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Speech Quality While Roaming in Next Generation Networks

In NGNs, handovers between different wireless access technologies provide seamless roaming during voice calls. The resulting speech quality depends on the audio bandwidth of the speech codecs used in the respective networks, as well as on degradations resulting from the handover, coding, and packet loss. We present the results of four listening experiments where speech quality is quantified as a function of network and codec characteristics, and compare them to estimations obtained from instrumental models. The results show when and under which circumstances a network handover and/or codec changeover should be scheduled in order to obtain better speech quality. This is important for the development of high-quality roaming strategies.

A Technique for Seamless VoIP-Codec Switching in Next Generation Networks

Vertical handovers in Next Generation Networks enable a new experience of mobility, since application layer sessions are maintained while roaming across different access networks. For real-time media services like Voice-over-IP, a change in the underlying network technology is particulary challenging. Ongoing calls are not suspended during the handover, however, the handover may go along with an audible gap during the transition time due to lost or delayed packets and an adaptation in call parameters such as those of the employed speech codec. This, in turn, may translate in an unfamiliar speech quality perception. This contribution introduces an ergonomic technique that aims at seamlessly switching the speech codec in Voice-over-IP calls during vertical handovers, based on SIP/SDP session renegotiation, the establishment of a parallel media stream and RTP packet filtering. Evaluation results are presented, showing that the proposed approach does not cause any interruption of the audio stream in about 90% of the test cases, clearly outperforming simple re-negotiation of session parameters that does not take a seamless transition into account (interruptions in all test cases). PESQ speech quality estimates reveal a quality advantage of 5.4% on average for the considered scenario.

A method for seamless codec changeover during active video calls

Seamless access to video services in Next Generation Mobile Networks (NGMNs) introduces new challenges in the video quality provisioning. Terminal mobility and network handover affect the Quality of Service (QoS) and the perception of the mobile users. To improve the Quality of Experience (QoE) in NGMNs, cross-layer adaptation of the video streaming is required. A technique to replace the applied video codec during ongoing real-time video transmissions, such as video call, is described in this paper. The presented solution enables to seamlessly switch between the video codecs independently from the applied coding schemes, allows to maintain ongoing sessions, and helps to reduce the changeover-related artifacts to a large degree. The proposed algorithm has been implemented and evaluated in a NGMN testbed. The technique enabled to significantly improve user perception during wireless transmission affected by packet loss, and performed best as compared to network handover and video bit rate reduction that are considered as alternative quality adaptation measures. This proves the relevance of the presented codec changeover solution for video quality provisioning in mobile networks.