Abstract: A cellular phone includes a random number generator. When the cellular phone performs location registration operation, a time of executing location registration is determined by random numbers. Therefore, in a situation where a large number of cellular phones must perform location registration substantially simultaneously, an event where a large number of the location registration operations overlap with one another can be avoided. Hence, simultaneous concentration of loads on a network is eliminated, and stable communications between the cellular phones and an administrative server are maintained.

Abstract: A cellular phone includes a random number generator. When the cellular phone performs location registration operation, a time of executing location registration is determined by random numbers. Therefore, in a situation where a large number of cellular phones must perform location registration substantially simultaneously, an event where a large number of the location registration operations overlap with one another can be avoided. Hence, simultaneous concentration of loads on a network is eliminated, and stable communications between the cellular phones and an administrative server are maintained.

Abstract: The mobile phone can connect to a preferred connected communication system A and a non-preferred communication system B. When the mobile phone connects to system A, if the detected received electric field intensity is less than a first threshold, the mobile phone connects to system B. When the mobile phone connects to system B, if the detected received electric field intensity is larger than a second threshold, the mobile phone connects to system A. The second threshold is set as larger than the first threshold, therefore, the frequency of switching between the communication systems is reduced.