This paper proposes an M/G/1 queueing model in which the server is replaced by another one with different service time distribution when the number of customers in the system exceeds a threshold value. A random set‐up time is needed to introduce the second server. This server works until the system becomes empty. This kind of system will appear, for instance, when the server is changed to a faster one as the number of waiting customers becomes too great. In data communication systems, the set‐up time may correspond to the time until the new carrier frequency for the faster transmission speed becomes stable, the time needed to assign a new link and/or the time necessary to inform a partner of a change in line speed. The behavior of the customers who receive the first and second kinds of service is analyzed using the theory of finite Markov chain and the decomposition property of the M/G/1 system with server vacations, respectively. Important performance measures, such as the distribution and average of the number of customers in the system and their average time in the system, are obtained. The influence of server characteristics, the conditions under which the server is replaced, and the distribution of set‐up time on the performance of the system are clarified.

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This paper describes a new network architecture for personal communication services that provides transparent connectivity to different physical networks. We call this system DUET (Distributed User‐agent for Easy Telecommuication). In this system both user information and customized service logic are managed independently by a distributed personal agent (PA) that is identified by the personal ID. The location information of users and terminals is managed by distributed area agents (AAs). Each PA interacts with other PAs and AAs to support flexible person‐to‐person communications in a heterogeneous network environment including mobile terminals. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt. 1, 80(3): 1–11, 1997