Towards Efficient Mobile M2M Communications: Survey and Open Challenges <s> M2M Traffic <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> BIB001 </s> Towards Efficient Mobile M2M Communications: Survey and Open Challenges <s> M2M Traffic <s> The 3GPP has raised the need to revisit the design of next generations of cellular networks in order to make them capable and efficient to provide M2M services. One of the key challenges that has been identified is the need to enhance the operation of the random access channel of LTE and LTE-A. The current mechanism to request access to the system is known to suffer from congestion and overloading in the presence of a huge number of devices. For this reason, different research groups around the globe are working towards the design of more efficient ways of managing the access to these networks in such circumstances. This paper aims to provide a survey of the alternatives that have been proposed over the last years to improve the operation of the random access channel of LTE and LTE-A. A comprehensive discussion of the different alternatives is provided, identifying strengths and weaknesses of each one of them, while drawing future trends to steer the efforts over the same shooting line. In addition, while existing literature has been focused on the performance in terms of delay, the energy efficiency of the access mechanism of LTE will play a key role in the deployment of M2M networks. For this reason, a comprehensive performance evaluation of the energy efficiency of the random access mechanism of LTE is provided in this paper. The aim of this computer-based simulation study is to set a baseline performance upon which new and more energy-efficient mechanisms can be designed in the near future. <s> BIB002
It is important to distinguish mobile M2M communications from mobile Human-based (H2H or H2M) communications. Small and infrequent data transmissions will be more common in M2M BIB002 BIB001 , and thus the knowledge developed for Human-based traffic, which is mostly bursty (web browsing), bulky (file transfer), or constant or variable bit rate streams (VoIP or video) can be difficult to apply directly to M2M. Laya et al. BIB002 mention that M2M and Human-based traffic differ further in traffic direction, since M2M traffic direction will be mainly uplink, while Human-based traffic is either balanced or mainly downlink. M2M applications will be duty-cycled and should have very short connection delay to guarantee fast access to the network when waken up, while Human-based applications tolerate longer connection delays but are very demanding once connections are established BIB002 . M2M applications might require very high priority with a detailed level of granularity due to the transmission of critical information, whereas priority for Human-based applications is mainly among applications for each user and not between different users BIB002 . Finally, M2M will have higher number of devices and may be required to operate for years or decades without maintenance, but users can recharge/replace batteries BIB002 .