## 2 M2M Network Architecture  
In the recent years, ETSI has been actively engaged in the development of a standard for M2M systems, with the objective to ensure interoperability between the diverse M2M components and the already existing technologies. To this direction, ETSI proposes a high-level horizontal architecture, dividing the system into three domains: (i) the device and gateway domain where the M2M devices communicate with a gateway through short-range area networks; (ii) the network domain that connects the gateway to the applications through long-range access and core communication networks; and (iii) the application domain where various application services are defined depending on the use case [19].  
Figure 1 illustrates the architecture of a wireless M2M system for mHealth applications. The ETSI framework consists of five key elements as described below [19]:  
* The M2M devices, which are devices capable of transmitting data autonomously or after receiving a data request. In the context of mHealth applications, the M2M devices are principally low-power medical sensors and actuators, placed close to or inside the patient's body (wearable or implanted sensors, respectively). Each device has an embedded wireless communication module, enabling their interconnection into a short-range network, as described in continuation.
* The M2M Area Network, also known as capillary network, which is the short-range network interconnecting the M2M devices and providing a link to the M2M gateway. Focusing on mHealth scenarios where wireless connectivity is considered among the body sensors, WBANs are the enabling technology for the M2M area network. Hence, in the rest of this paper, the terms M2M area network and WBAN will be employed interchangeably. IEEE 802.15.6 [20] is an emerging communication standard developed specifically for WBANs and suitable for healthcare applications. Other candidate technologies for the capillary network include ZigBee and IEEE 802.15.4, Bluetooth, and Bluetooth Low Energy. Proprietary solutions are also available, such as ANT, Sensium, Zarlink and Z-Wave.
* The M2M Gateway, which acts as a proxy between the M2M devices (interconnected through the WBAN) and the network domain. Practically, the gateway must be a portable device with advanced processing capabilities and multiple radio interfaces, able to operate in technologies employed by both the WBAN and the communication network. Typical example of M2M gateways include smartphones, Personal Digital Assistants (PDAs) and smart watches.
* The M2M Access Communication Network, which connects the M2M gateway to the Internet, eventually reaching the M2M application server. The communication network can be broken down to the access network and the core network. Common wireless technologies employed by the access network include Long Term Evolution (LTE), WiMAX and IEEE 802.11 WLAN. Core network technologies provide IP connectivity and include 3 GPP, 3 GPP2 and ETSI TISPAN core network systems.
* The M2M Application Server, which is the middleware layer that provides data to the specific business applications.  
Figure 1: Simplified Machine-to-Machine (M2M) architecture for wireless connectivity in mHealth scenarios.