With the expected explosion in the number of devices in the Internet-of-Things (IoT), the availability of spectrum for these devices to connect to the network is a challenging problem. A possible solution to this problem is the use of opportunistic machine-to-machine (M2M) communications where IoT devices exploit idle periods of primary users (i.e., users with higher priority on spectrum usage) to transmit their data. The feasibility of such opportunistic M2M communication depends on the temporal characteristics of the availability of unused spectrum. Considering the unlicensed bands where Wi-Fi devices are the primary users, we present a BMAP/G/1/nK queue-based model to characterize the duration and frequency of the periods available for opportunistic M2M communications. Our results show that M2M devices may co-exist with W-Fi networks, and even in Wi-Fi networks with high loads, there are adequately long and frequent idle periods that can be used to support opportunistic M2M communications.

Machine to machine (M2M) communications are expected to form one of the fundamental building blocks of the future Internet of Things (IoT). In view of the scarcity of spectrum and the service requirements of traditional users, providing network access to the extremely large number of devices in IoT and M2M scenarios is one of the fundamental problems for network designers and operators. As a possible solution to this issue, this paper explores the possibility of using the unlicensed industrial, scientific and medical (ISM) band for supporting M2M communications while co-existing with traditional users of this band. Since IEEE 802.11 or WiFi based networks are the most common networking technology in the ISM band, this paper presents an evaluation of “white spaces” in WiFi networks (i.e. periods where the WiFi network is not using the channel) that may be used opportunistically for M2M communications. Using a MMPP/G/1/K queue to model the operation of a WiFi access point, we characterize the WiFi white spaces in terms of their frequency, duration, and their probability distribution. Our results show that WiFi white spaces provide considerable transmission opportunities that may be exploited for M2M communications.

With the growing interest in the use of autonomous computing, sensing and actuating devices for various applications such as smart grids, home networking, smart environments and cities, health care, and machine-to-machine (M2M) communication has become an important networking paradigm. However, in order to fully exploit the applications facilitated by M2M communications, adequate support from all layers in the network stack must first be provided in order to meet their service requirements. This paper presents a survey of the requirements, technical challenges, and existing work on medium access control (MAC) layer protocols for supporting M2M communications. This paper first describes the issues related to efficient, scalable, and fair channel access for M2M communications. Then, in addition to protocols that have been developed specifically for M2M communications, this paper reviews existing MAC protocols and their applicability to M2M communications. This survey paper then discusses ongoing standardization efforts and open problems for future research in this area.