[ieee 2014 31st national radio science conference (nrsc) - cairo, egypt (2014.4.28-2014.4.30)] 2014...

31st National Radio Science Conference(NRSC2014)

April 28 – 30, 2014, Faculty of Engineering, Ain Shams University, Egypt

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978-1-4799-3821-6/14/$31.00 ©2014 IEEE

C21. E2MAC: An Energy-Efficient MAC Protocol for Wireless Sensor Networks

Hesham Sh. Elhelw1, Ahmed H. Zahra12, Khaled F. Elsayed3 1Faculty of Engineering, Cairo University, Giza, Egypt, [email protected]

2Faculty of Engineering, Cairo University, Giza, Egypt, [email protected] 3Faculty of Engineering, Cairo University, Giza, Egypt, [email protected]

ABSTRACT Wireless sensor networks applications witnessed a substantial growth in different fields like industry, security,

medicine and military fields. Energy efficiency is one of the most important aspects in wireless sensor networks MAC protocols design. This is due to the fact that in many applications reaching the nodes frequently to change their batteries is not feasible. This paper proposes a new MAC protocol called Energy-Efficient MAC (E2MAC) focuses mainly on achieving good performance in packet delay and packet delivery rate while being operated on very low level of energy consumption. E2MAC is designed based on a previously defined MAC protocol called LPEAS with the addition of adaptive wakeup time and next awake node first packet scheduling features. We compare the performance of IEEE 802.15.4, TMAC and LPEAS with E2MAC. The results show that E2MAC exhibits superior performance particularly in dense/high load networks.

Keywords: MAC protocols; wireless sensor networks; energy efficiency.

I. INTRODUCTION Wireless sensor networks (WSN) are widely used in many fields, such as industry, security, medicine and

military, to monitor and control physical parameters, such as temperature, humidity and pressure. Wireless sensor nodes are typically powered by batteries that are difficult to replace. Hence, energy-efficient operation is always a critical design goal at every layer in the protocol stack. The design of energy-efficient MAC could significantly affect the lifetime of any WSN. A widely used transceiver in the market such as the cc2420, drains 17.4 mA when it transmits a frame [1]. A greedier current drainer is idle listening which consumes 19.7 mA. Assuming such device is operated by two AA batteries of 1600 mAh, they would keep the system working for only 3.4 days unless energy-saving techniques are adopted.

At the MAC layer, several sources of energy waste are identified including collisions, overhearing, control overhead and idle listening [2]. Packet collision and overhearing naturally occur in a shared medium such as the wireless channel. Similarly, frequent control packets represent an unavoidable protocol overhead that is important for many purposes including organizing nodes association, nodes synchronization and data transactions. Further, idle listening is an operating mode during which a wireless node wastes energy without receiving any packets. Any energy-efficient MAC targets reducing the energy waste in one or more of the aforementioned sources.

This paper introduces E2MAC as a new energy-efficient MAC protocol for WSNs. E2MAC is based on LPEAS [3], which is based on IEEE 802.15.4 [4]. Similar to LEAPS, E2MAC is an asynchronous protocol in which each node has its own wakeup and sleep schedule but nodes share their expected wakeup time for communication coordination. E2MAC further introduces an adaptive wake-up duration algorithm and a novel next awake node first (NANF) packet scheduling technique. The performance of E2MAC is evaluated using simulations in Castalia [5] in which we implemented both LEAPS and E2MAC and compare their performance with TMAC and IEEE 802.15.4. E2MAC not only improves the energy consumption but also strikes a good compromise for normalized performance metrics such as network throughput and packet delay. These gains are attained while maintaining a simple implementation required for low price WSN nodes.

The rest of this paper is organized as follows. In Section II, we present a short overview for relevant energy-efficient MAC protocols for WSNs. In Section III, the design of E2MAC is presented followed by its evaluation in Section IV. Finally, Section V is dedicated for conclusions and future work.



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II. BACKGROUND AND RELATED WORK The design of energy-efficient MAC protocols for wireless sensors is a very active area of research. The

literature is rich with survey papers for energy efficient MAC protocols for WSNs, e.g. [6-7]. This section highlights some relevant proposals for WSN energy-efficient MAC protocols.

Sensor MAC (SMAC) is a synchronous protocol designed to reduce the effect of the major sources of energy waste. SMAC employs duty cycling to reduce the impact of idle listening. Hence, each node periodically wakes up for a pre-defined interval then sleeps for the rest of the period. Network nodes have a synchronized frame to wake up and sleep at the same time. However, there may be more than one frame in the network to accommodate network geographical expansion. To reduce collisions, SMAC uses three techniques including physical sensing, virtual sensing based on network allocation vector (NAV), and RTS/CTS transactions. Additionally, SMAC uses RTS/CTS information to sleep and avoids overhearing packets destined to other nodes. Finally to reduce retransmission effect, SMAC uses packet fragmentation to reduce the cost of retransmissions.

TMAC [8] mainly focuses on reducing idle listening effect by adaptively changing the wake-up time interval depending on the traffic load. TMAC defines TA as a threshold for idle listening after one of the following activities: firing of periodic frame timer for node wake up, reception of any data on radio channel, sensing of communication on the radio, end of transmission of owned data or acknowledgement packet, and knowledge (through overhearing prior RTS and CTS packets) that a data exchange of a neighbour has ended.

IEEE 802.15.4 is one of the most popular MAC protocols used for WSNs. IEEE 802.15.4 has many attractive advantages including implementation simplicity and low energy consumption. Hence, it is adopted in many WSN applications [4]. IEEE 802.15.4 has two modes of operation: beacon-enabled and non-beacon-enabled modes. In the former, radio time is divided into equally sized intervals bounded by the broadcast of beacon packets transmitted by a network coordinator, which is responsible for the network time synchronization. Each interval is split into two sub-intervals, active sub-interval during which nodes transmit and receive packets and inactive sub-interval during which nodes are allowed to sleep. In the non-beacon-enabled mode, nodes are not synchronized and hence wakeup and sleep at distinct times. In both modes IEEE 802.15.4 uses Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) algorithm as the MAC protocol.

The lack of synchronization in the non-beacon mode adds complexity to the protocol to coordinate between nodes in a distributed fashion. However, it suffers less contention on the medium since the number of active nodes at the same time is less than the case of synchronized protocols. Hence, transaction success rate is typically higher and less retransmission occurs. Another advantage is the possibility of network expansion is natural in comparison to the beacon-enabled mode that has to maintain synchronization among network nodes.

Long preamble emulation with acknowledgement after local synchronization (LPEAS) [3] is based on the non-beacon-enable mode of IEEE 802.15.4. LPEAS uses the long preamble concept to resolve medium contention and detect destination node wakeup time. In this context long preamble represents a train of Wakeup Request (WR) packets, which are sent from the transmitter to the receiver in order to start the hand-shake procedure required for the data transaction. Upon the reception of a WR packet, the receiver acknowledges the request by sending back a Wakeup Acknowledgement (WA) packet to the transmitter. If the transmitter does not receive WA after transmitting WR with a certain timeout, it retransmits another WR till receiving WA. Upon the success of a data exchange, LPEAS applies a local synchronization technique by which each device keeps a look up table for the expected wakeup time for all nodes in the network. The wakeup time of each node is detected via sleep information in WA packet. Hence, when a node wants to transmit a packet, the transmitter checks the estimated receiver wakeup time from the lookup table. The transmitter then starts transmitting the WR train of packets just before the receiver wakeup.

Figure 1 illustrates the operation of LPEAS when transmitting a packet for the first time for a receiver. At this time the transmitter does not have an estimate for the receiver wakeup time. Consequently, the transmitter starts blindly sending the train of WR packets till it receives the first WA packet which marks the receiver subsequent wakeup time.

Once the synchronization information is available, the transmitter sends WR just before the wakeup time of the receiver for following transactions. Such behaviour saves the energy of sending useless WR packets as in first



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transaction. In LPEAS, each node has its own independent wakeup time but all nodes share the same frame length and wakeup duration.

Fig. 1: LPEAS functionality

III. E2MAC DESIGN AND IMPLEMENTATION E2MAC introduces two new features to LPEAS including the adaptive wakeup time and next awake node first

(NANF) packet scheduling.

A. Adaptive wakeup Time The adaptive wakeup time feature enables E2MAC to change its activity to accommodate the network varying

load. By this addition, each E2MAC node may remain idle for TA seconds unless it receives packets destined to it. On receiving packets, E2MAC extends its wakeup time for another TA and this behaviour repeats until a complete TA passes without receiving any packet. Based on this design, TA is typically smaller than the fixed wakeup time of LPEAS.

Figure 2 illustrates the difference between LPEAS and E2MAC operation under high or low loading. At high network load, the first two packets are successfully received but LEAPS node sleeps before receiving the third one due to the end of its fixed wakeup time. Hence, such behaviour would add more delay time to the third packet because it will wait until the next wakeup event. Additionally, this behaviour may decrease the packet transmission success rate too due to possible buffer overflow at the transmitter. At low loading condition, LPEAS nodes would stay awake unnecessarily for prolonged intervals resulting in energy waste.

Although the adaptive wakeup time TA of E2MAC is smaller than the fixed wakeup time of LPEAS, the refresh of the timer enables E2MAC to accommodate high packet rate. Such refresh extends its cumulative wakeup time to receive all packets destined to the node that are near to each other in time. At low network load, E2MAC stays awake for a smaller interval in comparison to LEAPS and hence decreases the useless wakeup time resulting in a reduced energy consumption.

B. Next Awake Node First Scheduling Typically, the packets are serviced in a first-in-first-out (FIFO) order in the MAC queue. In our proposed next

awake node first (NANF) scheduling, the MAC would schedule the packets according to the expected wakeup order of receiving nodes independent of their arrival order from higher layers.

Figure 3 illustrates the impact of the proposed next awake node first on the performance. On using LPEAS, the transmitting node would not send the packet destined to node 1 before sending the packet destined to node 2. Hence, node 1 would wake up for one cycle without receiving any packet even though the network contains packets destined to it. Such behaviour increases both energy consumption and average packet delay in the system. When using NANF, E2MAC would schedule the packet transmission according to the activity of receiving nodes. Each time E2MAC receives a packet transmission request from the application layer it sorts its transmission buffer in an order corresponding to the destination wakeup time. Using such design enables E2MAC to transmit node 1 packet first although it received node 2 packet first from application layer. Hence, E2MAC would maintain a lower packet delay and energy consumption profile in comparison with LPEAS.



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Fig. 2: Adaptive wakeup time feature

Fig. 3: Next awake node first feature

IV. PERFORMANCE EVALUATION In our evaluation, we use Castalia simulation tool [5]. We develop both LPEAS and E2MAC. Since we can’t

evaluate E2MAC with all energy efficient MAC protocols for wireless sensor networks which are many, we select standard one, IEEE 802.15.4, and a well know protocol, TMAC, in addition to LPEAS protocol which E2MAC implementation is based on for comparative evaluation. The simulation field is assumed to be 100m x 100m over which nodes are uniformly located. The transmission range of each node covers the entire field area implying a single hop communication model. Each node may communicate directly to any other node in a single-hop fashion, i.e. a full mesh network topology. Each simulation spans a period of 300 seconds during which applications generate packets with constant rate. Additionally, the destination of each packet is selected randomly according to a uniform distribution. Results are averaged over all participated nodes and over five independent runs for each simulation trial. We assume TI CC2420 radio chip for all nodes [1]. We explore the impact of changing the number of nodes and the packet generation rate on our key performance indices (KPIs) including the consumed energy, packet success rate and average packet delay. In all energy consumption figures, the energy consumption of IEEE 802.15.4 PAN coordinator is not considered in the calculated average energy consumption since PAN



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coordinator in IEEE802.15.4 is always awake and hence, it consumes much energy than any other node and usually it is mains-operated.

A. Effect of Changing the Number of Nodes In this set of results, a packet rate of 10 packets per second is assumed for each simulated node. Figure 4 plots

the average consumed energy per node versus the number of nodes for E2MAC, TMAC, LEAPS and IEEE 802.11.5.4. The figure shows that the average consumed energy is almost not affected by changing number of nodes for all the simulated protocols except TMAC that shows a linear increase in the energy consumption. More importantly, E2MAC consumes the least amount of energy in comparison to all the simulated protocols. Such performance improvement is due to reducing the collision probability, adaptive wake up time. The high energy consumption of TMAC is interpreted by the extend node activity. Note that TMAC features many triggers that extend its wakeup time leading to extended activity.

Fig. 4: Average consumed energy with changing number of nodes

Figure 5 shows the average packet transmission success rate versus the number of nodes for E2MAC, TMAC, LEAPS and IEEE 802.11.5.4. Clearly, the successful delivery ratio decreases as the number of nodes increases. The figure also shows that the performance of IEEE 802.15.4 sharply drops as the number of nodes increases and loses its noticed superiority for small number of nodes. This deterioration is interpreted by the high collision probability due to the simultaneous attempts of IEEE 802.15.4 nodes to access the channel. On the contrary, LPEAS is less affected by increasing number of nodes than IEEE 802.15.4 because nodes wakeup time at different instances. Hence, nodes have a lower collision probability. E2MAC further improves LPEAS performance due to the adaptive wake-up time feature, which enables a receiving node to stay awake as long as needed to receive all destined packets to it. For a network consisting of ten nodes, E2MAC success ratio is approximately 1.7 better than IEEE 802.15.4. Although TMAC shows the highest success rate but this is interpreted by its extended activity. Hence, most of the packets destined to the TMAC node are received but at a higher energy cost.

Figure 6 plots the average packet delay per node versus the number of nodes for the simulated protocols. Similar to the success ratio, IEEE 802.15.4 achieves the lowest delay for small number of nodes but as the number of nodes increases, the performance gap increases in favor of E2MAC. TMAC slightly outperforms E2MAC due its extra extended awake time. For a network consisting of ten nodes, the average packet delay realized by E2MAC is approximately 20% of that realized by IEEE 802.15.4. This significant reduction is due to the adaptive wakeup and the NANF packet scheduling components. Such reduced delay makes E2MAC an attractive option for delay-sensitive application such as emergency.

B. Effect of Changing the Packet Generation Rate Fig. 7 shows the average consumed energy versus the packet generation rate in a six-node network. IEEE 802.15.4 average consumed energy is approximately constant independent of the input packet rate while TMAC, LPEAS and E2MAC perform differently. IEEE 802.15.4 nodes are synchronized with a super frame during which sleep and wakeup intervals are synchronized between all nodes. Hence, increasing the packet generation rate does



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not affect the node behaviour. On the other hand, both LPEAS and E2MAC have duty cycling frame for idle listening and reception but transmission is independent of this frame. Hence, new packets may trigger the transmitter node to wake up in case the destination node wakeup time occurs in the source node sleep time. With such transmission/reception profile, increasing input packet rate means additional time during which source node is required to be awake in order to transmit their packets. E2MAC additionally adapts its wakeup time according to the traffic activity. TMAC also naturally extends its radio activity due to many TA refresh triggers leading to a linearly increasing energy profile.

Fig. 5: Average packet transmission success rate with changing number of nodes

Fig. 6: Average packet delay with changing number of nodes

Fig. 7: Average consumed energy with changing input packet rate

Fig. 8 plots the average packet transmission success rate versus the application packet generation rate for the tested protocols. E2MAC also performs better than LPEAS and IEEE 802.15.4 especially at high packet rate. It is worth noting that increasing the packet rate implies a higher probability of having packets destined to every node from different sources. Hence, these source nodes will be waiting for destination node wakeup time at which they start transmitting the backlogged packets. In case of LPEAS or IEEE 802.15.4 since wakeup duration is fixed. Hence, destination node sleeps again once the wakeup time elapses independent of the traffic activity. On the contrary, E2MAC stays awake till all packets are delivered to it successfully. Again due to the extra extended wakeup time in TMAC, TMAC nodes achieve better success rate than other MAC protocols.

Fig. 9 shows average packet delay versus the packet generation rate for the simulated protocols. In light traffic scenarios, IEEE 802.15.4 shows the best performance until the packet rate reaches 6 packets/second after which the packet delay sharply increases. This result can be interpreted by observing the changes in the success rate performance of IEEE 802.15.4 in Fig. 7. By increasing the packet generation rate for IEEE 802.15.4, more collisions occur in the medium and hence, packets take longer time until they are successfully delivered or dropped. At high packet rate, E2MAC performance is better than LPEAS due to the added adaptive wakeup time



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and next awake node first techniques. TMAC performance is better than all other MAC protocols due to its extended activity.

Fig. 8: Average packet transmission success rate with changing input packet rate

Fig. 9: Average packet delay with changing input packet rate

C. Normalized Performance Evaluation The simulation results confirm the known tradeoff between the consumed energy and other performance KPIs

including the packet success rate and average packet delay. Hence, to fairly compare the simulated protocols and identify which compromises this tradeoff, we estimate network performance measures normalized to the consumed energy. Fig. 10 plots the normalized packet success rate versus the number of nodes for the simulated protocols. The Figure shows that E2MAC has the best performance, followed by LPEAS, IEEE 802.15.4, and then TMAC. More importantly, there is a relatively large performance gap between E2MAC that reaches 40% and 100% improvement in comparison to LEAPS and IEEE 802.15.4, respectively, at a network size of ten nodes.

Fig. 11 plots the normalized delay inverse versus the network size for the simulated protocols. Interestingly, this figure shows that IEEE 802.15.4 is superior at small network sizes. However, E2MAC performs better as the network size increases.

Fig. 10: Success rate per energy unit with changing number of nodes

Fig. 11: Inverse of packet delay multiplied by consumed energy with changing number of nodes

V. CONCLUSION AND FUTURE WORK This paper produces a new MAC protocol, E2MAC, based on LPEAS MAC by adding two main features;

adaptive wakeup time and next awake node first (NANF) scheduling. E2MAC shows good energy consumption without impacting packet delay and packet delivery success rate but even enhancing them especially in the



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dense/high load networks. Performance is compared with TMAC, LPEAS & IEEE 802.15.4 and IEEE 802.15.4 and the results show that lower delay and success rate per consumed energy are attained by E2MAC particularly in dense/high load networks. As a future work, we are considering analyzing how to enhance energy efficiency in light load networks. One solution can be changing TA adaptively in run-time to be short in light traffic networks and large in heavy load networks.

REFERENCES [1] Datasheet: 2.4GHz IEEE802.15.4/ZigBee-ready RF Transceiver, version 1.2, Chipcon, 2004. [2] Wei Ye, John Heidemann, Deborah Estrin, “An Energy-Efficient MAC Protocol for Wireless Sensor

Networks,” in Proc. IEEE INFOCOM, pp.1567-1576, vol. 3, 2002. [3] Tae Rim Park and Myung J. Lee, “Power Saving Algorithms for Wireless Sensor Networks on

IEEE.802.15.4,” in Proc. IEEE Commun. Mag., pp.148 -155, vol. 46, June 2008. [4] IEEE 802.15.4-2006, “Part 15.4: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)

Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs),” 2006. [5] User’s Manual: Castalia a simulator for Wireless Sensor Networks and Body Area Networks, version 3.2,

NICTA, 2011. [6] S. Hayat, N. Javaid, Z. A. Khan, A. Shareef, A. Mahmood, S. H. Bouk, “Energy Efficient MAC Protocols,”

in Proc. IEEE 9th International Conference on Embedded Software and Systems (HPCC-ICESS), June 2012. [7] K. Lakshmisudha, C. Arun, “Research on power optimization in Physical and MAC Layer of Wireless

Sensor Networks - A Survey,” in Proc. International Conference on Intelligent Systems and Signal Processing (ISSP), March 2013.

[8] T. V. Dam and K. Langendoen, “An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks,” in Proc. SenSys’03, Los Angeles, pp.171-180, Nov.2003.

[ieee 2014 31st national radio science conference (nrsc) - cairo, egypt (2014.4.28-2014.4.30)] 2014...

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