(PDF) Wireless sensors networks MAC protocols analysis

JOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 1, APRIL 2010 42 Wireless sensors networks MAC protocols analysis Lamia CHAARI and Lotfi KAMOUN Abstract—Wireless sensors networks performance are strictly related to the medium access mechanism. An effective one, require non-conventional paradigms for protocol design due to several constraints. An adequate equilibrium between communication improvement and data processing capabilities must be accomplished. To achieve low power operation, several MAC protocols already proposed for WSN. The aim of this paper is to survey and to analyze the most energy efficient MAC protocol in order to categorize them and to compare their performances. Furthermore we have implemented some of WSN MAC protocol under OMNET++ with the purpose to evaluate their performances. Index Terms— MAC protocol, Wireless sensors networks, energy-efficiency, performance, messages. —————————— —————————— 1 INTRODUCTION T HE wireless sensor networks (WSNs) are used in a wide range of applications to capture, gather and analyze live environmental data [1]. The wireless sen- This energy is wasted if there isn’t any transmission on the channel. As in the sensors networks of, the channel is most of the time free. The passive listens presents one of sor network architecture, as described in the majority of the major reasons of energy loss. the literature [2],[3][4] typically consists of a large number - Collisions: they concern the MAC contention proto- of miniature battery-powered sensor nodes scattered cols. A collision can occur when a node receives two sig- over an area of interest and forming a multi-hop commu- nals or more simultaneously from different sources that nication network. Each of these nodes has the capacity to transmit at the same time. When a collision occurs, the collect relevant data from the environment and to transfer energy provided for frame transmission and reception is them to the sinks nodes that transmits these data then by lost. Let's note that although there are MAC protocols that Internet or by satellite to the central processing node, in don't produce any collisions, as TDMA (Time Division order to analyse these data and to take the adequate deci- Multiple Access), the contention protocols are more used sions. In a wireless sensor network sensors nodes are a in multi-hops networks because of their simplicity and of low cost, resource constrained devices and are often posi- their capacity to operate in a decentralized context. tioned randomly. In many applications they are placed in - Overhearing: occurs when a node receives packets inaccessible locations, making battery replacement un- that are not destined to him or redundant broadcast. feasible. As a consequence, energy efficiency is an impor- - Protocol Overhead: can have several origins as the tant requirement in a medium access control protocol for energies lost at the time of transmission and reception of most wireless sensor networks. Radio energy consump- the control frames. For example, the RTS/CTS (Request tion is a major component contributing to the overall To Send /Clear To Send) used by some protocols trans- energy consumption at each node. port no information whereas their transmission consumes Many reasons related to MAC paradigms lead to ener- energy. Note that the traffic generated by control frames gy waste and WSN life reduction, such as: in sensors network is far from being negligible, it could - Idle listening: a node doesn’t know when will be re- represent until 70% of the global traffic [5]. ceiving a frame so it must maintain permanently its radio - Overmitting: occurs when a sensor node sends data in the ready to receive mode, as in the DCF method of the to a recipient who is not ready to receive them. Indeed, wireless networks protocol (IEEE 802.11). This mode con- the sent messages are considered useless and consume an sumes a lot of energy, nearly equal to the one consumed additional energy. in receipt mode. - Packets size: The size of the messages has an effect on the energy consumption of the emitting and receiving nodes. Thus, the size of the packets must not be too ele- ———————————————— vated nor too weak. Indeed, if it is small, the number of • L. Chaari is with Department of Electronic and Information Technology control packets increases the overhead. In the other case, Laboratory (LETI) at Sfax National Engineering School Tunisia. a high transmission power is necessary for large size • L. Kamoun is is with Department of Electronic and Information Technol- ogy Laboratory (LETI) at Sfax National Engineering School Tunisia. packets. © 2010 JOT http://sites.google.com/site/journaloftelecommunications/ JOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 1, APRIL 2010 43 - Traffic fluctuation: The fluctuations of the traffic load achieves low power operation, it doesn’t meet simple im- can lead to the waste a node’s energy reserves. Therefore, plementation, scalability, and tolerance to changing net- the protocol should be traffic adaptive. work conditions. As the size of the network increases, S- In [6], the authors introduce a comprehensive node MAC must maintain an increasing number of neighbors’ energy model, which includes energy components for schedules or incur additional overhead through repeated radio switching, transmission, reception, listening, and rounds of resynchronization. sleeping. In S-MAC, a node that has more data to send can mono- Several researchers have already proposed MAC proto- polize the wireless radio channel. This is unfair for other cols for wireless sensor networks for achieving energy nodes that have short packets to send but need to wait for efficient protocols and their common goal is to reduce the completion of the transmission of the long packet. sources of the energy waste. In section II of this paper an Many other MAC protocols have been proposed recently overview of WSNs energy-efficient MAC protocols is giv- which are based on, or inspired by, S-MAC en and taxonomy is elaborated. The rest of the paper is [11][12][13][14][15][16]. S-MAC requires some nodes to organized as follows. Section III describes the implemen- follow multiple sleep schedules causing them to wake up tation of some WSNs MAC protocol under OMNET++ more often than the other nodes. In [15], the authors add and shows some experimental results that confirm and adaptive listening when a node overhears a neighbor’s validate the previous theoretical results. Finally, conclu- RTS or CTS packets, it wakes up for a short period of time sions are drawn in section IV. at the end of their neighbor’s transmission to immediately transmit its own data. By changing the duty cycle, S-MAC can trade off energy for latency. In [14] a modification of 2 ENERGY-EFFICIENCY WSNS MAC PROTOCOLS the S-MAC protocol is proposed (S-MACL) to eliminate the need for some nodes to stay awake longer than the MAC protocols for WSNs must guarantee efficient access other nodes. The modified version improves the energy to the communication media while carefully managing efficiency and increases the life span of a wireless sensor the energy budget allotted to the node. The latter is typi- network. In [16], an adaptive S-MAC was proposed that enables low duty cycle operation, common sleep sche- cally achieved by switching the radio to a low-power dules to reduce control overhead, low latency and traffic mode based on the current transmission schedule. Ac- adaptive wakeup. To reduce control overhead and laten- cording to channel access policies, most of the existing cy, it introduces coordinated sleeping among neighboring protocols fall in two categories [7]: contention-based and nodes. TDMA-based protocols. In [22], the author’s compare adaptive-rate MACs to adaptive power schemes. 2.1 Contention-based MAC - Time out -MAC: As declared above, the SMAC protocol Contention-based MAC protocols are mainly based on does not work well when the traffic load fluctuates. To the Carrier Sense Multiple Access (CSMA) or Carrier overcome this problem, the TMAC protocol [9] introduces Sense Multiple Access/ Collision Avoidance the timeout value to finish the active period of a node. If a (CSMA/CA). The main idea is listening before transmit- node does not hear anything within the period corres- ting. The purpose of listening is to detect if the medium is ponding to the time-out value, it allows the node to go busy, also known as carrier sense. The typical contention- into sleep state. based MAC protocols are S-MAC [8], T-MAC [9], T-MAC, in variable workloads, uses one fifth the power DMAC[17], TEEM[18] ,UMAC [10] and BMAC[21]. of S-MAC. In homogeneous workloads, TMAC and S- - Sensor-MAC: As a slotted energy-efficient MAC proto- MAC perform equally well. T-MAC suffers from the same col, S-MAC [8] is a low-power RTS-CTS protocol for complexity and scaling problems of S-MAC. Shortening WSNs inspired by 802.11. S-MAC includes four major the active window in T-MAC reduces the ability to snoop components: periodic listening and sleeping, collision on surrounding traffic and adapt to changing network avoidance, overhearing avoidance, and message passing. condition. After the sleep period, the nodes wake-up and listen - DMAC[17]: The DMAC could be summarized as an im- whether communication is addressed to them, or they proved Slotted Aloha algorithm in which slots are as- initiate communication themselves. This implies that the signed to the sets of nodes based on a data gathering tree. sleep and listen periods should be (locally) synchronized During the receive period of a node, all of its child nodes between nodes. Each active period is of fixed size, 115 ms, have transmit periods and contend for the medium. It can with a variable sleep period. The length of the sleep pe- achieve very good latency compared to other sleep/listen riod dictates the duty cycle of S-MAC. At the beginning of period. However, collision avoidance methods are not each active period, nodes exchange synchronization in- utilized in DMAC. Hence, when a number of nodes that formation. Following the SYNC period, data may be have the same schedule try to send to the same node, col- transferred for the remainder of the active period using lisions will occur. RTS-CTS. The advantages of S-MAC are energy waste - UMAC: It is based on the SMAC protocol and provides caused by idle listening is reduced by sleep schedules and three main improvements on this protocol, e.g. various time synchronization overhead may be prevented by duty-cycles, utilization based tuning of duty-cycle, selec- sleep schedule announcements. Although S-MAC tive sleeping after transmission. The scheme does not as- JOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 1, APRIL 2010 44 sign the same duty cycle for nodes, and each node can be active during each transmission period, therefore, reduc- assigned different periodically listen and sleep schedules ing collision and eliminating idle-listening and overhear- with different duty cycle. Utilization based tuning of du- ing. Use of TDMA is viewed as a natural choice for sensor ty-cycle reflects to different traffic loads of every node in networks because radios can be turned off during idle a network. Such variation corresponds to the diversity of times in order to conserve energy However, deterministic performed tasks by a particular node and its location. TDMA scheduling1 requires a large overhead in order to Selective sleeping after transmission avoids the above maintain accurate synchronization between sensors and energy wastage. A node should go to sleep ”selectively”. to exchange local information, such as the network topol- When transmission is finished, a node checks if it is at ogy and the communication pattern. Furthermore, the scheduled sleep time, and goes to sleep if it’s at scheduled latency increases linearly with the total number of sensors sleep time. It does not introduce additional delays, since sharing the channel since TDMA assigns a separate time- traffic is not expected to this node. In consequence, the slot to each transmitting sensor. proposed protocol improves energy efficiency as well as - EYES MAC[19]: The TDMA-based EMACs protocol di- end-to-end latency. vides time into time slots, which nodes can use to transfer -Traffic Aware Energy Efficient MAC (TEEM) [18]: TEEM data without having to content for the medium or having makes two important modifications over the existing S- to deal with energy wasting collisions of transmissions. A MAC protocol: firstly by having all nodes turn off their node can assign only one slot to itself and is said to con- radios much earlier when no data packet transfer is ex- trol this slot. After the frame length, which consists of pected to occur in the networks, and secondly by elimi- several time slots, the node again has a period of time nating communication of a separate RTS control packet reserved for it. even when data traffic is likely to occur. The listen period A time slot is further divided in three sections: Communi- In TEEM consists of Syncdata and Syncnodata. The first cation Request (CR), Traffic Control (TC) and the data part of the listen period in TEEM contains data while the section. In the CR section other nodes can do requests to other part contains no data. Both parts are used for syn- the node that is controlling the current time slot. Nodes chronization. Each node will listen in the first Syncdata that have a request will pick a random start time in the part of its listen period whether someone has data to short CR section to make their request. The controller of a transfer or not. If there is no data in the Syncdata part time slot will always transmit a TC message in the time then it will send its own sync packet in the Syncnodata slot. When a time slot is not controlled by any node, all part. The TEEM protocol combines the Sync and RTS nodes will remain in sleep state during that time slot. The packets into one packet called SyncRTS. Whenever a node time slot controller also indicates in its TC message what wants to communicate with another node, it sends the communication will take place in the data section. If a SyncRTS packet in its Syncdata part. The destination node node is not addressed in the TC section nor its request receives the packet and starts the communication, while was approved, then the node will resume in standby state the other nodes synchronize themselves with a SyncRTS during the entire data section. The TC packet and go into sleep mode. TEEM MAC is a good message can also indicate that the controlling node is choice in small networks because there are fewer chances about to send an omnicast message. After the TC section of retransmission. the actual data transfer takes place. - Berkeley Media Access Control( B-MAC)[21]: B-MAC - Lightweight MAC [20] : This protocol is based on ideas uses clear channel assessment (CCA) and packet backoffs of the EMACs. LMAC protocol takes into account the for channel arbitration, link layer acknowledgments for physical layer properties. The intension of the protocol is reliability, and low power listening (LPL). B-MAC makes to minimize the number of transceiver switches, to make local policy decisions to optimize power consumption, the sleep interval for sensor nodes adaptive to the amount latency, throughput, fairness or reliability. To achieve low of data traffic. power operation, BMAC employs an adaptive preamble During its time slot, a node will always transmit a mes- sampling scheme to reduce duty cycle and minimize idle sage which consists of two parts: control message and a listening (an adaptive rate scheme). B-MAC supports on- data unit. the-fly reconfiguration and provides bidirectional inter- The control message has a fixed size and is used for sev- faces for system services to optimize performance, eral purposes. It carries the ID of the time slot controller, whether it is for throughput, latency, or power conserva- it indicates the distance of the node to the gateway in tion. By comparing B-MAC to S-MAC, we see that B- hops for simple routing to a gateway in the network, it MAC’s flexibility results in better packet delivery rates, addresses the intended receiver and reports the length of throughput, latency, and energy consumption than S- the data unit. MAC. The control data will also be used to maintain synchroni- zation between the nodes and therefore the nodes also 2.2 TDMA-based MAC transmit the sequence number of their time slot in the Although random access achieves good flexibility and frame. The transmission of the control data is carefully low latency for applications with low traffic loads, deter- timed by the nodes, although we do not assume that the ministic scheduling is actually the most effective way of nodes have clocks with high accuracy. All neighboring eliminating the sources of energy waste. With perfect nodes will put effort in receiving the control messages of scheduling, only one transmitter-receiver pair would be their neighboring nodes. When a node is not addressed in JOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 1, APRIL 2010 45 that message or the message is not addressed as an omni- addition, the coordination also allows TDMA to achieve cast message, the nodes will switch off their power con- better throughput under heavy traffic loads. suming transceivers only to wake at the next time slot. If a Although energy waste on collisions has been node is addressed, it will listen to the data unit which avoided, there are a number of negative aspects. Cluster, might not fill the entire remainder of the time slot. Both which is used in TDMA-based MAC, is difficult to dy- transmitter and receiver(s) turn off their transceivers after namically change its frame length and time slot assign- the message transfer has completed. ments, thus contributes to poor scalability. TDMA-based - Advanced Medium Access Control (A-MAC) [25]: is a protocols require strict time synchronization which re- TDMA-based MAC protocol developed for low rate and sults in high cost on hardware and high latency for data. reliable data transportation with the view of prolonging Recently there are others research that there have been the network lifetime, adapted from LMAC protocol. some hybrid proposals (ZMAC[23], GMAC[24]), which Compared to conventional TDMA-based protocols, which combine the advantages of contention-based MAC with depend on central node manager to allocate the time slot that of TDMA-based MAC. However hybrid MAC proto- for nodes within the cluster, the AMAC protocol uses cols are usually complex in transition mechanisms be- distributed technique where node selects its own time slot tween contention-based and TDMA-based, in addition, by collecting its neighborhood information. The protocol these protocols are more complex in implementation. uses the supplied energy efficiently by applying a sche- duled power down mode when there is no data transmis- sion activity. 3 WSNS MAC PROTOCOL PERFORMANCE ANALY- The protocol is structured into several frames, where each SIS frame consists of several time slots. Each node transmits a beacon message at the beginning of its time slot, which is In this section we have used discrete event simulator used for two purposes; as synchronization signal and OMNet++ we implemented LMAC and BMAC with a neighbor information exchanges. By using this message, framework for wireless networks. the controlled node informs which of its neighboring nodes will be participating in the next data session. The 3.1 LMAC performance analysis intended nodes need to stay in listening mode in order to LMAC, in state of sleep, the node always accepts packets be able to receive the intended packet, while other nodes of the network layer which permits to adapt the node turn to power down mode until the end of current time sleep interval with the data traffic. We consider the net- slot. The time slot assignment in A-MAC is divided into work as shown in Fig. 1. Table 1 illustrates simulation four states; initial, wait, discover, and active. A new node parameters. that enters a network starts its operation in initial state where node listens to the channel for its neighbor’s bea- con message in order to synchronize with the network. Node starts synchronization when it receives a beacon message from one of its neighbors and adjusts its timer by subtracting the beacon received time with beacon trans- mission time. Node remains in this state for aListenFrame frames in order to find the strongest beacon signal. Be- fore entering the wait state, node randomly chooses a number of waiting frame. Node enters the discover state when the waiting counter expired and start collecting its neighborhood information by listening for its neighboring node’s beacon messages for a period of aListen-Frame frames. Node enters active state when it successfully se- lects a time slot. Node enters sleep mode in two scenarios. First, after transmitting a beacon message and no more Fig. 1. A sample LMAC network topology data packet scheduled to be transmitted. Second, if re- ceived beacon message from it neighboring node indi- TABLE I cates no incoming data packet. Compared to LMAC, A- LMAC simulation parameters MAC allows node to transmit to multiple destinations. Control Channel TDMA requires strict synchronization among users carrierFrequency[en Hz] 868e+6 and a centralized control to coordinate the use of the Pmax [mw] 1.0 channels. Benefitting from the extra coordination, it is Seuil du signal d’atténuation [dBm] -110 easier for TDMA to achieve the users’ QoS demands, e.g. Coefficient alpha 3.0 the rate, delay or bit-error-rate (BER) requirements, while LMAC Layer consuming less resources. Even with the complexity of HeaderLength [en bit] 24 computing the optimal channel allocation and the in- queuLength [en bit] 50 crease of control messages, it is often worth-while for de- numSlots 4 lay-constrained or energy constrained applications. In Bit rate [bit/seconde] 19200 JOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 1, APRIL 2010 46 slotDuration [s] 0.2 Fig. 2. LMAC lifetime controlDuration [s] 0.02 3.2 BMAC performance analysis The “EndToEndDelayVec” which represents message BMAC node can be in one of the following three states: lifetime at the host is shown in fig;2. packets transmission state, channel verification state and sleep state. We study the effect of BMAC parameters (slot duration and bit rate) on the messages life time. Simula- tion results are shown respectively in fig.3 and fig.4. slotDuration=0.5s slotDuration=1s slotDuration= 5s slotDuration= 6s Fig. 3. LMAC lifetime (slot duration) JOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 1, APRIL 2010 47 Bitrate=50 bit/s Bitrate=500 bit/s Bitrate=1500 bit/s Bitrate=19200 bit/s Fig. 4. 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[22] Rooholah Hasanizadeh, and Saadan Zokaei Energy Efficiency of Adap- tive-Rate Medium Access Control Protocols for Sensor Networks, WORLD ACADEMY OF SCIENCE, ENGINEERING AND TECH- NOLOGY , VOLUME 19, JULY 2006, ISSN: 2070-3724 [23] A. Warrier, J. Min, and I. Rhee. "ZMAC: a Hybrid MAC for Wireless Sensor Networks". Technical report, Department of Computer Science, North Carolina State University, April 2005. [24] M. I. Brownfield. "Energy-efficient Wireless Sensor Network MAC Protocol”, PhD thesis, Faculty of Virginia Polytechnic Institute and State University, March 2006. [25] Rozeha A. Rashid, Wan Mohd Ariff Ehsan W. Embong, Azami Zaha- rim, Norsheila Fisal Development of Energy Aware TDMA-Based MAC Protocol for Wireless Sensor Network System”, European Jour- nal of Scientific Research, ISSN 1450-216X Vol.30 No.4 (2009), pp.571-578. First A. Author Dr Lamia CHAARI was born in Sfax, Tunisia, in 1972. She received the engineering and PhD degrees in electrical and electronic engineering from Sfax national engineering school (ENIS) in TUNISIA. Actually she is an assistant professor in multi- media and informatics higher institute in SFAX She is also a re- searcher in electronic and technology information laboratory (LETI). Her scope of research are communications, networking and signal processing which are specially related to wireless and new genera- tion networks. Second B. Author Jr. Lotfi Kamoun was born in Sfax Tunisia, 25 January. 1957. He received the electrical engineering degree from the Sciences and Techniques Faculty in Tunisia. Actually he is a Professor in Sfax national engineering school (ENIS) in TUNISIA. He is the director of electronic and technology information laboratory (LETI). His scope of research are communications, networking, Software radio and signal processing which are specially related to