## 3 The LOBIN Prototype: A Practical Study Case
### System Description
#### 3.1.3 Wireless Communications Infrastructure Subsystem  
The LOBIN WCIS as well as other surveyed WHMS such as CodeBlue [25], MASN [38], [PERSON] _et al._[42], and MEDISN [46] have decided to use IEEE 802.15.4 in their ANs because it is a technology with low bandwidth, low power consumption and low deployment and maintenance costs. Others WHMS have opted for the choice of technologies more developed and widely deployed at the time, such as IEEE 802.11 (e.g., AUBADE [31], MITHril LiveNet [17,18] or SmartVest [33]). The WCIS consists of a network of DPs that transmit data--coming from the WDADs and the LWTBs--_ad-hoc_ up to a Gateway, which forwards them to the Management Subsystem. The overall WCI Subsystem architecture is shown in Figure 8.  
Figure 7: Overall Location Subsystem architecture. DPs are deployed all over the targeted areas and they are plugged to the electricity supply network. From the hardware point of view, they are exactly the same as the BPs, the only difference being the software that runs on them. The Gateway incorporates a wireless interface and an Ethernet (IEEE 802.3) interface. They are in charge of forwarding all the data coming from the WCI to the Management Subsystem. They are plugged into the electricity supply network. They implement full routing functionality, exactly as DPs do. Moreover, WDADs and LWTBs can send data straight through them. Regarding the routing algorithm used in the WCIS, over the last few years much research has been carried out in order to design and develop routing algorithms that fit the many restrictions of IEEE 802.15.4 networks in an effective way [67, 68, 69, 70]. These restrictions have to do mainly with the limitations of the available hardware (e.g., battery, memory, computing capacity) and with some other issues associated with wireless communications, such as scarce bandwidth or collisions. The scenario considered in the LOBIN project adds some additional complexity to the problem, since it presents an asymmetric traffic pattern. Most of the information is sent from the patients to the Management Subsystem (uplink), since patients are sources of data; whereas only a few commands travel in the other direction, _i.e._, from the Management Subsystem to the patients (downlink). As a result, a routing algorithm that fits such special features has been designed and developed on top of IEEE 802.15.4 [71]. The routing algorithm used in the WCI Subsystem is a source routing algorithm based on Dynamic Source Routing (DSR) [72]. When a DP is turned on, it queries its neighbour DPs to discover how to reach the destination (_i.e._, the Gateway). Thereafter, it receives responses, which contain different paths to do so. The first received response is stored as the default route. The following responses (up to 2) are stored as back-up routes. Before storing such routes, it is checked that they do not contain the same DP several times in order to avoid loops. In addition, DPs exchange, periodically, status information with their neighbours. Nevertheless, since the targeted scenario is very stable, this period is set to a high value (namely, minutes) in order to reduce the overhead introduced into the WCI. If a DP realizes that the default path is down either while transmitting a packet or after exchanging routing information, it can use one of the other default paths to solve the problem immediately. If there is no back-up path available in the DP at any given moment, the path discovery procedure is triggered again [71]. Figure 8: Overall WCI Subsystem architecture. In order to save memory in the DPs, they are not aware of the end nodes (namely, WDAD or LWTBs) that are associated with them, _i.e._, they do not store such information in memory. However, end nodes do store the DP they are associated with. If, either because the end node roams from one DP to another or because the DP it is associated with goes down, the end node realizes its DP is no longer available and it looks for other DPs to send the data through.