OMA: a Multi-Channel MAC Protocol with Opportunistic Media Access In

2010 Sixth International Conference on Mobile Ad-hoc and Sensor Networks

OMA: a Multi-channel MAC Protocol with Opportunistic Media Access in Wireless Sensor Networks Jinbao Li, Desheng Zhang, and Longjiang Guo 1 School of Computer Science and Technology, Heilongjiang University, Harbin, Heilongjiang, China, 150080 2 Key Laboratory of Database and Parallel Computing of Heilongjiang Province, Harbin, Heilongjiang, China, 150080 [email protected], [email protected], [email protected] Abstract—To tackle control channel saturation problems, this paper the frequency of channel selection execution. Media access proposes OMA, an asynchronous duty cycle based multi-channel schemes fall in two categories: TDMA and CSMA according to MAC protocol with opportunistic media access for wireless sensor whether all the nodes in the WSNs are synchronized. networks. By adopting opportunistic media access, OMA effectively alleviates, if not completely eliminates, control channel saturation Dynamic channel selection and CSMA with duty cycling problems. More importantly, OMA is fully distributed with no are jointly considered as suitable schemes for WSNs because requirements of time synchronization or multi-radio. Therefore, of three reasons as follows. First, dynamic channel selection OMA is very easy to be implemented in resource-constrained sensor schemes require less number of channels than static schemes nodes. Via the theoretical analysis, the opportunistic probability with [7]. Second, CSMA involves no overhead of time synchroniza- which a node opportunistically access the control channel is obtained. tion required by TDMA. Third, duty cycling is a scheme to To validate the effectiveness of opportunistic media access, extensive address the idle listening problem in WSNs, which is consi- simulations and real testbed experiments were conducted. The simulation and experimental results show that when the number of dered as one of the largest sources of energy consumption in channels is large or the network loads are heavy, OMA improves WSNs [2]. However, these combined schemes sometimes fail energy efficiency and throughput significantly compared with other to offer satisfactory performance due to the Control Channel works in the literature. Saturation problems (CCS), which undermines the network performance of WSNs. We will verify CCS in Section III. I. INTRODUCTION In this paper, aiming at CCS, an asynchronous duty cycle Emerging as one of the dominant technology trends, wire- based multi-channel MAC protocol with Opportunistic Media less sensor networks (WSNs) have a wide range of potential Access, called OMA, is proposed. Involving no overhead of applications [1]. To support these applications, a larger number time synchronization and multi-radio, OMA is tailored to of MAC protocols have been proposed. Unfortunately, even overcome CCS. The key novelty of this work is to use the op- though multiple channel are available in existing sensor hard- portunistic media access to transmit control packets on the ware such as MICAz and Telos, mainstream MAC protocols in control channel to alleviate the control channel saturation the WSNs literature still focus on single channel solutions [2]. problem. Therefore, OMA is capable of alleviating the colli- Recently, to overcome the drawbacks of single channel MAC sions of control packets on the control channel, and improving protocols, some multi-channel MAC protocols (mcMAC) have network throughput and energy efficiency. The contributions been proposed to improve network performance via parallel of this work are as follows. communication, e.g., MMSN[2], Y-MAC[3], PMC[4], Hy-  To the best of authors’ knowledge, this paper makes the MAC[5], and TFMAC[6]. The mcMACs have several advan- first attempt to define and to verify the control channel tages as follows. First, since generally mcMACs employ one saturation problems under WSNs context. Control Channel (CC) to send control information and multiple Data Channels (DC) to send data, the overall channel utiliza-  This paper proposes a duty cycle based multi-channel tion is increased. Second, since communications on different MAC protocol, which applies an opportunistic media orthogonal channels do not interfere with each other, mcMACs access scheme to effectively handle the control channel have higher throughput and shorter latency. Third, because saturation problem. current off-the-shelf WSNs radios already offer multiple chan-  This paper analyzes the performance of OMA, and ob- nels [2] (e.g., the IEEE 802.11 a standard provides 12 channels, tains the opportunistic probability via correlating it with and MICA2 motes support more than 50 channels), mcMACs the real time network parameters. No previous work incur no more hardware cast, i.e., multi-radio. gives such an analysis. A mcMAC mainly consists of channel selection and media  The extensive simulation results show that compared access. Channel selection decides how to select idle channels with the other four protocols, OMA achieves 14% to for all the nodes efficiently in order to optimize the perfor- 179% more throughput ratios. OMA also has 6% to 20% mance of WSNs; whereas, media access decides when and how less energy consumption ratios. Furthermore, OMA is al- all the nodes access the channels that have been selected for so implemented in a real testbed. The experimental re- them to avoid the data packet collisions. Channel selection sults show that OMA achieves 31% to 75% more schemes can be classified as static and dynamic ones based on throughput ratios than compared schemes.

Authorized licensed use limited to: Rutgers University. Downloaded on July 23,2020 at 21:47:36 UTC from IEEE Xplore. Restrictions apply. II. RELATED WORK B. Asynchronous MAC Protocols In this section, related mcMACs for both WSNs and gen- Wu et al. [13] proposed DCA for ad hoc networks, which eral wireless network are surveyed from two categories, name- uses two radios, one for control information exchanging and ly, synchronous and asynchronous, respectively. the other for data communication. Adya et al. [14] proposed MUP for wireless networks. MUP employs two radios like A. Synchronous MAC Protocols DCA, but it allows both radios to send control information and Zhou et al. [2] proposed MMSN which is the first mcMAC data interchangeably. that takes into account the restrictions imposed by WSNs. Above protocols are based on multi-radio scheme. Exploit- Senders in MMSN switch their current channels to channels of ing multi-radio can simplify the design of protocols by dedicat- intended receivers at the beginning of every slot when they ing one radio on the CC to overhear the control information have packets to send. Kim et al. [3] proposed Y-MAC for exchanging continuously. However, multi-radio schemes lead WSNs where time is divided up into several fixed-length to not only larger node size but also more energy consumption. frames. The frames are composed of a broadcast period and a More importantly, increasing hardware cost makes the multi- unicast period. The difference between Y-MAC and above radio schemes unrealistic for large scale WSNs. mcMACs is that Y-MAC schedules receivers rather than senders to achieve low energy consumption. Salajegheh et al. [5] Luo et al. [15] exploited Distributed Information SHaring proposed HyMAC for WSNs where the communication period mechanism (DISH) and proposed CAM-MAC for ad hoc net- consists of a number of frames, which are divided up into works. In CAM-MAC, when a node-pair performs a channel scheduled slots and contention slots. The base station allocates reservation on the CC, all neighbors may send cooperative specific time slots and channels to all nodes for communica- packets to invalidate the reservation if they are aware of that tion. Jovanovic et al. [6] proposed TFMAC for WSNs where a the selected DC or receiver is unavailable. Luo et al. [16] pro- frame consists of a contention period and a contention-free posed ALTU based on altruistic cooperation, which introduces period that contains some equal sized time slots. It works simi- some specialized nodes called altruists whose only role is to larly with HyMAC except that the schedules are made by all acquire and share channel usage information. nodes rather than the base station. These two mcMACs are based on DISH. Nevertheless, in In general wireless networks, mcMAC has also received a every channel reservation, all the idle neighbors of the sender rapid growing interest in academia. So et al. [8] proposed and its receiver will send packets for invalidation if they as- MMAC for ad hoc networks by dividing up time into multiple sume that this reservation is invalid. This scheme involves slots, where all nodes exchange control information on the CC more packet transmission than necessary and easily results in for reservations of DCs at the front of each slot and switch to cooperative packet collisions, since many cooperative packets DCs for data communication at the rest of the slot. Chen et al. could be sent simultaneously. Thereby, this scheme will con- [9] proposed MAP for ad hoc networks. MAP works in the sume considerable energy under large-scale WSNs context. same way to MMAC but has variable-size data time slots, so it avoids the problem that data slot has to be set according to the Le et al. [4] proposed PMC which utilizes a control theory maximum data packet size. Tzamaloukas et al. [10] proposed approach to dynamically add available channel in a distributed CHAT for ad hoc networks using channel hopping scheme. method. In PMC, nodes work on current available channels by Under CHAT, all idle nodes switch among all channels using a CSMA, and decide whether to switch to the next available common hopping sequence. Moreover, both the sender and its channel based on certain parameters, which vary with channel receiver will stop hopping when they are aware of the fact that utilization from time to time. However, computing methods of they have to communicate with each other. Bahl et al. [11] these parameters need further discussion. Sun et al. [17] pro- proposed SSCH for ad hoc networks, which works in a differ- posed RI-MAC which is a receiver-initiated single-channel ent way to CHAT by adopting multiple hopping sequences for MAC protocol for WSNs. It attempts to minimize the time that different nodes. Under SSCH, a data communication starts a sender and its receiver occupy the wireless medium to find a when two nodes hop on the same channel. Tzamaloukas et al. rendezvous time for exchanging data. Wu et al. [7] proposed [12] proposed RICH-DP based on channel hopping for wire- TMCP which is a multi-channel protocol that does not require less networks, which differentiate itself with a receiver- time synchronization among nodes. However, this protocol is initiated collision-avoidance scheme. more like a topology control protocol than a MAC protocol. Zhou et al. [18] proposed CUMAC using cooperation for un- To sum up, above studies design protocols by time syn- derwater WSNs, but it requires a tone device on each node to chronization where let all control information or data be sent notify collisions, which increases the cost of WSNs. in some well-known slots and channels. For larger scale WSNs, however, synchronization itself remains an open issue C. Summary and it is not completely solved on low cost sensor nodes with From all mcMACs mentioned, it can be seen that currently cheap faulty clocks that are prone to drift. One common solu- there is no protocol that is based on single-radio multi-channel tion is to send SYNC packets periodically, but these SYNC asynchronous MAC protocol supporting duty cycling for WSNs. packets may induce considerable overhead, which consumes Therefore, this paper proposes OMA, which addresses all these more energy and makes channels more crowded. needs mentioned.

Authorized licensed use limited to: Rutgers University. Downloaded on July 23,2020 at 21:47:36 UTC from IEEE Xplore. Restrictions apply. 350 15 CBR IV. OMA PROTOCOL 325 20 CBR 25 CBR 300 30 CBR In this section, before OMA is described in detail, two as- 35 CBR 275 sumptions are made as follows. (1) Wireless bandwidth is di- 250 vided into one dedicated Control Channel (CC) for control 225 packets and Data Channels (DC) for data packets. All chan- 200 nels have the same bandwidth and quality. Every pair of the 175 channels are orthogonal to each other, so the packets simulta- 150 neously transmitted on different channels do not interfere with Aggregate MACThroughput (Kbps) 125 123456 each other. All the nodes have prior knowledge of the frequen- Total Number of Channels Figure 1. The simulation results for CCS verification cies of all the channels and the total number of DCs. (2) Each sensor node is equipped with a same single half duplex radio, which makes nodes can either send or listen on only one chan- III. MOTIVTION nel at a time but cannot do both actions simultaneously. In ad- To bridge the growing gap between energy wastage due to dition, the radio is capable of dynamically switching its channel. the control packets collisions on the CC and limited availabili- A. Media access scheme of OMA ty of energy through fixed-budget batteries, this section identi- fies and verifies the source of the control packets collisions on An overview of the unicast scheme of OMA is as follows where a message transmission includes five steps as follows. the CC under multi-channel WSNs. (1) When the sender in sleeping period has a message for the The Control Channel Saturation problem (CCS) means receiver that is also in sleeping period, wakes itself up, that under certain circumstances the CC is considered as a and then listens on the CC. (2) If the CC is idle, first com- bottleneck of the network performance. Fig.1 shows how the putes an opportunistic probability, denoted as , based on the CC becomes bottleneck, when the loads and the number of current network parameters (addressed in Section IV), and channels increase in the simulations. The mcMAC involved then accesses the CC with probability or enters sleeping pe- exploits a simple RTS and CTS handshake scheme on the CC riod with probability 1 to make the handshake in the next to make the channel reservation, and then the sender and the active period. (3) If chooses to access the CC, then in- receiver switch to the DC selected by the sender based on the itiates a data transmission by sending a RTS packet on the CC overhearing on the CC. Finally, they communicate with each including a DC number, say , selected by based on a cer- other on this DC. The specific detail of simulation setup is tain scheme (discussed in the next subsection). (4) After re- given in Section V. ceiving the RTS of the sender, sends a CTS packet to con- firm this transmission, and then switches to the DCn. (5) It is observed that when the number of channels is larger ter receives the CTS, switches to the DCn either, and then than 4 and the number of CBR (Constant Bit Rate) is larger employs DATA&ACK method to commence the data trans- than 25, the throughput of mcMAC becomes steady, which is mission with . Finally, they hop back to the CC to enter the because the collisions on the CC become more severe and thus sleeping period when this communication is over. it degrades the throughput. This is a clear indication that the When does not receive the CTS from within a prede- CC becomes a bottleneck of the network performance. There- termined time interval after sends out the RTS, assumes fore, if the CC is congested with retransmitted control packets, that is either on a DC for transmission or in sleeping period. all the DC are prevented from fully utilized. Thereby, the Therefore, wakes up by a series of RTS until the prede- spectrum resource is significantly wasted. This situation be- termined RTS retry times is reached. comes even worse under duty cycle based WSNs where (1) the deployment density of nodes is usually higher than general B. Channel selection scheme of OMA wireless networks; (2) the traffic loads are usually bursty, and When the sender has data to transmit to the receiver , the senders usually wake up the sleeping receivers by conti- selects a DC based on certain channel selection scheme and nuously transmitting a series of handshake packets. Therefore, then puts ID number of selected DC in the RTS. should se- when the saturation situation occurs, many senders with data lect the most likely idle DC based on some Channel Usage to transmit may re-send control packets on the CC simulta- Information (CUI) that can be obtained by overhearing on the neously, which make the CC more congested than it already is. CC. This scheme is leveraged by the most of mcMACs in the All the DCs are also blocked from further utilization. literature. However, there are several reasons why this overhearing based method is inefficient under multi-channel duty CCS is handled in this paper via an opportunistic media cycle based WSNs context. Firstly, since duty cycle scheme is access scheme, which reduces the probability that multiple the most efficient method to conserve energy, most of WSNs senders simultaneously transmit control packets on the CC. It is based on duty cycle. Therefore, when a node under the duty is unwise to directly transmit all these control packets on the cycle based WSNs is in sleeping period, it cannot overhear any CC under WSNs context, since when the detected event occurs control packet related to channel selection on the CC, which these control packets will quickly overwhelm the CC. This may lead to incomplete channel usage information. Secondly, will significantly increase the collision probability of the con- in the single radio scenario, when a node is on a DC for com- trol channels and degrade the network performance. This pa- munication, it cannot update its channel usage information in per tackles the CCS with opportunistic media access. time either, which may also lead to an inaccurate selection of

Authorized licensed use limited to: Rutgers University. Downloaded on July 23,2020 at 21:47:36 UTC from IEEE Xplore. Restrictions apply. idle DC where other nodes probably are transmitting. This where is the average data packet arrival rate at a node, and issue is also known as the multi-channel hidden terminals [8]. is the average number of data packets in a message (i.e., segment). Therefore, the total Time that a node Sends all these Based on above reasons, the overhearing based channel selection is inefficient under multi-channel duty cycle based Messages on DCs, denoted as , is given by WSNs. Thus, it is no better choice than the random selection ∙ , 2 when duty cycling as well as single-radio are employed and traffic loads are heavy. Thereby, to alleviate CCS, if not completely eliminate, OMA exploits random scheme to select DCs. where is the average duration length of one message By this scheme, when a node has data to transmit, it randomly communication on a DC, which consists of multiple data selects a DC with a new probability , and enters to sleep with packet transmissions. probability 1 to conserve energy. equals the ratio of the On the other hand, approximately, the Total time that a average available DCs number (denoted as ′) over the current total available DCs number (denoted as ), which is tho- node Receives all the Messages, denoted as , is given by roughly discussed in [19] where and are given, and ′ is ∙1 ∙, 3 continuously varying according to some networks parameters. Due to the limit of space, the final formula of ′ is not given in where is the probability that a node switches to a DC as a this work. Please check [19] for details. receiver, is the probability that a node is on the CC at an arbitrary time, and is the probability that a node is sleep- C. Handling CCS ing at an arbitrary time. With opportunistic media access scheme and random channel selection, OMA is capable of greatly alleviating, if not In the long run, the total time that a node sends messages is completely eliminating, CCS. When a sender has data to supposed to equal the total time that a node receives these transmit to the receiver, the sender has two opportunities to messages, when the networks is stable (i.e., after sufficiently relief the CCS in the WSN. To begin with, the sender com- long time). Therefore, via 2 and 3, we have putes an opportunistic probability according to the current conditions of networks, and then prepare to transmit control ∙ ∙1 ∙. 4 packets on the CC with probability , or enters to sleeping period with probability 1. Moreover, if the sender decides Assuming that 1/2 in the long run, we have to transmit control packets for channel selection, the sender computes another probability , and then actually transmit the 2 1 . 5 control packets on the CC to the receiver with probability , or backs off to enter the sleeping period with probability 1. The duty cycle (not the in Section IV.B) is given by Among the two opportunistic media accesses, the first op- the design of protocols, and is defined as the awake idle listen- portunistic CC access is to avoid the collisions on the CC, ing time on the CC (denoted by ) over the awake idle time while the second opportunistic DC access is to avoid the colli- on the CC plus sleeping time (denoted by ), i.e., sions on the DC. With these two opportunistic media accesses, not only can the transmitting node pairs efficiently alleviate . 6 the collisions on the CC that is the key issue we try to handle in this work, but also alleviate the collisions on the DC. We transform 6 into V. PERFORMANCE ANALYSIS / . 7 In this section, we make a theoretical analysis for the per- / / formance of OMA. In particular, we compute the opportunistic If we assume is an appropriate length of time, we have probability with which the senders transmit the control packets on the CC when the senders have data packets to . 8 transmit. Under OMA, the probability ensures that during the time interval of one control packet transmission, the expected number of control packets transmitting on the CC is where is the probability that a node is idle on the CC at equal to or less than 1. Therefore, OMA can greatly alleviate an arbitrary time. Moreover, via 8, we have collisions between control packets on the CC, and thus con- 1 1 serve more energy to extend the lifetime of WSNs. . 9 1/ / Assume that the packet arrival process is Poisson arrival Because the situation that a node is on the CC includes two process. Let be a sufficiently long time interval. During , cases (i.e., first, it is idle on the CC; second, it is exchanging the total Number of Arrival Messages at each node, denoted as control packets on the CC), we have , is given by . 10 , 1 Therefore, via 9 and 10, we have

Authorized licensed use limited to: Rutgers University. Downloaded on July 23,2020 at 21:47:36 UTC from IEEE Xplore. Restrictions apply. . 11 comparisons. The first one utilizes Consistent Media access, called CMA, which directly access the CC and is used to justify the effect of opportunistic media access. The second one Via 5 and 11, we have exploits a Fixed probability of 50% based Media Access, 12 ⁄ called FMA, which is used to justify the effect of opportunistic . 12 probability. 11/ Three groups of simulations are conducted to examine Therefore, in the long run, Expected Number of any node’s three metrics as follows: throughput, packet delivery ratio, and neighbors that are on the CC, denoted as , is given by energy consumption. In each group, different Total Number of 12 ⁄ Channels (TNC) and the network loads are considered. The ∙ ∙ . 13 11/ total number of channels includes the CC and all the DCs, and the network loads are varied via changes of the Number of where is the average number of neighbors of a receiver, i.e., CBR (NCBR, Constant Bit Rate) streams in the network. In network density. all simulation experiments, TNC is fixed at 4 when NCBR is Therefore, according to the Poisson arrival process, during varying, while NCBR is fixed at 30 when different TNCs are exploited in the simulations . the time interval of one control packet transmission, the probability that no message is arrival at all the neighbors of 1) Evaluation on throughput: The throughput is computed a sender is given by as the total amount of all useful data packets successful

∙∙ delivered via the MAC layer of the networks per unit time. ∙∙∙ ∙∙. 14 0! The effect of the total number of channels on throughput is shown in Fig.2 (a). Compared with others, OMA has lower Therefore, the upper bound of opportunistic probability , ∗ throughput when the total number of channels is small than 4. denoted as , with which a sender on the CC may send a con- Besides the duty cycling, this is also because OMA uses the trol packet is given by two-way handshake and opportunistic media access, which ⁄ ∙∙ will pay considerable cost if the total number of channels is ∗ min1,min1, ∙⁄ . 15 relatively small. When more channels are available, OMA, During every data transmission, all the senders on the CC CAM-MAC and PMC allow more nodes to simultaneously uniformly choose in interval 0, ∗, and opportunistically communicate on different DCs. This is because they employ send a control packet with probability or back off to enter dynamic channel selections, and thus outperform CSMA\CA the sleeping period with probability 1. Therefore, the ex- and MMSN. However, when the total number of channels pected number of control packets sent by all the senders on the becomes larger, OMA performs better than CAM-MAC and CC with opportunistic probability is equal to or less than 1. PMC. This is because CAM-MAC suffers from collisions of Note that and can be set according to the network dep- cooperative packets and PMC suffers from CCS, whereas OMA avoids using cooperative scheme and tackles CCS by loyment; can be obtained by a packet counter in the MAC opportunistic media access, so it achieves higher throughput. layer; and , as well as can be real-time estimated Moreover, OMA outperforms CMA and FMA due to its op- by a progressive weighted method. Therefore, is solvable portunistic scheme, which avoids the collisions on the CC. and is dynamically adaptive according the network parameters. The effect of loads on throughput is shown in Fig.2 (b). It VI. PERFORMANCE EVALUATION is observed that the throughputs of all the protocols rise with In this section, we performed both simulations and real the number of CBR streams. This is due to the fact that if more testbed experiments to evaluate the performance of OMA. node-pairs are involved in data communications, more parallel A. Simulation Results transmissions will occur on the DCs. Under light loads, OMA is suboptimal to other protocols. Nevertheless, the results We implemented a simulator using C++, which has 289 show that under heavy loads OMA performs progressively nodes whose transmission ranges are set to 40m. The nodes better than other protocols. This indicates that even though are uniformly deployed in a square area of size 200m 200m OMA is duty cycling, OMA still significantly benefits from with a node density of 38 (i.e., a node that is not in the edge of opportunistic media access when CCS becomes more serious networks has 37 neighbors). The many-to-many transmission model is used where the payload size is fixed at 32 Bytes and with the rise of the loads. Again, OMA outweighs CMA and the channel bandwidth is fixed at 250 Kbps. To investigate the PMA when loads are heavy. effect of opportunistic media access, OMA is compared with 2) Evaluation on packet delivery ratio: Packet delivery another four famous schemes: (1) CSMA\CA is a classic sin- ratio is computed as the ratio of the total number of packets gle channel MAC protocol; (2) MMSN [2] is a typical syn- that MAC layer successful delivered over the total number of chronous mcMAC with a static channel selection; (3) PMC [4] packets that the upper-layer requests MAC layer to deliver. is an asynchronous mcMAC with a dynamic channel selection; (4) CAM-MAC [15] is a synchronous mcMAC with DISH. The effect of the total number of channels on packet deli- Moreover, two varieties of OMA are also implemented for very ratio (PDR) is shown in Fig.3 (a). The results show that

Authorized licensed use limited to: Rutgers University. Downloaded on July 23,2020 at 21:47:36 UTC from IEEE Xplore. Restrictions apply. 600 600 all the packet delivery ratios increase with the rise of the total CSMA CSMA 550 550 MMSN MMSN number of channels. When the total number of channels is 500 PMC 500 PMC CAM-MAC CAM-MAC 450 450 smaller than 4, MMSN and PMC achieve better performances CMA CMA 400 FMA 400 FMA OMA OMA than CAM-MAC and OMA. One reason is that cooperative 350 350 schemes of CAM-MAC and opportunistic media access of 300 300 OMA undermine packet delivery ratio. However, when the 250 250 200 200 total number of channels is larger than 5, OMA outperforms 150 150 Aggregate MAC Throughput (Kbps) Throughput Aggregate MAC 100 Aggregate MAC Throughput (Kbps) 100 0123456789 16 20 24 28 32 36 40 44 48 52 others, but CMA and PMA still perform worse than MMSN TNC and PMC. This is because OMA involves fewer collisions on NCBR (a)Throughput vs. Total Number of Channels (b) Throughput vs. Loads the CC. In addition, CMA has many collisions due to its consistent media access and PMA does not consider the current Figure 2. Throughput evaluation

0.990 0.990 status of the networks and backs off with a fixed probability. CSMA CSMA 0.985 0.985 MMSN MMSN CMA 0.980 PMC 0.980 PMC FMA The effect of loads on packet delivery ratio is shown in 0.975 CAM-MAC 0.975 CAM-MAC OMA CMA 0.970 0.970 Fig.3 (b). It is observed that all the packet delivery ratios de- FMA 0.965 OMA 0.965 crease when the loads are heavier except that of OMA-50%, 0.960 0.960 0.955 0.955 which maintains steady around 96%-97%. This is because in 0.950 0.950 0.945 0.945 opportunistic media access, node-pairs more likely to avoid 0.940 0.940 The Packet Delivery Ratio the collisions on the CC, so they can find a DC for communi- Ratio Delivery Packet The 0.935 0.935 0.930 0.930 0123456789 16 20 24 28 32 36 40 44 48 52 cation in time. Moreover, PMA outperforms CMA, which TNC NCBR verifies values of backoff schemes. MMSN and PMC outper- (a) PDR vs. Total Number of Channels (b) PDR vs. Loads form PMA and CMA, which is still due to the fixed probability to access media and the flaws of consistent media access. Figure 3. Packet delivery ratio evaluation

) 3.0 2.9

Note that CAM-MAC has a lower packet delivery ratio than ) 2.9 2.8 all the protocols due to CCS, which causes the situation that 2.8 2.7 E-7mWhr the collisions on the CC between reservation packets and co- ( 2.7 E-7mWhr operative packets become more serious when loads are heavier. 2.6 ( 2.6 2.5 2.5

2.4 2.4 3) Evaluation on Energy Consumption: In this study, the 2.3 CSMA 2.3 CSMA 2.2 energy consumption for all the protocols is computed as the MMSN CMA MMSN CMA PMC FMA 2.1 2.2 PMC FMA energy consumed to successfully deliver a useful data byte. CAM-MAC OMA CAM-MAC OMA 2.0 2.1 The Energy Consumption The Energy Consumption 0123456789 16 20 24 28 32 36 40 44 48 52

TNC Energy Consumption The The effect of the total number of channels on energy consump- NCBR tion is shown in Fig.4 (a). The results show that energy con- (a) Energy vs. Total Number of Channels (b) Energy vs. Loads sumptions of all the protocols decrease with the rise of the Figure 4. Energy consumption evaluation total number of channels, but OMA outperforms others all the time due to its effective opportunistic media access to handle 400 400 350 350 CCS. All the results indicate that OMA can conserve more CAM-MAC CAM-MAC CMA 300 300 CMA energy to prolong the lifetime of WSNs by its duty cycle FMA FMA OMA scheme and effective solution to CCS. Note that CMA con- 250 250 OMA sumes more energy than others due to that its consistent media 200 200 150 access needs more energy to handle the collisions on the CC 150 100 100 caused by CCS. In addition, CAM-MAC consumes higher 50 50 Aggregate MAC Throughput (Kbps) Throughput Aggregate MAC energy than others due to its collisions of cooperative packets, 0 Aggregate MAC Throughput (Kbps) 0 123456 TNC 0123456 which undermines many communications when CCS is more NCBR serious. Note that when the total number of channels is becom- (a) Throughput vs. Total Number of Channels (b) Throughput vs. Loads ing larger, the gap between OMA and PMA on energy con- Figure 5. Testbed evaluation on throughput sumption is becoming larger. This may indicates that OMA with opportunistic media access is capable of achieving higher B. Testbed Results energy efficiency under the networks with more DCs. We built a sensor node platform, Hawk, to evaluate the The effect of the loads on energy consumption is shown in performance of OMA. Hawk employs C/OS, where each Fig.4 (b). All the energy consumptions increase when the node is equipped with an nRF905 radio and a MSP430 proces- loads rise. OMA maintains a relatively low energy consump- sor. A hawk node is shown in Fig.6 (a). The testbed consists of tion since opportunistic media access reduces the probability 10 hawk nodes which are completely connected as shown in of control packet collisions. Whereas other protocols suffer Fig.6 (b). Thereby, all the nodes are within communication from certain problems: MMSN consumes more energy to range of each other, similar to the work in [15]. The size of maintain time synchronization; PMC has many collisions on each packet is 32 Byte, and data transmission rate is 100 Kbps. the current channel when the loads are heavy; and CAM-MAC, All the nodes randomly choose a neighbor to transmit. The PMA as well as CMA still suffer from retransmissions resulted experiment was repeated for 10 times. When an experiment is by CCS and then consume more energy than other protocols. finished, all nodes send their total number of bytes received

Authorized licensed use limited to: Rutgers University. Downloaded on July 23,2020 at 21:47:36 UTC from IEEE Xplore. Restrictions apply. VIII. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (61070193, 60803015), the Key Scientif- ic and Technological Research Project of Heilongjiang Prov- ince (GC09A109), the China Postdoctoral Foundation (2008 0430902), the Science and Technology Innovation Research Project of Harbin (2008RFQXG107), the Heilongjiang Post- Sink doctoral Science Foundation (No.LRB08 -021), the Science and Technology Research of Heilongjiang Educational Com- (a) Hawk node (b) A snapshot in the testbed experiment mittee (1154Z1001), the Heilongjiang University Foundation Figure 6. Testbed experiments for the Outstanding Young Scholar, the Student Innovative during the experiment to a Sink, which is connected to a desk- Research Project (2010208) and Innovative Laboratory Project. top computer, and thus throughput can be obtained. Due to the time synchronization of MMSN and the complexity of PMC REFERENCES for parameter computations, only OMA, PMA, CMA and [1] D. Culler, D. Estrin, and M. Srivastava. Overview of Sensor Networks, CAM-MAC were implemented for throughput comparisons. in IEEE Computer, Special Issue on Sensor Networks, 2004. [2] G. Zhou, C. Huang, T.Yan, et al. A Multifrequency MAC Specially When the number of CBR is fixed at 5, the effect of the to- Designed for Wireless Sensor Network Applications, in ACM tal number of channels on throughput is shown in Fig.5 (a). It TRANSACTION on Embedded Computing Systems, 2010. is observed that CAM-MAC has higher throughput than OMA, [3] Y Kim, H Shin, et al. 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