Qinglin Zhao

A Simple and Approximate Model for Nonsaturated IEEE 802.11 DCF

We propose an approximate model for a nonsaturated IEEE 802.11 DCF network that is simpler than others that have appeared in the literature. Our key simplification is that the attempt rate in the nonsaturated setting can be approximated by scaling the attempt rate of the saturated setting with an appropriate factor. Use of different scaling factors leads to variants of the model for a small buffer and an infinite buffer. We develop a general fixed-point analysis that we demonstrate can have nonunique solutions for the infinite buffer model variant under moderate traffic. Nevertheless, in an asymptotic regime that applies to light traffic, we are able to prove uniqueness of the fixed point and predict the offered load at which the maximum throughput is achieved. We verify our model using ns-2 simulation and show that our MAC access delay results are the most accurate among related work, while our collision probability and throughput results achieve comparable accuracy to (D. Malone et al., 2007), (K. Duffy et al., 2007).

Modeling Nonsaturated IEEE 802.11 DCF Networks Utilizing an Arbitrary Buffer Size

We propose an approximate model for a nonsaturated IEEE 802.11 DCF network. This model captures the significant influence of an arbitrary node transmit buffer size on the network performance. We find that increasing the buffer size can improve the throughput slightly but can lead to a dramatic increase in the packet delay without necessarily a corresponding reduction in the packet loss rate. This result suggests that there may be little benefit in provisioning very large buffers, even for loss-sensitive applications. Our model outperforms prior models in terms of simplicity, computation speed, and accuracy. The simplicity stems from using a renewal theory approach for the collision probability instead of the usual multidimensional Markov chain, and it makes our model easier to understand, manipulate and extend; for instance, we are able to use our model to investigate the important problem of convergence of the collision probability calculation. The remarkable improvement in the computation speed is due to the use of an efficient numerical transform inversion algorithm to invert generating functions of key parameters of the model. The accuracy is due to a carefully constructed model for the service time distribution. We verify our model using ns-2 simulation and show that our analytical results based on an M/G/1/K queuing model are able to accurately predict a wide range of performance metrics, including the packet loss rate and the waiting time distribution. In contradiction to claims by other authors, we show that 1) a nonsaturated DCF model like ours that makes use of decoupling assumptions for the collision probability and queuing dynamics can produce accurate predictions of metrics other than just the throughput, and 2) the actual service time and waiting time distributions for DCF networks have truncated heavy-tailed shapes (i.e., appear initially straight on a log-log plot) rather than exponential shapes. Our work will help developers select appropriate buffer sizes for 802.11 devices, and will help system administrators predict the performance of applications.

An Equal-Spacing-Based Design for QoS Guarantee in IEEE 802.11e HCCA Wireless Networks

IEEE 802.11e standard develops a reference design for a sample scheduler and admission control unit to support the contention-free access. However, the reference design can not efficiently utilize the bandwidth. This paper proposes an equalspacing- based (equal-SP) design to address the problem. In the equal-SP design, which generalizes the reference design, each stream is scheduled with equal-spacing and different streams are scheduled with different spacings. The equal-SP design not only keeps all advantages of the reference design (i.e., it is simple, easy to implement, and can guarantee the delay requirement), but it is compatible with the standard and can also utilize the bandwidth efficiently.

A simple critical-load-based CAC scheme for IEEE 802.11 DCF networks

This paper proposes a simple and practical call admission control (CAC) scheme for one-hop IEEE 802.11 distributed coordination function (DCF) networks in heterogeneous environments. The proposed scheme is the first CAC scheme derived from an asymptotic analysis of the critical traffic load, where the critical traffic load represents the threshold for queue stability. The salient feature of our CAC scheme is that it can be performed quickly and easily without the need for network performance measurements and complex calculations. Using the proposed scheme, we specifically investigate the voice capacity of 802.11 DCF networks with unbalanced traffic. Extensive simulations covering both ad hoc and infrastructure-based networks, and a variety of nonsaturated traffic types, show that the proposed CAC scheme is very effective.

A Scalable and Accurate Nonsaturated IEEE 802.11e EDCA Model for an Arbitrary Buffer Size

IEEE 802.11e EDCA induces service differentiation by appropriate joint tuning of four adjustable contention parameters. Existing and emerging work has devoted considerable attention to the nonsaturated performance of EDCA networks due to the difficulty of predicting the joint influence of the four parameters. However, most existing nonsaturated EDCA models adopt complex extensions of a Markov-chain approach. In sharp contrast, this paper invokes an extension of a renewal-reward approach. Our extension has the following unparalleled advantages: good scalability, ease of understanding, fast computation speed, high accuracy, models joint differentiation of all four parameters, captures the impact of an arbitrary buffer size, and predicts a wide range of performance indicators including the buffer overflow probability and the MAC access delay distribution. Our nonsaturated EDCA model is a nontrivial augmentation of our previously proposed nonsaturated DCF model. Our results indicate that if we accurately model the nonsaturated collision probability, the same formulas used for the saturated performance descriptors can produce accurate results for nonsaturated operation, and therefore it is unnecessary to construct specific formulas for nonsaturated performance descriptors, as done in previous work. To illustrate the utility of our model, we also develop an admission control policy based on the proposed EDCA model for a CWmin-differentiation system. Simulations validate that this policy enables the system to run slightly below a critical point, beyond which the system performance deteriorates drastically.

A simple model for nonsaturated IEEE 802.11 DCF networks

We propose a simple approximate model for a nonsaturated IEEE 802.11 DCF network. Our model is much simpler than others that have appeared in the literature. The main simplification we introduce is that the attempt rate in the nonsaturated setting can be approximated by scaling the attempt rate of the saturated setting with the probability of a packet arrival. We verify our model using ns-2 simulation and show that our MAC access delay result is the most accurate amongst related work, while our collision probability and throughput results achieve comparable accuracy to [1].

Distance-Based Location Management Utilizing Initial Position for Mobile Communication Networks

This paper aims at improving the distance-based location management scheme for mobile communication networks. In location management, a mobile terminal (MT) is tracked based on its location-update area (LA). The improvement is brought about by joint optimization of LA center and LA size. For LA center optimization (LCO), we determine the optimal center position of the LA given the initial position of the MT upon each location update. The investigation of optimal LA center has eluded research to date. Based on the popular continuous-time random walk (CTRW) mobility model, we propose an analytical framework that uses a diffusion equation to determine the optimal LA center that minimizes the total cost of location management, consisting of the location update cost and terminal paging cost. This framework allows us to easily model the non-Markovian movement of the MT and evaluate the impact of various measurable physical parameters (such as length of road section, angle between road sections, and road section crossing time) and LA center. In particular, we show that proper LA center can significantly reduce the total cost. For example, for the circular LA and low Poisson call-arrival rate, optimizing the LA center alone has the potential of reducing the cost by up to 37 percent. Joint optimization of the LA center and terminal paging scheme can reduce the cost even further. Simulations results match the theoretical analysis to a gap within 3 percent, indicating that our theoretical model is very accurate.

A novel CAC scheme for homogeneous 802.11 networks

This paper proposes a new call admission control (CAC) scheme for one-hop homogeneous 802.11 DCF networks. Using the proposed scheme, we can perform admission control quickly and easily without the need for network performance measurements and complex calculations. The CAC rule is derived under asymptotic conditions, but our extensive numerical examples show that it works well for practical-sized networks with a finite retransmission limit and realistic nonsaturated traffic.

Location update cost of distance-based scheme for PCS networks with CTRW model

This letter concerns the optimization of the dynamic location-update area (LA) for the per-user distance-based scheme in personal communication service (PCS) networks. We have two main contributions: (1) under the general and popular one-dimensional (1-D) continuous-time random walk (CTRW) mobility model, we propose a novel framework to analyze the location update cost; (2) with this framework, we investigate the impact of call arrivals and the initial position of the mobile terminal (MT) on the position of the LA, which was neglected by previous work. Simulation shows that our theoretical model is very accurate.

Enhancing QoS Support in IEEE 802.11e HCCA

IEEE 802.11e standard develops a reference design to support the contention-free access. In the reference design, the packet transmission opportunity (TXOP) duration is calculated based on the mean data rate and the mean packet size, whereas the scheduled service interval (SI) is calculated based on the most stringent delay requirement. Such design can not effectively utilize the bandwidth and support the guaranteed packet loss requirement. This paper proposes a packet-loss-based and bandwidth-utilization-based (PB-based) design to address the two problems. In the PB-based design, the TXOP is calculated based on the data rate and packet size fluctuation, whereas the SI is calculated based on different delay requirements. In addition, when packet loss requirement is not taken into account, we propose an equal-spacing-based (ES-based) design to improve the bandwidth utilization. The proposed designs are helpful to provide more comprehensive QoS support.