# Routing in Mobile Wireless Sensor Networks: A Leader-Based Approach+
## 2 Related Work  
In this section, we revise some routing techniques that have been developed in recent years, specifically those that can be applied to MWSN.  
Many works address sink mobility; however, only a few consider the mobility of sensors nodes. These works are the ones addressed in this section. Furthermore, we limit this section to proactive protocols, a feature of ours.  
In order to avoid flooding and be energy efficient, hierarchical routing has been adopted in many works. For static WSNs, [PERSON] et al. proposed the Low-Energy Adaptive Clustering Hierarchy (LEACH) protocol [14], which defines a set of clusters in the network. In every cluster, a Cluster Head (CH) is in charge of aggregating the information of the cluster's nodes and route it towards the sink. In a variant of this protocol [15], a hierarchy of clusters is used to increase scalability.  
In MWSN, the cluster approach is followed in the Enhanced Cluster-Based Routing protocol (ECBR-MWSN) [16], which considers mobile sensor nodes; however, it assumes a static sink. The CH is chosen on the basis of a combination of the highest residual energy, lowest mobility and least distance from the sink. To face mobility, ECBR-MWSN elects a new CH periodically, which results in new paths to the sink. Although we do not follow the same cluster-based approach as ECBR-MWSN, our protocols perform periodic casts of the topology too.  
In the protocol proposed in [17], nodes cooperate with each other with the objective of enhancing the robustness of routing while using Wireless Broadcast Advantage (WBA) [18], as we also do. In short, WBA refers to the fact that when a node sends a broadcast message all the nodes that are in its transmission range will receive the message. They also assume that every node in the network has a pre-established path between the node and the sink. Special nodes, called _guard nodes_, are also used to support routing of the messages.  
Proactive routing, used in early, static WSNs (e.g., the Destination-Sequenced Distance Vector protocol, DSDV [14; 19]), has been implemented in [16; 17] for MWSN. A proactive routing protocol runs independently from the application to maintain and update routing tables, with the aim of reducing the end-to-end delay, a relevant Quality of Service (QoS) parameter.  
As mentioned before, [PERSON] et al. [13] uses a reconfigurable spanning tree. Once the spanning tree has been built, they apply a simple leader election mechanism to set the root of the tree. Contrary to this, in our approach leader election is the basis for the spanning tree construction.  
Ad-hoc On-Demand Distance Vector (AODV) protocol [20] is a classical on-demand routing protocol originally designed for Mobile Ad-hoc Networks (MANETs) that has been taken as a reference in many works. In AODV, a node floods route-request messages and waits for replay messages in order to update its routing tables. Additional periodic messages are used to detect disconnections.  
Some routing protocols avoid collisions by strongly relying on the Media Access Control MAC layer mechanisms, as it is the case of the Robust Ad-hoc Sensor Routing RaseR protocol [21], which uses Global Time Division Multiple Access (GTDMA). This requires perfect synchronization and a static membership in the set of nodes. In common with our approach, RaseR uses the number of hops as a criteria for routing.  
In a previous work [11], we presented a leader election based protocol to build and maintain a spanning tree. This protocol is a preliminary implementation of the approach we define in Section 3. The preliminary version was reactive, like LBR1, and it included handshaking in communications. In our current protocols, we have removed handshaking. Furthermore, as we have commented before, LBR2 is not reactive.  
In Section 4, we compare LBR1 and LBR2 to AODV and RASeR.