**9. Conclusions**

112 Wireless Sensor Networks – Technology and Protocols

Another vital aspect in WSN is mutual location of the nodes. Only BS can know its precise location among the other nodes (hence only BS is able to plot its migration vector). Not all WS node have capability to precisely define their position in space. Only a few of nodes have optional (built-in or add-in) GPS device, such nodes with GPS are exceptionally useful for remaining nodes without this capability, they can act as beacons – providing other nodes with their position at the same time enabling to set position on their own. Having three beacons in communicational range gives a node an opportunity to precisely set its position. After this operation such a node becomes an anchor which unfortunately cannot be used for any further nodes location designation. Node's (anchor) location could be determined either by Ad hoc On-Demand Distance Vector (AODV) Routing developed in Nokia Research Center, University of California, Santa Barbara and University of Cincinnati [17] or Angle of Arrival using Received Signal Strength [20, 21]. Node's position can be also relatively precisely determined based on loss of transmitted signal strength. When there are just two beacons and one anchor nodes location designation is still possible, but less precise than before. The least accurate location designation applies to a situation where there are just three anchors in node's proximity. In each case where anchor is used to location designation,

A constellation term, known from astronomy as a group of stars involved in a specific area of the celestial sphere that is shown in the relative position on the Earth night sky, could be also applied in WSN field, per analogy celestial bodies (nodes), sky (WSN), Earth (BS). Therefore constellation in WSN can also show only relative position, towards selected, known points. The advantage of constellation is that one need to know neither precise distance between nodes nor having any single beacon in a communicational proximity. One needs only select a reference point and then can calculate distance to it. Main disadvantage of this method is that it accumulates errors, the greater the determined distance the bigger inaccuracy. However these cumulative errors can be partially mitigated by introducing at

Another interesting, from energy saving standpoint, issue is how important (if at all) is sending information about zone number change (a hop zone, where a node is currently located). Normally, in a conventional WSN, when position of BS changes, zones and their numbering have to be designated again. Question is, whether we can avoid energetic cost on broadcasting info about all new zones? Let's concentrate on a situation, where we are focusing on a node located 15 hops from BS, does it make any sense to loose energy on broadcasting information that we are currently on 16th hop from BS, or maybe previous state of knowledge is good enough, in fact nothing important hasn't changed since the node still knows where to relay messages (set of nodes from preceding hop is still known). If this approach is taken *a priori* as reasonable solution, further investigation on where information about new zone is vital and where may be omitted. Shall a very

it is being recognized as a classifier rather than a positioning element.

**8.3. Flooding (new hop zones determination)** 

**8.2. Location of nodes in WSN** 

least one beacon.

Two methods of WSN energy preservation (theoretically and simulation proven) are directed and spatial communication. Both in many cases may save energy expense of a certain nodes and ultimately the whole network, however infrequently these contribute to WSN lifespan prolongation. Introducing a migrated BS into a static WSN environment may bring further energy savings for whole network and assure that network lifespan will be much longer that in a static one. There is no need to underline how important WSN lifespan issue is; there is a common knowledge the longer the weakest WSN link (a node with the least energy left) is active the longer WSN lifespan is. Usually having this knowledge in mind, algorithms made attempt to distribute messages load (packets sent from source to a sink) evenly across neighboring nodes to prevent premature death of this node. Nonetheless it could remediate such situations, preventing BS isolation outside (the communication range) of other nodes, but in certain conditions (e.g. where BS was located in a corner of WSN area with only a few neighbors – Fig. 2) were ineffective. Such a case could be improved having a mechanism allowing migration of base station(s). BS migration could be fixed (based on defined criteria) or adaptive. Both are having their advantages and drawbacks. However their disadvantages may be neglected if the only criterion is WSN prolongation.

When the base station migrates in a WSN, every time it musts ensure that it does not lost contact with network nodes. Also, in order to ensure the best survival of relay nodes (which actively support it work), it should stay as close as possible to the most active regions of the network. Constantly changing its location the base station also changes its neighbors. This implies that the base station (BS) should match its velocity with currently neighboring nodes to keep abreast. Any collective behavior is solely based on observable phenomena within neighborhood. In the tested issue nodes within BS neighborhood helps it to calculate the velocity vector (14). However, change the location of BS must occur in accordance with the relay activity of the WSN nodes. The integration of these behaviors results in a stable BS location, where most active regions of the network are at least some minimum distance from BS. In terms of energy savings this is an optimal solution, however, because of the intense burdening neighborhood nodes is advantageous to periodically change the BS location.

During our experiments in both simulated and first real life environments we found some intriguing and at the same time interesting issues that for now are worth to be mentioned (irregular and not quite circularly shaped hop zones, BS location determination in WSN area, new hop zones determination after BS movement) and in a near future will a the subject of our research.
