1530-437X (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2014.2364055, IEEE Sensors Journal > Sensors-10694-2014< 1 Abstract— Machine-to-Machine (M2M) networks enabling networked nodes (sensors or actuators) to exchange information have been intensively studied lately. Key components of M2M networks include sensors which can be deployed with a large quantity for monitoring or surveillance. This paper deals with M2M networks hierarchically structured to accommodate efficient data transmission from terminal nodes to a sink node via cluster heads. A novel Medium Access Control (MAC) protocol, more specifically backoff time decision rule, fit for hierarchical M2M networks is proposed and evaluated by simulations. Analytical success probability in channel access by cluster heads is derived and compared with simulation results. It is shown by simulation results that the proposed backoff time decision rule achieves smaller average latency and reduced per-node energy consumption in comparison with well-known MAC protocols, the DMAC and the SMAC, for sensor networks. Index Terms— M2M network, cluster head, hierarchical structure, MAC, backoff time decision I. INTRODUCTION ACHINE-to-Machine (M2M) communications are becoming a new paradigm for various devices such as sensors and actuators to directly communicate each other or access other devices. M2M networks for M2M communications can be implemented in a variety of ways for industrial sectors, smart homes, hospitals, and other facilities[1-2]. Particularly for sensing or surveillance purpose, an M2M network might possess hundreds or thousands of nodes (devices) densely located over small or medium area [3]. Such co-existence of plentiful nodes for sensing requires low cost implementation, low latency, and low per-node energy consumption. Manuscript received August 21, 2014. This research was supported by Ministry of Land, Infrastructure and Transport (MOLIT) as Railroad Specialized Graduate School and this research was supported by a grant from Railroad Technology Research Program (Technology development on the positioning detection of railroad with high precision) funded by Ministry of Land, Infrastructure and Transport of Korean government. Ieryung Park is with Cho Chun Shik Graduate School for Green Transportation, KAIST, Daejeon, Korea (e-mail: pleastop@kaist.ac.kr). Dohyun Kim is with Department of Electrical Engineering, KAIST, Daejeon, Korea (e-mail: dohyun_kim@kaist.ac.kr). Dongsoo Har is with Cho Chun Shik Graduate School for Green Transportation, KAIST, Daejeon, Korea (e-mail: dshar@kaist.ac.kr). When many nodes are directly connected to a sink node such as an access point of 802.11 standard or a coordinator of 802.15.4 standard, network complexity is significantly increased. To share communication channel of an M2M network with multiple nodes, Medium Access Control (MAC) protocol must be very efficient. However, with a single sink node, contention-based MAC protocols with excessive contentions among many nodes cause degraded network throughput and schedule-based MAC protocols with plentiful nodes incur large complexity in time scheduling [4-5]. Moreover, distribution of many nodes connected to a single sink node often involves long communication range between Tx node and Rx node [6]. Therefore, hierarchical structure of data transmission for which data from a cluster of locally gathered nodes are transmitted to their head (cluster head) and then forwarded to the sink node can be adopted for M2M networks in order to mitigate strict channel access control and potentially poor quality of data reception due to long communication range. This paper deals with MAC protocol for hierarchical M2M networks with clustered nodes. Specifically, a novel backoff time decision rule fit for hierarchical M2M networks is proposed and evaluated by simulations. In Section II, hierarchical structure of M2M networks with clustered nodes and backoff time decision rule for such networks are presented. In Section III, analysis of success probability based on backoff time decision rule is shown. Section IV gives performance evaluation by simulations. Finally, conclusion is in Section V. II. HIERARCHICAL STRUCTURE OF M2M NETWORKS AND BACKOFF TIME DECISION RULE A. Hierarchical M2M networks with clustered nodes Fig.1 shows hierarchical M2M networks. Locally gathered nodes can be associated with a cluster head. All the nodes including cluster heads and sink node have child-parent relations. Child nodes belonging to the same cluster contend to access the channel of the parent node. Cluster head can also be a child node to a parent node in upper level, e.g., level with smaller index in Fig.1. Data generated from terminal nodes that have no child nodes are transmitted to the cluster heads followed by data aggregation at the cluster heads for subsequent transmission to the sink node, or directly transmitted to the sink node. According to the DMAC [7] of MAC Achieving Low Latency and Energy Efficiency in Hierarchical M2M Networks with Clustered Nodes Ieryung Park, Dohyun Kim, and Dongsoo Har, Senior Member, IEEE M