## 3 Characteristics of Existing Evacuation Systems  
In this section, we will first briefly review the salient characteristics of some proposed evacuation systems. References [13; 14; 15; 16; 17; 18; 20; 21; 39; 42; 43; 44] rely on location information regarding the sensors to compute evacuation paths [51], while the deployment of sensors in work spaces and buildings in [12; 13; 14; 15; 16; 17; 20; 21; 25; 39; 42] enables the movement of evacuees and first responders. Except for [12; 13; 14; 15; 16; 20; 21], most approaches require maps either for computing or navigation purposes. Instead of physically sensed input, as in other work, some simulation studies [40; 41] use virtual hazards to simulate emergencies. Also, the dynamic spread and shrinking of hazards is considered in [15; 19; 20; 21; 23; 25; 39; 42]. While in [24; 28], opportunistic networks are considered, other papers assume that [24; 28; 29; 43; 44] mobile users themselves facilitate sensing and communications. In [24; 28; 29; 40; 41; 43; 44] localization technologies to be used are required. We note that in [24; 28; 29; 43; 44] personalized navigation directions can be provided to individuals, and the issue of congestion is also addressed in some papers [16, 17]. Table 1 also summarises and compares proposals that have been made for search and rescue systems. In [30, 34] searching victims or tracking their movements is the focus, while in [35, 36, 37], attention is payed to tracking rescuers (or firefighters) to guarantee their safety. Based on the presence of victims, in [38], the search and rescue space is reduced, and in [32, 33], a communication backbone is formed using robotics. The work in [26, 27] uses robots to search and guide victims, and in [31], the number of civilians in an emergency is assessed. Finally, the following papers, [45, 46, 47, 48, 49, 50], concentrate on the optimization of search and rescue cost. Figure 1 summarises the cost incurred as additional features and benefits are included in emergency response systems. wireless sensor network (WSN)-based emergency evacuation systems collect multi-domain sensing information to provide navigation services in a fully distributed manner and offer in-network knowledge discovery to respond to emergencies. This type of system relies on prior deployment of information and localization support. Decentralized evacuation systems offer better network resilience, scalability and lifetime, because they treat the sensing system separately from the decision system to prolong network lifetime and, also, involve mobile clients in sensing to adapt to intermittent connections and uncertain environments. However, this type of network must be supported by functionally separated system management and opportunistic networking and computing to extract the knowledge behind short-lived pieces of data. Agent-based simulators can help discover deep knowledge and conduct prediction and forecasting. Grid and cloud supported systems can provide parallel and auto-scaling computation to respond to emergencies. However, such systems must rely on a well-designed parallel programming model to divide adequately distribute computation tasks and Internet support. By involving more mobile entities, search and rescue systems can provide a richer response in case of emergencies using heterogeneous resource integration and control. In all these cases, we see that there is always a tradeoff between the quality and speed of the response to an emergency and the cost and sophistication of the technology. We also need to consider the similarities and differences between indoor and outdoor emergency response systems. Both indoor and outdoor cases present similar challenges mentioned in Section 1, but existing technologies for indoor systems are quite distinct from those for outdoor systems. Table 2 compares indoor and outdoor systems with respect to the technical challenges mentioned in Section 1. In addition to WiFi, 3G and wired networks, short-range ZigBee and Bluetooth technologies are also useful in an indoor system.