Throughput Enhancement through Efficient ChannelAssignment in Multi-radio Ad Hoc Networks

Ashish Sangwan* Ghanshyam Dass* Pradipta De**sangwan@cse. iitd. ernet. in csl 030163#cse. iitd. ernet. in pradipta. de@in. ibm. com

*Department of Computer Science, Indian Institute of Technology, Delhi**IBM, India Research Lab., Delhi 110016, India

Abstract - Use of a single wireless network inter-face on a device had been the standard practice fora long time due to hardware limitations. Designing amulti-hop ad hoc network with single NIC devices suf-fers from low bandwidth utilization due to co-channelinterference. Recent advances in wireless device tech-nology is spurring the emergence of several end userdevices with multiple radios on them. Wireless accessnetworks designed using these multi-radio devices hasbeen shown to give manifold improvement in through-put compared to single channel networks. However,these works have mainly focused on a gateway-basedmesh architecture. With growing number of multi-radio end user devices, we envision a future where puread hoc communication among these devices will be-come commonplace without a specific structure. Thispaper provides a distributed solution to the channelassignment problem to multiple NICs in such an adhoc scenario. The well-known routing protocols usedin single channel scenario, namely AODV is used forroute selection, but the potential for an improved rout-ing algorithm for multi-channel case has been high-lighted. We also provide an interface switching strat-egy so that the flows are not starved. We evaluateour channel assignment scheme through simulationson GloMoSim.

Keywords: Distributed Algorithms/Protocols,Design, Routing, AODV, Wireless Ad-Hoc Networks

Categories

[3c] - New signaling and control traffic in mul-timedia/multisystems [4f] - End to End serviceperformance evaluation

Huzur Saran*saran @cse. iitd. ernet. in110016, India

I. Introduction

Use of a single radio on a wireless access device hadbeen a prevalent practice due to limitations of hard-ware and vendor support. Single radio networks sufferfrom low network throughput in multi-hop topologydue to co-channel interference. This has spurred therecent research into using multiple radios on a singlewireless device. Each network interface card (NIC)is assigned a non-overlapping channel enabling simul-taneous interference-free transmission. Use of multi-ple NICs along with appropriate channel assignmentcan increase the overall network throughput manifold[4, 7, 9], and have been applied in architecting high-speed enterprise backbones based on multi-hop multi-NIC mesh topology.

Majority of the work in multi-channel multi-radionetworks has focused on building gateway-based wire-less access networks. With a surge in new devicesin the handheld market, presence of multiple NICsin end-users' wireless devices is a reality [6]. Thispresents the opportunity to explore the applicationof multi-radio devices in completely ad hoc environ-ments, like a disaster recovery environment or a hos-pital environment where multiple devices need to com-municate with each other simultaneously. In this ageof hi-tech hospitals, a doctor/nurse may need to main-tain simultaneous communication paths to several de-vices at the same time for patient monitoring as wellas sending/receiving alerts. Unlike the gateway-basedscenario for enterprise backbones, this poses a differentproblem scenario where tree-based architecture usedearlier may not apply.

The typical problems of designing a multi-radionetwork are all present in this new scenario: (a) as-signing channels to multiple NICs in a device so thatthere is minimal interference across hops, and (b) se-

lection of routes to maximize the throughput. Addi-tionally, when there is a limitation on the number ofNICs on a device and the number of flows is large, itintroduces the problem of timely switching of channelon an interface to prevent starvation of a flow througha node. In this paper, we mainly focus on designing adistributed channel assignment scheme that takes traf-fic load on a node as the key metric to assign channelsto the multiple radios. The routes are chosen basedon routes discovered by a well-known ad hoc rout-ing protocol AODV [8]. We show the improvementsgained through multiple radios and intelligent channelassignment even when using a standard routing pro-tocol. The starvation of flows is prevented by devisingan interface channel switching mechanism. We incor-porated interface switching in GloMoSim [3] networksimulator.

The rest of the paper is organized as follows. Sec-tion 2 provides a brief related work on the subject.Section 3 provides a modeling of a multi-channel multi-radio network and discusses how network is being ini-tialized and maintained. Section 4 describes the al-gorithms for the multi-radio ad hoc network, with afocus on the channel assignment scheme that we havedesigned. Section 5 discusses the performance of ouralgorithms on a 7 x 7 grid topology with randomlychosen flows. Section 6 concludes with a discussion ofthe future work.

II. Related Work

Channel assignment problem for multi-channel meshnetworks has attracted considerable attention of theresearch community. It has been proved that opti-mal channel assignment is a NP-hard problem [11].Several heuristics has been proposed with a typicalarchitecture of the network in context. For exam-ple, Raniwala et al. proposed a centralized load-awarechannel assignment and routing scheme for a gateway-based mesh network, that uses a multiple spanningtree based load balancing algorithm adaptive to chang-ing traffic loads [11]. Kyasanur et al. studies themulti-radio mesh network under the assumption of ahybrid setup, where a set of interfaces in a node canswitch channels dynamically to establish communica-tion with its neighbors, while the rest of the interfacesare bound to a specific channel [4]. They present a dis-tributed interface assignment strategy that includes

the cost of interface switching but is independent oftraffic characteristics.

On the other hand, Kodialam et al. has drawnthe theoretical bounds on achievable capacity for amulti-radio multi-channel wireless mesh network usinga primal-dual approach [7]. Similarly, [9] mathemati-cally formulate the joint channel assignment and rout-ing problem, taking into account the interference con-straints, number of channels and radios. They showthat the performance of their algorithm is within aconstant factor of that of any optimal algorithm forjoint channel assignment and routing problem. Un-like majority of these works which solves the chan-nel assignment problem in the context of a gateway-based wireless mesh networks or centralized networkswhere one node provides required information to othernodes, we propose a purely ad hoc environment. Weare proposing a distributed solution to the channelassignment problem in the context of a multi-radiomulti-hop ad hoc network without any specific struc-ture.

III. Network Model and Initialization

In this section, we present the network model for themulti-radio ad hoc network. We will introduce thedefinitions of terms that bring out the important con-cepts useful in the later sections.

We consider a fixed multi-hop wireless networkwith n nodes. We represent the network with a di-rected graph G = (V; E) where V represents the setof nodes in the network, E the set of directed links thatcan carry data (data links). We are given f source-destination pairs with desired flows between sourceand destination. We will be utilizing only local topol-ogy and local traffic load information to perform chan-nel assignment as used in [10]. This information iscollected from a (k + 1)-hop neighborhood, where k isthe ratio between the interference and communicationranges, and is typically between 2 and 3. We haveassumed that all the interfering nodes to a particularnode lies in its (k + 1)-hop neighborhood.

Definition 1 V v E V, p(v) denotes the priority ofthe node.

A. Heuristics to calculate p(v)Priority of the node will be an important measure in

our channel-assignment algorithm. We present someparameters from which the priority of node is calculated:-

* Traffic - Priority of the node increases with in-creasing Load on the node.

k-hop neighborhood of T

//O O\ OC

,/'\t,00/- *\ *,

<N u mb

-* Number of Routes - Priority of node is directly \ I

proportional to the number of routes passing o \through the node. t

T

0,

B. Network Initialization and Topology Discov-ery

Initially, there are f s-t pairs. First AODV will calcu-late the routes to follow and then Interface-assignmentalgorithm will do the assignment. After the routes areknown, every node calculates its priority by using anyone of the heuristic discussed in section IIIA. Andeach node will multicast its priority to its (k + 1)-hopneighborhood. This (k + 1)-hop neighborhood discov-ery is simple, as in a sense, node is broadcasting thepriority message with a hopcount variable in it. As theneighboring nodes receive this message, if hopcount isless than (k + 1) then they will send the message totheir neighbors with hopcount increased by 1. If aneighboring node receives the message with hopcountof (k + 1) then they will not send the message further.By this process, each node will know the highest prior-ity node in its (k+ 1)-hop neighborhood. This priorityis used in the formulation of channel-assignment algo-rithm. We need to do this priority calculation on aregular basis as the loads on the node can change incourse of time.

If a new node joins the network, then it will haveto wait till it starts receiving the priority messagesfrom other nodes that means the network is in prioritycalculation phase, which will be done regularly, andthe new node can start sending its priority now.

IV. Channel Assignment and RoutingHeuristic

As pointed out in Network Initialization section, ev-ery node knows its relative position in its (k + 1)-hopneighborhood. Each node in the network maintainstwo list of channels, Lr contains channels that areblocked for receiving and Lt contains channels thatcan not be used for transmitting. The node with thehighest priority in its (k + 1)-hop neighborhood starts

o o o X,/lo o o o(k+1)-hop neighborhood of S

k-hop neighborhood of S

node for which c is blocked for receiving

node for which c is blocked for receiving, transmittingS node for which c is blocked for transmitting

Figure 1: S communicates to T using channel c, k = 2.

the Channel Assignment procedure. Let this node berepresented by S. S choose the node to communicateby looking at the routing information it has. Let thenode chosen by S for communication be T. Then S willselect a free channel(if any) for transmitting and checkthat this channel is also free for receiving at T. For thischecking to be done, S transmits a CHECK containingchannel information packet to T and if channel is freeon T, i.e. channel does not belong to Lr of T, thenT sends an OK packet back to S otherwise it sends aREJECT packet specifying that it can not communi-cate on this channel. That is, channel should not bepresent in the S's Lt and Ts Lr

After finding such a channel, S multicasts the chan-nel usage information to its k-hop neighborhood re-questing the nodes to update their list of blocked chan-nel for receiving. Similarly, T multicasts its channelusage information to k-hop neighborhood by sending aparticular packet requesting the nodes to update theirlist of blocked channel for transmitting as shown in Fig1. S and T sets one of their interfaces on the selectedchannel. If no such channel is available which satisfiesabove stated constraints, then S and T chooses theleast loaded channel for communication. Then S willchoose another node to communicate from the routinginformation and start the same process. After assign-ing interfaces to all the routes passing through S, it willmulticast a RELEASE packet to its (k+ 1)-hop neigh-borhood specifying that it has completed its channel

T sends REJECT

sends CHECK to T

T send No rechannels available

S haso emore S has on moreT availa le to T availab e tocommu cate with commu cate with

S and T sets one NIC S and T sets one NIC

to selected channel to least loaded channel

Cha el assignment f S is completeS h moo des to communicate with

S sends RELEASE to its

(k+l)-hop neighborhood

Figure 2: Flowchart of packets exchanged when S does itschannel-assignment.

assignment as shown in Fig 2. Now, the next high-est priority node in every (k + 1)-hop neighborhoodwill start the channel assignment. For all the controlpackets as shown in Fig 2, there is a reserved controlchannel which can be later used for data communica-tion after channel assignment is complete.

When every node in the (k + 1)-hop neighborhoodof a node has finished the channel assignment i.e. thenode has received the RELEASE packet from eachnode in its (k + 1)-hop neighborhood then it can startsending the original data. Given algorithm only usesthe local information and no prior information aboutthe network topology. Our new algorithm is quite suit-able for a dynamic self-starting network.

Lemma 1 V v E V, v will start channel-assignmentwhen it is the node with highest p(v) value in its (k +1)-hop neighborhood which has not done the channelassignment yet.

A. Interface Switching

In previous subsection, we discussed the basic frame-work of our channel assignment algorithm. But whenthe number of interfaces at each node is smaller thanthe number of channels available, interfaces may haveto be switched between channels. In Fig 3, Tp, thepriority-shift time, is the interval after which priori-ties are recalculated and channel assignment is againdone. Ti denotes the interface switching interval, thatis, how frequently are we switching the interfaces. Ex-cept calculating priorities, Channel Assignment algo-

rithm discussed before is to repeated after Ti interval.This mechanism does not require a centralized ma-chine, since in every (k + 1)-hop neighborhood, nodewith the highest priority start it.

Ti

Tp

Figure 3: Priority-shift and Interface Switching Time.

Switching an interface from one channel to anotherincurs a switching delay which is of the order of hun-dreds of microseconds. So we have to intelligently fixTi and Tp. Value of Tp should depend upon how fre-quently the load is varying in network. In the worstcase, we need to calculate priorities each time we dointerface switching, i.e. Ti = Tp otherwise Ti < Tp.However, value of Ti is critical in the performance ofour algorithm as too small Ti may lead to high switch-ing delay & no throughput and large values of Ti maylead to the starvation of some flows and high end-to-end delays. Now we discuss the simple algorithmwhich we use for interface switching.

Let S had 3 interface available

S Q| A B | C 1 A

After Channel Assignment

S'QL A |B |C

Figure 4: At the start of a Ti, S performs an iteration ofswitching algorithm.

Each node maintains a Q of nodes to which has tocommunicate. In the interface -assignment algorithmwhenever it get its turn according to its priority, itdequeues one node from Q and assign one interfaceto the dequeued node and enqueue this node at therear of Q. If the number of interfaces available onthe node is less than the number of nodes it has tocommunicate with, then it can not do communicationwith all the nodes in Q simultaneously. After each Ti,node will be able to communicate with the numberof nodes present in front of Q equal to the interfaceson the node. Node dequeues these nodes from frontof Q and enqueues these nodes at rear of Q, so thatthe nodes which are starving now should get turn insubsequent Ti intervals. One iteration of the switchingalgorithm on node S is shown in Fig 4.

B. Routing Heuristic

As most of the routing protocols used today try toget shortest routes. But for a multi-channel multi-radio scenario, shortest path may not be optimal [12].Instead of choosing shortest paths, routing algorithmshould be such that:

* Route should be chosen such that the degree ofnodes in the route is less than the number ofNICs on the node. This will save a lot of NIC-switching delay.

* Route should be chosen so that it contains themaximum number of edge-disjoint nodes.

So route should be chosen such that it maximizes thesetwo metrics combinedly. This type of routing proto-col will also support our interface assignment strategy.For evaluation, current scheme uses AODV for routeselection, but this can be improved using the heuristicwith the stated goals, which will be part of our futurework.

V. Performance Evaluation

In this section, we present results of our performanceevaluation based on simulations. We have used Glo-MoSim [3] for our simulation purposes. We also imple-mented interface switching in GloMoSim which usesour switching algorithm. We consider a 7 x 7 gridtopology with 16 flows. Source-destination pair foreach flow are calculated randomly and each flow has afixed demand equal to the bandwidth of single link sothat the network saturates. We vary the number of ra-dios from 2 to 3, and the number of channels from 1 to8. Every channel has same fixed bandwidth associatedwith it. We have taken the interference to communi-cation distance ratio to be 2, i.e. k = 2. Grid unitdistance (between any two consecutive nodes) is suchthat the two consecutive nodes are always in communi-cation range and nodes 2-hop away are in interferencerange, and the nodes that are 3 or more hop away arenot in interference range as shown in Fig 5.

S Communication Range

Interference Range

Figure 5: Interference and communication ranges for S,k = 2.

4.5

4

- 2 Radios- 3 Radios

A 3.5

3p

t 2.5z

0 2

1.5

2 3 4 5 6Number of Channels ->

7 8

Figure 6: Throughput improvement with increasing Num-ber of Channels

Fig 6 shows the per-node throughput improvementsas the number of channels are increased. Per nodethroughput increases quickly as we move from channel1 to 3 and then 5 to 8. In between 3 to 5, through-put increases linearly. Throughput is normalized withthe single channel single radio throughput. Thesethroughputs are the throughputs with Ti of 10 secondsand Tp of time oc, i.e. there is no priority recalculationas the loads in the network are not time-varying.

514.5t

Ca 3.5

d) 35

v 2.5

cc

2-

0.5

2 3 4 5 6Number of Channels->

- 2 Radios-3 Radios

7 8

Figure 7: Average End-to-End Delay versus Number ofChannels

Fig 7 shows Average End-to-End delay versus thenumber of channels. As expected, on increasing thenumber of channels delay decreases swiftly and theresponse time also decreases. But as the number ofchannels are increased from more than 3, the decreaseis not large but there is still a decrement.

Fig 8 depicts the important behavior of the param-eter Ti on per node throughput. These simulationswere run for a period for 60 seconds. We normalize

- 5Channel 2Radio

4.5

4-

A

n 35

3

z 2.5

2

1.5

110 20 30 40

Switching Interval, T, (in seconds) ->50 60

Figure 8: Per Node Throughput versus Ti

the throughput with no switching throughput. Since,GloMoSim does not incorporates the interface switch-ing delay, we were not able to get optimal Ti, but we

got that interface switching increase the throughputin a considerable amount. High improvement justifiesthe intuition behind our switching algorithm.

Parameter Tp, the priority-shift time, can be tunedon the same frequency as how the traffic is varying inthe network. As priority of a node determines howmuch traffic does it have, so we need to calculate prior-ities repetitively only if the network has time-varyingtraffic.

VI. Conclusions

The recent developments in multi-radio devices hasencouraged research in architecting multi-radio multi-hop networks. Looking at the growing number ofmulti-radio end-user devices, we envision a future ofmulti-radio multi-hop ad hoc network with no inher-ent structure. In this paper, we present a distributedchannel assignment scheme for such a network using a

priority tagging mechanism for each node. The prior-ity determination for a node is dependent on the trafficload it carries. Currently our route selection uses thewell-known AODV routing protocol.

However, we identify possible improvements forAODV in a multi-radio scenario that we plan to inves-tigate in future. The efficiency of our channel assign-ment algorithm is demonstrated through simulationson GloMoSim. We have shown that per node through-put improves significantly when we increase the num-ber of channels and radios available per node. We havealso given a simple channel switching strategy whichsaves the network from arbitrary starvation. We incor-porated our interface switching strategy in GloMoSimand showed that it also increases the throughput asit tries to distribute the channels between flows regu-larly.

References[1] IEEE Std. 802.11-1999 "Part 11: Wireless LAN Medium

Access Control (MAC) and Physical Layer (PHY) specifica-tions", 1999.

[2] IEEE 802.16, Air Interface for Fixed Broadband WirelessAccess Systems.

[3] L. Bajaj, M. Takai, R. Ahuja, K. Tang, R. Bagrodia, andM. Gerla. GlomoSim: A Scalable Network Simulation Envi-ronment. Technical ReportCSD Technical Report, #990027,UCLA, 1997.

[4] P.Kyasanur, N.H.vaidya, "Routing and Interface Assign-ment in Multi-Channel Multi-Interface Wireless Networks",IEEE WCNC 2005.

[5] Atul Adya; Paramvir Bahl; Jitendra Padhye; Alec Wolman;Lidong Zhou, "A Multi-Radio Unification Protocol for IEEE802.11 Wireless Networks", ACM Mobicom 2003.

[6] Nokia N91, http://www.nokian9l.org/.[7] Murali Kodialam, Thyaga Nandagopal, "Characterizing the

Capacity Region in Multi-Radio Multi-Channel WirelessMesh Networks", ACM Mobicom 2005.

[8] Charles E. Perkins, Elizabeth M. Royer" Ad-hoc On-DemandDistance Vector Routing", 2nd IEEE Workshop on MobileComputing Systems and Applications, 1999.

[9] Mansoor Alicherry, Randeep Bhatia, Li Li, "Joint ChannelAssignment and Routing for Throughput Optimization inMultiradio Wireless Mesh Networks", ACM Mobicom 2005.

[10] Ashish Raniwala, Tzi-cker Chiueh, "Architecture and Al-gorithms for an IEEE 802.11-Based Multi-Channel WirelessMesh Network", IEEE Infocom 2005.

[11] Ashish Raniwala, Kartik Gopalan, Tzi-cker Chiueh, "Cen-tralized Channel Assignment and Routing Algorithms forMulti-Channel Wireless Mesh Networks", ACM SIGMO-BILE 2004.

[12] Richard Draves, Jitendra Padhye and Brian Zill, "Routingin Multi-Radio, Multi-Hop Wireless Mesh Networks", ACMMobicom, 2004.