Wireless sensor networks

Mitigating performance degradation in congested wireless sensor networks


CHAPTER 1: WIRELESS SENSOR NETWORKS


1.1 Introduction:

Wireless sensor networks (WSNs)[1] have attracted tremendous attention in both academia and industry in recent years. A WSN consists of one or more sinks and perhaps tens or thousands of sensor nodes scattered in an area. The upstream traffic from sensor nodes to the sink is many-to-one multi-hop convergent. A WSN consists of one or more sinks and perhaps tens or thousands of sensor nodes scattered in an area. The upstream traffic can be classified into four delivery models: event-based, continuous, query-based, and hybrid delivery. Due to the convergent nature of upstream traffic, congestion more probably appears in the upstream direction. Congestion that can leads to packet losses and increased transmission latency has Congestion control generally follows three steps congestion detection, congestion notification, and rate-adjusting. Congestion control protocol efficiency depends on how much it can achieve the following performance objectives: (i) First, energy-efficiency requires to be improved in order to extend system lifetime. Therefore congestion control protocols need to avoid or reduce packet loss due to buffer overflow, and remain lower control overhead that will consume additional energy more or less. (ii) Second, fairness needs to be observed so that each node can achieve fair throughput. Fairness can be achieved through rate-adjustment and packet scheduling (otherwise referred to as queue management) at each sensor node. (iii) Furthermore, support of traditional quality of service (QoS) metrics such as packet loss ratio and packet delay along with throughput may also be necessary. There are several congestion control protocols for sensor networks. They differ in the way that they detect congestion, broadcast congestion related information, and the way that they adjust traffic rate. CCF (Congestion Control and Fairness) exactly adjusts traffic rate based on packer service time along with fair packet scheduling algorithms, while Fusion in performs stop-and-start non-smooth rate adjustment to mitigate congestion. CODA jointly uses end-to-end and hop-by-hop controls. Both CODA and ARC employ AIMD-like (Additive Increase Multiplicative Decrease) coarse rate adjustment .The existing congestion control protocols for WSNs only guarantee simple fairness, which means that the sink receives the same throughput from all nodes. In fact, sensor nodes might be either outfitted with different sensors or geographically deployed in different place and therefore they may have different importance or priority need to gain different throughput. Therefore weighted fairness is required to make sensor nodes get a throughput proportional to their priority. This paper investigates the problem of upstream congestion control in WSNs. We propose a new priority-based congestion control protocol (PCCP). Our contribution includes: We use packet inter-arrival time and packet service time in order to produce a measure of congestion. By incorporating information about packet inter-arrival time and the packet service time, we can capture congestion level at the node or at the link through a parameter, referred to as congestion degree, which is defined as the ratio of service time over inter-arrival time.


1.2 Differentiate the data generated in the wireless sensor network based on priority:

This paper investigates the problem of upstream congestion control in WSNs. It proposes a new priority-based congestion control protocol (PCCP). Our contribution includes: Using packet inter-arrival time and packet service time in order to produce a measure of congestion. By incorporating information about packet inter-arrival time and the packet service time, we can capture congestion level at the node or at the link through a parameter, referred to as congestion degree, which is defined as the ratio of service time over inter-arrival time. PCCP realizes priority-dependent weighted fairness which allows sensor nodes to receive priority-dependent throughput. This model has not been considered by others. PCCP results in low buffer occupancy. As a result, it can avoid and/or reduce packet loss and therefore improve energy-efficiency. It achieves high link utilization and low packet delay. The project overview possesses the system models, presents PCCP in detail, provides simulation results.


In WSNs, sensor nodes might have different priority due to their function or location. Therefore congestion control protocols need guarantee weighted fairness so that the sink can get different, but in a weighted fair way, throughput from sensor nodes. 2) Congestion control protocols need to improve energy-efficient and support traditional QoS in terms of packet delivery latency, throughput and packet loss ratio PCCP tries to avoid packet loss while guaranteeing weighted fairness and supporting multi-path routing with lower control overhead. PCCP consists of three components: intelligent congestion detection (ICD), implicit congestion notification (ICN), and priority-based rate adjustment (PRA).ICD detects congestion based on packet inter-arrival time and packet service time. The joint participation of inter-arrival and service times reflect the current congestion level and therefore provide helpful and rich congestion information. To the best of our knowledge, jointly use of packet inter-arrival and packet service times as in ICD to measure congestion in WSNs has not been done in the past. PCCP uses implicit congestion notification to avoid transmission of additional control messages and therefore help improve energy-efficiency. In ICN, congestion information is piggybacked in the header of data packets. Taking advantage of the broadcast nature of wireless channel, child nodes can capture such information when packets are forwarded by their parent nodes towards the sink. Finally, PCCP designs a novel priority-base rate adjustment algorithm (PRA) employed in each sensor node in order to guarantee both flexible fairness and throughput, where each sensor node is given a priority index. PRA is designed to guarantee that the node with higher priority index gets more bandwidth. The nodes with the same priority index get equal bandwidth and a node with sufficient traffic gets more bandwidth than one that generates less traffic. The use of priority index provides PCCP with high flexibility in weighted fairness. For example, if the sink wants to receive the same number of packets from each sensor node, the same priority index can be set for all nodes. On the other hand, if the sink wants to receive more detailed sensory data from a particular set of sensor nodes, such sensor nodes can be assigned a higher priority index and therefore allocated higher bandwidth. PCCP is a hop-by-hop upstream congestion control protocol for WSNs. It uses packet inter-arrival and service times to accurately measure congestion at each sensor node. Introduces node priority index and realizes weighted fairness. Simulation results show that PCCP achieves high link utilization and flexible fairness. PCCP achieves small buffer size therefore it can avoid/reduce packet loss and therefore improve energy-efficiency, and provide lower delay. WMNs are multi-hop wireless networks formed by mesh routers and mesh clients. These networks typically have a high data rate and low deployment and maintenance overhead. Mesh routers are typically stationary and do not have energy constraints, but the clients are mobile and energy constrained. Some mesh routers are designated as gateway routers which are connected to the Internet through a wired backbone. A gateway router provides access to conventional clients and interconnects ad hoc, sensor, cellular, and other networks to the Internet. A mesh network can provide multi-hop communication paths between wireless clients, thereby serving as a community network, or can provide multi-hop paths between the client and the gateway router, thereby providing broadband Internet access to clients. As there is no wired infrastructure to deploy in the case of WMNs, they are considered cost-effective alternatives to WLANs (wireless local area networks) and backbone networks to mobile clients.


The existing wireless networking technologies such as IEEE 802.11 are used in the implementation of WMNs. The IEEE 802.11 is a set of WLAN standards that define many aspects of wireless networking. One such aspect is mesh networking, which is currently under development by the IEEE 802.11 Task Group. Recently, there has been growing research and practical interest in WMNs. There are numerous ongoing projects on wireless mesh networks in academia, research labs, and companies. Many academic institutions developed their own test for research purposes. These efforts are toward developing various applications of WMNs such as home, enterprise, and community networking. As the WMNs use multi-hop paths between client nodes or between a client and a gateway router, the existing protocols for multi-hop ad hoc wireless networks are well suited for WMNs [2]. The ongoing work in WMNs is on increasing the throughput and developing efficient protocols by utilizing the static nature of the mesh routers and topology.


1.3 Single-Hop and Multi-Hop Wireless Networks

Generally, wireless networks are classified as single-hop and multi-hop networks. In a single-hop network, the client connects to the fixed base station or access point directly in one hop. The well-known examples of single-hop wireless networks are WLANs and cellular networks. WLANs contain special nodes called access points (APs), which are connected to existing wired networks such as Ethernet LANs. The mobile devices are connected to the AP through a one-hop wireless link. Any communication between mobile devices happens via AP. In the case of cellular networks, the geographical area to be covered is divided into cells which are usually considered to be hexagonal. A base station (BS) is located in the center of the cell and the mobile terminals in that cell communicate with it in a single hop fashion. Communication between any two mobile terminals happens through one or more BSs. These networks are called infrastructure wireless networks because they are infrastructure (BS) dependent. The path setup between two clients (mobile nodes), say node A and node B, is completed through the BS.


In a multi-hop wireless network, the source and destination nodes communicate in a multi-hop fashion. The packets from the source node traverse through one or more intermediate/relaying nodes to reach the destination. Because all nodes in the network also act as routers, there is no need for a BS or any other dedicated infrastructure. Hence, such networks are also called infrastructure-less networks. The well-known forms of multi-hop networks are ad hoc networks, sensor networks, and WMNs. Communication between two nodes, say node C and node F, takes place through the relaying nodes D and E.


CHAPTER 2: WIRELESS MESH NETWORKS


2 .1 An Introduction to Wireless Mesh Networks:


2.2 Single-hop network scenario (cellular network)

In the case of single-hop networks, complete information about the clients is available at the BS and the routing decisions are made in a centralized fashion, thus making routing and resource management simple. But it is not the case in multi-hop networks. All the mobile nodes have to coordinate among themselves for communication between any two nodes. Hence, routing and resource management are done in a distributed way.


2.3 Ad hoc Networks and WMNs

In ad hoc networks, all the nodes are assumed to be mobile and there is no fixed infrastructure for the network. These networks find applications where fixed infrastructure is not possible, such as military operations in the battlefield, emergency operations, and networks of handheld devices. Because of lack of infrastructure the nodes have to cooperate among themselves to form a network. Due to mobility of the nodes in the network, the network topology changes frequently. So the protocols for ad hoc networks have to handle frequent changes in the topology. In most of the applications of ad hoc networks [3], the mobile devices are energy constrained as Mobile node Wireless link Communication path they are operating on battery.


2.4 Multi-hop network scenario (ad hoc network)

This requires energy-efficient networking solutions for ad hoc networks. But in the case of WMNs, mesh routers are assumed to be fixed (or have limited mobility) and form a fixed mesh infrastructure. The clients are mobile or fixed and utilize the mesh routers to communicate to the backhaul network through the gateway routers and to other clients by using mesh routers as relaying nodes. These networks find applications where networks of fixed wireless nodes are necessary. There are several architectures for mesh networks, depending on their applications. In the case of infrastructure backbone networking, the edge routers are used to connect different networks to the mesh backbone and the intermediate routers are used as multi-hop relaying nodes to the gateway router, but in the case of community networking, every router provides access to clients and also acts as a relaying node between mesh routers.


2.5 Architecture of WMNs

There are two types of nodes in a WMN called mesh routers and mesh clients. Compared to conventional wireless routers that perform only routing, mesh routers have additional functionalities to enable mesh networking.


The mesh routers have multiple interfaces of the same or different communications technologies based on the requirement. They achieve more coverage with the same transmission power by using multihop communication through other mesh routers. They can be built on general-purpose computer systems such as PCs and laptops, or can be built on dedicated hardware platforms (embedded systems). There are a variety of mesh clients such as laptop, desktop, pocket PCs, IP phones, RFID readers, and PDAs. The mesh clients have mesh networking capabilities to interact with mesh routers, but they are simpler in hardware and software compared to mesh routers. Normally they have a single communication interface built on them. The architecture of WMNs is the most common architecture used in many mesh networking applications such as community networking and home networking. The mesh routers shown have multiple interfaces with different networking technologies which provide connectivity to mesh clients and other networks such as cellular and sensor networks. Normally, long-range communication techniques such as directional antennas are provided for communication between mesh routers. Mesh routers form a wireless mesh topology that has self-configuration and self-healing functions built into them. Some mesh routers are designated as gateways which have wired connectivity to the Internet. The integration of other networking technologies is provided by connecting the BS of the network that connects to WMNs to the mesh routers. Here, the clients communicate to the BS of its own network and the BS in turn communicates to the mesh router to access the WMN.


2.6 Applications of WMNs

WMNs introduce the concept of a peer-to-peer mesh topology with wireless communication between mesh routers. This concept helps to overcome many of today's deployment challenges, such as the installation of extensive Ethernet cabling, and enables new deployment models. Deployment scenarios that are particularly well suited for WMNs include the following:


Campus environments (enterprises and universities), manufacturing, shopping centers, airports, sporting venues, and special events Military operations, disaster recovery, temporary installations, and public safety


Municipalities, including downtown cores, residential areas, and parks


Carrier-managed service in public areas or residential communities


Due to the recent research advances in WMNs, they have been used in numerous applications. The mesh topology of the WMNs provides many alternative paths for any pair of source and destination nodes, resulting in quick reconfiguration of the path when there is a path failure. WMNs provide the most economical data transfer coupled with freedom of mobility. Mesh routers can be placed anywhere such as on the rooftop of a home or on a lamppost to provide connectivity to mobile/static clients. Mesh routers can be added incrementally to improve the coverage area. These features of WMNs attract the research community to use WMNs in different applications:


Home Networking: Broadband home networking is a network of home appliances (personal computer, television, video recorder, video camera, washing machine, refrigerator) realized by WLAN technology. The obvious problem here is the location of the access point in the home, which may lead to dead zones without service coverage. More coverage can be achieved by multiple access points connected using Ethernet cabling, which leads to an increase in deployment cost and overhead. These problems can be solved by replacing all the access points by the mesh routers and establishing mesh connectivity between them. This provides broadband connectivity between the home networking devices and only a single connection to the Internet is needed through the gateway router. By changing the location and number of mesh routers, the dead zones can be eliminated shows a typical home network using mesh routers.


2.7 Issues in WMNs

Various research issues in WMNs are described in this section. As WMNs are also multi-hop wireless networks like ad hoc networks, the protocols developed for ad hoc networks work well for WMNs. Many challenging issues in ad hoc networks have been addressed in recent years. WMNs have inherent characteristics such as a fixed mesh backbone formed by mesh routers, resource-rich mesh routers, and resource-constrained clients compared to ad hoc networks. Due to this, WMNs require considerable work to address the problems that arise in each layer of the protocol stack and system implementation.


Capacity

The primary concern of WMNs is to provide high-bandwidth connectivity to community and enterprise users. In a single-channel wireless network, the capacity of the network degrades as the number of hops or the diameter of the network increases due to interference. The capacity of the WMN is affected by many factors such as network architecture, node density, and number of channels used, node mobility, traffic pattern, and transmission range. A clear understanding of the effect of these factors on capacity of the WMNs provides insight to protocol design, architecture design, and deployment of WMNs.


Orthogonal Frequency Division Multiplexing (OFDM): The OFDM technique is based on the principle of Frequency Division Multiplexing (FDM) with digital modulation schemes. The bit stream to be transmitted is split into a number of parallel low bit rate streams. The available frequency spectrum is divided into many sub-channels and each low bit rate stream is transmitted by modulating over a sub-channel using a standard modulation scheme such as Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). The primary advantage of OFDM is its ability to work under severe channel conditions, such as multi-path and narrow-band interference, without complex equalization filters at the transmitter and receiver. The OFDM technique has increased the transmission rate of IEEE 802.11a networks from 6 to 54 Mbps.


Ultra Wide Band (UWB): UWB technology provides much higher data rate (ranges from 3.1 to 10.6 GHz) compared to other RF transmission technologies. A significant difference between traditional radio transmission and UWB radio transmission is that traditional radio transmission transmits information by varying the power, frequency, or phase in distinct and controlled frequencies while UWB transmission transmits information by generating radio energy at specific times with a broad frequency range. Due to this, UWB transmission is immune to multi-path fading and interference, which are common in any radio transmission technique. UWB wireless links have the characteristic that the bandwidth decreases rapidly as the distance increases. On the other hand UWB provides hundreds of non interfering channels within radio range of each other. Hence, UWB is applicable for only short-range communications such as WPAN. Mesh architecture combined with UWB wireless technology allows a very easy installation of communications infrastructure in offices or homes by deploying many repeater modules. As these repeater modules require power to operate on, they have to be placed with ceiling lights or floor power boxes. The IEEE 802.15.3a TG a standard for WPAN uses a UWB physical layer technique consisting of a UWB impulse radio (operating in unlicensed UWB spectrum) and a chirp spread spectrum.


Multiple-Input Multiple-Output (MIMO): The use of multiple antennas at the transmitter and receiver, popularly known as MIMO wireless, is an emerging, cost-effective technology that makes high bandwidth wireless links a reality. MIMO significantly increases the throughput and range with the same bandwidth and overall transmission power expenditure. This increase in throughput and range is by exploiting the multi-path propagation phenomena in wireless communications. In general, the MIMO technique increases the spectral efficiency of a wireless communications system. It has been shown by Telatar that the channel capacity (a theoretical upper bound on system throughput) for a MIMO system increases as the number of antennas increases, proportional to the minimum of transmitter and receiver antennas. MIMO can also be used in conjunction with OFDM and is part of the IEEE 80 standard. Smart Antenna: The smart antenna technique improves the capacity of wireless networks by adding the directionality for transmission and reception of signals at the transmitter and receiver antenna. This also helps in increasing energy efficiency. In cellular networks, due to complexity and cost of smart antennas, it is implemented in BS alone. The directional antenna system is actively researched in ad hoc networks also. There are some directional antenna systems available that can be tuned to certain directions by electronic beam forming. This technique improves the performance of wireless.


2.8 Medium Access Control

The MAC (Medium Access Control) protocols for wireless networks are limited to single-hop communication while the routing protocols use multi-hop communication. The MAC protocols for WMNs are classified into single channel and multi-channel MAC. They are discussed in this section.


Single-Channel MAC: There are several MAC schemes which use single-channel for communication in the network. They are further classified as (1) contention-based protocols, (2) contention-based protocols with a reservation mechanism, and (3) contention-based protocols with a scheduling mechanism.


Contention-based protocols: These protocols have a contention-based channel access policy among the nodes contending for the channel. All the ready nodes in the network start contending for the channel simultaneously and the winning node gains access to the channel. As the nodes cannot provide guaranteed bandwidth, these protocols cannot be used in carrying real-time traffic, which requires QoS (quality of service) guarantees from the system. Some of the contention-based protocols are MACAW (a media access protocol for Wireless LANs), FAMA (Floor Acquisition Multiple Access protocol), BTMA (Busy Tone Multiple Access protocol), and MACA-BI (Multiple Access Collision Avoidance By Invitation), based protocols with a reservation mechanism: Because the contention-based protocols cannot provide guaranteed access to the channel, they cannot be used in networks where real-time traffic has to be supported. To support real time traffic, some protocols reserve the bandwidth a priori. Such protocols can provide QoS support for time-sensitive traffic. In this type of protocol, the contention occurs during the resource (bandwidth) reservation phase. Once the bandwidth is reserved, the nodes get exclusive access to the reserved bandwidth. Hence, these protocols can provide QoS support for time sensitive traffic. Some of the examples for these type of protocols are D-PRMA (Distributed Packet Reservation Multiple Access protocol), CATA (Collision Avoidance Time Allocation protocol), HRMA (Hop Reservation Multiple Access protocol) [8], and RTMAC (Real-Time Medium Access protocol). Contention-based protocols with scheduling mechanism: These protocols focus on packet scheduling at nodes and also scheduling nodes for access to the channel. The scheduling is done in such a way that all nodes are treated fairly and no node is starved of bandwidth. These protocols can provide priorities among flows whose packets are queued at nodes. Some of the existing scheduling-based protocols are DWOP (Distributed Wireless Ordering Protocol), DLPS (Distributed Laxity-based Priority Scheduling), and DPS (Distributed Priority Scheduling).


Contention-based protocols that use single-channel for communication cannot completely eliminate contention for the channel. In the case of WMNs the end-to-end throughput significantly reduces due to the accumulating effect of the contention in the multi-hop path. Further, an ongoing transmission between a pair of nodes refrains all the nodes which are in a two-hop neighborhood of nodes participating in the transmission from transmitting on the channel during the transmission period. To overcome these problems multi-channel MAC and multi-channel multi-radio MAC protocols are proposed.


Multi-Channel MAC (MMAC): Multi-channel MAC[10] is a link layer protocol where each node is provided with only one interface, but to utilize the advantage of multi-channel communication, the interface switches among different channels automatically. In MMAC the communication time is split into a number of beacon intervals. In the beginning of each beacon interval, during an ATIM (Ad hoc Traffic Indication Message) window period all the nodes in the network tune their radio to a common control channel and negotiate for the channel to be used for the remaining period of the beacon interval. Each node maintains a data structure called PCL (Preferred Channel List — usage of the channels within the transmission range of the node). When a source node S wants to send data to receiver node R, during the ATIM window node S sends an ATIM packet with its PCL. Upon receiving the ATIM packet from node S, node R compares the PCL of node S with its PCL and decides which channel is to be used during the beacon interval. Then node R sends an ATIM-ACK carrying the ID of the preferred channel. Node S, on receiving the ATIM-ACK, confirms the reservation by sending an ATIM-RES packet to node R when other nodes in the vicinity of node R hear the ATIM-ACK, they choose a different channel for their communication. The throughput of MMAC is higher than that of IEEE 802.11 when the network load is high. This increase in throughput is due to the fact that each node uses an orthogonal channel, thereby increasing the number of simultaneous transmissions in the network. Though MMAC increases the throughput, there are some drawbacks with it. When a node has to send a packet to multiple destinations, it can send only to one destination in a beacon interval, because the nodes have to negotiate during the ATIM window in the control channel. Due to this restriction the per-packet delay increases significantly. MMAC does not have any scheme for broadcasting.


Slotted Seeded Channel Hopping protocol (SSCH) [9] is another multichannel link layer protocol using a single transceiver. SSCH is implemented in software over an IEEE 80 compliant wireless Network Interface Card (NIC). SSCH uses a distributed mechanism for coordinating the channel switching decision. By this channel hopping at each node, packets of multiple flows in the interfering range of each other are transmitted simultaneously in an orthogonal channel. This improves the overall capacity of the multi-hop wireless network if the network traffic pattern has multiple flows in the interfering range of each other. Each node in the network finds the channel hopping schedule for it and schedules the packets within each channel. Each node transmits its channel hopping schedule to all its neighboring nodes and updates its channel hopping schedule based on traffic pattern. SSCH yields significant capacity improvement in both single-hop and multi-hop network scenarios. Multi-Radio


2.9 ROUTING

There are numerous routing protocols proposed for ad hoc networks in the literature. Because WMNs are multi-hop networks, the protocols designed for ad hoc networks also work well for WMNs. The main objective of those protocols is quick adaptation to the change in a path when there is path break due to mobility of the nodes. Current deployments of WMNs make use of routing protocols proposed for ad hoc networks such as AODV (Ad hoc On-Demand Distance Vector) , DSR (Dynamic Source Routing), and TBRPF (Topology Broadcast based on Reverse Path Forwarding)[11]. However, in WMNs the mesh routers have minimal mobility and there is no power constraint, whereas the clients are mobile with limited power. Such difference needs to be considered in developing efficient routing protocols for WMNs. As the links in the WMNs are long lived, finding a reliable and high throughput path is the main concern rather than quick adaptation to link failure as in the case of ad hoc networks.


Routing Metrics for WMNs: Many ad hoc routing protocols such as AODV [12] and DSR use hop count as a routing metric. This is not well suited for WMNs for the following reasons. The basic idea in minimizing the hop count for a path is that it reduces the packet delay and maximizes the throughput. But the assumption here is that links in the path either work perfectly or do not work at all and all links are of equal bandwidth. A routing scheme that uses the hop count metric does not take the link quality into consideration. A minimum hop count path has higher average distance between nodes present in that path compared to a higher hop count path. This reduces the strength of the signal received by the nodes in that path and thereby increases the loss ratio at each link. Hence, it is always possible that a two-hop path with good link quality provides higher throughput than a one-hop path with a poor/lossy link. A routing scheme that uses the hop count metric always chooses a single hop path rather than a two-hop path with good link quality. The wireless links usually have asymmetric loss rate as reported in. Hence, new routing metrics based on the link quality are proposed in the literature. They are ETX (Expected Transmission Count), per-hop RTT (Round-Trip Time), and per-hop packet pair.As per IEEE 802.11 standard, a successful transmission requires acknowledgment back to the sender. ETX considers transmission loss probability in both directions, which may not be equal as stated earlier. All nodes in the network compute the loss probability to and from its neighbors by sending probe packets. If pf and pr are respectively the loss probability in forward and reverse direction in a link, then the probability that a packet transmission is not successful in a link is given by

p = - ( - pf )( - pr ).

The expected number of transmissions on that link is computed as

ETX = -p . In [30]


The routing metrics based on link are compared with the hop count metric. The routing metric based on link quality performs better than hop count if nodes are stationary. The hop count metric outperforms the link quality metric if nodes are mobile. The main reason for this is that the ETX metric cannot quickly track the changes in the value of the metric. If the nodes are mobile, the ETX value changes frequently as the distance between the nodes changes.


As stated earlier, to improve the throughput the multi-radio multi-channel architecture is used in WMNs. In this case the routing metric based on link quality alone is not sufficient. It should also consider the channel diversity on the path. A new routing metric WCETT (Weighted Cumulative Expected Transmission Time) is proposed in [3], which takes both link quality and channel diversity into account. The link quality is measured by a per-link metric called ETT (Expected Transmission Time; expected time to transmit a packet of a certain size over a link). If the size of the packet is S and the bandwidth of the link is B, then

ETT = ETX * S B.


The channel diversity in the path is measured as follows. If X j is the sum of ETTs of the links using the channel j in the path, then channel diversity is measured as max =j =k X j, where k is the number of orthogonal channels used. The path metric for path p with n links and k orthogonal channels is calculated as n

WCETT (p) = ( - ß) * ETTi + ß * max =j =kX j,


i = where ß is a tunable parameter subject to 0 = ß = WCETT can achieve a good trade-off between delay and throughput as it considers both link quality and channel diversity in a single routing metric.


The WCETT metric considers the quality of links and the intra flow interference along the path. But it fails to take into account inter flow interference on the path. In [3], a new routing metric MIC (Metric of Interference and Channel switching) is proposed for multi-channel multiradio WMNs. This new metric considers the quality of links, inter flow interference, and intra flow interference altogether. This metric is based on Interference-Aware Resource Usage (IRU) and Channel Switching Cost (CSC) metrics to find the MIC for a given path. IRU captures the differences in the transmission rate and the loss ratios of the wireless link and the inter flow interference. The IRU metric for a link k which uses channel c is calculated as IRU k (c) = ETT k (c) * Nk (c), where ETT k (c) is the expected transmission time of the link k on the channel c, and Nk (c) is the number of nodes interfering with the transmission of the link k on channel c. The CSC metric captures the intra flow interference along the path. CSC for a node i is assigned a weight w if links in the path connected to it have different channels assigned. The path metric for a given path p, MIC (p), is calculated as follows:

MIC (p) = α ∗

IRUl +

CSCi.

(link l  ε  p)

(node i ε  p)


Here a is a positive factor which gives a trade-off between benefits of IRU and CSC.


Routing Protocols for WMNs

LQSR (Link Quality Source Routing) protocol. It is based on DSR and uses ETX as the routing metric. The main difference between LQSR and DSR is getting the ETX metric of each link to find out the path. During the route discovery phase, the source node sends a Route Request (RREQ) packet to neighboring nodes. When a node receives the RREQ packet, it appends its own address to the source route and the ETX value of the link in which the packet was received. The destination sends the Route Reply (RREP) packet with a complete list of links along with the ETX value of those links. Because the link quality varies with time, LQSR also propagates the ETX value of the links during the data transmission phase. On receiving a data packet, an intermediate node in the path updates the source route with the ETX value of the outgoing link. Upon receiving the packet, the destination node sends an explicit RREP packet back to the source to update the ETX value of links in the path. LQSR also uses a proactive mechanism to update the ETX metric of all links by piggybacking Link-Info messages to RREQ messages occasionally. This Link-Info message contains the ETX value of the entire links incident on the originating node.


A new routing protocol for multi-radio multi-channel WMNs called Multi-Radio Link Quality Source Routing (MR-LQSR) , which uses WCETT as a routing metric. The neighbor node discovery and propagating the link metric to other nodes in the network in MR-LQSR are the same as that in the DSR protocol. But assigning the link weight and finding the path weight using the link weight are different from DSR. DSR uses equal weight to all links in the network and implements the shortest path routing. But MR-LQSR uses a WCETT path metric to find the best path to the destination.


WCETT routing metric is used in a link state routing protocol, it is not satisfying the isotonicity property of the routing protocol and leads to formation of routing loops. To avoid the formation of routing loops by the routing metrics, they proposed Load and Interference Balanced Routing Algorithm (LIBRA), which uses MIC as the routing metric. In LIBRA a virtual network is formed from the real network and decomposed the MIC metric into isotonicity link weight assignment on the virtual network. The objective of MIC decomposition is to ensure that LIBRA can use efficient algorithms such as Bellman-Ford or Dijkstra's algorithm to find the minimum weight path on the real network without any forwarding loops.


Transport Layer

There are several reliable transport protocols proposed for ad hoc networks. Some of them are modified versions of TCP (Transmission Control Protocol) that work well in ad hoc networks and others are designed specifically for an ad hoc network scenario from scratch.


TCP is the de facto standard for end-to-end reliable transmission of data on the Internet. TCP was designed to run efficiently on wireline networks. Using the TCP protocol on a wireless network degrades the performance of the network in terms of reduction in throughput and unfairness to the connections. This degradation in performance is due to the following reasons. The Bit Error Rate (BER) in wireless networks is very high compared to wireline networks. Frequency of path break in wireless networks is high due to mobility of nodes in ad hoc networks. If the packets get dropped in the network due to these reasons, the TCP sender misinterprets this event as congestion and triggers the congestion control mechanism to reduce the congestion window size. This reduces the effective throughput of the network.


Other Transport Protocols for Wireless Networks: Transport protocol for wireless networks was proposed by not modifying the existing TCP protocol. This is done by introducing a thin layer called ATCP between the network layer and transport layer and it is invisible to transport layer. This makes nodes with ATCP and without ATCP interoperable with each other. ATCP gets information about congestion in the network from the intermediate nodes through ECN (Explicit Congestion Notification) and ICMP messages. Based on this, the source node distinguishes congestion and non-congestion losses and takes the appropriate action.


When the TCP sender identifies any network partitioning, it goes into persist state and stops all the outgoing transmissions. When the TCP sender notices any loss of packets in the network due to channel error, it retransmits the packet without invoking any congestion control. When the network is truly congested, it invokes the TCP congestion control mechanism.


2.10 Gateway Load Balancing

In WMNs the gateway nodes are connected to the backhaul network, i.e., to the Internet, which provides Internet connectivity to all nodes in the network. So the gateway may become a bottleneck for the connections to the Internet. As many clients in the network generate traffic to the gateway, the available bandwidth should be utilized effectively. The traffic generated by client nodes aggregates at gateway nodes in the WMN. If some of the gateway nodes are highly loaded and other gateway nodes are lightly loaded, it creates load imbalance between gateway nodes, which leads to packet loss and results in degradation in network performance. Hence, load balancing across gateway nodes in WMNs improves bandwidth utilization and also increases network throughput.


Load balancing across gateway nodes is obtained by distributing the traffic generated by the network to the backhaul network through all gateway nodes in the WMNs. The load balancing across multiple gateway nodes can be measured quantitatively by a metric called Index of Load Balance (ILB) which is calculated as follows.


Load index (LI) of a gateway i is defined as the fraction of the gateway's


, where ßk (i ) ,C (i ) is the fraction of node k's traffic that is sent through gateway i , Tk is the total traffic generated by node k, and C (i ) is the capacity of the backhaul link connected to the gateway node i . The ILB of the network is calculated as


ILB = max {LI (i)} - min {LI (i)}

max {LI (i)}


Therefore a perfectly balanced network has ILB equal to zero and a highly imbalanced network has ILB equal to one. The objective of all load balancing techniques is to obtain ILB values as small as possible. Several techniques for load balancing across gateways were proposed in the literature. Some of them are discussed in this section.


Moving Boundary-Based Load Balancing: A flexible boundary is defined for each gateway and the nodes which fall in the boundary are directed to communicate through that gateway. To adopt to variations in the traffic, the region of boundary is periodically redefined. The boundary can be defined in two different ways: (1) in a shortest path-based moving boundary approach, the boundary region for a gateway node is defined by distance of the node from the gateway, and (2) in a load index-based moving boundary approach, the gateways announce their load Index and the nodes join lightly loaded gateways. In this scheme the lightly loaded gateway serves more nodes and the heavily loaded gateway serves fewer nodes. Partitioned Host-Based Load Balancing: Here, the nodes in the network are grouped, and each group is assigned to a particular gateway. The main difference compared to the moving boundary-based load balancing is that no clear boundary is defined. This can be done in both a centralized and distributed way. In the centralized method, a central server assumes the responsibility of assigning the gateway to the nodes. The central server collects the complete information about the gateway nodes and traffic requirements of all the nodes and then allocates nodes to the gateways. In the distributed method, a logical network is formed by the gateway nodes. Each node is associated with a gateway node known as a dominating gateway through which traffic generated by this node reaches the Internet. The nodes in the network periodically update their dominating gateway about their traffic demand. The gateway nodes exchange information about their load and capacity information through the logical network. When a gateway is highly loaded, hand-over takes place, i.e., the gateway delegates some nodes to other gateways which are lightly loaded.


Probabilistic Stripping-Based Load Balancing: In the techniques discussed above, each node in the network utilizes only one gateway, which may not lead to perfect load balancing among the gateways. In a probabilistic stripping-based load balancing scheme, each node utilizes multiple gateways simultaneously, which gives perfect load balancing theoretically. In this technique each node identifies all the gateway nodes in the network and attempts to send a fraction of its traffic through every gateway. Hence, the total traffic is split among multiple gateways. This technique is applicable in the case where the load can be split for sending through multiple gateways.


2.11 SECURITY

As mentioned earlier, due to the unique characteristics of WMNs, they are highly vulnerable to security attacks compared to wired networks. Designing a foolproof security mechanism for WMNs is a challenging task. The security can be provided in various layers of the protocol stack. Current security approaches may be effective against a particular attack in a specific protocol layer, but they lack a comprehensive mechanism to prevent or counter attacks in different protocol layers. The following issues pose difficulty in providing security in WMNs.


Shared Broadcast Radio Channel: In a wired network, a dedicated transmission line is provided between the nodes. But the wireless links between the nodes in WMNs are broadcast in nature, i.e., when a node transmits, all the nodes within its direct transmission range receive the data. Hence, a malicious node could easily obtain data being transmitted in the network if it is placed in the transmission range of mesh routers or a mesh client. For example, if you have a WMN and so does your neighbor, and then there is a scope for either snooping into private data or simply hogging the available bandwidth of a neighboring, but alien node.


Lack of Association: In WMNs, the mesh routers form a fixed mesh topology which forms a backbone network for the mobile clients. Hence, the clients can join or leave the network at any time through the mesh routers. If no proper authentication mechanism is provided for association of nodes with WMNs, an intruder would be able to join the network quite easily and carry out attacks. Physical Vulnerability: Depending on the application of WMNs, the mesh routers are placed on lampposts and rooftops, which are vulnerable to theft and physical damage.


Limited Resource Availability: Normally, the mesh clients are limited in resources such as bandwidth, battery power, and computational power. Hence, it is difficult to implement complex cryptography based mechanisms at the client nodes. As mesh routers are resource rich in terms of battery power and computational power, security mechanisms can be implemented at mesh routers. Due to wireless connectivity between mesh routers, they also have bandwidth constraints. Hence, the communication overhead incurred by the security mechanism should be minimal.


2.12 Power Management

The energy efficiency of a node in the network is defined as the ratio of the amount of data delivered by the node to the total energy expended. Higher energy efficiency implies that a greater number of packets can be transmitted by the node with a given amount of energy resource. The main reasons for power management in WMNs are listed below.


Power Limited Clients: In WMNs, though the mesh routers do not have limitations on power, clients such as PDAs and IP phones have limited power as they are operated on batteries. In the case of Hybrid WMNs, clients of the other networks that are connected to them, such as sensor networks, can be power limited. Hence, power efficiency is of major concern in WMNs.


In multi-hop wireless networks, the transmission power level of wireless nodes affects connectivity, interference, spectrum spatial reuse, and topology of the network. Reducing the transmission power level decreases the interference and increases the spectrum spatial reuse efficiency and the number of hidden terminals. An optimal value for transmission power decreases the interference among nodes, which in turn increases the number of simultaneous transmissions in the network. Channel Utilization: In multi-channel WMNs, the reduction in transmission power increases the channel reuse, which increases the number of simultaneous transmissions that improves the overall capacity of the network. Power control becomes very important for CDMA-based systems in which the available bandwidth is shared by all the users. Hence, power control is essential to maintain the required signal-to-interference ratio (SIR) at the receiver and to increase the channel reusability.


2.13 Mobility Management

In WMNs the mobile clients get network access by connecting to one of the mesh routers in the network. When a mobile client moves around the network, it switches its connectivity from one mesh router to another. This is called hand-off or hand-over. In WMNs the clients should have capability to transfer connectivity from one mesh router to another to implement handoff technique efficiently. Some of the issues in handling hand-offs in WMNs are discussed below.


Optimal Mesh Router Selection: Each mesh client connects to one of the mesh routers in the WMN. Normally, each mesh client chooses the mesh router based on the signal strength it receives from the mesh routers. When a mobile client is in the transmission range of multiple mesh routers, it is very difficult to clearly decide to which mesh router the mobile client must be assigned.


Detection of Hand-off: Hand-off may be client initiated or network initiated. In the case of client initiated, the client monitors the signal strength received from the current mesh router and requests a handoff when the signal strength drops below a threshold. In the case of network initiated, the mesh router forces a hand-off if the signal from the client weakens. Here the mesh router requires information from other mesh routers about the signal strength they receive from the particular client and deduces to which mesh router the connection should be handed over.


Hand-Off Delay: During hand-off, the existing connections between clients and network get interrupted. Though the hand-off gives continuous connectivity to the roaming clients, the period of interruption may be several seconds. All ongoing transmissions of the client are transferred from the current mesh router to a new mesh router. The time taken for this transfer is called hand-off delay. The delay of a few seconds may be acceptable for applications like file transfer, but for applications that require real-time transport such as interactive VoIP (Voice-over-IP) or videoconferencing, it is unacceptable. Quality of Hand-Off: During hand-off some number of packets may be dropped due to hand-off delay or interruption on the ongoing transmission. The quality can be measured by the number of packets lost per hand-off. A good quality hand-off provides a low packet loss per hand-off. The acceptable amount of packet loss per hand-off differs between applications.


2.14 Adaptive Supports for Mesh Routers and Mesh Clients

Compared to other networking technologies where all the nodes in the network are considered to have similar characteristics, WMNs have different characteristics between mesh routers and mesh clients. The main differences between them which make the need for new networking protocols for WMNs are


Mobility: In many applications of WMNs, the mesh routers form a fixed backbone network by placing the mesh routers at fixed locations such as rooftops and lampposts. So the mesh routers are considered immobile, but the clients in the mesh network are highly mobile and can be connected to any mesh router based on signal strength received from different mesh routers.


Resource Availability: Normally, mesh routers are operated with electric power rather than battery power. They are placed in locations where the powerline is available, so the mesh routers do not have energy constraints. But the clients are operating with battery power and are considered energy constrained.


The existing protocols for ad hoc networks consider the characteristics of all nodes in the same way. The energy-aware protocols consider all nodes in the network battery operated. The protocols that take into account the mobility of nodes in the network consider all nodes in the network mobile. For example, a routing protocol designed for networks with high mobility and limited power when used in WMNs does not utilize the limited mobility and rich energy resource nature of mesh routers. Hence, it fails to improve the performance of WMNs. But due to the characteristics of mesh routers, the routing protocols become simple and efficient. So WMNs need efficient protocols that consider the differences between the mesh routers and mesh clients to improve the performance of WMNs.


Integration with Other Network Technologies

The integration of WMNs with other existing network technologies such as cellular, Wi-Fi, WiMAX, WiMedia, and sensor networks can be achieved by bridging functions at the mesh routers. These bridging functions can be provided by adding network interfaces corresponding to the networking technology that the mesh router has to support. There are several issues to be addressed in integrating multiple networking technologies with WMNs:


Complexity of Mesh Router: The integration of multiple networking technologies with the mesh network increases the complexity of the mesh routers. For each networking technology to be supported by a mesh router, a network interface should be provided. This increases the hardware and software complexity of the mesh routers. Cost of Mesh Router: The networking hardware or network interface for different networking technologies are not the same. Each networking technology needs specially designed hardware to operate on. Mesh routers have to be provided with the same number of interfaces as the number of networking technologies supported by them. This increases the cost of mesh routers.


Services Provided by Integrated WMNs: The services provided by different networking technologies are different. Services not provided by IEEE 80 can be provided by cellular networks. Similarly the services provided by sensor networks cannot be provided by cellular networks. The integration of other networking technologies with WMNs provides many services to the users that are not provided by WMNs alone. Depending on the service requirement, the required networking technologies can be integrated with WMNs. Inter-Operability of Network Technologies: The protocols for different network technologies are independent and operating them together is a difficult task. For example, the routing protocols used by a cellular network and an IEEE 80 network are not the same. Further, the MAC protocols used by different networking technologies are not inter-operable. So the inter-operability of different networking technologies necessitates new software architectures or middleware implementations over the mesh networking platform.


Though the integration of multiple networking technologies with WMNs is a difficult job, the services rendered by this necessitate the researchers to come up with a feasible solution. The development of new network architectures and middleware solutions may solve some of these problems. The problem of implementation of many network interfaces in a single mesh router can be solved by using software-defined radios. The software defined radio system is a software-based communication system for modulation and demodulation of radio signal. This is done by advanced signal processing techniques implemented in a digital computer or in a reconfigurable digital electronic system. This technique produces different radios that can receive and transmit a new form of radio protocol just by running different software rather than designing new hardware. This helps in reducing the number of networking interfaces in mesh routers.


2.15 Deployment Considerations

Scenario of Deployment: The capability required for deployments of different WMNs is not the same. For example, WMN deployment for community networking to share network resources among people is not the same as for rescue operations. Some of the deployment scenarios in which the deployment issues vary are


Emergency Operation Deployment: This kind of application scenario demands a quick deployment of a communication backbone network through which the mobile devices can communicate. For example, during disasters like flood, fire, and earthquake all the existing communication network infrastructure might be destroyed. Hence, a quick deployment of a backbone communication network is essential. Most importantly, the network should provide support for time-sensitive traffic such as voice and video. The network should also provide support for different networking technologies to communicate using this network. Hence, the mesh routers should provide interfaces for other existing technologies which allow people to communicate using any communication equipment they have. Commercial Broadband


Access Deployment: The aim of this deployment is to provide an alternate network infrastructure for wireless communications in urban areas and areas where a traditional cellular BS cannot handle the traffic volume. This scenario assumes significance as it provides very low cost per bit transferred compared to the cellular network infrastructure. Another major advantage of this application is the resilience to failure of a certain number of nodes. Addressing, configuration, positioning of relaying nodes, redundancy of nodes, and power sources are the major issues in deployment. Billing, provisioning of QoS, security, and handling mobility are major issues that the service provider needs to address.


Home Network Deployment: The deployment of a home area network needs to consider the limited range of the devices that are to be connected by the network. Given the short transmission ranges of a few meters, it is essential to avoid network partitions. Positioning of mesh routers at certain key locations of a home area network can solve this problem; also network topology should be decided so that every mesh router is connected through multiple neighbors for availability.


Cost of Deployment: The commercial deployment of a communications infrastructure using a WMN essentially eliminates the requirement of laying cables and maintaining them. Hence, the cost of deployment is much less than that of the wired infrastructure. Only the mesh routers have to be placed in appropriate locations for efficient coverage. The mesh router manufacturers are providing mesh routers for outdoor placements. Mesh routers can be placed on poles on the street, which reduces the cost of deployment of mesh networks.


Operational Integration with Other Infrastructure: Operational integration with other networking technologies such as satellite, cellular, and sensor networks can be considered to improve the performance or provide additional services to the end users. In the commercial world, the WMNs that service a given urban region can interoperate with the cellular infrastructure to provide better QoS and smooth hand-offs across the networks. Hand-offs to a different network can be done to avoid call drops when a mobile node with an active call moves into a region where service is not provided by the current network.


Area of Coverage: In most of the cases, the area of coverage of WMNs is determined by the nature of application for which the network is set up. For example, for home networks the coverage of the mesh routers is within the home or within the room in which the router is placed. But in the case of wireless service providers, mesh routers should be covering a number of homes on a street. Long-range communication by fixed mesh routers can be achieved by means of directional antennas. The mesh routers' and mobile clients' capabilities such as transmission range and associated hardware, software, and power source should match the area of coverage required.


Service Availability: Service availability is defined as the ability of a network to provide service even with failure of certain nodes. In WMNs the mesh routers form a fixed mesh backbone to provide multiple services to the mobile clients. These mesh routers may be placed in outdoor areas such as lampposts and rooftops. They are subject to failure due to power failure, environmental damage, physical damage, or theft. Due to this, the services provided by a WMN to mobile clients may not be available in certain areas. Hence, the mesh routers need to be placed in such a way that failure of some of them does not lead to lack of service in that area. In such cases, redundant inactive mesh routers can be placed in such a way that, in the event of failure of active mesh routers, the redundant mesh routers can take over their responsibilities.


Choice of Protocols: The choice of protocols at different layers of the protocol stack is to be done by taking into consideration the deployment scenario. The MAC protocol should ensure provisioning of security at link level for military applications. The routing protocol also should be selected with care. In the case of integration of different networking technologies, end-to-end paths may have different types of nodes with different capabilities. It requires routing protocols that consider the resource limitations of the nodes. At the transport layer, depending upon the environment in which the WMN is deployed, the connection-oriented or connectionless protocols should be chosen. If the clients connected to the WMN are highly mobile, a frequent hand-off of the clients with the mesh routers takes place. This causes the higher-layer protocols to take necessary action appropriately; also, packet loss arising due to congestion, channel error, link break, and network partition is to be handled differently in different applications. The timer values at different layers of the protocol stack should be adapted to the deployment scenario.


WMN Deployments/Testbeds: For the deployment of WMNs to be viable, they must be easy to install. This is particularly important for home applications where people are unwilling to install highly technical networks. Many task groups have been working on standardization of the protocols for WMNs, which leads to the development and interoperability of mesh networking products from different vendors. Many testbeds have been established to carry out research and development work in WMNs.


2.16 Academic Research Testbeds

Many academic research institutes established testbeds to study realistic behavior of WMNs. Some of them are discussed in this section.


MIT Roofnet: MIT Roofnet is an 80b multi-hop network designed to provide broadband Internet connectivity to users in apartments of Cambridge, MA. It has about 20 nodes connected through 80 interfaces in multi-hop fashion and connected to the Internet through an Ethernet interface available in the apartments. Research on Roofnet includes link-level measurements of 80 interfaces, finding high-throughput routes in the face of lossy links, adaptive bit-rate selection, and developing new protocols which take advantage of radio's unique properties. The main feature of Roofnet is that it is an unplanned network, i.e., no configuration or planning is required.


Cal Radio-I: California Institute for Telecommunications and Information Technology developed Cal Radio-I, which are a radio/ networking test platform for wireless research and development. This is a single integrated, wireless networking test platform which provides a simple, low-cost platform development from the MAC layer to a higher layer. All the MAC functionalities are coded in C language that runs on the DSP processor. Any modification to the MAC protocol can be done and tested in it. CalRadio-I functions as a test instrument, an AP, and as a Wi-Fi client.


2.17 Industrial Research in WMNs

Many companies started research in WMNs on their own and in collaboration with academic research institutions. Some of them recently came up with mesh networking products for implementing mesh network-based applications. In this section some of the industries working toward research aspects of WMNs and some of the industries providing mesh networking products are discussed.


Microsoft Research: Microsoft researchers at Redmond, Cambridge, and Silicon Valley are working to create wireless technologies that allow neighbors to connect their home networks together (community networking). They deployed their own mesh network testbed in their office building and local apartment complex. They developed a software module called the Mesh Connectivity Layer (MCL) which implements ad hoc routing and link quality measurement. Architecturally, MCL is a loadable Windows driver. It implements a virtual network adapter, so that the ad hoc network appears as an additional (virtual) network link to the rest of the system. The routing protocol used by MCL is LQSR, which improves network performance by supporting link-quality metrics for routing. 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. To lower-layer software, MCL appears to be just another protocol running over the physical link. This design has several significant advantages. First, higher-layer software runs unmodified over the ad hoc network. The testbed runs both IPv4 and IPv6 over the ad hoc network without requiring any modifications to the network layer. All network layer functionalities such as ARP, DHCP, and Neighbor Discovery work well. Second, the ad hoc routing runs over heterogeneous link layers as well. This implementation supports Ethernet-like physical link layers (e.g., 80 and 80.3), but the architecture accommodates link layers with arbitrary addressing and framing conventions. The virtual MCL network adapter can multiplex several physical network adapters, so the ad hoc network can be extended across heterogeneous physical links. Third, the design can support other ad hoc routing protocols as well.


Intel: A wide variety of research and development efforts at Intel are geared toward understanding and addressing the technical challenges for realizing multi-hop mesh networks. Intel's Network Architecture Lab is aimed at overcoming many of the challenges faced by WMNs. They developed low-cost and low-power AP prototypes or nodes to enable further research on security, traffic characterization, dynamic routing and configuration, and QoS problems. Intel is also working with other industries to develop standards and protocols that support WMNs and enable interoperability between products from multiple vendors. Intel is working to simplify the entire installation process, including network node placement and configuration so that end users and businesses can easily realize the full benefits of multi-hop mesh networking.


2.18 Mesh Networking Products

Strix Systems: The mesh networking products from Strix Systems are RF-independent supporting existing wireless standards 80 a/ b/g and 80 (WiMAX), designed to easily add in any future wireless technologies. The Strix Access/One® family of products delivers high-performance WMN systems by employing modular future-proof architecture supporting multi-radio, multi-channel, and multi-RF mesh networking technologies. The Access/One architecture delivers the industry's most scalable and flexible wireless networking platform by which the largest citywide and countrywide communication services can be built. Unlike competing single and other multiradio products, the Access/One design makes secure full-duplex transmission, instant path switching, and application classification a reality. Strix Access/One networks are deployed in many different environments and used for many different applications around the world, enabling users to access wireless broadband applications at any place, anywhere, anytime even while moving at 100 miles per hour. Strix Access/One is a scalable self-configuring and self-healing system designed to meet the needs of service providers, government agencies, and outdoor mobile enterprises.


Nortel: Nortel's WMN solution addresses the market requirements for networks that are highly scalable and cost-effective, offering end user security, seamless roaming beyond traditional WLAN boundaries, and provides easy deployment in areas that do not (or cannot) support a wired backhaul. Nortel's WMN solution is well-suited for providing broadband wireless access in areas that traditional WLAN systems are unable to cover. Nortel provides a number of products for WMN solutions, which include wireless AP, wireless bridge, WLAN security switches, and enterprise network management system. These products provide a number of applications for the mobile users such as secure mobile networking and voice connectivity featuring flexible seamless mobility across campus environments, IP telephony and converged multimedia applications, and low-cost, high-capacity point-to-point broadband transmission.


CHAPTER 3


WIRELESS MOBILE MESH NETWORKS:

Wireless mobile mesh networks are made up by several mobile nodes, fully wirelessly interconnected, which adopt multi-hop communication for data transmission. This chapter intends to argue why mesh networking technology represents a new issue to address for wireless networks by presenting the mesh networking fundamentals in wireless PANs, LANs, MANs, and WANs. For this purpose, we will first study the mesh networking characteristics while stressing the targeted applications, the network architecture, and the particularities of the routing, quality of service (QoS) provision, and management protocols. Then, details of the IEEE standardization efforts targeting the network coverage ranging from PANs to WANs are presented. We conclude by presenting some of the deployed solutions and discussing advanced design issues aiming at providing scalable, low-cost, and easily deployable Wireless Mobile Mesh Networks.


3.1 Introduction

The mobile ad hoc networks (or MANET) have gained researchers' attention for 30 years. MANET nodes share wireless links and can play the role of client and router at the same time without relying on any infrastructure; thus accomplishing large deployment ease and investments cost decrease. Besides, the ephemeral nature of MANETs particularly copes with critical applications such as disaster recovery and battlefield communications. Many research works have addressed the multi-hop communication issue in wireless networks, but the practical impact was not very important because users rarely operate in ad hoc mode. For instance, the targeted applications were limited to specialized missions inducing an unreasonable cost, while users searched mostly for cheap information sharing and Internet access. Client satisfaction has created a new research topic that aims at revising the MANET concept by considering the MANET network as a flexible and low-cost extension of wired infrastructure networks that integrates them. As a result, the wireless mesh networking paradigm, which inherits some MANET characteristics and targets civilian applications, was born. It is worth noticing that both the wired Internet and the public switched telephone network may be classed as mesh networks however, future wireless mesh networks should rely on a wireless infrastructure to interconnect mobile devices in a multi-hop fashion. Wireless mesh networks (WMNs) support home and enterprise networking applications; they also provide ubiquitous Internet access and enable the implementation of intelligent transportation systems and public safety applications. Besides, their deployment does not require important investments comparable to the deployment of wired solutions. In fact, wireless mesh routers can rapidly and easily integrate the wireless infrastructure as soon as the coverage needs to be extended. As a result, a growing number of cities have adopted this paradigm to attract visitors and citizens and start a long-lasting development process. Users can temporarily join the mesh network and act as clients and routers for other nodes, thus enhancing the network capacity, throughput, and reliability. Currently, one can find off-the-shelf and proprietary mesh networks solutions while IEEE standardization efforts are targeting network coverage ranging from PANs to WANs. The goal of this chapter is to present the mesh networking fundamentals in wireless PANs, LANs, MANs, and WANs. To this end, a general overview of the mesh networks architecture and characteristics is given while addressing general concepts such as the supported applications, the routing and management protocols, the QoS provision, and the security considerations. Then, the detail of the IEEE standardization efforts targeting the network coverage ranging from PANs to WANs is presented. We particularly address the physical layer and the MAC layer design issues for the mesh communication mode support while presenting the challenges that are particular to each network (PAN, LAN, MAN or WAN). An overview of the available commercial systems and deployed solutions is also given. We conclude by discussing some of the research issues aiming at designing scalable, low-cost, and easily deployable wireless mobile mesh networks.


3.2 WIRELESS MESH NETWORKING FUNDAMENTALS NETWORK ARCHITECTURE:

A wireless mesh network is a hierarchical network formed by fully wirelessly interconnected nodes, as illustrated. A fully meshed network is a network where every node directly connects to every other node; a partial mesh network is a network where each node is connected to a set of other nodes. We distinguish routers nodes that act as layer 3 gateways and support meshing functions. Such nodes are usually equipped with multiple network interfaces for different access technologies; they can guarantee wider coverage with less power consumption thanks to the support of multi-hop communications. The network resulting from the mesh routers interconnection is called a wireless backbone; it guarantees the connectivity between nomadic users and wired gateways. The wireless mesh network includes also Access Points (APs), which can be viewed as special mesh routers provided with a high-bandwidth wired connection to the Internet. The wireless network formed by the interconnection of the AP and the mesh routers is called a backhaul. The latter enables the access to external networks while providing high-bandwidth and seamless multi-hop communication at a low cost.


Finally, mesh clients are generally equipped with a radio interface supporting mesh networking functions; that is why they can act as routers for other mesh nodes. However, they do not provide the bridge/gateway functionalities needed for Internet access and interoperability with other networking technologies. Mesh clients can be laptops, pocket PCs, PDAs, IP phones, etc.


3.3 Characteristics

Mesh networks are gaining a growing interest thanks to their special characteristics that enable the deployment of new applications at lower cost. The most important characteristics are as follows:


Multi-Hop Communication: The multi-hop communication scheme guarantees larger coverage zones and an enhancement of the network capacity. In fact, line-of-sight constraint no longer matters because the intermediate nodes relay the information to their neighbors on short wireless links using a reduced power transmission. As a result, the interferences are decreased and the throughput is augmented. Besides, the multi-hop connectivity allows several devices to access the network at once by relying on other mesh nodes without affecting the overall network performance. Finally, mesh networks gain more capacity as the number of internal nodes increases and the data traffic can reach larger areas by crossing multiple hops until the final destination.


Wide Coverage and Cost Reduction: The wireless infrastructure supported by the mesh networks eliminates the deployment costs of a new wired backhaul through cities and rural areas. Moreover, the flexible infrastructure can easily be enforced by adding new wireless mesh routers anywhere, anytime the coverage needs to be enhanced. Only some APs need to be connected with the wired infrastructure to allow Internet access.


Mobility and Power Consumption: The mobility and power consumption vary with the nature of the mesh node. For example, mesh routers and APs have minimal mobility and reduced power constraints. However, mesh clients are mostly small mobile devices with reduced battery autonomy. Therefore, MAC and routing protocols supported by the backbone/backhaul do not need to be power efficient, but they cannot be implemented on simple mesh clients. Reliability: Mesh networks rely on multi-hop communication and can use every internal node to route traffic to the destination. Therefore, multiple paths exist between two communicating endpoints and temporary path failures can be easily tolerated. Besides, mesh clients that need to communicate with external destinations (e.g., Internet) can choose between multiple egress points toward the wired network, thus tolerating router failures and reducing potential congestions.


3.4 Supported Applications

The mesh networks support a large number of applications dedicated to personal, local, metropolitan, and wide areas networks.


Home Networking: Mesh networks can be deployed at home because they support bandwidth-greedy applications such as multimedia traffic transmission. Mesh nodes can be desktop PCs, laptops, high-definition TV, and DVD players. Wireless APs or mesh routers can easily be added to cover dead zones without requiring wiring or complex configurations.


Enterprise Networking: Traditional wirelesses LANs have been widely used in enterprises, but they have not succeeded in effectively reducing the deployment cost because the presence of a wired infrastructure is a must. Adopting mesh networks in enterprises enables the share of resources and an overall performance enhancement thanks to the multi-hop communication and the wireless infrastructure deployment. In fact, bottleneck congestion resulting from the one-hop access to the traditional APs is eliminated. Besides, the infrastructure can easily scale according to the network's needs without requiring complex configurations and wiring.


3.5 Routing Protocols

Wireless mesh networks are characterized by multi-hop communications and rely on a wireless backhauling system to access other external networks such as the Internet. Consequently, they need to address special constraints such as enhanced scalability, varying power constraints, and cross-layer design. These specificities require special routing capabilities that may be partially inherited from the ad hoc context, but that surely differ from those implemented in the wired and cellular networks. We believe that the specification of a wireless mesh routing protocol should provide new performance metrics that take into consideration the quality of the intermediate links while trying to minimize the path length. Meanwhile, the mesh routers and the mesh clients presenting different mobility and power constraints should implement an efficient hybrid routing protocol able to address those specificities. For instance, the Link Quality Source Routing (LQSR) based on the DSR protocol selects the routes with respect to the expected transmission count (or ETX), the per-hop round-trip tune (RTT), and the per-hop packet pair. Results showed that adopting the ETX for stationary nodes guarantees a good performance although adopting the minimum hop count as route selection criteria for mobile nodes gives better results. New performance metrics that achieve good performances in the mesh context present a research issue that needs to be investigated. In addition, fault-tolerance mechanisms that guarantee the rapid selection of a new path in case of link failure should be defined. Besides, the route selection should be based on the congestion status of the network to efficiently use the available resources. In fact, the mesh network presents multiple routes between communicating nodes so that alternative paths which offer the required QoS may be selected in case of mobility or link quality decrease. However, it is worth noticing that the route-establishment complexity increases as the network size grow. Meanwhile, the routing protocol should address the ephemeral nature of mesh nodes while guaranteeing the end-to-end QoS requirements, especially in the case of metropolitan and wide area mesh networks. When considering the ad hoc context, hierarchical routing protocols as presented in adopt a self organization scheme that groups the network nodes into clusters with a certain size. Each cluster is then managed by one or more cluster heads and nodes belonging to different clusters may communicate using other nodes as gateways. The routing mechanisms implemented inside a cluster may be proactive while intra-cluster routing may be on-demand. Such protocols achieve good performances especially when the node's density is high; however, they cannot be applied to the mesh context without adding some modifications. For instance, a mesh node selected as a cluster head may not present sufficient power and processing capabilities, thus becoming a bottleneck. Geographic routing which is topology-based resists mobility better, but requires important processing resources. In addition, delivery is not always guaranteed even if a path exists between the communicating nodes. Open research issues need to be addressed if this routing principle is applied to the mesh networking context.


3.6 Network Management

Mesh networks management needs to address nodes' specificities in terms of mobility, location, and power to provide an up-to-date vision of the network status. The resulting accurate management data will serve especially for enhancing the overall performances and making the wise decisions to overcome the encountered problems.


3.7 Mobility Management

Mobility management addresses the location management and the hand-over. Location management addresses the location registration and the call delivery; it guarantees that active nodes remain always reachable despite their mobility. The hand-over process, also known as hand-off, consists in transferring a communication; therefore, it requires a new connection generation and implements the control of the data flow. Advanced mobility management mechanisms have been proposed for cellular and IP networks; however, the adopted schemes are centralized because they rely on the base stations. As mesh networks present an ad hoc architecture, distributed or hierarchical location and hand-over management functions should be adopted while taking into consideration the nodes' nature (routers or clients) and their different mobility schemes. In fact, backbone nodes present reduced mobility while mesh clients frequently roam across different mesh routers. Proposing a multi-layer mobility management framework that addresses mesh specificities is a hot research topic that needs to be investigated. More specifically, location management functions may be used at MAC and routing layers to provide better performances and permit the development of new location-based applications for the mesh scenarios.


3.8 Power Management

Mesh networks are made up of mesh routers and mesh clients. While the routers present reduced mobility and power constraints, the clients are tiny pieces of equipment, such as IP phones and sensors, which are battery-dependent. Besides, it is always preferable to reduce the transmission power to save the resources and reduce the interferences while increasing the spectrum spatial-reuse efficiency. Consequently, power-efficient protocols need to be developed while paying particular attention to some constraints as the hidden nodes scenario to avoid the performance degradations at the MAC level.


3.9 Network Monitoring

Mesh routers need to calculate their own statistics to report them for monitoring servers. Servers should then analyze the data and process anomaly detection. They can then trigger alarms or reactively respond, depending on the scenario. Few networking management protocols have been proposed for the ad hoc context however, they do not address the scalability issue of the mesh networks. Besides, new data processing algorithms that address the mesh network's specificity need to be developed.


3.10 QoS Provision

A service in a communication network is defined by the International Telecommunication Union (ITU) as a service provided by the service plane to an end user (e.g., a host [end system] or a network element) and which utilizes the IP transfer capabilities and associated control and management functions for delivery of the user information specified by the service level agreement (SLA). In the telecommunications area, the quality of service is intrinsic, perceived, or assessed. Intrinsic QoS is a technical measure considered by engineers and network service providers; it is always objectively compared to the expected performance not affected by customers' perceptions. Perceived QoS reflects the end user's view about a service while assessed QoS is a factor that the customer decides whether or not to continue using the service. It is clear that the most challenging issue in providing QoS is to specify the requirements and then quantify them based on a set of measurable QoS parameters such as the delay, the jitter, and the bandwidth.


QoS routing algorithms deployed in the mesh networks adopt either an IntServ or a DiffServ approach according to the network size (coverage area and nodes numbers) and the mobility scheme. For instance, Mesh Dynamics proposes a technique for wireless mesh PANs called heartbeats, which relies on the information provided by each intermediate node to establish paths satisfying the QoS requirements from source to destination. Besides, proposes a QoS routing protocol called WMR (Wireless Mesh Routing) for a wireless mesh LAN infrastructure. WMR supports multimedia applications by guaranteeing minimum bandwidth and maximum end-to-end delay for all intra-BSS and inter-BSS communications; it also guarantees a per-flow granularity and processes a full, on-demand hop-by-hop routing with no route caching. To fulfill the broadband wireless access QoS requirements in MAN networks and address the scalability issues, the IEEE 80 standard defines four classes of service while presents a Wireless DiffServ architecture for the wireless mesh backbone.


CHAPTER 4

4. Mesh Networking in Wireless PAN's,LAN's,MAN's and WAN's

Mesh networks need to provide advanced security mechanisms to encourage client subscribing to reliable services. More specifically, the mesh traffic travels through multiple intermediate nodes on the particularly vulnerable wireless channels, thus increasing the hacking probability. Currently, mesh networks provide the same security services deployed in the WLANs and encrypt the backhaul communications which represent the important part of the whole traffic. However, they have some characteristics that render them particularly vulnerable. In fact, the adopted multi-hop communication which relies on the cooperation of the network nodes suffers from selfish behaviors. For instance, some selfish nodes may obtain free services while refusing to participate in routing and affecting the system availability. Besides, the lack of authentication provides attacking nodes with free-of-charge services. Consequently, hackers may cause denial of service by sending arbitrary traffic or advertise high rates, thus affecting network performance. Moreover, the routing service which adapts to the topology changes and the environment conditions can be attacked in several ways. In fact, malicious nodes can mislead targeted actors by pretending higher or reduced utility values to create an inaccurate representation of the network status, thus leading to serious denial of service attacks. To address this issue, each node should locally verify the consistency of the collected information and base its routing decision on the deduced conclusion.


4.1 Scheduling and Multimedia Support

Mesh networks adopt broadcast scheduling to coordinate transmissions between the communicating nodes. We mainly distinguish two types of scheduling which vary according to the scheduling-messages contention resolution procedure . For instance, in the distributed scheduling adopted by the IEEE 802.11 standard, the nodes share their scheduling data within the two-hop range and cooperate to avoid contention while resources are granted, thanks to a connection establishment procedure. However, mesh BS collects resource requests from the nodes within a certain range and then allocates the resources in a centralized manner. Such resource reservation procedures are implemented in the MAC layer to establish high-speed broadband mesh connections needed by multimedia applications. In fact, scheduling supplies guaranteed bandwidth and delay based on the flow priority requirements in both metropolitan and wide area networks. In PAN context, beacons are used to allow isochronous transmission by reserving Channel Time Allocation (CTA) slots. We may state that the QoS provision mechanisms proposed for mesh networks differ from one network to another. In the following sections, we further detail Wireless mesh PANs aim to provide short-range communications between small groups of fixed and mobile computing devices such as PCs, PDAs, peripherals, cell phones, pagers, and consumer electronics. As the network nodes have power constraints, the multi-hop communication is adopted to increase the coverage area while reducing transmission power and increasing the throughput. Besides, the nodes do not rely on an infrastructure as in wireless LANs; they have to play the role of clients and routers at the same time. Therefore, the network reliability and stability need to be guaranteed despite routers' mobility. In addition, wireless mesh PANs intend to provide multimedia applications that require the design of appropriate QoS routing protocols. More specifically, multimedia home networking with high-speed streaming media and streaming content download, environmental monitoring, automatic meter reading, and plenty of commercial and industrial-type applications monitoring need to be supported.


4.2 Challenges

The reliability of the QoS routing service is a major concern for wireless mesh PANs. In fact, in the ad hoc networks context, each node maintains a connectivity graph defining a path for every other node in the network. However, the node's mobility leads to a constant change in the routing tables and result in an important overhead as the number of the network members increases. To address these issues, mesh routings protocols select the next relay based on the local information stating which node has the strongest signal and is closest to the sender. Unfortunately, this local approach is efficient only in the case of small networks; besides, it is not able to guarantee QoS for mission-critical applications. A global approach based on the exchange of compact control messages for the routing tables updates needs to be found. On the other hand, the routing service needs to proactively adapt to the power constraints of the nodes to avoid paths breakage and QoS violations. The third wireless mesh PANs challenge is related to beacon alignment issues. In fact, traditional PANs use beacons to provide isochronous transmissions. A beacon is formed by CTA and Contention Access Period (CAP) time slots, as depicted in CTA time slots are reserved slots for regular transmissions of traffic with hard QoS constraints such as video streaming over a multi-hop network.


The Pico Net Controllers (PNCs) send the beacon synchronization pulses to coordinate the transmissions between the managed nodes. However, a node may not receive this pulse due to radio interference from other devices in other Pico nets. Consequently, the PNCs should coordinate their transmissions with their managed nodes despite the fact that interference may occur at anytime (during the beaconing period [B], the CAP, or the CTA period).


4.3 Architecture

A mesh PAN can either be organized in a full mesh topology or a partial mesh topology. When each node is directly connected to all others, we obtain a fully meshed network . In a partial mesh topology, only some nodes are directly connected to all others; the remaining ones are connected only to nodes with which they frequently communicate. A mesh PAN topology is made up of a PAN Coordinator (PAN-C) that is partially or fully connected with other Full Function Devices (FDDs). Each FDD is then interconnected with a set of Reduced Function Devices (RFDs). FDDs support enhanced functionalities such as routing and link coordination; RFDs are simple send/receive devices. This mesh topology allows better network coverage extension and provides enhanced reliability via route redundancy because nodes may act as routers and relay data in case of link breakage. In fact, data which has not reached its destination is forwarded to one or more neighbors by nodes that act as repeaters. Each node keeps a routing table that indicates which neighbor to contact when a packet with a particular address is forwarded. Moreover, an easier network configuration is fulfilled and the battery lives are extended due to short links usage.


4.4 Meshing and the Ultra Wide Band

The Ultra Wide Band (UWB) is a high-speed physical technique that particularly fits short-range communications. In fact, UWB enhances the meshing capabilities by having low power and cost constraints while guaranteeing precise location information and important throughput. This radio technology transmits signals with extremely wide spectrum (e.g., the bandwidth of the transmission can be several GHz wide) at a very low transmission power so that the resulting Power Spectrum Density (PSD) is very low, thus allowing a massive frequency reuse. The resulting reduction of the consumed power allows tiny devices to save their battery life while resisting fading and interference. However, UWB applies only to short-range communications because the bandwidth decreases rapidly as distance increases. Consequently, if the same throughput offered by the UWB needs to be provided for wireless mesh LANs or MANs, new physical layer transmission techniques need to be developed. UWB allows the coexistence of tens and even hundreds of simultaneous non-interfering channels within radio distance of each other. Using a mesh topology enables us to trade some channels to increase the overall performance. In fact, nodes A and B are direct neighbors distant by 10 m and having 100 Mbps as available bandwidth. Besides, node C is a common neighbor distant by m from A and B. This shorter distance implies 10 Mbps of available bandwidth between both A and C and B and C. If A wishes to communicate with B, it will be wise to choose the path A -> C -> B with an available bandwidth of 10 Mbps, which is two times faster than the direct one. Meshing also increases the coverage because nodes which are not in direct range can communicate by using other network members as relays. Using large UWB increases the available bandwidth as the number of nodes increases and a mesh topology guarantees a very easy and cheap deployment of communication networks for homes and offices.


4.5 A Meshed Adaptive Robust Tree (MART).

The Meshed Adaptive Robust Tree (MART) allows routing a packet through a shorter path; single points of failure can be avoided. For instance, if the link between H and B is broken, packets from H to C or to E can still be routed. However, paths are still non-optimal in most cases.


Samsung also proposes a key distribution scheme called KEYDS to provide security services. The mesh nodes that form the backbone should provide security services to the rest of the network entities. Every pair of backbone members shares a secret key that is used to secure the communication between them. Besides, a group key is shared among all backbone members to allow backbone message broadcasts. All mesh points should participate in the key pre-distribution scheme and should be able to perform common pair-wise keys computations. The initial setup of the distribution key management begins when each node within the mesh network obtains its ID. Then, every mesh point obtains the key block from KEYDS with a corresponding column of the incidence matrix. A member of the backbone, as any other mesh point, also obtains the key block from KEYDS.


In addition, every member of the backbone obtains the corresponding key block from the trivial key pre-distribution scheme. Then, every mesh point (except members of the backbone) obtains the final hash-value of the hash-chain and the lengths of the chain with respect to that final value. Finally, every member of the backbone obtains the start hash-value (the seed) of the hash-chain and the current length of the chain with respect to the final value given to the mesh points. Key refresh decisions are then taken by the backbone members when needed. When the network topology changes, the key pre-distribution scheme executes the mesh point exclusion, the mesh point association, and the lost mesh points' recovery to adapt to the new network needs.


CHAPTER 5


5 Wireless Mesh LAN

Wireless mesh LANs has an extended coverage area compared to mesh PANs; they always adopt an infrastructure-based architecture and rely on reduced-mobility APs. Therefore, the PANs router mobility is no longer a challenging issue. Nevertheless, mesh LANs need to provide QoS guarantees and address hand-off and roaming issues.


5.1 Introduction and Advantages

A wireless mesh LAN may be seen as a wireless LAN where all the APs are wirelessly interconnected. Traditional mobility management functionalities such as hand-over and roaming are supported; however, inter-AP communication within the same Extended Service Set (ESS) is done in a hop-by-hop fashion. The transmission scenario in a wireless mesh LAN is done as follows: the AP managing the source forwards the traffic to its neighboring AP instead of sending it to all the APs in the ESS. Then, the neighboring APs send the same packet to the next hop in the same way until the AP managing the destination is reached. At this time, the traffic is forwarded to the destination end node.


Abstraction of LAN Setup

If we compare traditional wireless LANs to wireless mesh LANs, we notice that the latter offers particular advantages related with the deployment costs, offered services, and nature of the supported applications. For instance, deploying a mesh node needs no special wiring and configuration. With little investment and easy configuration process, the network is more reliable because we can simply add as many wireless nodes as needed to increase the performances and cover new zones. Mesh LANs also guarantee load-balancing and optimal resources utilization because wireless nodes may act as routers or APs when the nearest AP is congested and route data to the closest low-traffic node. Fault tolerance is also provided because the clients communicate in a multi-hop fashion, exploiting the redundancy of paths in case of failures. The traffic is automatically rerouted while the failed routers are rapidly detected and recovered or replaced. Furthermore, deploying wireless mesh LANs addresses line-of-sight constraints, especially in outdoor environments. The provided applications in the mesh context fit particularly to the multi-hop architecture as explained as follows [7]:


Warehousing: Warehousing or broadband home networking applications can be supported by traditional wireless LANs. However, the APs are mainly installed on the roofs to provide good coverage; besides, an expensive deployment of a wired backhaul is needed. Adopting wireless mesh LANs optimally addresses the pre-described deployment issues. In fact, APs are wirelessly interconnected and can be added anytime and anywhere to improve the scalability, the reliability, and the network performance. Moreover, fault-tolerant paths can be used to route the traffic between the mesh nodes until the final destination while congestion resulting from the traditional access to the hub is eliminated.


5.2 Architecture Technologies

The mesh wireless LAN has two possible architectures. The infrastructure architecture is formed by different APs interconnected wirelessly within an ad hoc network. The resulting wireless backhaul reacts to any topology changes by processing automatic topology learning and dynamic path configuration. The IEEE 802.11 standard defines the physical and MAC functions needed by the interconnected APs to manage the mesh clients such as the reliable unicast or multicast/broadcast delivery. The infrastructure architecture aims at reducing deployment costs while enhancing network coverage and reliability. More specifically, it becomes easy to add new APs to enforce the existing backhaul network and cover dead zones without any need of wire deployment and complex configurations. The infrastructure meshing is the most used because it allows good scalability and supports gateway functions such as bridging, thus enabling the connection to the Internet and the integration with other network technologies.


The client meshing architecture does not require the backhaul; in fact, mesh nodes can play the role of APs and be clients and routers at the same time forming a dynamic ad hoc network. To do so, the mesh nodes communicate in a peer-to-peer fashion and perform layer 3 routing while supporting auto-configuration and providing end user services. Packets are transmitted within flat network architecture from one hop to another until the final destination; however, congestion occurs more frequently and the network performance rapidly decreases when the number of mobile nodes grows. The hybrid architecture combines the infrastructure and the client meshing to achieve enhanced performances. Mesh clients can be managed by APs, but may also directly communicate with other peers. This mode is still not used very often in case of Wi-Fi meshes.


5.3 Challenges

A wireless LAN implementing the IEEE 802.11 standards is formed by one or more APs responsible for central management and a set of mobile stations equipped with a 80 -compliant interface. An AP and the stations situated in its coverage zone form a cell or Basic Service Set (BSS). The mobile stations may also form an Independent Basic Service Set (IBSS) when they directly communicate in an ad hoc fashion without requiring a central AP. A set of APs may be interconnected by a wired distribution system, thus forming an ESS which can be viewed as a single 80 network segment. In the mesh context, the meshing APs have to form a wireless infrastructure; therefore, they need to implement auto-configuring mechanisms to automatically integrate the ad hoc network formed by the neighboring APs. Besides, the mesh traffic originated by a node is handled by the managing AP which is responsible for its delivery to the destination. This traffic may cross multiple intermediate nodes before reaching the recipient and each crossed node will introduce some latency, thus hardening the QoS provision in terms of minimum delay and jitter. Meanwhile, APs need to exchange data on wireless channels; therefore, mesh networks should guarantee the coexistence of intra-BSS and inter-BSS communication by eliminating possible interference while guaranteeing the required QoS. Hidden and exposed terminals problems should also be addressed. Last but not least, APs forward the arriving packets to their MAC layer, which adopts a drop-tail queue management without taking into consideration the number of crossed hops. This management strategy may lead to a severe unfairness problem because neighboring or smaller hop length flows arrive more frequently at APs and fill up the link layer buffer. Consequently, packets coming from far away nodes face a full buffer and will systematically be dropped.


CHAPTER 6


6.1 Problem Definition:

Data's generated in Wireless Sensor Networks may not all alike, some Data's are more important than other Data's and they may have Different Delivery Requirements. If Congestion occurs in the Wireless Network. Some or More Important data's may be dropped. But in our Project we handle this problem by addressing Differentiated Delivery Requirements. We propose a class of algorithms that enforce differentiated routing based on the congested areas of a network and data priority.


The basic protocol, called Congestion-Aware Routing (CAR), discovers the congested zone of the network that exists between high-priority data sources and the data sink and, using simple forwarding rules, dedicates this portion of the network to forwarding primarily high-priority traffic. Since CAR requires some overhead for establishing the high-priority routing zone, it is unsuitable for highly mobile data sources. To accommodate these, we define MAC-Enhanced CAR (MCAR), which includes MAC-layer enhancements and a protocol for forming high-priority paths on the fly for each burst of data. MCAR effectively handles the mobility of high-priority data sources, at the expense of degrading the performance of low-priority traffic.


6.2 Existing Problem:

Congestion may lead to indiscriminate dropping of data (i.e., high-priority (HP) packets may be dropped while low-priority (LP) packets are delivered). It also results in an increase in energy consumption to route packets that will be dropped downstream as links become saturated.


Congestion becomes worse when a particular area is generating data at a high rate. This may occur in deployments in which sensors in one area of interest are requested to gather and transmit data at a higher rate than others.


6.3 Problem Solution

We addressed data delivery issues in the presence of congestion in wireless sensor networks. We proposed CAR, which is a differentiated routing protocol and uses data prioritization.


Use energy more uniformly in the deployment and reduce the energy consumed in the nodes that lie on the conzone (Congested Zone), which leads to an increase in connectivity lifetime.


Conclusion:

In this I am going to conclude that one can transmit the data from one node to another regardless of the data which is to be transmitted. By addressing the priority either high or low we can send data according to the priority based. For this we proposed CAR which is used for knowing the priority of the data and routing protocol is differentiated. And usage of energy is distributed uniformly for the nodes in the congested zone which increases lifetime connectivity.


References

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