Aspects of Load-Balancing at MAC
Layer in a Wireless Mesh Network
by
Ivano Alocci
The thesis is submitted to University College Dublin for the degree of
Master Research of Computer Science
in the College of Engineering Mathematical and Physical Sciences
January 2010
School of Computer Science and Informatics
Prof. J. Carthy (Head of School)
Under the supervision of
Prof. J. Murphy
Abstract
Wireless communication technology has become popular in our daily life, especially wireless local networks such as the IEEE 802.11, also commonly known as Wi-Fi. It provides wideband access to the Internet and wired infrastructures for ubiquitous users equipped with laptop or other device such as handheld, PDA, cell-phone and other hybrid equipment. In the recent years, the research community in academia and industry alike have focused a significant amount interest on multi-hop wireless network where the backbone is completely based on wireless links. This solution should be suitable for hard-to-wire buildings such as museums, historical building, unfriendly terrains, and rural areas with high costs of network deployment, where self-configuration and self-deployment are the key features its success.
This thesis proposes a new load balancing protocol MAC layer optimization for wireless mesh network with multi-radio devices and in the presence of multiple gateways to provide access to external resources. Due to the presence of multiple radios in one node, a coordination protocol between different physical interfaces has been developed and most of the broad guideline present in the IEEE802.11s draft standard has been followed to implement a simulation model for this protocol. Particular attention has been given in reducing the overhead in the network due to the management frames sent at the data link layer. Thus, a lightweight signalling protocol for MAC layer has been designed with the main objective being to maximise the underlying network resources usage for management traffic. To achieve this objective, the beacon transmission mechanism has been leveraged to spread routing metrics information downstream for no extra overhead added. This way the beacon frame is overcharged with QoS metrics that are propagated in the network and used to dynamically adjust the load balancing.
A comprehensive set of simulations show the potential of this new approach in wireless mesh networks and its capacity to fully use network resources in the presence of multiple gateways in the network. This work also highlights some lessons learned that can be used as guidelines for future wireless mesh network design and deployment. For instance, we observed critical tradeoffs in the architecture such as the correlation between the required time to learn the network topology and the number of gateways which are known by the access point.
Table of Contents
Chapter 1.
Introduction ... 1
Chapter 2.
Background of Wireless Mesh Network (WMN) ... 4
2.1. General characteristic ... 4
2.2. Channel assignment in multi radio scenario ... 7
2.3. Routing protocol for WMN ... 9
2.3.1. AODV routing protocol ... 10
2.3.2. DSR routing protocol ... 12
2.3.3. OLSR routing protocol ... 13
2.4. Link Metrics for WMN... 14
2.5. Routing at MAC Layer vs. IP layer ... 17
2.6. Load balancing overview ... 17
Chapter 3.
Review of IEEE 802.11s ... 19
3.1. Overview ... 19
3.2. Frames header ... 22
3.2.1. Data Frame header ... 22
3.2.2. Management frames ... 23
3.3. Link establishment ... 26
3.3.1. Peer link algorithm ... 26
3.3.2. Peer Link validation ... 28
3.4. Airtime ... 28
3.5. Routing protocols at MAC layer ... 30
3.5.1. HWMP Routing Protocol ... 31
3.5.2. Ra-OLSR Routing Protocol ... 32
3.6. Data frame forwarding ... 33
3.6.1. Mesh deterministic access... 33
Chapter 4.
Proposed New Mesh Protocol ... 36
4.1. Architectural description ... 36
4.2. Channel selection ... 41
4.4. Load balancing mechanism ... 48
Chapter 5.
Implementation in QualNet ... 54
5.1. Design approach ... 54
5.2. Peer link procedure ... 61
5.3. Routing protocol and load balancing implementation ... 64
5.4. Beacon ... 67
Chapter 6.
Results & Analysis ... 70
6.1. Introduction ... 70
6.2. Channel selection results ... 72
6.2.1. Scenario “A” analysis ... 73
6.2.2. Scenario “B” analysis ... 76
6.3. Load balancing results ... 78
6.3.1. Scenario “A” analysis ... 80
6.3.2. Scenario “B” analysis ... 82
Chapter 7.
Conclusions & Future Work... 86
Table of figures
Figure 1: MUP Architecture diagram. ... 6 Figure 2: Typical 802.11s scenario, with all constitutive elements. ... 20 Figure 3: data frame header. ... 22 Figure 4: Addressing example for a Mesh Data frame. ... 23 Figure 5: peer link creation diagram. ... 27 Figure 6: Airtime's plot. ... 29 Figure 7: Cost function based on airtime link metric. ... 30 Figure 8: DMA access scheme. ... 34 Figure 9: Header structure for data frame for three different cases, a) all six fields in data header are used in forwarding, in b) only five fields are employed, in c) source and receiver for data traffic are MPs. ... 38 Figure 10: simplified architectural organization for mesh protocol. ... 39 Figure 11: detailed logical scheme. ... 40 Figure 12: Channel selection algorithm. ... 42 Figure 13: handling beacon, flow chart. ... 45 Figure 14: Loop detection mechanism, in the case ‘a’ only loop detect message is used, MP B is able to solve the loop; in case ‘b’ MP B is not able to solve the loop and uses a loop not detected frame to inform node A. ... 52 Figure 15: Loop event, in case A, multiple interface produces loop; case B, shows loop among three MP; the arrow shows the time when the data frame has been sent. ... 53 Figure 16: state machine for mesh management layer. ... 55 Figure 17: temporal shame for beacon interval and its transmission. ... 56 Figure 18: Simplified data header structure; in red box are represented the destination address field, and in black box the source address. ... 57 Figure 19: reception data frame. ... 58 Figure 20: flow chart for MAP process data frame. ... 59 Figure 21: flow chart for MAP process data frame. ... 60 Figure 22: peer link state machine... 62 Figure 23: List structure to save the switch table structure. ... 65 Figure 24: Mesh control layer routing frame, flow chart. ... 66 Figure 25: handle beacon, and update mesh portal list flow charts. ... 68 Figure 26: simulation scenarios. ... 72 Figure 27: Achieved throughput measured for different configurations in scenario A – the offered load is also illustrated as a diagonal line, signaling a steady regular increase the offered load. ... 74 Figure 28: Achieved throughput measured for different configurations in scenario "A". ... 74 Figure 29: Average number of links in the Routing Table per node in scenario “A”. ... 76 Figure 30: Achieved throughput measured for different configurations in scenario B – the offered load is also illustrated as a diagonal line, signalling a steady regular increase the offered load. ... 76Figure 31: Achieved throughput measured for different configurations in scenario "B". ... 77 Figure 32: number of links in the Routing Table per node in scenario "B". ... 78 Figure 33: Achieved throughput measured for different configurations in scenario A – the offered load is also illustrated as a diagonal line, signalling a steady regular increase the offered load. ... 80 Figure 34: Achieved throughput measured for different configurations in scenario A. ... 81 Figure 35: Average number of links in the Routing Table per node in scenario "A". ... 82 Figure 36: Achieved throughput measured for different configurations in scenario B – the offered load is also illustrated as a diagonal line, signalling a steady regular increase the offered load. ... 83 Figure 37: throughput measured for different configurations in scenario B. ... 83 Figure 38: Average number of links in the Routing Table per node in scenario B. ... 84
Chapter 1. Introduction
In the last decade the wireless technology has become very popular and the demand for wireless accesses to the Internet is constantly growing. Nowadays it is possible to have wireless access to the On-line resources from a large number of public places like airports, hotels’ hall, squares, etcetera, simply using a Wi-Fi connection, the most popular wireless technology. Wi-Fi Access Point (AP) requires a wired connection to the backbone, usually an Ethernet network, which allows the AP to act as a gateway to external resource. Today the cost of an AP is not significant in relation to the deployment cost; the prohibitive factor still resides in the local network, where wiring several APs to provide a good coverage may turn to be very costly [1] [2]. Another drawback could be found when there is the necessity to increase the network capacity by adding other Access Points; in doing so, the local wired network should be able to provide new resources for the new Access Points. However, wireless technology still seems to be the simplest and less expensive solution to provide connectivity in environments requiring the less possible infrastructure intrusion such as historical buildings where creating the wired backbone could be very difficult due to the limited possibilities to introduce alterations in the building.
Nowadays, a significant amount of research is directed to the so called Wireless Mesh
Network (WMN), where a multi hop strategy is adopted to provide a wireless
backbone. Several organizations are dealing with this topic; in particular IETF (Internet Engineering Task Force), with the Mobile Ad-Hoc Network (MANET) work group, have standardized a family of routing protocols for WMNs. In the same way, IEEE (Electronics and Electrical Engineering) with the 802.11 project is working on different aspects broadly related to wireless mesh networks. In particular the TG (Task
Group) 802.11i is dealing with security aspects in distributed network, TG 802.11n
focuses on MIMO (Multi-In Multi-Out) radio technology, while TG 802.11s is addressing a new standard for mesh networks. Moreover, some test-bed has been developed in cities such as New Orleans, San Mateo, Chaska, Philadelphia, Houston, and San Francisco to provide public service assistance or provide public broadband Internet access [3].
TG 802.11s aims to design large-scale WiFi-based networks and to solve the entailed issues associated with such endeavour; these networks are designed to bring up an all-wireless backbone, giving place to the Mesh Network. The first draft being proposed in 2004, still haven't reached a satisfactory level of depth to be ratified and approved as a standard. Its main characteristic, as already illustrated, is to be focused on the idea of creating a completely wireless backbone, and at the same time, to provide access to traditional wireless users. There is therefore much space to design advanced mechanisms within the backbone, while keeping a standard interface for the Wi-Fi users at the edge of the network.
The 802.11s has launched another strong innovation under its standardization activity. They have introduced routing functionalities at MAC (Media Access Control) sub-layer. This choice is able to provide more flexibility in the mesh network. For example, it becomes much easier to implement new upper-layer transport technologies even if not based on TCP/IP (e.g., ATM, DVB, UMTS, etc.). Any upper-layer transport technology is completely transparent to the network layer; therefore it is possible to upgrade the network functionalities without introducing extra load at layer 3.
While the 802.11s standards is working on the definition of a general framework for Wi-Fi-based wireless mesh networks, many critical open research issues are intentionally left outside of the standard. Issues like multi-path routing, load balancing, and QoS support are left open for optimisation and customisation by equipment manufacturers, mesh network operators, and solution integrators. As any other standard in the past (e.g., GSM) the details of resource allocation are usually developed by equipment manufacturers and operators upon the standard release that usually cover the main interfacing aspects.
This research work will focus mainly on the routing and load balancing aspects of a Mesh Network; as mentioned earlier, routing is one of the main elements introduced in the 802.11s draft standard and Load Balancing is a key element to deploy a high performance backbone. In fact, the standard proposes some routing/forwarding protocols and algorithms inspired by the works carried out in the IETF (Internet
Engineering Task Force) for mobile Ad-Hoc networks (MANET). The routing
protocols being in discussion in the 802.11s WG is a slightly altered version of OLSR (Optimal Link State Routing) protocol and AODV (Ad-hoc On-demand Distance
Vector) protocol. These protocols present some flaws that considerably limit the mesh
network capability to scale to hundreds of nodes, and their viability as an alternative to wirelessly cover a metropolitan area. In this context, this work aims at studying the scalability issues associated with routing protocols for wireless mesh networks and evaluating benefits coming from the Load Balancing scheme that works in conjunction with a routing algorithm. The performance limitation between the two previous elements and Peer Link procedure, a new method introduced by 802.11s TG to create a connection with neighbours nodes, are investigated as well.
The rest of this thesis is structured as follows. Chapter 2 provides an overview of WMNs by covering the most common problems. In Chapter 3, the new IEEE 802.11s draft standard is described with some level of details. Chapter 4 focuses on designing a new routing protocol for WMNs based on 802.11s standard and Load Balancing (LB), which is the main subject of this Masters of Research thesis. Chapter 5 deals with the implementation of a simulator for the LB and routing protocol for the WMNs described in the previous chapter while Chapter 6 illustrates the simulation results. Finally, Chapter 7 draws some important conclusions and future work perspectives.
Chapter 2. Background of Wireless Mesh
Network (WMN)
2.1. General characteristic
Multi-hop Wireless Mesh Network (WMN), is a well known subject in literature. The first study can be dated in the early 70’s [4]. It is common established that the end-to-end throughput, in a multi-hop wireless network, drops as the number of hops increase
[5] [6]. This behaviour can be explained by the fact that a wireless transceiver operates
in half-duplex mode. Therefore an incoming frame must be received fully before the wireless card can switch from “receive” mode to “transmit” mode. This is due to the fact that a half-duplex device is not allowed to receive and transmit in the same time. Thus, for a linear chain topology of n Wi-Fi nodes where only one transmission is allowed at any time in the network, the per-node throughput is on the order of O(1/n). In a general WMN deployed with a single channel (ad-hoc mode), the asymptotic per-node throughput scales as:
(1)
Where, the source and destination pair nodes are randomly selected as illustrated in Gupta Kumar work [5].
Another element responsible of limiting the network capacity can be found in the interaction between network congestion and the sub-optimal backoff algorithms in both the lower-layer MAC protocols [7] and the higher layer transport protocols [8]. Network congestion increases as node density increases, and this leads to rapid degradation in throughput. In other words, the IEEE 802.11 MAC is inherently unfair and can interrupt the flow of packets over multiple hops. Other reasons for the low throughput in 802.11 wireless networks can be indentified with the fact that only a small portion of the spectrum can be used to transmit or receive a message, and usually a single radio is available for each Wi-Fi devices [9].
MUP (Multi-radio Unification Protocol) protocol has been designated by Microsoft research group [9] as a possible solution for these limitations. In this publication an overview of the most common problems connected with multiple interfaces is provided. As the protocol’s name suggests, MUP is a link layer protocol that coordinates multiple IEEE 802.11 radio devices operating over multiple channels. It tries to optimize local spectrum usage via intelligent channel selection in a multihop wireless network. The MUP objective is to exploit the available spectrum as efficiently as possible and to extract the highest bandwidth possible from the existing technology. In other words, MUP is designed not to discover the optimal channel assignment in a multi-hop network; it is about using pre-assigned channels efficiently instead.
MUP has been designed following these architectural requirements:
• It must not require any hardware modifications; it should work with standard IEEE 802.11 cards.
• It must not require any changes to existing application, transport, or routing protocols.
• It does not require any global knowledge of network topology.
• It requires a priority mechanism such as that provided by the 802.11e standard
[10].
The below Figure 1 illustrates the architectural structure for the MUP protocol, in particular it shows the MUP block that provides an unification layer among multiple NIC (Network Interface Card). The MUP element allows getting the benefit from the frequency diversity. MUP also provides a mechanism by which one node can make decisions about which channel to use when communicating with its neighbour nodes.
IP and above layers
V-MAC
MUP ARP
MUP Table
NIC NIC NIC
Figure 1: MUP Architecture diagram.
The V-MAC block provides a single virtual MAC address to hide the complexity of multiple network interfaces from the upper layers of the protocol stack. For this reason MUP can be described as a multi-radio unification protocol. In fact, from the application perspective the system operates as if there is only a single wireless network interface.
Literature confirms that the link capacity in one-hop wireless networks scales if a spatial reuse technique is adopted. The typical approach to providing spatial reuse is based on reducing the re-use distance between co-channel users on different APs. In narrow-band systems, the extent of spatial reuse is directly proportional to the number of orthogonal channels available. Currently, only a very limited number of such orthogonal channels are available: 3 in 802.11b (2.4GHz) and 12 in 802.11a (5 GHz)
[11].
There are many works in the literature addressing the channel assignment issue. A distributed dynamic channel assignment algorithm for multi-hop network in single channel is defined in [12]. In [13] the authors study the network as an arbitrary undirected graph and presented two polynomial time algorithms for link scheduling. Two Integer Linear Programming (ILP) formulations for the fixed channel assignment problem in multi-radio scenario with the goal of maximizing the number of simultaneous transmissions in the network have been presented in [11]. The numerical results in [11] illustrate the benefits obtained by increasing the number of radios per node and the number of available channels in the network. The authors prove that the realization of the full potential of multiple radios and multiple channels requires the
use of an appropriate routing algorithm which is able to optimally use these channel assignments along with information about the traffic pattern. In particular, the authors suggest the use of appropriate weights on the links to tune the channel assignment in relation to the knowledge of the expected traffic patterns.
2.2. Channel assignment in multi radio scenario
As illustrated in the previous paragraph the main issue with the multi radio devices concerns how to efficiently manage these multiple interfaces. As previously cited in a multi interface or multi frequency wireless environment, reducing the interference in the network is only possible by using orthogonal channels. Unfortunately, IEEE 802.11b defines 14 radio channels of which only three are orthogonal and may consequently be used simultaneously with minimum interference. Thus, in this wireless scenario two different approaches based on Multi-Channel or Multi-Interface could be adopted.
Multi-channel approach is based on the use of a unique NIC per wireless node where
the node transmits, though not simultaneously, on several orthogonal channels. In the
Multi-interface approach each node is configured with several interfaces that can be
used simultaneously using orthogonal channels.
Independently form the type of configuration; three major problems need to be addressed in this scenario:
• Channel / interface assignment: which channel or interface should be assigned to a given information (i.e. data, signalling).
• Channel / interface management: how to switch between channels or interfaces and how to handle links information.
• Packets scheduling over channels / interfaces: which scheduling policy should be used for each channel or interfaces.
Two techniques were proposed to handle the assignment problem: a static assignment and a dynamic assignment. The static assignment technique is suitable whenever the number of NICs is equal to the number of the available orthogonal channels, the network topology is well known and does not change frequently and nodes mobility is reduced [14]. Dynamic assignment, on the other hand, is suitable whenever the number
of orthogonal wireless channels is greater than the number of NICs. It is also possible to combine the two methods within the same ESS (Extended Service Set). In fact, we can assign a permanent wireless channel for the signalling information and the other wireless channels to dynamically serve the different traffic flows [15] [16].
The interface management problem consists of gathering information about the available wireless channels (use ratio, signal quality, etc.) and updating routing information. Many works propose to set up “virtualization” techniques at the Link layer (MAC and LLC) [17]. These techniques aim to maintain specific data structures in order to create suitable mapping between the logical information of the Network Layer and the physical information of the Link and Physical layers. Many works dealt with the multi-channel approach [17].
The packet scheduling problem consists of identifying a scheduling algorithm that either enforce or relax the packet sequencing according to the load of the network to avoid congestion along the paths. In the IEEE 802.11 standard no rate adaptation mechanism is provided and no signalling mechanism is available to notify the channel conditions between the receiver and the transmitter; in other words there is no feedback channel. Auto Rate Fallback (ARF) [18] is one of the few algorithms used in commercial WLAN products. ARF works by increasing the transmission rate due to consecutive acknowledged transmissions and similarly decreasing the rate in the event of unsuccessful transmissions. In contrast, in [19], [20] Receiver Based Auto Rate (RBAR) is proposed as rate control algorithm. RBAR allows the receiver to select the appropriate rate for the data packet during the RTS/CTS (Ready to Send/Clear to Send) frame exchange. The sender stores the rate and the size of the data packet it have to send into the RTS frame. The receiver uses information available to it about the channel conditions to generate an estimate of the conditions for the impending data packet transmission. Then the receiver selects the appropriate rate based on that estimate, and transmits it using the CTS back to the sender. The other nodes overhearing RTS or STC frame are able to set their Network Allocation Vector (NAV) to the appropriate value.
FIFO (First In First Out) is the most common queue management strategy used in scheduling protocols. It is adopted in all Wi-Fi devices.
2.3. Routing protocol for WMN
Routing can be defined as the process of selecting paths in a network through which “transiting data traffic” should be sent. Routing is performed in different sorts of networks, including the telephone network, the Internet, and transport networks. Particularly, in a wireless scenario, routing protocol is a standard that controls how nodes decide how incoming packets are routed between devices in a wireless domain. In wireless networks, nodes do not have an ‘a priori’ knowledge of the topology of the network around them, but they have to discover it. The basic idea is that a new node announces its presence and listens to the broadcast announcements from its neighbours. The node learns about new neighbouring nodes and the ways to reach them, and may in turn announce their reachability to other nodes. As time goes on, each node knows about all other nodes and one or more ways to reach them.
The Internet Engineering Task Force (IETF) has defined Mobile Ad-Hoc Network (MANET) [21] to address the issue of routing in a WMN; this IETF working group has therefore defined a set of routing protocols for ad-hoc network.
MANET project is based on the traditional routing at IP layer, but in contrast with the routing protocol for wired network, they constantly broadcast routing messages to constantly updates routing paths/metrics and adjust to the quickly changing network conditions typical in a wireless environment [22]. In other words, these routing metrics should include additional characteristics that reflect the difference from wired link. Usually, there are different criteria to classify routing protocols, such as the type of algorithm adopted to find the path or the method to handle the link. Routing protocols that are usually used in ad-hoc wireless network can be classified in two major categories: Pro-active Routing and Reactive Routing.
Proactive protocols are based on the periodic distribution of routing tables throughout
the network to maintain fresh lists of destinations and their routes. The main disadvantages are the respective amount of data for maintenance and its slow responsiveness when it comes to rebuilding a stable logical topology upon a network failure.
Reactive protocols flood the network with RouteRequest packets to find “find a route”
on demand, only when there is a request from a flow to be routed through a new path. The main disadvantages are its high latency time in finding new routes and an excessive flooding that can lead to network clogging.
Hybrid protocols are also possible; they combine the advantages of proactive and of reactive routing. The routing is initially established with some proactively prospected routes and then serves the demand from additionally nodes through reactive flooding. The choice for one or the other method requires predetermination for typical cases. The previous three categories for routing protocol classification are the most used in literature but other sub-category are possible, such as Hierarchical routing protocol where the choice of proactive and reactive routing depends on the hierarchical level where a node resides; or power-aware routing protocol where the energy required to transmit a signal is also considered as a routing metric.
MANET group defines three routing protocols for wireless mesh networks, AODV (Ad-hoc On-demand Distance Vector) and DSR (Dynamic Source Routing) are based on traditional Distance Vector routing protocol; while OLSR (Optimized Link State
Routing Protocol) is based on Link State routing protocol. OLSR is a proactive or
table-driven routing protocol; AODV and DSR belong to reactive or on-demand routing protocol.
2.3.1. AODV routing protocol
AODV [23] is a reactive routing protocol, so it establishes a route to a destination only on demand. AODV has been developed by C. Perkins and S. Das in Nokia Research Centre of the University of California. The main characteristic of this protocol is its reactive nature. This also represents the main point of distinction from more traditional proactive protocols that - find routes between all source-destination pairs regardless of the use or need for such routes. The reason to adopt an on-demand approach for the routing protocol is the reduction of the routing load. High routing load usually has a significant performance impact in low-bandwidth wireless links [24].
In an on-demand routing protocol, the source node floods the network with
RouteRequest packets when it has to send a message and the route is not available for
routing protocols is that it uses a destination sequence number (DestSeqNum) to determine an up-to-date path to the destination and to prevent routing loops [25]. A node updates its path information only if the DestSeqNum of the current received packet is greater than the last DestSeqNum stored at the node.
When an intermediate node receives a RouteRequest, it either forwards it or prepares a
RouteReply if it has a valid route to the destination. When a RouteRequest message is
received multiple times the packet is discarded. The node identifies a duplicate packet by checking Broadcast identifier and Source identifier pair in the RouteRequest message. All intermediate nodes having valid routes to the destination, or the destination node itself, are allowed to send RouteReply packets to the source.
A RouteRequest message contains these fields in its payload: • Source identifier
• Destination identifier • Source sequence number • Destination sequence number • Broadcast identifier
• Time to live
Instead RouteReply message is formed with the sequent fields: • Hop counter
• Destination Identifier
• Destination sequence number • Source address
• Time to live
After the sending node has sent its RouteRequest, it could receive different RouteReply messages from its neighbour nodes, so it begins using the route that has the least number of hops through other nodes. When a link breakage in an active route is detected, a RouteError message is used to notify other nodes of the loss of the link. An important feature of AODV is the maintenance of timer-based states in each node, regarding the use of individual routing table entries. A routing table entry expires if not used in a certain amount of time.
Much of the complexity of the AODV protocol is in lowering the number of messages to conserve the capacity of the network. The recent specification of AODV [23] includes an optimization technique to control the RouteRequest flood in the route discovery process. It initially uses an expanding ring search to discover routes to an unknown destination. In the expanding ring search, increasingly larger neighbourhoods are searched to find the destination node. The search is controlled by the TTL
(Time-To-Live) field in the IP header of the RouteRequest packets.
The main advantage of AODV is the absence of routing traffic along existing links. Also, AODV is based on distance vector routing algorithm to find the best route to destination. Adopting distance vector method, AODV doesn't require much memory or calculation. However AODV requires more time to establish a connection, and the initial communication to establish a route is heavier than other wireless routing protocol.
2.3.2. DSR routing protocol
DSR [26] as AODV, is an on-demand protocol designed to restrict the bandwidth consumed by control packets in Ad-hoc wireless networks by eliminating the periodic table-update messages required in the table-driven approach. As all on-demand routing protocol, it does not require periodic “hello” packet transmission, which are used by a node to inform its neighbours of its presence. As previously explained for the AODV protocol it goes to flood the network with the RouteRequest message only if a new path is required.
The main difference with the previous protocol concerns the use of source routing instead of relying on the routing table at each intermediate device. Hence, intermediate nodes do not need to maintain up-to-date routing information in order to route the packets they receive, since the packet’s source has already made all of the routing decisions. In fact, DSR protocol requires accumulating the address of each node between the source and destination during route discovery. The source routing may provide high overhead in the WMN for long paths or large addresses, like IPv6. To reduce this drawback in DSR a flow Id option is adopted; the Id option allows packets to be forwarded on a hop-by-hop basis [27] [28].
The disadvantage of this protocol is that the route maintenance mechanism does not locally repair a broken link. If an old route is cached it could also result inconsistent during the route reconstruction phase. The connection setup delay is higher than in table-driven protocols. Even though the protocol performs well in static and low-mobility environments, the performance degrades rapidly with increasing low-mobility
[27].
2.3.3. OLSR routing protocol
OLSR [29] operates as a table driven and proactive protocol, thus exchanges topology information with other nodes of the network regularly. In contrast with the other proactive protocol, OLSR adopts two mechanisms to reduce the overhead in the WMN: Multipoint Relay Selectors and Multipoint Relay. Using multipoint relay
selector mechanism OLSR reduces the size of control packet; it declares only a subset
of link with its neighbours, instead of all links as in the traditional link state protocols. This sub-set of node is identified as Multipoint Relay (MPR). The protocol uses MPRs to facilitate efficient flooding of control messages in the network. In fact, only MPR can retransmit broadcast messages; therefore the number of retransmissions in a flooding or broadcast procedure is reduced [30].
OLSR defines two types of packets: “HELLO” message and Topology Control (TC) message. The first is used by a node to detect its neighbours; this packet is sent in broadcast but it is not propagated so only the neighbours which are one hop far from the “HELLO” sender node can receive this message. The other message is used to build an intra-forwarding database needed to route the packets. It is broadcasted periodically to declare its MPR selection set.
HELLO message carries the list of neighbour to which a bidirectional link exists, and a
list of partial links. In the TC message there is a list of nodes that have select the TC sender node as a MPR.
Being a proactive protocol, routes to all destinations within the network are known and maintained before use. Having the routes available within the standard routing table can be useful for some systems and network applications as there is no route discovery delay associated with finding a new route.
The first version of OLSR does not include any provisions for sensing of link quality; it simply assumes that a link is up if a number of hello packets have been received recently, otherwise it is down. This assumption is not correct in case of wireless networks, where links often exhibit intermediate rates of packet loss. The last version of OLSR adopts a link quality sensing called "fish-eye" or Radio-Aware OLSR.
Due to the fact that OLSR is a proactive protocol, it uses power and network resources in order to propagate data about possibly unused routes; for this reason OLSR is not suitable for sensor networks which try to stay in sleep mode for most of the time. OLSRv2 is currently being developed within the IETF; it maintains many of the key features of the original protocol including MPR selection and dissemination. Key differences are the flexibility and modular design using shared components.
2.4. Link Metrics for WMN
All routing protocols require a metric to assign a cost to each destination in its routing table. All the initial research on routing protocols was based on the hypothesis of wired networks, so the common metric adopted was the number of hops. After the wireless technology became popular, extensive research on routing in wireless networks has stared. As the wireless medium has different physical characteristics from the traditional wired medium, the number of hop was not sufficient to describe the quality of the links. For this reason, a set of new metrics has been proposed to address this problem. In general, in a wireless environment the type of node is one of the main elements to design a new metric. For example, the energy-efficiency a major design objective in sensor networks where sensor nodes are battery-operated.
Similarly, different parameters design objectives and performance metrics are more valued in wireless networks where the energy supply is not a major issue. There are two common metrics that are commonly used in wireless network: Expected
Transmission Count (ETX) and Expected Transmission Time (ETT).
ETX metric tries to find high-throughput paths in a multi-hop network. It minimizes the expected total number of packet transmissions required to successfully deliver a packet to the ultimate destination. The ETX metric includes the effects of link loss
ratios, asymmetry in the loss ratios between the two directions of each link, and interference among the successive links of a path. [31].
The ETX of a route is the sum of the ETX for each link in the route. The ETX of a link is calculated using the forward and reverse delivery ratios of the link as shown in the formula below.
(2)
Here, df is the forward delivery ratio, which is the measured probability that a data packet successfully arrives at the recipient. Instead dr represents the reverse delivery
ratio which is the probability that the ACK packet is successfully received. The
delivery ratio df and dr are measured using a sequence of multicast probe packets sent at constant rate.
ETX can be interpreted as the probability that a frame transmission is correctly received and acknowledged. This metric provides good channel estimation but it works well only in a single radio environment; instead it is not optimal in environment with different data rate and multiple radios [32].
ETT metric represents the expected time to transmit correctly a packet of fixed length “S” [33]. As shows in the below formula ETT is based on ETX metric,
(3)
Here; S, as previously presented, is the size of the probe packet and B is the bandwidth of the link. The link bandwidth can be measured using the Packet Pair method [34]. In Packet Pair method, each node have to periodically send two unicast probe packets with different sizes, one small and one large, to each of its neighbours. Each neighbour computes the delay interval between the two probe packets arrival and sends this delay back to the sender. This delay is used to estimate the link bandwidth [35].
Link bandwidth obtained by the Packet Pair method can be used as a metric as well. It main characteristic concern that it can differentiate between low and high bandwidth links which occur frequently due to the use of heterogeneous radios or variable link quality and rate control algorithms [34].
In [32] Weighted Cumulative ETT (WCETT), a modified version of ETT, is presented as possible metric for a new routing protocol designed for a multi radio device called Multi-Radio Link-Quality Source Routing. WCETT metric for a path of n hops is shown in the below formula.
(4)
Where, β is a tuneable parameter subject to 0≤β≤1 and Xj is given by
(5)
Hire, Xj represents the sum of ETT for the hops that are in the channel j.
The first part of WCETT formula can be interpreted as an estimation of the end-to-end delay experienced by a packet travelling along the path. Whereas, the second part of the formula reflects the channel diversity along the path. The WCETT metric aims to achieve a tradeoff between delay and throughput by balancing the usual criterion (minimum number of hops) with the notion of channel diversity. WCETT was implemented in Multi-Radio Link-Quality Source Routing (MR-LQSR) routing protocol [32]. In [32] all nodes are assumed to be stationary and the channel assignment is predetermined. The authors show that the classical shortest path routing is not suitable when multiple radios are deployed; exploiting channel diversity in multi-radio mesh networks significantly improves the network capacity and makes a better use of the channel resources.
Note that the energy required to transmit a signal could be approximated by the expression:
(6)
Where, Pr and Pt are respectively the received power and transmitted power, whereas d is the distance and α > 2 is the attenuation factor, which depends on the transmission medium. In free space α is set at 2, in a common environment an empirical value is adopted, usually between 3 and 5. In case of alpha equal two, due to the Friis equation, the power of the signal received reduces its value by the square of the distance. Therefore, one fourth of the energy is consumed in the first half of the distance between the two antennas.
2.5. Routing at MAC Layer vs. IP layer
Among the innovative elements in the IEEE 802.11s draft standard for WMN, the most important resides in the idea to move the routing functionality, from the layer 3 to the Data-Link layer. The motivations in implementing a routing functionality at MAC layer could be found in the layer’s syntax. In fact the APs are purely layer-2 devices, which are incapable of decoding IP packets, and adding layer-3 functionality to an AP has traditionally considered not acceptable [36]. Another advantage can be associated with the possibility to increase the coverage area and provide more flexibility for developing and updating a wireless network. Throughput increase, power optimization and backward compatibility are the other goals for the mesh routing. These benefits are due to the major quantity of information available at MAC layer and to a faster reaction to changes in the network. The other vantage to adopt a routing at MAC layer is that there is no necessity to subnet management and the layer-3 mobility is rather easy to address. However, such approach comes with the cost of implementing a complex virtual Ethernet layer, and protocols like DHCP, ARP, RARP that require a new design to avoid bandwidth inefficiency using layer-2 broadcast frames [33].
2.6. Load balancing overview
In a mesh wireless network, where multiple paths to the destination node are common case, load balancing is the best approach to increase network throughput and to reduce congestion. As introduced in [33] there are two ways to define the load balancing in a mesh network: Path load balancing and Gateway load balancing.
Path load balancing is based on the concept of distributing traffic among a set of
diverse paths. This type of load balancing technique can improve network performance and reliability, due to the availability of multiple paths to the same destination. In [37] and [38] this method is further examined with evidence that this load balancing method provides a negligible performance improvement in multi-hop networks because of route coupling of paths between common endpoints. The coupling effect is due to the interference between paths, which are in geographic proximity.
In Gateway load balancing traffic is distributed among a set of gateways in the wireless mesh network. This solution should have fewer problems with path coupling
effect problems; because route coupling of paths to different gateways from an endpoint in the mesh is expected to be less in a well-planned deployment.
The last type of load balance technique is adopted in MeshCluster routing protocol, developed by K. Ramachandran et al. MeshCluster protocol is based on AODV and, as previously cited, provides Gateway load balancing [33].
Chapter 3. Review of IEEE 802.11s
3.1. Overview
The Task Group (TG) “S” within the IEEE 802.11 working group was formed in July 2004 from the original Study Group of IEEE 802.11 born in September 2003. TG “S” attempts to standardize the Extended Service Set (ESS) which is a form of Wireless Mesh Network (WMN). The IEEE 802.11s Task Group aims to define MAC and PHY layers for mesh networks that improve coverage with no single point of failure [ HYPERLINK \l "SMF06" [36]. Therefore, the IEEE 802.11s [39] is an amendment to the original IEEE 802.11 standard, and it was expected to be completed in 2007. The open call for proposals (CFP) in 802.11s ended in June 2005 with 15 proposals received in total. In the July 2005 meeting, the proposals were reduced to six. In September 2005 the number was narrowed down to four proposals and afterwards consolidated into two. The two remaining proposals, SEE Mesh and Wi-Mesh Alliance, were merged and the final version was accepted as draft D0.01 after a unanimous ratification vote in March 2006. In March 2008 the standard is renamed draft D2 and submitted for ballot, but it failed with only 61% approval. In March 2009 all the comments are solved and a new Letter Ballot was required. In May the Draft D3.0 passed the WG ballot with 79% approval rating and 1,195 comments [40]. At the moment, in the last meeting of July 2009, TG “s” is working on Draft 3.0 resolving comments from its new Letter Ballot.
802.11s defines three new logical elements to deploy a WMN: MP (Mesh Point), MAP (Mesh Access Point) and MPP (Mesh Portal), these devices are shown in Figure 1, with a typical mesh scenario.
Figure 2: Typical 802.11s scenario, with all constitutive elements.
In the above Figure 2 the MPP is represented with a bicoloured box to highlight the presence of two different types of interfaces, in white the wired interface and in black the wireless interface. As well, MAP is represented with a bicoloured box to emphasize the logical difference between the wireless interface that provides AP (Access Point) functionalities and the other wireless interface that provides connection to the WMN. MPPs and MAPs; also perform bridging functionalities. Whereas the single black box represents MP nodes responsible of routing traffic within the wireless mesh network. The Figure shows how traffic flows in the WMN between mobile stations and wired networks.
The 802.11s provides a robust and transparent data layer that supports different types of upper-layer protocols and communication technologies. The frame forwarding and path selection are accordingly performed at MAC layer, avoiding in inter-dependency with upper layers. Also, additional functions are included concerning the 802.11i amendment which deal with security aspects in a wireless domain [41]. All the above mentioned elements could have more than one radio interface, meaning that each node has to be able to handle and coordinate the operation of more than one radio channel. MPs are the main elements in the mesh network used to develop the backbone, to increase the coverage area and capacity. Their main activities are concerned with finding neighbours, try to create a connection with them and forward the incoming frames. As mentioned earlier, the MP is the basic element in 802.11s. Unlike any other
802.11 entity, MPs may exchange and route frames over multiple wireless hops. Thus, MPs can communicate not only with other MPs at communication range but also with other MPs wireless hops away. On the one hand, for the purpose of our experiments, the MP may operate like a station that solely acts as an application end-point (sink or source of data traffic). Each MP may forward data frames and act as a relay for communication it is not involved in. Note that the MP itself does not provide AP (Access Point) functionalities for the common 802.11 stations [41].
Mesh nodes belonging to a wireless mesh network which use multi-hop connections may be able to communicate among themselves as long as a Mesh Path exists between them. For instance, if MP A is a member of a set of MPs, and that MP B is in the communication range of only MP A, a new link will be established between these latter MPs and MAC Service Data Units (MSDUs) will be forwarded throughout a set intermediary MPs. MP B learns the network topology by MSDU frames, which are sent from MP A. A set of concatenated adjacent MLs (Mesh Links) defines a Mesh Path. Accordingly, an MP to which an ML has been established is denoted as a peer MP. To set up an ML, two MPs invoke the 802.11s peer link management procedure over the 802.11 link.
From the one side, MAP provides AP services to the traditional 802.11 users, and from the other side it is similar to one MP. Therefore, it has some special functionality to perform this behaviour. For example, it has to change the frame structure and bridge it from 802.11a/b/g format into an 802.11s format. Another special goal for this element is to collect traffic from the legacy station and to perform a traffic aggregation and prioritization policy. In other words, this element allows one frame to enter into the mesh domain and to exit as well.
MPP has the goal to provide access to external network resources. It is a combination of MP functionalities and the IEEE 802.1D [42] bridging functionalities. Usually it is connected with an Ethernet local network that bridges the traffic to obtain enterprise network or the public Internet. Bridging the traffic to external networks and network announcement are chief among the different services provided by MPPs. More than one MPPs is usually used in a wireless mesh network with a reasonable scale (dozens of wireless mesh nodes).
3.2. Frames header
IEEE 802.11s draft standard adopts a modification in the legacy frame header to efficiently perform the data frame forwarding and management control at MAC layer. In the following sub-sections, these two elements will be explained.
3.2.1. Data Frame header
The 802.11s is designed to be a transparent support for any upper-layer protocols, allowing the Mesh services must support all kinds of unicast, multicast and broadcast traffic. Therefore, 802.11s introduces the Mesh header field to implement this feature in the original data frame. It includes four or sixteen octets as shown in Figure 3.
Figure 3: data frame header.
The first octet contains the Mesh Flags field. Its first bit indicates the presence of AE (Address Extension). All other bits are reserved for future use, and left open for specific implementation by the research community or the industry. The second octet defines the Mesh TTL (Time to Live), which is used to avoid having frames going through an endless looping; at each node, the frame’s TTL decrements the counter before forwarding to the next node; if it arrives to zero the frame is discarded. Octets number three and four provide Mesh E2E (End-to-End) sequence numbering. These fields are employed during the flooding procedure to avoid routing loops in the network. MPs use the Mesh E2E Sequence number field to avoid unnecessary retransmissions. Furthermore, the ultimate receiver of a frame uses the E2E sequence field to eliminate duplicates. With the AE flag being set, an MP uses the six-address scheme. The additional address fields identify certain intermediate MPs along the Mesh Path.
In particular, address 1 and Address 2 correspond respectively to the MP receiver
address (RA) and the MP transmitter address (TA) for a particular mesh link. These
and Address 4 correspond to the destination and source endpoints of a mesh path, respectively. The AE flag, as mentioned earlier, indicates the existence of the optional
address extension field including Address 5 and Address 6 in the Mesh Header which
correspond to the end-to-end destination address (DA) and source address (SA), when the end points are non-mesh node, the connection happens over a mesh via proxy MPs. Figure 4 illustrates an example of a Mesh Data frame transmitted and forwarded on a mesh path from an MAP to an MPP where the original source is an 802.11 station associated with the MAP and the final destination is an entity outside of the mesh that is reachable via the MPP.
Figure 4: Addressing example for a Mesh Data frame.
3.2.2. Management frames
In the above paragraph we discussed the Data Frame, which is used to transport data packets that are usually originated at end points external to the wireless mesh network. These packets are typically created by upper-layer protocols and sent through the mesh network to a destination node, identified by DA address as shown in Figure 4.
The 802.11s network supports as well a different type packet, namely management packets. This second type of packets is used to perform the layer-2 control functionalities, network synchronization, and nodes coordination. 802.11s management functionalities involve different management packets, of which the most common are mentioned below:
• Beacon Frame: is used to maintain the network.
• Disassociation Frame: is used when a node leaves the network. • Association Request Frame: is used when a node joins the network.
• Association Response Frame: is used by the network to reply to a join request.
• Probe Request Frame: is used, in dynamic association procedure, when a station needs information from another station.
• Probe Response Frame: is used during dynamic association procedure in response to a probe request frame.
These frames types are inherited from the original 802.11 standard, but some new fields have been added to provide 802.11s functionalities. Table 1 shows these fields and the frame in which they are used, while Table 2 provides a brief description of these fields. Beacon Disassociat ion Associatio n Request Associatio n Response Re‐ associatio n Request Re‐ associatio n Response Probe Reques t Probe Respons e Mesh ID X X X X X X X Mesh Capability X X X X X X X Peer Link Manageme nt X X X Mesh Neighbour List X X Mesh Portal Reachabilit y X X Beacon Timing X X Mesh TIM X X Mesh DTIM X MDAOP X X MSCIE X X X X X MSAIE X X RSNIE X
Description Mesh ID Used to advertise the identification of a mesh network Mesh Capability Used to advertise Mesh services Peer Link Management Transmitted by an MP to manage a peer link with a peer MP Mesh Neighbour List Used by an MP to advertise its peer MPs and their Power Management Mode Mesh Portal Reachability The Portal Announcement element is used for announcing the presence of an MP configured as a Portal MP in the mesh network Beacon Timing Used by a synchronizing MP to advertise an offset between its own TSF and the Mesh TSF, and to advertise the beacon timing information of zero or more of its neighbors Mesh TIM Used to maintain synchronization Mesh DTIM Used to maintain synchronization MDAOP This field is used for Mesh Deterministic Access MSCIE These three elements are used in Mesh security association (MSA) services to allow the establishment of link security between two MPs MSAIE RSNIE
Table 2: Management Fields element description.
The previous management packets are used to perform management functions such as beaconing and link creation. These operations are close to the same operations performed in the legacy 802.11, so it is reasonable to reuse the same frames. Whereas, in the new draft there are other functionalities, such as the routing protocol that requires its own management messages. These messages are designed to define a completely new set of frames that are encapsulated in the management action frame. In 802.11, this messages exchange provides a mechanism for specifying extended management actions; for example, it is used in the spectrum management to coordinate the channel assignment in wide wireless mesh network. Action frame header is composed of two fields: category and action details. In 802.11s the action details field is divided into two elements: the action value which identifies the specific action to perform and the real data suitable for the protocol. Typical functionalities performed using the action frames are: HWMP path selection, Congestion Control, Mesh Deterministic Access, Beaconing and Synchronization, RA-OLSR, Portal Announcement Protocol.
3.3. Link establishment
The peer link is one of the innovative elements in the IEEE 802.11s draft standard; it is used to create and close a connection with a neighbour node. The procedure can be divided into two different steps: “Peer Link creation” and “Peer Link validation”.
3.3.1. Peer link algorithm
The peer link creation deals with all the steps required to establish a link between two nodes. This procedure is described by a state machine; it has seven states, three timers and uses three different management frames (Peer Link Open, Peer Link Confirm, end
Peer Link Close). Further details concerning the peer link state machine are given in
Chapter 5.2.
IEEE 802.11s identifies the link between two MP as a logical instance which is defined by four elements:
• Local-MAC: is the MAC address of the MP.
• Peer-MAC: is the MAC address of the peer MP or the candidate peer MP. • Local-Link-ID: is an integer generated by the MP.
• Peer-Link-ID: is an integer generated by the peer MP or the candidate peer MP.
The Local-Link-ID shall be unique among all link identifiers used by the MP for its mesh link instances. The MP selects the Local-Link-ID to ensure that the same number won’t be used to identify a recent link instance. The Peer-Link-ID shall be supplied by the peer MP or candidate peer MP using the Peer Link Open and Peer Link Confirm management frame.
The mesh node can start the peer link management protocol in either of the following two cases. First, the management layer instructs the MP to passively listen to incoming requests from candidate peer MPs. The management layer achieve this by invoking “PassivePeerLinkOpen(localLinkID)” primitive function. This primitive creates a finite state machine to handle peer link establishment attempts initiated by other MPs. Second, the management layer reacts to an association request frame from a candidate peer MP and invokes the “ActivePeerLinkOpen(peerMAC, localLinkID)” primitive to
create an instance of a finite state machine establishing a link with candidate peer MP whose MAC address is peerMAC.
A link instance ends when the peer link is closed. The link closure can be caused by either an internal event or an external event. The IEEE 802.11s can close the link instance identified by the instance identifier localLinkID by issuing the “CancelPeerLink(localLinkID, ReasonCode)” primitive. On reception of a correct
Peer Link Close frame or on a failure of processing, the incoming peer link
management frame shall close the link instance. Such events are of external nature; internal events are not included in the standard.
Figure 5 illustrates a simple peer link creation diagram between two MPs to highlight the management frames exchange.
Figure 5: peer link creation diagram.
MPa starts the peer link procedure by sending a peer link open frame to the MPb; At
this point, MPb replies to the incoming message with the peer link confirm frame and
above handshake, the bidirectional link has been created and the peer link creation procedure is ended.
3.3.2. Peer Link validation
Although the peer link has been created, it cannot be used to forward traffic until a cost has been assigned to the link. The cost could be customized by different vendors but the 11s TG defines the Airtime as default metric for the peer link procedure.
As shown in the second part of Figure 4 a test frame, which has a pre-assigned pattern, is sent from MPb to MPa. The receiving node extracts the Frame Error Rate (FER)
from the test packet and, using the Link Metric Report frame, informs its peer on the cost value for the link.
When a cost for the link has been assigned, this value is used by the routing protocol (performed at MAC layer) to forward the incoming traffic.
3.4. Airtime
As already discussed in the sub-section dealing with the routing protocol for mesh network, in order to compute the forwarding table for individually addressed frames from the cached link state information generated by each MP, the latter shall first calculate the link cost for each link in the mesh network. This part of the 11s standard defines a default link metric that may be used by a path selection protocol to identify an efficient aware path. The cost function for the establishment of the radio-aware paths is based on airtime cost. Airtime cost reflects the amount of channel resources available by transmitting the frame over a particular link. Its value is obtunded according to the following formula.
(7)
Here, Ocp and Op are constants and represent internal delays, the rate r represents the rate at which the MP would transmit a frame of standard size (Bt) based on current channel condition. The frame error rate ef is the probability that the frame is corrupted due to transmission error when a frame of standard size (Bt) is transmitted at the current transmission bit rate (r), and its estimation is tied to specific vendors implementation. The typical values for the constants in Formula 1 are illustrated in
Chapter 5. Figure 6 shows the Airtime’s plot in function of its two main factors, FER and TX rate. This figure highlights how sensitive the airtime metric is to varying levels of FER and TX rate.
0.2 0.4 0.6 0.8 15 10 5 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 TX rate [Mbps] FER Co s t
Figure 6: Airtime's plot.
The below Figure 7 gives a typical example describing the way the airtime link is used to choose the path between MP1 and MP4; in this case the best solution will use two hops instead of using the single hop due to a higher minimum capacity in the sequence of links along the two-hops path.
Figure 7: Cost function based on airtime link metric.
As revealed earlier, the two hops path is able to provide high throughput. In fact the mean throughput available on the multiple hop path (MP1 MP5 MP 4) is 51 Mbps with an associated PER of 12 percent under the hypothesis that the two links are statistically independent. Therefore, with just an increment of 2 percent in FER, the two hops path is able to provide a throughput which is four time high than the correspondent throughput available on the single hop. In other words, the PER on the link between MP1 and MP5 is very low with a high transmission rate, so this link can be neglected compare to the others.
3.5. Routing protocols at MAC layer
Routing can be defined as the process of selecting a path in a network to be used to send data traffic. Routing is performed in different sorts of networks, including the telephone network, the Internet, and transport networks. In the particular wireless network scenario, routing protocol is a standard that controls how nodes come to agree on the way packets are routed between devices in a wireless domain.
In wireless networks, nodes do not have an ‘a priori’ knowledge of the topology of the network around them, but they have to discover it and keep tracking topology changes. The basic idea is that a new node announces its presence and listens to the broadcast announcements from its neighbours. The node learns about new neighbouring nodes and how to reach them, and may in turn announce their presence and reachability to other farther neighbouring nodes.
During network operation, each node knows about all other nodes and one or more ways to reach them. Routing protocols are usually used in ad-hoc wireless networks. They can be classified in two categories: Pro-active Routing and Reactive Routing. Pro-active Routing protocol maintains fresh lists of destinations and of their routes by periodically distributing routing tables throughout the network. The main disadvantages of such algorithms are:
• High control traffic overhead;
Reactive Routing protocol finds a route on demand by flooding the network with Route Request packets, when and as it’s needed. The main disadvantages of such algorithms are:
• High latency time in route finding;
• Excessive flooding can lead to network clogging.
Among the innovative elements in the 802.11s, the most important one resides in the idea to move the routing functionality from the IP layer to the Data-Link layer. The motivations in implementing such a routing functionality at MAC layer could be found in the layer’s syntax. In fact, the APs are purely layer-2 devices, which are incapable of decoding IP packets; also, adding layer-3 functionalities to an AP is typically considered not acceptable from a design point of view [36]. Another advantage can be associated with the possibility to increase the coverage area and provide more flexibility in developing and updating a wireless network. Throughput increasing, power optimization and backward compatibility are the other goals for the mesh routing. These benefits are due to the major quantity of information available at MAC layer and to a faster reaction to changes in the network.
The Dot11s standard defines a default routing protocol for the mesh network called Hybrid Wireless Mesh Protocol (HWMP) but other protocol are supported too. As optional protocol the 802.11s adopts the RA-OLSR (Radio-Aware Optimized Link
State Routing).
3.5.1. HWMP Routing Protocol
The HWMP is a mesh path selection protocol that combines the flexibility of on-demand path selection with the proactivity of a topology tree extensions approach. The combination of reactive and proactive elements of HWMP enables optimal and efficient path selection in a wide variety of mesh networks. This routing protocol uses a common set of protocol primitives, generation and processing rules inspired by AODV (Ad-Hoc On-Demand Distance Vector) protocol [23] adapted for MAC address -based path selection and link metric awareness.
HWMP supports two operation modes depending on its configuration. The first is indicated as “Proactive tree building mode”; this mode foresees that at least one MP is configured as root MP, which is the spanning tree root. The other is called “On
demand mode”; this mode allows MPs to communicate using peer-to-peer paths. This
configuration is used in situations where there is no root MP. These modes are not exclusive and they may be used concurrently to obtain a better performance.
In other words, as explained in [43], HWMP is located on layer 2 that means it uses MAC addresses. HWMP is a hybrid routing protocol relying on both reactive components and proactive components. The HWMP protocol is largely based on AODV concepts with adaptation to radio-aware link metrics and MAC addresses. The on-demand path setup is achieved through a path discovery mechanism that is very similar to the one featured by AODV. If a mesh point needs a path to a destination, it broadcasts a PREQ (path request) message into the mesh network. MPs will rebroadcast the updated PREQ whenever the received PREQ corresponds to a newer or better path to the source. Similarly, the requested destination MP will respond with a PREP (path reply) message whenever a received PREQ corresponds to a newer or better path to the source. Intermediate MPs that have already a valid path to the requested destination can respond with a PREP and update their respective routing tables.
The proactive component in HWMP is based on the original AODV extension with a proactive routing tree to specially designated MPs identified as root MPs. Any MP that is configured to be a root MP, will periodically broadcast proactive PREQ messages or RANN (root announcement) messages into the wireless mesh network, which will create and maintain a tree of paths to the root MP.
3.5.2. Ra-OLSR Routing Protocol
RA-OLSR protocol is a pro-active, link-state wireless mesh path selection protocol based on OLSR (Optimized Link State Routing) protocol [29] and uses radio-aware metrics in forwarding path and MPR (multipoint relay) set calculation. RA-OLSR enables the discovery and maintenance of optimal paths based on a predefined metric, given that each MP has a mechanism to determine the link cost metric for each of its neighbours. In order to propagate the metric link cost information between MPs, a metric field is used in RA-OLSR information elements. In disseminating topology information over the network, RA-OLSR uses only a subset of MPs in the network in flooding process. This specific subset of MPs is referred to as MPRs.
