# A Multi-Hop Clustering Mechanism for Scalable IoT Networks
## 1 Introduction  
The Internet of Things (IoT) relies on the interconnected objects that are able to communicate with each other and collect data about their context [1]. The IoT devices are assumed to be equipped with the network interfaces of low-power short-range wireless protocols, such as Bluetooth 4.0 and Zigbee, so that communications among themselves are possible. Furthermore, recent advances in the low-power long-range communication technologies, such as Bluetooth 5.0 and LTE-M, will facilitate even wider area communications for the IoT devices. The Gartner group predicts that up to 26 billion of things equipped with sensors and wireless communication capabilities will be connected to the Internet by 2020 for various purposes [2]. For application service such as smart city, factory automation or intelligent building management, the IoT devices placed over a particular area may collect and transmit the sensed or their context data to the servers in the Internet in a periodic manner or in real-time. The size of data may vary from the small ones such as temperature or humidity to the relatively larger ones, such as images. For a largescale IoT network, having each IoT device connected to the server with an individual connection may result in the excessive requirement of Internet connections, as well as the waste of wireless resources of an IoT network. Conservation of the Internet and the wireless network resources may be possible by grouping the devices of an IoT network into clusters and having a representative device collects the data from the rest of the devices in each cluster and communicates with the server on behalf of the other devices instead of having a per-device Internet connection and communication [3; 4; 5]. Hereinafter, let us call those representative IoT devices responsible for the data transmission to the Internet servers as the coordinators and the entire set of IoT devices as nodes. By configuring the IoT networks in this way, the Internet connection is required for the coordinators only instead of the entire nodes in the IoT network. To minimize the number of Internet connections, the clusters need to be configured as large as possible. As the size of the cluster becomes larger, though, the data transmission delay, as well as the wireless resource consumption increases, and congestion may occur, especially near the coordinators. Since it may lead to the degradation of quality of application services, the clusters for IoT networks need to be configured to cover as many nodes as the wireless capacity of IoT network allows. Clustering of a network into groups has already been extensively studied in the context of sensing data transmissions in the sensor networks. The sensors are equipped with limited energy resources, but are required to operate without recharging or replacing batteries for extended periods of time. Clustering techniques for sensor networks, therefore, have been introduced to support the scalability with energy efficient communication between sensors and eventually to prolong the life time of a network [4]. Note, differing from the typical sensors, the nodes of IoT networks are expected to be the appliances which are equipped with continuous supply of power to perform some intrinsic functionality other than mere sensing. For the IoT networks, therefore, minimizing the number of Internet connections, instead of the energy efficient communications, is the critical issue for the conservation of network management cost. In this paper, therefore, we propose a clustering mechanism for the IoT networks that have a continuous supply of power. Especially, we assume the leverage of low-power wireless communications, which are supposed to be commonly available at the IoT devices, for the communication between the nodes and their coordinators for the flexible and scalable formation of clusters. Extending the hop counts between a coordinator and its member nodes leads to the increase of delay in the wireless network.