Towards Efficient Mobile M2M Communications: Survey and Open Challenges <s> M2M Support in Wireless Networks <s> It is envisioned that home networks will shift from current machine-to-human communications to the machine-to-machine paradigm with the rapid penetration of embedded devices in home surroundings. In this article, we first identify the fundamental challenges in home M2M networks. Then we present the architecture of home M2M networks decomposed into three subareas depending on the radio service ranges and potential applications. Finally, we focus on QoS management in home M2M networks, considering the increasing number of multimedia devices and growing visual requirements in a home area. Three standards for multimedia sharing and their QoS architectures are outlined. Cross-layer joint admission and rate control design is reported for QoS-aware multimedia sharing. This proposed strategy is aware of the QoS requirements and resilience of multimedia services. Illustrative results indicate that the joint design is able to intelligently allocate radio bandwidth based on QoS demands in resource-constrained home M2M networks. <s> BIB001 </s> Towards Efficient Mobile M2M Communications: Survey and Open Challenges <s> M2M Support in Wireless Networks <s> To enable full mechanical automation where each smart device can play multiple roles among sensor, decision maker, and action executor, it is essential to construct scrupulous connections among all devices. Machine-to-machine communications thus emerge to achieve ubiquitous communications among all devices. With the merit of providing higher-layer connections, scenarios of 3GPP have been regarded as the promising solution facilitating M2M communications, which is being standardized as an emphatic application to be supported by LTE-Advanced. However, distinct features in M2M communications create diverse challenges from those in human-to-human communications. To deeply understand M2M communications in 3GPP, in this article, we provide an overview of the network architecture and features of M2M communications in 3GPP, and identify potential issues on the air interface, including physical layer transmissions, the random access procedure, and radio resources allocation supporting the most critical QoS provisioning. An effective solution is further proposed to provide QoS guarantees to facilitate M2M applications with inviolable hard timing constraints. <s> BIB002 </s> Towards Efficient Mobile M2M Communications: Survey and Open Challenges <s> M2M Support in Wireless Networks <s> Machine-to-machine communications is characterized by involving a large number of intelligent machines sharing information and making collaborative decisions without direct human intervention. Due to its potential to support a large number of ubiquitous characteristics and achieving better cost efficiency, M2M communications has quickly become a market-changing force for a wide variety of real-time monitoring applications, such as remote e-healthcare, smart homes, environmental monitoring, and industrial automation. However, the flourishing of M2M communications still hinges on fully understanding and managing the existing challenges: energy efficiency (green), reliability, and security (GRS). Without guaranteed GRS, M2M communications cannot be widely accepted as a promising communication paradigm. In this article, we explore the emerging M2M communications in terms of the potential GRS issues, and aim to promote an energy-efficient, reliable, and secure M2M communications environment. Specifically, we first formalize M2M communications architecture to incorporate three domains - the M2M, network, and application domains - and accordingly define GRS requirements in a systematic manner. We then introduce a number of GRS enabling techniques by exploring activity scheduling, redundancy utilization, and cooperative security mechanisms. These techniques hold promise in propelling the development and deployment of M2M communications applications. <s> BIB003 </s> Towards Efficient Mobile M2M Communications: Survey and Open Challenges <s> M2M Support in Wireless Networks <s> This paper focuses on the use capillary M2M (Machine-to-Machine) networks for the estimation of spatial random fields. The capillary network is coordinated by an M2M gateway which is in charge of (i) collecting the set of spatially correlated sensor observations; and (ii) aggregating (compressing) such observations prior to their re-transmission to a remote application server. We consider a realistic scenario where a contention-based Medium Access Control scheme is used for sensor-to-GW communications. In this context, we attempt to determine the optimal duration of the two transmission phases, namely, sensor-to-gateway and gateway-to-application server, in such a way that the best trade-off in terms of packet collisions and compression level is attained. By doing so, the distortion in the reconstructed random field can be minimized. We also investigate how the optimal operating point is affected by the specific data aggregation scheme, both of them being based on the Karhunen-Loeve transform, or the spatial correlation properties of the underlying field. <s> BIB004 </s> Towards Efficient Mobile M2M Communications: Survey and Open Challenges <s> M2M Support in Wireless Networks <s> Attaining very high energy efficiency is one of the big challenges to implement Machine-to-Machine (M2M) networks whose lifetime (without human intervention) must be measured in years. In this paper, we consider a synchronized duty-cycled M2M network composed of a large number of devices that periodically wake up their radio interfaces to transmit data to a coordinator. We theoretically analyze the delay and energy efficiency of two contention-based Medium Access Control (MAC) protocols for this kind of networks. One protocol is based on Frame Slotted ALOHA, and the other is based on a tree-splitting contention resolution algorithm. Computer-based simulations have been carried out to ensure the accuracy of the theoretical models and to evaluate and compare the performance of the two alternatives for M2M applications based on low power Wi-Fi devices. <s> BIB005
M2M devices using radio technologies will face well-known problems from wireless and cellular networks. Potential issues on the air interface including channel interference, channel quality fluctuation, and noise will be very common due to the multitude of devices and the characteristics of M2M traffic BIB001 BIB002 , and they can introduce coordination problems in the medium access. According to Lu et al. BIB003 , reliability is critical for general acceptance of M2M, since unreliable processing, sensing, or transmission leads to false or lost data, and ultimately to M2M communications' failure from the user's perspective. Although end-to-end service reliability is still a challenge, it is being addressed by standardization efforts. As the number of devices competing for the same channel increases, the number of simultaneous accesses will increase, and packet collisions, and signal interference in general, will be more common and result in more packet/data loss. Optimizing the uplink channel access and radio resource allocation is a way to achieve further improvement in performance and resource usage efficiency, avoiding constant transmission deferrals originated from the packet collision avoidance mechanisms and data loss originated from packet collisions, or providing general QoS guarantees. Gallego et al. BIB005 introduce contention-based MAC protocols for sensor-to-gateway communications in wireless M2M, and analyse them in terms of delay and energy efficiency. The authors consider an M2M wireless network composed of a large number of devices that periodically wake up their radio interfaces to transmit data to a coordinator, that is, to a gateway. Zhang et al. BIB001 propose a joint rate and admission control scheme for QoS provision in M2M communications, using an IEEE 802.11 network, by exploiting heterogeneous networks and accurate predictions of QoS. Wireless networks usually use solely collision avoidance mechanisms, which introduce well-known problems, such as the hidden node problem or the exposed node problem, that other networks do not face, such as cellular networks. Further work needs to be carried using wireless networks, in order to take advantage of the high data rate and low latency common in those networks. Techniques that efficiently aggregate the data to be transmitted can be explored to further optimize bandwidth utilization and energy consumption in M2M communications. Two data aggregation schemes based on the Karhunen-Loève transform for M2M in a wireless network are proposed by Matamoros et al. BIB004 . Their system includes several sensors, one gateway, and one application server. The sensors transmit the data to the gateway, which transmits all the data to the application server. While gateway-to-application server communications use a reservation-based MAC protocol, the sensor-to-gateway communications use a contention-based MAC scheme and, thus, packet collisions may occur. They determine the optimal duration of the sensor-to-gateway and gateway-to-server transmission phases, in such a way that the best trade-off between the number of packet collisions and compression level from data aggregation is attained.