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

Yıl 2019, Sayı: 16, 638 - 645, 31.08.2019

Fatih Abut

https://doi.org/10.31590/ejosat.585736

Öz

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.

Anahtar Kelimeler

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

Öz

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.

Anahtar Kelimeler

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

Kaynakça

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