183-197.

[7] Zemlianov A, Veciana G.D. Capacity of ad hoc wireless networks withinfrastructure support. IEEE Journal on Selected Areas in Comunications-vol. 23, 2005: 657-667.

[8] Ingelrest Fran, Simplot-Ryl David, Stojmenovic Ivan. Routing and Broad-casting in Hybrid Ad hoc Networks. Canada: University of Ottawa, 2004.

[9] Ratnasanry S, Francis P, Handley M, et al. A scalable content-addressablenetwork. ACM SIGCOMM Computer Communication Review[C]. SanDiego: ACM Press, 2001.

[10] Rowstron A, Druschel P. Pastry: Scalable, distributed object locationand routing for large-scale peer-to-peer systems. Proceedings of the Int’lConf. on Distributed Systems Platforms (Middleware 2001). Heidelberg:Springer-Verlag, 2001.

[11] Jiang Song, Guo Lei, Zhang Xiaodong. LightFlood: an Efficient Flood-ing Scheme for File Search in Unstructured Peer-to-Peer Systems. Pro-ceedings of the 2003 International Conference on Parallel Processing(ICPP’03), 2003.

[12] Jiang Song, Zhang Xiaodong. FloodTrail: an Efficient File SearchTechnology in Unstructured Peer-to-Peer Systems. GLOBECOM2003,2003.

[13] Stoica I, Morris R, Karger D, et al. Chord: A scalable peer-to-peerlookup service for Internet applications[J]. ACM SIGCOMM ComputerCommunication Review. San Diego: ACM Press, 2001.

53