User-level Performance Evaluation of VoIP under Different Background TCP Traffic Conditions in ns-2

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 applied 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 voice frames into the same IP packet improves the MOSs up to 14.61% over a 1.5 Mbps ADSL link.

Keywords

• Simulation,ns-2,VoIP,TCP,E-model,Mean Opinion Score

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

Abstract

IP üzerinden ses (Voice over IP; VoIP), giderek daha fazla önem kazanmakta ve geleneksel telefonun yerini almaktadır. Örneğin, dünya genelinde aylık 300 milyondan fazla aktif kullanıcı, popüler VoIP uygulaması Skype’ı kullanmaktadır. Ancak, VoIP ortamında ses kalitesi büyük bir sorun teşkil etmektedir. Bu çalışmanın amacı, arka plan TCP trafiğinin kullanıcı düzeyinde bir VoIP konuşmasının algılanan ses kalitesi üzerindeki etkilerini G.711 kodeğini kullanarak incelemektir. VoIP sunucusu tarafından statik tamponlama ve optimal tamponlamayı içeren iki farklı tamponlama politikası uygulanmıştır. Karşılaştırma amacıyla, VoIP sunucusu tarafından herhangi bir tamponlama politikası uygulanmadığında da aynı deneyler tekrarlanmıştır. Kaynak, transit ve hedef alt ağdan oluşan üç sekmeli bir ağ topolojisi simüle edilirken, tüm ağın uçtan uca kapasitesi 1.5 Mbps Asimetrik Dijital Abone Hattı (ADSL) bağlantısı ile sınırlandırılmıştır. Arka plan trafiğinin değişken koşullarını simüle etmek için farklı segment boyutlarında birden fazla eşzamanlı TCP bağlantısı kurulmuştur. Popüler Network Simulator 2'ye (ns-2) eklenen bir geliştirme olan ns2voip framework’ü kullanılarak VoIP kullanıcı seviyesi performansını analiz etmek için kapsamlı simülasyon deneyleri yapılmıştır ve ses kalitesi Ortalama Görüş Puanı (OGP) hesaplanarak değerlendirilmiştir. Sonuçlar, ses kalitesinin 1500 Bayt segmentli tek bir TCP akışında bile arka plan TCP trafiğinden güçlü bir şekilde olumsuz etkilendiğini göstermektedir. Ayrıca, arka plan TCP trafiğinin boyutu, VoIP konuşmalarının ulaşılabilir OGS'lerini önemli ölçüde etkilemektedir. Bununla birlikte, birden fazla konuşma çerçevelerinin tek bir IP paketi içerisine toplanmasının OGS’yi artırabileceği de gözlenmiştir. Özellikle, arka plan TCP akışlarının sayısına ve segment boyutuna bağlı olarak, en uygun sayıdaki ses çerçevelerinin aynı IP paketinde toplanması, OGS'leri 1.5 Mbps ADSL bağlantısı üzerinden %14.61'e kadar iyileştirdiği görülmüştür.

Keywords

• Simülasyon,ns-2,VoIP,TCP,E-model,Ortalama Görüş Puanı

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