Modelling PTT Packet Delay in the GPRS/GSM Uplink

Push-to-Talk (PTT) is a new mobile service which enables fast-access two-way communication between two or more users. This paper provides an approximate evaluation model for PTT delay performance using the General Packet Radio Service (GPRS) over Global System for Mobile Communications (GSM) networks and taking account of GSM and GPRS traffic loading. Cases with and without provision of strict priority for PTT traffic over GPRS data are considered. Our PTT/GPRS/GSM model is based on the following three modelling assumptions: (1) a quasi-stationary assumption on the behaviour of GPRS under GSM voice loading, (2) a M/G/1 processor sharing (PS) queueing model to analyse PTT packet queueing delay performance, and (3) independence between retransmission traffic and original traffic. The model is validated by simulation. We present numerical and simulation mean delay results to illustrate the effect of traffic load and various design parameters on PTT packet delay.