Nawaporn Wisitpongphan - Profile on Academia.edu (original) (raw)

Papers by Nawaporn Wisitpongphan

Smart automotive lighting for vehicle safety

IEEE Communications Magazine, 2000

ABSTRACT It is believed that vehicle-to-vehicle (V2V) communications and accurate positioning wit... more ABSTRACT It is believed that vehicle-to-vehicle (V2V) communications and accurate positioning with sub-meter error could bring vehicle safety to a different level. However, to this date it is still unclear whether the envisioned V2V standard, dedicated short-range communications, can become available in commercially available vehicle products, while widely available consumergrade GPS receivers do not provide the required accuracy for many safety applications. In this article, combining visible light communications and visible light positioning, we propose the use of smart automotive lighting in vehicle safety systems. These lights would be able to provide the functions of illumination and signaling, reliable communications, and accurate positioning in a single solution. The proposed solution has low complexity, and is shown to be scalable in high vehicle density and fast topology changing scenarios. In this article, we also present several design guidelines for such a system, based on the results of our analytic and empirical studies. Finally, evaluation of our prototype provides evidence that the system can indeed detect potential risks in advance and provide early warnings to the driver in real-world scenarios, lowering the probability of traffic accidents.

It is now well known that Internet traffic exhibits self-similarity, which cannot be described by... more It is now well known that Internet traffic exhibits self-similarity, which cannot be described by traditional Markovian models such as the Poisson process. In this work, we simulate a simple network with a full implementation of TCP-Reno. We also assume Poisson arrivals at the application layer specifically to determine whether TCP can cause self-similarity even when input traffic does not exhibit long-range dependence. Our study shows that, at some loads TCP can induce the appearance of self-similarity. In particular, when load is low and loss is rare, traffic looks Poisson. When load is high and the network is overloaded, TCP congestion control can smooth out the burstiness of the aggregate stream so that traffic at the bottleneck tends to Poisson. However, when load is intermediate and the network is prone to occasional bouts of congestion, as is typical of many networks, traffic can become self-similar. Moreover, factors such as round trip time and number of streams passing through the bottleneck can cause the network to become congested at different loads, and consequently affect the range of load over which self-similarity can be observed. The impact of one self-similar TCP stream has also been observed. In particular, our study show that if one or more streams passing through the bottleneck is self-similar and the aggregate flow does not exceed the capacity, traffic observed at the bottleneck will also be selfsimilar.

Mobile ad hoc wireless networks have been extensively studied as they make possible various inter... more Mobile ad hoc wireless networks have been extensively studied as they make possible various interesting future applications. However, most of the existing studies focus on showing the overall impact of mobility from topology changes and signal variations, while they do not clearly show how inter-operability among protocol layers affects the network performance. To address this issue, we developed a testbed which enables us to investigate the cross-layer interaction among protocol layers at Carnegie Mellon University (CMU). In this paper, we report on the testbed as well as on our experiences and findings from it.

In this paper, we compare the performance of two possible switching techniques for ad hoc wireles... more In this paper, we compare the performance of two possible switching techniques for ad hoc wireless networks: namely, reservation-based (RB) and non-reservation-based (NRB) switching. Fundamental network performance metrics, such as throughput and delay, are studied to understand the tradeoffs between the two switching schemes. In particular, we identify the circumstances where each of these two schemes is better than the other.

Mobile ad hoc wireless networks have been extensively studied as they make possible various inter... more Mobile ad hoc wireless networks have been extensively studied as they make possible various interesting future applications. However, most of the existing studies focus on showing the overall impact of mobility from topology changes and signal variations, while they do not clearly show how inter-operability among protocol layers affects the network performance.

In this paper, we report the first complete version of a multi-hop broadcast protocol for vehicul... more In this paper, we report the first complete version of a multi-hop broadcast protocol for vehicular ad hoc networks (VANET). Our results clearly show that broadcasting in VANET is very different from routing in mobile ad hoc networks (MANET) due to several reasons such as network topology, mobility patterns, demographics, traffic patterns at different times of the day, etc. These differences imply that conventional ad hoc routing protocols such as DSR and AODV will not be appropriate in VANETs for most vehicular broadcast applications. We identify three very different regimes that a vehicular broadcast protocol needs to work in: i) dense traffic regime; ii) sparse traffic regime; and iii) regular traffic regime. We build upon our previously proposed routing solutions for each regime and we show that the broadcast message can be disseminate efficiently. The proposed design of the distributed vehicular broadcast (DV-CAST) protocol integrates the use of various routing solutions we have previously proposed.

Disjoint multipath source routing in ad hoc networks: transport capacity

While much of the effort on routing in ad hoc wireless networks is focused on source routing and ... more While much of the effort on routing in ad hoc wireless networks is focused on source routing and flooding, in this work we explore the possibility of applying packet switching concept to an ad hoc wireless environment. It is well known that packet switching is a robust and efficient technique for data that can tolerate some delay. Similar to the traditional packet switching technique, we propose a disjoint multipath source routing (DMPSR) technique which allows packets originating from the same source to be statistically multiplexed onto multiple disjoint routes. Each route can relay packets from multiple sources. In this paper, we analyze the transport capacity of this scheme and compare the result to that obtained in P. Gupta et al. (2000) and O.K. Tonguz et al. (2003), which consider single path source routing. Given a network of size A with N mobile nodes, each capable of transmitting at R bps, our results show that the maximum transport capacity of this scheme converges to R√(AN) when load balancing is employed (i.e., number of routes being relayed through any given node is the same for all the nodes). If, however, no load balancing is used, then the transport capacity of DMPSR will be greater than that of the traditional source routing when the spatial density of the network is below some critical threshold. Beyond the critical threshold, however, transport capacity is significantly less than the one obtained in O.K. Tonguz et al. ( 2003) and P. Gupta et al. (2000) since a mobile node has to spend more time relaying messages for other sources.

In this paper, the performance of two possible switching schemes for ad hoc wireless networks, na... more In this paper, the performance of two possible switching schemes for ad hoc wireless networks, namely reservation-based (RB) and non-reservation-based (NRB) is investigated. It is shown that route reservation would be a scheme of choice in a scenario where delay is a constraint, the network is dense, and/or nodes move with moderate speed. However, a non-reservation scheme is preferable in a scenario where the network is sparse, there is heavy traffic load, nodes are static, and/or delay is not a major concern.

In order to guarantee a certain level of quality of service (QoS), routing protocol should be sma... more In order to guarantee a certain level of quality of service (QoS), routing protocol should be smart enough to pick a stable and good quality communication route in order to avoid any unnecessary packet loss. Intuitively, if the chosen communication route is error-prone, it is very likely that the transmitted packets over consecutive links may get corrupted or even lost. It is well-known that retransmissions incur additional delay and increase the overhead in the network. This is very undesirable for delay sensitive applications, especially in an ad hoc wireless network environment where devices are typically power-limited and topology is constantly changing. Unlike most of the existing routing protocols for ad hoc wireless networks, in which the basic criterion for route selection is the minimization of the overall route length, in this paper we consider bit error rate (BER)-based routing, where the chosen route is the one which guarantees the lowest BER at the ending node. In other words, we consider providing QoS in terms of BER at the destination node. Our analysis shows that, according to this criterion, shortest path (SP) routing is not always the optimal choice as it often selects a path with unacceptable BER which can directly affect the end-to-end packet delay and goodput.

Current ad hoc wireless routing protocols typically select the route with the shortest-path or mi... more Current ad hoc wireless routing protocols typically select the route with the shortest-path or minimum hop count to the destination. However, routes chosen according to such criteria tend to include longer hop-length links with possibly bad signal quality. It is expected that the routing protocol can be improved by eliminating the usage of routes with bad links. In this work, we modify the ad hoc on-demand distance vector (AODV) routing protocol to avoid routing through bad quality links. We also integrate the "hand-off" concept into our protocol to prevent link breakages during route maintenance. The herein reported OPNET simulation results seem very promising and they show that our protocol introduces a much lower routing overhead while still providing equal or better performance than the original AODV protocol in terms of throughput and delay.

IEEE Wireless Communications, 2007

Accident (a) Accident Accident Forward with highest p (b) Accident T = 0 T = τ T Accident

In this paper, we propose a simple semi-analytical approach for the evaluation of the impact of m... more In this paper, we propose a simple semi-analytical approach for the evaluation of the impact of mobility on the bit error rate (BER) performance of multi-hop ad hoc wireless networks. Analytical expressions, relating the BER at the end of a multi-hop route with the mobility characteristics of the nodes and the routing strategy, are derived. Two node mobility models are considered: direction-persistent (DP) and direction-nonpersistent (DNP). In particular, two network switching scenarios are analyzed: (i) opportunistic non-reservation-based switching (ONRBS), where a message flows from source to destination by opportunistically choosing the available shortest consecutive links; and (ii) reservation-based switching (RBS), where, after the creation of a multi-hop route from source to destination, the message is "forced" to flow over the reserved links, regardless of their actual lengths. The network performance is evaluated in ideal (without inter-node interference, INI) and realistic (with INI) cases. The improved robustness against mobility offered by ONRBS, with respect to RBS, is analyzed and quantified.

IEEE Transactions on Mobile Computing, 2007

This paper investigates whether and when route reservation-based (RB) communication can yield bet... more This paper investigates whether and when route reservation-based (RB) communication can yield better delay performance than non-reservation-based (NRB) communication in ad hoc wireless networks. In addition to posing this fundamental question, the requirements (in terms of route discovery, medium access control (MAC) protocol, and pipelining, etc.) for making RB switching superior to NRB switching are also identified. A novel analytical framework is developed and the network performance under both RB and NRB schemes is quantified. It is shown that if the aforementioned requirements are met, then RB schemes can indeed yield better delay performance than NRB schemes. This advantage, however, comes at the expense of lower throughput and goodput compared to NRB schemes

Routing protocols developed for ad hoc wireless networks use broadcast transmission to either dis... more Routing protocols developed for ad hoc wireless networks use broadcast transmission to either discover a route or disseminate information. More specifically, reactive routing protocols has to flood the network with a route request (RREQ) message in order to find an optimal route to the destination. Several applications developed for vehicular ad hoc wireless networks (VANET), which is a subset of MANET, rely on broadcast to propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem.

IEEE Transactions on Mobile Computing, 2007

This paper investigates whether and when route reservation-based (RB) communication can yield bet... more This paper investigates whether and when route reservation-based (RB) communication can yield better delay performance than non-reservation-based (NRB) communication in ad hoc wireless networks. In addition to posing this fundamental question, the requirements (in terms of route discovery, medium access control (MAC) protocol, and pipelining, etc.) for making RB switching superior to NRB switching are also identified. A novel analytical framework is developed and the network performance under both RB and NRB schemes is quantified. It is shown that if the aforementioned requirements are met, then RB schemes can indeed yield better delay performance than NRB schemes. This advantage, however, comes at the expense of lower throughput and goodput compared to NRB schemes.

Scalable broadcast strategies for ad hoc routing protocols

In ad hoc wireless networks, routes are discovered either by means of a periodic update between 1... more In ad hoc wireless networks, routes are discovered either by means of a periodic update between 1-hop neighbors or by using a reactive routing protocol which relies on an explicit route discovery process. Although the major advantage of a reactive protocol is the reduction in the routing overhead, the conventional flooding mechanism used to propagate the route request (RREQ) message during the route discovery process does not scale well to large networks. In addition to the overhead problem, the way in which the route request gets propagated across the network affects the network performance in two different ways: (i) it determines how fast the route can be acquired; (ii) it is also an important factor in determining the availability and quality of the route. In this paper, we propose three novel RREQ broadcast techniques, i.e., weighted p-persistence, slotted p-persistence, and slotted p-persistence schemes. Our simulation results show that the proposed schemes can achieve up to 75% routing overhead reduction while keeping the route acquisition delay close or equal to the conventional flooding scheme.

IEEE Journal on Selected Areas in Communications, 2007

A Vehicular Ad Hoc Network (VANET) may exhibit a bipolar behavior, i.e., the network can either b... more A Vehicular Ad Hoc Network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on I-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time).

Previous work on mobile ad hoc wireless networks has primarily focused on studying the impact of ... more Previous work on mobile ad hoc wireless networks has primarily focused on studying the impact of node mobility on the performance of various routing protocols and the development of mobility models to represent node mobility. This is due to the traditional viewpoint that the main reason for degradation in network performance as a result of node mobility is due to the traffic control overhead required for maintaining accurate routing tables in the case of table-driven protocols and maintaining routes in the case of on-demand protocols [X. Hong et al., 2001]. Little work exists on the effect of mobility on fundamental communication and network performance metrics such as the bit error rate (BER) of a multi-hop route joining a source-destination pair, and minimum required node spatial density of an ad hoc wireless network for full connectivity. This paper discusses the impact of mobility on these parameters and points out their importance in real-life scenarios for average pedestrian and vehicular speeds. We quantify the effect of node mobility and message length on the BER of an average multi-hop route using a recently developed communication-theoretic framework for ad hoc wireless networks [O.K. Tonguz et al., Feb 2003]. BER of an average multi-hop route directly affects the ability of an ad hoc wireless network to support applications requiring a specific BER, for a given node transmission power and node spatial density.

Vehicular Ad Hoc Wireless Network (VANET) exhibits a bipolar behavior in terms of network topolog... more Vehicular Ad Hoc Wireless Network (VANET) exhibits a bipolar behavior in terms of network topology: fully connected topology with high traffic volume or sparsely connected topology when traffic volume is low. In this work, we develop a statistical traffic model based on the data collected on I-80 freeway in California in order to study key performance metrics of interest in disconnected VANETs, such as average re-healing time (or the network restoration time). Our results show that, depending on the sparsity of vehicles, the network re-healing time can vary from a few seconds to several minutes. This suggests that, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times.

IEEE Wireless Communications, 2010

Smart automotive lighting for vehicle safety

IEEE Communications Magazine, 2000

ABSTRACT It is believed that vehicle-to-vehicle (V2V) communications and accurate positioning wit... more ABSTRACT It is believed that vehicle-to-vehicle (V2V) communications and accurate positioning with sub-meter error could bring vehicle safety to a different level. However, to this date it is still unclear whether the envisioned V2V standard, dedicated short-range communications, can become available in commercially available vehicle products, while widely available consumergrade GPS receivers do not provide the required accuracy for many safety applications. In this article, combining visible light communications and visible light positioning, we propose the use of smart automotive lighting in vehicle safety systems. These lights would be able to provide the functions of illumination and signaling, reliable communications, and accurate positioning in a single solution. The proposed solution has low complexity, and is shown to be scalable in high vehicle density and fast topology changing scenarios. In this article, we also present several design guidelines for such a system, based on the results of our analytic and empirical studies. Finally, evaluation of our prototype provides evidence that the system can indeed detect potential risks in advance and provide early warnings to the driver in real-world scenarios, lowering the probability of traffic accidents.

It is now well known that Internet traffic exhibits self-similarity, which cannot be described by... more It is now well known that Internet traffic exhibits self-similarity, which cannot be described by traditional Markovian models such as the Poisson process. In this work, we simulate a simple network with a full implementation of TCP-Reno. We also assume Poisson arrivals at the application layer specifically to determine whether TCP can cause self-similarity even when input traffic does not exhibit long-range dependence. Our study shows that, at some loads TCP can induce the appearance of self-similarity. In particular, when load is low and loss is rare, traffic looks Poisson. When load is high and the network is overloaded, TCP congestion control can smooth out the burstiness of the aggregate stream so that traffic at the bottleneck tends to Poisson. However, when load is intermediate and the network is prone to occasional bouts of congestion, as is typical of many networks, traffic can become self-similar. Moreover, factors such as round trip time and number of streams passing through the bottleneck can cause the network to become congested at different loads, and consequently affect the range of load over which self-similarity can be observed. The impact of one self-similar TCP stream has also been observed. In particular, our study show that if one or more streams passing through the bottleneck is self-similar and the aggregate flow does not exceed the capacity, traffic observed at the bottleneck will also be selfsimilar.

Mobile ad hoc wireless networks have been extensively studied as they make possible various inter... more Mobile ad hoc wireless networks have been extensively studied as they make possible various interesting future applications. However, most of the existing studies focus on showing the overall impact of mobility from topology changes and signal variations, while they do not clearly show how inter-operability among protocol layers affects the network performance. To address this issue, we developed a testbed which enables us to investigate the cross-layer interaction among protocol layers at Carnegie Mellon University (CMU). In this paper, we report on the testbed as well as on our experiences and findings from it.

In this paper, we compare the performance of two possible switching techniques for ad hoc wireles... more In this paper, we compare the performance of two possible switching techniques for ad hoc wireless networks: namely, reservation-based (RB) and non-reservation-based (NRB) switching. Fundamental network performance metrics, such as throughput and delay, are studied to understand the tradeoffs between the two switching schemes. In particular, we identify the circumstances where each of these two schemes is better than the other.

Mobile ad hoc wireless networks have been extensively studied as they make possible various inter... more Mobile ad hoc wireless networks have been extensively studied as they make possible various interesting future applications. However, most of the existing studies focus on showing the overall impact of mobility from topology changes and signal variations, while they do not clearly show how inter-operability among protocol layers affects the network performance.

In this paper, we report the first complete version of a multi-hop broadcast protocol for vehicul... more In this paper, we report the first complete version of a multi-hop broadcast protocol for vehicular ad hoc networks (VANET). Our results clearly show that broadcasting in VANET is very different from routing in mobile ad hoc networks (MANET) due to several reasons such as network topology, mobility patterns, demographics, traffic patterns at different times of the day, etc. These differences imply that conventional ad hoc routing protocols such as DSR and AODV will not be appropriate in VANETs for most vehicular broadcast applications. We identify three very different regimes that a vehicular broadcast protocol needs to work in: i) dense traffic regime; ii) sparse traffic regime; and iii) regular traffic regime. We build upon our previously proposed routing solutions for each regime and we show that the broadcast message can be disseminate efficiently. The proposed design of the distributed vehicular broadcast (DV-CAST) protocol integrates the use of various routing solutions we have previously proposed.

Disjoint multipath source routing in ad hoc networks: transport capacity

While much of the effort on routing in ad hoc wireless networks is focused on source routing and ... more While much of the effort on routing in ad hoc wireless networks is focused on source routing and flooding, in this work we explore the possibility of applying packet switching concept to an ad hoc wireless environment. It is well known that packet switching is a robust and efficient technique for data that can tolerate some delay. Similar to the traditional packet switching technique, we propose a disjoint multipath source routing (DMPSR) technique which allows packets originating from the same source to be statistically multiplexed onto multiple disjoint routes. Each route can relay packets from multiple sources. In this paper, we analyze the transport capacity of this scheme and compare the result to that obtained in P. Gupta et al. (2000) and O.K. Tonguz et al. (2003), which consider single path source routing. Given a network of size A with N mobile nodes, each capable of transmitting at R bps, our results show that the maximum transport capacity of this scheme converges to R√(AN) when load balancing is employed (i.e., number of routes being relayed through any given node is the same for all the nodes). If, however, no load balancing is used, then the transport capacity of DMPSR will be greater than that of the traditional source routing when the spatial density of the network is below some critical threshold. Beyond the critical threshold, however, transport capacity is significantly less than the one obtained in O.K. Tonguz et al. ( 2003) and P. Gupta et al. (2000) since a mobile node has to spend more time relaying messages for other sources.

In this paper, the performance of two possible switching schemes for ad hoc wireless networks, na... more In this paper, the performance of two possible switching schemes for ad hoc wireless networks, namely reservation-based (RB) and non-reservation-based (NRB) is investigated. It is shown that route reservation would be a scheme of choice in a scenario where delay is a constraint, the network is dense, and/or nodes move with moderate speed. However, a non-reservation scheme is preferable in a scenario where the network is sparse, there is heavy traffic load, nodes are static, and/or delay is not a major concern.

In order to guarantee a certain level of quality of service (QoS), routing protocol should be sma... more In order to guarantee a certain level of quality of service (QoS), routing protocol should be smart enough to pick a stable and good quality communication route in order to avoid any unnecessary packet loss. Intuitively, if the chosen communication route is error-prone, it is very likely that the transmitted packets over consecutive links may get corrupted or even lost. It is well-known that retransmissions incur additional delay and increase the overhead in the network. This is very undesirable for delay sensitive applications, especially in an ad hoc wireless network environment where devices are typically power-limited and topology is constantly changing. Unlike most of the existing routing protocols for ad hoc wireless networks, in which the basic criterion for route selection is the minimization of the overall route length, in this paper we consider bit error rate (BER)-based routing, where the chosen route is the one which guarantees the lowest BER at the ending node. In other words, we consider providing QoS in terms of BER at the destination node. Our analysis shows that, according to this criterion, shortest path (SP) routing is not always the optimal choice as it often selects a path with unacceptable BER which can directly affect the end-to-end packet delay and goodput.

Current ad hoc wireless routing protocols typically select the route with the shortest-path or mi... more Current ad hoc wireless routing protocols typically select the route with the shortest-path or minimum hop count to the destination. However, routes chosen according to such criteria tend to include longer hop-length links with possibly bad signal quality. It is expected that the routing protocol can be improved by eliminating the usage of routes with bad links. In this work, we modify the ad hoc on-demand distance vector (AODV) routing protocol to avoid routing through bad quality links. We also integrate the "hand-off" concept into our protocol to prevent link breakages during route maintenance. The herein reported OPNET simulation results seem very promising and they show that our protocol introduces a much lower routing overhead while still providing equal or better performance than the original AODV protocol in terms of throughput and delay.

IEEE Wireless Communications, 2007

Accident (a) Accident Accident Forward with highest p (b) Accident T = 0 T = τ T Accident

In this paper, we propose a simple semi-analytical approach for the evaluation of the impact of m... more In this paper, we propose a simple semi-analytical approach for the evaluation of the impact of mobility on the bit error rate (BER) performance of multi-hop ad hoc wireless networks. Analytical expressions, relating the BER at the end of a multi-hop route with the mobility characteristics of the nodes and the routing strategy, are derived. Two node mobility models are considered: direction-persistent (DP) and direction-nonpersistent (DNP). In particular, two network switching scenarios are analyzed: (i) opportunistic non-reservation-based switching (ONRBS), where a message flows from source to destination by opportunistically choosing the available shortest consecutive links; and (ii) reservation-based switching (RBS), where, after the creation of a multi-hop route from source to destination, the message is "forced" to flow over the reserved links, regardless of their actual lengths. The network performance is evaluated in ideal (without inter-node interference, INI) and realistic (with INI) cases. The improved robustness against mobility offered by ONRBS, with respect to RBS, is analyzed and quantified.

IEEE Transactions on Mobile Computing, 2007

This paper investigates whether and when route reservation-based (RB) communication can yield bet... more This paper investigates whether and when route reservation-based (RB) communication can yield better delay performance than non-reservation-based (NRB) communication in ad hoc wireless networks. In addition to posing this fundamental question, the requirements (in terms of route discovery, medium access control (MAC) protocol, and pipelining, etc.) for making RB switching superior to NRB switching are also identified. A novel analytical framework is developed and the network performance under both RB and NRB schemes is quantified. It is shown that if the aforementioned requirements are met, then RB schemes can indeed yield better delay performance than NRB schemes. This advantage, however, comes at the expense of lower throughput and goodput compared to NRB schemes

Routing protocols developed for ad hoc wireless networks use broadcast transmission to either dis... more Routing protocols developed for ad hoc wireless networks use broadcast transmission to either discover a route or disseminate information. More specifically, reactive routing protocols has to flood the network with a route request (RREQ) message in order to find an optimal route to the destination. Several applications developed for vehicular ad hoc wireless networks (VANET), which is a subset of MANET, rely on broadcast to propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem.

IEEE Transactions on Mobile Computing, 2007

This paper investigates whether and when route reservation-based (RB) communication can yield bet... more This paper investigates whether and when route reservation-based (RB) communication can yield better delay performance than non-reservation-based (NRB) communication in ad hoc wireless networks. In addition to posing this fundamental question, the requirements (in terms of route discovery, medium access control (MAC) protocol, and pipelining, etc.) for making RB switching superior to NRB switching are also identified. A novel analytical framework is developed and the network performance under both RB and NRB schemes is quantified. It is shown that if the aforementioned requirements are met, then RB schemes can indeed yield better delay performance than NRB schemes. This advantage, however, comes at the expense of lower throughput and goodput compared to NRB schemes.

Scalable broadcast strategies for ad hoc routing protocols

In ad hoc wireless networks, routes are discovered either by means of a periodic update between 1... more In ad hoc wireless networks, routes are discovered either by means of a periodic update between 1-hop neighbors or by using a reactive routing protocol which relies on an explicit route discovery process. Although the major advantage of a reactive protocol is the reduction in the routing overhead, the conventional flooding mechanism used to propagate the route request (RREQ) message during the route discovery process does not scale well to large networks. In addition to the overhead problem, the way in which the route request gets propagated across the network affects the network performance in two different ways: (i) it determines how fast the route can be acquired; (ii) it is also an important factor in determining the availability and quality of the route. In this paper, we propose three novel RREQ broadcast techniques, i.e., weighted p-persistence, slotted p-persistence, and slotted p-persistence schemes. Our simulation results show that the proposed schemes can achieve up to 75% routing overhead reduction while keeping the route acquisition delay close or equal to the conventional flooding scheme.

IEEE Journal on Selected Areas in Communications, 2007

A Vehicular Ad Hoc Network (VANET) may exhibit a bipolar behavior, i.e., the network can either b... more A Vehicular Ad Hoc Network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on I-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time).

Previous work on mobile ad hoc wireless networks has primarily focused on studying the impact of ... more Previous work on mobile ad hoc wireless networks has primarily focused on studying the impact of node mobility on the performance of various routing protocols and the development of mobility models to represent node mobility. This is due to the traditional viewpoint that the main reason for degradation in network performance as a result of node mobility is due to the traffic control overhead required for maintaining accurate routing tables in the case of table-driven protocols and maintaining routes in the case of on-demand protocols [X. Hong et al., 2001]. Little work exists on the effect of mobility on fundamental communication and network performance metrics such as the bit error rate (BER) of a multi-hop route joining a source-destination pair, and minimum required node spatial density of an ad hoc wireless network for full connectivity. This paper discusses the impact of mobility on these parameters and points out their importance in real-life scenarios for average pedestrian and vehicular speeds. We quantify the effect of node mobility and message length on the BER of an average multi-hop route using a recently developed communication-theoretic framework for ad hoc wireless networks [O.K. Tonguz et al., Feb 2003]. BER of an average multi-hop route directly affects the ability of an ad hoc wireless network to support applications requiring a specific BER, for a given node transmission power and node spatial density.

Vehicular Ad Hoc Wireless Network (VANET) exhibits a bipolar behavior in terms of network topolog... more Vehicular Ad Hoc Wireless Network (VANET) exhibits a bipolar behavior in terms of network topology: fully connected topology with high traffic volume or sparsely connected topology when traffic volume is low. In this work, we develop a statistical traffic model based on the data collected on I-80 freeway in California in order to study key performance metrics of interest in disconnected VANETs, such as average re-healing time (or the network restoration time). Our results show that, depending on the sparsity of vehicles, the network re-healing time can vary from a few seconds to several minutes. This suggests that, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times.

IEEE Wireless Communications, 2010