ITU studies have shown that voice quality is very much affected by delay times and echo. For the classical telephone network, end-to-end delay times have been set at 25 ms in the national domain; 100 ms in the international domain. For the echo signal, there are two limits of 30 ms and 150 ms. In this range, the interfering echo can be compensated.
In packet-switched networks such as Internet telephony( VoIP), switching- and network-specific delays generally occur in voice communication. In the audio codec and packetization in the transmitter, the delay can be between 20 ms and 25 ms; for buffering in the network area, delays of up to 700 ms can occur, and for synchronization, depacketization, and decoding, delays of up to 150 ms can occur again.
When transmitting graphical information in real time, the acceptable value is 30 ms. In wide-area networks, the delay time should not exceed 100 ms, and in LANs it should be less than 30 ms. The delay times for voice transmissions are specified by the International Telecommunication Union (ITU) in Recommendation G.114.