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

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 ve optimal olmak üzere iki farklı tamponlama politikası uygulanmıştır. Karşılaştırma amacıyla, VoIP sunucusu tarafından herhangi bir tamponlama politikası uygulanmadığında da aynı deneyler tekrarlanmıştır. Kaynak, transit ve hedef alt ağlarından 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 (ns-2) programına 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.

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

Voice over IP (VoIP) is gaining more and more importance and displaces the traditional telephony. For example, more than 300 millionmonthly active users worldwide use the popular VoIP application "Skype". However, a big problem in the VoIP environment is the voicequality. The purpose of this study is to investigate the effects of background TCP traffic on perceived voice quality of a VoIP conversationat the user-level using the G.711 codec. Two different playout buffering policies including static buffering and optimal buffering havebeen applied by the VoIP server. For comparison purposes, the same experiments have also been repeated when no playout bufferingpolicy has been used by the VoIP server. A three-hop network topology consisting of a source, a transit, and a destination subnetworkwas 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 conditionsof background traffic. By using the ns2voip framework, an enhancement to the popular Network Simulator 2 (ns-2), extensive simulationexperiments for analyzing the VoIP user-level performance have been carried out and the voice quality has been evaluated by calculatingthe Mean Opinion Score (MOS). The results show that the voice quality is strongly negatively affected by background TCP traffic, evenin the presence of a single TCP flow with 1500 Byte segments. Also, the size of background TCP segments significantly influences theachievable MOSs of VoIP conversations. However, it has also been observed that aggregating multiple speech frames into a single IPpacket can increase the MOS. Particularly, depending on the number and segment size of background TCP flows, aggregation of theoptimal number of speech frames into the same IP packet improves the MOSs up to 14.61% over a 1.5 Mbps ADSL link.

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