In this paper, we focus on a simple yet important objective: to provide Erlang-like models for GPRS/EDGE radio engineering. To this end, we develop a constructive model based on an original discrete-time Markov process that captures the detailed behavior of the radio allocator when a finite number of users are performing ON/OFF sessions in the cell. After a preliminary focus on infinite-length sessions, which provide a simpler analytical framework, we include finite-length sessions and study their specific effects.