Keywords

1 Introduction

Wireless Sensor Networks (WSNs) has been under worldwide concentration in recent years, thanks to the expansion in Micro-Electro-Mechanical System (MEMS) technology which has made the development of integrated smart sensors feasible [1]. The sensor nodes in the network establish communication by using the wireless medium, which may be either radio frequency waves, infrared medium or any other media in which there is no use of the wires is done [2]. These nodes are capable of monitoring physical or environmental conditions, such as temperature, sound, pressure, etc. Nodes can also cooperatively pass their data through the network to a main location. There are four top basic functions that must be performed by wireless sensor networks: sensing, processing, storage of data, and communication.

Fundamentally, the number of the sensor nodes that are involved in functional WSNs is large, due to the small size and resource limitation of sensor nodes. Nodes depend on limited battery power as their primary power resource, which may get depleted easily due to the complex operations these nodes have to perform. Additionally, hostile deployment regions make it unrealistic to recharge or replace the batteries frequently. Therefore, the main challenge in WSNs is to reduce the energy consumption and maximize network’s lifetime. Major sources of energy waste in WSNs are basically of five types [3, 4], which are shown as follows.

  • Collision: When a transmitted packet is corrupted because of interference, it has to be discarded and retransmitted by the sender when available. This causes wastage in both sender and receiver as it expands energy without any benefit [5]. Collision also increases latency which adversely affects the network transmission speed.

  • Overhearing: When node receives a packet that is not destined to it, “overhearing” or “on-listening” happens [6]. This received packet must be discarded and the whole process, receiving and discarding the packet, ends up with a waste of energy.

  • Packet Overhead: In WSNs, packet header and other additional overheads (like control messages) occupy a large proportion of the medium, while data packets are normally small in size. Sending minimal number of control packets will decrease energy wastage [5].

  • Idle Listening: It appears when node listens to an idle channel in order to receive possible traffic, and is particularly costly in energy in the case of applications that do not require virtually a large data exchange [6]. Due to the fact of WSNs’ low traffic loads, idle listening is regarded as the major energy wastage in a sensor node [5].

  • Over Emitting: “Over emitting” or “on-emission” happens when receiver node is not ready for receiving data which has been sent already by sender node.

Many researches aiming at this topic have been conducted for proposing energy efficient Medium Access Control (MAC) protocols. MAC protocols can directly regulate the communication module, which can significantly affect the performances of WSNs in many perspectives.

In this paper, we investigate some MAC protocols that are broadly utilized in applications for WSNs. The remainder of this paper is structured as follows. Section 2 summarizes the characteristics of MAC protocols. Section 3 categorizes and surveys some typical MAC protocols and their features. Section 4 gives a comparative analysis of the protocols’ performance. Section 5 concludes the paper.

2 Characteristics of MAC Protocols

Medium Access Control (MAC) protocol is used to establish and regulate data communication [8]. MAC is the sub-layer of data link layer, the second layer in Open Systems Interconnection (OSI) model. MAC plays an important role in making decision when a node can access the shared medium, framing, addressing and flow control etc. It also ensures that nodes share the communication medium fairly and efficiently. Some major characteristics of MAC Protocol for WSNs are discussed below.

  • Energy Efficiency: Energy is a scarce resource for WSNs, due to the limited battery power and the difficulty of recharging or replacing the battery. As MAC layer regulates the activities of the radio layer, which consumes the most energy, then we can deduce that the MAC protocol can avoid energy wastage, hence reach the goal of energy efficiency.

  • Adaptability: In most applications of WSNs, traffic density varies significantly over time and from part of the network to another [9, 10]. This network is dynamic in many aspects, such as size, density and topology. In this case, MAC protocol designers must take these uncertain factors into consideration.

  • Latency: Many applications of WSNs require delay-bounded delivery of data, such as target tracking and precise data monitoring. In these applications, the detected events must be reported to the sink node in real time, and in this way the appropriate action could be taken immediately [7, 11].

  • Throughput: This is the total data amount that successfully transmitted between a transmitter and a receiver in a definite time. Data throughput requirement can be a crucial feature in applications that process a large amount of data.

  • Fairness: In many applications of WSNs, it is necessary to ensure sink nodes to receive information from all sensor nodes fairly [11]. This property remains very important in conventional wireless networks in the fact that each node wants the same chance as other nodes for transmitting or receiving data.

3 Classification of MAC Protocols

MAC protocols are generally divided into two categories – schedule based and contention based MAC protocol. Schedule based protocol can avoid collisions, overhearing and idle listening by making transmit & listen periods scheduled. This collision-free protocol is efficient in terms of energy, but requires strict time synchronization [11]. The contention based protocol, also known as unscheduled protocol [12], has relax time synchronization requirement, and can easily adjust to the topology changes as some new nodes may join and other may die few years after deployment [13, 16].

3.1 Schedule Based MAC Protocols

These protocols access the medium by defining a schedule for the transmission, reception or being idle by the nodes in the network. Nodes communicate during specific allotted time slot and stay idle otherwise. Some widely used protocols are discussed as follows.

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    Low-Energy Adaptive Clustering Hierarchy Protocol (LEACH). LEACH protocols involve different characteristics for communication in WSNs [14, 17]. LEACH is the initial and most popular energy efficient hierarchical clustering protocol for WSNs that was developed for reducing energy waste [15]. Clustering is an energy efficient communication algorithm utilized when sensor nodes broadcast the sensed data to the sink nodes. Every cluster has a special node which is responsible for managing the data transmission activities of other nodes in this cluster, called cluster head. Data transfer from a lower clustered layer to a higher one, and the hierarchical structure precedes the data faster to the base station. LEACH protocol fully takes this advantage. Moreover, cluster head rotation prolongs the network lifetime by equilibrating the rate of energy usage by all the nodes in network [8]. It also enhances the scalability and reliability in the network by limiting different communication inside the different local clusters [11]. LEACH is a crucial and fundamental one of the clustering hierarchical MAC protocols in WSN [1].

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    Traffic Adaptive MAC Protocol (TRAMA). The traffic adaptive medium access [15] is a TDMA based protocol that has been designed for reducing the energy consumption by means of avoiding collision in WSN. Switching nodes to low-power sleeping state when they are idle also contributes to the network’s energy efficiency. This protocol is composed of three main parts:

    • The Neighbor protocol is for gathering the corresponding information about the neighboring nodes.

    • Schedule Exchange Algorithm plays its role in transmitting information and schedule between the two-hop neighbors.

    • Referring to the neighborhood and schedule information, the Adaptive Election Algorithm decides the transmitting and receiving nodes for the current time slot. The other nodes in the same slot are switched into low power mode, reducing idle listening effectively.

    Although TRAMA protocol successfully achieves the goal of energy efficiency, the latency it brings is considerable compared to the other contention based MAC protocols [11]. This protocol is suitable for applications that require high energy efficiency and throughput.

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    Wise-MAC Protocol. Wise-MAC protocol uses “Preamble Sampling” mechanism to minimize the energy loss due to passive listening [6]. In this mechanism, receiver nodes listen to the channel periodically during relatively short periods to detect activities on the channel. If the channel is busy, the receiver node continues listening, waiting for a packet intended for itself, until the channel returns to free state [20]. As for the transmitter, a “Wake up” preamble is transmitted before each message to activate the receiver, in this way the message can be successfully received. To avoid collisions, Wise-MAC randomly chooses the “Wake up” preamble with a non-persistent CSMA technique, hence reduces energy waste. However, the transmission of preambles will consume energy either at the transmitter level or the receiver one. In order to remedy this energy loss, Wise-MAC dynamically determines the length of the preambles so that is as small as possible [20].

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    Bitmap-assisted Efficient and Scalable TDMA-based MAC Protocol (BEST-MAC). BEST-MAC is proposed for adaptive traffic in hierarchical WSNs that can be deployed in the smart cities [21]. It can flexibly handle the varying amount of data traffic by using large number of small size data slots. The implementation of Knapsack optimization technique significantly reduces sensor nodes’ job completion time, thus decreasing the average packet delay. The link utilization of the networks is also well enhanced due to these aforementioned attributes. Nevertheless, this protocol enables nodes to be identified by a unique 1 byte short address, which reduces the control overhead and minimizes energy consumption.

3.2 Contention Based MAC Protocols

In contention based MAC protocols the medium access is distributed, and there is no central coordination for nodes to access the medium [3, 18]. These protocols mostly follow the operational model of CSMA, incorporating handshaking signals and a back-off mechanism to avoid collisions [19]. Some protocols are discussed here.

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    Sensor MAC Protocol (S-MAC). Sensor MAC protocol is specifically designed for WSNs with the purpose of reducing energy losses [22]. Locally managed synchronizations and periodic active-asleep schedules based on these synchronizations form the basic idea behind the S-MAC protocol [23]. In this protocol, sensor node periodically goes to the fixed listen/sleep cycle. A time frame is basically divided into two parts: one for listening session and the other for a sleeping session [25]. Only for a listen period, sensor nodes can communicate with other nodes and send control packets, such as SYNC, RTS (Request to Send), CTS (Clear to Send) and ACK (Acknowledgement). Specially, by exchanging SYNC packet all neighboring nodes can synchronize together, and by using RTS/CTS exchange two nodes can communicate with each other.

    S-MAC protocol effectively saves energy by utilizing sleep and wake up technique. It also simplifies the network’s implementation and prevents time synchronization overhead with sleep schedule announcements. However, the sleep and listen periods are predefined and constant, which lowers the transmission efficiency under variable traffic condition.

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    TimeOut MAC Protocol (T-MAC). T-MAC adopts contention-based scheme that lays on improvement of S-MAC protocol by enabling active nodes to have adaptive duty-cycles for the operation [22]. In T-MAC, nodes wake up to broadcast with its nearby nodes and then switch to sleep mode until the next frame starts. The listen period ends when no activation event has occurred for a time threshold TA, whose decision is presented along with some solutions to the early sleeping problem defined in [22, 24]. This protocol can deal with variable traffic load due to active/asleep schedule and high energy efficiency for low data rate applications. The handicap of this protocol is that T-MAC has higher transmission latency as compared to the S-MAC protocol.

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    Berkeley MAC. Berkeley MAC refers to Berkeley Medium Access Control for low-power sensor networks [26], is highly configurable and can be implemented with a small code and memory size. It mainly consists of three parts: clear channel assessment (CCA), packet back-off and link layer acknowledgements [13]. When a node is ready to transmit packet, it has to wait during a back-off period before the operation of CCA. If the channel is accessible, the node transmits its packet, or a second back-off begins. Every node must check the channel regularly by using low-power listening (LPL) [8]. If the channel is found free and the node has no packet to transmit, then the node goes to sleep state [26]. The B-MAC protocol does not use a RTS-CTS scheme, which is utilized in many ad-hoc networks and causes considerable overhead. However, the adaptive preamble of the B-MAC protocol inevitably results in overhead, which may lower its energy efficiency [26].

  4. (4)

    Priority-based Adaptive MAC Protocol (PA-MAC). In this protocol, the fixed dedicated beacon channel (BC) is assigned for the beacon, while the rest of the communication operates through the data channel (DC) [27]. The data traffic is prioritized by using a priority-guaranteed carrier-sense multiple access with collision avoidance (CSMA/CA) procedure in the contention access period (CAP). In contention free period (CPF), continuous and massive data packets are transmitted to the coordinator. This traffic prioritization scheme, along with the classification of the data transfer procedure, lowers the contention complexity and avoids collision and retransmission of the packets effectively. In this way, PA-MAC protocol can highly rise the quality of service and energy efficiency of WSNs.

4 Comparative Analysis

We compare the typical state-of-art MAC protocols for WSNs proposed so far in the literature. First, we categorize the MAC protocols to two types, schedule based and contention based. Then we analyze the performance of every protocol in various aspects, such as latency, adaptivity, QoS, robustness, and most important of all, energy efficiency. Table 1 summarizes the comparison result of MAC protocols.

Table 1. Comparison of MAC protocols
Full size table

From this comparison table and our comparative analysis, some conclusive comments can be drawn. BEST-MAC outperforms other schedule based MAC protocols due to its extremely low transmission latency. As for contention based MAC protocols, PA-MAC can be implemented by application calls for rigidly guaranteed quality of service.

In summary, every aforementioned MAC protocol takes energy efficiency into consideration specifically, and accomplishes this goal by optimizing different property of the networks. Due to the advancement of algorithm and technology, the newly proposed MAC protocol can outperform the typical ones in many aspects apparently.

5 Conclusion

In WSNs, sensor nodes are basically supported by energy-constraint batteries, so increasing energy efficiency becomes the paramount goal for many applications. A well designed medium access control protocol can contribute to this by regulating the distribution of the medium and the activity of sensor nodes. In this paper, we have provided a brief introduction of WSNs, and analyze the main sources of energy waste. We have discussed some typical MAC protocol suitable for WSNs and their characteristics. According to comparison, choice of MAC protocol depends on the requirements of applications.