hybrid architecture and integrated routing in a scalable...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 25, NO. 11, NOVEMBER 2007 3443

Hybrid Architecture and Integrated Routing in aScalable Optical–Wireless Access Network

Wei-Tao Shaw, Shing-Wa Wong, Ning Cheng, Member, IEEE, Koussalya Balasubramanian, Xiaoqing Zhu,Martin Maier, Member, IEEE, and Leonid G. Kazovsky, Fellow, IEEE

Abstract—We propose a hybrid optical–wireless access networkthat consists of reconfigurable optical backhaul and wirelessmesh networks (WMNs). The complementary characteristicsof wireless and optical networks are combined to provide abroadband and ubiquitous last-mile connection. Wireless meshrouters are deployed to penetrate the vicinity of end users fora flexible and ubiquitous connection. It eliminates massive andgeographically scattered deployment of physical infrastructure toreach the end users. The broadband optical backhaul consists ofoptical ring and multiple tree networks, connecting the centralhub and WMNs. The ends of the optical tree networks connect tothe wireless gateway routers of WMNs. A hybrid time-division-multiplexing (TDM)/wavelength-division-multiplexing (WDM)optical backhaul is realized by wavelength-multiplexing multipleTDM-passive-optical-network streams. This hybrid architectureprovides graceful scalability, cost effectiveness, and bandwidthefficiency. To adapt to a change of the overall demand in differentdistricts, reconfigurability is implemented in the optical backhaulutilizing tunable optical transceivers. An experimental test bedis implemented to evaluate the reconfigurable scheme. Giventhe synergy of the optical backhaul and WMNs, we propose anintegrated-routing algorithm to achieve load balancing on thishybrid architecture. The simulation using NS2 shows an approx-imately 25% throughput improvement with load balancing.

Index Terms—Hybrid routing algorithm, hybrid time-division-multiplexing (TDM)/wavelength-division-multiplexing (WDM) re-configurable optical backhaul, time-division-multiplexing passiveoptical network (TDM-PON), wireless mesh networks (WMNs).

I. INTRODUCTION

O PTICAL and wireless networks were initially developedfor different communication purposes and scenarios.

Optical networks are aimed at high-bandwidth long-distancecommunications, whereas wireless networks are intended forflexible communications in personal local areas and cellularsystems where high bandwidth is not required. In the last twodecades, however, due to the growth of Internet traffic andchanges in the way people communicate, optical and wireless

Manuscript received February 27, 2007; revised July 25, 2007. This paperis based on work supported by the National Science Foundation under Grant0520291 and Grant 0627085.

W.-T. Shaw, S.-W. Wong, N. Cheng, K. Balasubramanian, X. Zhu, andL. G. Kazovsky are with the Department of Electrical Engineering, StanfordUniversity, Stanford, CA 94305 USA (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

M. Maier is with the Institut National de la Recherche Scientifique, Montreal,QC H5A 1K6, Canada (e-mail: [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.2007.909202

technologies have been applied to new applications, such asaccess networks.

Various optical- and wireless-access solutions have beendeveloped to address the different challenges of the accessnetworks, such as bandwidth, cost, and network availability.For example, the passive optical networks (PONs) [1], [2] haveemerged to replace the copper-wire access networks for band-width enhancement, and IEEE 802.16 (WiMax) [3] has beendeveloped to provide cost-effective and flexible Internet access.Today, the optical and wireless technologies have converged atthe access segment, as shown in Fig. 1.

Since current optical- and wireless-access technologies aimto address different issues, it is difficult for any single tech-nology to resolve all the challenges in the access segment. Forexample, albeit optical access enables broadband services, thededicated infrastructure to user’s house leads to a significantdeployment cost, and the network availability is confined withinthe residential and business units. Similarly, despite its ubiqui-tous and flexible connectivity, the limited bandwidth of a wire-less access network prevents simultaneous access from manyusers. In light of the complementary characteristics of opticaland wireless technologies, a combination of optical and wire-less technologies may result in a desirable compromise amongall these issues. We thus proposed a hybrid optical–wireless ac-cess network that can provide a blanket coverage of broadbandand flexible connection for both fixed and mobile users. Thisheterogeneous architecture consists of wireless mesh networks(WMNs) and reconfigurable-optical-backhaul networks. In thefront end, the wireless mesh routers are deployed to penetratea user’s vicinity, facilitating a ubiquitous connectivity andminimizing the deployment cost. The multihop characteristicsof WMNs enable blanket service coverage with a backhaulconnection shared by multiple mesh routers.

In the back end, the reconfigurable optical backhaul, whichconsists of optical ring and multiple tree networks, connectsthe WMN and the central hub that centrally manages the entirenetwork. We leverage time-division-multiplexing (TDM)-PONtechnology in the optical backhaul due to its bandwidth/costsharing, statistical multiplexing, and scalability in topologyand medium access control (MAC). This scalability is criticalin enhancing the performance of WMN by flexibly increas-ing backhaul connection [4]. Multiple TDM-PON streams arewavelength-multiplexed on the optical backhaul. By utilizingtunable transceivers, the reconfigurable optical backhaul canfacilitate bandwidth reallocation among different districts inadaptation to the variation of traffic demand. An experimentaltest bed is implemented to evaluate the reconfigurable scheme.

0733-8724/$25.00 © 2007 IEEE

3444 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 25, NO. 11, NOVEMBER 2007

Fig. 1. Convergence of optical and wireless at access network.

Fig. 2. Wireless mesh network.

The integration of point-to-multipoint optical backhaul andWMNs facilitates multiple routes between the central hub andeach end user. An integrated-routing paradigm that can dynam-ically choose the optimum route is therefore essential for sucha hybrid optical–wireless network. We propose an integrated-routing algorithm to achieve load balancing as congestionoccurs in the WMN. Simulation results show the performanceimprovement compared to the minimum-hop routing algorithmwith a single-gateway association of the WMNs.

II. INTRODUCTION TO THE INFRASTRUCTURE WMNS

The WMN is a wireless network employing multihop com-munications to forward traffic en route to and from wiredInternet entry points [5]. It can be categorized into the in-frastructure WMNs and the client WMNs, and the differenceis described in [4]. Since the proposed hybrid network employsthe infrastructure WMN, in the rest of this paper, “WMN” isreferred to as the “infrastructure WMN.”

Unlike the conventional wireless ad hoc networks, the meshrouters in WMN have no mobility, less power constraint, bet-ter computational capability, and forward traffic to/from theInternet through multiple gateway routers [6]. As shown inFig. 2, the wireless mesh routers automatically establish andmaintain the connectivity, as indicated by the dashed line inFig. 2, and the gateway router has a backhaul connection tothe Internet. The upstream traffic from the end user is firstcollected by the nearby mesh router, i.e., router 2. Then, thepackets are relayed to one of the nearby gateway routers, i.e.,the gateway router A or B. Furthermore, having an access tomultiple nearby gateway routers, a mesh router can access agateway router through multiple paths, too. This multigatewayassociation and the multipath features enhance reliability andenable load balancing in WMN [4].

New technologies and protocols in the physical (PHY) layer,MAC, and routing protocols are indispensable to optimizethe performance of the WMN. In the PHY layer, for ex-ample, smart antenna, multiinput multioutput (MIMO), andmultichannel/interface systems are being explored to enhancenetwork capacity. MAC protocols based on distributed time-and code-division-multiplexing access are expected to improvethe bandwidth efficiency from carrier-sense-multiple-access-conditional-access protocols [4]. In WMN, since packets arerouted among mesh routers in the presence of interference,shadowing, and fading, a cross-layer design is required tooptimize the routing in WMN.

Currently, IEEE 802.11a/b/g (Wi-Fi) technologies are widelyexploited in commercial products and academic research ofWMN due to their low cost, technological maturity, and high-product penetration [7]–[9]. Wi-Fi-based WMN will be widelydeployed in urban areas such as San Francisco, the SiliconValley, Minneapolis, Toronto, etc. As originally designed forwireless local area networks, however, the PHY and MAClayers of the Wi-Fi are not optimized for WMN applications.Hence, proprietary wireless technologies and WiMAX (IEEE802.16) have been proposed to enhance capacity, reliability,and mobility of WMN [5], [8], [10]. Ultrahigh-bandwidthstandards, such as the IEEE 802.16m, which aims to provide1 Gb/s and 100 Mb/s shared bandwidths for residential andmobile users [11], can be employed to further enhance thebandwidth and mobility of WMN. Due to the technologicaland bandwidth enhancements of WMN and to the numerousgateway routers geographically scattered in a large area suchas a city, WMN requires a high-capacity, point-to-multipoint,low-loss, and bandwidth-/cost-shared backhaul. We envisionthat optical technology is a promising candidate to fulfill theserequirements.

III. HYBRID OPTICAL–WIRELESS-ACCESS-NETWORK ARCHITECTURE

The network scalability is a challenging issue for WMN[4], [5]. If more wireless mesh routers are deployed in a WMN,the throughput per router and overall network capacity degradedue to the increased hop count [6]. Although using more RFchannels and radios interfaces per router will compensate forthe performance degradation [4], the inherent hop-count limita-tion will still exist. Instead of exhausting wireless resources,therefore, the throughput and capacity can be enhanced byscaling the number of gateway routers correspondingly withthat of the mesh routers. In a hybrid optical–wireless network,

SHAW et al.: HYBRID ARCHITECTURE AND INTEGRATED ROUTING IN AN OPTICAL–WIRELESS ACCESS NETWORK 3445

Fig. 3. Hybrid optical–wireless-access-network architecture.

since every gateway router is connected to the optical backhaul,a highly scalable, cost-effective, and point-to-multipoint opticalbackhaul is desirable to address the coupled scalability betweenmesh and gateway routers and the performance enhancement ofWMN. The integration of point-to-multipoint optical backhauland wireless mesh networks requires an integrated routingalgorithm to select the optimum routing path. In the followingsections we will introduce the proposed hybrid network ar-chitecture, optical backhaul technology, and integrated routingalgorithm.

The proposed hybrid optical–wireless-access-network archi-tecture is shown in Fig. 3. The optical backhaul consists ofring and multiple tree networks that are rooted at the ringnetwork. The ends of the tree networks are connected to thegateway routers of WMN. Under this hybrid architecture, theupstream traffic is first aggregated at a nearby mesh routerand then forwarded to one of the gateway routers. In Fig. 3,for example, router 4 aggregates the traffic of the nearby endusers and relays it over routers 3, 2, and 1 to reach the gatewayrouter A. Once the traffic reaches the gateway router, it is trans-parently forwarded toward the central hub over the optical back-haul. For the downstream traffic, packets are first routed to oneof the gateway routers, such as the gateway router B in Fig. 3,and then forwarded on a specific route, e.g., through routers 5,3, and 4, to reach the end user. The specific gateway routerand route should be calculated based on real-time networkconditions. Fig. 3 also shows the protocol stack of the hybridarchitecture.

A. Architecture and Technologies of the Optical Backhaul

As mentioned, the network performance can be enhanced bystrategically deploying more gateway routers to limit the maxi-mum hop count in WMN. As the bandwidth demand increases,therefore, an optical backhaul should enable scalable gateway-router deployment and capacity enhancement. Furthermore, anoptically transparent backhaul that allows the bandwidth and

cost to be shared by multiple gateway routers is desirable. Toachieve these goals, we leverage the TDM-PON technologyin the backhaul. The point-to-multipoint MAC protocol of theTDM-PON can connect to various numbers of Optical NetworkUnits (ONUs) within a limit to gracefully address the scalabilityissue in WMN. New gateways can seamlessly be added withoutinterruption of the network operation. The logical point-to-point link simplifies traffic transportation on the backhaul.

In the optical backhaul, multiple TDM-PON streamsare multiplexed using dense-wavelength-division-multiplexing(DWDM) technique. Hence, a DWDM transceiver is requiredfor the upstream traffic, which is a modification from theconventional TDM-PON. In Fig. 4, wavelengths λ1d, λ2d,λ1u, and λ2u that carry downstream and upstream signals aredropped and added at the optical backbone node. The opticalaggregation nodes consist of passive optical splitters. In Fig. 4,the four gateway routers on the tree network are managed byPON1 and PON2. These PONs are grouped under a centralizedmanagement in the central hub for bandwidth reallocation,integrated routing, flow control, and network management.Note that, although not shown, optical amplifiers may berequired for the upstream and downstream transmissions tocompensate for the component loss of the optical backbone andaggregation nodes if a large number of ONUs are connected ona tree network.

Since the backhaul and WMN use different technologiessuch as the Ethernet PON (EPON) and Wi-Fi, an interoper-ability issue exists at the interface between the ONU and thewireless gateway router. To address this issue, we can fusethe backhaul and WMN at either the networking layer usingan IP router or at the data link layer using an application-specific integrated circuit designed to translate a format if thepacket types are compatible. Furthermore, fusing two differenttechnologies, the interface performs route computation andcongestion monitoring in the local WMN. Later, we will showthat this interface plays an important role of the proposedintegrated-routing algorithm.


Fig. 4. Distribution network and interface between the optical backhaul and WMNs.

Fig. 5. Central-hub structure.

B. Reconfigurable Optical Backhaul

In a hybrid optical–wireless network, the bandwidth demandfrom different districts can drastically vary within a certaintime period. Instead of overprovisioning based on the peakdemand, it is desirable to reallocate bandwidth among multipleTDM-PON systems, which can be achieved by using a tunableoptical transceiver in the ONU. As shown Fig. 4, if PON1 ishighly loaded and PON2 is not, by tuning the transmitting andreceiving wavelengths of ONU3 from 1D, 1U to 2D, and 2U,ONU3 is enabled to join PON2 under the same tree network toreduce PON1’s loading.

Fig. 5 shows the central-hub structure. In the central hub, anetwork terminal (NT) is devised to manage N TDM-PONs. Inthe NT, the system-bandwidth-management module continuallymonitors the buffer depth of each optical line termination(OLT) for the downstream traffic. As any PON is heavilyloaded, which is reflected by the OLT buffer depth, the system-bandwidth-management module will instruct the heavily loadedPON to deregister some ONUs and to reregister them to the

lightly loaded PON(s). The number of ONUs to be moved de-pends on the average loading of the PONs, which are calculatedby the traffic estimators (TEs). Before an ONU is deregistered,its packets that are queued in the OLT will first be emptied.After an ONU is deregistered, the incoming traffic to that ONUis temporarily stored in the queue of NT until the reregistrationis complete. Note that the ONU deregistration and registrationcan readily be achieved by the multipoint-control-protocol-data-unit messages (deregistration and registration processes)[2] and physical layer operations, administration, and mainte-nance message (deactivate and ONU-activation procedure) [1]for EPON and Gigabit PON, respectively.

In optical access networks, tunable transceivers have beenproposed for different purposes such as bandwidth-efficiencyimprovement [14], network scalability [15], and inventorysimplification [16]. Due to the rapid advancement of opticaltechnology, various tunable-laser technologies have beendeveloped [16]. To optimize the cost issue, one of thepromising solutions is tunable long-wavelength vertical-cavity


Fig. 6. (a) Upgrade the wireless links between the master tower and the first aggregation tower. (b) Upgrade the wireless links between the first and secondaggregation towers and links between the first aggregation and access towers. (c) Upgrade the bandwidth on the first tree networks by adding new TDM-PONstream at a different wavelength. (d) Gradually upgrade some wireless links between the access tower and wireless gateway router.

surface-emitting lasers [17]. Lower cost results from integratedmanufacturing, easy packaging, and testing. Recently, an inte-grated fast wavelength-selective photodetection was developed[18]. Its tuning time is in the nanosecond range, and themonolithic design facilitates cost reduction.

With slow tunable transceivers, the tuning time can be a ma-jor overhead during reconfiguration, which leads to a network-performance degradation. For example, as the traffic loadingof a PON oscillates, the reconfiguration will frequently betriggered and the network efficiency decreases. To minimizethe performance degradation, we can increase the buffer sizeand set a higher threshold for the reconfiguration trigger. Thus,as reconfiguration is triggered, the tuning time will take arelatively small fraction compared with the time needed toempty the packets stored in the OLT buffer, and the networkefficiency will be improved. With this approach, although theefficiency is improved, the packets will suffer more delay fromlonger queuing time in the buffer. In the near future, we willinvestigate the use of alternative end-to-end route to forwardhigh-priority traffic to enhance the QoS during reconfiguration.

C. Upgrade of Hierarchical Wireless Access Networks

The proposed optical backhaul provides a promising upgradepath for hierarchical wireless access network architecture suchas the one to be built in San Francisco [19]. This proposednetwork consists of three wireless layers: the backhaul layer,capacity injection layer, and the wireless mesh layer [19]. Thelink rates in the backhaul and capacity injection layers arehundreds of Mbps using proprietary wireless technology [19].Given the aggregated traffic throughout the city and technical

upgrade in the wireless mesh layer in the near future [8],the throughput limitation will occur in the capacity injectionand backhaul layers. The proposed optical backhaul can beemployed to gradually upgrade the two layers, as shown inFig. 6(a)–(d).

D. Performance Simulation of the ReconfigurableOptical Backhaul

To evaluate the performance improvement, simulation wasconducted to compare the performance of reconfigurable andfixed architectures. Assume that both the reconfigurable andfixed architectures consist of two EPONs and the aggregatedtraffic is Poisson random traffic. First we simulate the averagepacket delay of the two PONs in both architectures. The loadingof PON2 is fixed at 0.2 and that of PON1 is changed from0 to 1.8. Quantitative results of this simulation was summa-rized and presented in [20]. The results show that in the fixedarchitecture, the overall throughput is limited and packets beginto drop after the OLT1 buffer overflows. With reconfiguration,the overall throughput of the reconfigurable architecture isoptimized, i.e., the loadings of two PONs are balanced, and theOLT1 buffer depth begins to decrease.

E. Experimental Test Bed of ReconfigurableOptical Backhaul

To demonstrate the reconfiguration scheme and its feasibility,we have implemented an experimental test bed with commer-cially available optical devices. To facilitate the reconfigurationprocess, the reconfiguration-control interfaces (RCIs) arerequired at the ONU and the central hub, as shown in Fig. 5.


Fig. 7. Experimental testbed and transient response of the tunable filter at ONU.

After the bandwidth-management module detects an unbal-anced traffic loading among OLTs, the RCI will be instructed tomove some ONUs from a heavily loaded PON (e.g., PON1) to alightly loaded PON (e.g., PON2) through the following steps.

1) The RCI at the central hub sends the wavelength informa-tion (e.g., λ2d,u) of a lightly loaded OLT (e.g., OLT2) tothe RCI at the to-be-reconfigured ONU.

2) The RCI at the central hub instructs the heavily loadedOLT (e.g., OLT1) to deregister the to-be-reconfiguredONU.

3) After deregistration, the RCI at the ONU tunes the wave-lengths of tunable transceiver to the new wavelength ofthe designated OLT (e.g., from λ1u,d to λ2u,d).

4) After reconfiguration is finished, the RCI at the centralhub instructs the designated OLT (e.g., OLT2) to discoverand register the ONU.

The test bed is shown in Fig. 7, which consists of two OLTswith fixed optical transceivers and one ONU with a tunabletransceiver. We assume that the PON1 and PON2 are heavilyand lightly loaded, respectively, and that the ONU is originallyregistered to PON1. The ONU deregistration, discovery andregistration processes and the RCI are encoded in a field-programmable gate array (FPGA). The tunable transmitterat the ONU used is the same as the one in [21], which tunesfrom 1552.5 to 1550.92 nm within 80 ns. The tunable receiverat the ONU is implemented by a MEMS-based tunable filterprovided by Nortel, Inc. By changing the control voltage from0 to 35 V, this tunable filter can tune from 1591 to 1525 nmat 25 ◦C with a full-width at half-maximum less than 350 pm,and throughout the tuning range, the loss is less than 1 dB.The transient response of the tuning wavelength from 1591 to1586 nm is shown in Fig. 7, showing that it takes 33.6 µs for thefilter to stabilize after the control voltage is changed. Since thetuning time is much longer than that of the tunable transmitter,the tunable filter dominates the overall ONU-reconfigurationperiod. We thus programmed the reconfiguration period of50 µs at the RCIs in the FPGA. The experimental result isshown in Fig. 8, where the ONU deregistration from theOLT1, reconfiguration period, and the ONU discovery andregistration performed by the OLT2 are demonstrated. Note

that in Fig. 8, the message propagation delay is insignificantcompared to the reconfiguration period that is in the scale oftens of microseconds.

IV. HYBRID-ROUTING ALGORITHM FOR TRAFFIC LOAD

BALANCING AND THROUGHPUT ENHANCEMENT

A. Integrated-Routing Algorithm

On the hybrid optical–wireless network, numerous routesexist between the central hub and each end user, and theoptical backhaul introduces little cost compared to the WMNdue to its high capacity and shared bandwidth. Furthermore,the entire network is under the same system management. Inlight of these characteristics, we propose an integrated-routingalgorithm that computes the optimum route based on the up-to-date wireless-link state and average traffic rate. For eachwireless mesh router, the algorithm will select the optimumONU/gateway and route on WMN in given network condi-tions. Specifically, this algorithm will adapt to each router’straffic and perform dynamic load balancing among WMNsunder a district. The proposed integrated-routing paradigm isdescribed by the following steps and simplified architecturein Fig. 9(a).

1) Wireless-Link-State Update: Each wireless mesh routerperiodically probes the link states with the neighborrouters. The probing can be done by measuring theretransmission ratio [22] or the transmission-loss rate[23] in both directions. The measurement verifies theconnectivity and reflects the quality and capacity of thewireless links that vary over time. Each mesh router thenbroadcasts the up-to-date link states with a hop-countlimit Hmax. The hop-count Hmax is assigned accordingto the wireless gateway/router ratio, i.e., a small ratiowill require to a large hop-count limit. As such, onlythe nearby gateways will receive the broadcast from eachmesh router. Compared to wireless ad hoc networks, theoverhead of link-state update under the hybrid architec-ture is significantly lower because the fixed infrastructureand network engineering lead to less channel variation. Ahigh gateway/router ratio will further reduce the overhead


Fig. 8. Experimental results. (1) OLT1 sends deregistration message. (2) ONU receives the deregistration message. (3) ONU sends the ACK. (4) OLT1 receivesthe ACK. (5) After a 50-µs reconfiguration period, OLT2 sends the discovery gate message. (6) ONU receives the discovery gate message. (7) ONU sends theregistration request. (8) OLT2 receives the registration request. (9) OLT2 sends the registration message. (10) ONU receives the registration message. (11) ONUsends the registration ACK. (12) OLT2 receives the registration ACK.

by limiting the propagation of an update broadcast onthe WMN.

2) Local-WMN Route Calculation: Based on the link-stateupdates, the interface between the ONU and the gatewayrouter calculates the optimum route for each mesh routerwithin the hop-count limit in both the downstream andupstream directions using the shortest path algorithmwith the link states as cost [22], [23].

3) Route-Cost Report: Each ONU reports to the central hubthe calculated route cost in both directions for each meshrouter within Hmax.

4) Gateway Association: After receiving the reports from allONUs, the route-assignment module will associate everywireless mesh router with the gateway router that has thelowest cost. The comparison result among a mesh routerand the different gateway routers is stored for futurereference to enable load balancing.

5) Congestion Monitoring: At the interface between theONU and gateway router, an LPF is dedicated to eachassociated mesh router to measure the average flow ratesin both directions. Based on the measurement, a capac-ity table is continuously updated to monitor the overallloading of the local WMN. To calculate the capacity, theinterference in the local WMN can be measured usingthe technique proposed in [24]. If loading at a certainarea exceeds a congestion threshold, the interface willlocate the k furthermost router(s) RT{1, 2, . . . , k}, ofwhich the flows pass the hot zone and can sufficientlyreduce the congestion if the flows are removed. If somerouters among RT{1, 2, . . . , k} exceed the minimum hop-count threshold Hmin, these routers will be categorized asRTflow_controll{1, 2, . . . , i} and the rest of the routers asRTload_balancing{1, 2, . . . , j}; note that i + j = k.

6) Congestion Report: The interface that detects congestion[e.g., interface 1 in Fig. 9(a)] sends a congestionreport, including RTload_balancing{1, 2, . . . , j} andRTflow_control{1, 2, . . . , i}, to the route-assignmentmodule.

7) Alternative Gateway Lookups: For each router inRTload_balancing{1, 2, . . . , j}, the route assignment mod-ule checks the gateways that received the link-state update[e.g., interface 2 in Fig. 9(a)] based on the stored compar-ison result described in step 4.

8) Gateway Reassociation: The reassociation begins withgateways with the lowest cost. The interfaces of thegateway will then check whether the loading thresholdwill be exceeded if the flows are added. If the threshold isexceeded, then the route-assignment module will negoti-ate with the gateway with the second lowest cost, and soon. If f routers among RTload_balancing{1, 2, . . . , j} failto be associated to a new gateway, they will be includedin RTflow_control.

9) Flow Control: After RTload_balancing{1, 2, . . . , j}are reassociated, flow control will be executed inRTflow_control.

B. Simulation Result and Performance Analysis

To evaluate the performance improvement by the algorithm,we implement the simulation with NS-2 [25]. The simulationscenario consists of four gateways and 192 mesh routers, asshown in Fig. 9(b), where the distance between any two ad-jacent nodes is 100 m. The IEEE 802.11b protocol provided bythe NS-2 is used, which supports a data rate of up to 11 Mb/s.Poisson random traffic and two-ray radio-propagation modelare used throughout the simulation. We set a uniform


Fig. 9. (a): Integrated-routing algorithm based on hierarchical structure. (b): Simulation scenario. (c): Aggregated throughputs versus hot-zone loading in the hotzone using the minimum hop and the integrated-routing algorithm. (d): Average delays versus hot-zone loading using the minimum hop and the integrated-routingalgorithm.

transmission power and receiver sensitivity among all the nodesin which the transmission and IRs are equal to 120 and 180 m,respectively. Each node has only one radio interface and isequipped with an omnidirectional antenna. Note that in thissimulation scenario, the shortest path routing with link state ascost is reduced to the minimum hop routing. We inject uniformbackground traffic to each mesh router, and the average flowrate out of each gateway to the nearby 48 mesh routers is 1%of the date rate. Assume that there is a hot zone, as shownFig. 9(b). We inject additional loading on routers in the hotzone and increase the overall loading from 0.1 to 1.5 Mb/s. Thecongestion threshold is set to be 8% (0.88 Mb/s) of the datarate, i.e., after the flow rate of certain router exceeds 0.88 Mb/s,the interface will detect congestion and send a congestion

report to the central hub. We investigate the throughput and thepacket delay performance of mesh routers in the hot zone.The simulation results in Fig. 9(c) and (d) show that afterthe additional loading in the hot zone exceeds 0.77 Mb/s(in the presence of 0.11 Mb/s background traffic), the flowsto the boundary routers are shifted to the other three gatewaysto balance the loading based on the proposed algorithm. As aresult, the throughput and delay are improved by about 25%.

V. CONCLUSION

We have proposed a hybrid optical–wireless access network.It consists of reconfigurable optical backhaul and WMN andaims to provide a broadband, ubiquitous, and blanket-coverage


access service. The reconfigurable TDM/WDM optical back-haul aims to provide a promising solution to address the scal-ability issue in WMN and to upgrade for hierarchical wirelessaccess networks. The TDM-PON technology is leveraged dueto its scalable MAC, flexible architecture, cost effectiveness,and technological maturity. Reconfigurability is implementedin the optical backhaul for bandwidth reallocation to improveresource utilization. A simulation was conducted to show theperformance improvement, and an experimental test bed hasbeen built to demonstrate its feasibility. We also propose anintegrated-routing algorithm for this hybrid architecture, whichcan locate the optimum ONU/gateway and route in the WMNbased on network condition and average traffic rate. Simulationresults show throughput and delay improvement, which haveresulted from the load balancing among multiple gateways.

In the near future, we will investigate the performance ofthe integrated-routing algorithm with next-generation WMNtechnology and explore essential issues such as QoS, net-work resilience, and multicast on the hybrid optical–wireless-network architecture.

REFERENCES

[1] ITU-T Recommendation ITU-T G.984, “GPON Gigabit Capable OpticalAccess Network.”

[2] IEEE Std. 802.3ah 2004.[3] A. Ghosh, D. R. Wolter, J. G. Andrews, and R. Chen, “Broadband wireless

access with WiMax/802.16: Current performance benchmarks and futurepotential,” IEEE Commun. Mag., vol. 43, no. 2, pp. 129–136, Feb. 2005.

[4] I. Akyildiz, “A survey on wireless mesh networks,” IEEE RadioCommun., vol. 43, no. 9, pp. S23–S30, Sep. 2005.

[5] R. Bruno, M. Conti, and E. Gregori, “Mesh networks: Commodity multi-hop Ad Hoc networks,” IEEE Commun. Mag., vol. 43, no. 3, pp. 123–131,Mar. 2005.

[6] J. Jun and M. L. Sichitiu, “The nominal capacity of wireless mesh net-works,” IEEE Wireless Commun., vol. 10, no. 5, pp. 8–14, Oct. 2003.

[7] J. Bicket, D. Aguayo, S. Biswas, and R. Morris, “Architecture andevaluation of an unplanned 802.11b mesh network,” in Proc. Mobicom,Aug. 2005, pp. 31–42.

[8] [Online]. Available: http://www.tropos.com/[9] [Online]. Available: http://www.belairnetworks.com/

[10] [Online]. Available: http://www.radiant.net/[11] [Online]. Available: http://www.ieee802.org/16/tgm/[12] J. Broch et al., “A performance comparison of multi-hop wireless ad-hoc

network routing protocols,” in Proc. ACM/IEEE MobiCom, Dallas, TX,1998, pp. 85–97.

[13] J. Li, C. Blake, D. S. J. De Couto, H. I. Lee, and R. Morris, “Capacityof ad hoc wireless networks,” in Proc. ACM SIGMOBILE, Rome, Italy,Jul. 2001, pp. 61–69.

[14] T. Chen, H. Woesner, Y. Ye, and I. Chlamtac, “Wireless gigabit ethernetextension,” in Proc. Broadband Netw., 2005, vol. 1, pp. 425–433.

[15] Y.-L. Hsueh, W.-T. Shaw, L. G. Kazovsky, A. Agata, and S. Yamamoto,“SUCCESS PON demonstrator: Experimental exploration of next-generation optical access networks,” IEEE Commun. Mag., vol. 43, no. 8,pp. S26–S33, Aug. 2005.

[16] J. Buus, “Tunable lasers in optical networks,” J. Lightw. Technol., vol. 24,no. 1, pp. 5–11, Jan. 2006.

[17] C. J. Chang-Hasnain, “Tunable VCSEL,” IEEE J. Sel. Topics QuantumElectron., vol. 6, no. 6, pp. 978–987, Nov. 2006.

[18] R. Chen, D. A. B. Miller, K. Ma, and J. S. Harris, Jr., “Novel electricallycontrolled rapidly wavelength selective photodetection using MSMs,”IEEE J. Sel. Topics Quantum Electron., vol. 11, no. 1, pp. 184–189,Jan./Feb. 2005.

[19] [Online]. Available: http://www.sfgov.org/site/tech_connect_page.asp[20] [Online]. Available: http://www.teknovus.com/files/TK3722%20PB1.pdf[21] W.-T. Shaw et al., “MARIN: Metro-access ring integrated network,” in

Proc. Globecom, San Francisco, CA, Nov. 2006, pp. 1–5.[22] D. S. J. De Couto, D. Aguayo, J. Bicket, and R. Morris, “A high-

throughput path metric for multi-hop wireless routing,” in Proc.MobiCom, San Diego, CA, Sep. 2003, pp. 134–146.

[23] R. Draves, J. Padhye, and B. Zill, “Routing in multi-radio, multi-hopwireless mesh networks,” in Proc. MobiCom, Philadelphia, PA, Sep. 2004,pp. 114–128.

[24] J. Padhye, S. Agarwal, V. N. Padmanabhan, L. Qiu, A. Rao, and B. Zill,“Estimation of link interference in static multi-hop wireless networks,” inProc. IMC, 2005, pp. 305–310.

[25] [Online]. Available: http://www.isi.edu/nsnam/ns/[26] [Online]. Available: http://motorola.canopywireless.com/

For photograph and biography of Wei-Tao Shaw, please see page 3442 ofthis issue.

For photograph and biography of Shing-Wa Wong, please see page 3442 ofthis issue.

For photograph and biography of Ning Cheng, please see page 3442 of thisissue.

Koussalya Balasubramanian received the Bachelor of Technology degreefrom the Madras Institute of Technology, Chennai, India. She is currentlyworking toward the M.S. degree at Stanford University, Stanford, CA.

Her research interests include hybrid communications (wireless–optical)and networking. She is currently working with the Photonics and Network-ing Research Laboratory, Department of Electrical Engineering, StanfordUniversity.

Xiaoqing Zhu received the B.E. degree in electron-ics engineering from Tsinghua University, Beijing,China, in 2001 and the M.S. degree in electricalengineering from Stanford University, Stanford, CA,in 2002, where she is currently working toward thePh.D. degree within the Department of ElectricalEngineering.

During the summer of 2003, she was with the IBMAlmaden Research Center, San Jose, CA, workingon intelligent document extraction and analysis. Shespent the summer of 2006 interning at Sharp Labs

of America, Camas, WA, working on distributed resource allocation for mediastreaming over wireless home networks, which remains her current researchinterest. She has worked on several research projects granted by the NationalScience Foundation.

Dr. Zhu was the recipient of the Stanford Graduate Fellowship from 2001to 2005.

Martin Maier (M’04) received the Dipl.-Ing. andDr.-Ing. degrees (both with distinctions) from theTechnical University of Berlin, Berlin, Germany, in1998 and 2003, respectively.

In the summer of 2003, he was a PostdoctoralFellow with the Massachusetts Institute of Technol-ogy (MIT), Cambridge. He was a Visiting Profes-sor with Stanford University, Stanford, CA, fromOctober 2006 to March 2007. He is an AssociateProfessor with the Institut National de la RechercheScientifique, Montreal, QC, Canada. Currently, his

research activities focus on evolutionary upgrades of optical access and metronetworks and their seamless integration with broadband wireless access net-works. He is the author of the book Metropolitan Area WDM Networks—AnAWG-Based Approach (Kluwer, 2003) and the forthcoming book OpticalSwitching Networks (Cambridge Univ. Press, 2007).

For photograph and biography of Leonid G. Kazovsky, please see page 3442of this issue.

hybrid architecture and integrated routing in a scalable...

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