This paper presents a two-tier coordinated checkpointing algorithm which can reduce the number of messages by being composed of two levels in mobile computing. Thus mobile devices have a high mobility and are lack of resources (e.g., storage, bandwidth, and battery power), traditional distributed algorithms like coordinated checkpointing algorithms could not be applied properly in mobile environment. In our proposed two-tier coordinated checkpointing algorithm, the messages to be transferred are requested by the mobile hosts and are handled by the appropriate MSS's (Mobile Support Stations). And the broadcast messages are handled by MSS instead of relaying the messages to all the mobile hosts directly as with the previous algorithms. This can reduce the communication cost and maintain the overall system consistency. In wireless cellular network, mobile computing based on a two-tier coordinated checkpointing algorithm reduces the number of synchronization messages. We perform performance comparisons by parametric analysis to show that a two-tier coordinated checkpointing algorithm can reduce communication cost compared to the previous algorithms in which the messages are directly sent to the mobile hosts.

The default server strategy is commonly used to manage the location and state of mobile hosts in cellular networks. With this strategy, connections can be established after the client obtains the location information of the mobile host by querying the default server. Unfortunately, the communication cost increases if the query requests are frequent and the distance between the default server and the client is long. Still more, no connection to a mobile host can be established when the default server of the destination mobile host fails. These problems can be solved by replicating the default servers and by letting the nearest replicated default server process the query request which is sent from a client [9]. It is important to allocate replicated default servers efficiently in networks and determine the number of replicated default servers. In this paper, we suggest and evaluate a default server replication strategy to reduce communication costs and to improve service availabilities. We consider optimal replication degree as well as location for replicating the default servers in n-grid and tree networks.