VoIP Quality Assessment Technologies
Mousa Al-Akhras, Iman Al · InTech eBooks · 2011
VoIP Technologies 1999).VoIP can be used in many applications, including: call centre integration, directory services over telephones, IP video conferencing, fax over IP, and Radio/ TV Broadcasting (Collins, 2003;Miloslavski et al., 2001;Ortiz, 2004; Schulzrinne & Rosenberg, 1999).VoIP technology was adopted by many operators as an alternative to circuit switching technology.This adoption was motivated by the above advantages and to share some of the high revenue achieved by telecommunication companies.However, to be able to compete with the highly reputable PSTN networks, VoIP networks should be able to achieve comparable quality to that achieved by PSTN networks.Although VoIP services often offer much cheaper solutions than what PSTN does, but regardless of how low the cost of the service is, it is the user perception of the quality what matters.If the quality of the voice is poor, the user of the traditional telephony will not be attracted to the VoIP service regardless of how cheap the service is.This comes from the fact that customers who are used to the high-quality telephony networks, expect to receive a comparable quality from any potential competitor.IP networks were originally designed to carry non real-time traffic such as email or file transfer and they are doing this task very well, however, as IP networks are characterised by being best-effort networks with no guarantee of delivery as no circuit is established between the sender and the receiver, therefore they are not particularly appropriate to support real-time applications such as voice traffic in addition to data traffic.The best-effort nature of IP networks causes several degradations to the speech signal before it reaches its destination.These degradations arise because of the time-varying characteristics (e.g.packet loss, delay, delay variation (jitter), sharing of resources) of IP networks.These characteristics which are normal to data traffic, cause serious deterioration to the real-time traffic and prevent IP networks from providing the high quality speech often provided by traditional PSTN networks for voice services.Sharing of resources in IP networks causes no resources to be dedicated to the voice call in contrast to what is happening in traditional circuit switching telephony such as PSTN where the required resources are allocated to the phone call from the start to the end.With the absence of resource dedication, many problems are inevitable in IP networks.Among the problems is packet loss which occurs due to the overflow in intermediate routers or due to the long time taken by packets to reach their destinations (Collins, 2003).Real-time applications are also sensitive to delay since they require voice packets to arrive at the receiving end within a certain upper bound to allow interactivity of the voice call (ITU-T, 2003a;b).Also, due to their best-effort nature, packets could take different routes from the same source to the same destination within the same session which causes packets interarrival time to vary, a phenomenon known as jitter.Due to the problem of jitter, it is not easy to play packets in a steady fashion to the listener (Narbutt & Murphy, 2004;Tseng & Lin, 2003;Tseng et al., 2004).The above challenges cause degradation to the quality of the received speech signal before it reaches its destination.Many solutions have been proposed to alleviate these problems and the quality of the received speech signal as perceived by the end user is greatly affected by the effectiveness of these solutions.Another approach is to reserve resources across the path from the sender to the receiver.A mechanism called Call Admission Control (CAC) is needed to determine whether to accept a call request if it is possible to allocate the required bandwidth and maintain the given QoS target for all existing calls, or otherwise to reject the call (Mase, 2004).Among the solutions that have been proposed to implement CAC and to manage the available bandwidth efficiently are: Resource Reservation Protocol (RSVP), Differentiated Service (DiffServ), How to referenceIn order to correctly reference this scholarly work, feel free to copy and paste the following: Mousa Al-Akhras and Iman Al Momani (2011).