In this paper, we analyze the impacts of random forwarding on network performance in wireless sensor networks (WSNs). Specifically, network lifetime and packet delay are investigated for two alternative forwarding schemes: random forwarding and deterministic forwarding. In the random forwarding, a probabilistic function is used to select a next-hop from the set of candidate nodes, whereas the candidate node with the greatest value (or minimum cost if it is favored) in terms of routing metric is selected in the deterministic forwarding. In order to gain the insight into the nature of those forwarding schemes, we establish a Markov chain model for a simple two-hop network. Based on the analytic model, we obtain the network lifetime for two alternative schemes and identify the conditions in which one scheme outperforms the other. Moreover, the simulation results using ns2 validate that such conditions exist even in more realistic scenarios.

We propose an opportunistic transmission strategy for wireless sensor networks (WSNs) that operate in a strict energy-constrained environment. Our proposed transmission strategy named binary-decision based transmission (BDT) attempts to transmit at good channel conditions while meeting the delay constraint for delay-sensitive applications and the buffer size constraint for non-delay-sensitive applications, under time- varying wireless channel. The Markov decision process (MDP) is used to find an optimum threshold for transmission decision. Our BDT scheme initiates transmission only when the channel quality exceeds the optimum threshold, so that unsuccessful transmissions causing a waste of energy are avoided whenever possible.

In this work we propose an energy-efficient transmission strategy for wireless sensor networks that operate in a strict energy-constrained environment. Our transmission algorithm consists of two components: a binary-decision based transmission and a channel-aware backoff adjustment. In the binary-decision based transmission, decision on whether to transmit or not is absolutely dependent on the current channel conditions. Specifically, transmission is initiated only when the channel quality exceeds a specified threshold, so that unsuccessful transmissions causing a waste of energy are avoided whenever possible. Using the Markov decision process (MDP) formulation we obtain the optimum threshold for successful transmission. A channel-aware backoff adjustment, the second component of our proposal, is introduced to favor sensor nodes seeing better channel in terms of transmission priority. By intelligently combining these two ingredients our transmission algorithm attempts to maximize energy efficiency. Extensive simulations are performed to verify the performance of our proposal over fading wireless channels. Numerical results show that our transmission algorithm outperforms the existing approaches in terms of energy efficiency, thereby further prolonging the network lifetime.

A novel anti-collision algorithm is proposed to maximize identification performance in slotted Aloha based radio frequency identification (RFID) systems. We observe much higher throughput can be achieved by identifying tags in a divide-and-conquer style, where the set of entire tags is partitioned into multiple subsets of roughly equal size and each subset is then identified in sequence. To quantify the throughput gain by partitioning, we introduce the notion of partitioning gain defined as an amount of throughput increased from partitioning. Our algorithm starts with initial blind partitioning and then attempts to estimate the tag population by identifying the first subset of tags. Once the estimation is obtained the remaining subsets of tags are repartitioned into an optimum number of subsets in order to maximize the partitioning gain. The proposed partitioning technique enables a faster yet accurate estimation on the number of contending tags and yields much higher throughput against previous non-partitioning approaches. Probabilistic analytical models are developed to investigate performance of our partitioning algorithm. Extensive simulations are also performed to validate the analytical results and demonstrate superiority of our algorithm. Numerical results show that throughput performance of our algorithm, let alone outperforming existing proposals, exceeds the commonly referenced theoretical limit of 1/e.

Providing quality-of-service (QoS) guarantees over wireless links requires thorough understanding and quantification of the interactions among the traffic source, the wireless channel, and the underlying link-layer error control mechanisms. We account for such interactions in an analytical model that we use to investigate the delay distribution and the packet discard rate (PDR) over a wireless link. Our analysis accommodates the inherent autocorrelations in both the traffic source as well as the channel error characteristics. An on-off fluid process is used to model the arrival of packets at the transmitter. These packets are temporarily stored in a first-in-first-out (FIFO) buffer before being transmitted over a channel with a time-varying and autocorrelated service rate. Using fluid analysis, we first derive the distribution for the queueing delay at the transmitter. As part of this analysis, we solve a fundamental fluid problem, namely, the probability distribution for the workload generated by a two-state fluid source over a fixed time interval. We then use the delay analysis to derive the PDR at the receiver. A closed-form expression for the effective bandwidth subject to a delay constraint is provided as a function of the source, channel, and error scheme parameters. This expression enables fast assessment of the bandwidth requirement of real-time traffic over QoS-based wireless networks. Numerical results and simulations are used to verify the adequacy of the analysis and to study the interactions among various system parameters.

Providing quality-of-service (QoS) guarantees over wireless packet networks poses a host of technical challenges that are not present in wireline networks. One of the key issues is how to account for the characteristics of the time-varying wireless channel and for the impact of link-layer error control in the provisioning of packet-level QoS. We accommodate both aspects in analyzing the packet-loss performance over a wireless link. We consider the cases of a single and multiplexed traffic streams. The link capacity fluctuates according to a fluid version of Gilbert-Elliott channel model. Traffic sources are modeled as on-off fluid processes. For the single-stream case, we derive the exact packet-loss rate (PLR) due to buffer overflow at the sender side of the wireless link. We also obtain a closed-form approximation for the corresponding wireless effective bandwidth. In the case of multiplexed streams, we obtain a good approximation for the PLR using the Chernoff-dominant eigenvalue (CDE) approach. Our analysis is then used to study the optimal forward error correction code rate that guarantees a given PLR while minimizing the allocated bandwidth. Numerical results and simulations are used to verify the adequacy of our analysis and to study the impact of error control on the allocation of bandwidth for guaranteed packet-loss performance.

Providing quality of service (QoS) guarantees over wireless links requires thorough understanding and quantification of the interactions among the traffic source, the wireless channel, and the underlying error control mechanisms. In this paper, we account for such interactions in a network-layer model that we use to investigate the delay performance for an on/off traffic stream transported over a wireless link. The capacity of this link fluctuates according to a fluid version of Gilbert-Elliot's model. We derive the packet delay distribution via two different approaches: uniformization and Laplace transform. Computational aspects of both approaches are discussed. The delay distribution is then used to quantify the wireless effective bandwidth under a given delay guarantee. Numerical results and simulations are used to verify the adequacy of our analysis and to study the impact of error control and bandwidth allocation on the packet delay performance.

Several technical issues must be resolved before ATM services can be efficiently extended to the wireless environment. Key issues include incorporating the characteristics of the time-varying wireless channel in the provisioning of the cell-level QoS, and improving the transport performance using error control mechanisms. We analyze the cell loss and delay performance over a wireless ATM link. We consider both cases of a single and multiplexed ATM connections. The link capacity fluctuates according to a fluid version of Gilbert-Elliot channel model. Traffic sources are modeled as on-off fluid processes. The analytical framework incorporates the effects of error control schemes (i.e., ARQ and/or FEC), which are used to improve the transport performance over the wireless link. For the single-stream case, we derive the mean delay and the cell loss rate (CLR) due to buffer overflow at the sender side of the wireless link. We also obtain a closed-form approximation for the corresponding wireless effective bandwidth. In the case of multiplexed streams, we obtain a good approximation for the CLR using the Chernoff-dominant eigenvalue (CDE) approach. The expressions for the CLR and effective bandwidth are then used to study the optimal FEC code rate that guarantees the requested QoS while maximizing the utilization of the wireless bandwidth. Numerical results and simulations are used to verify the adequacy of our analysis and to study the impact of error control on the allocation of bandwidth for guaranteed cell loss and delay performance.

Mobile computing has the potential for managing information globally. Data management issues in mobile computing have received some attention in recent times, and the design of adaptive broadcast protocols has been posed as an important problem. Such protocols are employed by database servers to decide on the content of broadcasts dynamically, in response to client mobility and demand patterns. In this paper we design such protocols and also propose efficient retrieval strategies that may be employed by clients to download information from broadcasts. The goal is to design cooperative strategies between server and client to provide access to information in such a way as to minimize energy expenditure by clients. We evaluate the performance of our protocols analytically.

Analysis of the draft IEEE 802.11 wireless local area network (WLAN) standard is needed to characterize the expected performance of the standard's ad hoc and infrastructure networks. The performance of the medium access control (MAC) sublayer, which consists of distributed coordination function (DCF) and point coordination function (PCF), is determined by simulating asynchronous data traffic in a 1 Mbps ad hoc network, and asynchronous data and packetized voice traffic in a 1 Mbps infrastructure network. The simulation models incorporate the effect of burst errors, packet size, RTS threshold and fragmentation threshold on network throughput and delay. The results show that the IEEE 802.11 WLAN can achieve a reasonably high efficiency when the medium is almost error-free, but may degrade appreciably under harsh fading. The results also show that time-sensitive traffic such as packet voice can be supported together with other intensive traffic such as packet data. However, an echo canceller is required for packet voice systems.