## 4 ETSI M2M Architecture  
The ETSI M2M architecture is currently the reference architecture for global, end-to-end, M2M service level communications, and is being adopted by main European telcos. The system architecture is based on current network and application domain standards, and it is extended with M2M Applications and Service Capabilities layers (SCLs). SCLs are Service Capabilities (SCs) on the Network domain, M2M Device, or M2M Gateway. SCs provide functions to be shared among different M2M applications. The functions of a SCL include, but are not limited to, registration of applications, provision of means for storage, policy-based selection of communication means for information delivery, support for multiple management protocols, or support of remote management of gateways and devices [45]. Figure 1 shows the high level ETSI M2M system architecture as defined in ETSI Technical Standard (TS) 689 [8]. The key entities in M2M are [46]:  
* M2M Device: a device that runs application(s) using M2M capabilities and network domain functions;
* M2M Gateway: guarantees M2M devices interconnection to the network and their inter-operability;
* M2M Applications: applications that run the service logic and use Service Capabilities accessible via open interfaces;
* M2M Area Network: provides connectivity between M2M devices, compliant and non-compliant with ETSI M2M, and ETSI M2M gateways;
* M2M Network and Application Domain: provides connectivity between the M2M gateways and M2M applications;
* M2M Network Applications: applications, in the Network and Applications domain, that run the service logic and use Service Capabilities accessible via open interfaces. The Network and Application Domain is formed by the Access Network, the Transport Network, and the M2M Core. The Access Network provides connectivity between the M2M Device Domain and the Core Network, and the Transport Network provides connectivity within the Network and Application Domain. Satellite, UTRAN, WLAN, UWB, or WiMAX technologies are used in the Access Network. The M2M Core is composed by the Core Network (CN) and M2M Network SCs. The CN provides IP connectivity, interconnections, and roaming capabilities within the M2M Core. Technologies provided by 3 GPP or TISPAN can be used in the CN. The M2M Device domain is formed by M2M devices, M2M Area Networks, and M2M gateways. The M2M devices can connect directly to the Network and Application domain using the Access Network, or they can connect first to an M2M gateway using the M2M Area Network. In the first case, the devices run an M2M application and have an M2M SCL. In the latter, the M2M gateway runs an M2M application and an SCL, and provides access to the Access Network for the M2M Device, acting on its behalf, since the M2M Device has only an M2M Application running, but no SCL, and it is not compliant with ETSI. The Area Network provides connectivity between M2M devices and M2M gateways, and can be built on Bluetooth, UWB, ZigBee, M-BUS, or IEEE 802.15.4 technologies. The TS also specifies several interfaces not shown in the figure. The interface between an M2M Application in the M2M Device Domain and the M2M SC in the Network and Application Domain is termed mIa. The interface between an M2M Device or M2M Gateway and the M2M SC in the Network and Application Domain is named mId. Finally, the dIa interface is defined between an M2M Application in an M2M Device or M2M Gateway and the M2M SC at the same device. To better illustrate some entities, we describe a storyboard and map it in M2M, similar to [47]. [PERSON] is concerned with his well-being. He is an elderly person and needs to control his heart condition. During his daily life he wears a wearable monitoring system that monitors his heart rate. Every morning, he picks up his smartphone, which connects to the wireless sensor to retrieve the measurements. These measurements are stored online, enabling a service that later provides his doctors access to his heart rate history. Figure 2 shows an overview of this storyboard.