Nowadays it is quite evident that a technological revolution was happening in the last few years, spreading over many areas of daily life, which are currently supported by digital technologies. In the telecommunications world, where phone calls play a primary role, migration from the analogue Public Switched Telephone Network (PSTN) to the Integrated Services Digital Network (ISDN) has been witnessed, though both of them were still based on the circuit switching paradigm. Since the emergence of the packet-switching technology was one the most important factors responsible for the enormous success of the current digital telecommunications services, the widespread use of the Internet Protocol (IP) has reached traditional telephony communications in a global scale. As a consequence, Voice over IP (VoIP) is rapidly taking over legacy technologies and the existing IP infrastructure soon appeared suitable for transporting telephony voice signals because of its low cost associated with the flexibility and diversity of other possible services. Call centre integration, directory services over telephones, IP video conferencing, Fax over IP (FoIP) and Radio/TV broadcasting are among an ever increasing number of services and applications (Al-Akhras & Momani, 2011).
According to statistics from the Organization for Economic Co-operation and Development (OECD), the use of VoIP services has been steadily increasing (Borges, 2009). For example, the French Autorité de Régulation des Communications Électroniques et des Postes (ARCEP)1 unveils that, from the third quarter of 2006 to the corresponding quarter of 2010, the use of the Public Switched Telephone Network (PSTN) for VoIP services rose from 5% to 37% (Postes, 2010). According to (Related, 2008), it is expected that VoIP will replace conventional telephony in a couple of decades and there are market analysis foreseeing that the number of VoIP consumers is increasing from 70 million in the second quarter of 2008 to nearly 200 million by 2012 (Point Topic, 2009). Perspectives outlined by (www.3G.co.uk, 2006) state that the emerging VoIP traffic over the cellular network will represent 23% of the total voice traffic time by 2015 in occidental Europe. There is also a current evolution of voice services towards VoIP over WiFi (VoWiFi) which has recently emerged as promising technology (Mondal, Huang, J. Li, & Kuzmanovic, 2010).