  Voice over IP (VoIP) is an emerging communication service allowing voice
transmission over a private or a public IP network. VoIP allows significant
benefits for customers and service providers including cost savings, phone and
service portability, mobility, and the integration with other applications.
Nevertheless, the deployment of the VoIP technology encounters many challenges
such as interoperability issues, security issues, and QoS concerns. Among these
disadvantages, QoS issues are considered the most serious due to the QoS
problems that may arise on IP networks, and the stringent QoS requirements of
voice traffic. The aim of this paper is carry out a deep analysis of the QoS
concerns of the VoIP technology. Firstly, we present a brief overview about the
VoIP technology. Then, we discuss the QoS issues related to the use of the IP
networking technology for voice traffic transmission. After that, we present
the QoS concerns related voice clarity. Finally, we present the QoS mechanisms
proposed to make the IP technology able to support voice traffic QoS
requirements in terms of voice clarity, voice packet delay, packet delay
variation, and packet loss.
