## 1 Introduction  
Machine-to-Machine (M2M) communications describe mechanisms, algorithms and technologies that enable networked devices, wireless and/or wired, and services to exchange information or control data seamlessly, without explicit human intervention. In this context, a machine is a device or piece of software, as opposed to a human. M2M communications are expected to revolutionize telecommunication operators' business due to emergence of new networked applications, which will attract new clients and increase the data flowing in their networks, creating more billing opportunities. The Internet of Things (IoT) with its unlimited range of applications that rely on everyday objects becoming intelligent connected devices [1] is a major driver for M2M applications. The National Intelligence Council foresees that food packages, furniture, and even paper documents can be Internet nodes by 2025 [2]. IDC envisions 212 billion \"things\" by the end of 2020, where 30.1 billions are expected to be connected autonomously [3]. On the other hand, the sensing and connectivity capabilities of smartphones and their pervasiveness in people's lives make them critical pieces of this IoT and future applications. Thus, mobile M2M communications will certainly play a significant role in the M2M ecosystem and in cellular communications. The Cisco Visual Networking Index Global Mobile Data Traffic Forecast Update [4] estimates that in 2013 mobile M2M communications represented 1% of all mobile data with 4.9% of all connected devices, and by 2018 mobile M2M communications will represent 6% of all mobile data with 19.7% of all connected devices. The staggering market of mobile M2M communications in vehicles leads Machina Research to expect an exponential growth in the number of vehicles with M2M connection capabilities in the next years [5], which demands further research in this area due to the mobile nature of vehicles. Vehicles can have embedded M2M devices, or users can carry devices, including smartphones, with M2M capabilities. This year in a survey, 80% of inquired drivers (5000 in total) expected that the car of the future provides them the same connected experience they have at home, at work, or on the move via their mobile phone [6]. Furthermore, more than 70% of them said they were interested in using or were already using internet services in their cars. The number of vehicles with built-in connectivity is predicted to increase from 10% of the overall market in 2013 to 90% by 2020 [7]. The same study predicts that the M2M market in the automotive sector will generate $422 billion by 2022, and services, including both connectivity and applications supported by it, shall account 88%. Mobile M2M communications will revolutionise a very broad range of businesses like large sensing applications, intelligent transport systems, or monitoring and control applications, e.g., personal health monitoring, city automation, automotive systems, and smart grid [8; 9; 10; 11; 12; 13]. Key challenges in mobile M2M will be the support of a large variety and diversity of devices, most of them with resource constraints (memory, processing, bandwidth, energy, _etc._), and the traffic volume and traffic pattern they generate. M2M traffic differs from Human-to-Machine (H2M) or Human-to-Human (H2H) by the large number ofshort payload transactions [14]. The uplink traffic will be dominant in the first since M2M devices will rather be data generators, data resulted from monitoring or sensing application, than data consumers. Furthermore, the communication model in H2H is predominately request-response, while for M2M it is expected a partition between request-response and publish-subscribe. Within mobile M2M communications, smartphones are bound to play a special role. They are equipped with wide environment and context sensors, as well as with multiple personal, local, and wide area communication capabilities, namely NFC, Bluetooth (Zigbee is expected in the near future), WiFi and several cellular technologies.