## 2 Related Work  
In wireless sensor networks, several routing protocols [4; 5; 6; 18; 20; 21; 22; 23] have been proposed for providing timeliness in real-time applications. The protocols based on spatiotemporal approaches improve the timeliness. These approaches consider the information on the end-to-end distance (spatial) and transmission delay time (temporal) of relay nodes to make the routing decision. This chapter briefly reviews these real-time routing protocols based on spatiotemporal approaches. SPEED [4] is the first developed spatiotemporal real-time routing protocol in wireless sensor networks. SPEED is a stateless and localized routing protocol providing a soft real-time communication service by a desired uniform delivery speed across the network under the assumption that the end-to-end delay is proportional to the distance between the source and the destination. It exploits only the local information, i.e., the location and delay information about the neighbor nodes, and not the global topology. Stateless nondeterministic geographic forwarding (SNGF) is a routing module responsible for selecting an appropriate forwarding node in the neighborhood table that meets the desired delivery speed, which in turn balances the load in each flow between multiple concurrent routes. Delay estimation modules are intended to estimate the single-hop delay that helps in making routing decisions. A backpressure re-routing algorithm helps in re-routing packets around large-delay links (i.e., congestion) with minimum control overhead. To achieve energy efficiency, POWER-SPEED [19] does not use control packets to select the next-hop neighbor according to the destination based on the downstream QoS condition estimation based on the spatiotemporal historic data. Then, POWER-SPEED adjusts the power level of the wireless transmitter to a minimum value to send packets to the next-hop neighbor. As another protocol to achieve energy efficiency, an energy-aware real-time routing protocol named EA-SPEED [21] uses the residual energy information as well as the single-hop delay and the location information in routing decisions to select the next-hop neighbor to a destination. Then, the weight function of neighbor nodes for the routing decision module is formed based on the information of these three parameters. For supporting fault tolerance, another extension of SPEED named FT-SPEED [23] is proposed to solve the void problem of SPEED by using a void announce scheme and a void bypass scheme. The void announce scheme prevents packets reaching the void through other routing paths, while the void bypass scheme forwards packets around two sides of the void. MMSPEED [5] is an extension of SPEED that provides service differentiation and a probabilistic QoS guarantee. For delivery timeliness, multiple network-wide packet delivery speed options are provided for different traffic types according to their end-to-end deadlines. In supporting service reliability, probabilistic multipath forwarding is used to control the number of forwarding paths based on the required end-to-end arrival probability. In order to provide reliability, MMSPEED exploits the principle of redundancy, i.e., the more paths that a packet is delivered via, the higher the probability of reaching its final destination. As an extension of MMSPEED for enhancing energy efficiency, Improved MMSPEED (IMMSPEED) [18] uses a multi-path and multi-speed approach, similar to MMSPEED. However, IMMSPEED selects neighbor nodes with higher residual energies as next-hop nodes to a destination. This is different from MMSPEED, which selects neighbor nodes randomly. THVR [6] and OMLRP [22] attempt to increase the performance of real-time communication with multi-hop network information. Using multi-hop lookahead, sensor nodes gather the information of network topology and delay. In these protocols, a routing decision is made based on the multi-hop (more than two) velocity.