## 2 Previous Work
### Routing Protocols  
Many of the proposed routing protocols that assume sink mobility have been devised for energy efficiency, regardless of the transmission delay [25]. [PERSON] _et al._ proposed the Sensor Networks with Mobile Agents (SENMA) in [26], a network architecture for low power and large scale sensor networks. In SENMA, mobile agents or sinks are the only receiving terminals in data collections. Thus, when a node detects an event, it must wait for a mobile agent to be in its transmission range to send the collected data, avoiding the need of multi-hop transmission. The Scalable Energy-Efficient Asynchronous Dissemination (SEAD) protocol was introduced in [27] by [PERSON] _et al._, which assigns specific fixed nodes as the sink's access nodes. The access node is used to represent the moving sink. Sensor nodes deliver data to the access node, which in turn delivers it to the sink without exporting the sink's location information to the rest of the nodes. The protocol assumes that each sensor node is aware of its own geographic location.  
MobiRoute [28] is a routing protocol that focuses on scenarios where all the sensor nodes are fixed and have limited energy. Sink mobility is used to increase the network lifetime. The MULE architecture [29] also exploits mobility for energy efficient non-real-time data collection in sparse sensor networks. [PERSON] and [PERSON] proposed the Transmission Scheduling Algorithm Sensor Networks with Mobile Sink (TSA-MSSN) for WSN-based applications with large latency tolerance, minimizing the energy consumption costs. More recently, [PERSON] and [PERSON] [30] studied the impact of different data collection modes on energy, including network assisted sink navigation.  
Concerning those proposals that study sink mobility combined with the IEEE 802.15.4 standard, [PERSON] and [PERSON] [31] conducted a performance evaluation considering mobile sinks and using the AODV routing protocol without major modifications. They considered mainly the delay-energy metric for assessing the performance of different data collection schemes. [PERSON] _et al._[32] presented different strategies for reducing the mobility-related overhead in 802.15.4/ZigBee-based WSNs, assessing the effectiveness of different path-constrained mobile sink trajectories and their suitability in real environments.  
Our MRLG algorithm differs from these proposals since for target tracking applications, it is required that the sink receives near real-time feedback about the intruder position in order to pursue him (during short periods of time). Thus, the sink trajectory cannot be constrained. Besides, it does not aim at meeting minimum energy consumption requirements and is thus outside the family of routing protocols that help extending the WSN lifetime.  
Recently, the Collection Tree Protocol (CTP) [33] has been proposed for WSNs. CTP is a sink-announcement based protocol where sink announcements are fully propagated throughout the WSN to reach all nodes, allowing the creation of a route tree from all the sensor nodes towards the sink. Any sensor node willing to send or forward a report packet to the sink will consult its routing table to see if there is a valid path towards the sink, and then send the information using that route. In our experiments we compared MRLG against the CTP routing protocol to assess performance gains.  
Overall, we find that although the WSN literature is quite broad, only a few routing protocols offer mobile sink support and low-delay traffic delivery [34].