integrated fiber-wireless fiwi access networks supporting inter-onu communications-tnb

714 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 5, MARCH 1, 2010

Integrated Fiber-Wireless (FiWi) Access NetworksSupporting Inter-ONU Communications

Yan Li, Jianping Wang, Member, IEEE, Chunming Qiao, Fellow, IEEE, Ashwin Gumaste, Member, IEEE,Yun Xu, and Yinlong Xu

Abstract—Integrated fiber-wireless (FiWi) access networksprovide a powerful platform to improve the throughput ofpeer-to-peer communication by enabling traffic to be sent fromthe source wireless client to an ingress optical network unit (ONU),then to the egress ONU close to the destination wireless client, andfinally delivered to the destination wireless client. Such wireless-op-tical-wireless communication mode introduced by FiWi accessnetworks can reduce the interference in wireless subnetwork,thus improving network throughput. With the support for directinter-ONU communication in the optical subnetwork, throughputof peer-to-peer communication in a FiWi access network canbe further improved. In this paper, we propose a novel hybridwavelength division multiplexed/time division multiplexed passiveoptical network (WDM/TDM PON) architecture supportingdirect inter-ONU communication, a corresponding decentralizeddynamic bandwidth allocation (DBA) protocol for inter-ONUcommunication and an algorithm to dynamically select egressONU. The complexity of the proposed architecture is analyzedand compared with other alternatives, and the efficiency of theproposed system is validated by the simulations.

Index Terms—Dynamic bandwidth allocation (DBA), fiber-wire-less (FiWi), load balancing, passive optical network (PON),peer-to-peer, wavelength assignment, wireless mesh network(WMN).

I. INTRODUCTION

NOWADAYS, access can mainly be divided into wiredaccess and wireless access. Optical access which can

provide huge bandwidth is an attractive wired access approachto meet the increasing bandwidth requirement, but it is costlyto achieve deep fiber penetration. Wireless access can supportflexible and ubiquitous communication in small community

Manuscript received November 30, 2008; revised June 20, 2009, August 23,2009, and November 27, 2009. First published December 15, 2009; current ver-sion published March 05, 2010. This work was supported in part by the CityUniversity of Hong Kong under Grant 7002468.

Y. Li is with the Department of Computer Science, University of Scienceand Technology of China, Hefei, Anhui 230000, China, and also with the De-partment of Computer Science, City University of Hong Kong, Kowloon, HongKong (e-mail: [email protected]).

J. Wang is with the Department of Computer Science, City University of HongKong, Kowloon, Hong Kong (e-mail: [email protected]).

C. Qiao is with the Department of Computer Science and Engineering, Uni-versity of Buffalo, Buffalo, NY 14260–2000 USA (e-mail: [email protected]).

A. Gumaste is with the Department of Computer Science and Engineering,India Institute of Technology, Bombay 400076, India (e-mail: [email protected]).

Y. Xu and Y. Xu are with the Department of Computer Science, Univer-sity of Science and Technology of China, Hefei, Anhui 230000, China (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2009.2038598

areas with a low deployment cost. However, interference andlow bandwidth limit its deployment scalability. In light ofthe complementary properties of optical access and wirelessaccess, integrated Fiber-Wireless (FiWi) access networks [1],[2] are gaining rapid popularity as a promising candidate forfuture access networks to achieve a perfect balance betweenprovision of enormous access bandwidth and cost-effectivenessof infrastructure deployment.

A FiWi access network consists of a wireless subnetwork asthe front end and an optical subnetwork as the back end. In thispaper, we will consider a FiWi access network with a wirelessmesh network (WMN) [3] and a hybrid wavelength divisionmultiplexed/time division multiplexed passive optical network(WDM/TDM PON) [4].

One of our design objectives is to support various types ofcommunications using such a FiWi access network. For ex-ample, some wireless clients may be surveillance video cam-eras which need to transmit videos to the Internet, some wire-less clients may receive streaming video from the Internet, andmoreover, some other wireless clients may share files amongthem. Therefore, the FiWi access network must be able to ef-ficiently support upstream, downstream, and peer-to-peer com-munications.

Given that the network throughput in a WMN is severelylimited due to the interference in the wireless subnetwork, thispaper will address the major challenges of how to utilize thehigh bandwidth provided by the optical subnetwork to effi-ciently support peer-to-peer communication from one wirelessclient to another wireless client, which has been a focus in IEEE802.11s standard [5]. Our idea is briefly explained as follows.

The integration of PONs and WMNs in FiWi access networksprovides an opportunity to reduce the impact of interference onthe network throughput. In a FiWi access network, peer-to-peercommunication among wireless clients can be efficiently car-ried through a wireless-optical-wireless mode where the trafficis sent from the source wireless client to an optical networkunit (ONU), referred to as the ingress ONU, then the ingressONU transmits the packet to another egress ONU, and at last theegress ONU delivers the packet to the destination wireless client[6]. Such a wireless-optical-wireless communication mode canpotentially reduce the delay for some peer-to-peer communica-tions since the transmission in the PON network can be muchfaster than that in the multi-hop wireless network.

In order to efficiently support peer-to-peer communicationamong wireless clients through the wireless-optical-wireless

mode, efficient inter-ONU communication in the optical sub-network must be supported. In a conventional PON, inter-ONUcommunication is carried through the optical line terminal(OLT) which may affect the transmission of upstream traffic

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LI et al.: INTEGRATED FIBER-WIRELESS (FIWI) ACCESS NETWORKS SUPPORTING INTER-ONU COMMUNICATIONS 715

and downstream traffic. In this paper, we investigate new ap-proaches to supporting direct inter-ONU communication in aFiWi access network such that inter-ONU traffic does not haveto go through OLT. The main issues addressed in this paper arelisted below.

• WDM/TDM PON architecture design for efficient support

of inter-ONU communication. In order to efficiently sup-port such wireless-optical-wireless peer-to-peer communi-cation, a new WDM/TDM PON architecture design to sup-port direct inter-ONU communication without the involve-ment of the OLT is needed.

• Wavelength assignment (and traffic grooming) in the

optical subnetwork. Due to interference and limited band-width in the wireless subnetwork, traffic carried by eachONU to/from the wireless subnetwork is much less thanthe capacity provided by each wavelength. Therefore,multiple ONUs may share a wavelength through TDMA,which raises the issue of wavelength assignment (andtraffic grooming).

• Dynamic Bandwidth Allocation (DBA) protocol design

supporting inter-ONU communication. Dynamic band-width allocation protocol that determines which ONU (ofa given group) can send/receive data on the bandwidthshared by other ONUs within the group plays an importantrole in supporting efficient inter-ONU communication.

• Dynamic ingress ONU and egress ONU selection and

routing in WMNs. Dynamic selection of the ingress ONUand egress ONU for a pair of wireless clients will also playa role on reducing the interference, achieving load balancein the wireless subnetwork, and improving the throughputof DBA protocol.

In this paper, we study the above fundamental problems inthe optical subnetwork of a FiWi access network and our con-tributions are summarized as follows:

• We design a novel arrayed waveguide grating (AWG)-based [7], [8] WDM/TDM PON architecture efficientlysupporting inter-ONU communication. To our best knowl-edge, the proposed architecture is the first WDM/TDMPON supporting direct inter-ONU communication.

• Given the estimated traffic load arrived at ONUs from thewireless subnetwork, an efficient load balancing wave-length assignment which determines a subset of ONUs foreach wavelength is proposed in this paper.

• We propose a new decentralized DBA protocol to sup-port bandwidth allocation for inter-ONU communication.The proposed decentralized DBA protocol can maximizethroughput and provide fairness among ONUs.

• The proposed FiWi access network supports dynamicegress ONU selection at the ingress ONU to reduce thedelay of peer-to-peer communication among wirelessclients.

The rest of the paper is organized as follows. Section IIreviews the related work. Section III presents a WDM/TDMPON architecture supporting direct inter-ONU communica-tion. Section IV discusses wavelength assignment. Section Vproposes a decentralized DBA protocol to assure efficient com-munication among ONUs. Section VI presents dynamic egressONU selection at the ingress ONU. Simulation results are pre-sented in Section VII. We conclude the paper in Section VIII.

II. RELATED WORK AND PRELIMINARY KNOWLEDGE

Related Work on PON Architecture Design Supporting

Inter-ONU Communication: In conventional PON architec-tures, it is generally assumed that there is little inter-ONUtraffic. Thus, inter-ONU communication is implemented bysending the traffic from the source ONU to the OLT, whichthen sends it back to the destination ONU [9]. This implemen-tation, however, suffers from optical to electronic to optical(OEO) conversion delay and round-trip transmission delay.To accommodate the emerging need for direct inter-ONUcommunication, some PON architectures are developed tosupport such communication. In [10], [11], inter-ONU com-munication is conducted by broadcasting optical signals fromone ONU to all ONUs through star coupler (SC) which isdeployed at the remote node (RN). Such designs cause largepower loss and can only be applied to TDM PON. The authorsin [12] propose a WDM EPON architecture supporting directinter-ONU communication in which upstream transmissionand inter-ONU communication are alternately taken place, e.g.,the transmission cycle is divided into two sub-cycles, one forupstream transmission, the other for inter-ONU communica-tion, which makes it inappropriate for a FiWi access networkwith demanding upstream, downstream and peer-to-peer com-munication. In [13], a novel WDM PON with internetworkingcapability is developed. A AWG, where is thenumber of ONUs, is placed at the RN and two distributed fibersare connected to each ONU in such architecture. Based onthe cyclic property of AWG, direct inter-ONU communicationcan be implemented. In [13], if two ONUs are connected tothe same port of the AWG router to share wavelength forinter-ONU communication, they can not receive signals fromeach other since the wavelength they use to transmit inter-ONUtraffic can not be routed to the port they are connected to. Thus,this architecture can not be generalized to a hybrid WDM/TDMPON to act as the back end of FiWi access networks.

Related Work on DBA Protocols: Many centralized ordecentralized DBA protocols have been proposed for up-stream transmission and downstream transmission in PONnetworks [14]–[17]. [12] proposes a centralized DBA protocolfor inter-ONU communication where bandwidth allocationrequests are sent to the OLT and bandwidth allocation grant issent back to the ONUs from the OLT. Such an approach mayaffect upstream and downstream communication. On the otherhand, this protocol is vulnerable to OLT failure since it dependson OLT to conduct bandwidth allocation.

Related Work on Wavelength Assignment in WDM/TDM

PON: A hybrid optical-wireless access network supportingload balancing with WDM/TDM PON as the optical sub-network is proposed in [18]. ONUs can dynamically sharewavelengths by tuning to different wavelengths. Such a dy-namic wavelength sharing among ONUs is feasible becausepassive splitters are deployed at the RN, which is subject topower loss. In our proposed AWG-based WDM/TDM PON,ONUs sharing a common wavelength will receive/transmitsignal from/to a specific port of the AWG. Thus, static wave-length assignment must be conducted to determine whichONUs are allocated into the same set.



Fig. 1. WDM/TDM PON architecture supporting inter-ONU communication.

In this paper, we do not consider the specific routing inWMN. There are several routing algorithms have been pro-posed in FiWi access networks. Delay-aware routing in a FiWiaccess network has been studied in [19]. Capacity and delayaware routing in a FiWi access network is discussed in [20].An integrated routing algorithm which can adapt to the changeof overall demand among different service districts is proposedin [21]. Fault tolerant routing in a FiWi access network is con-sidered in [22]. A LP-based routing algorithm for peer-to-peercommunication among wireless clients is introduced in [6].However, the algorithm proposed in [6] just selects the ONUwhich is the closest one to the destination wireless client asegress ONU. Such a selection of egress ONU may not resultin high throughput due to the potential contention at the egressONUs. In this paper, we consider dynamic egress ONU se-lection at ingress ONU to reduce the contention at the egressONUs. Thus, higher throughput can be achieved.

III. WDM/TDM PON ARCHITECTURE SUPPORTING

INTER-ONU COMMUNICATION

In this section, we first introduce a new WDM/TDM PONarchitecture supporting direct inter-ONU communication, thencompare the proposed architecture with existing PON architec-tures supporting direct inter-ONU communication.

A. Overall Architecture

We first introduce the main optical components at the OLT,the RN and ONUs in our proposed WDM/TDM PON architec-ture as shown in Fig. 1. Suppose that there are ONUs andavailable wavelengths on each fiber.

At the OLT: An array of fixed transmitters and an array offixed receivers are equipped for transmitting and receiving databy using a AWG. are assigned for upstreamtransmission, are used for downstream transmis-sion and where FSR is the freespectrum range of the AWG. Note that though one may also use

through for downstream as well, using throughmay simplify the design of the ONUs.

At the RN: An passive combiner and a AWGare deployed at the RN as a wavelength router. Input portis used for both upstream and downstream traffic and input

port is used for data and bandwidth request of inter-ONUcommunication.

At each ONU: A pair of fixed transmitter and receiver areequipped for upstream transmission and downstream transmis-sion at designated upstream and downstream wavelengths, re-spectively. In addition, a tunable transmitter and a fixed receiverare used for inter-ONU communication. More specifically, the

ONUs are partitioned into groups, and each group is as-sociated with output port of the AWG and thus assignedwavelength and for upstream and downstream traffic,respectively. In addition, each group is assigned a wavelength

for and for to receive inter-ONUtraffic using a fixed receiver. In order to send inter-ONU trafficto other groups, a tunable transmitter is used. For example, ifany ONU would like to send traffic to the first group, it has totune its tunable transmitter to .

The inter-ONU traffic from all ONUs are combined into onefiber through an passive combiner which is connectedto port of the AWG. Only one ONU can send trafficusing a given at any given time, and this is controlled bythe collision-free bandwidth allocation protocol to be describedlater. Similarly, collision-free transmission of upstream trafficon among all ONUs belonging to the same group is alsoensured by using the bandwidth allocation protocol which isconducted at the OLT as conventional PON, which is not thefocus of this paper.

B. Comparison With Other PON Architectures Supporting

Direct Inter-ONU Communication

We now compare the proposed WDM/TDM PON architec-ture with two representative PON architectures [12], [13] dis-cussed in Section II. For the simplicity of presentation, we use

to refer to the architecture proposed in [12], to refer tothe one in [13], and to refer to the one in this paper. We com-pare these three architectures from the following perspectives:

• Optical components at each ONU. Two distributed fibersare connected to each ONU and four Txs/Rxs are equippedat each ONU in all three architectures.

• Optical components at the OLT. An array of receivers andan array of transmitters are equipped at the OLT for up-stream transmission and downstream transmission in all



Fig. 2. Architecture comparison.

three architectures. An AWG is employed at the OLTin while a AWG is deployed at the OLT in forrouting upstream and downstream signals, where .However, a AWG is needed at the OLT in givenits wavelength assignment.

• Optical components at the RN. In , only a small scaleAWG is needed at the RN. In , a AWG and anSC are deployed at the RN. In , an combiner anda AWG are deployed as an optical router at the RN,where .

• Concurrent upstream and inter-ONU transmission. As wementioned in Section II, in , concurrent transmissioncan not be supported given that upstream transmission andinter-ONU communication are alternately taken place. Inboth and , concurrent transmission can be supportedby routing function at the AWG.

• Supporting decentralized DBA protocol. In , centralizedDBA algorithm is conducted at the OLT. In both and

, decentralized DBA protocol is supported given that thetraffic request of each ONU can be broadcasted to all otherONUs independent of OLT. However, in , for one ONU,to receive request message from all other ONUs, it needsto tune its receiver onto different wavelengths frequently aseach ONU will broadcast its request message on its fixedwavelength. Since the size of request message is relativelysmall, the overhead and delay due to frequent tuning willbe significant in such a design.

• Supporting hybrid WDM/TDM PON. As we mentionedbefore, WDM/TDM PON is a desirable back end solutionfor FiWi access networks, which motivates us to design theWDM/TDM PON architecture . The WDM PONcan not be generalized to an effective WDM/TDM PONsince that concurrent upstream and inter-ONU transmis-sion can not be supported. The WDM PON can not beeasily generalized to a WDM/TDM PON which can sup-port direct inter-ONU communication. In , if two ONUsare connected to the same port of the AWG router to sharewavelength for inter-ONU communication, they can not re-ceive signals from each other since the wavelength they useto transmit inter-ONU traffic can not be routed back to theport that they are connected to.

The above comparison is summarized in Fig. 2. Through theabove comparison, we can observe that our proposed architec-ture is a cost-effective WDM/TDM PON architecture for effi-ciently supporting direct inter-ONU communication.

IV. WAVELENGTH ASSIGNMENT IN WDM/TDM PON

In this section, we introduce how to assign ONUs to wave-lengths such that network throughput can be maximized. Weassume that upstream traffic and downstream traffic are sym-metric. For example, for a communication network, the resi-dent subscribers may request symmetric upstream bandwidthand downstream bandwidth. Thus, we only need to consider thewavelength assignment in one direction.

Suppose that there are ONUs in a PON network and thereare available wavelengths for upstream transmission (down-stream transmission). Let be the set ofwavelengths. Let be the bandwidth provided by a wavelength.Let be the split ratio of a wavelength which means that onewavelength can be shared by at most ONUs, where we as-sume . Let be the estimated traffic load (meanvalue of traffic load) of where such infor-mation can be obtained statistically.

Wavelength assignment is to assign ONUs onto wave-lengths such that the traffic load on each wavelength is lessthan . Since the traffic load at each ONU is only estimated,in order to provide resilience to traffic fluctuation, we prefer tokeep enough residual bandwidth on each wavelength. In otherwords, load balanced wavelength assignment is preferred. Let

be 1 if is associated with wavelength , otherwisebe 0. The problem can be formulated as follows:

(1)

subject to:

(2)



(3)

(4)

In the above formulation, the objective is to maximize theminimum residual bandwidth on all wavelengths. Since allwavelengths are associated with the same bandwidth , (1) canbe rewritten as follows:

(5)

Equation (2) indicates that each ONU can only associate withone wavelength. (3) indicates one wavelength can be shared byat most ONUs and (4) shows that the traffic load on a wave-length can not be more than its capacity.

The wavelength assignment problem is an NP-hard problemsince the load balancing problem which is known to beNP-complete [23] can be reduced to the wavelength assignmentproblem. In this paper, we modify SortedGreedyLoadBalance

algorithm which is used for load balancing problem to solvethe wavelength assignment problem. The modified algorithm isreferred to as .

We would like to note that the proposed wavelength assign-ment is a static wavelength assignment, which determines whichport of the AWG that each ONU’s distributed fiber should con-nect to. Thus, it is not suitable to change frequently. If the trafficprofile in the wireless subnetwork changes dramatically suchthat some ONU groups are overloaded and some ONU groupsare underutilized, we have the following approaches to resolvethe unbalanced load among ONU groups:

• Adjust the traffic sent to each ONU through dynamicrouting in the wireless subnetwork such that traffic amongONU groups are balanced.

• Relay traffic among ONU groups. Suppose that traffic inthe ONU group using wavelength is more than thebandwidth that a wavelength can provide and traffic in theONU group using wavelength is far less than the band-width that a wavelength can provide, then ONUs usingcan send some traffic to ONUs by using which willthen send the traffic to the OLT. In other words, we can useinter-ONU communication scheme proposed in this paperto shift traffic from one ONU group to another such thathigh network throughput can be achieved.

Algorithm 1

Initialize all ;

Sort ONUs by decreasing traffic load, ;

For to ;

For to ;

Choose withand the minimum value of ;

Set ;

TABLE INOTATIONS USED IN THE DBA PROTOCOL

V. DECENTRALIZED DBA PROTOCOL FOR

INTER-ONU COMMUNICATION

Since multiple ONUs may request to communicate with theONUs which are in the same set simultaneously, collision mayhappen. Therefore, bandwidth allocation protocol is neededto assure efficient inter-ONU communication. Furthermore,to protect the communication against the OLT failure, decen-tralized DBA protocol is desirable since centralized protocoldepends on the OLT to conduct bandwidth allocation.

In this section, we propose a decentralized bandwidth alloca-tion protocol which is based on the proposed WDM/TDM PONarchitecture for inter-ONU communication. We first introducethe general framework of the proposed decentralized DBA pro-tocol and the parameters used in this section, then introduce howrequest message is generated and transmitted, and how DBA al-gorithm is performed at each ONU. We summarize the proper-ties of the proposed decentralized DBA protocol at last.

A. General Framework of Decentralized DBA Protocol

The proposed decentralized DBA protocol works in a syn-chronized manner where request message transmission is fol-lowed by data transmission in each transmission cycle. In eachtransmission cycle, at most ONUs are able to transmit datato their destination ONUs. The general framework of our de-centralized DBA protocol includes the following three stages ineach transmission cycle:

• Transmitting request messages. In this stage, the requestmessage from each ONU will reach all other ONUs in-cluding the ONUs in its own group. We will introduce theformat of request message and the mechanism of transmit-ting request messages shortly.

• Executing bandwidth allocation algorithm. In this stage,based on the gathered request messages, each ONU willindependently execute the same decentralized bandwidthallocation algorithm which will be introduced shortly.

• Transmitting data. During this period, ONUs will sendtraffic to other ONUs according to the decision made bythe bandwidth allocation algorithm.

B. Request Message Construction and Transmission

We now introduce how request messages are constructed andtransmitted. The notation used in the DBA protocol is given inTable I. Each source ONU’s weight is initially setto be . If has backlogged traffic and loses the bid atcurrent transmission cycle, is updated to ,otherwise is reset to .



At the beginning of one transmission cycle, we assumethat each ONU has selected a destination ONU accordingto intra-ONU scheduling [24], [25] which schedules trafficqueued at the ONU. If is the selected destination of

in current transmission cycle, the request message ofincludes the following information , where

are the indexes of source ONU and destination ONU,respectively. In one transmission cycle, if has no trafficto other ONUs, e.g., , the request messageof is depicted as to avoid the illusion that therequest message of is lost.

The key challenge of transmitting the request messages is thatthe request message from each ONU needs to reach all otherONUs. However, such a broadcast nature is hard to be satis-fied since ONUs receive inter-ONU request message and dataon fixed and different wavelengths. In this paper, we propose apipeline approach to broadcast inter-ONU request messages.

Before we introduce the mechanism of request message trans-mission for the general case where there are multiple ONUs ineach group, we start with a simple case where and thereis only one ONU in each group. In such a case, request messagetransmission period is divided into time slots. In the firsttime slot, each ONU sends its own request message to its adja-cent next ONU, e.g., sends its request message towhere if and if . From time slot2 to time slot , each ONU forwards its received requestmessage in the previous time slot to its adjacent next ONU. Ac-cording to such a pipeline approach, ’s request messagereaches in time slot . At the end of timeslot ’s request message will reach . Thus,all other ONUs have the request message of at the endof time slot . Since transmissions in groups can takeplace simultaneously, after time slots, each ONU receivesthe request messages of all other ONUs.

When there are multiple ONUs in each group , re-quest message from an ONU also needs to reach other ONUsin the same group. We need time slots to finish broadcastingrequest messages. For each group , we have the following twophases:

• In the first phase, each ONU in group has one time slot totune to wavelength and sends its request message to allONUs in its adjacent next group, group , where if

or if . In other words, after time slots,group ’s adjacent next group has the bandwidth requestsfrom ONUs in group for . All these requestmessages are buffered at each ONU.

• In the second phase, one ONU in each group will repre-sent the group to forward received request messages to itsadjacent group. Without loss of generality, we assume that

in group for will forward requestmessage. In each time slot sendsone of its received request messages which have not beenforwarded to its adjacent next group.

In the proposed request message transmission scheme,ONUs in a group only communicate with ONUs of itsadjacent group. Thus, frequent tuning is avoided. Wenow use the following example to demonstrate how theproposed request message transmission scheme works.Suppose that 7 ONUs are divided into three ONU

Fig. 3. Request message transmission in one transmission cycle.

groups, , and. In time slot 1, ,

and send their request messages toand

, respectively. In time slot 2,, and send their request messages

to and, respectively. In time slot 3, and

start to forward their received request messages intime slot 1 to their next groups respectively, and sendsits own request message to group . In timeslot 4, and start to forward their receivedrequest messages in time slot 2 and starts to forwardits received request message in time slot 1. In time slot 5, 6,and 7, , and continue to forward receivedrequest messages. Such a request message transmissionprocess is given in Fig. 3.

C. Decentralized DBA Algorithm

In this section, we introduce a decentralized DBA algorithmby which an ONU can determine whether it can send its trafficat current transmission cycle or not after receiving the requestmessages of all other ONUs.

Since one ONU can send at most traffic to its destina-tion in a transmission cycle, the maximum traffic amount that

can be allowed to sent to its selected destination ,e.g., , is set to be if , otherwise, .

Suppose that the destination ONU of the request messageoriginated from is , which belongs to group . Forall request messages with destination ONU belonging to group

, if has the highest values, can send traffic toits destination , otherwise, changes its weight to

, and waits for the next transmission cycle.The proposed decentralized DBA protocol can achieve high

network throughput and fairness:• It can achieve high throughput. The protocol assures that

if there is traffic destined to a group, one ONU will sendtraffic to that group. Among all flows destined for ONUs inthe same group, if the source ONUs have continuously lost



the bid for times, the flow with the highest trafficdemand will be transmitted to its destination. Suppose that

and have lost the bid for the samenumber of times and they select the destination ONUs inthe same group, and . Suppose that

, then we have, then will transmit

the traffic. Thus, high network throughput can be achieved.• It can resolve contention and break the tie. Among all flows

destined for the ONUs in the same group, if the sourceONUs have continuously lost the bid for timesand these flows have the same traffic demand, the one withhighest source ONU index will send its traffic to its desti-nation since the weight of each sourceis set to be .

• It can assure fairness. If has continuously lost thebid for times, it will have higher priority than otherONUs which have continuously lost the bid for

times. Suppose that has continuously lostthe bid for times, its weight is updated to

and its traffic demand is . Without loss of generality,we assume that has traffic demand and it hascontinuously lost the bid for times. Wehave

. Thus, will have higherpriority than in current transmission cycle.

VI. DYNAMIC EGRESS ONU SELECTION FOR PEER-TO-PEER

COMMUNICATION AMONG WIRELESS CLIENTS

If the destination ONUs of many requests belong to the samegroup, such requests contend for the same wavelength andinter-ONU communication may experience longer delay. Asa result, peer-to-peer communication among wireless clientswill experience longer delay. One remedy solution will be todynamically determine the ingress ONU and egress ONU forpeer-to-peer communication among wireless clients. In thispaper, we assume that the ingress ONU for each pair of wirelessclients to communicate through wireless-optical-wireless modeis given and we propose an algorithm to dynamically selectegress ONU. The motivation is to select an egress ONU foreach communication request between a pair of wireless clientssuch that the traffic load on the wavelength assigned to thategress ONU is light.

To support dynamic egress ONU selection at ingress ONU,each ONU needs to maintain the following two tables:

• Traffic demand table. Each ONU will maintain a traffic de-mand table which records the volume of backlogged trafficdestined to each group. Such information can be easilymaintained with the obtained request messages and the re-sult of decentralized DBA algorithm. Let be the currenttraffic demand on where is initially setto 0. After receiving request messages in one transmissioncycle, is updated to , where denotes

the index of the wavelength which is used by to re-ceive data from other ONUs. In one transmission cycle, if

is allowed to send data to on , then isupdated to .

Fig. 4. Example of egress ONU selection.

• Candidate egress ONU table. Each ONU willmaintain a candidate egress ONU table. Foreach wireless client maintains an entry

in the table as the candidate

egress ONUs for , where each candidate egress ONUcan send traffic to the wireless client within relativelysmall delay. Such candidate egress ONUs can bedetermined by the routing in the WMN subnetwork,which is out of the scope of this paper.

With the above information, when a packet arrives at itsingress ONU, if it is peer-to-peer traffic among wireless clients,the ingress ONU will check the destination wireless clients’scandidate egress ONU. Since each candidate egress ONU cansend traffic to the wireless client within relatively small delay,among all candidate egress ONUs, the one which receivesinter-ONU traffic on the wavelength with the minimum back-logged traffic demand will be selected as the egress ONU forthe packet.

We now use an example shown in Fig. 4 to demonstrate dy-namic egress ONU selection. Fig. 4(a) shows a FiWi access net-work in which and share wavelength to receivedata from other ONUs while and share wave-length to receive data from other ONUs. For simplicity, weassume that there are five wireless clients in WMNand a packet with destination arrives at . Fig. 4(b)shows the traffic demand table at when the packet ar-rives at . Fig. 4(c) shows the candidate egress ONU tableat . In our example, will select as the egressONU for the arrived packet. The reason is that the current trafficdemand on wavelength (109 Mb) which is used by toreceive inter-ONU traffic is much less than the current traffic



Fig. 5. Topology of simulated small scale FiWi.

demand on wavelength (156 Mb) which is used by toreceive inter-ONU traffic.

VII. SIMULATION

In this section, we conduct the simulation to demonstrate theefficiency of our work from the following perspectives:

• to demonstrate that supporting direct inter-ONU commu-nication can further improve the system throughput ofpeer-to-peer communication in FiWi access networks.

• to demonstrate the effectiveness of wavelength assignmentin the optical subnetwork.

• to compare the performance of the proposed WDM/TDMPON supporting direct inter-ONU communication withconventional WDM/TDM PON without supporting directinter-ONU communication.

• to demonstrate the effectiveness of dynamic egress ONUselection for peer-to-peer communication among wirelessclients.

A. Simulation Setting

In the simulation, we first construct a small scale FiWi accessnetwork which is shown in Fig. 5 to show throughput enhance-ment brought by supporting direct inter-ONU communicationin FiWi access networks.

The simulation is conducted in ns-2.34, which is the most re-cent version of ns-2. In the WMN subnetwork of the constructedFiWi access network, there are two gateways (ONUs) (we as-sume that a single device integrates the functions of both ONUand gateway) and eight wireless mesh routers which are uni-formly distributed in 400 200 m square region. Each ONU(gateway) manages a subregion with four mesh routers. DSDVprotocol which is provided in ns-2 is used for wireless routing.Two CBR traffic flows, e.g., and , are gener-ated in our simulation.

To simulate the realistic wireless communication environ-ment, we set the main parameters for wireless physical layeras shown in Table II. Each wireless mesh router equips a single802.11a radio which works on 5.18 GHz frequency. TwoRay-Ground propagation model is selected for wireless channel. Thephysical date rate is set to 54 Mbps and 64-QAM is selected as

TABLE IIPARAMETERS FOR PHYSICAL LAYER

our modulation scheme. The signal to noise ratio (SNR) is setto 10 dB to assure the bite error rate (BER) of wireless channelis about . We set the transmission power of each node to

dB to create a communication range of approximately120 meters. The sensibility range in our simulation is 400 metersby setting CST to dBm. The achievable data rate of 802.11aradio is no more than 10 Mbps in our simulation scenario, whichis consistent with the result reported by other researchers usingns-2 simulator that maximum achievable data rate of 802.11aradio is around 17% of the raw data rate. The size of each wire-less packet is set to 1000 bytes.

The achievable throughout of the wireless subnetwork is lessthan 10 Mbps in the simulation. To fully utilize the bandwidthprovided by the optical subnetwork, some fixed end users canbe connected directly to the ONUs for Internet access. Tosimulate such a deployment, we assume that each ONU has 2wavelengths (one for upstream transmission and one for down-stream transmission), each with 16 Mbps allocated bandwidthfor the peer-to-peer wireless traffic from the constructed WMNand the upstream/downstream traffic from fixed users/OLT.Since each ONU has one dedicated wavelength for inter-ONUcommunication and there are only two ONUs, the queueingdelay experienced by inter-ONU traffic in this small scaleFiWi access network only depends on the available bandwidthfor inter-ONU communication and inter-ONU traffic will notexperience queueing delay caused by contention to the samedestination ONU from other ONUs. The propagation delayfrom the OLT to each ONU is set to 100 s. Each PON frameis set to be 15000 bytes. In the simulation, we assume thatwhen a wireless packet arrives at an ONU, there is some trafficfrom other fixed users or wireless clients and a PON frame isformed immediately for transmission. We note that if trafficarrived at the ONU is light and a PON frame can not be formeduntil enough wireless packets arrive, the performance gain ofenabling wireless-optical-wireless mode and direct inter-ONUcommunication may be reduced.

Since the proposed work to efficiently support directinter-ONU communication is independent of the specific set-ting and routing in wireless subnetwork, in order to demonstratethe efficiency of proposed work in this paper, we then conductsimulations assuming that traffic from 100 wireless clients hasbeen sent to 16 ONUs where these 100 wireless clients maybelong to several different wireless networks. By making suchassumption, we can clearly demonstrate the efficiency of theproposed work independent of the underlying routing protocolin the wireless subnetwork and we can simulate larger trafficload. As we mentioned earlier, if inter-ONU traffic is carriedby sending traffic to the OLT at first and then transmitting backto the peer ONU, the impact of inter-ONU communication onupstream traffic is the same as that on downstream traffic. Thus,we only simulate upstream communication and inter-ONUcommunication in a WDM/TDM PON. In this part of thesimulation, we simulate a larger PON network where there are



Fig. 6. Throughput of peer-to-peer communication versus system traffic load.

Fig. 7. Mean end to end packet delay versus system traffic load.

16 ONUs operating on four different wavelengths each withcapacity of 1 Gbps for upstream transmission and inter-ONUcommunication. ONUs in the same group will operate inTDMA mode to share wavelength for upstream transmission.The propagation delay between the OLT and ONUs varies inthe interval of [50 s, 100 s]. We assume that traffic arrivedat each ONU from the wireless subnetwork follows Poisson

distribution.

B. Simulation Results

The results shown in Figs. 6 and 7 are based on the con-structed small scale FiWi network. In both figures, we demon-strate the throughput enhancement and mean end-to-end delayreduction of peer-to-peer communication by supporting directinter-ONU communication in FiWi access networks. In the op-tical subnetwork, a FiWi access network without supportingdirect inter-ONU communication and the proposed FiWi ac-cess network supporting direct inter-ONU communication aresimulated. It is obvious that peer-to-peer communication canbe better supported in the FiWi access networks with enablingwireless-optical-wireless mode and the performance of peer-to-

Fig. 8. Mean packet queuing delay at ONU versus system traffic load.


peer communication can be further improved by supporting di-rect inter-ONU communication.

The following results are based on the constructedWDM/TDM PON with 16 ONUs. To demonstrate the effective-ness of the proposed load balancing wavelength assignment in aWDM/TDM PON, we assume that each ONU’s expected meantraffic load (or offered bandwidth) to each ONU is given. In theWDM/TDM PON, the ratio of upstream traffic to inter-ONUtraffic is set to be 1:1. In order to simulate traffic burst, weassume that the actual traffic load of each ONU fluctuates in therange of [ Mbps, Mbps]. Fig. 8 shows the meanpacket queuing delay and corresponding confidence intervalwith 99% confidence probability under the fluctuated trafficload. We compare the performance of balanced wavelengthassignment with that of a naive wavelength assignment wherefour ONUs are assigned to share a wavelength regardless oftheir estimated traffic load. From Fig. 8, we can see that withload balancing wavelength assignment, traffic will experienceless delay at ONUs even if the estimated traffic load has mod-erate fluctuation.

Fig. 9 shows that the mean packet queuing delay at ONUs ina conventional WDM/TDM PON which does not support directinter-ONU communication and in the proposed WDM/TDM



Fig. 10. ONU fairness versus system traffic load.

PON supporting direct inter-ONU communication. We assumethat the ratio of upstream traffic to inter-ONU traffic is 1:1. Forthe peer-to-peer traffic arrived at each ONU from the wirelesssubnetwork, we suppose that the egress ONUs are uniformlydistributed among for simplicity. Fig. 9shows that the mean packet queuing delay in the proposedWDM/TDM PON is less than that in conventional WDM/TDMPON. Furthermore, with the increase of network traffic load,this superiority is more and more significant. We note the meanqueuing delay shown in Fig. 9 is for peer-to-peer wirelessusers. In practical FiWi access networks, upstream transmis-sion, downstream transmission and inter-ONU communicationmay simultaneously take place. Upstream and downstreamtraffic also benefits from the capability of supporting directinter-ONU communications, though these were not shown.

Fig. 10 shows the fairness of the proposed DBA protocol forinter-ONU communication. We suppose that only inter-ONUtraffic is generated and the egress ONUs are determined byuniformly distributing them among for sim-plicity. We use Jain’s fairness index [26] to estimate ONU fair-ness which varies from (worst case) to 1 (best case). Asshown in Fig. 10, the good fairness can still be achieved evenunder high traffic load in the proposed DBA.

In Fig. 11, we assume that 100 wireless destinations aredeployed in the WMN subnetwork. Each ONU in a FiWiaccess network maintains a candidate egress ONU table. Todemonstrate the effectiveness of our proposed dynamic egressONU selection, Fig. 11 compares the mean packet queuing ofthe proposed dynamic egress ONU selection with that of theclosest egress ONU selection which selects the one close tothe destination in candidate ONU list. It can be seen that themean packet queuing delay of the FiWi access network withproposed dynamic egress ONU selection is smaller than theclosest egress ONU selection. This is because that the proposeddynamic egress ONU selection considers the real-time trafficload at each ONU and always selects the egress ONU wherethe traffic load on the wavelength assigned to that egress ONUis light.


VIII. CONCLUSION

One of the challenging issues in FiWi access networks is howto efficiently utilize the high bandwidth provided in the opticalsubnetwork to improve the throughput of wireless subnetworks.In this paper, we focus on the relevant issues in the opticalsubnetwork in order to efficiently support peer-to-peer com-munication among wireless clients in FiWi access networks.We propose a novel WDM/TDM PON architecture whichsupports direct inter-ONU communication and a decentralizedDBA protocol to assure efficient inter-ONU communication.We also propose algorithms for wavelength assignment anddynamic egress ONU selection to improve network throughput.In conclusion, this paper presents innovative approaches toimprove the throughput of peer-to-peer communication in FiWiaccess networks.

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Yan Li received the B.S. degree in computer sciencefrom the University of Science and Technology ofChina (USTC), Hefei, in 2006, where she is currentlyworking toward the Ph.D. degree.

Jianping Wang (M’03) received the B.S. and M.S.degrees in computer science from Nankai University,Tianjin, China, in 1996 and 1999, respectively, andthe Ph.D. degree in computer science from the Uni-versity of Texas at Dallas in 2003.

She is currently an Assistant Professor with theDepartment of Computer Science, City Universityof Hong Kong. Her research interests include opticalnetworks and wireless networks.

Chunming Qiao (F’10) received the B.S. degreein computer science and engineering from theUniversity of Science and Technology of Chinain 1985, the M.S. degree in computer science andengineering from Shanghai Jiao Tong Universityin 1987, and the Ph.D. degree in computer sciencefrom the University of Pittsburgh, Pittsburgh, PA,in 1993.

He is currently a Professor with the State Uni-versity of New York, Buffalo. His research interestsinclude optical networks, wireless and mobile

networks, survivable networks, and TCP/IP technologies.

Ashwin Gumaste (M’04) received the Ph.D. degreein electronic engineering from the University ofTexas at Dallas in 2003.

He is currently the James R. Isaac Chair in the De-partment of Computer Science and Engineering withthe Indian Institute of Technology, Bombay. His re-search has been funded by vendors, providers, systemintegrators, and government agencies.

Yun Xu received the Ph.D. degree in computerscience from the University of Science and Tech-nology of China (USTC), Hefei, in 2002.

He is currently an Associate Professor with theDepartment of Computer Science, USTC, wherehe is leading a group of research students in somehigh-performance computing and bioinformaticsresearch. His research interests include designand analysis of parallel algorithms, heterogeneouscomputing and multi-core processors, computingperformance tuning, and biological sequence

analysis and mining.

Yinlong Xu received the B.S. degree in mathematicsfrom Peking University, Peking, China, in 1983, andthe M.S. and Ph.D. degrees in computer science fromthe University of Science and Technology of China(USTC), Hefei, in 1989 and 2004, respectively.

He is currently a Professor with the School ofComputer Science and Technology, USTC, where heis leading a group of research students in some net-working and high-performance computing research.His research interests include network coding,wireless network, combinatorial optimization, and

design and analysis of parallel algorithm.
