Physical layer capture (PLC) in 802.11b refers to the successful reception of the stronger (higher signal strength at receiver) frame in a collision. PLC causes significant imbalance in the throughputs of sources. Existing 802.11b simulators, including ns2 and Qualnet, assume that PLC occurs only if the stronger frame arrives first at the receiver. We show empirically that in reality PLC occurs even if the stronger frame arrives later (but within the physical layer preamble of the first frame). Consequently, throughput unfairness in reality can be significantly (up to 15%) higher than with the former PLC model. We have modified the ns2 simulator to account for this and Qualnet will be incorporating a fix in their next release. To identify which frames were involved in collisions, when their transmissions started, and which of them were retrieved, we have devised a novel technique using multiple sniffers and instrumented device drivers to reconstruct from the air interface all tx/rx events in a WLAN to within 4 � × accuracy. This allows us to quantify the causal links from the PHY layer through the MAC layer to the observed application layer imbalance. It also shows that the arrival times of colliding frames routinely differ by as much as 20 � × due to inherent uncertainties of 802.11b firmware clock synchronization and rx/tx turnaround delays, and that the frame to arrive first can be either the stronger or the weaker with equal likelihood. 1.

A new modeling framework is introduced for the analytical study of medium access control (MAC) protocols operating in multihop ad hoc networks. The model takes into account the e#ect of physical-layer parameters on the success of transmissions, the MAC protocol on the likelihood that nodes can access the channnel, and the connectivity of nodes in the network. A key feature of the model is that nodes can be modeled individually, i.e., it allows a per-node setup of many layer-specific parameters. Moreover, no spatial probability distribution or a particular arrangement of nodes is assumed; the model allows the computation of individual (per-node) performance metrics for any given network topology and radio channel model. To show the applicability of the modeling framework, we model multihop ad hoc networks using the IEEE 802.11 distributed coordination function and validate the results from the model with discreteevent simulations in Qualnet. The results show that our model predicts results that are very close to those attained by simulations, and requires seconds to complete compared to several hours of simulation time.

A performance comparison is presented between two types of code-division multiple-access wireless local area networks: centrally controlled and ad hoc networks. Based on a finite-population model, the network throughput, the average packet delay, and the network first exit time are derived for both systems. Two aspects of the performance comparison are addressed: 1) the comparison between the centrally controlled and the ad hoc architecture; and 2) the impact of spreading gain and error control coding on both systems. The efficiency of bandwidth utilization is investigated by normalizing the network performance with respect to the consumed bandwidth. Evaluations of these performance comparisons are also provided.

We present an acknowledged anycast primitive that allows a node to wirelessly transmit a packet and efficiently determine that at least one neighbor successfully received it. The initiator transmits a single packet to a unicast, multicast, or broadcast address and all nodes that match the destination respond with identical acknowledgment packets automatically generated by the hardware. Although these acknowledgments interfere, they usually do so non-destructively, so the initiator can decode their superposition. We call such an exchange a Backcast and show that this operation is feasible using a commodity radio, general because it enables multiple network services, efficient because it is independent of the neighborhood size and runs in constant time, and scalable because it works with no fewer than a dozen interfering acknowledgments. 1

this paper, we derive general expressions for the stability (the cusp points and the bifurcation sets) of various types of slotted ALOHA systems, taking into account multiple packet reception capability of a receiver as well as the capture e#ect. In fact, some classes

A performance comparison is presented between two types of CDMA random access systems: cellular and ad hoc. Based on a finite population model, the network throughput is derived for both systems. Two as- pects of the performance comparison are addressed: (1) the throughput comparison; (2) the impact of spread- ing gain and error control coding on the throughput in both systems. Evaluations of these performance comparisons are also provided.

Abstract — While physical layer capture has been observed in real implementations of wireless devices accessing the channel like 802.11, log-utility fair allocation algorithms based on accurate channel models describing the phenomenon have not been developed. In this paper, using a general physical channel model, we develop an allocation algorithm for log-utility fairness. To maximize the aggregate utility, our algorithm determines channel access attempt probabilities of nodes using partial derivatives of the utility. Our algorithm is verified through extended simulations. The results indicate that our algorithm could quickly achieve allocations close to the optimum with 8.6 % accuracy error on average.

A new analytic framework based on a linear algebra approach is proposed for examining the performance of a direct sequence spread spectrum (DS/SS) slotted ALOHA wireless communication network systems with delay capture. The discrete-time Markov chain model has been introduced to account for the effect of randomized time of arrival (TOA) at the central receiver and determine the evolution of the finite population network performance in a single-hop environment. The proposed linear algebra approach applied to the given Markov problem requires only computing the eigenvector P of the state transition matrix and then normalizing it to have the sum of its entries equal to 1. MATLAB computation results show that systems employing discrete TOA randomization and delay capture significantly improves throughput-delay performance and the employed analysis approach is quite easily and staightforwardly applicable to the current analysis problem. 128 Mun Geon Kyeong ETRI Journal, volume 18, number 3...

ix Dedication x Acknowledgements xi 1 Introduction 1 2 Receiver-Initiated Multiple-Access Protocols 10 2.1 Receiver-Initiated Collision Avoidance . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2 Receiver-Initiated Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2.1 Protocols with Simple Polling . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.2.2 Protocols with Dual-Use Polling . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.2.3 Protocols with Broadcast Polling . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.3 Correct Collision Avoidance in RIMA protocols . . . . . . . . . . . . . . . . . . . . . 25 2.4 Performance Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.4.1 Approximate Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.4.2 Average Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 2.5 Simulation Results . . . . . . . . . . . . ...

Abstract not found