1. Wireless Mesh Networks

1.1. IntroductionWireless Mesh Network (WMN) is a promising wireless technology for several emerging and commercially interesting applications, e.g., broadband home networking, community and neighborhood networks, coordinated network management, intelligent transportation systems. It is gaining significant attention as a possible way for Internet service providers (ISPs) and other end-users to establish robust and reliable wireless broadband service access at a reasonable cost.WMNs consist of mesh routers and mesh clients as shown in Fig. 1.1. In this architecture, while static mesh routers form the wireless backbone, mesh clients access the network through mesh routers as well as directly meshing with each other. [1]

Figure 1.1 an illustration of wireless mesh network architecture.

Wireless communication is without a doubt a very desirable service as emphasized by the tremendous growth in both cellular and wireless local area networks (WLANs). The cellular networks offer wide area coverage, but the service is relatively expensive and offers low data rates, even the third generation of cellular networks (3G) offers low data rates (2Mbps) compared to WLANs. On the other hand, the WLANs have rather limited coverage (and the associated reduced mobility). Furthermore, in order to increase the coverage of WLANs, a wired backbone connecting multiple access points is required.Wireless metropolitan area networks (WMANs) partially bridges this gap, offering high data rates with guaranteed quality of service to a potentially large customer base (up to tens of miles from the base station). The main drawback of WMANs is their (current) lack of mobility support and the line of sight (LOS) requirement.Wireless mesh networks (WMNs) have the potential to eliminate many of these disadvantages by offering low cost, wireless broadband Internet access both for fixed and mobile users.[2]

In this chapter, we will provide a comprehensive introduction to the recent developments in the protocols and architectures of wireless mesh networks (WMNs) and also discusses the opportunities and challenges of wireless mesh networks.

1.2. WMN ArchitectureWMN is quite different network architecture when comparing to the traditional Internet, cellular networks, or WLAN.The first widely deployed wireless data network standard has been IEEE’s 802.11 standard. The 802.11 standard is a suite of protocols defining an Ethernet-like communication channel using radios instead of wires. [3]A typical 802.11 network has two types of network elements: stations (STAs) and access points (APs).Stations can be mobile devices such as laptops, personal digital assistants, IP phones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface. Access points (APs), normally routers, are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with. [4]The 802.11 standard specifies two network architectures: Infrastructure basic service set BSS and Independent basic service set IBSS. Either 802.11 stations communicate directly with each other to form an IBSS or with the AP to form a BSS. An extended service set (ESS) is a set of connected BSS. Access points in an ESS are connected by a distribution system (DS) via wired links.Figure 1.2 shows an example of an ESS.

Figure 1.2. Extended service Set :[5]

The drawback of this architecture is highly expensive infrastructure costs, since an expensive cabled connection to the wired Internet backbone is necessary for each AP. On the other hand, constructing a wireless mesh network decreases the infrastructure costs, since the mesh network requires only a few points of connection to the wired network.(1)

In wireless Mesh Networks (WMNs) APs turn into mesh access points (MAPs). Mobile stations are sometimes referred as mesh clients. The new IEEE 802.11s standard for WMNs introduces a third class of nodes called mesh points (MPs) [6]. MPs and MAPs support WLAN mesh services, allowing them to forward packets on behalf of other nodes to extend the wireless transmission range. Mesh clients can associate with MAPs but not with MPs. Mesh portals are MAPs connected to a distribution system or a non IEEE 802.11 network. [7]There are three different types of WMN:

Infrastructure WMN (Figure 1.3): is a hierarchical network consisting of mesh clients, mesh routers and gateways. Mesh routers constitute a wireless mesh backbone, to which mesh clients are connected as a star topology, and gateways are chosen among mesh routers providing Internet access.[8]

Client WMN (Figure 1.4): In this type the mesh clients form the network and no MAP is required. Client WMNs are also known as mobile ad-hoc networks (MANETs). [7]

Hybrid WMN (Figure 1.5): Mesh clients can perform mesh functions with other mesh clients as well as accessing the network.[9]

Figure 1.3. Infrastructure Wireless Mesh Network [10]

Figure 1.4. Client Wireless Mesh Network [10]

Figure 1.5. Hybrid Wireless Mesh Network [10]

Because of the system architecture, wireless mesh networks have different requirements to the physical layer, MAC layer and routing protocols than the traditional IEEE 802.11 WLAN, in the next few sections we will present an overview of the WLAN protocols and which changes have to be made for WMN.

1.3 Physical Layer

1.3.1 IEEE 802.11 WLAN Physical Layer

The 802.11 physical layer (PHY) is the interface between the MAC and the wireless media where frames are transmitted and received. The PHY provides three functions. First, the PHY provides an interface to exchange frames with the upper MAC layer for transmission and reception of data. Secondly, the PHY uses signal carrier and spread spectrum modulation to transmit data frames over the media. Thirdly, the PHY provides a carrier sense indication back to the MAC to verify activity on the media. [11]

The PHY layer is composed of physical layer convergence (PLCP) and physical medium dependent (PMD) layers .PLCP is an interface to MAC

layer and PMD is equipped with a transmission interface to send and receive files over the air.The physical layer (PHY) is the layer 1 element of OSI protocol stack. IEEE 802.11 introduced three PHY standards in 1997 and two supplementary standards in 1999:• Frequency-hopping spread-spectrum (FHSS)• Direct-sequence spread-spectrum (DSSS)• Infrared light (IR)• 802.11b: High-rate Direct Sequence (HR/DSSS)• 802.11a: Orthogonal Frequency Division Multiplexing (OFDM) [12]

To further increase the throughput a new physical layer was introduced in 802.11g, which supports up to 54 Mbps. IEEE 802.11a/g uses orthogonal frequency-division multiplexing (OFDM).The newest standard IEEE 802.11n will use OFDM in combination with multiple antennas. Thereby data rates of more than 100 Mbit/s will be possible. [13]

A comparison of the most used IEEE 802.11 standards is described below in Table 1.1

802.11n802.11g802.11b802.11a802.11standard

> 100 Mbps54 Mbps11 Mbps54 Mbps2 MbpsMax. Data

Rate2.4 or 5 GHz2.4 GHz2.4 GHz5 GHz2.4 GHzFrequency

OFDM/MIMOOFDMDSSSOFDMFHSS/DSSS

Physical layer

CSMA/CACSMA/

CACSMA/CACSMA/CACSMA/CAMAC Layer Table 1.1 Comparison of wide-spread 802.11 standards [13]

1.3.2 WMN Physical Layer

The key functions of physical layer techniques involve two aspects: efficient spectrum utilization and robustness to interference, fading, and shadowing. In order to increase capacity and mitigate the impairment by fading, delay-spread, and co-channel interference, antenna diversity and smart antenna techniques can be used in WMNs.

It is still necessary to further improve the performance of physical layer techniques. Multiple-antenna systems have been researched

for years. However, their complexity and cost are still too high to be widely accepted for WMNs.

To best utilize the advanced features provided by physical layer, higher layer protocols, especially MAC protocols, need to be re-designed. Otherwise, the advantages brought by such physical layer techniques will be significantly compromised.[14]

1.4 MAC Layer1.4.1 IEEE 802.11 WLAN MAC Layer

The 802.11 MAC layer provides functionality to allow reliable data delivery for the upper layers over the wireless PHY media. The data delivery itself is based on an asynchronous, best-effort, connectionless delivery of MAC layer data. There is no guarantee that the frames will be delivered successfully. [11]

The 802.11 standard defines two media access protocols: DCF and PCF. Distributed coordination function (DCF) allows multiple stations to interact access the medium without a central control. On the other hand, the point coordination function (PCF) lets a central entity—the point coordinator (or PC, usually located in the AP)—control the medium. The PC is responsible for managing access to the medium.All 802.11-compliant devices must support DCF, whereas the support for PCF is optional. However, PCF has never been, in fact, implemented by any 802.11 equipment manufacturers. [3]

Carrier senses multiple access: The basic access mechanism, called DCF is typically the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. CSMA protocols are well known and Ethernet is the most famous one, which is a protocol based on the CSMA/CD access mechanism (CD for collision detection). Contrarily to CSMA/CD mechanism which is based on the collision detection, the CSMA/CA allows an access to a shared medium by avoiding collisions. [15]The CSMA-CD scheme works well for a wired medium. However, this approach is unsuitable for the wireless medium for multiple reasons. Implementing collision detection would require the implementation of

a full-duplex radio (capable of transmitting and receiving at the same time).

Two, the wireless medium is inherently open to interference from a wide variety of sources, especially since 802.11 operates in the unlicensed frequency spectrum[3]

The Hidden node problem (Figure 1.6): The hidden node problem that is unlikely to occur in a wired LAN is another challenge for WLANs. If two stations (A, C) are unreachable and if there is a station (B) in the middle of those two that is reachable from both [12], CSMA requires that, before starting transmission, a terminal “senses” the medium to ensure that the medium is idle and therefore available for transmission. In our case, assume that A is already transmitting data to B. Now, C also wishes to send data to B. Before beginning transmission, it senses the medium and finds it idle since it is beyond the transmission range of A. It therefore begins transmission to B, thus leading to collision with A’s transmission when the signals reach B. [3].

Figure 1.6 Hidden Node Problems [12]

The Exposed Node Problem (Figure 1.7):

Consider what happens when B wants to send data to A and C wants to send data to D (Figure 4.10). As is obvious, both communications can go on simultaneously since they do not interfere with each other. However, the carrier-sensing mechanism raises a false alarm in this case. Suppose B is already sending data to A. If C wishes to start sending data to D, before beginning it senses the medium and finds it busy (due to B’s ongoing

transmission). Therefore, C delays its transmission unnecessarily. [3]

Figure 1.7 Exposed Node Problems [12]

Interframe Spaces (Spaces between successive frames): IEEE 802.11 standard defines four types of IFS timers classified by ascending order, which are used to define different priorities:

Short interframe space (SIFS) is the smallest time required to give priority to the completion of a frame exchange sequence, since other STAs wait longer to seize the medium.[12] , SIFS is specific to PHY layers.

Point coordination IFS (PIFS) is used by the AP (called coordinator in this case) to gain the access (to the medium before any other station. It reflects an average priority to transmit the time-bounded traffic.[3]PIFS = SIFS + 1 Slot time.

Distributed IFS (DIFS) is the IFS of weaker priority than the two previous; it is used in the case of data asynchronous transmission.[3]DIFS = SIFS + 2 Slot time.

Extended IFS (EIFS) is the longest IFS. It is used by a station receiving a packet which is corrupted by collisions to wait more time than the usual DIFS in order to avoid future collisions. [3]

Slot Time: Time is quantized in slots. Slot time is specific to PHY layers. [12].

Table 1.2 shows the main parameters of SIFS and Slot Time for different PHY layer technologies.

ValuePhysical layerParameters50FHSSSlot Time20DSSS, HR/DSSS 9OFDM 8IR 

  28 +/- 10 %FHSSSIFS Time

10DSSS, HR/DSSS 16OFDM 10IR 

Table 1.2 Main Parameters (Values are in microseconds). [15]

Distributed Coordination Function (DCF)The primary 802.11 MAC function is the so-called Distributed Coordination Function (DCF). The DCF is a random access scheme based on the Carrier Sense Multiple Access with Collision Avoidance protocol (CSMA/CA). [17]The DCF will be implemented in all STAs, for use within both IBSS and infrastructure network configurations. In this protocol, the STA, before transmitting, senses the medium. If the medium is free for a specified time, called distributed interframe space (DIFS), the STA executes the emission of its data [18]. When a station receives a unicast frame it waits for the duration of SIFS (which is shorter than DIFS) and sends back an acknowledgement message (ACK).However If the medium is busy because another STA is transmitting, the STA defers its transmission, and then it executes a backoff algorithm within a contention window (CW). This behavior of the CSMA/CA protocol is sketched in the Figure 1.8.The backoff mechanism used in the DCF is discrete and the time following a DIFS is divided into temporal slots.[18]

Figure 1.8 IEEE 802.11 unicast data transfer. [13]Distributed Coordination Function (DCF) with RTS/CTSIn order to solve the hidden and exposed terminal problems in CSMA, researchers have come up with many protocols, which are contention based but involve some forms of dynamic reservation collision resolution. Some schemes use the Request- To-Send/Clear-To-Send (RTS/CTS) control packets to prevent collisions. [20]The principle operation of the mechanism is described as follows: A station wanting to send frames begins by initially transmitting a

short control packet called request to send (RTS), which contains the source, the destination and duration of the transmission(ie.NAV).[15]

The destination station answers (if the medium is free) with a control packet called clear to send (CTS), containing the same duration information(ie.NAV) .[15]

All stations receiving either the RTS and/or the CTS will set their Virtual Carrier Sense indicator (called NAV, for Network Allocation Vector), for the given duration, and will use this information together with the Physical Carrier Sense when sensing the medium. Therefore, these nodes consider the channel busy for the NAV mentioned.[21]

If the source does not receive the CTS packet, it assumes that a collision occurred and will retransmit the RTS packet after a random waiting period.[15]

If the recipient receives the CTS packet correctly, the source emits an acknowledgement to announce to the recipient that the packet CTS was received. Then the communication will be able to take place.[15]

Figure 1.9 shows a transmission between two stations A and B, and the NAV setting of their neighbors.

Figure 1.9 CSMA/CA with RTS/CTS mechanism. [15]Point Coordination Function (PCF)

As an optional access method, the 802.11 standard defines the PCF, which enables the transmission of time-sensitive information. With PCF, a point coordinator within the access point controls which stations can transmit during any give period of time. Within a time period called the contention free period, the point coordinator will step through all stations operating in PCF mode and poll them one at a time. For example, the point coordinator may first poll station A, and during a specific period of time station A can transmit data frames (and no other station can send anything). The point coordinator will then poll the next station and continue down the polling list, while letting each station to have a chance to send data. [16]

1.4.2 WMN MAC Layer

In general, the shared wireless medium in WMNs requires the use of appropriate MAC protocols to mitigate the medium contention issues as well as to allow for efficient use of the limited bandwidth. Specialized MAC protocols could also help alleviate the hidden/ exposed terminal problem. [10]Distributed coordination Function DCF can be used in WMN, However it shows bad performance, it does not solve the hidden and exposed terminal problem and does not provide fairness.

As a consequence MAC protocols for single-hop WLANs do not work properly in WMNs. Consequently specialized MAC protocols should be used in WMNs.

Single channel MAC protocolsEven though multiple non-overlapped channels exist in the 2.4GHz and 5GHz spectrum, most IEEE 802.11-based multi-hop ad hoc networks today use only a single channel. [22]

There are three approaches in this case:

Improving existing MAC protocols. Currently several MAC protocols have been proposed for multi-hop ad hoc networks by enhancing the CSMA/CA protocol. These schemes usually adjust parameters of CSMA/CA such as contention window size and modify backoff procedures. They may improve throughput for one-hop communications. However, for multi-hop cases such as in WMNs, these solutions still reach a low end-to-end throughput, because they cannot significantly reduce the probability of contentions among neighboring nodes.[23]

Cross-layer design with advanced physical layer techniques. Two major schemes exist in this category: MAC based on directional antenna and MAC with power control [23]. The first set of schemes eliminates exposed nodes, the second set of schemes reduce them .However hidden nodes still exist and may become worse.

Proposing innovative MAC protocols.[23]

New introduced WMN MAC protocols mainly try to provide QoS mechanisms and enhance fairness.

Multi-channel MAC protocols

Despite significant advances in physical layer technologies, today's wireless LAN still cannot offer the same level of sustained bandwidth as their wired brethren. The advertised 54 Mbps bandwidth for IEEE 802.11a/g wireless LAN interface is the peak link-layer data rate. When all the overheads, MAC contention, 802.11 headers, 802.11 ACK, packet errors are accounted for the actual goodput available to applications is almost halved. In addition, the maximum link layer data rate falls quickly with increasing distance between the transmitter and the receiver. The bandwidth problems is further aggravated for multi-hop ad hoc networks due to interference from adjacent hops on the same path as well as from

neighboring paths .Fortunately, the IEEE 802.11b/g standards and IEEE 802.11a standard provide 3 and 12 non-overlapped frequency channels respectively, which could be used simultaneously within a neighborhood. The ability to utilize multiple channels substantially increases the effective bandwidth available to wireless network nodes. [22]

A multi-channel MAC may belong to one of the following categories: Multi-channel single-transceiver MAC: Since only one transceiver

is available, only one channel is active at a time in each network node. However, different nodes may operate on different channels simultaneously in order to improve system capacity.

Multi-channel multi-transceiver MAC: In this scenario, a radio includes multiple parallel RF front-end chips and baseband processing modules to support several simultaneous channels. On top of the physical layer, there is only one MAC layer to coordinate the functions of multiple channels.

Multi-Radio MAC: In this scenario, a network node has multiple radios each with its own MAC and physical layers. Communications in these radios are totally independent. Thus, a virtual MAC protocol such as the multi-radio unification protocol (MUP) is required on top of MAC to coordinate communications in all channels. In fact one radio can have multiple channels. However, for simplicity of design and application, a single channel is used in each radio. [23]

Wireless mesh network standard IEEE 802.11s support an optional multi-channel single-transceiver MAC protocol called Common Channel Framework (CCF) [24]The CCF assumes that each node is equipped with a single half-duplex transceiver and nodes in the network share a common control channel. Using the CCF, node pairs, select a different channel and switch to that channel for a short period of time, after which they return to the common channel. During this time, node exchange one or more frames. The channel coordination itself is carried out on the common channel by exchanging control frames or management frames that carry information about the destination channel. As shown in Figure 1.10, mesh points are synchronized to each other and utilize the common control channel .once on the common channel, an arbitrary MP can initiate transmission by sending request-to-switch (RTX) frame carrying information of the destination data channel on which the communication will take place.

The destination MP accepts this request by transmitting a clear-to-switch (CTX) frame carrying the same destination data channel .if the receiving MP accepts the RTX request, the MP pair switches to the destination channel together, which causes the channel switching delay.Then the sender transmits the data and the receiver responds with an ACK. To increase the utilization of the common channel a channel coordination window (CCW) is defined, in which the common channel is solely used for RTX/CTX. Outside the CCW the common channel can also be used for data transfers. [24, 25]

Figure 1.10 Common Channel Frameworks in IEEE 802.11s [24]

1.5 Routing Protocols

1.5.1 Routing Protocols overview

Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network, electronic data networks (such as the Internet), and transportation networks. This section is concerned primarily with routing in data networks using packet switching technology.

Routing schemes differ in their delivery semantics:

Unicast delivers a message to a single specified node;

Broadcast delivers a message to all nodes in the network.

Multicast delivers a message to a group of nodes that have expressed interest in receiving the message.

Anycast delivers a message to any one out of a group of nodes, typically the one nearest to the source.

Small networks may involve manually configured routing tables (static routing), while larger networks involve complex topologies and may change rapidly, making the manual construction of routing tables unfeasible. Adaptive routing attempts to solve this problem by constructing routing tables automatically, based on information carried by routing protocols, and allowing the network to act nearly autonomously in avoiding network failures and blockages [4].In traditional wired networks either distance-vector protocols or link-state protocols are used. Distance Vector protocols determine best path on how far the destination is, Distance can be hops or a combination of metrics calculated to represent a distance value. While Link State protocols are capable of using more sophisticated methods taking into consideration link variables, such as bandwidth, delay, reliability and load. Distance-vector routing protocols are simple and efficient in small networks, and require little, if any management. However, they do not scale well, and have poor convergence properties. Link State Routing protocols provide greater flexibility and sophistication than the Distance Vector routing protocols.Some of the link-state routing protocols are the OSPF, IS-IS and EIGRP, RIP and BGP are well-known distance-vector protocols [26].

Routing protocols can be classified as interior routing protocols or exterior routing protocols.Most known interior routing protocols are:

Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP) Open Shortest Path First (OSPF) Intermediate System to Intermediate System (IS-IS)

Most known exterior routing protocols are: Border Gateway Protocol (BGP) Constrained Shortest Path First (CSPF)

Another classification of routing protocols is useful to WMN; pro-active routing protocols maintain fresh lists of destinations and their routes by periodically distributing routing tables throughout the network, while reactive routing protocols find a route on demand by flooding the network with Route Request packets. The Hybrid routing protocol

combines the advantages of proactive and reactive routing , The routing is initially established with some proactively prospected routes and then serves the demand from additionally activated nodes through reactive flooding.[4]

1.5.2 Routing in Wireless Mesh Network

Routing metrics in wireless mesh network The cost of a route is calculated using what are called routing metrics. Routing metrics are assigned to routes by routing protocols to provide measurable values that can be used to judge how useful (how low cost) a route will be. Routes may have more than one metric and the metrics used may be exchanged between routers, or it may be entirely local to that one router. Routes may have more than one metric and the metrics used may be exchanged between routers, or it may be entirely local to that one router.[27]

Expected Transmission Count (ETX)This metric calculates the expected number of transmissions (including retransmissions) needed to send a frame over a link, by measuring the forward and reverse delivery ratios between a pair of neighboring nodes. To measure the delivery ratios, each node periodically broadcasts a dedicated link probe packet of a fixed size. The probe packet contains the number of probes received from each neighboring node during the last period. Based on these probes, a node can calculate the delivery ratio of probes on the link to and from each of its neighbors. The expected number of transmissions is then calculated as:ETX = 1

df∗dr

Where df and dr are the forward and reverse delivery ratio, respectively. With ETX as the route metric, the routing protocol can locate routes with the least expected number of transmissions.[1]

Expected Transmission Time (ETT)

ETT estimates the MAC layer duration needed for successfully transmitting a packet. ETT is a bandwidth-adjusted ETX, and is generated by multiplying the link bandwidth to obtain the time spent in transmitting the data packet. ETT has the form of ETT = ETX * S

B , where S denotes the size of the data packet and B is the data transmission rate of the link.[28]

Weighted Cumulative Expected Transmission Time (WCETT)WCETT is a path metric that is calculated as the sum of the ETT's of all the hops on the path. This gives an estimate of the end-to-end delay experienced by a packet traveling along the path based on the loss rate and bandwidth. Thus WCETT for a path with n hops is given by:

Where k is the number of channels in the network and Xj is the sum of transmission times of hops on channel j.The WCETT is a measure of the quality of a path and hence it serves as a suitable metric for choosing packet sizes. In a set of paths between a source and destination, the path with the lowest WCETT value is most likely to deliver the maximum number of packets with least delay. [29]

Ad-hoc Routing Protocols

Ad hoc On-demand Distance vector Routing (AODV)The Ad hoc On-Demand Distance Vector (AODV) algorithm enables dynamic, self-starting, multi-hop routing between participating mobile nodes wishing to establish and maintain an ad hoc network. AODV allows mobile nodes to obtain routes quickly for new destinations, and does not require nodes to maintain routes to destinations that are not in active communication.Route Requests (RREQs), Route Replies (RREPs), and Route Errors (RERRs) are the message types defined by AODV. When a route to a new destination is needed, the node broadcasts a RREQ to find a route to the destination. A route can be determined when the RREQ reaches either

the destination itself, or an intermediate node with a 'fresh enough' route to the destination. A 'fresh enough' route is a valid route entry for the destination whose associated sequence number is at least as great as that contained in the RREQ. The route is made available by unicasting a RREP back to the origination of the RREQ. Each node receiving the request caches a route back to the originator of the request, so that the RREP can be unicast from the destination along a path to that originator, or likewise from any intermediate node that is able to satisfy the request.[30]

Figure 1.11 AODV Route Discovery

Figure 1.11 shows an example of AODV route discovery in a network that contains 10 nodes. The source node want to find a route to the destination node, so source node broadcasts a RREQ request to the destination, which is re-broadcasted by other nodes. When the RREQ reaches the destination, the destination node replies with a unicast RREP message.The AODV routing protocol is designed for mobile ad hoc networks with populations of tens to thousands of mobile nodes. AODV can handle

low, moderate, and relatively high mobility rates, as well as variety of data traffic levels.

Hybrid Wireless Mesh Protocol (HWMP)802.11s defines a default mandatory routing protocol called Hybrid Wireless Mesh Protocol. HWMP is inspired by a combination of Radio Metric ad hoc on-demand Distance Vector Routing Protocol (RM-AODV) and tree-based routing Protocol. IEEE 802.11s denotes HWMP as path selection protocol instead of routing protocol because it uses layer 2 addressing schemes. [24]The word “Hybrid” in this protocol refers to the fact that it supports both Reactive and Proactive routing.HWMP supports two operation modes:On demand: The mode is used in situations where there is no root MP configured. If no root portal is configured, RM AODV is used. For destinations within the mesh the route discovery works like normal AODV. If the destination is outside the mesh, the source receives no RREP upon a RREQ. Therefore it sends the messages to the route portal after a timeout. The portal forwards them to the connected network [24]Proactive: Route is discovered before any request or demand and as a result when request arrived for a particular destination node it is fulfilled. Root Portals (also calledMesh Portal) are configured to send announcement called root announcement (RANN) periodically. The root MP periodically floods a RANN message into the network. The information contained in the RANN is used to disseminate path metrics to the root MP. Upon reception of a RANN, each MP that has to create or refresh a path to the root MP sends a unicast path request (PREQ) to the root MP via the MP from which it received the RANN. The unicast PREQ follows the same processing rules defined in the on demand mode. The root MP sends path reply PREP in response to each PREQ. The unicast PREQ creates the reverse path from the root MP to the originating MP, while the PREP creates the forward path from the MP to the root MP. When the path from an MP to a root MP changes, it may send a PREP with the addresses of the MPs that have established a path to the root MP through the current MP.

A mesh portal connects mesh networks to outside network like internet. A designated mesh portal (MPP) is selected as designated root MPP. This selection is done either by configuration or by selection process. As a result we have a tree structure with a root and thus it allows proactive routing toward MP. [24] Figure 1.12 shows a HWMP route discovery example.

Figure 1.12 HWMP Route Discovery

1.5.3 SummaryTraditional routing protocols are not feasible for IEEE 802.11s WMN. AODV is not ideal for WMNs, since it uses hop-count as a routing metric. Also layer 3 routing does not fit into the concept MAPs, which are layer 2 devices. HWMP eliminates these shortcomings.

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