We consider a packet-switching network as a queueing system with constant service times: that means that the lengths of the packets are equal. We study such a network by isolating a particular path which we treat as tandem queueing system. Two aspects will be examined: the study of the response time of such a system, and the throughput of such a system when time-outs are used to destroy packets. The results we obtain show that when the interference between paths is neglected the response time is independent of the order of the stations. The control policy we examine proves the necessity to have a limitated memory capacity at each node of a packet-switching network. These results confirm the intuitive options taken in a network like CYCLADES.