Limitations and Future Work

After the summary of this thesis, this section discusses limitations of the current pro-tocol implementation and possible future work.

The first limitation of this thesis is that the protocol is implemented only partially, e.g., the transport component does not implement the implicit ACK mechanism and the discovery does not implement the extended search patterns. Further implemen-tation limiimplemen-tations are covered in Chapter 7. First, this limitations are founded in the complexity of the different protocol aspects, as described in the design space. Second, this is due to the fact that the transport mechanism is not the focus of this thesis. How-ever, a more advanced transport mechanism can improve the overall performance and energy efficiency of PACLD. Therefore, a higher flexibility of the data rate and a more robust behavior at high route lengths are a topic for future work.

A limitation of the caching mechanism, implemented in this protocol, is the low flexi-bility of the caching policy to select an object for removal. This restriction is based in PACLD’s behavior to evict objects of approximately the same size as the new object.

Having only a maximum of 20 objects in the cache and given a high variety of differ-ent object sizes, on average, the caching policy can choose only between two objects to evict. Eviction of multiple small objects in order to free memory for one large object is not considered in the current implementation.

The performance of the protocol, in case of high request frequency and poor distribution of the requested object, is limited by the character of the base station as a bottleneck.

These conditions mainly are expected after deployment of the sensor network when all caches are empty. However, most request models are expected to be handled well by the protocol. Nevertheless, future work may research optimizations of concurrent requests which interfere and, therefore, may result in mutual blocking.

CHAPTER 9. CONCLUSION 97

One possible extension of PACLD can be a cluster-based request mechanism, where only the cluster-head requests an object and, afterwards, uses a broadcast-based dis-semination protocol to cheaply propagate the object to all member nodes of its cluster.

The concept of source caches, storage of information where sources can be found, can by investigated in future work, in order to decrease search costs of the discovery mech-anism while, at the same time, increasing the success probability. Information about the location of objects within the network can be stored for this improved discovery.

Thereby, this information base can be built reactively by storing information of past discoveries. However, this information base can also be built proactively by advertise-ments. Different scopes can be investigated. First, only neighborhood information can be maintained. Second, information about larger parts of the network, up to the com-plete network, can be maintained at each node. Another aspect to research is the effect of a reduced number of nodes which maintain a source cache. Moreover, this concept can be studied with additional system assumptions, like for example, geographic loca-tion informaloca-tion.

Another subject for future work is the concept of inter-cache communication. This thesis already proposed a simple mechanism to prevent cache clustering, i.e., neigh-boring nodes which cache the same data object. However, more sophisticated mecha-nisms can be researched in order to achieve a better distribution of object copies in the network and in order to serve as a tool for the source cache mechanism as presented above. First, different network structures can be investigated as a base for this com-munication, e.g., communication along the edges of trees is well researched in sensor networks. Second, the restriction to implicit communication, where only information is drawn from overhearing the network traffic, can be compared to explicit communi-cation, where a special inter-cache communication protocol is developed. As a result of the cache communication, replication of objects can be initiated, and the caching deci-sion can be decided.

In addition to the topics for future work presented above, the protocol behavior can be studied and optimized under different system conditions. Despite the limitations of the prototype implementation, PACLD provides a mechanism to supply nodes in a sensor network efficiently with a high number of large data objects.

APPENDIX A. TOPOLOGY I

Table A.1: Node coordinates for topology with 100 nodes

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