Tham khảo tài liệu 'satellite communicationsever increasing widespread part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 166 Satellite Communications ever increasing widespread use of IP technologies will greatly contribute to enhancing mobility and nomadicity between private and corporate access. The data to be transmitted are characterized mainly in terms of bit rate error rates Bit Error Rate - BER or Packet Error Rate - PER delay jitter average and maximum packet sizes. The salient Quality of Service QoS features of the two types of applications which are envisaged are presented hereafter - Real-time delay-sensitive but not or very little loss-sensitive applications especially VoIP. A strictly minimum bit rate of kbps assuming ITU ACELP codec is required Nguyen et al. 2001 . It has been shown however that optimal bandwidth occupation for VoIP over satellite is around 12 kbps. VoIP bit rate also varies depending on the codec on whether RTP Real-Time Protocol is compressed and on the overhead introduced by the header of the protocol suite Ethernet IP UDP RTP . For instance with RTP a total of 54 bytes are transmitted out of which only 20 belong to the payload. The bit rate can thus be considered to range between and 13 kbps. A minimum MOS Mean Opinion Score requirement of as suggested in WISECOM WISECOM 2007 ensures a good voice quality. In this respect ITU-T Recommendation IUT-T 2000 specifies a maximum latency value of 150 ms for one-way VoIP communications. As for the jitter the mean packet inter-arrival time at the receive side must be roughly close to that at the transmit side with a small standard deviation. Lastly in terms of packet corruption and loss some experiments have shown that the satellite link is quite robust to packet corruption in clear sky or moderately degraded channel environment with a BER of up to a 10 5 that is Frame Erasure Rate or FER of 2 Nguyen et al. 2001 . The performance of VoIP over satellite as obtained by Nguyen et al. Nguyen et al. 2001 from the COMSAT laboratories will be used as a reference test bench. - .
