## 2 Related Work
### Location Service  
Location service is a popular solution to handle mobility in ad-hoc and sensor networks. In location service, the actor updates its location information to a subset of sensor nodes and the source can query these selected nodes to obtain the location of the actor. These selected nodes are defined as location servers and connect the source and the actor. The location service can be further divided into _quorum-based_ and _home-based_ protocols based on the methods to construct the rendezvous area.  
In the quorum-based approach, such as XYLS [8], the actor updates its location to a vertical area, and the source sends its location query to a horizontal area. The intersection of these two areas becomes the rendezvous area where the query will be satisfied. The Local Update-based Routing Protocol (LURP) proposed by [PERSON] et al. [9] provides an algorithm which constrains the location updating within a local area. When the actor moves out of the local area, another area is built. The Adaptive Location Update-based Routing Protocol (ALURP) [10] makes an improvement compared with the LURP protocol. In ALURP, the mobile sink adaptively adjusts its location updating range when it moves in the destination area. MLS in [11] takes advantage of the fact that in most applications, the actor knows its own mobility strategy, such as its destination, trajectory and speed. Therefore, the actor can share its mobility strategy in the update packet to improve the performance of location service. The authors in [12] adopt quorum-based schemes to real irregular fields with voids as well as non-rectangular shapes and propose an energy-balanced Sink Location Service scheme.  
In home-based protocols, such as GLS [13] and GHLS [14], the rendezvous area is selected via a public hash function. The input of the hash function is a node ID, and the output can be either node IDs or geographic locations. GLS (Grid Location Service) and HLS (Hierarchical Location Service) coupled to the GPSR routing protocol (Greedy Perimeter Stateless Routing) are compared in [15]. This paper presents the efficiency and the scalability performance study of both GLS and HLS and analyze their applicable scenes respectively. The Line-based data dissemination protocol (LBDD) [16] uses a virtual vertical line or strip area constructed in the central field of the network which can divide the whole network into two equal parts. The source transmits the data packet towards the virtual area. When the actor requests event reports, it also sends a query to this rendezvous area. Then it travels linearly until it encounters the node storing the packets. [PERSON] et al. [17] proposed a  
Figure 1: Expanding Ring Search and Trail-based Search.  
directional routing and scheduling scheme (DRSS). In DRSS the source could hold or relay data to a predetermined area called rendezvous area. These event packets are stored in this area until a mobile sink is nearby to collect.  
The major disadvantage of location service is the requirement of periodical location update which does not consider the occurrence of the events. The overhead of location update may waste considerable energy when there is no event occurs. Moreover, the packets should be forwarded to the rendezvous area first and then to the mobile actor, which increases the transmission delay and generates unbalanced energy consumption. At last, the location of actors may be hard to be obtained in many applications [18].