## 2 Related Work  
Several researchers have proposed routing protocols for utilizing multiple sink nodes [12-16], but only [15,16] proposed a geographic routing. In [15] a grid scenario was assumed, ignoring the routing holes problem, and no details about the real implementation is given. The so-called Greedy Forwarding scheme based routing protocol for multiple sink WSNs is a novel research issue [16]. The advantages of multiple sink wireless sensor networks compared with single sink sensor networks are as follows:  
* They are more reliable due to the fact that invalidation of a sink node will drag down the whole network in single sink WSNs. * Usually there exists a serious node energy bottleneck (around sinks) if a single sink collects reports from too many sensors. * They relieve the unbalanced energy consumption among sensor networks. * They avoid mobile sink schemes that result in large energy consumption and serious communication interference [11]. * Multiple sinka reduce payoff of data fusion in very large and complex WSN applications [17]. * They offer more versatile functional applications and communication cooperation. In some applications, different users (sinks) may require different environmental variables (temperature, humidity, light intensity, _etc._) or data formats (image, sound, video, _etc._). In this time, all nodes need to cooperate with each other during the communication process. * In some cluster routing protocols, such as LEACH [5] or PEGASIS [7], each cluster head node needs to communicate with a sink node directly. If only one sink node was deployed, cluster head nodes must work with high transmission power, which not only consumes too much node energy, but also the interference problem of the long distance transmission cannot be ignored. * In some location-based routing protocols [18] (such as GPSR [19]), the routing holes problem is unavoidable, but it is is expected to be solved effectively in a multiple sink network structure because the sink deployment dispersion will help a sender find a next hop node. * They provide more real-time data transport of networks, which has a significant effect in multimedia WSNs [20]. At present, the multiple sink sensor networks have been tried in a few applications, such as polar environmental monitoring [21], underwater WSNs [22], _etc_. These applications provide valuable experience for further research on multiple sink networks systems. In [23], the problem of routing packets in dynamically changing networks is considered, concentrating on two different modes: anycasting and multicasting. In anycasting, a packet has a set of destinations but only has to reach one of them, whereas in multicasting, a packet has a set of destinations and has to reach all of them. Due to the more balanced energy consumption of clustering-based routing protocols, they are usually employed for large scale WSNs. The LEACH protocol presented in [24] is an elegant solution where clusters are formed to fuse data before transmitting it to the base station. By randomizing the cluster heads chosen to transmit to the base station, LEACH achieves an 8-fold improvement compared to direct transmissions, as measured in terms of when nodes die. But in LEACH all cluster head nodes communicate directly with the base station, which exhausts the nodes far away from base station soon. TEEN considers that cluster heads at increasing levels in the hierarchy need to transmit data over correspondingly larger distances [6]. Combined with the extra computations they perform, they end up consuming energy faster than the other nodes. In order to evenly distribute this consumption, all the nodes take turns in becoming the cluster head for a time interval \(T\), called the cluster period. TEEN is well suited for time critical applications and is also quite efficient in terms of energy consumption and response time. PEGASIS (Power-Efficient GAthering in Sensor Information Systems) [7] is a near optimal chain-based protocol that is an improvement over LEACH.