Enhancing IEEE 802.11MAC in congested environments

IEEE 802.11 is currently the most deployed wireless local area networking standard. It uses carrier sense multiple access with collision avoidance (CSMA/CA) to resolve contention between nodes. Contention windows (CW) change dynamically to adapt to the contention level: Upon each collision, a node doubles its CW to reduce further collision risks. Upon a successful transmission, the CW is reset, assuming that the contention level has dropped. However, the contention level is more likely to change slowly, and resetting the CW causes new collisions and retransmissions before the CW reaches the optimal value again. This wastes bandwidth and increases delays. In this paper we analyze simple slow CW decrease functions and compare their performances to the legacy standard. We use simulations and mathematical modeling to show their considerable improvements at all contention levels and transient phases, especially in highly congested environments

Medium Access Control Protocols for Ad-Hoc Wireless Networks: A Survey

Studies of ad hoc wireless networks are a relatively new field gaining more popularity for various new applications. In these networks, the Medium Access Control (MAC) protocols are responsible for coordinating the access from active nodes. These protocols are of significant importance since the wireless communication channel is inherently prone to errors and unique problems such as the hidden-terminal problem, the exposed-terminal problem, and signal fading effects. Although a lot of research has been conducted on MAC protocols, the various issues involved have mostly been presented in isolation of each other. We therefore make an attempt to present a comprehensive survey of major schemes, integrating various related issues and challenges with a view to providing a big-picture outlook to this vast area. We present a classification of MAC protocols and their brief description, based on their operating principles and underlying features. In conclusion, we present a brief summary of key ideas and a general direction for future work

Optimization of Efficiency and Energy Consumption in p-persistent CSMA-based Wireless LANs

Wireless technologies in the LAN environment are becoming increasingly important. The IEEE 802.11 is the most mature technology for Wireless Local Area Networks (WLANs). The limited bandwidth and the finite battery power of mobile computers represent one of the greatest limitations of current WLANs. In this paper we deeply investigate the efficiency and the energy consumption of MAC protocols that can be described with a p-persistent CSMA model. As already shown in the literature, the IEEE 802.11 protocol performance can be studied using a p-persistent CSMA model [Cal00]. For this class of protocols, in the paper we define an analytical framework to study the theoretical performance bounds from the throughput and the energy consumption standpoint. Specifically, we derive the p values (i.e., the average size of the contention window in the IEEE 802.11 protocol) that maximizes the throughput, $p^C_{opt}$, and minimizes the energy consumption, $p^E_{opt}$. By providing analytical closed formulas for the optimal values, we discuss the trade-off between efficiency and energy consumption. Specifically, we show that power saving and throughput maximization can be jointly achieved. Our analytical formulas indicate that the optimal $p$ values depend on the network configuration, i.e., number of active stations and length of the messages transmitted on the channel

Improving the Performance of Wireless LANs

This book quantifies the key factors of WLAN performance and describes methods for improvement. It provides theoretical background and empirical results for the optimum planning and deployment of indoor WLAN systems, explaining the fundamentals while supplying guidelines for design, modeling, and performance evaluation. It discusses environmental effects on WLAN systems, protocol redesign for routing and MAC, and traffic distribution; examines emerging and future network technologies; and includes radio propagation and site measurements, simulations for various network design scenarios, numerous illustrations, practical examples, and learning aids

An efficient and fair reliable multicast protocol for 802.11-based wireless LANs

Many applications are inherently multicast in nature. Such applications can benefit tremendously from reliable multicast support at the MAC layer since addressing reliability at the MAC level is much less expensive than handling errors at the upper layers. However, the IEEE 802.11 MAC layer does not support reliable multicast. This void in the MAC layer is a limiting factor in the efficacy of multicast applications. In this work, we propose a Slot Reservation based Reliable Multicast protocol that adds a novel reliability component to the existing multicast protocol in the 802.11 MAC. Our protocol builds on the existing DCF support in the IEEE 802.11 MAC to seamlessly incorporate an efficient reliable multicast mechanism. Intelligent assignment of transmission slots, minimal control packet overhead and an efficient retransmission strategy form the basis of our protocol. We evaluate the performance of our protocol through extensive simulations. Our simulation results show that our protocol outperforms another reliable multicast protocol, Batch Mode Multicast MAC, in terms of delivered throughput in various scenarios. We enhance our protocol to add a fairness component in the presence of parallel unicast and multicast flows and provide unicast friendly multicast operation. We then evaluate the performance of our Slot Reservation Based Reliable Multicast Protocol with Fairness through extensive simulations and see that the scheme ensures fairness among parallel unicast and multicas

Performance modelling and enhancement of wireless communication protocols

In recent years, Wireless Local Area Networks(WLANs) play a key role in the data communications and networking areas, having witnessed significant research and development. WLANs are extremely popular being almost everywhere including business,office and home deployments.In order to deal with the modem Wireless connectivity needs,the Institute of Electrical and Electronics Engineers(IEEE) has developed the 802.11 standard family utilizing mainly radio transmission techniques, whereas the Infrared Data Association (IrDA) addressed the requirement for multipoint connectivity with the development of the Advanced Infrared(Alr) protocol stack. This work studies the collision avoidance procedures of the IEEE 802.11 Distributed Coordination Function (DCF) protocol and suggests certain protocol enhancements aiming at maximising performance. A new, elegant and accurate analysis based on Markov chain modelling is developed for the idealistic assumption of unlimited packet retransmissions as well as for the case of finite packet retry limits. Simple equations are derived for the through put efficiency, the average packet delay, the probability of a packet being discarded when it reaches the maximum retransmission limit, the average time to drop such a packet and the packet inter-arrival time for both basic access and RTS/CTS medium access schemes.The accuracy of the mathematical model is validated by comparing analytical with OPNET simulation results. An extensive and detailed study is carried out on the influence of performance of physical layer, data rate, packet payload size and several backoff parameters for both medium access mechanisms. The previous mathematical model is extended to take into account transmission errors that can occur either independently with fixed Bit Error Rate(BER) or in bursts. The dependency of the protocol performance on BER and other factors related to independent and burst transmission errors is explored. Furthermore, a simple-implement appropriate tuning of the back off algorithm for maximizing IEEE 802-11 protocol performance is proposed depending on the specific communication requirements. The effectiveness of the RTS/CTS scheme in reducing collision duration at high data rates is studied and an all-purpose expression for the optimal use of the RTS/CTS reservation scheme is derived. Moreover, an easy-to-implement backoff algorithm that significantly enhances performance is introduced and an alternative derivation is developed based on elementary conditional probability arguments rather than bi-dimensional Markov chains. Finally, an additional performance improvement scheme is proposed by employing packet bursting in order to reduce overhead costs such as contention time and RTS/CTSex changes. Fairness is explored in short-time and long-time scales for both the legacy DCF and packet bursting cases. AIr protocol employs the RTS/CTS medium reservation scheme to cope with hidden stations and CSMA/CA techniques with linear contention window (CW) adjustment for medium access. A 1-dimensional Markov chain model is constructed instead of the bi-dimensional model in order to obtain simple mathematical equations of the average packet delay.This new approach greatly simplifies previous analyses and can be applied to any CSMA/CA protocol.The derived mathematical model is validated by comparing analytical with simulation results and an extensive Alr packet delay evaluation is carried out by taking into account all the factors and parameters that affect protocol performance. Finally, suitable values for both backoff and protocol parameters are proposed that reduce average packet delay and, thus, maximize performance

DIFS modifications to support QoS in IEEE 802.11g DCF ad-hoc networks

This paper describes and investigates the QoS provisioning technique used in IEEE 802.11g ad-hoc structure. This research then propose better scheme to support QoS by modifying the DCF Interframe Space (DIFS) to use new values to bias towards the high priority traffic flow and distinguish it from the low priority traffic.Simulations are done using NS-2 and the findings presented. Results showed that better throughput can be achieved to provide better traffic flows on high priority traffi

Medium access control protocol design for wireless communications and networks review

Medium access control (MAC) protocol design plays a crucial role to increase the performance of wireless communications and networks. The channel access mechanism is provided by MAC layer to share the medium by multiple stations. Different types of wireless networks have different design requirements such as throughput, delay, power consumption, fairness, reliability, and network density, therefore, MAC protocol for these networks must satisfy their requirements. In this work, we proposed two multiplexing methods for modern wireless networks: Massive multiple-input-multiple-output (MIMO) and power domain non-orthogonal multiple access (PD-NOMA). The first research method namely Massive MIMO uses a massive number of antenna elements to improve both spectral efficiency and energy efficiency. On the other hand, the second research method (PD-NOMA) allows multiple non-orthogonal signals to share the same orthogonal resources by allocating different power level for each station. PD-NOMA has a better spectral efficiency over the orthogonal multiple access methods. A review of previous works regarding the MAC design for different wireless networks is classified based on different categories. The main contribution of this research work is to show the importance of the MAC design with added optimal functionalities to improve the spectral and energy efficiencies of the wireless networks