## 2 State of the Art
### Architectures and Communications Technologies for WHMS  
Figure 1 shows a complete architecture for WHMS that fits the different approaches in [8, 9, 10, 11]. However, wearable healthcare systems are so diverse that they will implement different combinations of such communications segments depending on the target application. In an application aiming at allowing the user or patient to monitor its health status on its own by using a smart portable device, such as a smartphone or a tablet, the Backhaul/WAN communications segment may not be needed; the sensors in charge of measuring the appropriate physiological parameters could communicate directly with the smart device making a BAN up. In case the raw data coming from the BAN is not processed locally at the smart device but at the \"cloud\" (e.g., to save battery in the smart device), the smart device would have to communicate with the back-end servers through the backhaul communications segment, so the communications architecture would be composed of the BAN and the Backhaul/WAN. In telemedicine applications aiming at remote diagnosis or remotely monitoring the health status of a patient or group of patients being at home, the backhaul network would be definitely needed and the information coming from the BAN may have to traverse an AN before reaching the backhaul segment of the communications architecture. If such application also takes into account information coming from devices or just things (as in the so-called Internet of Things) around the users, the gateway of the BAN and the objects around the user might form a PAN directly connected to the WAN and so to the medical information systems. The communications technologies to be used will tightly depend on the specific requirements of each communications segment. For the WAN/Backhaul segment, cellular technologies (e.g., GSM, GPRS, UMTS) as well as broadband wired technologies (e.g., ADSL, cable) are the most appropriate, the first ones being the most widely used in practice, as it will be shown later in this section and in Section 2.2. Wi-Fi (IEEE 802.11) seems to be the most appropriate candidate for the AN. However, a wireless mesh network based on IEEE 802.11 or on IEEE 802.15--e.g., to provide coverage in indoor environments such as a hospital--that enables the communications with an eventual back-end system through a gateway, is also considered as an AN. As it will be also illustrated later in this section and when reviewing the related projects in Section 2.2, Bluetooth is one of the most widely used communications technologies for PAN, since it provides reasonably high bandwidth (able to aggregate the traffic coming from the BAN) and it is widely implemented in commercial devices, such as smartphones or laptops. However, other communications technologies, such as IEEE 802.15.4/Zigbee or RFID, or even a combination of all them, can be also used to effectively deploy PANs. Although many wired solutions are used to implement BAN in practice, as it will be shown at the end of this section and in Section 2.2, Wireless BANs (WBANs) are of special interest to this work since they meet better some key requirements of wearable healthcare systems, such as wearability, noninvasiveness, and comfortability. However, WBAN also need to overcome some issues related to reliability or security. Therefore, much research is currently being carried out to identify challenges at every layer of the communications stack and to propose solutions [9, 12, 13, 14, 15]. As a matter of fact, the IEEE 802.15 working group has recently launched the IEEE 802.15.6 standard exclusively targeting WBAN [16]. Table 1 summarizes and compares the most relevant communications for WBAN (both for Implantable--IWBAN and Wearable--WWBAN). It is worthwhile to remark that Bluetooth 4.0 Low Energy, despite not having been used in this kind of applications yet, represents a good candidate for WBANs, coupling its suitable technical features with its estimated high commercial penetration.