PERFORMANCE ANALYSIS OF MULTI-HOP WIRELESS NETWORKS

A thesis submitted

to Kent State University in

partial fulfillment of the requirements

for the degree of Master of Science

by

Hassan Hadi Latheeth AL-Maksousy

December 2012 Thesis written by

Hassan Hadi Latheeth AL-Maksousy

B.S., Al-Mustansiriya University, 2006

Approved by

Dr. Hassan Peyravi , Chair, Master Thesis Committee

Dr. Javed Khan , Members, Master Thesis Committee

Dr. Feodor F. Dragan

Accepted by

Dr. Javed Khan , Chair, Department of Computer Science

Dr. Raymond A. Craig , Associate Dean, College of Arts and Sciences

ii TABLE OF CONTENTS

LIST OF FIGURES ...... vii

LIST OF TABLES ...... viii

Acknowledgements ...... ix

1 Introduction ...... 1

1.1 Multi-hop Wireless Networks ...... 2

1.2 Issues and Difficulties ...... 3

1.2.1 Unpredictable Link Properties ...... 5

1.2.2 Node Mobility ...... 5

1.2.3 Limited Battery Life ...... 6

1.2.4 Hidden and Exposed Terminal Problems ...... 6

1.2.5 Route Maintenance ...... 7

1.2.6 Security ...... 8

1.3 Multi-hop Wireless Network Protocols ...... 8

2 Previous and Current Work ...... 9

2.1 IEEE 802.11s: Mesh Networks ...... 9

2.1.1 Approaches to Wireless Mesh ...... 12

iii 2.1.2 The Single Radio Approach (Everything on the Same Channel) 12

2.1.3 The Dual Radio Approach (Sharing the Backhaul) ...... 13

2.1.4 The Multi Radio Approach (A Structured Wireless Mesh) . . 13

2.1.5 Physical Layer ...... 14

2.1.6 Frame Structure of IEEE 802.11s ...... 15

2.1.7 Mesh Coordinated Channel Access ...... 17

2.1.8 Routing ...... 18

2.1.9 Applications ...... 19

2.1.10 Issues ...... 20

2.2 IEEE 802.15.5 ...... 22

2.3 IEEE 802.16j ...... 24

2.3.1 Physical Layer ...... 25

2.3.2 OFDMA and SOFDMA ...... 27

2.3.3 Frame Structure of IEEE 802.16j ...... 28

2.3.3.1 Frame Structure for Transparent Relay ...... 28

2.3.3.2 Frame Structure for Non Transparent Relay . . . . . 29

2.3.4 MAC Layer ...... 29

2.3.5 Routing ...... 32

2.3.6 Issues ...... 32

2.3.7 QoS Scheduling ...... 34

2.4 Previous Work ...... 35

iv 2.4.1 Throughput ...... 36

2.4.2 End-to-End Delay ...... 36

2.4.3 Fairness ...... 37

3 Multi-hop Wireless Network Performance ...... 39

3.1 Bandwidth Degradation ...... 39

3.2 Radio Frequency Interference ...... 40

3.3 Network Latency ...... 41

3.4 Fairness Provisioning Technique ...... 43

3.4.1 Background on Fairness ...... 43

3.4.2 Jain Fairness Index ...... 44

3.4.3 Max-Min Fairness ...... 44

3.4.4 Proportional Fairness ...... 45

4 The Simulator ...... 46

4.1 Software ...... 46

4.2 QualNet Architecture ...... 48

4.3 Models ...... 51

4.3.1 Nodes and Topology ...... 51

4.3.2 Applications ...... 52

4.3.2.1 Constant Bitrate ...... 52

4.3.2.2 File Transfer Protocol ...... 53

v 4.3.2.3 Source Traffic Model ...... 54

4.3.3 Protocols ...... 54

4.3.3.1 IEEE 802.11e ...... 55

4.3.3.2 IEEE 802.11s ...... 55

4.3.3.3 TDMA ...... 55

4.3.3.4 TDMA characteristics ...... 56

4.4 Simulation Parameters ...... 57

4.4.1 Duration ...... 57

4.4.2 QoS Metrics ...... 57

4.4.3 Physical Layer ...... 58

4.4.4 MAC Layer ...... 58

4.4.5 Network Layer ...... 59

5 Results ...... 60

6 Future Work and Conclusion ...... 65

6.1 Future Work ...... 65

6.2 Conclusion ...... 66

Glossary ...... 68

BIBLIOGRAPHY ...... 72

vi LIST OF FIGURES

1 Relay and mesh...... 3

2 Hidden terminal...... 6

3 Exposed terminal...... 7

4 IEEE 802.11s frame structure ...... 16

5 IEEE 802.16j transparent frame structure...... 29

6 IEEE 802.16j non-transparent frame structure...... 30

7 Throughput degradation...... 40

8 QualNet architecture...... 46

9 (a) Linear, and (b) tree...... 51

10 Throughput and normalized throughput...... 60

11 Average throughput on varying topologies...... 61

12 Fairness index of average end-to-end delay on varying topologies. . . . 61

13 Average end-to-end delay per node on varying topologies...... 62

14 Average end-to-end delay per node on varying topologies...... 63

15 Average throughput and Jain fairness index for varying protocols. . . 63

16 Average end-to-end delay and Jain fairness index for varying protocols. 63

17 Controlling rate for better fairness...... 64

vii LIST OF TABLES

1 Scenario input data...... 52

viii Acknowledgements

I would first like to thank my adviser Dr. Hassan Peyravi for all of his guidance, support, and advice. He has broadened my knowledge and helped me to become a better researcher. I was truly fortunate to have him as my adviser. I would also like to thank the committee members Dr. Javed Khan and Dr. Feodor F. Dragan for the valuable comments and suggestions they have offered me. Their feedback has improved the quality of this thesis.

I am so thankful for my parents. I am thankful for their unconditional love and for raising me the right way, making me the person I am today.

Finally, I would like to thank the Kent community at large and especially the department of Computer Science. I would like to extend a special thanks to Marcy

Curtiss, the gradute secretary. They have all made me feel welcome and have sur- rounded me with the warmth of a family. Kent State University will always have a special place in my heart.

ix CHAPTER 1

Introduction

Broadband access to the Internet has become necessary in all of todays networks to support high quality multimedia services, such as video, voice, high definition

TV or interactive games. Network communications with end devices are becoming increasingly wireless. Many standards for wireless networking are now taking the next step to support mesh architectures in which data is commonly forwarded on paths consisting of multiple wireless hops. These multi-hop network architectures have great flexibilities, however it will be shown that they suffer from a performance limitation.

There is a significant body of the literature addressing the fairness problem with respect to throughput and delay performance in multi-hop wireless networks. How- ever, none has shown the impact of other important factors such as network topology, level of aggregation, and source traffic models. In this thesis, we study the effect of these important factors under normal and extreme conditions and show major limita- tions in terms of hop counts, traffic load and contention domain in terms of horizontal

(spread) and vertical (hop count) contentions. The result show that supporting de- lay and throughput sensitive applications require careful deployment of the underling

1 2 relay nodes.

1.1 Multi-hop Wireless Networks

Multi-hop wireless networks (MWNs) are being used as alternative solutions to the wired back-haul links. They are similar to the mobile ad hoc networks (MANETs), but differ in terms of mobility, topology, and architecture. Nodes in MWNs are relatively

fixed with relatively robust connectivity and often follow a hierarchical architecture.

MWNs can be classified into relay architecture, where nodes form a sink tree, or mesh topology, in which multiple connections exists among mesh nodes.

Most of wireless networks operate in high frequency bands above 2 GHz. However, at higher carrier frequency cell size needs to be decreased because of an increase in path loss [1] and increased power consumption. In order to conserver power, base stations (BSs) need to be placed closer together. Therefore more BSs are needed to achieve the same coverage. MWNs overcome these problems by extending ranges through increasing capacity and reduce power consumption through route diversity.

A MWN can be arranged in one of two architectures: relay or mesh. The relay and mesh topologies have some of the same elements. They both have BSs and subscriber stations (SSs). The SSs are trying to connect with the BS, but each topology does this in a different way.

In relay, the network infrastructure includes relay stations (RSs) that are mostly installed, owned and controlled by a service provider. A RS is not directly connected 3 to wire infrastructure and has the minimum functionality necessary to support multi- hop communication. The important aspect is that traffic always leads from or to a

BS. SS to SS communication paths that do not include a BS are not considered.

SSs may forward traffic to another SS, RS or BS, and can communicate directly with each other. Nodes are comprised of mesh routers and mesh clients. Therefore, the routing process is controlled not only by BSs but also by SSs. Each node can forward packets on behalf of other nodes that may not be within direct wireless transmission range of their destination. A system that has a direct connection to backhaul services outside the mesh network is termed a mesh BS. All the other systems are called a mesh SS [2]. So, every SS can be RS but not every RS can be a SS.

BS RS RS SS SS SS BS Relay Mesh

Figure 1: Relay and mesh.

1.2 Issues and Difficulties

There are many benefits for using multi-hop technology such as rapid deployment with lower-cost back-haul, extend coverage due to multi-hop forwarding in hard-to- wire areas, enhanced throughput due to shorter hops and extended battery life due 4 to lower power transmission. [2]

MWNs suffer from some drawbacks including:

1. Performance: MWNs do not match the performance of a wired network.

2. Routing complexity: These networks need path management which creates ex-

tra delay due to multi-hop relaying. For example, we must also specify the

maximum number of hops for each node which should not be exceeded, because

more hops mean more transmission time.

3. Increased channel contention: When a packet follows a multi-hop path to an

access point (AP), it uses the wireless channel of the AP two or more times.

This would increase the contention of the channel and potentially allow reduced

data throughput for the source as well as other clients in the vicinity.

4. Resource consumption at relay nodes: Packets following multi-hop paths con-

sume resources such as battery power and bandwidth. This leaves no incentives

for wireless clients to operate in such an altruistic mode. Each client in the

wireless local area network (WLAN) can choose independent policies on when

they would like prefer it to serve as this kind of proxy.

5. Security threats: it has been argued that deployment of such multi-hop mecha-

nisms does not add any security problems that current single-hop environments

do not already have [3]. However, allowing an intermediary to forward data 5

packets on behalf of a client may potentially open the WLAN to new security

threats.

MWNs differ from traditional wired internet infrastructures. The differences intro- duce unique issues and difficulties for supporting quality of service (QoS) in MANET environment. These issues include features and unintended consequences. Examples of features include unpredictable link properties, node mobility and limited battery life. Whereas, hidden and exposed terminal problems, route maintenance and secu- rity can be categorized as unintended consequences. These issues are briefly described below.

1.2.1 Unpredictable Link Properties

Packet collision is intrinsic to wireless networks. Signal propagation faces diffi- culties such as signal fading, interference, and multi-path cancellation. All of these properties make measures such as bandwidth and delay of a wireless link unpre- dictable.

1.2.2 Node Mobility

Mobility of the nodes creates a dynamic network topology. Links will be dynami- cally formed when two nodes come into the transmission range of each other and are torn down when they move out of range. 6

1.2.3 Limited Battery Life

Mobile devices generally depend on finite battery sources. Resource allocation for

QoS provisioning must consider residual battery power and rate of battery consump- tion corresponding to resource utilization. All the techniques for QoS provisioning should be power-aware and power-efficient.

1.2.4 Hidden and Exposed Terminal Problems

In a media access control (MAC) layer with the traditional carrier sense multiple access (CSMA) protocol; multi-hop packet relaying introduces the hidden terminal and exposed terminal problems.

A B C D

Figure 2: Hidden terminal.

The hidden terminal problem occurs when signals of two nodes, A and C, are out of one anothers transmission ranges collide at a common receiver, node B. With the same nodal configuration, an exposed terminal problem will result from a scenario where node B attempts to transmit data to A while node C is transmitting to node 7

D. In such a case, node B is exposed to the transmission range of node C, deferring its transmission even though it would not interfere with the reception at node A.

A B C D

Figure 3: Exposed terminal.

1.2.5 Route Maintenance

The dynamic nature of the network topology and the changing behavior of the communication medium make the precise maintenance of network state information very difficult, causing the routing algorithms in MANETs have to operate with in- herently imprecise information. In addition, in ad hoc networking environments, nodes can join or leave at any time. The established routing paths may be broken, even during the process of data transfer, creating the need for maintenance and re- construction of routing paths with minimal overhead and delay. QoS-aware routing would normally require reservation of resources at the routers (intermediate nodes).

However, with the changes in topology the intermediate nodes also change and new paths are created. This causes reservation maintenance with updates in the routing 8 path to become cumbersome.

1.2.6 Security

Security can be considered a QoS attribute. Without adequate security, unau- thorized access and usage may violate QoS negotiations. The nature of broadcasts in wireless networks may potentially result in more security exposure. The physical medium of communication is inherently insecure. There is a need to design security- aware routing algorithms for MANETs.

1.3 Multi-hop Wireless Network Protocols

There are various standards being deployed in MWNs, and in this thesis we will focus on the IEEE 802.11 family and time division multiple access (TDMA). IEEE

802.11 is found in nearly every access point currently deployed. Also, we could take advantage of using the network simulation software QualNet which can model IEEE

802 family and TDMA protocols. CHAPTER 2

Previous and Current Work

This chapter delves into the previous and current work from the research commu- nity. The protocols have been studied in great detail, and the following sections will show the specifications of each. The chapter will conclude with examples of previous attempts to improve performance.

2.1 IEEE 802.11s: Mesh Networks

IEEE 802.11s is an amendment to the IEEE 802.11 standard to create a wireless distributed system (WDS), also known as mesh. It has automatic topology learning and wireless path configuration. The intended size of network is small to medium, containing approximately 32 forwarding nodes, although it can be larger. By using dynamic, radio-aware path selection in the mesh, data delivery can happen on single- hop and multi-hop paths, in unicast and broadcast/multi-cast modes. In addition, this amendment is extensible to allow support for diverse applications and future innovations. For example 802.11i offers security and it is compatible with higher layer protocols.

9 10

Each node in 802.11s networks establishes the optimal path to its closest neighbor.

The wireless environment changes such as the addition of a new node or link conges- tion data paths are re-evaluated based on latency throughput, noise, etc. Wireless mesh networks (WMNs) are self-tuned to maintain their performance at the peak level. If a data path is lost, or if radio frequency interference (RFI) affects perfor- mance, the network self-heals by re-routing traffic so that nodes stay connected and data paths remain optimized. All self-tuning and self-healing processes are dynamic, occurring in the background and in real time. Processes are transparent to the user and are without need for human intervention for outdoor deployment. The forwarding capabilities of mesh architecture allow the wireless network to switch traffic around large physical object, such as a building. Instead of attempting to radiate through impeding objects, a WMN can easily forward packets around an object via interme- diate relay nodes. This approach is very useful in dense urban environments that contain many obstructions and in rural areas where hills or mountains become an obstacle to conventional wireless network. [4]

The 802.11s standard extends the existing 802.11 MAC layer in several ways. In order to improve network capacity, multiple channels and multiple radios are used.

BSs’ traffic and forwarding mesh traffic are handled efficiently. There is also an intra- mesh congestion control. Optionally, a mesh coordinated channel access (MCCA) is applied. MCCAs is discussed further below. Flows with longer hops suffer from band- width starvation and fair link access disparities along the path towards the gateway. 11

A WMN consists of one or more wired gateways to the Internet, a number of wireless

APs that relay client traffic via other wireless APs towards the gateway and other wireless clients. Generally, APs act as wireless routers (relay nodes) and normally participate in forming a minimum spanning tree rooted at a gateway.

Although WMNs are flexible and easy to deploy, they lack scalability. Cross-layer optimization of existing protocols from the application layer, to the transport layer, to the MAC layer, and to the physical layer is needed to satisfy the QoS require- ments [5]. In the absence of a centralized control achieving such fairness becomes far more challenging. Generally, carrier sense multiple access with collision avoidance

(CSMA/CA) including 802.11 family protocols does not work well in MWNs. This is mainly due to the hidden and exposed node problems and successive binary expo- nential back-off contention resolution algorithm [5]. IEEE 802.11 MAC protocol is originally designed for single hop wireless networks.

In multi-hop networks, throughput decreases exponentially with the increase in number of hops. This is mainly due to traffic aggregation and saturation tree builds up toward the gateway. It has been shown [6] that current protocols that are adapted for

WMNs can lead to starvation for flows farther away from the gateways. IEEE 802.11s has numerous benefits, including rapid deployment with lower-cost backhaul because it extends an existing standard. It is easy to provide coverage in hard-to-wire areas.

The standard is self-healing, resilient, and extensible. With some optimal conditions,

802.11s can promote greater range due to multi-hop forwarding, higher bandwidth 12 due to shorter hops, and better battery life due to lower power transmission.

2.1.1 Approaches to Wireless Mesh

Wireless mesh approaches [7] vary, but most have their technology roots in the original concept of the WDS. A WDS is a wireless AP mode that uses wireless bridg- ing, where APs communicate only with each other and don’t allow wireless clients to access them and wireless repeating where APs communicate with each other and with wireless clients. It is intrinsic to all mesh networks that user traffic must travel through several nodes before exiting the network (for example, the wired local area network (LAN)). The number of hops that user traffic must make to reach its des- tination will depend on the network design, the length of the links, the technology used, and other variables.

2.1.2 The Single Radio Approach (Everything on the Same Channel)

The single radio model is the weakest approach to wireless mesh. It uses only one radio on a single channel in the AP, shared by the wireless clients and the backhaul traffic (forwarded between two APs). As more APs are added to the network a higher percentage of the radio bandwidth is dedicated to repeating backhaul traffic leaving very little capacity for wireless clients. This phenomenon is due to the simple fact that wireless is a shared medium. Consequently an AP cannot send and receive at the 13 same time or send when another AP within range is transmitting. This contention for the available shared bandwidth is based on Ethernet-like collision avoidance rules for wireless (CSMA/CA).

2.1.3 The Dual Radio Approach (Sharing the Backhaul)

With the dual radio approach, one radio is dedicated to wireless client support while the other radio is dedicated to wireless backhaul support. The backhaul channel is shared for both ingress and egress traffic. A slight improvement is gained for wireless clients but overall network performance is still poor because the backhaul forms a bottleneck.

2.1.4 The Multi Radio Approach (A Structured Wireless Mesh)

In the multi-radio or structured mesh-approach there are several dedicated link interfaces of least three radios in use per network node. One radio for wireless client traffic and a second radio for ingress wireless backhaul traffic. This approach offers significantly better performance than either the single or dual radio approaches. It allows for dedicated backhaul links that can transmit and receive simultaneously because each link is on a separate channel. 14

2.1.5 Physical Layer

Some advanced physical-layer techniques have been available for WMNs Wireless radios of existing WMNs are able to support multiple transmission rates by a com- bination of different modulation and coding rates. With such modes, adaptive error resilience can be provided through link adaptation. Schemes such as orthogonal fre- quency division multiplexing (OFDM) and ultra-wide band techniques are being used to support high-speed transmissions. In order to further increase capacity and miti- gate the impairment by fading, delay-spread, co-channel interference, multi-antenna systems such as antenna diversity, smart antenna, and multiple input multiple out- put (MIMO) systems, have been proposed for wireless communications. Although these physical-layer techniques are also desired by other wireless networks, it is a more challenging problem to develop such techniques for WMNs. For example, mesh networking among multiple nodes makes the system model much more complicated than that of a conventional MIMO system in WLANs or cellular networks.

In order to achieve much better spectrum utilization and viable frequency plan- ning for WMNs, frequency-agile or cognitive radios are being developed to dynam- ically capture the unoccupied spectrum. The Federal Communications Commission

(FCC) has recognized the promising technique and is pushing to enable it to a full realization. Implementing cognitive radios on a software radio platform is one of the most powerful solutions, because all components of a radio, such as radio frequency

(RF) bands, channel access modes, and channel modulations, are programmable. The 15 software radio platform is not a mature technology yet, although physical test-beds are currently available. However, in the long run it will be a key technology for wireless communications because it can enable the programmability of all advanced physical layer techniques.

2.1.6 Frame Structure of IEEE 802.11s

IEEE 802.11s extends the previous 802.11 frame format. In the IEEE 802.11 standard, the header is 30 bytes long, followed by a payload from 0 to 2312 bytes, and concludes with a CRC32 of 4 bytes. While the error-detection code portion remains unchanged, IEEE 802.11s modifies the header and payload parts of the frame.

The header of the original standard is composed of version, frame type, frame subtype, frame control, duration, destination address, source address, base station identification, and sequence number. The frame control bits determine the frame type as one of four options: management frames, control frames, data frames or reserved frames. IEEE 802.11s utilizes only the management and data frames. When IEEE

802.11s specifies data frames, it adds mesh header frames to the payload section, for further processing. The management frames are used along with either and action frame subtype or a multi-hop action frame subtype. These are used to direct mesh- specific commands.

Another characteristic of the new frames [8] is the use of the FromDS and ToDS

flags. In IEEE 802.11, those bits marked frames as being originated from or destined 16

Frm Dur Seq QoS HT FCS Address 1 Address 2 Address 3 Address 4 Payload Ctrl ID Ctrl Ctrl Control CRC32

Mesh Mesh Flags Sequence Number

Mesh Mesh Source Ext Source Address Mode Address

Figure 4: IEEE 802.11s frame structure to a distribution system, which is the infrastructure that interconnects APs.

In a WDS, the backhaul connecting the access points is, as the name implies, wireless. A WDS frame is used to exchange frames between them and has both

FromDS and ToDS frames activated. These frames can be found at bits 3 and 4 of the mesh flags in the payload section of the frame. Its original role is to allow transmissions between stations connected to two different APs in the same WLAN.

IEEE 802.11s also sets FromDS and ToDS flags in frames transmitted inside a mesh cloud. The IEEE 802.11 standard defines frames where FromDS and ToDS flags are set to 1 as data frames using the four-address format. This definition will be changed to A data frame using the four-address MAC header format, including but not limited to mesh data frames when the IEEE 802.11s amendment is approved.

The fact that WDS implementations are vendor specific may potentially raise issues of compatibility with the emerging standard. As we described the use of the DS flags, we should notice that both are set to zero in ad hoc IEEE 802.11 frames. 17

In an ad hoc network, peer-to-peer transfers can happen opportunistically in a way that should not be confused with that proposed by a mesh network, where frame forwarding, for example, multi-hop forwarding, capabilities are present.

2.1.7 Mesh Coordinated Channel Access

MCCA is an optional scheme based on the reservation of contention free time slots.

Lower contention (more deterministic) mechanism improves QoS for periodic flows.

The mesh coordination function consists of a mandatory and an optional scheme.

For the mandatory part, The mesh coordination function relies on the contention- based protocol known as enhanced distributed channel access (EDCA), which itself is an improved variant of the basic IEEE 802.11 distributed coordination function

(DCF). Using DCF, a station transmits a single frame of arbitrary length. Using

EDCA, a station may transmit multiple frames whose total transmission duration may not exceed the transmission opportunity (TXOP) limit. The intended receiver acknowledges any successful frame reception. Additionally, EDCA differentiates four traffic categories with different priorities in medium access and thereby allows for limited support of QoS. 18

2.1.8 Routing

The IEEE 802.11s standard bases routing on the MAC address, uses radio-aware routing metric, unicast, multicast and broadcast, and single/multiple radio devices.

The default routing protocol for IEEE 802.11s is hybrid wireless mesh protocol

(HWMP). It can perform both reactive and proactive routing. Reactive routing is based on the ad hoc on-demand distance vectoring (AODV) routing protocol for

MANETs and other wireless ad hoc networks and dynamic source routing (DSR), which is similar to AODV in that it forms a route on-demand when a transmitting computer requests one. However, it uses source routing instead of relying on the routing table at each intermediate device. Reactive routing computes a path only if one is needed for sending data between two mesh points (MPs). This keeps the over- head small, but there will be extra route discovery delay and data buffering. With proactive routing, routes are maintained to all destinations, regardless of whether or not these routes are needed. This protocol has low delay, but high routing overhead.

Furthermore, in HWMP, the proactive tree-based routing is totally centralized and constrained by the root node. Even if there is a short path between two MPs, the proactive routing mode still routes the packets via the root node, which results in a bottleneck at the root node and wastes precious wireless resources due to non- optimized routing path. Meanwhile, the on-demand routing mode still initiates a path discovery process to look for an optimized routing path before the packets are sent out, which results in a broadcast storm problem and wastes power resources on 19 other MPs. Thus, the hybrid routing protocol in IEEE 802.11s still cannot support route optimization between two MPs.

An optional routing protocol is radio aware optimized link state routing (RAOLSR).

It offers minimal latency and is ideal for high density or large networks. Optimized link state routing (OLSR) avoids the extra work of finding the destination by retain- ing a routing entry for each destination all the time, thus providing low single-packet transmission latency.

OLSR can easily be extended to QoS monitoring by including bandwidth and channel quality information in link state entries. Thus, the quality of the path (e.g., bandwidth, delay) is known prior to call setup. However, when the network is sparse, every neighbor of a node becomes a multi-point relay. The OLSRs then reduces to a pure link state protocol. In effect, this means there will be high control overhead

(reduced by MPR usage), higher computation, greater storage requirements, and complex implementation.

2.1.9 Applications

1. Residential: in this model, the primary purposes for the mesh network are to

create low-cost, easily deployable, high performance wireless coverage through-

out the digital home. The mesh network is intended to eliminate RF dead-spots

and areas of low-quality coverage. High bandwidth applications tend to be used

but also simple ones, for example, video streaming and wireless printers. 20

2. Office: the objective in the office case is to create low-cost, easily deployable

wireless networks that provide reliable coverage and performance. A wireless

mesh LAN becomes useful in areas where Ethernet cabling does not exist or

is cost prohibitive. Companies reduce their costs in association with cable

and time of installation. Furthermore, they can benefit also from increase in

employee productivity through expanded connectivity to key network resources.

3. Campus, community, public access: mesh networks can in this case provide

connectivity over large geographic areas in low cost, higher and-width Internet

access in contrast to the traditional methods and location based services for

information and safety purposes.

4. Public safety: access to emergency and municipal safety personnel such as fire,

police, and hospital is important if a corresponding incident occurs. The net-

work can be used for video surveillance, tracking emergency workers with biosen-

sors, voice and data communication between emergency personnel and so on.

2.1.10 Issues

In a public hotspot, nodes may enter or leave a network at any time and they may wander around while there is active communication. Node mobility changes the number of nodes, the amount of tracks, and the network topology. The most critical impacts are link breaks and route changes, which inevitably result in packet losses 21 and communication delays. [9]

Nodes share the same medium in a WLAN hotspot. Nodes are not required to stay within an APs radio range, centralized medium access scheduling at an AP is infeasible in a multi-hop hotspot. It is essential that nodes access the medium with a decentralized scheduling scheme such as the DCF of IEEE 802.11. Without global coordination, medium sharing among nodes is potentially unfair [10]. The dynamics due to node mobility and decentralized access of nodes often cause unacceptable variations in the QoS of ongoing end-to-end communications in these networks [11],

[12], [13].

The existing end-to-end QoS assurance techniques [14] [15] [16] can be broadly grouped into two camps, integrated services (IntServ) based and differentiated services

(DiffServ) based. IntServ based mechanisms aim to assure each flow with its specific

QoS along its specific route with per-flow resource reservation at each node along the route. DiffServ mechanisms, on the other hand, do not perform resource reservation and per-flow operations. A number of service classes are provisioned with certain resources to provide different levels of QoS and applications choose to be serviced in any of these classes. DiffServ mechanisms for QoS assurances at a node are defined as per hop behaviors (PHBs), e.g. the expedited forwarding (EF) PHB [17] and the assured forwarding (AF) PHB. [18]

End-to-end QoS assurances are defined as per domain behaviors (PDBs) with

PHBs as their building blocks. [15] There are a number of reasons to believe that the 22 existing IntServ and DiffServ proposals are difficult to implement in a MWN. Diffi- culties with IntServ solutions are centered on resource reservation. As the available bandwidth for each node varies with time, it is necessary but hard for a node to es- timate its available bandwidth for resource reservation. Since the medium is shared, it is also necessary that resource reservation be done with global coordination. Once reservations are made, violations may occur due to bandwidth fluctuations as well as route changes as such reservations are pinned to a route [19].

DiffServ solutions do not perform resource reservation. DiffServ assurances, how- ever, largely depend on available resources in a network. The EF PHB assures low queuing delays at a node if its service rate is no less than the EF traffic arrival rate.

The AF PHB provides assured throughput at a node with packet priority marking and selective queue management. Packets marked with high priorities are assured to be always serviced before low priority ones. If the marking rate does not exceed the bandwidth, a throughput equal to the marking rate is assured at each node. AF alone, however, does not address end-to-end throughput assurances. Both EF and AF face a difficult resource provisioning problem in a dynamic multi-hop network [20].

2.2 IEEE 802.15.5

Mesh topologies are also being used in wireless personal area networks (WPANs).

The IEEE 802.15.5 standard describes architecture for high-rate mesh WPANs that is based on single-hop MAC and physical layer activity. It is at first an extension 23 for the MAC layer, including features such as multiple beacon operation, distributed reservations, and optional frequency reuse to increase the capacity of the mesh. ID assignment and wireless path selection are additional mesh support services.

IEEE 802.15.5 offers two unique features:

1. It goes beyond the coverage of personal area through the use of mesh networking

technology, and,

2. unlike other IEEE 802 standards, it defines a recommended practice, which is

the least n or native kind of IEEE standard, for a mesh sub-layer on top of

existing 802.15.x MAC and physical layers.

The standard includes mesh support for both high and low rate WPANs. These are realized through the 802.15.3b standard for high rates and 802.15.4b for low rates.

Every mesh piconet controller (MPNC) transmits a beacon. Each beacon carries an

ID, synchronization information, neighborhood information, neighbors, and medium access information. When a new MPNC is switched on it scans for beacons. It also selects an idle space for beacon transmission, which will incorporate channel sensing, information from neighbors, and provides local viewpoint of occupancy to neighbors, to help increase their knowledge. However, this information of occupancy is only disseminated over three hops, and could lead to a hidden node problem.

The IEEE 802.15.5 standard can be implemented using full or partial mesh topolo- gies. The intended range of operation for IEEE 802.15.5 is less than 10m, such as the human body or a single, small room. The benefits of 802.15.5 in the MAC layer 24 are mobility, hidden and exposed nodes, loop prevention, and the ability to broad- cast data. Easier network configuration is due to automatic path selection and self- organization. Furthermore, there is enhanced reliability via route redundancy and extension of network coverage without increasing transmit power or receive sensitiv- ity while reducing effects of interference, which would lead to unnecessary capacity decrease. It needs also to be mentioned better device battery life due to fewer re- transmissions.

2.3 IEEE 802.16j

The 802.16j standard uses a tree topology. However, a single point of breakage can leave the network unstable. If a relay connection is broken or a bottleneck occurs, then this leaves a whole branch of the tree disconnected. The tree based topology is vulnerable in large coverage networks and careful resource allocation is needed to ensure a constant uniform traffic flow over the whole network.

The two relay modes defined in this standard are transparent and non-transparent.

Transparent mode supports centralized scheduling, which is performed only in the BS.

It uses a connection identifier (CID) based forwarding scheme which enacts only two hops. However, it does not provide coverage extension, even though it can maintain a low RS cost. The non-transparent mode supports centralized or distributed scheduling which can be done in the BS or a RS. To accomplish this, it uses a tunnel based or CID based forwarding scheme, but it will require two or more hops, as compared to the 25 transparent mode. Ultimately, non-transparent mode provides BS coverage extension, but at a high RS cost. The relaying techniques include the conventional techniques time domain relaying, frequency domain relaying and hybrid time/frequency domain relaying and the current technique that is interest among the research community is co-operative technique.

There are three main benefits provided by relay based architecture over single hop architecture. The first, throughput enhancement, is expected to increase the system capacity by deploying RSs in a manner that enables more aggressive spatial reuse. Next, the relay technology is expected to improve the coverage reliability in geographic areas that are severely shadowed from the BS and/or to extend the range of a BS. Third, relay based systems have the potential to deliver cost gains over traditional single hop wireless access systems. Using RSs, an operator could deploy a network with wide coverage at a lower cost than using only (more) expensive BSs to provide good coverage and system capacity.

2.3.1 Physical Layer

The physical layer of Worldwide interoperability for microwave access (WiMAX)

IEEE 802.16j standard is based on the IEEE 802.16-2004 and IEEE 802.16e-2005 standards and was designed from IEEE 802.11a. In the IEEE 802.16-2009 standard, an adaptive burst profile mechanism is used, so that WiMAX systems are able to

flexibly adjust the modulation and power scheme for individual SSs depending on 26 the radio conditions. The relay physical layer provides definition of physical layer design, for example, subchannelization, modulation, coding, etc., for links between

BS and RS and between RSs. The IEEE 802.16j standard has extended the past

IEEE 802.16e frame structure to support in-band BS-to-RS communication. The frame structure supports a typical two-hop relay-enhanced communication, where some SSs are attached to a RS and communicating with a BS via the RS, and some SSs connected directly to the BS. TDMA, as the access method and the policy for selecting scheduled links in a given time slot, will definitely impact the system performance.

The wireless metropolitan area network (MAN) scalable orthogonal frequency di- vision multiple access (SOFDMA) physical layer supports frequency range of below

11 GHz in licensed bands, and supports time division and frequency division du- plexing alternatives. In the IEEE 802.16j-2009 standard, an adaptive burst pro-file mechanism is used, so that the worldwide interoperability for microwave access sys- tems is able to flexibly adjust the modulation and power scheme for individual SSs depending on the radio conditions. Four modulation schemes are defined in burst profiles to suit different signal to noise ratio situations: binary phase shift keying, quadrature phase shift keying, 16 quadrature amplitude modulation, and 64 quadra- ture amplitude modulation. Some other techniques and technologies, such as forward error correction, multiple input multiple output antennas, adaptive antenna systems, and automatic repeat request are also defined to improve performance 27

2.3.2 OFDMA and SOFDMA

The most interesting technologies adopted in WiMAX are OFDM, orthognal fre- quency division multiple access (OFDMA) and SOFDMA technology which are spec- ified in the IEEE 802.16-2009 standard. [21] [22] The operating spectrum of OFDMA is divided into multiple narrow frequency bands; here it groups sub-carriers into sub- channels, each of which consists of multiple sub-carriers.

The multiple access is achieved by assigning different sub-channels to different users in the network for simultaneous transmission. The available resource can be viewed as transmission blocks (or simply blocks) in a two dimensional structure with mini slots in one dimension and sub channels in the other. The scheduling prob- lem is to how to assign such blocks to each link in the network to optimize certain objectives. OFDM is very similar to frequency division multiplexing (FDM), but it effectively squeezes multiple modulated carriers tightly together by keeping the signals orthogonal so that they do not interfere with each other and thus reduces the required bandwidth. OFDMA is a multiple access / multiplexing scheme that provides multiplexing operation of data streams like OFDM. Flexibility to efficiently use the system resources is introduced with SOFDMA. It allows smaller fast Fourier transform (FFT) sizes to further improve performance and reduce the cost for lower bandwidth channels. One key aspect in OFDMA systems is to take advantage of the so called multi-user diversity as defined by Knopp and Humblet. Multi-user diversity means that each user experiences different, time varying and frequency varying signal 28 quality due to time and frequency fading inherent to wireless channels. As OFDMA allows dynamical assignment of different sub carriers and OFDMA symbols to dif- ferent users, dynamic sub-carrier allocations and adaptive power allocation (APA) to multiple users can be used to enhance the system capacity.

2.3.3 Frame Structure of IEEE 802.16j

In IEEE 802.16j frame structure [23] , frame is divided in to two subframes, downlink subframe (DL) and uplink subframe (UL). These subframes are divided in two zones called as DL/UL access zone and DL transparent zone / UL relay zone. It is divided in to zones to support BS-RS and RS-MS communication. These frames have two different structure, transparent mode frame structure and non transparent mode frame structure.

2.3.3.1 Frame Structure for Transparent Relay

Only two-hop topology is supported by transparent relay mode, where it has only single access zone and one transparent zone in both the DL and UL and a transition gap between the two zones. Scheduling is performed via MAPs that are transmitted by the BS.RS have some small buffering capability such that multiple hops via the relay can be scheduled in different frames. 29

Frame DL Sub-frame DL Sub-frame TTG

NULL

NULL BS Frame BS

Frame DL Sub-frame DL Sub-frame TTG

MS -> BS RS -> BS

BS -> MS RS Frame RS BS -> RS

Figure 5: IEEE 802.16j transparent frame structure.

2.3.3.2 Frame Structure for Non Transparent Relay

More than two hop topology is supported by non transparent relay, where the DL must include at least one DL access zone and may include at least one or more relay zones. The UL on the other hand may include one or more UL access zone and one or more relay zones. It uses centralized or distributed scheduling.

2.3.4 MAC Layer

The BS would use the standard MAC a common interface that makes the networks interoperable to nearly instantaneously allocate uplink and downlink bandwidth to subscribers according to their needs. In IEEE 802.16j, the RS adds MAC protocol to 30

Frame DL Sub-frame DL Sub-frame DL Access Zone DL Relay Zone UL Access UL Relay

TTG Zone Zone BS Frame BS

Frame DL Sub-frame DL Sub-frame DL Access Zone DL Relay Zone UL Access UL Relay TTG Zone Zone

NULL NULL RS Frame RS

Relay TTG Relay RTG

Figure 6: IEEE 802.16j non-transparent frame structure. 31 support multi-hop communication between RS, and also BS must support multiple

RS.

The MAC layer of IEEE 802.16j includes the following phases:

1. Network topology acquisition before handover, which includes network topology

advertisement, and MS scanning or association of neighbor BSs.

2. Handover execution phase, which mainly includes cell reselection, handover

initialization and handshake process, connection release, and target network

reentry. MAC layer provides routing and path management. The relay MAC

(R-MAC) sub-layer has been introduced in the IEEE 802.16j standard. It pro-

vides efficient MAC PDU relaying/forwarding and control functions, (such as

scheduling, routing, and flow control).

It is applicable to the links between MR-BS and RSs and between RSs. The IEEE

802.16 MAC uses a scheduling algorithm for which the SS need compete once (for initial entry into the network). After that it is allocated an access slot by the BS.

The time slot can enlarge and contract, but remains assigned to the SS which means that other subscribers cannot use it.

The IEEE 802.16 scheduling algorithm is stable under overload and over-subscription

(unlike 802.11). It can also be more bandwidth efficient. The scheduling algorithm also allows the BS to control QoS parameters by balancing the time slot assignments among the application needs of the SSs. 32

2.3.5 Routing

Relay path routing is the process of determining the most suitable route to BS by considering constrains such as bandwidth available, radio resource, interference, etc. In centralized path routing, the path information is stored in the BS; however, in distributed path routing, the path information is populated in RSs. The relay path information is embedded in the data burst by the BS using the source routing mechanism, and each RS just navigates the given path from the received data burst.

In order to reduce the latency and use the radio resource effectively, distributed path routing is preferred as the BS using centralized path routing cannot control the whole network. Throughput of the wireless link depends on both the bandwidth of the link and the physical layer loss rate; therefore, the path selection method should take both the loss rate and the link bandwidth into account.

2.3.6 Issues

Handover related issues in IEEE 802.16j networks include excessive scanning and association activities, optimizing scanning interval, efficient exploitation of downlink and uplink signals, wastage of ranging slots, prolonged handover connection disrup- tion time, network re-entry activity due to ping-pong effects, internet protocol (IP) connectivity delay during network re-entry, and optimizing based load distribution.

The IEEE 802.16j MAC protocol was designed for PMP broadband wireless access application. The purpose of this layer is to fragment and segment the MAC service 33 data units into the MAC protocol data units to control the QoS and the scheduling and retransmission of MAC protocol data units. The IEEE 802.16j MAC protocol is connection oriented where it includes inherently connectionless services that are mapped into connection. Each connection is referenced by a 16-bit connection iden- tifier.

Each of the SS has a 48-bit MAC address, but this serves mainly as an equipment identifier, since the primary addresses used during operation are the channel IDs. The

MAC protocol data units are exchanged between the BS and SS, and each of the MAC protocol data units consists of fixed length MAC header, variable length payload, and cyclic redundancy check (CRC). MAC defines two header formats, namely the generic header and bandwidth request header, which are followed by three types of MAC sub headers including the grand management sub header, fragmentation sub header, and packing sub header. An important functionality of MAC common part sub layer is QoS. To ensure QoS bandwidth request, grand schemes are defined to allocate bandwidth in a frame to frame basis. There are two classes of BS to allocate bandwidth: one per connection, called grand per connection, and one per

SS called grand per SS. The grand per SS was recommended by the IEEE 802.16j committee. [24] 34

2.3.7 QoS Scheduling

It is one of the key functionality of the MAC common part sub layer (MAC-

CPS), where it provides QoS constrains to the MAC PDUs. The QoS constrains states latency, jitter, data rate, error rate; system availability should be met for all service flow. IEEE 802.16j standard defines five scheduling services; unsolicited grand service, real time polling service (rtPS), non-real time polling service (nrtPS), best-effort service, and extended real time polling service (ertPS). The main QoS parameters are; maximum sustained rate, maximum latency, and tolerated jitter.

Schedulers are designed to meet four main important criteria such as:

1. The scheduler should use all available unsolicited grand service slots if there is

traffic, it should optimize system throughput.

2. The scheduler should guarantee the delay constraints or maximum latency.

3. It should minimize delay jitter.

4. The scheduler should minimize number of bursts and mobile application part

overhead.

Hence the scheduler first calculates the number of slots to allocate to each SS and selects on which sub-channel and time interval the data will be transmitted.

Schedulers can be broadly classified into channel unaware schedulers and channel aware schedulers. 35

In channel unaware schedulers, the decision is independent of the channel condi- tion and only focuses on ensuring the QoS requirements of the different service classes.

In channel aware approaches on the other hand try to take advantage of the channel condition in order to maximize system throughput.

Two QoS issues are service flow and bandwidth request. A service flow is defined as a one-way flow of MAC service data units on a connection associated with specific

QoS parameters such as latency, jitter, and throughput. There are three basic types of service flows: provisioned service flows, admitted service flows, and active service

flows. The provisional service flow is defined in the system with a service flow iden- tifier. It might not have any traffic presence. In admitted service flow based on the external request from the specified service, the available bandwidth in admitted. In active service flow a service flow will be activated when all the checks are completed and the resources are allocated.

2.4 Previous Work

Many people have tried to enhance throughput, end-to-end delay and fairness, and here are some of their works. 36

2.4.1 Throughput

In [25], Das et al. propose to increase the number of radios and available channels to increase network throughput. In [26], Duffy et al. have presented an analytical model based on finite-load Markov chain to estimate the throughput of terminals when traffic flows are known. The proposal requires detailed knowledge of terminal traffic flows, which is difficult in a small scale network such as WMN. The authors propose to use IEEE 802.11e TXOPs [27] according to the capacity information to ensure fairness in the network.

2.4.2 End-to-End Delay

Several queuing schemes have been analazed to achieve WMN fairness in [28].

The authors state that the simple method of separating original and relayed traffic with adequate weights does not achieve ideal fairness. Ideal fairness requires per flow queuing. The intuition from the single-hop case does not automatically generalize to the multi-hop case. Delay and rate aware scheduling in the context of 802.11-based multi-hop networks has been proposed using a probabilistic approach with priority backoff [29]. However, very little work has been done to deterministically guarantee delays in MWNs; previous work on QoS had focused mainly on guaranteeing rates

[30], [31]. 37

2.4.3 Fairness

In [6] some performance objectives such as maximizing spatial reuse and removing spatial bias have been used to address multi-hop fair access. A distributed algorithm has been used to show the capability of achieving fairness without any modifications to the transport protocol. It has been demonstrated that, by applying the algorithm, a near perfect fairness can be achieved with high throughput for both CSMA and

CSMA/CA. However, the topology used was restricted to a static mesh network with only a single chain of nodes and already known traffic patterns.

A contention window based algorithm has been introduced in [32]. It distinguishes different medium access rates for different nodes by varying their contention window sizes. Although the algorithm improves fairness, it comes at the expense of high computation due to the exchange of load information between neighboring nodes.

In [33], the capacity of WMNs has been formulated in terms of collisions domain which was defined as the set of nodes that need to be inactive in order for the wireless link to transmit successfully. It has been shown that the effective load on a collision domain is less than or equal to the sum of traffic on its links. A constant effective

MAC capacity has been assumed in this study which may not be realistic due to the interference and other physical limitations in MWNs.

A heuristic space time division multiple access (STDMA) algorithm has been pro- posed in [34] to improve the throughput and fairness in WMNs. The technique is 38 difficult to implement mainly due to the centralized nature of STDMA and complex- ities associated with time synchronization along the paths in multi-hop networks [5].

In [35], fairness has been improved through a queue management technique in which an arriving packet at each MP is either admitted into the queue or dropped, depending on its source node granted buffer quota. While the algorithm improved the through- put of nodes located further away from the gateway, it did not solve the fairness problem. A centralized traffic congestion control algorithm to mitigate unfairness has been proposed in [36]. A single path routing has been used that may not be scalable on a multi-path routing due to a large number of potential paths a flow can choose.

Fairness disparity between upstream and downstream traffic in 802.11 has been investigated in [26]. Some features of IEEE 802.11e like TXOP, and minimum con- tention window (CWmin) adjustment, similar to the approach in [26], has been used to restore fairness at relay aggregation points. It has been observed in [37] that, in

MWNs, flows with the same hop count may experience different throughput (symmet- rical unfairness). The authors proposed a distributed routing algorithm to improve this symmetrical unfairness. However, it is not clear if the algorithm can be used to remove spatial bias.

A distributed slot allocation algorithm has been used in [38] to improve fairness in MWNs. While the scheme did not make any assumption about the existence of a centralized coordination of traffic patterns, a two-hop interference region has been assumed. CHAPTER 3

Multi-hop Wireless Network Performance

The multi-hop dilemma can be defined as bandwidth degradation, radio interfer- ence and network latency problems caused by multiple traffic hops within a WMN.

3.1 Bandwidth Degradation

The problem of multi-hop bandwidth degradation is most severe when the back- haul is shared, as in the single and dual radio approaches. In these cases, each time the aggregated traffic hops from AP to AP the throughput is almost cut in half.

There are two main theories for this degradation pattern. Whether you take the best-case theory of 1/n degradation, where n is the number of hops, or the worst case theory of 1/2n−1 degradation, the amount of bandwidth degradation is still substan- tial. The best-case scenario assumes that all of the nodes are arranged in a linear fashion, where each node can only communicate with its two neighboring nodes. In a real-world mesh deployment, any node can hear more than two nodes. This is where bandwidth degradation more closely resembles the worst case scenario.

Our experiments show that the throughput falls between the best-case and worst

39 40

Figure 7: Throughput degradation. case scenarios. Except when the nodes are closer together, the normalized throughput for nodes 2 and 3 is nearly 1.0. When these nodes are near the edge of the transmission range, the throughput becomes less than 1.0, which also negatively affects the other nodes.

3.2 Radio Frequency Interference

RFI is a serious issue that affects the performance of wireless networks. It can be defined simply as undesired signal (or signals) that interfere with the normal operation of other radio communication devices. In todays wireless networks, IEEE

802.11b and 802.11g are the two most common technologies used by enterprises and service providers for wireless user coverage. And since a majority of wireless mesh 41 deployment use IEEE 802.11b for their wireless backhaul infrastructure, it’s easy for their network backhaul bandwidth to become affected by RFI from neighboring devices that operate in the same band. RFI can also cause transmission errors. These errors can compound. Furthermore, it is important to note that interference may vary in different parts of the network. As previously mentioned, the single and dual radio approaches operate on the same backhaul channel across the network, so when any part of the network is affected by interference, the overall network performance degrades. Also, these approaches cannot accommodate configuration changes (for example, adjusting the channel) in that part of the network that will allow it to avoid the interference.

WLANs are not the only radio devices operating in the unlicensed 2.4 GHz and 5

GHz frequency bands. Other types of devices operating in the band include security systems, intercom systems, cordless telephones, and many others. In addition, there are some electronic devices that can leak radio signals in the unlicensed band (for example, microwave ovens, computers, and mobile telephones). These devices can cause interference in a variety of ways. The interference may be temporary (in the case of microwave ovens) or continues (from a wireless video security camera).

3.3 Network Latency

One application that is a key driver for WLAN is voice over internet protocol

(VoIP). The creation and deployment of converged WLAN and IP telephony solutions 42 that support both voice and data services is likely to drive the widespread adoption of WLANs throughout the enterprise. In order to support these voice and video applications, there is very little tolerance for network latency and jitter. As packet traverses the network from node to node, processing delays are naturally introduced.

In large wide-area mesh networks, numerous termination points to the wired network would normally be required in order to avoid excessive delay or latency-but this would defeat the benefits of a WMN. Furthermore, as latency increases, voice and video applications are heavily affected (especially while wireless clients are roaming) this can result in complete connection loss. Another problem arises when using a

Layer 3 (routing) protocol for mesh information. Using the Layer 3 protocol, where the overhead can be small and data friendly for small to medium deployments, it is not well-suited for large scale environments that support large numbers of users who are roaming and making the most of voice and video applications. There are various causes of delay or latency that can affect the overall performance of the wireless network. Some of these causes include chatter, congestion, timeouts and retransmissions. Bandwidth Delay Product is a concept for measuring the capacity of a network link—something to keep in mind when designing and deploying large networks. 43

3.4 Fairness Provisioning Technique

3.4.1 Background on Fairness

A fairness reference model provides a formal idealized objective that can be used as a target and benchmark for protocol design and as a tool for studying alternatives for a networks fairness and performance objectives. For a single wired node, the fairness objective is immediate and is defined by max-min fairness [39] and realized by fair queuing [40]. Yet for a wireless network, even in a simple case with a single

AP, the fairness objective must consider the resource which is to be fairly allocated.

If the resource is throughput, then IEEE 802.11 performance degrades consid- erably as all flows are constrained by the maximum throughput of the bottleneck link [41]. On the other hand, if the resource is time [42], then all users are assured an equal time share of channel access, so that users with high quality channels can obtain throughput gains independent of the channel qualities of others. Likewise, users with poor channels are also guaranteed their fair time share. In this way, time- share fairness provides the desirable performance isolation property and avoids the performance anomaly of throughput fairness. For MWN, there are multiple possible fairness objectives, including extending max-min fairness to multiple resources [39] as targeted in the asynchronous transfer mode (ATM) fairness literature and propor- tional fairness [43] [44] as targeted by transfer control protocol (TCP), for example,

TCP achieves throughput inversely proportional to round-trip-time thereby penaliz- ing longer-distance higher-path-length flows. In MWN, defining the fairness objective 44 must also address contention neighborhoods and variable rate channels.

3.4.2 Jain Fairness Index

The Jain fairness index can be used to quantitatively measure resource sharing and it is independent of the amount of a resource and the number of users. The index of fairness can readily show the performance of a network, and it makes comparisons easy to show. It uses a continuous range from 0 to 1. The formula is:

P 2 ( xi) F airnessIndex = P 2 n xi

Where x is normalized throughput and n is the number of users.

3.4.3 Max-Min Fairness

Max-min fairness is a simple, well-recognized approach to define fairness in net- works [45]; it aims at allocating as much as possible to users with low rates, and, at the same time, not unnecessarily wasting resources. It was used in window flow control protocols [46], then became very popular in the context of bandwidth sharing policies for ABR service in ATM networks [47]. It is now widely used in various areas of networking [48], [46], [49], [50], [51], [52], [53], [54], [55], [56]. One of the simplest max-min fairness examples, given in [45], is single-path rate allocation. Suppose we have a network consisting of links with fixed capacities, and a set of source destina- tion pairs that communicate over a single path each, and with fixed routing. The 45 problem is to allocate a rate to each source-destination pair, while keeping the rate on each link below capacity. Here, we call a rate allocation max-min fair if one cannot increase the rate of a flow without decreasing the rate of an already smaller flow. A definition dual to a max-min fair allocation is min-max fair allocation, and is used in the context of workload distribution, where the goal is to spread a given workload evenly to all the parties [57] and where rates have to be allocated to available links as evenly as possible.

3.4.4 Proportional Fairness

Proportional fairness is a compromise-based scheduling algorithm. Its based upon maintaining a balance between two competing interests: Trying to maximize total wireless network throughput while at the same time allowing all users at least a minimal level of service. This is done by assigning each data flow a data rate or a scheduling priority (depending on the implementation) that is inversely proportional to its anticipated resource consumption. [58] CHAPTER 4

The Simulator

The experiments were conducted in a simulated environment. This chapter de- scribes the software, the simulation models, and the parameters used in the models.

4.1 Software

The data for this thesis were derived from simulation created using the QualNet

5.2 platform. QualNet can be used to create custom network models to predict, among other things, wireless network performance.

Command Graphical User Interface Line Architect Interface Design Visualize Analyzer Packet Tracer File Editor Mode Mode

Model Libraries External Interfaces

HLA IPNE DIS QualNet Simulation Kernel

Figure 8: QualNet architecture.

QualNet provides a comprehensive environment for designing protocols, creat- ing and animating network scenarios, and analyzing their performance. QualNet is

46 47 composed of the following tools:

1. QualNet Architecture: A graphical experiment design and visualization tool.

The architecture has two modes: the design mode, for designing experiments,

and the visualize mode, for running and visualizing experiments.

2. QualNet Analyzer: A graphical statistics analyzing tool.

3. QualNet Packet Tracer: A graphical tool to display and analyze packet traces.

4. QualNet File Editor: A text editing tool.

5. QualNet Command Line Interface: Command line access to the simulator.

The key features of QualNet that enable creating a virtual network environment are:

1. Speed: QualNet can support real-time speed to enable software-in-the-loop,

network emulation, and hardware-in-the-loop modeling. Faster speed enables

model developers and network designers to run multiple what-if analyses by

varying model, network, and traffic parameters in a short time.

2. Scalability: QualNet can model thousands of nodes by taking advantage of

the latest hardware and parallel computing techniques. QualNet can run on

cluster, multi-core, and multi-processor systems to model large networks with

high fidelity. 48

3. Model Fidelity: QualNet uses highly detailed standards-based implementation

of protocol models. It also includes advanced models for the wireless environ-

ment to enable more accurate modeling of real-world networks.

4. Portability: QualNet and its library of models run on a vast array of platforms,

including Windows, Linux, and Mac OS X operating systems, distributed and

cluster parallel architectures, and both 32- and 64-bit computing platforms.

Users can now develop a protocol model or design a network in QualNet on their

desktop or laptop Windows computer and then transfer it to a powerful multi-

processor Linux server to run capacity, performance, and scalability analyses.

5. Extensibility: QualNet can connect to other hardware and software applica-

tions, such as OTB, real networks, and third party visualization software, to

greatly enhancing the value of the network model.

4.2 QualNet Architecture

The kernel of QualNet is a, Scalable Network Technologies-proprietary, parallel discrete-event scheduler. It provides the scalability and portability to run hundreds and thousands of nodes with high-fidelity models on a variety of platforms, from laptops and desktops to high performance computing systems. Users do not directly interact with the kernel, but use the QualNet application programming interface

(API) to develop their protocol models. 49

QualNet includes support for a number of model libraries that enables one to design networks using protocol models developed by Scalable Network Technologies.

QualNet graphical user interface (GUI) consists of Architect, Analyzer, Packet

Tracer, and File Editor. Architect is a network design and visualization tool. It has two modes: design and visualize. In design mode, you can set up terrain, network connections, subnets, mobility patterns of wireless users, and other functional param- eters of network nodes. You can create network models by using intuitive, click and drag operations. You can also customize the protocol stack of any of the nodes. You can also specify the application layer traffic and services that run on the network.

In visualize mode, you can perform in-depth visualization and analysis of a network scenario designed in design mode. As simulations are running, users can watch pack- ets at various layers flow through the network and view dynamic graphs of critical performance metrics. Real-time statistics are also an option, where you can view dynamic graphs while a network scenario simulation is running. You can also assign jobs to run in batch mode on a faster server and view the animated data later. You can perform what-if analysis by setting a range of values for a particular protocol parameter and comparing the network performance results for each of them.

Analyzer is a statistical graphing tool that displays the metrics collected during the simulation of a network scenario in a graphical format. You can customize the graph display. All statistics are exportable to spreadsheets in CSV format. 50

Packet Tracer provides a visual representation of packet trace files generated dur- ing the simulation of a network scenario. Trace files are text files in XML format that contain information about packets as they move up and down the protocol stack.

File Editor is a text editing tool that displays the contents of the selected file in text format and allows the user to edit files.

The QualNet command line interface enables a user to run QualNet from a DOS prompt (in Windows) or from a command window (in Linux or Mac OS X). When

QualNet is run from the command line, input to QualNet is in the form of text

files which can be created and modified using any text editor. Building and running scenarios with the command line interface takes less memory and scenarios typically run faster than with the GUI. With the command line interface the users have the

flexibility to interface with visualization and analysis tools of their choice.

QualNet can also interact with a number of external tools in real-time. The

HLA/DIS module, which is a part of the Standard Interfaces Model Library, al- lows QualNet to interact with other HLA/DIS compliant simulators and computer- generated force (CGF) tools like OTB. The QualNet STK interface, which is a part of the Developer Model Library, provides a way to interface QualNet with the Satel- lite Toolkit developed by Analytical Graphics, Inc. and function in a client-server environment. 51

Figure 9: (a) Linear, and (b) tree.

4.3 Models

4.3.1 Nodes and Topology

Each simulation consists of six nodes. Five of the nodes have some form of data generation, while the remaining node performs as a data sink. we considered two different topologies; line and tree.

These topologies are found in modern networks. For example, linear topologies are used for long distance connections, tree topologies can be found connecting metropoli- tan areas, and meshes are used to knit neighborhoods or campuses together.

The nodes represent any user device that needs to connect to a wireless network.

They also act as access points for other devices, because of the multi-hop functionality of the network.

The table shows the basic parameters for each application. These are the most pertinent settings. 52

Table 1: Scenario input data.

CBR FTP-GEN SOURCE-TRAFFIC Nodes 6 6 6 No. of Packets 6122 6122 N/A Packet Size 1024 1024 512-1024 Pareto 3 Interval 33ms N/A 33-66ms Pareto 3

Used GUI to set up each scenario. Created topology. Adjusted device and net- work parameters. Attached and set-up applications on nodes. Gathered data from generated text files, put into spreadsheet.

4.3.2 Applications

4.3.2.1 Constant Bitrate

Constant bitrate (CBR) is a term used in telecommunications, relating to the quality of service. When referring to codecs, constant bit rate encoding means that the rate at which a codecs output data should be consumed is constant. CBR is useful for streaming multimedia content on limited capacity channels since it is the maximum bit rate that matters, not the average, so CBR would be used to take advantage of all of the capacity. CBR would not be the optimal choice for storage as it would not allocate enough data for complex sections (resulting in degraded quality) while wasting data on simple sections.

The problem of not allocating enough data for complex sections could be solved by choosing a high bitrate (for example, 256 kbit/s or 320 kbit/s) to ensure that there 53 will be enough bits for the entire encoding process, though the size of the file at the end would be proportionally larger.

Most coding schemes, such as Huffman coding or run-length encoding, produce variable-length codes, making perfect CBR difficult to achieve. This is partly solved by varying the quantization (quality), and fully solved by the use of padding. (How- ever, CBR is implied in a simple scheme like reducing all 16-bit audio samples to 8 bits.)

In the case of streaming video as a CBR, the source could be under the CBR data rate target. So in order to complete the stream, its necessary to add stuffing packets in the stream to reach the data rate wanted. These packets are totally neutral and dont affect the stream.

4.3.2.2 File Transfer Protocol

File transfer protocol (FTP) is a standard network protocol used to transfer files from one host or to another host over a TCP-based network, such as the Internet.

FTP is built on client-server architecture and uses separate control and data connec- tions between the client and the server. [2] FTP users may authenticate themselves using a clear-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure trans- mission that hides (encrypts) the username and password, and encrypts the content,

FTP is often secured with SSL/TLS (FTPS). SSH File Transfer Protocol (SFTP) is 54 sometimes also used instead.

4.3.2.3 Source Traffic Model

A source traffic model is a stochastic model of the traffic flows or data sources in a communication network, for example a cellular network or a computer network.

A packet generation model is a traffic generation model of the packet flows or data sources in a packet-switched network. For example, a web traffic model is a model of the data that is sent or received by a users web-browser. These models are useful during the development of telecommunication technologies, in view to analyze the performance and capacity of various protocols, algorithms and network topologies.

4.3.3 Protocols

Protocols describe how devices interact on a network. These experiments focus on four multi-hop wireless technologies: IEEE 802.11s, IEEE 802.11e, IEEE 802.11 and TDMA. In particular, the results are the effects of changing the topologies and applications for each protocol. The following paragraphs show how each protocol was affected by these changes.

IEEE 802.11 is a set of standards for implementing WLAN computer communi- cation in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee (IEEE 802). The base version of 55 the standard IEEE 802.11-2012 has had subsequent amendments. These standards provide the basis for wireless network products using the Wi-Fi brand.

4.3.3.1 IEEE 802.11e

802.11e is an approved amendment to the IEEE 802.11 standard that defines a set of QoS enhancements for WLAN applications through modifications to the

MAC layer. The standard is considered of critical importance for delay-sensitive applications, such as Voice over WLAN and streaming multimedia. The amendment has been incorporated into the published IEEE 802.11-2007 standard.

4.3.3.2 IEEE 802.11s

IEEE 802.11s, as mentioned previously, is an IEEE 802.11 amendment for mesh networking, defining how wireless devices can interconnect to create a WLAN mesh network, which may be used for static topologies and ad hoc networks.

4.3.3.3 TDMA

Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share 56 the same transmission medium (for example, radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital

Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications

(DECT) standard for portable phones. It is also used extensively in satellite systems, combat-net radio systems, and passive optical networks for upstream traffic from premises to the operator; for usage of dynamic TDMA packet mode communication.

TDMA is a type of time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers.

4.3.3.4 TDMA characteristics

TDMA shares single carrier frequency with multiple users and uses non- contin- uous transmission to make handoff simpler. Slots can be assigned on demand in dynamic TDMA. It has less stringent power control than CDMA due to reduced intra- cell interference, and higher synchronization overhead than CDMA. Advanced equalization may be necessary for high data rates if the channel is frequency selective.

TDMA will create inter symbol interference. Cell breathing (borrowing resources from adjacent cells) is more complicated here than in CDMA. TDMA has frequency/slot 57 allocation complexity and it employs a pulsating power envelope which will interfer- ence with other devices.

4.4 Simulation Parameters

4.4.1 Duration

Each of the scenarios ran for 5 simulated minutes during which the test application generates traffic nearly the entire time. Each application starts 1 second after the creation of the simulation. QualNet uses a generated random number to create a more realistic simulation. A seed value of 1 was used for each of these tests.

4.4.2 QoS Metrics

QoS is usually defined as a set of service requirements that needs to be met by the network while transporting a packet stream from a source to its destination. The network is expected to guarantee a set of measurable specified service attributes to the user in terms of end-to-end delay statistics, bandwidth, probability of packet loss, delay variance (jitter), etc. Energy efficiency and service coverage are two other

QoS attributes that are more specific to wireless ad hoc networks due to the limited battery source.

The QoS metrics could be concave or additive. We have gathered two types of bandwidth. First, we have normalize throughput which is calculated by dividing the 58 number of bytes received from the actual bytes that has been sent. Second is the

QualNet throughput, which the simulator calculates internally.

The average end-to-end delay is the accumulation of all delays of the links along the path and fairness index.

4.4.3 Physical Layer

QualNet allows for detailed control over many environmental and simulation set- tings. For these experiments, the simulator’s physical layer was set to emulate 802.11b radio equipment using omni-directional antenna. QualNet can also simulate electro- magnetic noise, which was set to a factor of 10 in these scenarios. The radio networks were set to a 2 Mbps data rate.

4.4.4 MAC Layer

It is in the MAC Layer settings were the protocol could be set for each experiment.

QualNet has all four of the protocols used in these experiments, namely 802.11e,

802.11s, 802.11, and TDMA. The MAC Propagation Delay for all protocols was 1 micro-second. The TDMA protocol required additional parameters, which were 5 milliseconds for the slot duration and 6 slots per frame. 59

4.4.5 Network Layer

In the network layer adjustments, all experiments were set to use the IPv4 network protocol. The IP fragmentation unit was 2048 bytes, in order to keep the experiment packets whole. The IP output queue was set to strict policy, and the priority routing protocol was Bellman Ford. CHAPTER 5

Results

The results for these experiments were gathered from output files generated by

QualNet. The data was put into a spreadsheet, where all of the calculations were performed. The graphs were generated from the spreadsheet results. The following are some of the significant examples from the data.

Through the results of the experiments, it is shown that fairness increases when load on the network decreases. It is even possible to almost reach a fairness index of

1 when the load is only 20%. Jain Fairness Index was used in this metric.

Figure 10: Throughput and normalized throughput.

At first look, it is obvious the average throughput decreases as the load decreases.

And for all of the nodes which make up to 2 hops, this average is accurate.

60 61

Figure 11: Average throughput on varying topologies.

However, nodes that require more hops actually show an improved throughput as the load decreases since they now have more time on the network.

Figure 12: Fairness index of average end-to-end delay on varying topologies.

Additionally, as the load decreases, there are fewer packets in the pipe. This leads to shorter wait times, and ultimately lowers average end-to-end delay. Even though the topology is different, the sink node still must handle the requests from five other 62 nodes as well as its own needs. The average end-to-end remains nearly the same.

However, in linear, as the load increases, the further the node is from the sink, the longer the end-to-end delay. In the tree topology, end-to-end delay times are more evenly distributed for some nodes.

Figure 13: Average end-to-end delay per node on varying topologies.

The number of hops inversely affects the fairness and throughput, while directly affecting the end-to-end delay. Changing the topology to another type will affect the number of hops. However, if the topology is changed from linear to tree, for example, there will now be contention in the network. The contention is because two or more nodes are trying to send to the same node trying to send to the same node.

Contention will also increase the end-to-end delay for a particular node though it won’t affect the average end-to-end delay time.

In IEEE family, IEEE 802.11e performed a slightly better in terms of throughput because it was designed for QoS and IEEE performed better in terms of short end- to-end-delay because it was designed for better route finding.

By looking at this data, we found one of the best ways to solve the fairness issue 63

Figure 14: Average end-to-end delay per node on varying topologies.

Figure 15: Average throughput and Jain fairness index for varying protocols.

Figure 16: Average end-to-end delay and Jain fairness index for varying protocols. 64 is to control the rate for each node in regards to its hop count from the gateway. For example, if the first node sending, node 2 in our experiment, has an inter-arrival time of t and duration of T , then node 3 will have inter-arrival 2t and duration 2T , and so on in a linear fashion. As shown in figure 17, we get a very fair network, in this case with 90% load, where the average normalized throughput is 0.99 fairness overall.

Figure 17: Controlling rate for better fairness. CHAPTER 6

Future Work and Conclusion

6.1 Future Work

While we have studied the impact of network topology, traffic characteristic, and traffic load on MWNs and we have include standards such as IEEE 802.11, IEEE

802.11s, IEEE 802.11e and TDMA. Even though some results were produced, further testing is needed in a couple of areas. Firstly, the simulation software QualNet only provides for IEEE 802.11, 802.11e, and 802.11s, along with TDMA. There is no support yet for IEEE 802.15.5 nor 802.16j. The scope was limited to only the IEEE

802.11 family and TDMA protocols. Secondly, only static SS location was studied.

A more complex simulation which includes multiple MWN protocols and dynamic SS placement, in other words the nodes are moving, may yield more limitation details.

Additionally, each SS used a single radio channel to communicate. It would be good to see the effect on performance of dual radio channel mode as well.

65 66

6.2 Conclusion

Even though MWNs been extensively studied, the full limits of this technology have not been observed. MWNs are currently widely available in various networks.

We know the performance degrades, but we dont know the severity. The fairness and performance of the 802.11 family and TDMA networks carry the most profound information. MWNs must keep hop counts below three, otherwise performance suffers greatly, due to the MAC layer not being designed for multi-hop. The networks are only fair when they are incredibly inefficient; for example 20load will allow all SS an almost equal share of the resources. This is too low for high bandwidth data such as that generated by video conferencing. One goal of MWNs is to increase connectivity, which will make the network more robust, and hopefully reduce the number of hops to a BS. With this connective increase, there is little change to the average end-to-end delay. On surface inspection, this is due to there being only one data sink, only one connection to the larger network. There is, however, and improvement in the distribution of end-to-end delay. Whereas in a linear topology the furtherest node has the longest end-to-end delay, other topologies allow for the nodes to more equally share this burden. As the our results have shown, MWNs have significant limitations for delay- sensitive traffic. Their QoS can only be guaranteed if we closely monitor three aspects: short node distance, low network traffic load, and lowest possible hop counts through changing topology. 68 Topology, traffic source and workload remain to be the major issue in network performance. We focused on 67 infrastructure MWNs for applications such as campus networks, enterprise networks, health care, and many other areas. Our evaluation metrics included throughput and end-to-end delay, along with a fairness index for both. We have shown that MWNs have signaficant limitations for delay-sensitive traffic. There will also be a poor QoS provisioning when MWNs have too many nodes. While this technology will certainly be used becuase of low cost deployment, these limitations will need to be overcome to provide a truly robust network. Glossary

AF assured forwarding. 22, 23

AODV ad hoc on-demand distance vectoring. 19

AP access point. 4, 12–14, 17, 22, 40, 44

APA adaptive power allocation. 29

API application programming interface. 49

ATM asynchronous transfer mode. 44, 45

BS base station. 2, 3, 11, 25–27, 29, 30, 32–34

CBR constant bitrate. 53

CID connection identifier. 25

CRC cyclic redundancy check. 34

CSMA carrier sense multiple access. 6, 38

CSMA/CA carrier sense multiple access with collision avoidance. 12, 14, 38

CWmin minimum contention window. 39

DCF distributed coordination function. 18, 22

68 Glossary 69

DiffServ differentiated services. 22, 23

DL downlink subframe. 29, 30

DSR dynamic source routing. 19

EDCA enhanced distributed channel access. 18

EF expedited forwarding. 22, 23

FCC Federal Communications Commission. 15

FDM frequency division multiplexing. 28

FFT fast Fourier transform. 28

FTP file transfer protocol. 54

GUI graphical user interface. 50

HWMP hybrid wireless mesh protocol. 19

IntServ integrated services. 22, 23

IP internet protocol. 33, 42

LAN local area network. 13, 21

MAC media access control. 6, 11, 12, 17, 19, 23, 24, 30, 32–36, 38 Glossary 70

MAN metropolitan area network. 27

MANET mobile ad hoc network. 2, 5, 7, 8, 19

MCCA mesh coordinated channel access. 11, 18

MIMO multiple input multiple output. 15

MP mesh point. 19, 20, 39

MPNC mesh piconet controller. 24

MWN multi-hop wireless network. 2, 4, 5, 8, 12, 22, 37–39, 44, 66

OFDM orthogonal frequency division multiplexing. 15, 28

OFDMA orthognal frequency division multiple access. 28, 29

OLSR optimized link state routing. 20

PDB per domain behavior. 22

PHB per hop behavior. 22, 23

QoS quality of service. 5, 6, 8, 12, 18, 20, 22, 32, 34–37

R-MAC relay MAC. 32

RAOLSR radio aware optimized link state routing. 20

RF radio frequency. 15, 20 Glossary 71

RFI radio frequency interference. 11, 41, 42

RS relay station. 2, 3, 25–27, 29, 30, 32, 33

SOFDMA scalable orthogonal frequency division multiple access. 27, 28

SS subscriber station. 2, 3, 26, 27, 32, 34, 35, 66

STDMA space time division multiple access. 38, 39

TCP transfer control protocol. 44

TDMA time division multiple access. 8, 9, 27, 66

TXOP transmission opportunity. 18, 37, 39

UL uplink subframe. 29, 30

VoIP voice over internet protocol. 42

WDS wireless distributed system. 10, 13, 17

WiMAX worldwide interoperability for microwave access. 26, 28

WLAN wireless local area network. 4, 5, 15, 17, 22, 42, 43

WMN wireless mesh network. 11, 12, 15, 37, 38, 40, 43

WPAN wireless personal area network. 23, 24 BIBLIOGRAPHY

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