Performance of VoIP over GPRS

We study the performance of VoIP over GPRS by simulation of standard GPRS system with some improvement for real time attributes. It is showed that the VoIP over GPRS has more capacity than circuit switch voice while guaranteeing acceptable QoS. With one radio frequency, the circuit switch system can provide 7 voice connections, while VoIP over GPRS can support 12 voice connections if they use the same 13Kbps speech coder. If VoIP over GPRS uses 5.3Kbps speech coder, it can support 34 voice connections. The uplink voice packet loss comes from the contention for the random access channels and shortage of idle data traffic channels. Because of its random contention characteristics, the access channel effective usage is low. We apply a modified PRMA to increase the access channel utilization, and the maximum voice connections can be increased by 17% compared to the reservation slotted ALOHA.