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], forazio.briante, ffrancisco.vazquez, [email protected] antonella.molinaro, [email protected] 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 MAC PROTOCOLS (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 coordi- nator 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.