A model for GPRS/EDGE network engineering is developed in order to obtain an Erlang-like law. It is based on a discrete-time Markov chain. It captures the main features of the GPRS/EDGE radio resource allocation and assumes an ON/OFF traffic (infinite sessions with geometric distributions of ON/OFF sizes) performed by a finite number of users over the cell. The Markov chain is simplified by Taylor series expansion and a simple and accurate Erlang-like law is obtained.