Elsevier

Computer Networks

Volume 55, Issue 15, 27 October 2011, Pages 3351-3363
Computer Networks

Low energy operation in WSNs: A survey of preamble sampling MAC protocols

https://doi.org/10.1016/j.comnet.2011.06.022Get rights and content

Abstract

The limited energy resources of sensor nodes are among the most important constraints in Wireless Sensor Networks (WSNs). Consequently, the Medium Access Control (MAC) layer design is crucial, due to its influence on the transceiver, which is the most energy-consuming component of a sensor node. Among the different MAC protocols designed for WSNs, preamble sampling techniques provide extremely low energy consumption at low loads and have a notably simple operation and a lack of synchronisation requirements, which are characteristics that are especially appealing to WSNs. In this work, a survey of the different types of MAC protocols designed for WSNs is presented with a special focus on preamble sampling MAC protocols. The aim of this work is to give a detailed overview and classification of the most relevant preamble sampling MAC protocols, being motivated by the extremely large number of MAC protocols designed for WSNs in recent years. Moreover, a simple set of guidelines for matching the most suitable MAC protocol category to a given application is provided in this work.

Introduction

Wireless Sensor Networks (WSNs) are networks formed by small and low-capability devices that are able to sense environmental metrics and to communicate them wirelessly to a central unit, known as a sink [1]. There is a wide range of potential applications in various areas, including industrial, military, environmental, health and home automation applications, which can profit from using WSNs to collect data. However, the large number of constraints on the sensor nodes and the special characteristics and uses of WSNs impose several challenges on the design of a WSN. Some of these characteristics are summarised next.

The most important characteristic of WSNs is their high application dependence [1]. Different requirements and constraints are directly imposed by the application. This issue complicates the design of a general protocol stack to be used in different deployments and applications and leads to the definition of different approaches. What follows is a review of the general characteristics and constraints of these networks; however, it is important to note that they all depend on the application and deployment scenario.

The deployment of dense WSNs in large, remote and difficult-to-access areas requires keeping the size and cost of the sensor nodes as low as possible. This constraint implies that the energy, computational and memory resources of the sensor nodes are usually limited. Therefore, simplicity and low energy consumption are important requirements in the design of WSN protocols. In addition, the limited energy resources of sensor nodes, combined with variable environmental conditions, cause a high level of network dynamics, which should also be taken into account.

Another interesting feature is the traffic patterns that are usually found in WSNs. In these networks, it is common that all of the sensor nodes periodically transmit information (the value of the metric of interest) to the central unit, causing a communication pattern notably different from the traditional point-to-point approach. There is also the case of event reporting (for example, when a metric surpasses a given threshold) to the sink. In this instance, only those nodes that detect the event will try to send a notification to the central unit. Finally, the sink can also query the network about certain information. For instance, the sink can ask the network in which location a given metric has surpassed a certain value. In this case, the data flows from the sink to all of the sensor nodes in a data-centric approach, in contrast to the more common communication pattern based on destination addresses.

The scalability of WSNs is also a challenge because deployments can be formed by a large number of sensor nodes. This challenge also causes extra difficulty in some scenarios in which a global identifier should be assigned to each device.

To summarise, the constraints and characteristics of WSNs are the following:

  • High application dependence

  • Large, remote, dense and difficult-to-access deployments

  • Low-cost and small devices

  • Devices with limited memory, transmitting and computing resources

  • Devices with limited energy resources

  • High network dynamics

  • Periodic, event-based and query-based communication patterns

  • Data flowing from all or a group of sensors to the central unit and vice versa

Among the different constraints, limitations on energy resources are the most important because this limitation directly affects the network lifetime. This relationship is particularly important given that, in typical WSN deployments, it is too costly or even impossible to access the sensor nodes to replace the batteries. In such a situation, it is obvious that the WSN protocol stack must be designed to obtain the highest possible energy savings and thus extend the network lifetime, while maintaining the performance of the target application. Therefore, the design of the Medium Access Control (MAC) layer is of crucial importance because it controls the most energy consuming component of a sensor node: the transceiver. Asynchronous MAC protocols and, particularly, preamble sampling are especially appealing to WSNs because of their simplicity and lack of synchronisation requirements.

In this work, a survey of the basic preamble sampling technique and its extensions is provided. There are other general surveys of MAC protocols for WSNs presented in the literature, such as the review performed in 2006 by Dermikol et al. [2], the extensive survey performed in 2007 by Kredo et al. [3], the survey performed by Langendoen in 2008 [4] and the more recent review presented by Bachir et al. [5]. The main difference between this work and those articles is in the specific focus placed here on the preamble sampling technique and its extensions. That focus allows us to provide a deeper study on this specific and prolific area. Moreover, this detailed study has been extended with a discussion of the benefits and drawbacks of each approach to provide a set of guidelines to help researchers and developers of WSN protocols and applications to select the most suitable MAC category for a given application.

The rest of this paper is organised as follows: Section 2 describes the importance of the MAC layer design in a WSN. Next, in Section 3, a general overview of the different categories of MAC protocols for WSNs is provided. In Section 4, the basic preamble sampling and its extensions are presented. A suggested procedure to select the MAC protocol based on the application is subsequently provided in Section 5. Finally, some conclusions are drawn.

Section snippets

The importance of the MAC layer in WSNs

The MAC layer is responsible for coordinating transmissions to a shared channel by defining how and when a node will attempt transmission. The medium access procedure can be based on a schedule assignment (which can be fixed during the entire network lifetime or during a certain amount of time) or, conversely, can be based on a random procedure in which the transmission attempt time is decided independently at each node. It is also possible to define hybrid approaches that combine both

A general overview of MAC protocols for WSNs

In previous years, a large number of MAC protocols that are specially designed for WSNs have been defined [3]. To save energy, especially the energy wasted because of idle listening, the most common approach is to put the transceiver into sleep mode for as much time as possible because sleep mode consumes substantially less energy than the other available modes (idle, transmitting or receiving). In sleep mode, a sensor node is not able to receive/transmit packets from/to the medium. This

Preamble sampling MAC protocols

In the basic preamble sampling technique, sensor nodes sleep and periodically wake up only to sample the channel. If the channel is determined to be busy, the nodes remain awake; otherwise, they return to sleep. The time between channel samples, called the check interval (Tci), is fixed and is known by all of the nodes in the network. To ensure the correct reception of packets, each message is preceded by a long preamble transmission that must overlap with the listening time of the receiver;

Selection guideline

As previously pointed out, the selection of the MAC protocol to use strictly depends on the application to be provided. The usual recommendation is to use TDMA-like MAC protocols for high traffic loads, protocols with common active periods for applications with periodic traffic and asynchronous protocols for low traffic loads [5]. In this section, a more elaborated guideline to select the best MAC category is provided. The guideline takes into account the preamble sampling extensions reviewed

Concluding remarks

Given the high application dependence of WSNs, there is not one MAC protocol able to satisfactorily work in every deployment. However, preamble sampling is able to provide some interesting capabilities, which are especially appealing to WSNs that usually work in low traffic load conditions and that are formed by low-capability and energy-constrained devices.

The benefits and disadvantages of basic preamble sampling have been outlined and used as the basis for the extended review of the different

Acknowledgment

This work has been partially supported by the Spanish Government under projects TEC2008-06055 (Plan Nacional I+D) and CSD2008-00010 (Consolider-Ingenio Program), and by the Catalan Government (SGR2009⧹#00617).

Cristina Cano obtained the Telecommunications Engineering Degree at the Universitat Politecnica de Catalunya (UPC) in February 2006. Next, she received an M.Sc. (2007) and a Ph.D. (2011) on Information, Communication and Audiovisual Media Technologies from the Universitat Pompeu Fabra (UPF). Currently, she is working at UPF on topics related to Wireless Networks, Wireless Sensor Networks, Quality of Service, MAC layer design and Multi-Channel operation.

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    Cristina Cano obtained the Telecommunications Engineering Degree at the Universitat Politecnica de Catalunya (UPC) in February 2006. Next, she received an M.Sc. (2007) and a Ph.D. (2011) on Information, Communication and Audiovisual Media Technologies from the Universitat Pompeu Fabra (UPF). Currently, she is working at UPF on topics related to Wireless Networks, Wireless Sensor Networks, Quality of Service, MAC layer design and Multi-Channel operation.

    Boris Bellalta received a B.Sc. degree in Telecommunications from the Universitat Politecnica de Catalunya (UPC) in 2002 and a Ph.D. from the Universitat Pompeu Fabra (UPF) in 2007, where he combined Ph.D. studies with a fulltime assistant professor position. Since 2007, he is a post-doc researcher and fulltime lecturer at UPF. His main research interests are in the area of wireless communications, MAC protocols and queuing models.

    Anna Sfairopoulou graduated in Computer Science from the University of Ioannina, Greece on 2000. She received her M.Sc. (2004) and Ph.D. (2008) degree on Computer Science and Digital Communications from the Universitat Pompeu Fabra (UPF). Since 2002, she has been working at the UPF as a fulltime researcher and teaching assistant and from 2008 as a Visiting Professor in the Department of Information and Communication Technologies of the UPF. Her research interests are focused in the area of performance analysis and QoS optimisation of VoIP and streaming video over 802.11 networks, MAC protocols for 802.11 and sensor networks, routing protocols and P2P overlay architectures for real-time applications.

    Miquel Oliver received a degree in Telecommunications Engineering (1994, UPC), a degree in Business Administration (2009, UOC) and a Ph.D. in Electrical Engineering (1999, UPC). He received a postdoctoral award, joining the Wireless Information Networks Laboratory (WINLAB) at Rutgers University (NJ, USA). He has been involved in research projects regarding wireless access networks following neutrality according to his ongoing research. His research topics include radio resource management for cellular systems, quality of service in wireless data networks, IP mobility and radio access protocols, including wireless sensor networks and distributed peer-to-peer protocols. He has also recently researched the ICT impact upon society, as well as business models for incoming technologies and networks.

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