Taka Sakurai

MAC Access Delay of IEEE 802.11 DCF

The MAC access delay in a saturated IEEE 802.11 DCF wireless LAN is analyzed. We develop a unified analytical model and obtain explicit expressions for the first two moments as well as the generating function. We show via comparison with simulation that our model accurately predicts the mean, standard deviation, and distribution of the access delay for a wide range of operating conditions. In addition, we show that the obtained generating function is much more accurate than others that have appeared in the literature. Using our model, we prove that the binary exponential backoff mechanism induces a heavy-tailed delay distribution for the case of unlimited retransmissions. We show using numerical examples that the distribution has a truncated power-law tail when a retransmission limit exists. This finding suggests that DCF is prone to long delays and not suited to carrying delay-sensitive applications

Performance Analysis of the IEEE 802.11 MAC Protocol for DSRC Safety Applications

In this paper, we evaluate and improve the performance of the medium-access control (MAC) protocol for safety applications in a dedicated short-range communication (DSRC) environment. We first develop an analytical model to study the IEEE 802.11 distributed coordination function (DCF) MAC protocol that has been adopted by the IEEE 802.11p standard for DSRC. Explicit expressions are derived for the mean and standard deviation of the packet delay, as well as for the packet delivery ratio (PDR) at the MAC layer in an unsaturated network formed by moving vehicles on a highway. The proposed model is validated using extensive simulations, and its superior accuracy is compared with that of other existing models is demonstrated. Insights gained from our model reveal that the principal reason for the low PDR of the DCF protocol is packet collision due to transmissions from hidden terminals. We then present a novel protocol based on the DCF that uses an out-of-band busy tone as a negative acknowledgment to provide an efficient solution to the aforementioned problem. We extend our analytical model to the enhanced protocol and show that it preserves predictive accuracy. Most importantly, our numerical experiments confirm that the enhanced protocol improves the PDR by up to 10% and increases the supported vehicle density by up to two times for a range of packet arrival rates while maintaining the delay below the required threshold level.

Packet loss analysis of the IEEE 802.15.4 MAC without acknowledgements

Transmission of the acknowledgement frame after a packet reception is optional in the IEEE 802.15.4 standard. Considering a set of sensor nodes, each of which has a packet to transmit at the beginning of an active period, we provide an analysis that yields the packet loss statistics of the non-acknowledgement mode of the standard. The analysis is based on a non-stationary Markov chain model and its accuracy is verified by ns-2 simulations

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).

Accurate delay distribution for IEEE 802.11 DCF

We derive the access delay generating function of the IEEE 802.11 DCF protocol. Our analysis corrects a model recently presented by O. Tickoo and B. Sikdar (see Proc. IEEE INFOCOM 2004, p.1404-13). We demonstrate that numerical transform inversion can be used to efficiently obtain values of the distribution. Simulations show that our analytical results are highly accurate.

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.

Performance analysis of the IEEE 802.11 MAC protocol for DSRC with and without Retransmissions

We develop an accurate analytical model for a dedicated short range communication (DSRC) network that uses the IEEE 802.11 distributed coordination function (DCF) MAC protocol, as adopted by the forthcoming IEEE 802.11p specification for DSRC. The specific focus is on broadcast vehicle-to-vehicle safety messages. We derive explicit expressions for the mean of the total packet delay and the packet delivery ratio (PDR) in an unsaturated network formed by moving vehicles on a highway. Our model is validated using extensive simulations and we show that our model yields better predictive accuracy than other existing models. The model is then used to investigate the performance of a modified DCF that uses a fixed number of sequential retransmissions to improve the reliability of packet delivery. We find that with sequential retransmissions, the PDR improves at low vehicle density (i.e. low traffic load), but degrades at heavy loads where higher collisions induced by the retransmissions outweighs the benefit of repeated attempts.

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.

An Analysis of Different Backoff Functions for an IEEE 802.11 WLAN

We compare the performance of different backoff functions for the multiple access protocol in an IEEE 802.11 wireless LAN (WLAN). We provide a unified analytical model with explicit expressions for the mean and standard deviation of the access delay for generalized exponential, polynomial and linear backoff functions. Using our model, we show that linear and polynomial backoffs with appropriate parameter settings can improve upon binary exponential backoff specified in the 802.11 WLAN standards, in terms of throughput, access delay statistics and packet drop rate.

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.