The increasing popularity of wireless networks over the last years indicates that there will be a demand for communicating devices providing high capacity communication together with QoS requirements. There are two types of wireless networks, infrastructure and Ad Hoc networks. The variation of topology caused by the mobility of hosts in the Ad Hoc networks results in a long latency, large jitter and low throughput. In infrastructure wireless networks, a base station (BS) or an Access Point (AP) is in charge of the data transmission. Therefore, the wireless hop can be considered as another hop of the transmission path. With the rapid growth of wireless traffics, the future wireless network is expected to provide services for heterogeneous data traffics with different quality of service requirements. Most proposed schemes do not have mechanisms to adapt to environment changes. In real situation, bandwidths, error rates, and loss rates of wireless links vary frequently. The QoS issues are very important in modern networks. There are many proposed service models and mechanisms to support QoS in wireline networks. Most of these QoS mechanisms are not suitable for direct application to the wireless network because of the characteristics of wireless communication which includes: 1) high error rates and bursty errors, 2) location-dependent and time-varying wireless channel capacity, 3) scarce bandwidth, 4) user mobility, and 5) power constraints of the mobile hosts. All of these above characteristics make the development of QoS in wireless networks very difficult and challenging. We try to cope with the bandwidth variations caused by the high error rate and bursty errors in wireless links, and the location-dependent and time-varying natures of wireless channel capacity. Furthermore, we expect to utilize the scarce wireless bandwidth more efficiently. In our proposed scheme, the higher priority flow is capable of broadcasting a message to inform the lower priority flows to change their priorities to adapt to environment variations. We will base on the differentiated service model and propose a Wireless Differentiation (WD) scheme for UDP flows and a Wireless Differentiation with Prioritized ACK (WDPA) scheme for connections with TCP flows which provide QoS support for IEEE 802.11b and do not change the basic access mechanism of IEEE 802.11b.

Το ζήτημα της κατανάλωσης ενέργειας σε ασύρματα ad hoc δίκτυα αποτελεί ένα ερευνητικό πεδίο το οποίο έχει λάβει ιδιαίτερη προσοχή τα τελευταία χρόνια. Οι σταθμοί σε ένα ασύρματο ad hoc δίκτυο λειτουργούν επί το πλείστον με μπαταρίες με αποτέλεσμα η εξοικονόμηση ενέργειας στο δίκτυο να αποτελεί πρώτιστο στόχο για την βιωσιμότητα των σταθμών του δικτύου και την αξιόπιστη παραλαβή των πακέτων δεδομένων από τους παραλήπτες τους. Μέχρι σήμερα, έχουν αναπτυχθεί διάφορα ενεργό-αποδοτικά πρωτόκολλα τα οποία μπορούν να υιοθετηθούν σε ένα ασύρματο ad hoc δίκτυo. Τα πρωτόκολλα αυτά λειτουργούν σε διαφορετικό επίπεδο το καθένα (ελέγχου πρόσβασης στο μέσο, δικτύου, υψηλότερα). Η παρούσα εργασία συντάχθηκε με σκοπό την μελέτη αυτών των ενεργό-αποδοτικών πρωτοκόλλων που μπορούν να υιοθετηθούν σε ασύρματα ad hoc δίκτυα, την καταγραφή των ιδιαίτερων χαρακτηριστικών κάθε πρωτοκόλλου καθώς και την ανάπτυξη ενός νέου πρωτοκόλλου ελέγχου της ισχύος μετάδοσης σε ασύρματα ad hoc δίκτυα με κύριο στόχο την περαιτέρω μείωση της κατανάλωσης ενέργειας στο δίκτυο με παράλληλη αύξηση του throughput του δικτύου. Συγκεκριμένα, στην παρούσα μελέτη, αναλύονται και παρουσιάζονται τα ακόλουθα ζητήματα. Στο Κεφάλαιο 1 παρουσιάζονται οι στόχοι της παρούσας εργασίας καθώς και η συνεισφορά της στο ζήτημα της κατανάλωσης ενέργειας στα ασύρματα ad hoc δίκτυα. Στο Κεφάλαιο 2 αρχικά πραγματοποιείται μία εισαγωγή στην αρχιτεκτονική των ασυρμάτων ad hoc δικτύων και ακολούθως παρουσιάζονται οι διάφορες κατηγορίες των ασυρμάτων ad hoc δικτύων, δίνοντας ιδιαίτερη έμφαση στην 802.11 τεχνολογία και πρωτόκολλα. Στο Κεφάλαιο 3 παρουσιάζονται υπάρχοντα ενεργό-αποδοτικά πρωτόκολλα τα οποία μπορούν να χρησιμοποιηθούν σε ασύρματα ad hoc δίκτυα. Τα πρωτόκολλα αυτά παρουσιάζονται ταξινομημένα ανάλογα με το επίπεδο στο οποίο το καθένα λειτουργεί (επίπεδο ελέγχου πρόσβασης στο μέσο, επίπεδο δικτύου, υψηλότερα επίπεδα). Στο Κεφάλαιο 4 περιγράφεται το προτεινόμενο MAC πρωτόκολλο ελέγχου της ισχύος μετάδοσης το οποίο μπορεί να χρησιμοποιηθεί σε ασύρματα ad hoc δίκτυα. Παρουσιάζεται η λειτουργία του πρωτοκόλλου, περιγράφοντας το χαρακτηριστικό της αργής εκκίνησης που το προτεινόμενο πρωτόκολλο υιοθετεί για την μετάδοση των RTS πλαισίων, καθώς και τον CTS μηχανισμό που χρησιμοποιείται. Στο τέλος του κεφαλαίου, πραγματοποιείται μια σύγκριση της λειτουργίας του προτεινόμενου ενεργό-αποδοτικού πρωτοκόλλου με υπάρχοντα πρωτόκολλα MAC επιπέδου που παρουσιάστηκαν στο Κεφάλαιο 3. Στο Κεφάλαιο 5 πραγματοποιείται αξιολόγηση της απόδοσης του προτεινόμενου πρωτοκόλλου. Η αξιολόγηση της απόδοσης του προτεινόμενου πρωτοκόλλου πραγματοποιείται μέσω της εκτέλεσης πειραμάτων με την βοήθεια του εξομοιωτή δικτύων network simulator 2. Η αξιολόγηση του προτεινόμενου πρωτοκόλλου πραγματοποιείται ως προς διάφορες παραμέτρους – κατανάλωση ενέργειας στο δίκτυο, διασπορά της καταναλόμενης ενέργειας, μέση καθυστέρηση παραλαβής των πακέτων, throughput του δικτύου, πλήθος συγκρούσεων RTS πλαισίων, λόγος ληφθέντων προς απεσταλμένα πακέτα – έχοντας ως βάση το πρόβλημα της εκκένωσης των πακέτων στο δίκτυο και για την περίπτωση δύο διαφορετικών προσεγγίσεων. Μία στην οποία κάθε σταθμός χρησιμοποιεί στατική ισχύ για τις μεταδόσεις του και μία στην οποία κάθε σταθμός ρυθμίζει την ισχύ μετάδοσής του στην ελάχιστη απαιτούμενη τιμή για την ορθή παραλαβή των πακέτων από τους παραλήπτες τους. Τέλος στο Κεφάλαιο 6 παρουσιάζονται τα συμπεράσματα που προκύπτουν από την παρούσα εργασία καθώς και διάφορα ανοιχτά ζητήματα προς περαιτέρω έρευνα. / Power consumption at wireless ad hoc networks has received last years much attention. Mobile stations in a wireless ad hoc network operate in the most cases with batteries, so power conservation at the network is a major objective for the viability of the mobile stations of the network and the reliable delivery of packets to their destinations. Till today, various power-aware protocols have been developed for wireless ad hoc networks. Each one of these protocols operates at different layer (medium access control, network, higher layers). The main objectives of this study are the review of the power-aware protocols that can be used at wireless ad hoc networks, the detection of the special characteristics of each protocol, and the development of a new power control MAC-layer protocol for wireless ad hoc networks. The main objective of this new protocol is the farther reduction of the power consumption in the network, with parallel increment at network throughput. More specifically speaking, in this study, the following points are represented and analyzed: in Chapter 1 the various objectives of this study are represented as the contribution of the study in point of power consumption at wireless ad hoc networks. In Chapter 2, initially an introduction at the architecture of wireless ad hoc networks is performed, while afterwards the various categories of wireless ad hoc networks are represented, by emphasizing the 802.11 technology and protocols. In Chapter 3 various energy-aware protocols for wireless ad hoc networks are represented. These protocols are represented categorized by the layer at which every one operates (medium access control layer, network layer and higher layers). In Chapter 4 the proposed power control MAC protocol for mobile ad hoc networks is represented. The operation of the protocol is represented, by describing the slow start mechanism used for RTS frames transmissions, and the CTS mechanism used. At the end of Chapter 4, a comparison between the operation of the proposed power-aware protocol and previous MAC-layer power aware protocols is performed. In Chapter 5, the performance evaluation of the proposed protocol is performed. The performance evaluation of the proposed protocol was done via execution of experiments with network simulator 2. The performance parameters that were measured are: the power consumption in the network, the variance of the consumed energy, the average delay for packets delivery, the network throughput, the number of RTS frames collisions and the received-to-sent packets ratio. The performance of the proposed protocol was evaluated in the setting of the evacuation problem and for two different approaches. One in which every node uses static power for its transmissions and one in which every node adjusts its transmission power at the minimum required value for coherent reception of the packets from the recipients. Finally in Chapter 6 are represented the results that are derived from this study and various open issues for further investigation.

Hop count, i.e., the number of wireless hops a packet has to go through to reach the destination, is a fundamental metric in multi-hop wireless ad-hoc networks. Network performance, such as throughput, end-to-end delay, energy consumption, and so on, depends critically on hop count. Previous work on modeling hop count is limited in making unrealistic simplifying assumptions either at the physical or network, or both layers of the communication protocol stack. A key contribution of this thesis is to present an analytical model to derive the probability distribution of hop count under realistic assumptions at both physical and network layers. Specifically, the model considers a log-normal shadowing radio propagation capable of accommodating the random signal fading observed in most wireless communication environments, and the widely used geographic routing at the network layer. Validation of the model is achieved by a comprehensive set of simulation experiments including a trace driven simulation of a real-word vehicular ad-hoc network. The model reveals that the presence of randomness in radio propagation reduces the required number of hops to reach a given destination significantly. To demonstrate the utility of the proposed hop count model, the thesis proposes three new applications which address some of the key challenges in multi-hop wireless networks. The first application derives the per-node packet forwarding load in multi-hop wireless sensor networks and reveals that the nodes in the vicinity of the base station has a significantly less forwarding load than previously thought under simplifying radio propagation and routing assumptions. The second application demonstrates that using hop count as a measure of distance traveled by a data packet, geocasting can be achieved in multi-hop wireless networks in situations when some of the network nodes do not have access to reliable location information. Finally, the proposed hop count model is used to evaluate the performance of the third application which demonstrates that the overhead of geographic routing can be reduced significantly by embracing a position update philosophy which adapts to the mobility and communication patterns of the underlying ad-hoc network.

Hop count, i.e., the number of wireless hops a packet has to go through to reach the destination, is a fundamental metric in multi-hop wireless ad-hoc networks. Network performance, such as throughput, end-to-end delay, energy consumption, and so on, depends critically on hop count. Previous work on modeling hop count is limited in making unrealistic simplifying assumptions either at the physical or network, or both layers of the communication protocol stack. A key contribution of this thesis is to present an analytical model to derive the probability distribution of hop count under realistic assumptions at both physical and network layers. Specifically, the model considers a log-normal shadowing radio propagation capable of accommodating the random signal fading observed in most wireless communication environments, and the widely used geographic routing at the network layer. Validation of the model is achieved by a comprehensive set of simulation experiments including a trace driven simulation of a real-word vehicular ad-hoc network. The model reveals that the presence of randomness in radio propagation reduces the required number of hops to reach a given destination significantly. To demonstrate the utility of the proposed hop count model, the thesis proposes three new applications which address some of the key challenges in multi-hop wireless networks. The first application derives the per-node packet forwarding load in multi-hop wireless sensor networks and reveals that the nodes in the vicinity of the base station has a significantly less forwarding load than previously thought under simplifying radio propagation and routing assumptions. The second application demonstrates that using hop count as a measure of distance traveled by a data packet, geocasting can be achieved in multi-hop wireless networks in situations when some of the network nodes do not have access to reliable location information. Finally, the proposed hop count model is used to evaluate the performance of the third application which demonstrates that the overhead of geographic routing can be reduced significantly by embracing a position update philosophy which adapts to the mobility and communication patterns of the underlying ad-hoc network.

Network coding is a technique used to increase the capacity of a network by combining messages sent over the network. The combined messages could be separated by using sufficient original messages which were used to combine the messages. Network coding can be implemented in different layers of the 051 stack, but to date a complete comparison between different implementations of network coding has not been done. The goal of this dissertation is to implement a wireless node model with network coding in the MAC layer and evaluate the performance characteristics of reference networks that implement the new node model. This will serve as the first step of a greater goal, namely finding the most favourable position in the 051 stack to implement network coding. The characteristics of the different implementations of network coding are presented in this dissertation. Simulations were done in OPNET® to find further attributes concerning the implementation of network coding in the MAC layer. The simulation process used is presented and explained, and the results from the simulations are analysed. Network coding in the simulations was implemented opportunistically. The results show that the more often different nodes send frames to the coding node, the better network coding performs. The work contributes to finding the best layer for implementing network coding for its increased throughput. A benchmark network was created so that network coding could be implemented in all the layers of the 051 stack, and then be compared to each other. An implementation of network coding in the MAC layer was simulated and analyzed. We conclude that, because there are so many different purposes for which networks are used, a single instance of network coding is unlikely to be similarly beneficial to all purposes. There still remains work to find the most favourable position for network coding in the 051 stack for all the different types of network coding. / Thesis (M. Ing. (Computer and Electronical Engineering))--North-West University, Potchefstroom Campus, 2011

A wireless ad hoc network consists of a number of mobile nodes that temporarily form a dynamic infrastructure-less network. New routing protocols that can adapt to the frequent topology changes induced by node mobility and varying link qualities are needed. During the last decade dozens of different ad hoc routing protocols have been proposed, optimized and partially compared, mainly through simulation studies. This thesis takes an experimental approach to the evaluation of ad hoc routing protocols. We argue that real world experiments are needed in order to complement simulation studies, and to gain practical experience and insights that can provide feedback to routing protocol design and existing simulation models. For example, we discovered a performance discrepancy for the AODV protocol between real world experiments and corresponding simulation studies. This so called ``communication gray zone'' problem was explored and countermeasures were implemented. As a result we could eliminate this performance problem to a large extent. We have implemented a software-based testbed called APE to carry out efficient and systematic experimental evaluation of ad hoc routing protocols. Experiments with up to 37 participating ad hoc nodes have demonstrated APE's ability to scale efficiently and assess repeatability between test runs. APE is part of our methodology for test repeatability in a real world ad hoc routing protocol testbed. It addresses the repeatability issue induced by stochastic factors like the radio environment and node mobility. Using APE, we have performed a systematic experimental evaluation of three ad hoc routing protocols (AODV, OLSR and LUNAR). Our results show that TCP does not work satisfactorily even in very small networks with limited mobility.

Network coding is a technique used to increase the capacity of a network by combining messages sent over the network. The combined messages could be separated by using sufficient original messages which were used to combine the messages. Network coding can be implemented in different layers of the 051 stack, but to date a complete comparison between different implementations of network coding has not been done. The goal of this dissertation is to implement a wireless node model with network coding in the MAC layer and evaluate the performance characteristics of reference networks that implement the new node model. This will serve as the first step of a greater goal, namely finding the most favourable position in the 051 stack to implement network coding. The characteristics of the different implementations of network coding are presented in this dissertation. Simulations were done in OPNET® to find further attributes concerning the implementation of network coding in the MAC layer. The simulation process used is presented and explained, and the results from the simulations are analysed. Network coding in the simulations was implemented opportunistically. The results show that the more often different nodes send frames to the coding node, the better network coding performs. The work contributes to finding the best layer for implementing network coding for its increased throughput. A benchmark network was created so that network coding could be implemented in all the layers of the 051 stack, and then be compared to each other. An implementation of network coding in the MAC layer was simulated and analyzed. We conclude that, because there are so many different purposes for which networks are used, a single instance of network coding is unlikely to be similarly beneficial to all purposes. There still remains work to find the most favourable position for network coding in the 051 stack for all the different types of network coding. / Thesis (M. Ing. (Computer and Electronical Engineering))--North-West University, Potchefstroom Campus, 2011

The applications of ad hoc wireless networks envisaged in this thesis are those related to issues of disaster management, rehabilitation, security and defense. The circumstances in such situations warrants the deployment of a quick ad hoc network that is simple and uses minimum resoures to get started. The communication within the network has to be reliable and it has to be simple so that it can be deployed in extremely compex topography and other climatic conditions. Since large batteries cannot be assumed to be at our disposal for the sake of communication at all the times, energy conservation by way of energy efficient schemes is a paramount issue. Ad hoc wireless networks are broadcast networks by nature. For all the communications, transmissions by the nodes are broadcast into the air. A networkwide broadcast is distinguished from this. When a node wants its data to reach all the other nodes in the network then it initiates a networkwide broadcast. There may be nodes in the network that are not directly reachable by the node that wants to do a networkwide broadcast. Networkwide broadcast is used by ad hoc wireless networks for routing protocols, updating of network status information, network organization and multicasting. Most importantly, the applications envisaged out of this thesis need all their data communication as networkwide broadcast alone. In an ad hoc wireless network, a networkwide broadcast is usually effected by the flooding mechanism, which is inherently inefficient, since all the nodes in the network have to transmit the same information. It is possible to exploit the topology of the network in such a way, that only a few of the nodes need to transmit the information to complete a networkwide broadcast. The thesis deals with a new scheme for a networkwide broadcast implemented in the media access control (MAC) layer of an ad hoc wireless network. The new scheme is developed by extending the concept of handshaking signals used in unicast, to the networkwide broadcast scenario. In the case of unicast, where there is an intended recipient, handshaking is done for reliability and happens through the RTS and CTS packets. This idea is extended to suit the networkwide broadcast scenario and the consequences are discussed in detail in the thesis. Intuitively, adding more packets for handshaking increases the number of bytes transmitted. But the results obtained are interesting, since the network transmits fewer bytes per networkwide broadcast, on an average, with the newly proposed scheme. A comparison is done with the implementation of simple flooding following the IEEE 802.11 standard. These results have been demonstrated by simulations. The average improvement is nearly 2.5 times reduction in the number of bytes transmitted per networkwide broadcast. The performance of a networkwide broadcast in an ad hoc wireless network is usually affected badly by losses due to transmssion error in the medium. In a medium with errors, persistence improves reliability. This reliability helps in bringing robustness. The advantage of the proposed scheme is that it uses the idea of persistence to ensure the networkwide broadcast reachabilityto be almost independent of transmission error rate. The MAC layer ensures that the broadcast packet reaches each and every node that is connected to the node that initiates the etworkwide broadcast. The effects of collision are also overcome. Our simulations establish that the scheme works correctly, and gives good performance.

<p>A <i>wireless ad hoc network </i>consists of a number of mobile nodes that temporarily form a dynamic infrastructure-less network. New routing protocols that can adapt to the frequent topology changes induced by node mobility and varying link qualities are needed. During the last decade dozens of different ad hoc routing protocols have been proposed, optimized and partially compared, mainly through simulation studies.</p><p>This thesis takes an experimental approach to the evaluation of ad hoc routing protocols. We argue that real world experiments are needed in order to complement simulation studies, and to gain practical experience and insights that can provide feedback to routing protocol design and existing simulation models. For example, we discovered a performance discrepancy for the AODV protocol between real world experiments and corresponding simulation studies. This so called ``communication gray zone'' problem was explored and countermeasures were implemented. As a result we could eliminate this performance problem to a large extent. </p><p>We have implemented a software-based testbed called APE to carry out efficient and systematic experimental evaluation of ad hoc routing protocols. Experiments with up to 37 participating ad hoc nodes have demonstrated APE's ability to scale efficiently and assess repeatability between test runs. APE is part of our methodology for test repeatability in a real world ad hoc routing protocol testbed. It addresses the repeatability issue induced by stochastic factors like the radio environment and node mobility. Using APE, we have performed a systematic experimental evaluation of three ad hoc routing protocols (AODV, OLSR and LUNAR). Our results show that TCP does not work satisfactorily even in very small networks with limited mobility.</p>

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