Map: a scalable monitoring system for dependable 802.11 wireless networks [dependability in the ubiquitous wireless access]

Many enterprises deploy 802.11 wireless networks for mission-critical operations; these networks must be protected for dependable access. This article introduces the MAP project, which includes a scalable 802.11 measurement system that can provide continuous monitoring of wireless traffic to quickly identify threats and attacks. We discuss the MAP system architecture, design decisions, and evaluation results from a real testbed.     

Optimal admission control in multimedia mobile networks with handover prediction [dependability in the ubiquitous wireless access]

Admission control is one of the key traffic management mechanisms that must be deployed in order to meet the strict requirements for dependability imposed on the services provided by modern wireless networks. We study the problem of optimizing admission control policies in mobile multimedia cellular networks when predictive information regarding the movement of mobile terminals is available. For the optimization process we deploy a novel reinforcement learning approach based on the concept of afterstates. The results obtained define theoretical limits for the gain that can be expected when using handover prediction, which cannot be established by deploying heuristic approaches. Numerical results show that the performance gain is a function of the anticipation time with which the admission controller knows the occurrence of handovers, and an optimal anticipation time exists. We also compare an optimal policy obtained deploying our approach with a previously proposed heuristic prediction scheme, showing that there is still room for technological innovation.     

Identity-based key management in mobile ad hoc networks: techniques and applications [dependability in the ubiquitous wireless access]

Security is one of the major issues in MANETs. Their natural characteristics make them vulnerable to numerous severe attacks. It is widely acknowledged that cryptography provides a set of strong techniques against most vulnerabilities. Several cryptographic mechanisms for MANETs can be found in the literature. Among them, identity-based cryptographic mechanisms and key management schemes are proposed to simplify key management and to reduce the memory storage cost. This article presents the most important ID-based key management schemes, discussing their approaches, strengths, and weaknesses, and comparing their main features. It also presents the main IDbased key management application fields on MANETs. In this way it can be useful for users and researchers as a starting point on ID-based key management and its possible uses in MANETs.     

Dependability issues with ubiquitous wireless access [guest editorial]

Recent years have witnessed a proliferation of the number of wireless technologies available to access the Internet, ranging from wireless local area networks to cellular and broadcast systems, and ad hoc and mesh networks. While the emergence of these new technologies can enable truly ubiquitous Internet access, it also raises issues with the dependability of the Internet service delivered to users. Dependability in this context refers to the ability of a wireless access system to deliver specified services on which users can rely.     

An overview of MIMO-oriented channel access in wireless networks [medium access control protocols for wireless LANs]

The integration of MIMO technology in WLANs has recently been the locus of extensive research. The main objectives of this technology are to improve channel reuse and or reduce energy consumption. In this article, we give an overview of MIMO systems and their use in WLANs. We highlight the different types of gains that MIMO offers and then discuss some of the work that has been done on MAC design. We conclude by outlining several open issues thai must be addressed for MIMO-based systems.     

Providing QOS support at the distributed wireless MAC layer: a comprehensive study[medium access control protocols for wireless LANs]

Holding the promise of making ubiquitous mobile access to IP-based applications and services a reality, wireless local area networks have been deployed in an unlimited way over the last few years. Due to their robust characteristics, distributed MAC protocols are the most widely used mechanisms to arbitrate access to the wireless channel. However, their ability to achieve high medium usage efficiency while providing services with meaningful performance assurances is being challenged by a wide range of existing and emerging applications that have lately migrated from other telecommunication networks to wireless environments. This article aims to provide a comprehensive study of the limitations and merits of mechanisms that have been proposed toward embedding QoS support to distributed wireless MAC protocols. A hybrid scheme that incorporates signaling and information sharing is proposed, and extensive simulation experiments are run to assess the efficiency of the access schemes in maximizing utilization of the wireless bandwidth while providing QoS support for heterogeneous applications.     

MCCA: a high-throughput MAC strategy for next-generation WLANs [medium access control protocols for wireless LANs]

Wireless LAN technology has been shown to he a revolutionary development during the last decade. Recently popularized IEEE 802.11a/g-based products can support up to 54 Mb/s physical layer rate and provide wireless access to the Internet. However, in order to deal robustly with the unreliable wireless nature, the 802.11 medium access control protocol has a relatively large overhead and hence, the throughput performance is much worse than the underlying physical layer rate. Moreover, along with many emerging applications and services over WLANs, such as voice over WLAN and audio/video streaming, the demand lor faster and higher- capacity WLANs has been growing recently. In this article, we propose a new medium access control protocol for the next-generation high-speed WLANs. The proposed medium access control, called multi-user polling controlled channel access, is composed of two components: multi-layer frame aggregation, which performs aggregation at both the medium access control and the physical layers; and multi-user polling, used to reduce the contention overhead and in turn, achieve higher network utilization. Multi-user polling controlled channel access is compared with the 802.11e-enhanced distributed channel access medium access control. Highly enhanced medium access control efficiency can be achieved by applying multi-user polling controlled channel access. We show the improved medium access control performance in terms of the aggregate throughput of non-QoS Hows with relevant QoS requirements.     

Variable bit rate VOiP in IEEE 802.11e wireless LANs [medium access control protocols for wireless LANs]

WLANs have become a ubiquitous networking technology deployed everywhere. Meanwhile. VoIP is one popular application and a viable alternative to traditional telephony systems due to its cost efficiency. VoIP over WLAN (VoWLAN) has been emerging as an infrastructure to provide low-cost wireless voice services. However, VoWLAN poses significant challenges due to the characteristics of contention-based protocols and wireless networks. In this article we propose two mechanisms to provide quality of service for variable bit rate VoIP in IEEE 802.11e contention-based channel access WLANs: access time-based admission control and access point dynamic access. Simulation results are conducted to study these schemes.     

A near-optimum cross-layered distributed queuing protocol for wireless LAN [medium access control protocols for wireless LANs]

Distributed queuing collision avoidance (DQCA) is a distributed MAC protocol for WLAN systems that offers near optimum performance. The protocol implements a reservation scheme that ensures collision-free data transmissions for high traffic load and switches smoothly and automatically to a random access mechanism when the traffic load is light, improving the delay performance for this situation. In this article the DQCA protocol operation is thoroughly described, and its algorithm rules are given. Furthermore, an enhanced cross-layer scheduling mechanism is also proposed for inclusion in the protocol procedure. This mechanism employs a virtual priority function to reschedule transmissions according to a cross-layer design. Two possible configurations are described in this article by including a PHY-MAC dialog involving channel stale information and the waiting time of the packets in the system, offering a trade-off between throughput maximization and fairness. The performance in terms of throughput and mean delay of DQCA and the two cross-layer schemes has been evaluated through simulations, and a significant enhancement over legacy IEEE 802.11 operation is achieved. The obtained results emphasize the advantages of the proposed schemes and the importance of cross-layer design in wireless communication systems.     

Medium access control protocols for wireless LANs [guest editorial]

The six papers in this special issue focus on medium access control protocols for wireless LANs. The papers are summarized here.     

The problem of medium access control in wireless sensor networks

In this article we revisit the problem of scheduled access through a detailed foray into the questions of energy consumption and throughput for MAC protocols in wireless sensor networks. We consider a static network model that rules out simultaneous transmission and reception by any sensor node and consequently requires partitioning of nodes into disjoint sets of transmitters and receivers at any time instant. Under the assumption of circular transmission (reception) ranges with sharp boundaries, a greedy receiver activation heuristic is developed relying on the network connectivity map to determine distinct receiver groups to be activated within disjoint time intervals. To conserve limited energy resources in sensor networks, the time allocation to each receiver group is based on the residual battery energy available at the respective transmitters. Upon activating each receiver group separately, the additional time-division mechanism of group TDMA is imposed to schedule transmissions interfering at the non-intended destinations within separate fractions of time in order to preserve the reliable feedback information. The two-layered time-division structure of receiver activation and group TDMA algorithms offers distributed and polynomial-time solutions (as required by autonomous sensor networks) to the problems of link scheduling as well as energy and throughput-efficient resource allocation in wireless access. The associated synchronization and overhead issues are not considered in this article.     

Multipacket reception in random access wireless networks: fromsignal processing to optimal medium access control

There has been considerable interest in the idea of cross-layer design of wireless networks. This is motivated by the need to provide a greater level of adaptivity to variations of wireless channels. This article examines one aspect of the interaction between the physical and medium access control layers. In particular, we consider the impact of signal processing techniques that enable multipacket reception on the throughput and design of random access protocols     

Multi-objective cooperative medium access control protocols in wireless Ad-Hoc networks

Wireless Networks - Relay-based cooperative communications have been emerging as a novel paradigm in many wireless protocols. The IEEE 802.11 medium access control (MAC) protocols have attracted...     

Spatial correlation-based collaborative medium access control in wireless sensor networks

Wireless Sensor Networks (WSN) are mainly characterized by dense deployment of sensor nodes which collectively transmit information about sensed events to the sink. Due to the spatial correlation between sensor nodes subject to observed events, it may not be necessary for every sensor node to transmit its data. This paper shows how the spatial correlation can be exploited on the Medium Access Control (MAC) layer. To the best of our knowledge, this is the first effort which exploits spatial correlation in WSN on the MAC layer. A theoretical framework is developed for transmission regulation of sensor nodes under a distortion constraint. It is shown that a sensor node can act as a representative node for several other sensor nodes observing the correlated data. Based on the theoretical framework, a distributed, spatial Correlation-based Collaborative Medium Access Control (CC-MAC) protocol is then designed which has two components: Event MAC (E-MAC) and Network MAC (N-MAC). E-MAC filters out the correlation in sensor records while N-MAC prioritizes the transmission of route-thru packets. Simulation results show that CC-MAC achieves high performance in terms energy, packet drop rate, and latency.     

Performance of secondary users in opportunistic spectrum access

An analytical model to evaluate the performance of a secondary user for opportunistic spectrum access in cognitive networks under non-saturation traffic conditions is developed. Numerical results are given to show the performance of secondary users in cognitive networks.     

Multicarrier DS-CDMA: a multiple access scheme for ubiquitous broadband wireless communications

We identify some of the key problems that may be encountered when designing a broadband multiple access system with bandwidth on the order of tens or even hundreds of megahertz. We commence with a comparative discussion in terms of the characteristics of three typical code-division multiple access schemes: single-carrier direct-sequence CDMA (SC DS-CDMA), multicarrier CDMA (MC-CDMA), and multicarrier DS-CDMA (MC DS-CDMA). Specifically, their benefits and deficiencies are analyzed when aiming to support ubiquitous communications over a variety of channels encountered in indoor, open rural, suburban, and urban environments. It is shown that when communicating in such diverse environments, both SC DS-CDMA and MC-CDMA exhibit certain limitations that are hard to circumvent. By contrast, when appropriately selecting the system parameters and using transmit diversity, MC DS-CDMA becomes capable of adapting to such diverse propagation environments at a reasonable detection complexity.     

A taxonomy for medium access control protocols in wireless sensor networks

Wireless sensor networks (Wsns) tend to be highly optimized due to severely restricted constraints. Various medium access control (Mac) protocols forWsns have been proposed, being specially tailored to a target application. This paper proposes a taxonomy for the different mechanisms employed in those protocols. The taxonomy characterizes the protocols according to the methods implemented to handle energy consumption, quality of service and adaptability requirements. We also present an overview of the transceptors found inWsns, identifying how events on communication affect the energy consumption. Based on the taxonomy, we classify existingMac protocols. Finally, we discuss challenging trends inMac protocols forWsns, such as security issues and software radios.