MAC Layer Fairness in IEEE 802.11 DCF BasedWireless Mesh Networks

Sandip Chakraborty and Sukumar NandiDepartment of Computer Science and Engineering,

Indian Institute of Technology, Guwahati,Guwahati, Assam, India

Email: {c.sandip,sukumar}@iitg.ernet.in

Abstract—Fairness provisioning in IEEE 802.11 DCF basedWireless Mesh Networks is a very challenging task due to unfairnature of 802.11 DCF based MAC protocol. In this paper,notion of proportional fairness is used in WMN where eachnode gets channel share proportional to their load. A loadestimation strategy is proposed at each WMN node, which in turnestimates required channel share for every node. A probabilisticapproach is used to tune the contention window based on thedifference between actual channel share and required channelshare. Simulation result shows that the proposed scheme worksbetter than the standard IEEE 802.11 based DCF MAC in termsof fairness and throughput.

Index Terms—Wireless Mesh Network, Fairness, DCF, MAC

I. INTRODUCTION

Wireless Mesh Networks (WMN) are next generation wire-less broadband networks that draws significant attention thesedays. WMNs are multihop networks that comprise of threetypes of nodes - mesh clients, mesh routers and mesh gate-ways. There are several candidate technologies to implementwireless mesh networks, such as IEEE 802.11 and IEEE802.16, out of which IEEE 802.11 DCF based WMN is morepopular due to its simplicity and low deployment cost. How-ever IEEE 802.11 DCF based WMN faces inherent problemof unfairness due to the multihop nature of the network, asaddressed in [1], [2], [3] and [4].There are some works exists in literature that targets to

provide flow-fairness in IEEE 802.11 DCF based WMN. In[5], the authors describe a distributed min-max fairness amongthe nodes based on weight estimation of each node in a multiradio multi channel environment. In [6], the authors addressthe problems of using IEEE 802.11 DCF in the context ofwireless mesh networks and propose a weighted contentiongraph based approach to provide end-to-end flow fairness.They have calculated maximal capacity for each flow andused that capacity to maintain fairness among the flows. Yooand Kim [7] propose a centralized flow coordination basedapproach to provide proportional fairness in enterprise wirelessmesh networks. In [8], the authors propose a fair schedulingapproach in multiple gateway based wireless mesh networks.All the existing works as mentioned above consider flow

fairness among stationary mesh routers only, and so theyconsider max-min fairness among mesh nodes. However incase of heterogeneous wireless mesh network, load of differentnodes varies significantly based on their traffic forwarding

GW1 2

f1 f2

f1 f1+f2 f1+f2

Fig. 1. Multihop Network with different Traffic Load

properties, and hence, mesh clients should get less channelshare than mesh routers. Similarly mesh gateways work asbottleneck of the network, and so it should get maximumchannel share. Besides, flow fairness alone can not resolveMAC layer contention, and so without a MAC layer supportit may waste bandwidth when there are over-injected data. Asan example, a simple network is shown in Figure 1. Assumingthat flow fairness is implemented at the network with all flowsof equal priority, there are f1 number of flows originatedfrom (or destined to) the clients of node 1 and f2 numberof flows originated from (or destined to) the clients of node2. Node 1 and node 2 receive traffic load from their ownclients as f1 × c and f2 × c respectively, where c is themaximal bandwidth share that can be allocated to each flow.So node 1 has to forward f1 × c amount of traffic whereasnode 2 has to forward (f1 + f2) × c amount of traffic. Thegateway has to forward (f1 + f2) × c amount of traffic.As all the three nodes are in the interference range of eachother, if the total available bandwidth is B, node 1, node 2and GW node should get f1

3×f1+2×f2× B, f1+f2

3×f1+2×f2× B

and f1+f2

3×f1+2×f2×B amount of bandwidth share respectively.

However, IEEE 802.11 DCF implicitly provides equal accessprobability for all nodes. Thus when the load between twocontending nodes varies significantly, total network throughputcan be dragged down. This problem can be solved if everynode can get channel share proportional to their traffic load.Thus per node proportional fairness can be best suited forWMN.

In [9], the authors show the capacity of Wireless MeshNetworks. They have discussed the impact of relayed traffic onfairness, and finally shown that for WMNs, the throughput ofeach node decreases as O(1/n), where n is the total number ofnodes in the network. In a later work [10], they introduce a per-flow reservation based mechanism, both at MAC and networklayer, to support fairness in WMN without degradation of

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throughput at higher load. However per-flow queuing requireshigh implementation cost, and also may induce the scalabilityproblem in large scale WMN. There are some works exist inliterature that target to provide MAC layer fairness at IEEE802.11 DCF based wireless ad hoc networks. In [11], theauthors propose a graph theory based algorithm to providemax-min fairness. In their scheme, interference graph is ex-tracted from network graph and communication graph, andthen max-min optimization is applied on interference graph toschedule nodes. In [12], the authors propose a scheme whereevery node estimates number of active nodes in its vicinity andaccordingly decides its mode of contention: aggressive, normalor restrictive. The Contention Window (CW) is tuned basedon the contention mode of the node. However, these schemesprovide equal channel share distribution among all contendingnodes and thus are not applicable directly for WMN, wherenodes should get channel share proportional to their trafficload.The proposed scheme provides per-node proportional fair-

ness at the MAC layer which in turn provides approximatemax-min fairness among different flows with equal priority,as shown by simulation results. The nodes with higher loadget higher channel share that makes equal distribution ofbandwidth among different flows. Thus if a normal FIFOservice queue is used to forward packets from upper layer,each flow is treated as with equal priority and bandwidthis distributed among all flows equally. Thus the problemwith heterogeneous traffic demand and bandwidth wastagewith high traffic load can be solved by providing per-nodeproportional fairness at MAC layer.

II. FAIRNESS PROVISIONING IN IEEE 802.11 DCF MAC

A wireless mesh network can be represented as a connectedgraph G(V, E), where V is the set of nodes, and E is theset of links between them. Here the term ‘node’ representsboth mesh routers and mesh clients. As described earlier, eachmesh gateways, mesh router, mesh client and mesh gatewayshould get channel share proportional to its load. The completeprotocol works in five steps as follows;

• Every mesh router estimates its total traffic load whichcomprises of traffics from two types of sub-flows - fromits own clients, and from other routers as relayed traffic.

• Each node (mesh routers and mesh clients) estimates itsrequired channel share from its knowledge of contentiondomain. Every node can get this information by overhear-ing data and control frames.

• Each node calculates the actual channel share it received,by overhearing data and control frames.

• Based on this estimation, the nodes enter one of the threemodes - aggressive (if estimated required channel shareis more than actual channel share), normal (if estimatedshare equals actual share) or restrictive (if estimatedrequired channel share is less than actual channel share).

• The nodes tune its CW, as suggested in [12], to balancethe estimated required channel share and its actual chan-nel share.

As discussed, every nodes estimates required channel sharefrom the knowledge of its contention domain. The followingsection defines the notion of contention domain formally andfrom it, the load estimation procedure is discussed.

A. Contention Domain

The Collision Domain of ith link is the set of links formedby the ith link and all other links that have to be inactive forthe ith link to have a successful transmission [10]. SimilarlyContention Domain of node N, CDN = {VN , EN} is definedas the collision domain EN associated with the links of nodeN and the set of corresponding nodes VN sending data throughthose links. A wireless mesh architecture is shown in Figure2, where the black node is the gateway node. The contentiondomain for the gateway node is shown when it receives trafficfrom other nodes.Let N2

i be the set of nodes in two-hop neighborhood, N1i be

the set of nodes in one hop neighborhood; and Vi be the setof nodes in the current contention domain of node i. Then

1) If k ∈ Vi; then k ∈ N2i

2) If k ∈ N1i ; then node i can decide whether k ∈ Vi

by overhearing the RTS and DATA frames. The sourceaddress in RTS and DATA frames should be the addressof k in this case.

3) If k ∈ N2i ; then node i can decide whether k ∈ Vi by

overhearing the CTS and ACK frames. The destinationaddress in CTS and ACK frames should be the addressof k in this case.

The above three conditions are good enough to get informa-tion about the active nodes in a contention domain. The firstcondition says that contention is up to two hop, whereas thesecond and third conditions say that information about nodesin a contention domain can be populated by overhearing dataand control frames. The channel share in a contention domainshould satisfy following two constraints-

1) The total normalized channel share in a contention do-main should be less than or equals to unity.

2) Each node in a contention domain should get channelshare proportional to their traffic load.

Based on above two constraints, required channel share fornode i (RSi) is formulated as;

RSi =SLi∑

j∈CDi

SLj

(1)

where, SLi is total traffic load for node i, and CDi is thecontention domain for node i

B. Load Analysis and Load Estimation

Let us consider mesh topology as shown in Figure 2. Toshow the amount of traffic forwarded from each router, equalnumber of active clients, say G, are assumed under each meshrouter. In this paper it is assumed that the traffics are towardsor from the mesh gateway. In the figure traffic is assumedonly towards mesh gateway. It has been assumed that datais forwarded following a tree structure, both in up-link and

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Fig. 2. Load Analysis in a Complex Mesh Topology

down-link direction, so that no loop is introduced in estimationprocedure.1) Load Estimation at Each Mesh Router: In WMN, there

are mainly two types of traffic - up-link traffic and down-linktraffic. Based on this, two terms are defined, Uplink ActivityFactor (UAF) and Downlink Activity Factor(DAF) as follows;

• Uplink Activity Factor (UAF) for router r defined as;

UAFr ← Cur +

∑i∈Rr

UAFi (2)

Where Cur is the rate of up-link traffic received from the

clients of router r, and Rr is the set of mesh routerswho have forwarded up-link data to router r in last timeinterval.

• Downlink Activity Factor (DAF) for router r defined as;

DAFr ← Cdr +

∑i∈Cr

DAFr (3)

Where Cdr is the rate of down-link traffic forwarded to

the clients of router r and Cr is the set of mesh routerswho have forwarded down-link data to router r in lasttime interval.

• Activity Factor (AF) for router r is defined as;

AFr = UAFr + DAFr (4)

As each individual router requires UAF and DAF values ofrouters who are currently forwarding relayed data, the value ofthese two parameters need to be propagated along the network.These values are piggybacked in the DATA and ACK frames.The piggybacked value can be a positive value or ‘-1’. If thepiggybacked value is a positive value that it is valid UAF orDAF value. The next task is to differentiate between UAFand DAF values. Each router also maintains two lists LUAF

and LDAF which store the UAF and DAF values of all itsneighbors. The following distributed mechanism is used tocalculate UAF and DAF values efficiently at every router.

1) Each mesh router i initialize its UAF and DAF valuesas follows;UAFi ← Cu

i and;

DAFi ← Cdi

2) The gateway always forwards DATA frame with piggy-backed value ‘-1’.

3) When router i receives a DATA frame from router j, itdoes following,

a) If the piggybacked value is a positive value, then it isthe UAF vale of router j, it updates LUAF for routerj. In this case, it executes following steps,

i) Router i calculates its own UAF value using equa-tion (2).

ii) It forwards DATA frame using UAFi as the pig-gybacked value.

iii) In response to that DATA frame, it sends ACK torouter j with ‘-1’ as the piggybacked value.

b) If the piggybacked value is ‘-1’, then it executesfollowing steps,

i) It calculates its own DAF value using equation(3).ii) In response to the DATA frame from router j, it

sends ACK frame to router j with DAFi as thepiggybacked value.

iii) It forwards the DATA frame to the next hop routerwith ‘-1’ as the piggybacked value.

4) When router i receives ACK frame from router j it doesfollowing,

a) If the piggybacked value is a positive value, then it isthe DAF value of router j. It updates its LDAF forrouter j.

Thus a positive value in DATA frame indicates a UAF value,and a positive value in ACK frame indicates a DAF value forcorresponding mesh routers who are forwarding those frames.Clearly the UAF value propagates in O(1) message passingwhereas the DAF value propagates in O(h) message passing;where h is number of hops between mesh gateway and themesh router for the intended mesh client. After finding outthe traffic load of a router in terms of activity factor, the nexttask is to estimate number of active mesh routers and meshclients in a contention domain.

C. Estimation of Active Mesh Routers and Active Mesh Clients

To estimate the number of active routers within the con-tention range, every mesh router maintains a list. Whenever amesh router overhears a MAC frame, it inserts the ID of thesender of the flow into the list if the ID does not exist. If theID already exists, the router simply refreshes the time of theentry containing this ID. In order to prevent stale entries, thesimilar approach presented in [12] is used here.Mesh routers can estimate up-link and down-link traffic

rates for its clients by maintaining a history. Similarly, everymesh client also knows the AF value of its router by readingthe piggybacked value in the MAC frames.

D. Estimation of Required Channel Share

The required channel share can be estimated for meshrouters and mesh clients using above estimation and equation

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(1), as follows.1) Estimation of Required Share by Mesh Clients: Let CSi

R

be the required share of client i and AF be the activity factorof the router working as the access point for the client i. Then;

CSiR =

1

AF(5)

2) Estimation of Required Share by Mesh Routers: Let,RSi

R be the required share for router i, Vi is the contentiondomain, Ri is the set of active mesh routers in contentiondomain Vi and AFi is the Activity Factor of router i. Thenusing equation (1);

RSiR =

AFi∑k∈Ri

AFk

(6)

3) Estimation of Actual Share by Mesh Routers and Clients:The actual share estimation can be done by maintaininga communication history at each node. Whenever a nodeoverhears a DATA or ACK frame transmitted over the medium,it appends sender ID of the frame into its history. If a nodeoverhears both the DATA and ACK frames, belonging to samehandshaking, it adds the ID only once. By maintaining sucha history a node knows its actual share wi, by checking howmany times (say m) its own ID appears in the latest windowwith nw frames. Therefore,

wi =m

nw

(7)

E. Tuning CW based on the Estimation

Based on the estimation of actual share and required share,a node enters one of the three modes - aggressive, restrictiveor normal to compensate for the over or under usage in theimmediate past. A similar approach like [12] is used to tunethe CW probabilistically. When a node generates the back-offtimer, rather than using the CW directly, the node uses a scaledvalue S × CW , where S is the scaling factor determined bythe node’s mode. A node also records number of times it hasbeen in the aggressive or in the restrictive modes since thelatest occurrence of the normal mode, which are representedby Naggressive and Nrestrictive, respectively. Since a nodecannot enter aggressive mode from the restrictive mode, andvice-versa, without passing through normal mode, Naggressive

and Nrestrictive are reset to zero whenever the node’s modebecomes normal. If a node is in the aggressive mode, theNaggressive is incremented by one when any other nodestransmit a packet. On the other hand, if a node is in therestrictive mode, Nrestrictive is incremented by one when thenode itself transmits a packet. Therefore the scaling factor Sshould like this,

S =

⎧⎨⎩

1 normal mode2×Nrestrictive restrictive mode1/(2×Naggressive) aggressive mode

(8)

To limit the scaling factor, whenever Naggressive orNrestrictive reaches a maximum value, it is not allowed toincrease any further.

III. SIMULATION RESULTS

The proposed scheme is simulated using NS-3.9 [13] net-work simulator. 802.11 has been considered as physical layerstandard with 2Mbps data rate for mesh routers and gateways,and 1Mbps for mesh clients. 50 mesh routers are placed ina 100 × 100 space. Random direction 2D mobility model isused to simulate client mobility. Initially clients are distributeduniformly under the routers, and then they start moving usinga constant velocity of 3 m/s. Figure 3(a) shows the UDPthroughput for four flows in 802.11 DCF based MAC, andFigure 3(b) shows the corresponding UDP throughput in theproposed fair WMN MAC. Here flow 1 and flow 2 are down-link flows, and flow 3 and flow 4 are up-link flows. It canbe seen from the figure that the proposed scheme showssubstantial improvement in terms of fairness over standardIEEE 802.11 DCF MAC. The fairness index is calculated asgiven by Jain’s index [14],

F (x) =(∑

xi)2

n(∑

x2i )

(9)

where xi is the throughput for flow i, and n is the totalnumber of such flows. The output for UDP traffic is shownin Figure 4(a). It can be seen from the figure that the fairWMN MAC shows more fairness than the IEEE 802.11 DCFMAC. To simulate the behavior for TCP traffic, a seamlesshandover scheme is implemented at each mesh router thatredirects the data frames to the new access point, when aclient switches from one access point to another. Based onthe similar topology as described earlier, Figure 4(b) showsthe fairness improvement in case of TCP traffic.An important observation is that the proposed scheme

improve fairness without any loss in throughput at high load.Table I shows the throughput comparison for individual flowsin case of TCP traffic, where each flow inject traffic at a rate of256 Kbps. After the saturation (which comes with five flows),the throughput for some of the flows drops significantly incase of 802.11 DCF MAC due to congestion effect. Howeverthe proposed fair MAC scheme distributes overall throughputamong different flows, and so all flows get almost equalamount of throughput that makes overall aggregate throughputhigher than the throughput observed in IEEE 802.11 DCFMAC. Table II shows the same for UDP traffic, up-link anddown-link respectively. As the mobility effect is different forup-link and down-link flows for connection-less UDP flows, sothese two cases are shown separately. In case of UDP traffic,after the saturation point (with five flows), the total throughputbecomes constant, and is also higher than 802.11 DCF MACthroughput. Furthermore, all flows get almost equal shareof the total throughput. For down-link traffic, the aggregatethroughput is lower because of mobility effect. In all threecases, total aggregate throughput is higher in case of proposedfair WMN MAC.A comparison with existing per-flow reservation based fair

allocation [10] is shown in Figure 5. This work is choosenfor comparison as it is a pioneer work that considers theeffect of MAC layer relayed traffic on fairness. The proposed

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Fig. 3. Simulation results for UDP Traffic

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Fig. 4. Fairness Index

TABLE ITHROUGHPUT FOR TCP FLOWS

Number ofFlows

802.11 DCF MAC (Kbps) Fair WMN MAC(Kbps)

Individual Flow ThroughputAggregateThroughput Individual Flow Throughput

AggregateThroughput

4 208.2, 216.8, 192.5, 188.6 736.1 211.2, 231.1, 197.3, 204.3 800.9

5 212.6, 188.3, 211.4, 132.6, 142.5 887.4 218.1, 192.4, 210.6, 192.3, 198.6 1012.0

6 132.1, 166.6, 92.4, 188.3, 112.0, 121.2 812.6 132.6, 143.6, 140.4, 162.0, 132.7, 141.0 851.9

8 91.0, 88.1, 146.9, 21.1, 112.3, 14.5, 87.2, 92.4 653.5 92.1, 98.6, 96.4, 81.0, 82.5, 74.2, 77.1, 96.2 698.1

10 21.1, 12.6, 65.1, 0, 56.1, 28.6, 62.4, 0, 61.8, 72.6 380.3 31.2, 34.5, 36.2, 53.1, 38.6, 58.1, 34.2, 51.8, 28.3,52.5

418.5

scheme provides flow fairness at per the scheme proposedin [10], as shown in Figure 5(a). The aggregate throughputcomparison is shown in Figure 5(b). While per-flow queuingat Network Layer alone reduces total aggregate throughputsignificantly at high load, per-flow queuing at MAC layerimproves the result. The proposed scheme improves totalaggregate throughput further, by reducing high implementationcost for per-flow queuing. Furthermore, no explicit queuemanagement is required at MAC layer, which makes theproposed scheme more scalable with increasing number of

flows.

IV. CONCLUSION

In this paper MAC layer fairness problem in IEEE 802.11DCF based wireless mesh networks is described and a pro-portionally fair MAC protocol is proposed over IEEE 802.11DCF MAC. A load estimation strategy is used to estimate loadat each mesh router and mesh client. The algorithm estimatesthe current share and required share for each node, and basedon the difference between these two, it enters one of the threemodes, and accordingly tune CW to probabilistically achieve

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TABLE IITHROUGHPUT FOR UDP FLOWS

Uplink Traffic

Number ofFlows

802.11 DCF MAC (Kbps) Fair WMN MAC(Kbps)

Individual Flow ThroughputAggregateThroughput Individual Flow Throughput

AggregateThroughput

4 255.2, 253.8, 251.5, 255.6 1016.1 255.2, 255.1, 254.3, 254.3 1018.9

5 242.6, 231.8, 241.6, 242.6, 232.1 1190.7 238.9, 242.2, 240.6, 242.1, 238.4 1202.2

6 122.7, 116.8, 172.9, 198.2, 212.0, 191.8 1183.8 202.1, 203.6, 200.6, 192.0, 202.3, 201.5 1203.1

7 212.6, 132.1, 121.6, 172.1, 144.2, 208.6, 196.5 1187.7 172.1, 172.3, 172.8, 178.1, 176.1, 162.5, 169.4 1203.3

Downlink Traffic

4 228.4, 211.8, 202.7, 198.1 841.0 218.2, 221.1, 224.1, 214.7 878.1

5 231.6, 191.2, 237.1, 222.5, 202.4 1084.8 228.1, 221.5, 220.1, 222.6, 218.7 1111.0

6 128.1, 196.2, 152.1, 198.9, 162.6, 171.7 1009.6 207.3, 191.5, 192.4, 182.9, 182.5, 181.8 1138.4

7 212.6, 182.1, 132.5, 102.8, 112.4, 172.6, 154.2 1069.2 172.6, 162.7, 162.7, 152.9, 172.9, 152.6, 154.5 1130.9

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Fig. 5. Comparison With Per-Flow Queuing

fairness at MAC layer. This type of fairness is essentiallyper-node proportional fairness that considers current load atevery node. The per-node proportional fairness in turn givefairness among different flows. Simulation result shows thatthe proposed scheme gives better result than IEEE 802.11 DCFMAC in terms of aggregate throughput and fairness.

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