Ns-2'de Farklı Arka Plan TCP Trafik Koşulları Altında VoIP'nin Kullanıcı Düzeyinde Performans Değerlendirmesi

Abstract

Voice over IP (VoIP) is gaining more and more importance and displaces the traditional telephony. For example, more than 300 million monthly active users worldwide use the popular VoIP application "Skype". However, a big problem in the VoIP environment is the voice quality. The purpose of this study is to investigate the effects of background TCP traffic on perceived voice quality of a VoIP conversation at the user-level using the G.711 codec. Two different playout buffering policies including static buffering and optimal buffering have been applied by the VoIP server. For comparison purposes, the same experiments have also been repeated when no playout buffering policy has been used by the VoIP server. A three-hop network topology consisting of a source, a transit, and a destination subnetwork was simulated whereas the end-to-end capacity of the entire network was limited by a 1.5 Mbps Asymmetric Digital Subscriber Line (ADSL) link. Multiple simultaneous TCP connections with different segment sizes were established to simulate the various conditions of background traffic. By using the ns2voip framework, an enhancement to the popular Network Simulator 2 (ns-2), extensive simulation experiments for analyzing the VoIP user-level performance have been carried out and the voice quality has been evaluated by calculating the Mean Opinion Score (MOS). The results show that the voice quality is strongly negatively affected by background TCP traffic, even in the presence of a single TCP flow with 1500 Byte segments. Also, the size of background TCP segments significantly influences the achievable MOSs of VoIP conversations. However, it has also been observed that aggregating multiple speech frames into a single IP packet can increase the MOS. Particularly, depending on the number and segment size of background TCP flows, aggregation of the optimal number of speech frames into the same IP packet improves the MOSs up to 14.61% over a 1.5 Mbps ADSL link.