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  1. 1. Multi-Radio Hybrid Wireless-Optical Broadband Access Networks N. S. C. Correia†, J. Coimbra†, G. Schutz ¥ † Center for Electronic, Optoelectronic and Telecommunications, University of Algarve, Faculty of Science and Technology, 8005-139 Faro, Portugal Phone: +351 289800900, Fax: +351 289819403, e-mail: {ncorreia,jcoimbra}@ualg.pt ¥ University of Algarve, Institute of Engineering, 8005-139 Faro, Portugal Phone: +351 289800165, Fax: +351 289888405, email: gschutz@ualg.pt Abstract1— The wireless-optical broadband-access network interfaces are attracting the research community due to the(WOBAN) architecture has been proposed as a flexible solution considerable improvement in network throughput andto meet the ever-demanding needs in access networks. At the availability of cost-effective wireless devices [6]. Currentwireless front-end multi-channel communication, with routers IEEE 802.11b/g and 802.11a standards, for example, providehaving multiple radio interfaces tuned to non-overlapping 3 and 12 orthogonal channels, respectively, that can workchannels, can be used to improve network throughput in a cost- simultaneously with negligible inter-channel interference [7].effective way. In this paper we address integrated routing and Although equipping mesh routers with multiple radios canchannel assignment in multi-radio WOBANs. Our goal is to alleviate the capacity problem, channel assignment to radiosevaluate how equipping mesh routers with multiple radios can becomes a challenge. As far as known, research efforts ineffectively alleviate the capacity problem in WOBANs. The WOBAN architectures do not consider multi-radio routers inoptimal distribution of radios is also analyzed. the sense that the assignment of channels to routers is not considered. I. INTRODUCTION In this paper we address integrated routing and channel assignment in multi-radio WOBANs. Our goal is to evaluate Optical and wireless networks were initially developed for how equipping mesh routers with multiple radios candifferent communication scenarios [1]. Optical networks have effectively alleviate the capacity problem in WOBANs. Thebeen mainly used for high-bandwidth and long-distance optimal distribution of radios is also analyzed.communications while the wireless technology is used at The rest of this paper is organized as follows. Section IIwireless local networks with flexibility and low bandwidth provides an overview on WOBAN architectures and multi-needs. The present growing demand for bandwidth-intensive radio wireless mesh front-end. Section III defines andservices, and the way people now communicate, are mathematically formulates the multi-radio WOBAN problemaccelerating research on efficient and cost-effective access being addressed in the paper. Section IV analyses resultsinfrastructures whereas optical-wireless combinations are while Section V concludes the article.seen as promising approaches. The wireless-opticalbroadband-access network (WOBAN) architecture has been II. WOBAN ARCHITECTURErecently proposed as a flexible solution to meet such ever-demanding needs in access networks [2,3,4]. The WOBAN At the front end a WOBAN consists of a multi-hop wirelessarchitecture provides a flexible and cost-effective solution mesh network while at the back end an optical accesswhere fiber is provided as far as possible from the central network provides connection to the Internet [2,4]. At the backoffice (CO) to the end users and then wireless access is end the dominant technology is the passive optical networkprovided at the front end. Because of such excellent (PON) having optical line terminals (OLTs), located at thecompromise early versions are being deployed as municipal CO, and optical network units (ONUs) to provide connectionaccess solutions to eliminate the need for wired connection to to wireless gateway routers. Different PON segments areevery customers wireless router thus saving on network supported, with each segment radiating from a single OLT atdeployment cost [5]. the CO to multiple ONUs near end-users. The PON interior The network scalability is a very challenging issue in elements are basically passive combiners, couplers andWOBAN architectures [1]. An increase in the number of splitters. Since no active elements exists between the OLTswireless mesh routers may lead to more hops, decreasing the and the ONUs the PONs are considered robust networks. Inper router throughput and degradating the performance of the traditional time-division-multiplexed (TDM) PONs annetwork. A way to reduce degradation is to either use more upstream and a downstream wavelength channel is used forradio interfaces per router, tuned to non-overlapping bidirectional communication [3]. The WOBAN architecturechannels, or to properly scale the number of gateway routers is illustrated in Figure 1. To provide connection betweento that of the mesh routers. Multi-channel communication in PONs a reconfigurable optical backhaul, allowing easywireless mesh networks with routers having multiple radio bandwidth reallocation, can be used. In [1] a reconfigurable optical ring is proposed.This work was supported by the Foundation for Science and Technologyfrom Portugal within CEOT (Center for Electronic, Optoelectronic andTelecommunications) and by the project PTDC/EEA-TEL/71678/2006.
  2. 2. considerable improvement in network throughput [6]. Current IEEE 802.11b/g and 802.11a standards provide 3 and 12 orthogonal channels, respectively, that can work simultaneously with negligible inter-channel interference [7]. In such multi-radio networks each radio interface has the capability of switching over orthogonal channels and transmission or reception is possible at any channel at a time. That is, considering a specific channel, only links that are located out of their mutual interference range can transmit at the same time. Consequently, a careful channel assignment becomes critical in such networks so that more simultaneous transmission occurs, leading to a global system capacity improvement. Fig. 1. WOBAN Architecture. Although having low installation and maintenance cost, dueto the passive infrastructure, in the traditional PON the OC isshared by all end users. That is, a packet sent to a specificuser, connected to a particular ONU, is broadcast to allONUs. As users demand for more bandwidth, wavelength-division-multiplexing (WDM) PONs may become a betteralternative. A WDM PON solution supports multiplewavelengths over the same fiber infrastructure, but activecomponents become necessary. Incorporating WDM in aPON allows the support of much higher bandwidth andscalability when compared to the standard PON whichoperates in single-wavelength mode. Regarding the wireless infrastructure, standard WiFi orWiMAX technology can be used for wireless mesh Fig. 2. Illustration of interference in wireless front-end: a)connectivity. These wireless access technologies have been Network with feasible links based on interference ranges; b)employed worldwide. Several wireless routers, also called Network with transmission links after channel assignment.base stations (BSs), provide multihop connectivity for user To better illustrate interference consider a wireless networktraffic delivery toward a few wireless gateway routers that are modeled by a directed graph , where is the setconnected to the ONUs of the optical back end. An ONU can of nodes, each equipped with one or more interface cardsdrive multiple gateways. An individual user scattered over (referred here as radios), and is the set of feasiblesuch geographical area will associate with a nearby wireless transmission links. A node can transmit to node only ifrouter for Internet access. When multiple routers are available the distance between and , denoted by , is smallerthe less overloaded one can be chosen. A mesh router cangain access to a gateway router through multiple paths. In the than the transmission range of node , denoted by . That is,upstream direction traffic can be delivered to any of the a feasible transmission linkgateways while in the downstream direction traffic is sent to exists if and only if , where and are thea specific wireless router. Note that the gateways/ONUs can source and destination nodes of link , respectively. Feasiblebe strategically placed over the geographical region to better transmission links are illustrated in Figure 2a) for an exampleserve the wireless community [8]. network. Feasible transmission links are potential transmission links and can become one after a channel being WOBAN is envisioned primarily for residential and assigned to it for transmission to occur, illustrated in Figurebusiness users in municipal networks where wireless devices 2b).have limited mobility [2,4]. In this case the location ofwireless mesh routers and gateways/ONUs is known in While the transmission range of a node, , defines theadvance and the channel of operation for access points can be maximum physical range of a radio signal, the interferencecarefully planned. Access solutions using WOBAN are range determines the area in which other nodes will not beexpected in many cities around the world in a near future. For able to receive or transmit signals successfully. Thus, sincea clear understanding of WOBAN advantages see [3]. is a directed graph, if we consider a specific link then another link can interfere with , unable to transmit onA. Multi-Radio Wireless Front-End the same channel simultaneously, if and only if node As previously stated, multi-channel communication in or node are located in the interferencewireless mesh networks with routers having multiple radio range of node , i.e.interfaces are attracting the research community due to the (1)
  3. 3. where . Note that the network conditions can be traffic transmission from different primary routes. This candifferent if we switch with because of the wireless nature happen because nearby communications interfere with eachof the network. In the example of Figure 2, and other and the small number of radios does not allow the usecan not transmit simultaneously at the same channel because of non-interfering channels. The precomputed primary routes is at the interference range of , and vice versa. are the shortest ones. This is because traffic demands, and theSimultaneous transmission would be possible for effective transmission rate of radios, oscillate too much. Intransmissions and . Note that, if was at the this fast changing scenario, planning the network routes usingtransmission range of then and could not another criteria can be too risky. Besides this, unacceptablesimultaneously transmit using the same channel. For long routes could be computed. Note that, choosing thesimplicity, the example illustrates the case when but shortest routes indirectly leads to a network that is able tousually the interference range is assumed to be twice the support more traffic due to the reduced number of hops and,transmission range. therefore, reduced use of resources per route. Also, as already In summary, the interference among concurrent stated, gateways and ONUs are usually placed near moretransmissions can dramatically affect the throughput in traffic intensive areas. Note, however, that the mathematicalwireless networks and a careful channel assignment is formulation provided next is general and can be applied toneeded. When using multi-radio technologies in WOBAN, any set of primary routes.different radio channel assignments will lead to different The maximum channel transmission rate is normalized as athroughputs flowing from wireless router aggregators to unit constant and local traffic demand at nodes is definedgateways/ONUs. This is due to the fact that different channel proportionally and denoted by , .assignments result into different sets of active transmission B. Problem Formulationlinks, called induced graph, and also result into differentinterferences. Note that an active transmission link between Let us define as the set of precomputed primary routes. Atwo neighbour nodes exists only if their radio interfaces share specific primary route included in is denoted by . Everya common channel, being able to communicate with each route can be defined as a connected serie of links,other. written as . We define and as the set of links used by primary route and the set of III. MULTI-RADIO WOBAN PROBLEM DEFINITION links used by primary route having node as its source or destination, respectively.A. General Assumptions − Traffic Flow over Channels: Let us consider the wireless network modeled by a directedgraph , where is the set of nodes, each equipped Let be a binary variable indicating if the traffic ofwith one or more interface cards (referred here as radios), and flows through channel at link . For traffic is the set of feasible transmission links. For simplicity the to flow through channels of primary routes,transmission and interference ranges are considered here to (2)be the same for all radios inside a specific node (the work iseasily extended to consider different ranges). We define amatrix , having lines and columns, that summarizes − Limitation on the Number of Radios:independence among links. That is, if can not Let be a binary variable indicating if the nodetransmit, on the same channel, simultaneously with (not transmits or receives through channel . Then,independent). (3) The subset of routers aggregating local user traffic, andresponsible for injecting the user packets into the wirelessmesh of the WOBAN, is denoted by ⊂ . The subset of That is, if at least one primary route uses a particularmesh routers acting as gateways, and attached to an ONU, is channel at some specific node then a radio becomesdenoted by ⊂ . The set of available channels per radio, necessary and is forced to be one. The number of radiosthat can work simultaneously, is denoted by . The total in use is limited bynumber of radios is denoted by R. A global number of radios, (4)and not the number of radios on a per-node basis, allows theallocation of radios to critical regions. − Channels Time Division Coefficients: In the following sections we assume precomputed primaryroutes. A single primary route is assigned to each node for The transmission bandwidth of a specific channel can benormal traffic delivery. These routes have a gateway router as time divided between primary traffic flowing through thedestination. The nodes can have a different number of radio channel and primary traffic of links at the interference rangeinterfaces but, for simplicity, the set of channels is the that are using the same channel. Note that, although a singlesame for all radios. Time-division can be used to allocate route is computed for each node, so that local packets are assigned a specific gateway as destination, a node can be an
  4. 4. intermediate for the delivery of traffic originated at someother node. Thus, local and intermediate traffic, at a specificnode, can be directed to different gateways. At someinterference range links will use the same channel if no extrachannels and/or radios are available. Let χ be the load ofthe most overloaded channel of all links. Then, χ (5) This constraint accounts for all traffic flowing throughprimary routes at the interference range. − Objective Function: Fig. 4. Average number of hops from radios to nearest gateway. The maximization of the network scalability is achieved bythe following objective function V. CONCLUSIONS χ (6) We have analyzed the capacity problem in multi-radio − Binary and Non-Negative Assignments: WOBANs. Results show that the use of multiple radios per χ (7) router is much more effective when 12 orthogonal channels are available while for 3 channels the spectrum rapidly IV. ANALYSIS OF RESULTS becomes exhausted. The optimal distribution of radios can easily be obtained using the presented approach. In this section we analyze the results obtained, usingCPLEX, for the multi-radio WOBAN capacity planning REFERENCESproblem. Results were obtained for the SFNet, a WOBAN inSan Francisco discussed in [4]. This network has 25 [1] W.-T. Shaw, et al., “Hybrid Architecture and Integratednodes/routers, 5 gateways included. Routing in Scalable Optical-Wireless Access Network”, IEEE Journal of Lightwave Technology, Vol. 25, No. 11 Simulations were done for 3 and 12 orthogonal channels (Nov. 2007), pp. 3443-3451.available. Results in Figure 3 show that the spectrum rapidly [2] Suman Sarkar, et al., “A novel delay-aware routingbecomes exhausted when 3 channels are available and an algorithm (DARA) for a hybrid wireless-opticalincrease in the number of radios per node rapidly starts to broadband access network (WOBAN)”, IEEE Network,have no effect on the network capacity problem. The lowest Vol. 22, No. 3 (May/Jun 2008), pp. 20-28.value for χ that can be obtained is 2.5. When 12 channels [3] Suman Sarkar, et al., “Hybrid Wireless-Opticalare available the use of multiple radios per node can be much Broadband-Access Network (WOBAN): A Review ofmore benefic. Relevant Challenges”, IEEE Journal of Lightwave The results in Figure 4 show the average number of hops Technology, Vol. 25, No. 11 (Nov. 2007), pp. 3329-from radios to the nearest gateway. This average distance 3340.tends to decrease as the number of radios in the network [4] Suman Sarkar, et al., “DARA: Delay-Aware Routingincreases meaning that the nodes/routers near the gateways Algorithm in a Hybrid Wireless-Optical Broadbandare the ones needing extra radios (are more congested). The Access Network (WOBAN)”, IEEE ICC 2007, pp. 2480-optimal distribution of radios is obtained when the average 2484.distance to gateways stabilizes. [5] Suman Sarkar, et al., “Hybrid Wireless-Optical Broadband-Access Network (WOBAN): Network Planning and Setup”, IEEE Journal of Lightwave Technology, Vol. 26, No. 6 (Aug. 2008), pp. 12-21. [6] Jihui Zhang, et al., “Joint routing and scheduling in multi-radio multi-channel multi-hop wireless networks”, BROADNETS 2005, Vol. 1, pp. 631-640. [7] Krishna N. Ramachandran, et al., “Interference-Aware Channel Assignment in Multi-Radio Wireless Mesh Networks”, IEEE INFOCOM 2006, pp. 1-12. [8] Suman Sarkar, et al., “Optimum Placement of Multiple Optical Network Units (ONUs) in Optical-Wireless Hybrid Access Network”, IEEE/OSA OFC 2006. Fig. 3. Load of most overloaded channel.