## 5 Analysis of Prominent Clustering Routing Protocols in WSNs
### Data-Transmission Based Clustering Routing Protocols
#### 5.2.4 TTDD  
The Two-Tier Data Dissemination (TTDD) approach, presented by [PERSON] _et al._[29], is a low-power protocol for efficient data delivery from multiple sources to multiple mobile sinks. It exploits a geographic routing based on grid of cells as the routing method. Instead of passively waiting for queries from sinks, sensor nodes can proactively establish a structure to set up forwarding information. Ultimately, the sensing field is figured as a set of grid points.  
In TTDD, a source divides the field into a grid of cells and each cell is square. A source, at one crossing point of the grid, propagates data announcements to reach all the other crossings, called dissemination points, on the grid as shown in Figure 10. A source calculates the locations of its four neighboring dissemination points and sends a data announcement message to the four neighboring dissemination points using simple greedy geographical forwarding, _i.e._, it forwards the message to the neighbor node that has the smallest distance to the neighboring dissemination point. Similarly, the neighbor node continues forwarding the data announcement message till the message stops at a node that is closer to the dissemination point than all its neighbors. During this process, each intermediate node stores the source information and this process continues until the message stops at the border of the network. After this process, the grid structure is obtained.  
The sink can flood a query within a local area to discover nearby dissemination nodes. Once the query reaches a local dissemination node, it is forwarded on the grid to the upstream dissemination node from which this intermediate node receives data announcements. The query is forwarded by the upstream toward the source, until finally arrives at the source. During the above process, each dissemination node stores the location of the downstream dissemination node, thus this information is used to direct data back to the sink.  
When a sink moves in the network, trajectory forwarding is employed to relay data to the mobile sink from its immediate dissemination node. In trajectory forwarding, each sink is associated with two sensor nodes: a primary agent and an immediate agent. A sink picks a neighboring sensor node as its primary agent which receives data directly from the immediate dissemination node, and subsequently relays data to the sink. Initially, the primary agent and the immediate agent are the same sensor node. When a sink is about to move out of the range of its immediate agent, it selects another neighboring node as its new immediate agent and sends the information of the new immediate agent to its primary agent, thus future data are forwarded to the new immediate agent.  
The character and advantage of TTDD can be concluded as follows: (1) It deals with the problems caused by multiple mobile sinks and sink moving in large-scale WSNs; (2) Despite that it is effective in high mobility scenarios, the overhead to build and maintain the overlay is significant, especially in periodic reporting scenarios, which are more traffic intensive than event-based reporting. Therefore, TTDD is better suited to event-detecting WSNs with sporadic rather than continuous traffic [75].  
There are some disadvantages in TTDD as follows: (1) The routing of a forwarding path in TTDD is not the shortest path, thus it may lead to large latency for the long path; (2) The grid structure formation and query flooding cost large energy consumption; (3) TTDD requires that sensor nodes are stationary and location-aware and assumes the availability of an accurate positioning system that may not yet available in a real WSN. If mobile sensor nodes are allowed to move in the network, how would TTDD perform is still an open question.  
Figure 10: The Grid-based Topology in TTDD.