Omar Carvalho Branquinho(*)
Pontiﬁcia Universidade Catolica de Campinas (*)Voluntary Researcher-Computation Institute-UNICAMP P.O. Box 317 13086-900 Campinas - SP - Brazil firstname.lastname@example.org
Pedro Henrique Gomes
University of Campinas - UNICAMP Avenida Albert Einstein, 1251 13084-971 Campinas - SP - Brazilpedro.email@example.com
Rafael Seraphin Seste
University of Campinas - UNICAMP Avenida Albert Einstein, 1251 13084-971 Campinas - SP - Brazil firstname.lastname@example.org
Tatiane Silvia Leite
University of Campinas - UNICAMP Avenida Albert Einstein, 1251 13084-971 Campinas - SP - Brazil email@example.com
Abstract— It is well known that the quality of real-timeapplications, like VoIP, in wireless networks suffers great degradation due to the signal instability in mobile environments. In this paper the quality of VoIP applications on WiFi networks is evaluated using an emulation system with variation of the test environment following the Rayleigh distribution. With this system it is possible to maintain control of the environment and modify it,emulating real situations of WiFi networks in obstructed places. Index Terms— Flat Fading, QoS, Rayleigh, VoIP, Weibull, WiFi.
To test the quality of VoIP applications on 802.11b networks a wireless network emulation system was built in the Wireless Communication Laboratory at PUC-Campinas, in partnership with WCN-Intel Laboratory at UNICAMP. This project included the development of customized softwarethat controls the emulation of ﬂat fading channel with Rayleigh distribution. II. W I F I A. 802.11 standard
I. I NTRODUCTION Currently Voice over Internet Protocol (VoIP) and Wireless Local Area Networks (WLANs) are two technologies of great prominence in the market and in scientiﬁc research. The current status indicates that the convergence of these two technologies will create a newparadigm in the market of telecommunications and also in many Internet applications. VoIP presents many advantages when compared to the traditional Public Switched Telephone Network (PSTN) mainly with regard to its cost, bandwidth efﬁciency and creation of differentiated services that can be offered to the users. The WLANs, in turn, offer the so desired mobility, easy installation and use and also lowcost when compared to many wired network solutions. The low cost of VoIP allied to the mobility and popularity of the WLANs, more speciﬁcally WiFi, became VoIP over WiFi (VoWiFi) a promising application and the target of many studies, mainly regarding the performance of the network and the quality of service (QoS) required for the VoIP. There are three great problems inherent to the WLANs that canharm VoWiFi performance: the inefﬁciency of the 802.11 MAC protocol, the signal instability caused by electromagnetic phenomena and the competition for bandwidth usage between voice trafﬁc and data trafﬁc. The article focuses on the analysis of the second problem: signal instability.
All WiFi equipments implement 802.11 protocol family, speciﬁed by IEEE. The standard 802.11a supports a maximumof 54 Mbps transfer rate, at 5.3 GHz. The standard 802.11b and 802.11g are more used than the previous one and support, respectively, a maximum of 11 Mbps and 54 Mbps transfer rate, both at 2.4 GHz. WiFi networks use the CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance) MAC (Medium Access Control) protocol, where the transmission collision is prevented. The stations access randomly themedium; they do not have any kind of privileges with regard to the service that is being transmitted . Thus, real-time application as VoIP and video streaming suffer as much as Web applications and ﬁle transfer with network performance degradation. This scenario is not fair, because real-time applications do not work properly in networks with package loss, delays and jitter; on the other hand...