Performance Evaluation of Reservation Frame Slotted-ALOHA for Data Collection M2M Networks

Performance evaluation of Reservation Frame Slotted-ALOHA for Data Collection M2M Networks

F. Vazquez-Gallego,´ J. Alonso-Zarate´ A. M. Mandalari, O. Briante , A. Molinaro, G. Ruggeri Centre Tecnologic` de Telecomunicacions de Catalunya Universita´ Mediterranea di Reggio Calabria, Italy, Castelldefels (Barcelona), Spain, [email protected], {orazio.briante, {francisco.vazquez, jesus.alonso}@cttc.es antonella.molinaro, giuseppe.ruggeri}@unirc.it

Abstract—In this paper, we consider a Machine-to-Machine Therefore, the network is abruptly set into saturation condi- (M2M) wireless network composed of a group of devices which tions when all devices wake up to execute a data collection duty cycle to save energy. These devices operate in low-power round (DCR). This has been referred to as the delta-traffic sleeping mode for most of the time and, periodically, they wake-up model. Since the total number of devices that may transmit to listen to a poll packet transmitted by a data collector. Upon this simultaneously can be potentially large, an energy efficient and broadcast poll, all devices try to get access to the uplink channel low-complexity Medium Access Control (MAC) protocol is to transmit a burst of data packets. Therefore, the idle network is suddenly set into saturation conditions when all devices wake needed to manage this delta traffic condition. up and attempt to get access to the channel simultaneously. The Medium Access Control (MAC) protocol used to coordinate these Frame Slotted ALOHA (FSA) has been identified in the transmissions has a strong influence on the energy efficiency of past as a good approach to handle the delta traffic in this kind the network, and thus the lifetime of the devices. Frame Slotted of networks due to its simplicity and good performance when ALOHA (FSA) has been identified in the literature as a simple optimally configured [2]-[3]. With FSA, time is organized yet efficient MAC protocol for such kind of communications. into access slots upon the transmission of a request from the HoFwever, when the devices have to transmit more than one data coordinator. Every device selects at random one of the access packet per channel invocation, the Reservation Frame Slotted- slots to transmit data. If two devices select the same access ALOHA (RFSA) may be more efficient, since it guarantees the slot, then a collision occurs and a retransmission is scheduled collision-free transmission of data for a device once it succeeds for for a next frame. In a frame by frame basis, eventually all the fist time. Existing analyzes of both FSA and RFSA are valid devices will manage to successfully transmit their data packets. for steady conditions and not for abrupt idle-to-saturation traffic patterns. Motivated by this fact, in this paper we evaluate the FSA has been deeply investigated under stationary traffic energy efficiency of RFSA through computer-based simulations conditions. The analysis in [4] has been carried out varying the to show its better performance compared to FSA. Results show probability φ that an arbitrary station generates a new packet that RFSA can attain up to 48% energy gains compared to FSA, in a frame showing that, when φ = 1, there is an optimum thus extending the lifetime of data-collection M2M networks.1 frame length, i.e., number of access slots, which optimizes the average throughput of the entire network. This optimal I.INTRODUCTION frame length is equal to the number of contending devices. The works in [2] and [3] consider the case of every device Machine-to-Machine (M2M) communications represent has exactly one data packet to transmit to the coordinator. one of the fastest growing segments of Information and Therefore, with a single successful channel invocation, a device Communication Technologies (ICT). Communication among is successful. Unfortunately the traffic pattern generated by autonomous devices will facilitate new and promising appli- M2M applications is quite peculiar and frequently bursty, cations such as smart cities, smart grids, eHealth, etc. [1]. therefore the results in [4], [2], [3] should be extended to M2M communications pose some unique requirements among perfectly fit such a context. which ultra-high energy efficiency and very low-complexity can be identified. Typically, devices are low-cost equipment While some M2M applications will have a small data powered with batteries or, at most, some kind of energy transmission requirement and one data packet may suffice, harvester which can capture energy from the environment. some other applications may require the transmission of longer Therefore, communication protocols for M2M networks need bursts of data packets per DCR. As an example, we could to be specifically designed to meet these requirements as well consider a group of video surveillance cameras deployed in as those defined by the particular application of the network. a building transmitting still images every few seconds. In In this paper, we focus on M2M wireless networks where these cases, Reservation Frame Slotted-ALOHA (RFSA) can a group of devices are synchronized to switch off their radio improve the performance of the network when compared to interfaces for certain periods of time in order to save energy FSA. This is the main motivation for the work presented while they sense some parameters from the environment. in this paper. RFSA is a straightforward evolution of FSA Periodically, they wake up to transmit a burst of data packets that has been proposed in the past in other contexts such with the sensed information to a coordinator upon request. as Radio Frequency Identification (RFID) systems [6] [7], vehicular networks [8] [9], and satellite communications [10]. 1This work has been partially funded by the European Projects ADVAN- The idea is very simple and consists in letting a device reserve TAGE (FP7-607774) and NEWCOM# (FP7-318306). a slot for data transmission once it successfully gets access Time+ to the channel for the first time. To the best of our knowl- Frame 1 Frame 2 edge, the RFSA performance has been analysed in steady- Coordinator state conditions, assuming that devices generate packets with RFD FBP FBP a random distribution. In this case, RFSA achieves better Device 1 throughput than FSA. However, the performance of RFSA in the case of M2M data burst transmission under delta traffic Device 2 conditions remains an open research issue. To fill the gap, in Device 3 this paper we analyse RFSA and FSA performances in these Device 4 cases, in terms of average access delay, energy consumption Device 5 and average throughput through comprehensive computer- based simulations. Results show the superior performance of RFSA for its application in low-complexity energy-constrained IFS Slot IFS IFS Slot IFS (a) Frame Slotted ALOHA M2M networks. We have considered radio transceivers that Time+ are compliant with the physical layer of IEEE 802.15.4 [11], Frame 1 Frame 2 which is becoming the de-facto standard for industrial M2M Coordinator deployments. RFD FBP FBP The remainder of this paper is organized as follows. In Device 1 Section II, we describe the system model and summarize Device 2 the operation of FSA and RFSA protocols. In Section III, Device 3 we formulate the energy consumption of the coordinator and Device 4 the devices. Section IV is devoted to evaluate the simulated performance of the two protocols. Finally, Section V concludes Device 5 the paper.

IFS Slot IFS IFS Slot IFS

II.SYSTEM MODEL AND MACPROTOCOLS (b) Reservation Frame Slotted ALOHA

We consider a single-hop wireless network composed of Fig. 1. Example of the data collection round considering 5 transmitters and one coordinator and n end-devices. Devices can operate in 5 slots per frame: (a) FSA, and (b) RFSA. five modes: i) transmitting, ii) receiving, iii) idle listening, iv) standby, or v) sleeping. The associated power consumptions are Ptx, Prx, Pσ, Psby and Psleep, respectively, being Pσ = Prx. We also assume that the energy and time required by a device to switch between operation modes are negligible. Periodically, all devices wake up and wait for the coordinator to send a Request for Data (RFD) packet. This packet initiates a data collection round (DCR) where devices contend Fig. 2. Packets Format. to transmit a fixed number of L data packets. The RFD is followed by a sequence of time frames divided into m slots. In the first frame, each device selects one of the m slots to to inform the devices of the state of the m slots in the past transmit the first of its L data packets without performing a frame. The packets format are shown in Fig. 2. The coordinator clear channel assessment (CCA) before transmission, i.e., with- can classify the state of each slot as: (i) Empty, no device out carrier sensing. When a device succeeds in transmitting attempted to access the slot and no data packet has been its first data packet, i.e., the coordinator has received the data received; (ii) Success, just one device accessed the slot and packet correctly, the following two operations can be executed the data packet has been successfully (without errors) received, depending on the MAC under consideration: (iii) Failure, one or more devices transmitted in the slot and no packets could be successfully decoded (due to bit errors 1) FSA: the device randomly selects one of the m slots or collisions, respectively), and (iv) Reserved, used only by of subsequent frames to transmit the next of the se- RFSA to acknowledge the success in the slot and reserve its quence of L packets until all of them are successfully use in the next frame to the transmitting device. transferred, thus contending independently for each packet of the burst. A guard time, called Inter Frame Space (IFS), is left at the 2) RFSA:the slot with a successful transmission is re- beginning and the end of each frame, in order to compensate served to the successful device for the next L − 1 propagation, processing, and turn-around times due to the frames. When the device transmits the complete se- radio transceiver switching between transmitting and receiving quence of L data packets, it releases its slot, which modes. can be used by another device in the subsequent frames. The described procedures are repeated, frame after frame, L The two access procedures are shown in Fig. 1. until the coordinator is able to decode the data packets from all the devices. Once a device has successfully transmitted its According to the described operation, at the end of each burst, it switches to sleep mode until the next data-transmission frame, the coordinator broadcasts a Feedback packet (FBP) round to save energy. III.ENERGY CONSUMPTION ANALYSIS A. Energy Consumption of a Single Device The average energy consumed by one device to transmit its L data packets to the coordinator depends on the average number of transmission attempts necessary to complete the burst transmission. As illustrated in Fig. 3, a device which transmits its data packet in slot i (i = 1 . . . m) in a given frame goes through the following states: (i) it remains in stand-by mode waiting for slot i, (ii) it transmits the data packet in slot i, (iii) it switches to sleep mode from slot (i+1) to slot m, and (iv) it switches the Fig. 3. Energy consumption in the data collection round. radio on to receive the FBP at the end of the frame. Therefore, d TABLE I. SYSTEM PARAMETERS the energy Eframe consumed by a device to transmit a data packet in slot i of a given frame can be expressed as Parameter Value Parameter Value d tx rx MAC Header 8 bytes Data-Rate 250 kbps E = 2·EIFS+(i−1)·Esby+E +(m−i)Esleep+E frame pkt FPB Data Payload 104 bytes FBP Payload m · 2bits/slot (1) P 0.12µW CRC 2byte where E = P · T is the energy consumed in idle sleep IFS σ IFS P 100.8mW T 192µsec T E = P · T tx IFS listening during the IFS time IFS; sby sby slot is the P = P 66.9mW P 525µW i Etx = rx σ sby energy consumed in stand-by mode waiting for slot ; pkt PHY preamble 160µsec SLOT time 3.808msec Ptx ·Tslot is the energy consumed for data packet transmission in slot i; Esleep = Psleep · Tslot is the energy consumed in sleep mode after transmission, Tslot is the slot duration; and rx Then, the total energy consumed by the coordinator dur- EFBP = Prx · TFBP is the energy consumed to receive the co ing a data collection round, denoted by Econsumed, can be FBP and TFBP is the FBP packet transmission time. Then, the expressed as total energy consumed by a single device in a data collection d co tx co round, denoted by Econsumed, can be expressed as Econsumed = ERF D + NF ramesTOT · Eframe (5) d rx d Etx = P ·T Econsumed = ERF D + NF ramesT x · Eframe (2) where RF D tx RF D is the energy consumed to transmit the RFD packet, and NF ramesTOT is the average number of rx where ERF D = Prx · TRF D is the energy consumed by a frames needed to complete a data-collection round. device to receive the RFD packet, and NF ramesT x is the average number of frames needed to successfully transmit the Finally, we compute the amount of energy consumed by the burst of L packets (NF rames ≥ L). coordinator for each bit successfully received from a device in T x a DCR (Energy per bit) as: Finally, we can compute the amount of energy (Energy per bit) consumed by a device for each bit successfully delivered Eco = Eco /(L · n · N bits ) (6) to the coordinator in a DCR as : bit consumed DataP acket

d d bits Ebit = Econsumed/(L · NDataP acket) (3) All the parameters expressed in (2) and (5) have deter- ministic values, except for the values of NF ramesT x and bits where NDataP acket is the bits length of a data packet (Fig. 2). NF ramesTOT . These are random variables whose analytical calculation is not trivial and is currently under study. In the next section, we compute the energy values through simulation. B. Energy Consumption of the Coordinator The average energy consumed by the coordinator to suc- IV. PERFORMANCE EVALUATION cessfully receive the L data packets from each of the n devices A. Simulation assumptions depends on the average number of frames needed to complete a data collection round. In this section, we evaluate the performance of FSA and RFSA by using MATLAB. We consider a single-hop network As shown in Fig. 3, the coordinator goes through the formed by a coordinator and a variable number of thousands following steps in every frame: (i) it listens to the channel of devices. We averaged the simulation results of 100 runs for for m slots, and (ii) it transmits a FBP at the end of every each test case to achieve a conﬁdentiality interval bounded to frame. Therefore, the energy consumed by the coordinator in co 1% for each point in all the reported curves. a single frame, denoted by Eframe, can be expressed as The parameters used to run the simulations are summarized co rx tx Eframe = 2 · EIFS + m · Eslot + EFPB (4) in Table I; they are taken from standard parameters in IEEE 802.15.4 [11] and from the CC2520 radio transceiver data- rx where Eslot = Prx · Tslot is the energy consumed to listen a sheet [12]. The FBP payload includes 2 bits per slot to provide tx slot, EFPB = Ptx · TFBP is the energy needed to transmit the ternary feedback for FSA and quaternary feedback for RFSA FBP packet. on the status of each slot, i.e., empty, success, collision, and

500 100 20 FSA, 1000 devices FSA, 1000 devices RFSA, 1000 devices 450 RFSA, 1000 devices 18 FSA, 1500 devices 90 FSA, 1500 devices RFSA, 1500 devices RFSA, 1500 devices 400 16 80 350 14 70 300 12

250 60 10 Time [s] 8 200 50 Energy [uJ/bits] Throughput [Kbit/s] 150 6 40 FSA, 1000 devices RFSA, 1000 devices 100 4 30 FSA, 1500 devices RFSA, 1500 devices 50 2 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 Number of Slots Number of Slots Number of Slots a) b) c) Fig. 4. FSA and RFSA comparison vs. number of slots (with L = 10). (a) Average data collection round (DCR) length, (b) Average network throughput, (c) Energy per bit consumed by an end-device.

0.4 FSA, 1000 devices the average DCR duration is approximately 196s, and it is RFSA, 1000 devices 0.35 FSA, 1500 devices 121s when using RFSA. This represents a gain factor of 38%. RFSA, 1500 devices The gain becomes greater as the number of devices increases 0.3 as well; note that when n = 1500 devices, the gain increases up to 48%. 0.25

0.2 In Fig. 4 (b) we show the throughput achieved by the coordinator. Also in this case, RFSA outperforms FSA. The Energy [mJ/bits] 0.15 throughput trend is determined by the DCR delay; indeed it shows a maximum when the DCR duration is minimum. In 0.1 the FSA case, when increasing the number of slots per frame

0.05 the throughput increases because the collision probability 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 Number of Slots decreases as well, but when the frame becomes longer than necessary the throughput decreases because of a higher number Fig. 5. FSA and RFSA comparison vs. number of slots (with L = 10). of unused slots. In the RFSA case, instead, before reaching Energy per bit consumed by the coordinator. its maximum value, the throughput first increases with the number of slots like in FSA but then it decreases because of the long delay suffered by those ”unlucky” devices forced reserved for RFSA only. We assume ideal channel conditions to wait for the other winning devices to send all the data burst so a packet failure is only due to collision; there is no capture before releasing the channel. Above the maximum value the effect, therefore, if two packets are transmitted simultaneously, throughput decreases again because of a higher number of a collision occurs and no packet can be decoded. unused slots. The FSA and RFSA protocols are compared in terms The Energy per bit consumed by the end-devices and by of (i) the average delay needed to receive all data bursts the coordinator are shown in Fig. 4 (c) and Fig. 5, respectively. by all devices, i.e., the mean duration of the DCR; (ii) the In all cases RFSA outperforms FSA. The energy spent by the average network throughput that represents the number of bits coordinator is strongly influenced by the DCR delay, hence successfully received by the coordinator per DCR; and (iii) the it shows a similar trend. The same considerations made for average energy per bit spent by the coordinator and the devices the DCR delay may be applied to explain how the energy in a round. These parameters are evaluated as a function of the consumed by the coordinator varies with m. The energy spent number of slots (m) in a frame and of the numbers of packets by the devices also follows the trend of the DCR duration; (L). it first decreases when the frame size increases because of reduced collisions and, after reaching a minimum, it slightly B. Effects of the number of slots increases due to the small amount of energy spent by a device in sleep mode after a successful data burst transmission. We consider that every device has L = 10 data packets ready to transmit to the coordinator upon reception of each It is interesting to observe that, given a number n of devices RFD at the beginning of a DCR. The number of devices in the network, the performance of both protocols strongly n is set equal to 1000 and 1500, and the number of slots depends on the number of slots per frame m. Both protocols per frame varies between 500 and 2500. The average DCR present a number of slots mt which maximizes the throughput duration using FSA and RFSA is represented in Fig. 4(a). and a number of slots me which minimizes the average energy Results show that conventional FSA requires more time to consumption of the devices. Specifically the values of mt and complete a DCR than RFSA. The difference between the two me for both protocols are reported in Table II. Interestingly protocols becomes more apparent when the number of slots the value mt which maximizes the throughput of FSA differs is smaller, and thus contention is greater in each frame. For from the value m = n which, accordingly to [4], maximizes example, when m = 500 slots and n = 1000 devices, in FSA the throughput under a stationary traffic load. TABLE II. OPTIMALCONDITIONSFOR L = 10 Max Throughput Min Energy Protocol n d d mt Throughput [Kbit/s] Ebit [µJ/bits] me Throughput [Kbit/s] Ebit [µJ/bits] 1000 800 52.39 6.46 1000 50.94 6.32 FSA 1500 1100 52.38 9.06 1400 51.32 8.87 1000 1200 92.61 3.32 1200 92.61 3.32 RFSA 1500 1800 92.37 4.76 1800 92.37 4.76

550 60 14 FSA, 1 packet FSA, 1 packet 500 FSA, 5 packet(s) 55 13 FSA, 5 packet(s) FSA, 10 packet(s) FSA, 10 packet(s) 450 FSA, 15 packet(s) 50 12 FSA, 15 packet(s) FSA, 20 packet(s) 400 FSA, 20 packet(s) 45 11 350 300 40 10 250 35 9 Time [s] 200 30 8 Energy [uJ/bits]

150 Throughput [Kbit/s] FSA, 1 packet 25 FSA, 5 packet(s) 7 100 FSA, 10 packet(s) 20 6 50 FSA, 15 packet(s) FSA, 20 packet(s) 0 15 5 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 Number of Slots Number of Slots Number of Slots a) b) c) Fig. 6. FSA performance vs. data burst size L (with n = 1000 devices). (a) Average data collection round (DCR) length, (b) Average network throughput, (c) Energy per bit consumed by an end-device.

14 14 3500 RFSA, 1 packet RFSA, 1 packet RFSA, 1 packet RFSA, 5 packet(s) RFSA, 5 packet(s) RFSA, 5 packet(s) RFSA, 10 packet(s) RFSA, 10 packet(s) 3000 12 12 RFSA, 10 packet(s) RFSA, 15 packet(s) RFSA, 15 packet(s) RFSA, 15 packet(s) RFSA, 20 packet(s) RFSA, 20 packet(s) RFSA, 20 packet(s) 2500 10 10

2000 8 8

1500 Time [s] 6 6 Energy [uJ/bits]

1000 Throughput [Kbit/s]

4 4 500

0 2 2 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 Number of Slots Number of Slots Number of Slots a) b) c) Fig. 7. RFSA performance vs. data burst size L (with n = 1000 devices). (a) Average data collection round (DCR) length, (b) Average network throughput, (c) Energy per bit consumed by an end-device.

The performance of FSA smoothly changes varying the FSA and RFSA strongly depend on L. Specifically, for each number of slots between mt and me, therefore a non-precise given protocol, the differences are more evident when passing tuning would cause only a small degradation of the perfor- from L = 1 to L = 5 and much less evident when passing mance. Contrarily the performance of RFSA presents an abrupt from L = 15 to L = 20. Clearly, for L = 1 the performances change close to mt. From Table II it can be argued that of the two protocols coincide. For each value of L an optimal when n = 1000 then me = mt, therefore it is possible frame size mt can be found which maximizes the throughput to obtain both the maximum throughput and the minimum and a value me which minimizes the energy consumption. energy consumption. Choosing a number of frame equal to Table III and Table IV show the values of mt and me, for mt will cause only a small increase in the energy consumption. FSA and RFSA respectively. When FSA is considered mt is As a conclusion, we can say that RFSA always achieves always lower than me, when RFSA is considered mt tends to better performance than FSA in terms of delay, throughput me at the increasing of L. For RFSA, under the considered and energy, but its throughput is more sensitive to the tuning settings (n = 1000), it is possible to choose a configuration in the number of slots per frame. which is optimum for both the energy consumption and the throughput. The throughput performance of RFSA presents an C. Effect of the data burst size L abrupt change close to mt, and this behaviour is more evident at the increasing of L. FSA presents a similar but less abrupt In Fig. 6 and Fig. 7 the average DCR duration, the average trend. It is confirmed that in all cases RFSA outperforms FSA throughput, and the energy performance are respectively shown but requires a more precise tuning of the frame size. for FSA and RFSA with a fixed number of devices n = 1000, as a function of the number of slots m (from 500 to 2500) V. CONCLUSION when varying the number of packets L (in [1, 5, 10, 15, 20]) transmitted by each device. Results confirm that, for a given In this paper the performance of FSA and RFSA has number of devices in the network, the performances of both been evaluated when the offered traffic is bursty as in typical TABLE III. FSA - BEST SETTINGS VARYING L Max. Throughput Min. Energy L d d mt Delay [s] Ebit [µJ/bits] Throughput [Kbit/s] me Delay [s] Ebit [µJ/bits] Throughput [Kbit/s] 1 500 21.13 8.99 43.16 800 23.06 8.59 39.55 5 700 91.55 7.03 49.81 900 94.13 6.81 48.44 10 800 173.69 6.46 52.39 1000 179.37 6.32 50.94 15 800 254.27 6.29 53.80 1000 259.39 6.11 52.73 20 800 332.99 6.18 54.77 1000 338.19 5.97 53.93

TABLE IV. RFSA - BEST SETTINGS VARYING L Max. Throughput Min. Energy L d d mt Delay [s] Ebit [µJ/bits] Throughput [Kbit/s] me Delay [s] Ebit [µJ/bits] Throughput [Kbit/s] 1 500 21.13 8.99 43.16 800 23.06 8.59 39.55 5 600 64.33 5.16 70.88 800 65.92 4.90 69.16 10 1200 97.06 3.32 92.61 1200 97.06 3.32 92.61 15 1100 122.33 2.83 111.82 1100 122.33 2.83 111.82 20 1100 152.00 2.64 119.99 1100 152.00 2.64 119.99

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