A DTN mode for reliable internet telephony

IP telephony suffers from the well-known fact that the Internet does only provide a best-effort service. Thus, minimum transmission quality cannot be guaranteed and, especially during times of network congestion or occasional link failures, UDP-based VoIP becomes unusable. To overcome these limitations, we have developed a rate-adaptive transmission system for highly scalable speech and audio codecs that uses the Delay Tolerant Networking (DTN) approach for very low bit rates. In this way, as soon as the available transmission capacity falls below the minimum coding rate, our system switches to a Push-To-Talk (PTT) like conversational mode. Subjective conversational quality tests have shown that the algorithm allows for a good and effective conversation (MOS-CQS is 3.5) even in cases where traditional UDP based VoIP telephony fails (MOS-CQS is 1).