Showing papers in "Wireless Networks in 2006"

Energy-efficient, collision-free medium access control for wireless sensor networks

Abstract: The traffic-adaptive medium access protocol (TRAMA) is introduced for energy-efficient collision-free channel access in wireless sensor networks. TRAMA reduces energy consumption by ensuring that unicast and broadcast transmissions incur no collisions, and by allowing nodes to assume a low-power, idle state whenever they are not transmitting or receiving. TRAMA assumes that time is slotted and uses a distributed election scheme based on information about traffic at each node to determine which node can transmit at a particular time slot. Using traffic information, TRAMA avoids assigning time slots to nodes with no traffic to send, and also allows nodes to determine when they can switch off to idle mode and not listen to the channel. TRAMA is shown to be fair and correct, in that no idle node is an intended receiver and no receiver suffers collisions. An analytical model to quantify the performance of TRAMA is presented and the results are verified by simulation. The performance of TRAMA is evaluated through extensive simulations using both synthetic-as well as sensor-network scenarios. The results indicate that TRAMA outperforms contention-based protocols (CSMA, 802.11 and S-MAC) and also static scheduled-access protocols (NAMA) with significant energy savings.

Energy balanced data propagation in wireless sensor networks

Abstract: We study the problem of energy-balanced data propagation in wireless sensor networks. The energy balance property guarantees that the average per sensor energy dissipation is the same for all sensors in the network, during the entire execution of the data propagation protocol. This property is important since it prolongs the network's lifetime by avoiding early energy depletion of sensors. We propose a new algorithm that in each step decides whether to propagate data one-hop towards the final destination (the sink), or to send data directly to the sink. This randomized choice balances the (cheap) one-hop transimssions with the direct transimissions to the sink, which are more expensive but "bypass" the sensors lying close to the sink. Note that, in most protocols, these close to the sink sensors tend to be overused and die out early. By a detailed analysis we precisely estimate the probabilities for each propagation choice in order to guarantee energy balance. The needed estimation can easily be performed by current sensors using simple to obtain information. Under some assumptions, we also derive a closed form for these probabilities. The fact (shown by our analysis) that direct (expensive) transmissions to the sink are needed only rarely, shows that our protocol, besides energy-balanced, is also energy efficient.

Fault-tolerant and 3-dimensional distributed topology control algorithms in wireless multi-hop networks

Abstract: The topology of a multi-hop wireless network can be controlled by varying the transmission power at each node. The lifetime of such networks depends on battery power at each node. This paper presents a distributed fault-tolerant topology control algorithm for minimum energy consumption in multi-hop wireless networks. This algorithm is an extension of cone-based topology control algorithm [19, 12]. The main advantage of this algorithm is that each node decides on its power based on local information about the relative angle of its neighbors and as a result of these local decisions, a fault-tolerant connected network is formed on the nodes. It is done by preserving the connectivity of a network upon failing of, at most, k nodes (k is a constant) and simultaneously minimize the transmission power at each node to some extent. In addition, simulations are studied to support the effectiveness of this algorithm. Finally, it is shown how to extend this algorithm to 3-dimensions.

Connectivity in one-dimensional ad hoc networks: a queueing theoretical approach

Abstract: In this paper we analyze connectivity issues in one-dimensional ad hoc networks. Starting with a deterministic channel model, we show how an equivalent GI|D|∞ queueing model may be used to address network connectivity. In this way, we obtain exact results for the coverage probability, the node isolation probability and the connectivity distance for various node placement statistics. We then show how a GI|G|∞ model may be used to study broadcast percolation problems in ad hoc networks with general node placement in the presence of fading channels. In particular, we obtain explicit results for the case of nodes distributed according to a Poisson distribution operating in a fading/ shadowing environment. In the latter case, heavy traffic theorems are applied to derive the critical transmission power for connectivity and broadcast percolation distance in highly dense networks. The impact of signal processing schemes able to exploit the diversity provided by small-scale fading by means of multiple antennas is considered. The analysis is then extended to the case of unreliable ad hoc networks, with an in-depth discussion of asymptotic results.

Uplink scheduling in CDMA packet-data systems

Abstract: Uplink scheduling in wireless systems is gaining importance due to arising uplink intensive data services (ftp, image uploads etc.), which could be hampered by the currently in-built asymmetry in favor of the downlink. In this work, we propose and study algorithms for efficient uplink packet-data scheduling in a CDMA cell. The algorithms attempt to maximize system throughput under transmit power limitations on the mobiles assuming instantaneous knowledge of user queues and channels. However no channel statistics or traffic characterization is necessary. Apart from increasing throughput, the algorithms also improve fairness of service among users, hence reducing chances of buffer overflows for poorly located users.The major observation arising from our analysis is that it is advantageous on the uplink to schedule "strong" users one-at-a-time, and "weak" users in larger groups. This contrasts with the downlink where one-at-a-time transmission for all users has shown to be the preferred mode in much previous work. Based on the optimal schedules, we propose less complex and more practical approximate methods, both of which offer significant performance improvement compared to one-at-a-time transmission, and the widely acclaimed Proportional Fair (PF) algorithm, in simulations. When queue content cannot be fed back, we propose a simple modification of PF, Uplink PF (UPF), that offers similar improvement.

A call admission and rate control scheme for multimedia support over IEEE 802.11 wireless LANs

Abstract: Quality of service (QoS) support for multimedia services in the IEEE 802.11 wireless LAN is an important issue for such WLANs to become a viable wireless access to the Internet. In this paper, we endeavor to propose a practical scheme to achieve this goal without changing the channel access mechanism. To this end, a novel call admission and rate control (CARC) scheme is proposed. The key idea of this scheme is to regulate the arriving traffic of the WLAN such that the network can work at an optimal point. We first s how that the channel busyness ratio is a good indicator of the network status in the sense that it is easy to obtain and can accurately and timely represent channel utilization. Then we propose two algorithms based on the channel busyness ratio. The call admission control algorithm is used to regulate the admission of real-time or streaming traffic and the rate control algorithm to control the transmission rate of best effort traffic. As a result, the real-time or streaming traffic is supported with statistical QoS guarantees and the best effort traffic can fully utilize the residual channel capacity left by the real-time and streaming traffic. In addition, the rate control algorithm itself provides a solution that could be used above the media access mechanism to approach the maximal theoretical channel utilization. A comprehensive simulation study in ns-2 has verified the performance of our proposed CARC scheme, showing that the original 802.11 DCF protocol can statically support strict QoS requirements, such as those required by voice over IP or streaming video, and at the same time, achieve a high channel utilization.

Power efficient range assignment for symmetric connectivity in static ad hoc wireless networks

Abstract: In this paper we study the problem of assigning transmission ranges to the nodes of a static ad hoc wireless network so as to minimize the total power consumed under the constraint that enough power is provided to the nodes to ensure that the network is connected. We focus on the MIN-POWER SYMMETRIC CONNECTIVITY problem, in which the bidirectional links established by the transmission ranges are required to form a connected graph.Implicit in previous work on transmission range assignment under asymmetric connectivity requirements is the proof that MIN-POWER SYMMETRIC CONNECTIVITY is NP-hard and that the MST algorithm has a performance ratio of 2. In this paper we make the following contributions: (1) we show that the related MIN-POWER SYMMETRIC UNICAST problem can be solved efficiently by a shortest-path computation in an appropriately constructed auxiliary graph; (2) we give an exact branch and cut algorithm based on a new integer linear program formulation solving instances with up to 35-40 nodes in 1 hour; (3) we establish the similarity between MIN-POWER SYMMETRIC CONNECTIVITY and the classic STEINER TREE problem in graphs, and use this similarity to give a polynomial-time approximation scheme with performance ratio approaching 5/3 as well as a more practical approximation algorithm with approximation factor 11/6; and (4) we give the results of a comprehensive experimental study comparing new and previously proposed heuristics with the above exact and approximation algorithms.

Priority scheduling in wireless ad hoc networks

Abstract: Ad hoc networks formed without the aid of any established infrastructure are typically multi-hop networks. Location dependent contention and hidden terminal problem make priority scheduling in multi-hop networks significantly different from that in wireless LANs. Most of the prior work related to priority scheduling addresses issues in wireless LANs. In this paper, priority scheduling in multi-hop networks is discussed. We propose a scheme using two narrow-band busy tone signals to ensure medium access for high priority source stations. The simulation results demonstrate the effectiveness of the proposed scheme.

Energy efficient node-to-node authentication and communication confidentiality in wireless sensor networks

Abstract: A distributed Wireless Sensor Network (WSN) is a collection of low-end devices with wireless message exchange capabilities. Due to the scarcity of hardware resources, the lack of network infrastructures, and the threats to security, implementing secure pair-wise communications among any pair of sensors is a challenging problem in distributed WSNs. In particular, memory and energy consumption as well as resilience to sensor physical compromise are the most stringent requirements. In this paper, we introduce a new threat model to communications confidentiality in WSNs, the smart attacker model. Under this new, more realistic model, the security features of previously proposed schemes decrease drastically. We then describe a novel pseudo-random key pre-deployment strategy ESP that combines all the following properties: (a) it supports an energy-efficient key discovery phase requiring no communications; (b) it provides node to node authentication; (c) it is highly resistant to the smart attacker. We provide both asymptotic results and extensive simulations of the schemes that are being proposed.

Relaying in mobile ad hoc networks: the Brownian motion mobility model

Abstract: Mobile ad hoc networks are characterized by a lack of a fixed infrastructure and by node mobility. In these networks data transfer can be improved by using mobile nodes as relay nodes. As a result, transmission power and the movement pattern of the nodes have a key impact on the performance. In this work we focus on the impact of node mobility through the analysis of a simple one-dimensional ad hoc network topology. Nodes move in adjacent segments with reflecting boundaries according to Brownian motions. Communications (or relays) between nodes can occur only when they are within transmission range of each other. We determine the expected time to relay a message and compute the probability density function of relaying locations. We also provide an approximation formula for the expected relay time between any pair of mobiles.

A new architecture for 3G and WLAN integration and inter-system handover management

Abstract: WLAN has strong potential to provide a perfect broadband complement to the 3G wireless systems. This has raised much interest in their integration. In this paper, a novel architecture using the Network Interoperating Agent (NIA), and Integration Gateway (IG) is proposed to integrate the 3G systems and WLANs of various providers that may not necessarily have direct service level agreement (SLA) among them. The proposed architecture is scalable as it eliminates the need for the creation of bilateral SLA among the 3G and WLAN operators. In addition, inter-system handover (ISHO) protocols using the concept of the dynamic boundary area is proposed to support seamless roaming between 3G and WLAN. The dynamic boundary area is determined based on the speed of the user and WLAN cell size. The ISHO procedures are initiated when a mobile user enters the boundary area of the WLAN and are completed before the user leaves the coverage area of the serving WLAN. This ensures that the roaming from WLAN to 3G is transparent to the applications. The performance evaluation shows that the proposed boundary area based ISHO algorithm outperforms the existing 3G/WLAN ISHO algorithms.

PANDA: a novel mechanism for flooding based route discovery in ad hoc networks

Abstract: Flooding technique is often used for route discovery in on-demand routing protocols in mobile ad hoc networks (MANETs) such as Dynamic Source Routing (DSR) and Ad hoc On-demand Distance Vector (AODV) routing. In this paper we present a Positional Attribute based Next-hop Determination Approach (PANDA) to improve the performance of flooding-based route discovery in MANETs using positional attributes of the nodes. These attributes may be geographical, power-aware, or based on any other quality of service (QoS) measure. We identify the "next-hop racing" phenomena due to the random rebroadcast delay (RRD) approach during the route discovery process in DSR and AODV, and show how the PANDA approach can resolve this problem. We assume that each node knows its positional attributes, and an intermediate node can learn the positional attributes of its previous-hop node via the received route-request message. Based on the attributes such as the relative distance, estimated link lifetime, transmission power consumption, residual battery capacity, an intermediate node will identify itself as good or bad candidate for the next-hop node and use different rebroadcast delay accordingly. By allowing good candidates to always go first, our approach will lead to the discovery of better end-to-end routes in terms of the desired quality of service metrics. Through simulations we evaluate the performance of PANDA using path optimality, end-to-end delay, delivery ratio, transmission power consumption, and network lifetime. Simulation results show that PANDA can: (a) improve path optimality, and end-to-end delay, (b) help find data paths with only 15%-40% energy consumption compared to the RRD approach at a moderate cost of increased routing messages, (c) balance individual node's battery power utilization and hence prolong the entire network's lifetime.

Energy-aware on-demand routing protocols for wireless ad hoc networks

Abstract: Energy use is a crucial design concern in wireless ad hoc networks since wireless terminals are typically battery-operated. The design objectives of energy-aware routing are two folds: Selecting energy-efficient paths and minimizing the protocol overhead incurred for acquiring such paths. To achieve these goals simultaneously, we present the design of several on-demand energy-aware routing protocols. The key idea behind our design is to adaptively select the subset of nodes that are required to involve in a route-searching process in order to acquire a high residual-energy path and/or the degree to which nodes are required to participate in the process of searching for a low-power path in networks wherein nodes have transmission power adjusting capability. Analytical and simulation results are given to demonstrate the high performance of the designed protocols in energy-efficient utilization as well as in reducing the protocol overhead incurred in acquiring energy-efficient routes.

An OFDM-TDMA/SA MAC protocol with QoS constraints for broadband wireless LANs

Abstract: Orthogonal frequency division multiplexing (OFDM) is an important technique to support high speed transmission of broadband traffic in wireless networks, especially broadband wireless local area networks (LANs). Based on OFDM, a new multiple access scheme, called OFDM-TDMA with subcarrier allocation (OFDM-TDMA/SA), is proposed in this paper. It provides more flexibility in resource allocation than other multiple access schemes such as OFDM-TDMA, OFDM-frequency division multiple access (OFDM-FDMA), and orthogonal frequency division multiple access (OFDMA). With OFDM-TDMA/SA, a medium access control (MAC) is designed for broad-band wireless LANs. It optimizes bit allocation in subcarriers so that maximum bits are transmitted in each OFDM symbol under a frequency selective fading environment. The OFDM-TDMA/SA MAC protocol also supports three classes of traffic such as guaranteed, controlled-load, and best effort services. Based on the optimum subcarrier bit-allocation algorithm and considering heterogeneous QoS constraints of multimedia traffic, a hierarchical scheduling scheme is proposed to determine the subcarriers and time slots in which a mobile terminal can transmit packets. In such a way, the OFDM-TDMA/SA MAC protocol significantly increases system throughput in a frequency selective fading environment and guarantees QoS of multimedia traffic. Computer simulation is carried out to evaluate the performance of the OFDM-TDMA/SA MAC protocol. Results show that the new MAC protocol outperforms other MAC protocols for OFDM-based wireless LANs.

A randomized energy-conservation protocol for resilient sensor networks

Abstract: In this paper we present PEAS, a randomized energy-conservation protocol that seeks to build resilient sensor networks in the presence of frequent, unexpected node failures. PEAS extends the network lifetime by maintaining a necessary set of working nodes and turning off redundant ones, which wake up after randomized sleeping times and replace failed ones when needed. The fully localized operations of PEAS are based on each individual node's observation of its local environment but do not require per neighbor state at any node; this allows PEAS to scale to very dense node deployment. PEAS is highly robust against node failures due to its simple operations and randomized design; it also ensures asymptotic connectivity. Our simulations and analysis show that PEAS can maintain an adequate working node density in presence of as high as 38% node failures, and a roughly constant overhead of less than 1% of the total energy consumption under various deployment densities. It extends a sensor network's functioning time in linear proportional to the deployed sensor population.

Capacity of multiservice WCDMA networks with variable GoS

Abstract: Traditional definitions of capacity of CDMA networks are either related to the number of calls they can handle (pole capacity) or to the arrival rate that guarantees that the rejection rate (or outage) is below a given fraction (Erlang capacity). We extend the latter definition to other quality of service (QoS). We consider best-effort (BE) traffic sharing the network resources with real-time (RT) applications. As is often the case in CDMA systems, we assume that the BE traffic access is done using a time-shared channel (such as the HDR or the HSDPA). BE applications can adapt their instantaneous transmission rate to the available one and thus need not be subject to admission control or outages. Their meaningful QoS is the average delay (i.e. the sojourn time). The delay aware capacity is then defined as the arrival rate of BE calls that the system can handle such that their expected delay is bounded by a given constant. We compute in this paper both the blocking probability of the RT traffic having an adaptive Grade of Service (GoS) as well as the expected delay of the BE traffic for an uplink multicell WCDMA system. This yields the Erlang capacity for former and the delay capacity for the latter.

User independent paging scheme for mobile IP

Abstract: Multi-step paging has been widely proposed in personal communications services (PCS) systems to reduce the signaling overheads. Similar ideas can be applied to Mobile IP to provide IP paging services. However, current proposed multi-step paging schemes are user dependent under which the partition of paging areas and the selection of paging sequence are different for each user. The performance of a user dependent paging scheme for individual users may be affected by many factors. It is often difficult to achieve perfect performance for each user. In addition, when multiple users are paged at the same time, user dependent paging schemes may consume significant system resources. This paper introduces a user independent paging scheme where the paging criterion is not based on individual user information. The goal of user independent paging is to provide satisfactory overall performance of the whole system, when personalized optimal performance for each user is hard to obtain. The user independent paging scheme is proposed for IP mobility for its easy implementation and convenient combination with paging request aggregation. The paging criterion adopted is the mobility rate of each subnet determined by the aggregated movements of all mobile users. In order to implement the proposed scheme, a concept of "semi-idle state" is introduced and the detailed solution for obtaining mobility rate is presented. Analytical results show that when paging one user at a time, the performance of the proposed user independent paging scheme is comparable to that of the paging schemes based on perfect knowledge of user movement statistics. When paging multiple users simultaneously and when the knowledge on individual user behavior is not perfectly accurate, the proposed scheme has remarkable advantages in terms of reducing the overall paging cost.

Cross-layer modeling of adaptive wireless links for QoS support in heterogeneous wired-wireless networks

Abstract: Future wired-wireless multimedia networks require diverse quality-of-service (QoS) support. To this end, it is essential to rely on QoS metrics pertinent to wireless links. In this paper, we develop a cross-layer model for adaptive wireless links, which enables derivation of the desired QoS metrics analytically from the typical wireless parameters across the hardware-radio layer, the physical layer and the data link layer. We illustrate the advantages of our model: generality, simplicity, scalability and backward compatibility. Finally, we outline its applications to power control, TCP, UDP and bandwidth scheduling in wireless networks.

A game-theoretic look at simple relay channel

Abstract: In this paper, we address the crosslayer problem of joint medium access control (MAC) and routing in ad hoc wireless networks from the novel perspective of stochastic dynamic games. As a starting point to understand the efficient modes of wireless network operation, we look at the basic problem of multi-hop communication over the simple topology of a single relay channel. A stochastic game is formulated for transmitter and relay nodes competing over collision channels to deliver packets to a common destination node using alternative paths. We rely on a reward mechanism to stimulate cooperation for packet forwarding and evaluate the conflicting multiple access and routing strategies of direct communication and relaying through a detailed foray into the questions of cooperation incentives, throughput, delay and energy-efficiency. Under the separate models of selfish and cooperative network operation, we study the interactions among the equilibrium strategies and present a detailed performance analysis based on multiple system parameters that involve the packet arrival rates, throughput rewards, delay and energy costs.

Effective location based services with dynamic data management in mobile environments

Abstract: With the proliferation of mobile computing technologies, location based services have been identified as one of the most promising target application. We classify mobile information service domains based on feature characteristics of the information sources and different patterns of mobile information access. By carefully examining the service requirements, we identify the dynamic data management problem that must be addressed for effective location based services in mobile environments. We then devise a general architecture and cost model for servicing both location independent and location dependent data. Based on the architecture and cost model, we propose a set of dynamic data management strategies that employs judicious caching, proactive server pushing and neighborhood replication to reduce service cost and improve response time under changing user mobility and access patterns. Detail behavior analysis helps us in precisely capturing when and how to apply these strategies. Simulation results suggest that different strategies are effective for different types of data in response to different patterns of movement and information access.

A geometrical-based microcell mobile radio channel model

Abstract: In this paper we present a geometric multipath propagation model for a microcell mobile environment. The proposed model provides the statistics for the direction-of-arrival (DOA) of multipath components. These statistics are required to test adaptive array algorithms for cellular applications. The proposed model assumes that (1) a line-of-sight (LOS) path exists between the transmitter and the receiver, (2) the scatterers lie within a circle of radius R around the mobile station, and (3) the base station lies within this circle. The distances between the scatterers and the mobile station are subject statistically to a hyperbolic distribution. The model also provides the multipath power delay profiles (PDP), which are used to evaluate the bit error rate (BER) and the signal to interference ratio (SIR) for the direct-sequence code division multiple access (DS-CDMA). We derive and simulate the joint probability density functions (pdfs) of the power--DOA and the power--Doppler shift. Further we determine expressions for the BER performance and for the SIR of a DS-CDMA system over the proposed channel model. A simplified expression based on the improved Gaussian approximation (SEIGA) is used to evaluate the BER and the SIR in a wideband multipath channel. Although the proposed model is applicable for downlink as well, in this paper we will analyze the uplink environment only.

Impact of interference on the wireless ad-hoc networks capacity and topology

Abstract: It is wide spread belief that wireless mobile ad-hoc networks will be a further evolutionary step towards ubiquitous communication and computing. Due to the mobility of the network nodes, the strongly varying radio propagation conditions and the varying data traffic load these networks constitute a very dynamic environment. One essential step in evaluating the true benefit of this new technology consists of estimates and constraints concerning the scalability and performance of such networks. Using a simple model we discuss analytically the effect of interference on the link quality and connectivity of large networks. It turns out that the outage probability rapidly increases with increasing traffic load. Furthermore, we investigate the connectivity of the network under varying traffic load and find a percolation phase transition at a particular value of the traffic load. We discuss the dependence of these effects on parameters characterizing the receiver and the radio propagation conditions.

Novel assignment strategies for spatial reuse TDMA in wireless ad hoc networks

Abstract: Spatial reuse TDMA has been proposed as an access scheme for multi-hop radio networks where real-time service guarantees are important. The idea is to increase capacity by letting several radio terminals use the same time slot when possible. A time slot can be shared when the radio units are geographically separated such that small interference is obtained. In reuse scheduling, there are several alternative assignment methods. Traditionally, transmission rights are given to nodes or to links, i.e., transmitter/receiver pairs. We present a comparison of these two approaches and show that both have undesirable properties in certain cases, e.g. link assignment gives a higher delay for low traffic loads but can achieve much higher throughput than node assignment. Furthermore, we propose a novel assignment strategy, achieving the advantages of both methods. Simulation results show that the proposed method can achieve the throughput of link assignment for high traffic loads as well as the lower delay characteristics of node assignment for low traffic loads.

The effects of synchronization on topology-transparent scheduling

Abstract: Topology-transparent scheduling is an attractive medium access control technique for mobile ad hoc networks (MANETs) and wireless sensor networks (WSNs). The transmission schedule for each node is fixed and guarantees a bounded delay independent of which nodes are its neighbours, as long as the active neighbourhood is not too dense. Most of the existing work on topology-transparent scheduling assumes that the nodes are synchronized on frame boundaries. Synchronization is a challenging problem in MANETs and in WSNs. Hence, we study the relationships among topology-transparent schedules, expected delay, and maximum delay, for successively weaker models of synchronization: frame-synchronized, slot-synchronized, and asynchronous transmission. For each synchronization model, we give constructive proofs of existence of topology-transparent schedules, and bound the least maximum delay. Perhaps surprisingly, the construction for the asynchronous model is a simple variant of the slot synchronized model. While it is foreseen that the maximum delay increases as the synchronization model is weakened, the bound is too pessimistic. The results on expected delay show that topology-transparent schedules are very robust to node density higher than the construction is designed to support, allowing the nodes to cope well with mobility, and irregularities of their deployment.

End-to-end QoS framework for heterogeneous wired-cum-wireless networks

Abstract: With information access becoming more and more ubiquitous, there is a need for providing QoS support for communication that spans wired and wireless networks. For the wired side, RSVP/SBM has been widely accepted as a flow reservation scheme in IEEE 802 style LANs. Thus, it would be desirable to investigate the integration of RSVP and a flow reservation scheme in wireless LANs, as an end-to-end solution for QoS guarantee in wired-cum-wireless networks. For this purpose, we propose WRESV, a lightweight RSVP-like flow reservation and admission control scheme for IEEE 802.11 wireless LANs. Using WRESV, wired/wireless integration can be easily implemented by cross-layer interaction at the Access Point. Main components of the integration are RSVP-WRESV parameter mapping and the initiation of new reservation messages, depending on where senders/receivers are located. In addition, to support smooth roaming of mobile users among different basic service sets (BSS), we devise an efficient handoff scheme that considers both the flow rate demand and network resource availability for continuous QoS support. Furthermore, various optimizations for supporting multicast session and QoS re-negotiation are proposed for better performance improvement. Extensive simulation results showthat the proposed scheme is promising in enriching the QoS support of multimedia applications in heterogeneous wired-cum-wireless networks

A robust and energy-efficient data dissemination framework for wireless sensor networks

Abstract: Wireless sensor networks (WSNs) are appealing in obtaining fine-granular observations about the physical world. Due to the fact that WSNs are composed of a large number of low-cost and energy-constrained sensor nodes, along with the notorious time-varying and error-prone nature of wireless links, scalable, robust and energy-efficient data dissemination techniques are needed for the emerging WSN applications such as environment monitoring and surveillance. In this paper, we examine this emerging field from the point of view of supply chain management and propose a hybrid data dissemination framework for WSNs. In particular, for each sensing task, the whole sensor field is conceptually partitioned into several functional regions based on the supply chain management methodology. Different routing schemes are applied to different regions in order to provide better performance in terms of reliability and energy consumption. For this purpose, we also propose a novel zone flooding scheme, essentially a combination of conventional geometric routing and flooding techniques. Our hybrid data dissemination framework features low overhead, high reliability, good scalability and flexibility, and preferable energy efficiency. Detailed simulation studies are carried out to validate the effectiveness and efficiency of our scheme.

Performance study of proactive flow handoff for mobile ad hoc networks

Abstract: A Mobile Ad hoc Network (MANET) is a collection of wireless mobile computers forming a temporary network without any existing wire line infrastructure. Searching for feasible paths, or routing, is very challenging in mobile ad hoc networks because of frequent topology changes caused by users' mobility. Location information is required by some applications and can be used to facilitate routing implementation. In this paper, we propose a proactive flow handoff method based on nodes' location information. In summary, location information is utilised to reduce the control overhead in route discovery phase, to search quickly for a feasible path upon link breakage, and to hand off a flow to a stable path if the active one breaks based on predication. Keeping "always-on" end-to-end connectivity once a flow is established is the main advantage of this routing method. A through simulation study is performed to demonstrate the efficiency of this method.

An adaptive delay and synchronization control scheme for Wi-Fi based audio/video conferencing

Abstract: The prevalence of the IEEE 802.11 b technology has made Wi-Fi based Audio/Video (AV) conferencing applications a viable service. However, due to the "best-effort" transport service and other unpredictable factors such as user mobility, location and background traffic, the transport channel behavior often fluctuates drastically. It thus becomes rather difficult to configure an appropriate de-jitter buffer to maintain the temporal fidelity of the AV presentation. We propose in this paper an adaptive delay and synchronization control scheme for AV conferencing applications over campus-wide WLANs. Making use of a distributed timing mechanism, the scheme monitors the synchronization errors and estimates the delay jitters among adjacent Media Data Units (MDUs) in real-time. It piece-wisely controls the equalization delay to compensate for the delay jitters experienced by MDUs in a closed-loop manner.We investigate the performance of the proposed scheme through trace-driven simulations. We collected network traces from a production campus-wide IEEE 802.11 b WLAN by emulating real conferencing sessions. Simulation results show that the scheme is capable of dynamically balancing between synchronization requirements and latency requirements in all scenarios. Small synchronization phase distortions, low MDU loss percentages and low average end-to-end delay can be achieved simultaneously. In particular, compared with solutions using a static setting, the proposed scheme is able to achieve a reduction of around 100ms in end-to-end delay with the same amount of MDU losses under some media-unfriendly situations.

An improved GGSN failure restoration mechanism for UMTS

Abstract: Universal mobile telecommunications system (UMTS) provides packet-switched data services for mobile users. To efficiently deliver packets in the UMTS core network, the PDP contexts (i.e., the routing information) are maintained in the volatile storage (e.g., memory) of SGSN, GGSN, and UE. The GGSN routes packets between the UMTS core network and external data networks, and thus has heavy traffic and computation loading, which may result in PDP contexts lost or corrupted, and the QoS of the UMTS network may degrade significantly. To resolve this issue, 3GPP 23.007 proposes a mechanism for GGSN failure restoration. In this mechanism, the corrupted PDP contexts can be restored through the PDP Context Activation procedure. However, this incurs extra signaling cost to the network. To reduce the network signaling cost and delay for restoration of the corrupted PDP contexts, this paper proposes an improved mechanism "GGSN Failure Restoration" (GFR) with different backup algorithms. The analytic models and simulation experiments are conducted to evaluate GFR. Our study indicates that the GFR mechanism can significantly reduce the cost for the PDP context restoration.

Efficient multicast search under delay and bandwidth constraints

Abstract: The issue of a multicast search for a group of users is discussed in this study. Given the condition that the search is over only after all the users in the group are found, this problem is called the Conference Call Search (CCS) problem. The goal is to design efficient CCS strategies under delay and bandwidth constraints. While the problem of tracking a single user has been addressed by many studies, to the best of our knowledge, this study is one of the first attempts to reduce the search cost for multiple users. Moreover, as oppose to the single user tracking, for which one can always reduce the expected search delay by increasing the expected search cost, for a multicast search the dependency between the delay and the search cost is more complicated, as demonstrated in this study. We identify the key factors affecting the search efficiency, and the dependency between them and the search delay. Our analysis shows that under tight bandwidth constraints, the CCS problem is NP-hard. We therefore propose a search method that is not optimal, but has a low computational complexity. In addition, the proposed strategy yields a low search delay as well as a low search cost. The performance of the proposed search strategy is superior to the implementation of an optimal single user search on a group of users.