Chapter2_Mesh - Wireless Mesh Networks Wireless WIRELESS...

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Unformatted text preview: Wireless Mesh Networks Wireless WIRELESS MESH NETWORKS The term 'wireless mesh networks' describes wireless The networks in which each node can communicate directly with one or more peer nodes. The term 'mesh' originally used to suggest that all nodes The were connected to all other nodes, but most modern meshes connect only a sub-set of nodes to each other. subI.F. Akyildiz,, “Wireless Mesh Networks; A Survey”, Akyildiz,, Survey” Computer Networks Journal, March 2005. Slides from GIT I. F. Akyildiz Still, this is quite different than traditional wireless Still, networks, which require centralized access points to mediate the wireless connection. Even two 802.11b nodes that are side-by-side in Even side- byinfrastructure mode must send data to each other through the access point. 2 Wireless Mesh Networks Nodes are comprised of mesh routers and mesh clients. Each node operates not only as a host but also as a router, forwarding packets on behalf of other nodes that may not be within direct wireless transmission range of their destinations. A WMN is dynamically self-organized and self-configured, with the nodes in the network automatically establishing and maintaining mesh connectivity among themselves Wireless Mesh Networks Extend the range and link robustness of existing Wi-Fi’s by Extend Wiallowing mesh-style multi-hopping meshmultiA user finds a nearby user and hops through it - or possibly user multiple users - to get to the destination Every user becomes a relay point or router for network traffic Every Mesh networks consist of multiple wireless devices equipped with Mesh with COTS802.11 a/b/g cards that work in ad-hoc fashion a/b/g ad802.11 capable antennas placed on rooftops allow a large area 802.11 coverage 3 4 Network Architecture WMNs consist of two types of nodes: WMNs Mesh Routers and Mesh Clients. Clients. A wireless mesh router contains additional routing wireless functions to support mesh networking. It is equipped with multiple wireless interfaces built It on either the same or different wireless access technologies. A wireless mesh router can achieve the same wireless coverage as a conventional router but with much lower transmission power through multi-hop multicommunications. 5 WMN COMPONENTS Examples of mesh routers based on different embedded systems: (a) PowerPC and (b) Advanced Risc Machines (ARM) 6 1 WMN COMPONENTS WMN COMPONENTS Examples of mesh clients: (a) Laptop, (b) PDA, (c) Wi-Fi IP Phone and (d) Wi-Fi RFID Reader. 7 8 WMN COMPONENTS Wireless Mesh Networks Conventional nodes (e.g., desktops, laptops, PDAs, Conventional PDAs, PocketPCs, phones, etc.) equipped with wireless PocketPCs, network interface cards (NICs) can connect directly (NICs) to wireless mesh routers. Customers without wireless NICs can access WMNs by Customers connecting to wireless mesh routers through, e.g., Ethernet. Thus, WMNs will greatly help users be always-on-line Thus, always- onanywhere anytime. 9 10 Wireless Mesh Networks Moreover, the gateway/bridge functionalities in mesh Moreover, routers enable the integration of WMNs with various existing wireless networks such as cellular, wireless sensor, wireless-fidelity (Wi-Fi), worldwide interwireless(Wiinteroperability for microwave access (WiMAX) networks. (WiMAX) Consequently, through Consequently, existing networks are impossible services of an integrated WMN, users of provided with otherwise these networks. Network Architecture Classification 1. INFRASTRUCTURE MESHING 2. CLIENT MESH NETWORKING 3. HYBRID MESH NETWORKING 11 12 2 INFRASTRUCTURE MESHING . INFRASTRUCTURE MESHING It provides backbone for conventional clients It and enables integration of WMNs with existing wireless networks, through gateway/bridge functionalities in mesh routers. Conventional clients with Ethernet interface Conventional can be connected to mesh routers via Ethernet links. For conventional clients with the same radio For technologies as mesh routers, they can directly communicate with mesh routers. 13 14 This includes mesh routers that form an infrastructure This for clients that connect to them. This can be built using various types of radio This technologies The mesh routers form a mesh of self-configuring, The selfself-healing links among themselves. selfWith gateway functionality, mesh routers can be With connected to the Internet. INFRASTRUCTURE MESHING INFRASTRUCTURE MESHING If different radio technologies are used, clients must If communicate with the base stations that have Ethernet connections connections to mesh routers. These are the most commonly used. These For example, community and neighborhood networks can be built For using infrastructure meshing. The mesh routers are placed on the roof of houses in a The neighborhood, which serve as access points for users inside the homes and along the roads. Typically, two types of radios are used in the routers, i.e., for Typically, for backbone communication and for user communication, respectively. respectively. The mesh backbone communication can be established using long-range The longcommunication techniques including directional antennas. 15 16 Client WMNs Client meshing provides peer-to-peer networks among client Client peer- todevices. Client nodes constitute the actual network to perform routing and Client and configuration functionalities as well as providing end-user endapplications to customers. A mesh router is not required for these types of networks. mesh A packet destined to a node in the network hops through multiple packet nodes to reach the destination. They are formed using one type of radios on devices. They Moreover, the requirements on end-user devices is increased when Moreover, endcompared to infrastructure meshing; the end-users have to endperform additional functions such as routing and selfselfconfiguration. 17 Client WMNs 18 3 HYBRID WMNs This architecture is the combination of infrastructure and This client meshing. Mesh clients can access the network through mesh routers as Mesh well as directly meshing with other mesh clients. While the infrastructure provides connectivity to other While networks such as the Internet, Wi-Fi, WiMAX, cellular, and WiWiMAX, sensor networks; the routing capabilities of clients provide improved connectivity and coverage inside the WMN. The hybrid architecture will be the most applicable case!!! The Hybrid WMNs 19 20 CHARACTERISTICS Multi-hop Wireless Network MultiSupport for Ad Hoc Networking, and Capability Support of Self-Forming, Self-Healing, and Self-Organization SelfSelfSelfMobility Dependence on the Type of Mesh Nodes Mobility Multiple Types of Network Access Multiple Dependence of Power-Consumption Constraints on the Dependence PowerType of Mesh Nodes Compatibility and Interoperability with Existing Compatibility Wireless Networks WMNs vs AD HOC Networks Dedicated Routing and Configuration: In ad-hoc networks, end-user devices also perform routing and In adendconfiguration functionalities for all other nodes. However, WMNs contain mesh routers for these functionalities. However, Hence, the load on end-user devices is significantly decreased, Hence, endwhich provides lower energy consumption and high-end application highcapabilities to possibly mobile and energy constrained end-users. endMoreover, the end-user requirements are limited which Moreover, enddecreases the cost of devices that can be used in WMNs. WMNs. 21 22 WMNs vs AD HOC Networks Multiple Radios: * Mesh routers can be equipped with multiple radios to perform perform routing and access functionalities. * This enables separation of two main types of traffic in the wireless domain. * While routing and configuration traffic is performed between mesh between routers, access to the network from end-users can be carried in a enddifferent radio. * This significantly improves the capacity of the network. * On the other hand, these functionalities are performed in the same the channel in ad-hoc networks constraining the performance. ad23 WMNs vs AD HOC Networks Mobility: Since ad-hoc networks provide routing using the end-user devices, Since adendthe network topology and connectivity depends on the movement movement of users. This imposes additional challenges to routing protocols as well as This network configuration and deployment. Since mesh routers provide the infrastructure in WMNs, the Since WMNs, coverage of the WMN can be engineered easily. While providing continuous connectivity throughout the network, While the mobility of end-users is still supported, without compromising endthe performance of the network. 24 4 WMNs vs AD HOC Networks Compatibility: WMNs contain many differences when compared to ad hoc WMNs networks. Ad hoc networks can be considered as a subset of WMNs. Ad WMNs. More specifically, the existing techniques developed for ad-hoc More adnetworks are already applicable to WMNs. WMNs. As an example, through the use of mesh routers and routingAs routingcapable end-users, multiple ad hoc networks can be supported endin WMNs, but with further integration of these networks. WMNs, Application Scenarios 1. Broadband Home Networking: Realized through IEEE 802.11 WLANs Realized Problem Problem location of the access points. location Homes have many dead zones without service coverage. Homes Solutions based on site survey are expensive and not practical for home Solutions for networking, while installation of multiple access points is also expensive and not convenient because of Ethernet wiring from access points to backhaul network access modem or hub. Moreover, communications between end nodes under two different Moreover, access points have to go all the way back to the access hub. Not an efficient solution, especially for broadband networking. Mesh networking can resolve all these issues in home networking. Mesh 25 26 Broadband Home Networking Application Scenarios 2. Community and Neighborhood Networking: In a community, the common architecture for network access is based on In based cable or DSL connected to the Internet, and the last-hop is lastwireless by connecting a wireless router to a cable or DSL modem. modem. This type of network access has several drawbacks: This * Even if the information must be shared within a community or neighborhood, all traffic must flow through Internet. Internet. This significantly reduces network resource utilization. utilization. * Large percentage of areas in between houses is not covered by wireless covered services. * An expensive but high bandwidth gateway between multiple homes or multiple neighborhoods may not be shared and wireless services must be set up individually. As a result, network service costs may increase. * Only a single path may be available for one home to access the Internet access or communicate with neighbors. 27 WMNs mitigate the above disadvantages through flexible mesh WMNs connectivities between homes. 28 Community Networking Application Scenarios 3. Enterprise Networking: Within an office or all offices in an entire building, or among offices Within in multiple buildings. IEEE 802.11 WLANs are widely used in various offices currently. IEEE However, they are still isolated islands. Connections among them are achieved through wired Ethernet Connections (still costly) In addition, adding more backhaul access modems only increases In capacity locally, but does not improve robustness to link failures, failures, network congestion and other problems of the entire enterprise network. Multiple backhaul access modems can be shared by all nodes in the Multiple the entire network, and thus improve the robustness and resource utilization of enterprise networks. WMNs can grow easily as the size of enterprise expands. WMNs 29 30 5 Enterprise Networking Application Scenarios Metropolitan Area Networks: The physical-layer transmission rate of a node in WMNs is much higher The physicalthan that in any cellular networks, e.g., an IEEE 802.11g node can can transmit at a rate of 54 Mbps. Moreover, the communication between nodes in WMNs does not rely on a Moreover, wired backbone. Compared to wired networks, e.g., cable or optical networks, Compared wireless mesh MAN is an economic alternative to broadband networking, networking, especially in underdeveloped regions. Wireless mesh MAN covers a potentially much larger area than Wireless home, enterprise, building, or community networks. Thus, the requirement on the network scalability by wireless mesh MAN Thus, mesh is much higher than that by other applications. 31 32 Metropolitan Area Networks METROPOLITAN AREA NETWORKS 33 34 Application Scenarios Transportation Systems: Instead of limiting IEEE 802.11 or 802.16 access to stations Instead and stops, mesh networking technology can extend access into buses, ferries, and trains. Thus, convenient passenger information services, remote monitoring of Thus, monitoring in-vehicle security video, and driver communications can be supported. insupported. To enable such mesh networking for a transportation system, two key To techniques are needed: the high-speed mobile backhaul from a vehicle high(car, bus, or train) to the Internet and mobile mesh networks networks within the vehicle. Transportation Systems 35 36 6 Application Scenarios Building Automation: In a building, various electrical devices including power, light, elevator, air In light, conditioner, etc., need to be controlled and monitored. Building Automation Currently this task is accomplished through standard wired networks, which is Currently networks, very expensive due to the complexity in deployment and maintenance of a wired maintenance network. Recently Wi-Fi based networks have been adopted to reduce the cost of such Recently Winetworks. However, this effort has not achieved satisfactory performance yet, because However, yet, deployment of Wi-Fis for this application is still rather expensive due to wiring Wiof Ethernet. If BACnet (Building Automation and Control Networks) access points are If replaced by mesh routers, the deployment cost will be significantly reduced. significantly The deployment process is also much simpler due to the mesh connectivity The connectivity among wireless routers. 37 38 Application Scenarios Health and Medical Systems: In a hospital or medical center, monitoring and diagnosis data need to be In need processed and transmitted from one room to another for various purposes. various Data transmission is usually broadband, since high resolution medical images and Data medical various periodical monitoring information can easily produce a constant and large volume of data. Traditional wired networks can only provide limited network access to certain Traditional access fixed medical devices. Wi-Fi based networks must rely on the existence of Ethernet connections, which Wiconnections, may cause high system cost and complexity but without the abilities to eliminate abilities dead spots. However, these issues do not exist in WMNs. However, WMNs. Application Scenarios Security Surveillance Systems: As security is turning out to be a very high concern, security As surveillance systems become a necessity for enterprise buildings, buildings, shopping malls, grocery stores, etc. In order to deploy such systems at locations as needed, WMNs In are a much more viable solution than wired networks to connect all devices. Since still images and videos are the major traffic flowing in the Since the network, this application demands much higher network capacity than other applications. 39 40 Critical Factors Influencing Network Performance Typical examples: * MIMO systems, Critical Factors Influencing Network Performance 2. Scalability 3. Mesh Connectivity 4. Broadband and QoS 5. Compatibility and Inter-Operability Inter6. Security 7. Ease of Use 1. Radio Techniques: * Directional and smart antennas (Key Technology for IEEE 802.11n) (Key * Multi-radio/multi-channel systems Multi- radio/multi* Reconfigurable radios * Frequency agile/cognitive radios and * Even software radios 41 42 7 MAC LAYER Differences between WMNs MACs and the Wireless Networks MACs * SINGLE CHANNEL MACs Improving Existing MAC Protocols MAC protocols are proposed for multi-hop WMNs by enhancing existing multiMAC protocols. For example, in an IEEE 802.11 mesh networks, these schemes usually usually adjust parameters of CSMA/CA, e.g., contention window size, and modify backoff procedures. However, these solutions only achieve a low end-to-end throughput, end- tobecause they cannot significantly reduce the probability of contentions contentions among neighboring nodes. As long as contention occurs frequently, whichever method is taken to taken modify backoff or contention resolution procedures, the end-to-end end- tothroughput will still be significantly reduced due to the accumulating effect accumulating on the multi-hop path. multi- MACs for WMNs are concerned with more than one hop communication MACs * MAC must be distributed, needs to be collaborative, and must must work for multipoint-to-multipoint communication. multipoint- to* Network self-organization is needed for better collaboration between selfneighboring nodes and nodes in multi-hop distances. multi* Mobility affects the performance of MAC. 43 44 SINGLE CHANNEL MACs Cross-layer design with advanced physical layer techniques Cross- SINGLE CHANNEL MACs Proposing Innovative MAC Protocols: Determined by their poor scalability in an ad hoc multi-hop network, multirandom access protocols such as CSMA/CA are not an efficient solution. solution. Thus, revisiting the design of MAC protocols based on TDMA or CDMA is CDMA indispensable. To date, few TDMA or CDMA MAC protocols are available for WMNs, WMNs, probably because of two factors: * The complexity and cost of developing a distributed and cooperative cooperative MAC with TDMA or CDMA. * The compatibility of TDMA (or CDMA) MAC with existing MAC protocols. 45 46 1. MACs based on Directional Antennas Eliminate exposed nodes if antenna beam is assumed to be perfect. perfect. Due to the directional transmission, more hidden nodes are produced. Also face other difficulties such as cost, system complexity, and practicality of fast steerable directional antennas. SINGLE CHANNEL MACs 2. MACs with Power Control They reduce exposed nodes, especially in a dense network, using low transmission power, and thus, improve the spectrum spatial reuse factor in WMNs. WMNs. However, hidden nodes may become worse because lower transmission power level reduces the possibility of detecting a potential interfering node. SINGLE CHANNEL MACs For example, in IEEE 802.16, the original MAC protocol is a centralized TDMA scheme, but a distributed TDMA MAC for IEEE 802.16 mesh is still missing. In IEEE 802.11 WMNs, how to design a distributed WMNs, TDMA MAC protocol overlaying CSMA/CA is an interesting but a challenging problem!!! 47 48 8 Multi-Channel MACs Multi-Channel Single-Transceiver MAC: MultiSingleIf cost and compatibility are the concern, one transceiver on a radio is a preferred hardware platform. Multi-Channel MACs Multi-Channel Multi-Transceiver MACs MultiMultiA radio includes multiple parallel RF front-end chips frontand baseband processing modules to support several simultaneous channels. On top of the physical layer, only one MAC layer module is needed to coordinate the functions of multiple channels. To date, no multi-channel multi-transceiver MAC multimultiprotocol has been proposed for WMNs. WMNs. Since only one transceiver is available, only one channel is active at a time in each network node. However, different nodes may operate on different channels simultaneously. To coordinate transmissions between network nodes under this situation, protocols such as the multi-channel MAC and the multiseed-slotted channel hopping (SSCH) scheme are needed. seedSSCH is actually a virtual MAC protocol, since it works on top of top IEEE 802.11 MAC and does not need changes in the IEEE 802.11 MAC. 49 50 Multi-Channel MACs Multi-Radio MACs MultiThe network node has multiple radios each with its The own MAC and physical layers. Communications in these radios are totally independent. Communications Thus, a virtual MAC protocol such as the multi-radio Thus, multiunification protocol (MUP) is required on top of MAC to coordinate communications in all channels. In fact, one radio can have multiple channels in this In case. However, for simplicity of design and application, a However, single fixed channel is usually applied in each radio. MAC LAYER-Open Research Issues LAYERScalable Single-Channel MACs: SingleMACs: The scalability issue in multi-hop ad hoc networks has multi- hop not been fully solved yet. not Most of existing MAC protocols only solve partial problems of the overall issue, but raise other problems. To make the MAC protocol really scalable, new distributed and collaborative schemes must be proposed distributed to ensure that the network performance (e.g., to throughput and even QoS parameters such as delay and parameters delay jitter) will not degrade as the network size delay increases. increases. 52 51 MAC LAYER-Open Research Issues LAYERScalable Multi-Channel MACs MultiMulti-channel MAC protocols for radios with multiple transceivers have not been thoroughly explored, possibly due to the relatively high cost of such radios. However, as the cost goes down, a multi-channel multi-transceiver MAC will be a rather promising solution for WMNs. It is obvious that a multi-channel MAC protocol can achieve higher throughput than a single-channel MAC. However, to really achieve spectrum efficiency and improve the per-channel throughput, the scalable MAC protocol needs to consider the overall performance improvement in multiple channels. Thus, developing a scalable multi-channel MAC is a more challenging task than a single-channel MAC. 53 MAC LAYER-Open Research Issues LAYERMAC/Physical Cross-Layer Design CrossWhen advanced physical layer techniques, such as MIMO and cognitive radios, are used, novel MAC protocols, especially multi-channel MAC, need to be proposed to utilize the agility provided by the physical layer. Network Integration in the MAC Layer Mesh routers in WMNs are responsible for integration of various wireless technologies. Thus, advanced bridging functions must be developed in the MAC layer so that different wireless radios, such as IEEE 802.11, 802.16, 802.15, etc., can seamlessly work together. Reconfigurable/software radios and the related radio resource management schemes may be the ultimate solution to these bridging functions. 54 9 MAC LAYER-Open Research Issues LAYERMAC Protocol Implementation. The functions of a MAC protocol are distributed in software, firmware, and hardware. Modifying functions in the firmware or hardware is much more complicated and costly than doing that in software. A solution to this problem is to develop a new MAC protocol architecture in which the proposed new MAC functions can be completely implemented in the software. Currently, several IEEE 802.11 chipset manufacturers have eliminated firmware in their MAC implementation architecture, and the hardware of some MAC chipsets is software programmable like a software defined radio (SDR). With such an architecture, a true software MAC. can be implemented. Routing Layer Optimal routing protocol for WMNs must capture the following features: Multiple Performance Metrics. * Many existing routing protocols use minimum hop-count as a hopperformance metric to select the routing path. * This has been demonstrated to be ineffective in many situations. situations. * For example, when a link on the minimum hop-count path has bad hopquality or experiences congestion, it becomes a bottleneck to the end-to-end throughput. - toend * To solve this problem, other performance metrics, e.g., link quality and round trip time (RTT), must be considered in the routing protocol. 56 55 Routing Layer Scalability * Setting up a routing path in a very large wireless network may take a long time. * Furthermore, even when the path is established, the node states on the path may change. * Thus, it is critical to have a scalable routing protocol in WMNs. WMNs. Routing Layer Adaptive Support of Both Mesh Routers and Mesh Clients. Mesh Considering the minimal mobility and no constraint of power consumption in mesh routers, a routing protocol much simpler than ad hoc network routing protocols can be developed for mesh routers. However, the routing protocol for mesh clients is usually complicated due to the support of mobility and power efficiency. Consequently, it is necessary to design a routing protocol that can adaptively support both mesh routers and mesh clients. 57 58 Robustness * To avoid service disruption, WMNs must be robust to link failures or congestion. * Thus, routing protocols need to be fault tolerant with link failures and can achieve load balancing. Routing Layer Routing Protocols with Various Performance Metrics: LQSR aims to select a routing path according to link quality metrics. metrics. Three performance metrics, i.e., expected transmission count (ETX), per(ETX), perhop RTT, and per-hop packet pair are implemented separately in LQSR. perThe performance of the routing protocol with these three performance performance metrics is compared with the method using the minimum hop-count. hopFor stationary nodes in WMNs, ETX achieves the best performance, WMNs, while the minimum hop-count method outperforms the three link quality hopmetrics when nodes are mobile. This result illustrates that the used link quality metrics are still not enough for WMNs when mobility is concerned. Routing Layer Multi-Radio Routing: MultiMulti-radio per node is a preferred architecture in the network Multilayer, because the capacity can be increased without modifying The MAC protocol. A multi-radio LQSR (MR-LQSR) is proposed multi(MRwhere a new performance metric, called weighted cumulative Expected transmission time (WCETT), is incorporated. WCETT takes into account both link quality metric and the minimum hop-count and achieves good tradeoff between delay and hopthroughput. MR-LQSR assumes that all radios on each node are tuned to nonMRnoninterfering channels with the assignment changing infrequently. In other words, MR-LQSR relies on the MAC layer to perform MRchannel selection. 59 60 10 Routing Layer Multi-Path Routing Multi* The main objectives of using multi-path routing are to perform The multibetter load balancing and to provide high fault tolerance. * Multiple paths are selected between source and destination. * When link is broken on a path due to a bad channel quality or or mobility another path in the set of existing paths can be chosen. chosen. * Thus, without waiting for setting up a new routing path, the the end-to-end delay, throughput, and fault tolerance can be improved. end- to* However, given a performance metric, the improvement depends on the availability of node-disjoint routes between source and nodedestination * Another drawback of multi-path routing is its complexity multi* As a result, how to design a cost-effective multi-path routing costmultiprotocol with appropriate performance metrics needs further study. 61 Routing Layer Hierarchical Routing: In hierarchical routing, a certain self-organization scheme is employed to group network nodes into clusters. Each cluster has one or more cluster heads. Nodes in a cluster can be one or more hops away from the cluster head. Since connectivity between clusters are needed, some nodes can communicate with more than one cluster and work as a gateway. Routing within a cluster and routing between clusters may use different mechanisms. 62 Routing Layer Hierarchical Routing: For example, inter-cluster routing can be a proactive protocol, while intra-cluster routing can be on demand. When the node density is high, hierarchical routing protocols tend to achieve much better performance because of less overhead, shorter average routing path, and quicker set-up procedure of routing path. However, the complexity of maintaining the hierarchy may compromise the performance of the routing protocol. Implementation difficulty, because a node selected as a cluster head may not necessarily have higher processing capability and channel capacity than the other nodes. Unless being intentionally designed so, the cluster head may become a bottleneck. 63 Routing Layer Geographic Routing: Compared to topology-based routing schemes, geographic routing schemes forward packets by only using the position information of nodes in the vicinity and the destination node. Thus, topology change has less impact on the geographic routing than other routing protocols. Early geographic routing algorithms are a type of single-path greedy routing schemes in which packet forwarding decision is made based on the location information of current forwarding node, its neighbors, and the destination node. However, all greedy routing algorithms have a common problem, i.e., delivery is not guaranteed even if a path exists between source and destination. 64 Routing Layer Geographic Routing: Partial flooding and keeping the past routing information can help to guarantee delivery. However, these approaches increase communication overhead and lose the stateless property of single-path greedy routing. In order to keep the stateless property and guarantee delivery, planar-graph based geographic routing algorithms are proposed recently. However, these algorithms usually have much higher communication overhead than single-path greedy routing algorithms. EXISTING ROUTING SOFTWARE IMPLEMENTATIONS The Mobile Mesh software is covered by the GNU General Public License (Version 2). License TBRPF, or Topology Broadcast based on Reverse-Path Forwarding, is a proactive, link-state TBRPF Reverselinkrouting protocol designed for mobile ad-hoc networks, which provides hop-by-hop routing along adhop- byminimum hop paths to each destination. It seems it is patent-protected unless it becomes a patentIETF standard. OSPF is a link-state routing protocol. It is designed to be run internal to a single Autonomous OSPF linksingle System. Each OSPF router maintains an identical database describing the Autonomous describing System's topology. From this database, a routing table is calculated by constructing a calculated shortest-path tree. shortestGNU Zebra is free software that manages TCP/IP-based routing protocols. It is released as GNU TCP/IPpart of the GNU Project, and is distributed under the GNU General Public License. It General supports BGP-4 protocol as described in RFC1771 (A Border Gateway Protocol 4) as well as BGPRIPv1, RIPv2, and OSPFv2. LocustWorld develops a free bootable CD solution based on the AODV protocol, and also LocustWorld protocol, develops and sells a complete ready-to-deploy MeshBox running its software, most (but not ready- toall) of which is available under the GPL. The MeshBox and mesh software have been used in a number of community networks in the UK. 4g MeshCube. The German company 4G Mobile Systems has developed a tiny MeshCube running 4g Debian Linux on a MIPS processor, using MITRE Mobile Mesh routing software. This is a software. ready-to-deploy gateway with both a wireless and a wired interface. With a power ready- toconsumption of 4W (and potentially lower), it is ideal for deployment with an autonomous deployment sustainable power source. 65 66 11 Routing Layer- Open Research Issues LayerScalability. Hierarchical routing protocols can only partially solve this problem due to their complexity and difficulty of management. Geographic routing relies on the existence of GPS or similar positioning positioning technologies, which increases cost and complexity of WMNs. WMNs. Thus, new scalable routing protocols need to be developed. Routing Layer - Open Research Issues Routing/MAC Cross-Layer Design. CrossA routing protocol needs to interact with the MAC layer in order to improve its performance. Adopting multiple performance metrics from layer-2 into routing protocols layeris an example. However, interaction between MAC and routing layers is so close that merely exchanging parameters between them is not adequate. Merging certain functions of MAC and routing protocols is a promising promising approach. It is particularly meaningful for multi-radio or multi-channel routing, multimultibecause the channel/radio selection in the MAC layer can help the path the selection in the routing layer. Better Performance Metrics. New performance metrics need to be developed. Also, it is necessary to integrate multiple performance metrics into a routing protocol so that the optimal overall performance is achieved. 67 68 Transport Layer Reliable Data Transport * * TRANSPORT PROTOCOLS TCP Variants: TCP variants TCP New transport protocols. Non-Congestion Packet Losses. NonThe classical TCPs do not differentiate congestion and nonnoncongestion losses. As a result, when non-congestion losses occur, the network nonthroughput quickly drops due to unnecessary congestion avoidance. In addition, once wireless channels are back to the normal operation, the classical TCP cannot be recovered quickly. Feedback mechanism can be used to differentiate different packet losses. 69 70 TRANSPORT PROTOCOLS Unknown Link Failure Link failure occurs frequently in mobile ad hoc networks, since all nodes are mobile. As far as WMNs are concerned, link failure is not as critical as in mobile ad hoc networks, because the WMN infrastructure avoid the issue of single-point-of-failure. However, due to wireless channels and mobility in mesh clients, link failure may still happen. To enhance TCP performance, link failure needs to be detected. One possible approach is to include a link failure notification scheme in the transport protocol. 71 TRANSPORT PROTOCOLS Network Asymmetry. Network asymmetry is defined as the situation where the forward direction of a network is significantly different from the reverse direction in terms of bandwidth, loss rate, and latency Since TCP is critically dependent on ACK, so its performance can be severely impacted by network asymmetry. Schemes such as ACK filtering, ACK congestion control, etc., are proposed to solve the network asymmetry problem However, whether they are applicable to WMNs needs investigation. 72 12 TRANSPORT PROTOCOLS Large RTT Variations. In WMNs, mesh routers and mesh clients are connected as an ad hoc network, so dynamic change of routing path is common. Considering mobility, variable link quality, traffic load, and other factors, the change may be frequent and may cause large variations of RTT. This will degrade the TCP performance, because the normal operation of TCP relies on a smooth measurement of RTT. 73 TRANSPORT PROTOCOLS New Transport Protocols: To further improve performance of transport protocols, researchers have started to develop entirely new transport protocols. The ad hoc transport protocol (ATP) is proposed for ad hoc networks. Transmissions in ATP are rate-based, and quick start is used ratefor initial rate estimation. The congestion detection is a delay-based approach, and thus delayambiguity between congestion losses and non-congestion losses is nonavoided. 74 TRANSPORT PROTOCOLS New Transport Protocols: Moreover, in ATP, there is no retransmission timeout, and congestion control and reliability are decoupled. By using an entirely new set of mechanisms for reliable data transport, new transport protocol like ATP achieves much better performance (e.g., delay, throughput, and fairness) than the TCP variants. However, for WMNs, an entirely new transport protocol is not WMNs, Favorable solution. WMNs will be integrated with the Internet and many other wireless networks, and thus, transport protocols for WMNs needs to be compatible with TCPs in such networks. 75 TRANSPORT PROTOCOLS Real-Time Delivery RealRCP protocols can be classified into two types: additive-increase additivemultiplicative-decrease (AIMD)-based or equation-based. multiplicative(AIMD)equationAn adaptive detection rate control (ADTFRC) scheme for mobile ad hoc networks an end-to-end multi-metric joint detection approach an end- tomultiis developed for TCP-friendly rate control schemes. TCPTo really support real-time delivery for multimedia realtraffic, the accuracy of the detection approach is still insufficient. Also all non-congestion packet losses due to different nonproblems are processed in the same way, which may degrade the performance of the rate control scheme. 76 Cross-layer Solution to Network Asymmetry. Cross- TRANSPORT PROTOCOLSPROTOCOLSOPEN RESEARCH ISSUES TRANSPORT PROTOCOLSPROTOCOLSOPEN RESEARCH ISSUES Adaptive TCP. WMNs will also be integrated with the Internet and various wireless networks such as IEEE 802.11, 802.16, 802.15, etc. The characteristics of these networks may be significantly heterogeneous due to different network capacity and behaviors of error control, MAC, and routing protocols. Such heterogeneity renders the same TCP ineffective for all networks. Applying different TCPs in different networks is a complicated and costly approach, and cannot achieve satisfactory performance 77 78 All problems of TCP performance degradation are actually related to protocols in the lower layers. For example, it is the routing protocol that determines the path for both TCP data and ACK packets. To avoid asymmetry between data and ACK packets, it is desired for a routing protocol to select an optimal path for both both data and ACK packets. Moreover, the link layer performance directly impacts packet loss ratio and network asymmetry. In order to reduce the possibility of network asymmetry, the MAC layer and error control may need to treat TCP data and ACK packets differently. 13 Application Layer Internet Access. Various Internet applications provide important timely information to people, make life more convenient, and increase work efficiency and productivity. In a home or small/medium business environment, the most popular network access solution is still DSL or cable modem along with IEEE 802.11 access points. However, comparing with this approach, WMNs have many potential advantages: low cost, higher speed, and easy installation. 79 Application Layer Distributed Information Storage and Sharing. Backhaul access to the Internet is not necessary in this type of applications, and users only communicate within WMNs. WMNs. A user may want to store high-volume data in disks owned by other highusers, download files from other users' disks based on peer-topeer- toPeer networking mechanism, and query/retrieve information located located in distributed database servers. Users within WMNs may also want to chat, talk on the video phones, and play games with each other. 80 Application Layer Information Exchange across Multiple Wireless Networks. For example, a cellular phone may want to talk to a Wi-Fi phone through WMNs, or a user on a Wi-Fi WiWMNs, Winetwork may expect to monitor the status in various sensors in a wireless sensor networks. Application Layer Three research directions: 1. Improve Existing Application Layer Protocols. Due to ad hoc and multi-hop wireless network architecture of multiWMNs, protocols in the lower layers cannot provide perfect support WMNs, for the application layer. For example, as perceived by the application layer, packet loss may not always be zero, packet delay may be variable with a large jitter, etc. Such problems may fail many Internet applications that work smoothly in a wired network. Therefore, existing application layer protocols need to be improved. improved. 81 82 Application Layer 2. Propose New Application Layer Protocols for Distributed Information Sharing. Currently, many peer-to-peer protocols are available for peer- toinformation sharing on the Internet. However, these protocols cannot achieve a satisfactory performance in WMNs, since WMNs have much different WMNs, characteristics than the Internet. New application layer protocols need to be developed. Application Layer 3. Develop Innovative Applications for WMNs. WMNs. Such applications must bring tremendous benefits to users, and cannot achieve best performance without WMNs. WMNs. Such applications will enable WMNs to be a unique networking solution instead of just another option of wireless networking. 83 84 14 Mobility Management Centralized mobility management schemes are not applicable in WMNs. WMNs. Thus, a distributed mobility management scheme is needed for WMNs. WMNs. However, because of the existence of a backbone network a distributed scheme for WMNs can be simpler than that for mobile ad hoc networks. How to take advantages of the network backbone to design a light-weight distributed mobility management scheme for WMNs lightneeds further investigation. 85 Mobility Management Location service is a desired feature by WMNs. WMNs. Location information can enhance the performance of MAC and routing protocols. It can help to develop promising location-related applications. locationProposing accurate or efficient algorithms for location service is still an open research topic Mobility management is closely protocols, the development of schemes is another interesting related to multiple layers of network network multi-layer mobility management multiresearch topic. 86 Power Management The goal of power management in WMNs varies with network nodes. Usually, mesh routers do not have a constraint on power consumption; consumption; power management aims to control connectivity, interference, spectrum spectrum spatial-reuse, and topology. spatialIn contrast to mesh routers, mesh clients may expect protocols to be to power efficient. For example, some mesh clients are IP phones or even sensors; power power efficiency is the major concern for them. Thus, it is quite possible that some applications of WMNs require power management to optimize both power efficiency and connectivity, which which results in a complicated problem. Network Monitoring The statistics in the MIB (management information base) of mesh nodes, especially mesh routers, need to be reported to one or several servers in servers order to continuously monitor the network performance. In addition, data processing algorithms in the performance monitoring monitoring software on the server analyze these statistical data and determine determine potential abnormality. Based on the statistical information collected from MIB, data processing algorithms can also accomplish many other functions such such as network topology monitoring. To reduce overhead, schemes for efficient transmission of network network monitoring information are expected. In addition, in order to accurately detect abnormal operation and quickly and derive network topology of WMNs, effective data processing algorithms WMNs, need to be developed. 88 87 SECURITY WMNs lack efficient and scalable security solutions, because their security is easier to be compromised due to: distributed network architecture, vulnerability of channels and nodes in the shared wireless medium, and dynamic change of network topology. Different attacks in several protocol layers can easily fail the network. Attacks may occur in the routing protocol such as advertising wrong routing updates. SECURITY The attacker may sneak into the network, impersonate a legitimate node, and does not follow the required specifications of a routing protocol. Same types of attacks as in routing protocols may also occur in MAC protocols. For example, the backoff procedures and NAV for virtual carrier sense of IEEE 802.11 MAC may be misused by some attacking nodes, which cause the network to be always congested by these malicious nodes. Attackers may also sneak into the network by misusing the cryptographic primitives. 89 90 15 SECURITY In a cryptographic protocol, the exchange of information among users occurs frequently. The users employ a fair exchange protocol which depends on a trusted third party. However, this trusted party is not available in WMNs due to lack of due infrastructure. infrastructure. A widely accepted counter-attack measure is authentication counterand authorization. For wireless LANs, this is taken care of by authentication, authorization, and accounting (AAA) services directly over the access acces point or via gateways. 91 SECURITY However, AAA is performed through a centralized server such as RADIUS (remote authentication dial-in user service). dialSuch a centralized scheme is not applicable in WMNs. WMNs. Moreover, security key management in WMNs is much more difficult thanin wireless LANs, because there is no central authority, trusted third party or server to manage security keys. Key management in WMNs need to be performed in a distributed but secure way. Therefore, a distributed authentication and authorization scheme with secure key management needs to be proposed for WMNs. WMNs. 92 SECURITY To further ensure security of WMNs, two more strategies WMNs, need to be considered: embedding security mechanism into network protocols such as secure routing and MAC protocols or developing security monitoring and response systems to detect attacks monitor service disruption, and respond quickly to attacks. attacks. For a secure networking protocol, a multi-protocol layer security multischeme is desired, because security attacks occur simultaneously in different protocol layers. For a security monitoring system, a cross-layer crossframework also needs to be developed. How to design and implement a practical security system, including including cross-layer secure network protocols and various intrusion detection crossalgorithms, is a challenging research topic. 93 Cross-Layer Design CrossThe methodology of layered protocol design does not necessarily lead to an optimum solution. This is particularly the case in WMNs due to unreliable physical links, dynamic network topology, distributed network architecture, etc. The physical channel in WMNs is variable in terms of capacity, bit error rate, etc. Although different coding, modulation, and error control schemes can be used to improve the performance of the physical channel, there is no way to guarantee fixed capacity, zero packet loss rate, or reliable connectivity. 94 Cross-Layer Design CrossIn order to provide satisfactory network performance, MAC, routing, and transport layer protocols need to interactively work together with the physical layer. In WMNs, because of the ad hoc feature, network topology WMNs, constantly changes due to mobility and link failures. Such a dynamic network topology impacts multiple protocol layers. layers. Thus, in order to improve the protocol efficiency, cross-layer design crossbecome indispensable, as discussed before in the open research issues of different protocol layers. Cross-Layer Design CrossCross-layer design can be performed in two ways. Cross1. Improve the performance of a protocol layer by considering parameters in other protocol layers. Typically, parameters in the lower protocol layers are reported to reported higher layers, e.g., the packet loss rate in the MAC layer can layer be reported to the transport layer so that a TCP protocol is able to differentiate congestion from packet loss; e.g., physical layer can report the link quality to a routing protocol as an additional performance metrics for the routing algorithms. 95 96 16 Cross-Layer Design Cross2. Merge several protocols into one component, e.g., in ad hoc networks, MAC and routing protocols can be combined into one one protocol in order to closely consider their interactions. The advantage of the first way is that it does not totally abandon The abandon the transparency between protocol layers. However, the second way can achieve much better performance through closer interaction between protocols. Certain issues must be considered when carrying out cross-layer protocol design: crossCross-layer design have risks due to loss of protocol layer Crossabstraction,incompatibility with existing protocols, unforeseen impact on the future design of the network, and difficulty in maintenance and management. 97 Academic Research Testbeds Carnegie-Mellon University's mobile ad hoc network testbed. CarnegieIt consists of 7 nodes: 2 stationary nodes, 5 car mounted nodes that drive It around the testbed site, and 1 car mounted roving node that enters and enters leaves the site. Packets are routed between the nodes using the DSR protocol which also which integrates the ad hoc network into the Internet via a gateway. They experiment with the network behavior under different levels of traffic load, including audio and video streams, and designing protocol enhancements to provide these streams with QoS promises. 98 Academic Research Testbeds Interesting results were observed in the experiments at CMU: Local (i.e., link layer) retransmission algorithms is a critical part of any multihop ad hoc network. If the retransmission algorithms implemented above the link layer, it must be adaptive in order to accommodate network congestion and periods of high contention in the wireless channel. Delivering routing protocol control packets as rapidly as possible possible is important for high end-to-end performance, and this implies end- tothat packets with routing implications should be scheduled for transmission ahead of users data packets. Academic Research Testbeds MIT's Roofnet This is an experimental multi-hop 802.11b mesh network. multiIt consists of about 50 wireless nodes to interconnect the Ethernet Ethernet networks (with Internet gateways) in apartments in Cambridge, MA. MA. A primary feature of Roofnet is that it requires no configuration or planning. One consequence of an unplanned network is that each node can route route packets through any of a large number of neighbors, but the radio radio link to each neighbor is often of marginal quality; finding the best multi-hop routes through a rich mesh of marginal links turns out to multibe a challenge. 100 99 Academic Research Testbeds MIT's Roofnet The average TCP throughput and latency of all Roofnet nodes to their nearest gateway were measured in April 2004. When 1 hop is considered for 18 nodes, the average throughput and latency are 357.2 Kbytesps and 9.7 ms. However, when 4 hops are considered for 7 nodes, the average throughput is only 47.3 Kbytesps and the average latency is 43.0 ms. The low multihop throughput reflects the problem typical in all 802.11 MAC based multihop networks. Academic Research Testbeds SUNY Stony Brook It is 4-node multi-channel 802.11b testbed. 4multiEach node is equipped with 2 cards whose channels were determined determined based on the load-aware channel assignment algorithm. loadThe multi-channel network achieves 2.63 times the throughput as multicompared to the single channel network. The number of non-overlapping channels in 802.11b standard, noni.e. 3, is the limiting factor for this performance. The performance, however, does not reach 3 times of the singlesinglechannel network performance because of the inter-channel interinterference that cannot be completely eliminated. 102 101 17 Academic Research Testbeds Substantial interference was observed between two 802.11b cards placed on the same machine despite operating on non-overlapping channels. In addition, the degradation due to inter-channel interference was found independent of the guard band. One way to reduce the interference is to use USB cards instead of PCI/PCMCIA cards and place them side-by-side in similar configuration as in Orinoco AP-1000 access points. Another possibility is to equip cards with external antennas and place the external antennas slightly away from each other. Yet another option is to use the upcoming Engim chipsets which solve the interference problem at RF-level. 103 Academic Research Testbeds California Institute for Telecommunications and Information Technology (Cal-(IT)^2) (CalA basic Wi-Fi MAC development platform called CalRADIO-I WiCalRADIOThis is a Wi-Fi research and development device that consists of a WiTI 5410 DSP, a 16-bit stereo CODEC, external Flash and SRAM 16memories, and support of a RF LAN module. It provides a convenient platform for development of RF radios from the physical layer up to the application layer. The key benefit of the board is that all aspects of the MAC are coded in C language and, therefore, are altered easily for research in queueing, security, power management, MIMO, queueing, cognitive radio, and so forth. 104 Academic Research Testbeds It also utilizes basic Symbol Technologies' Wi-Fi test Wiboard as the base for modifying board and re-spinning to incorporate reNew features. CalRADIO-I is evolving into a CalRADIO-II develop platform with CalRADIOCalRADIObasic DSP board and multiple RF front-end modules such as front802.11x, 802.16, cell and general RF. This will eventually allow the capability of publishing standards in standards software/firmware and hardware. Academic Research Testbeds The Broadband and Wireless Network (BWN) Lab at Georgia Tech The WMN, called BWN-Mesh, consists of 15 IEEE 802.11b/g based BWNmesh routers, among which several of them are connected to the next next generation Internet testbed (also available in the BWN Lab) as backhaul access to the Internet. The testbed consists of laptops and desktops equipped with IEEE 802.11b and IEEE 802.11g cards located in various rooms on the floor floor Where the BWN Lab resides. By changing the topology of the network, experiments investigating the effects of inter-router distance, backhaul placement interAnd clustering are performed along with mobility experiments using the using Laptops in the testbed. 105 106 BWN-Mesh Testbed at Georgia Tech Academic Research Testbeds Moreover, experiments with existing protocols (i.e., TCP, AODV, and IEEE 802.11g as transport, routing, and MAC protocols) for BWN-Mesh testbed have demonstrated that these protocols do BWNnot perform well in terms of end-to-end delay and throughput in end- toWMNs. WMNs. Currently, the research is focused on adaptive protocols for transport transport layer, routing and MAC layers and their cross-layer design. crossThese protocols are developed and evaluated on the BWN-Mesh BWNtestbed. 107 108 18 Academic Research Testbeds The approaches being explored in the BWN lab are not merely limited to Wi-Fi mesh networks but also applicable for wireless Wisensor networks (WSNs) and wireless sensor and actor networks (WSNs) (WSANs). WSANs). Thus, the BWN-Mesh testbed is integrated with the already BWNexisting BWN Sensor Network Testbed, which consists of MICA motes, with TinyOS distributed software operating system, and light, temperature, acoustic actuator, magnometer, and magnometer, accelerometer sensors. Academic Research Testbeds In align with this effort, BWN Lab is also trying to integrate the current Wi-Fi mesh networks with other Wiwireless networks such as WiMAX. WiMAX. Consequently, this integrated testbed will enable the design and evaluation of protocols applicable to heterogeneous wireless networks including WMNs, next WMNs, Generation Internet, WSNs, WSANs, and WiMAX. WSNs, WSANs, WiMAX. 109 110 Industrial Practice Microsoft Research Lab (MSR) It implements ad-hoc routing and link quality measurement in a adsoftware module called the mesh connectivity layer (MCL). Architecturally, MCL is a loadable Windows driver. It implements a virtual network adapter, so that the ad-hoc adnetwork appears as an additional (virtual) network link to the rest of the system. MCL routes by using a modified version of DSR called LQSR. Industrial Practice MCL is a routing protocol well-suited for low mobility, wellunconstraint power consumption and small diameter networks. The MCL driver implements an interposition layer between the link layer and the network layer. To higher layer software, MCL appears to be just another Ethernet link, albeit a virtual link. 111 112 Industrial Practice No modification to either network stack is required. Network layer functionality (for example ARP, DHCP, and Neighbor Discovery) works fine. Ad-hoc routing runs over heterogeneous link layers. AdMicrosoft's implementation supports Ethernet-like physical link layers Ethernet(e.g., IEEE 802.11 and 802.3) but the architecture accommodates link layers with arbitrary addressing and framing conventions. Industrial Practice INTEL: Low-cost and low-power access point prototypes have Lowlowbeen developed to enable further research on security, traffic characterization, dynamic routing and configuration, and QoS problems. A demonstration was discussed in various occasions, consisting of a of collection of Centrino laptop computers and IXP425 network processor based routers running AODV and 802.11b MAC protocols. The testbed results confirm that 802.11 MAC limits full exploitation of multihop throughput. 113 114 19 Industrial Practice As a means to enhance multihop throughput, it advocates spatial reuse through carrier sensing threshold tuning. Another potential solution experimented was the concept of heterogeneous networks: an 802.11 mesh network comprised of 4 high-end nodes, such as Intel XScale based nodes, is overlaid on a high50-sensor node (motes) network scattered throughout a large 50conference room. Sensor data can enter and exit the 802.11 backbone at multiple interchanges (the XScale nodes) in order to bypass the intermediate sensors. Industrial Practice NORTEL: A peer-to-peer architecture—with smart antennas, integrated routers peer- toarchitecture— and adaptive routing and security capabilities—to backhaul data wirelessly capabilities— to wired broadband networks. This minimizes the need for expensive expensive wired backhaul connections, such as T1 lines. Nortel Networks Wireless Mesh Network solution is comprised of three Nortel three main network elements: – Wireless Access Point 7220 (Wireless AP) The Wireless AP performs traffic collection and distribution functions functions for traffic within the Community Area Network (CAN) and incorporates: routing and wireless transit functions; security functions functions for validating connections to other Wireless APs; security functions APs; for controlling access by user devices; and low-cost advanced antenna lowdesigns for extended reach, simplified deployment and reliability. reliability. 116 115 Industrial Practice – Wireless Gateway 7250 The Wireless Gateway advertises reachability for one or more IP subnets assigned to Wireless LAN CAN subscribers and network entities. – In addition, the Wireless Gateway hides Wireless LANLANspecific mobility and provides data security for the mesh transit links (between Wireless AP 7220s). – Optivity Network Management System The Optivity Network Management System provides centralized facilities for monitoring and managing network operations, including discovery and visualization of Wireless AP 7220 and Wireless Gateway 7250, fault management, and real-time performance metrics. real117 Industrial Practice NORTEL 118 Industrial Practice MeshNetworks' MeshNetworks' Initial attempt to commercializing mesh technology was its Quadrature Division multiple access (QDMA) radio platform. The QDMA radio is designed for mobile ad hoc broadband networking. networking. It uses multi-channel MAC and PHY to overcome the effects of Doppler multishifting, rapid Raleigh fading and multipath encountered in a mobile system. mobile The MeshNetworks' scalable routing protocol is implemented above QDMA MeshNetworks' radios. The scalable routing technology utilizes a hybrid ad hoc routing algorithm that combines both proactive and reactive routing techniques. Industrial Practice To adapt the routing protocol to a given radio platform, adaptive adaptive Transmission protocol (ATP) is implemented to tightly bind the scalable scalable routing protocol to the underlying radio platform. MeshNetworks provides a software-only overlay solution that lets native software802.11b clients in existing networks work in mesh-mode. meshWhile it will not add any mobile broadband capabilities beyond what 802.11b what can already support, it will extend the range and link robustness of robustness existing Wi-Fi Networks by allowing mesh-style multi-hopping. WimeshmultiSecurity features in MeshNetworks' QDMA-based systems include MeshNetworks' QDMAa hardware firewall on a chip that makes it impossible for a client to access somebody else's packets. 119 120 20 Industrial Practice Tropos Networks employs a cellular Wi-Fi network architecture to support Wi“infrastructure mesh” networking, using its a layer-3 network operating system (NOS) called Tropos Sphere, layerthat runs on standard 802.11 hardware and software. Tropos Sphere operates on every (small sized) Tropos Wi-Fi cell Wiand contains the key communications, path selection, and security security functions that allow the Wi-Fi cells to inter-operate and form a Wiintercompletely wireless network like a wireless routed LAN. Tropos uses a lightweight control protocol for supporting a large number of Wi-Fi cells. Wi- Industrial Practice It uses a proprietary predictive path optimization protocol to improve endimprove enduser throughput and continuously optimize performance to compensate for compensate the changing RF environment. This protocol is called predictive wireless routing protocol (PWRP), which is (PWRP), analogous to traditional wired routing protocols such as OSPF). However, PWRP does not use routing tables or rely on hop-count only to hopselect transmission paths. Rather, it compares packet error rates and other network conditions to conditions determine the best path at a given moment. Since the system is largely a layer-3 solution that relies on the standard layer802.11 MAC protocol for a large mesh network, many of the throughput performance impairments remain unresolved. 121 122 INDUSTRIAL PRACTICE PacketHop's Core Technology (SRI International Lab) It consists of a network controller, performing gateway, QoS, security, and roaming functions, QoS, a network management system, and the Windows software for ad hoc mesh routing. This is largely a layer-3 solution that runs on layer802.11 and multi-mode broadband radios. multiPacketHop is in collaboration with Nortel to complement Nortel's “infrastructure mesh” solution with its ad hoc meshing capability. 123 INDUSTRIAL PRACTICE Kiyon Inc., is in field trials with building automation and “small office home office'' (SOHO) customers of its broadband wireless mesh routers. Its technology is a layer-2/3 solution that layerimplements a hybrid CSMA/CA and distributed TDMA MAC protocol atop an 802.11g/a physical layer. 124 INDUSTRIAL PRACTICE This is tightly coupled with a multi-metric “Attribute Routing'' multiprotocol, aiming at high and steady multihop throughput in a mesh network. As the new generation 802.11 radios adopt the soft MAC approach, e.g., Atheros, Broadcom and more recently Intel, Kiyon's Atheros, enhanced MAC/routing protocols can be implemented in host software and downloaded into these standard 802.11 chipsets at runtime. Kiyon supports both infrastructure and client mesh, stationary or mobile wireless networks. INDUSTRIAL PRACTICE Several routers form a broadband backbone of the network. Each of the routers is equipped with Kiyon's routing and MAC protocol software plus a standard IEEE 802.11g/a radio. For client access to the broadband backbone, several options can be adopted. The first option is called the “wired host route”, in which a client can connect to the backbone via an Ethernet connection. 125 126 21 INDUSTRIAL PRACTICE Any IP capable devices (e.g., a RFID reader, BACnet controller or database server) can be connected to the wireless network this way. way. No software modification on the client is required. The second option is called the “wireless host route”, in which a client device connects to the wireless network via a wireless LAN LAN interface such as an 802.11 b/g/a NIC. In this arrangement, the client is “homed” on one of the routers in the network that satisfies the defined routing metric, such as signal signal strength. INDUSTRIAL PRACTICE The client has the option to run Kiyon's software or not. With Kiyon's software, a client becomes a full function router. Without Kiyon's software, a client device running standard 802.11 station software can originate and terminate traffic. Mobility of client devices is supported in both cases. The third option is a form of hierarchical network, in which a number of standard 802.11 access points serve as the access layer for client devices. Each of these access points is attached to one of the backbone routers via an Ethernet connection. 128 127 Standard Activities IEEE 802.11 Mesh Networks Currently, IEEE 802.11 wireless networks can achieve a peak rate of 11 Mbps (802.11b) and 54 Mbps (802.11a/g). Also under development is a high-bandwidth extension to highthe current Wi-Fi standard. WiResearchers expect 802.11n to increase the speed of Wi-Fi connections by 10 to 20 times. Wi- Standard Activities Although many home users will not benefit from the additional speed right away, because of limits on their cable or DSL connections, enterprises are hoping the technology will allow them to reduce the burden of laying and maintaining Ethernet cabling throughout the building. There are many academic testbeds and commercial deployment of mesh networks using IEEE 802.11 wireless LAN technology. However, mesh networking is at the same stage as wireless LANs were in the early 1990's; they are expensive and proprietary. 129 130 Standard Activities To become commoditized, the economic pressures are driving the standard processes. Furthermore, protocols for 802.11 ad hoc mode are insufficient for multi-hop and mesh networking, because of lack of scalability multiin the MAC protocol, resulting in poor network performance. A working group within IEEE 802.11, called 802.11s, has been formed recently to standardize the extended service Set (ESS). 802.11s aims to define a MAC and PHY layers for meshed networks that extended coverage with no single point of failure. In such networks, 802.11 cellular wireless LAN access points relay relay information from one to another in a router-like hop-by-hop fashion. routerhop- by- Standard Activities As users and access points are added, the capacity increases, as in the Internet, giving rise to a scalable and redundant architecture. Early discussions in this working group include definition of WMNs, WMNs, usage cases, QoS, architecture specifications, security, routing QoS, protocols,and development of new MAC protocols. Several task groups have been formed to tackle these issues. 131 132 22 Standard Activities Wi-Fi mesh networking can be implemented in two basic modes: Wiinfrastructure and client meshing. The former is an infrastructure ESS mesh, in which access points are interconnected through wireless links that enable automatic topology topology learning and dynamic path configuration. Clients are associated with access points and need not be aware of the mesh. Infrastructure meshing creates wireless backhaul mesh among access access points or wireless routers. This reduces system backhaul costs while increasing network coverage and reliability. 133 Standard Activities To provide an IEEE 802.11 ESS Mesh, 802.11s will define an architecture and protocol based on the IEEE 802.11 MAC to create an IEEE 802.11 wireless distribution system (WDS) that supports both broadcast/multicast and unicast delivery at the MAC layer using radio-aware metrics over self-configuring multi-hop topologies. radioselfmulti- hop From the view of access points, the infrastructure meshing also forms an ad hoc network among access points. The other mode of meshing, i.e. client meshing, is a layer-3 layerad hoc IBSS (independent basic service set), in which all devices devices operate in ad hoc mode in a flat network, using IP routing. 134 Standard Activities There is no distinction between access points and client. Client meshing enables wireless peer-to-peer networks to form between and among client devices and does not require any network infrastructure to be present. In this case, clients can hop through each other to reach other clients in the network. To maximize the benefit that meshing can offer, both modes should be supported simultaneously and seamlessly in a single network Standard Activities IEEE 802.15 Mesh Networks IEEE 802.15.3a standard is based on MultiBand OFDM Alliance (MBOA)'s physical layer that uses ultra wide band (UWB) to reach up to 480 Mbps. A competing proposal of a Direct Sequence-UWB (DS-UWB) claims support for up to 1.3 Gbps. It is intended for high throughput personal area networking (PAN) that has communication distances of around 10 meters (or less), with applications in home networking space, with imminent wireless extensions to USB, IEEE 1394, and with the capability to address the convergence of PC, consumer electronics and IP mobile phones. Vendors planning to produce 802.15.3a products have formed the WiMedia Alliance, a branding and testing organization that will certify standards compliance. 135 136 Standard Activities UWB networks hold many advantages over other wireless networks, such as covert communications, low power and cost requirement, accurate location information, and extra high bandwidth. However, the communication range is rather short. Mesh networks have been predicted to be the killer application for UWB radio systems. A new MAC proposed by MBOA, which deviates substantially from the original IEEE 802.13a MAC proposal, has added strong support for mesh networking and mobility, paving the way for UWB to enter the enterprise network. 137 Standard Activities The MBOA MAC uses piconet structure, combined with a decentralized resource-handling ability to allow for resourcethe reservation of timeslots for 802.15.3-like TDMA 802.15.3for high priority connections requiring determinism while assigning contention-based, best-effort access contentionbestperiods. 138 23 Standard Activities IEEE 802.15.4 is intended for telemetry with low data rate, long battery life and low device cost requirements. The ZigBee Alliance is developing higher-level protocols that will run higherover 802.15.4 MAC and PHY layers that operate in unlicensed bands bands worldwide. Raw data rates of 250Kbps can be achieved at 2.4GHz (16 channels), 40Kbps at 915MHz (10 channels), and 20Kbps at 868MHz (1 channel). The transmission distance is expected to range from 10 to 75 meters, depending on power output and environmental characteristics. 139 Standard Activities The ZigBee network layer supports multinetwork topologies including star, cluster tree, and mesh. In a mesh topology, a special node called coordinator is responsible responsible for starting the network and for choosing key network parameters. parameters. The routing algorithm uses a request-response protocol to eliminate requestsub-optimal routing. subRecently a new working group, i.e., IEEE 802.15.5, is established to established determine the necessary mechanisms in the physical and MAC layers layers to enable mesh networking in wireless PANs. . PANs 140 Standard Activities IEEE 802.16 Mesh Networks While IEEE 802.11 networks fulfill the need for data services in a local area (i.e. last several hundreds of feet), IEEE 802.16 aims at serving the broadband wireless access in metropolitan Area networks (i.e., last mile), supporting point-to-multipoint connection oriented QoS communications to extend fiber optic backbones. The original 802.16 standard operates in the 10-66 GHz frequency band and requires line-of-sight towers. Standard Activities The 802.16a extension, ratified in January 2003, uses a lower frequency of 2-11 GHz, enabling nonline-of-sight connections. With 802.16a, carriers will be able to connect more customers to a single tower and substantially reduce service costs. To allow consumers to connect to the Internet while moving at vehicular speeds, researchers are developing an extension to IEEE 802.16 standard called 802.16e. 141 142 Standard Activities To enhance the 802.16 mesh, several proposals have been submitted to the standard committee. A group within 802.16, the Mesh Ad Hoc committee, is investigating ways to improve the performance of mesh networking. It is understood that only a small amount of meshing is required to see a large improvement in the coverage of a single base station. More importantly, the following issues are considered in specifying the 802.16 mesh MAC protocol: * avoiding hidden terminal collisions, * selection of links, * synchronization, * power versus data rate tradeoffs, and * greater routing-MAC interdependence. routing143 ADVANTAGES OF WMNs Price: Price: 802.11 radios have become quite cheap, but the radios are often still often among the most expensive elements of such a network. The fact that that each mesh node runs both as a client and as a repeater potentially means potentially saving on the number of radios needed and thus the total budget. Ease and simplicity: Ease If you have a box that is pre-installed with wireless mesh software and preuses standard wireless protocols such as 802.11b/g, the setup is extremely simple. Since routes are configured dynamically, it is often enough to simply drop the box into the network, and attach whatever whatever antennas are required for it to reach one or more existing neighboring neighboring nodes (assuming that we can solve the issue of IP address allocation). allocation). Organization and business models: Organization The decentralized nature of mesh networks lends itself well to a decentralized ownership model wherein each participant in the network network owns and maintains their own hardware, which can greatly simplify the simplify financial and community aspects of the system. 144 24 ADVANTAGES OF WMNs Network robustness: Network The character of mesh topology and ad-hoc routing promises greater adstability in the face of changing conditions or failure at single nodes, single which will quite likely be under rough and experimental conditions. conditions. Power: Power: The substrate nodes of a mesh network can be built with extremely low extremely power requirements, meaning that they can be deployed as completely completely autonomous units with solar, wind, or hydro power. Power generating units are typically connected to points of infrastructure and human presence. This makes them valid locations for network nodes. As a secondary benefit, the presence of integrated network nodes within nodes power networks may allow for better monitoring and management. ADVANTAGES OF WMNs Integration: Integration: Mesh hardware is typically small, noiseless, and easily encapsulated in weatherproof boxes. This means it also integrates nicely outdoors as well as in human human housing. Reality fit: Reality Reality rarely comes as a star, ring, or a straight line. In difficult terrain -- be that urban or remote -- where not every user can see one or few central points, chances are one can see one or more neighboring users. 145 146 FURTHER ADVANTAGES Provides a value-added entry into the high-speed wireless packet and data Provides valuehighbusiness Utilizes 802.11 technology—the interface of choice for high-speed wireless Utilizes technology— highpacket data. Offers high-speed wireless packet data access across a wider coverage area Offers highToday's cellular systems do not provide the bandwidth available in WLANs. Today's WLANs. Today's isolated hotspot 802.11 deployments do not satisfy user desire for Today's ubiquitous access or for mobility. Emergence of small-form factor terminals with 802.11 wireless interfaces Emergence smallmeans impending demand for adding mobility to WLAN packet data services. services. Minimizes cost of capital, installation and commissioning Minimizes Utilizes low-cost 802.11 technology. Utilizes lowUses wireless links for backhaul to eliminate costs associates with installation Uses with of wired interconnect. 147 FURTHER ADVANTAGES Auto-configuration algorithms in Wireless Access Point eliminate Autocosts associated with engineering and organization of the wireless wireless backhaul network. Minimizes cost of operations Minimizes Uses wireless links for backhaul to eliminate costs associated with Uses with ongoing leasing of facilities. Auto-configuration, self-organizing and self-healing are intrinsic Autoselfselfto the Wireless Mesh Network solution Centralized OAM&P minimizes staffing requirements. Centralized Highly flexible in terms of capacity, coverage and availability Highly Increasing capacity, coverage and/or availability simply means Increasing deploying more Wireless Access Points. Wireless Access Points maybe deployed indoors or outdoors. Wireless 148 Ugly Truths 1. Ugly Truths 3. That does not sound too bad when you are putting together a wireless wireless sensor network with limited bandwidth and latency considerations. considerations. It is DISASTROUS if you wish to provide the level of latency/throughput latency/throughput people are accustomed to with their wired networks. Consider the case of just 10 client stations at each node of a 4 hop mesh network. The clients at the last rung will receive -at best- 1/(16,0000) of the besttotal bandwidth at the root. Radio is a shared medium and forces everyone to stay silent while one Radio while person holds the stage. Wired networks, on the other hand, can and do hold multiple simultaneous conversations. 2. In a single radio ad hoc mesh network, the best you can do is (1/2)^^n at each hop. So in a multi hop mesh network, the max available bandwidth available to you degrades at the rate of 1/2, 1/4, 1/8. By the time you are 4 hops away the max you can get is 1/16 of the total available bandwidth. 149 150 25 Ugly Truths 4. Why has this not been noticed as yet? Because first there are not a lot of mesh networks around and second, they have not been tested under high usage situations. Browsing and email do not count. Try video - where both latency and bandwidth matter - or VOIP where the bandwidth is a measly 64Kbps but where latency matters. Even in a simple 4 hop ad hoc mesh network with 10 clients, VOIP clients, phones will not work well beyond the first or second hop – the latency and jitter caused by CSMA/CA contention windows (how wireless systems avoid collisions) will be unbearable. Mesh networks are a great concept. But the challenge lies in managing managing the dynamics of mesh networks so users receive an acceptable level of level performance in terms of both latency and throughput. It is time to focus on solving some real problems to make mesh networks scale and provide stable performance. 151 Is this a Disruptive Technology? Disruptive In its vision of WLANs and wireless access networks In of the very near future, Mesh Networks sees every client device also becoming a relay point or router for network traffic. One immediate benefit is that such networks can in One effect see around corners. Even line-of-sight network technologies like 802.11 Even line- ofcan become non-line-of-sight - almost overnight if non- line- ofMesh Networks can deliver what it is promising. And the next-generation networks the company is And nextbuilding will also power mobile broadband services. 152 What is a Disruptive Technology? Disruptive "In real-world terms, it has to meet at least two of "In realthree criteria: Be ten times cheaper than any alternative, have ten times higher performance, and ten times higher functionality. All three is best." There are two parts to Mesh Networks' supposedly There disruptive technology. WMNs as Disruptive Technology One is QDMA (quad-division multiple access), a One (quadproprietary radio technology developed for and currently used by WMNs. WMNs. QDMA's most notable characteristics are that it is IP QDMA's from end to end and supports high-speed mobile highbroadband access and infrastructure-free "ad hoc infrastructurepeer-to-peer networking." peer- toThe company claims it can deliver up to 6 Mbps to The each user in a QDMA wireless network. The technology also has built-in GPS capabilities and The builtQoS (quality of service) for IP voice and video. 153 154 REMARKS For QoS in the strict sense, there are doubts. For But for Video and Data with a certain bandwidth, they will take But off in wireless as the physical wireless capacity becomes really really broadband and reliable for mobile end users. Because, currently, GSM/GPRS systems are widely used Because, here for data and multimedia (to a certain degree, such as small small video, image and sound captures with the camera of the cellular cellular phone) communication, although its data rate is very low GPRS GPRS data rates were around 14.4-20 14.4- REMARKS However, the interest of the people and the success in However, the implementation draws big companies' attention. Especially the future application of Mesh networks is Especially considered to realize Mobile WiFi by integrating Mesh Enabled Architecture (MEA) architecture with 802.11 access points. MEA consists of wireless cards, cheap mesh wireless MEA routers, portable wireless routers, and intelligent access points 155 156 26 Wireless Mesh Networks Case Studies Medford, OR (70,000 people) 24 square miles – broadband communications ROI – 8 months possible $770,000 to install Garland, TX (221,000 people) 57 square miles – broadband communications Cost Avoidance – subscription fees / cell towers Cost 27 ...
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