In the analysis of a packetized communication system such as a slotted ALOHA, one needs to deal with discrete-time point processes and related queues. We examine how Poisson processes, M/M/1 queue, M/G/infinity, etc., should be translated in their discrete-time versions. We then give a new definition of processor-sharing and its properties. Correlation functions of input and output processes in an M/G/infinity are also studied. (Author)