Abstract. In the present scenario, a demand for the highly reliable and synchronous systems is seen. As a result, there has been a gradual shift to distributed systems from the centralized systems. There are few disadvantages for this system too. The most important one is that in a distributed system, the different nodes maintain their own time using local clocks and their time values may not be same for the different nodes. I.e. there is no global clock within the system so that that the various activities in the distributed environment can be synchronized. The various clocks in the system even if set to a common time value at an instant, drift apart due to unavoidable reasons. Hence some kind of continuous mechanism for synchronization is needed so that they can coordinate and work together to achieve the objectives of the distributed system. Two types of synchronization are possible-external synchronization and internal synchronization. In a real time scenario, it is important for the system to be synchronous with each other and with a common external reference time. This is called external synchronization. But in certain systems, it is only necessary for the nodes in the system to be synchronized with each other. This is called internal synchronization. In many applications, the relative ordering of events is more important than actual physical time. Here event ordering is done without clock time values. Hence, depending on the area and type of application, clock synchronization techniques used differs.In certain real time applications, the system requires to be both internally and externally synchronized. In such cases a centralized algorithm called the 'Cristian's' algorithm is used for synchronization. But this algorithm fails in situations where the time server fails. This paper suggests some methods to make the synchronization process distributed so that the disadvantages of the Cristian's algorithm can be nullified.

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This paper presents an effective algorithm for selecting cluster heads in mobile ad hoc networks using ant colony optimization. A cluster in an ad hoc network consists of a cluster head and cluster members which are at one hop away from the cluster head. The cluster head allocates the resources to its cluster members. Clustering in MANET is done to reduce the communication overhead and thereby increase the network performance. A MANET can have many clusters in it. This paper presents an algorithm which is a combination of the four main clustering schemes- the ID based clustering, connectivity based, probability based and the weighted approach. An Ant colony optimization based approach is used to minimize the number of clusters in MANET. This can also be considered as a minimum dominating set problem in graph theory. The algorithm considers various parameters like the number of nodes, the transmission range etc. Experimental results show that the proposed algorithm is an effective methodology for finding out the minimum number of cluster heads.

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