Processing delays, known as latency, can occur due to the encoding / decoding process and the time it takes to transmit large blocks of data over the network. Excessive latency increases the chances of people talking over each other and increases the likelihood that video may not stay in synch with the audio. While higher latency is typically most noticeable from an audio perspective, poor audio can severely reduce the overall quality of the entire video conferencing experience.
Latency affects the quality of an interactive voice or video conferencing call by slowing down the response from the other party. After latency reaches 200 ms the delay effect is noticeable, and parties have trouble interrupting each other, and maintaining the flow of a normal conversation. The 200 ms value represents the delay from the speaker to the listener, including all the delays of the encoding, transmission and decoding of the signal. The network is only involved with the transmission portion of this delay. Network latency should be kept below 100ms to insure that speaker-to-listener latency stays below 200ms.
Latency is affected by congestion and by geographic distance. Congestion can be managed through bandwidth management and QoS, but geographic distance and the speed of light are harder to manage. Using a satellite incurs a long delay because of the distance signals have to travel up and back to a geostationary satellite. In some global routing cases it is possible to get better routes. The path traffic takes from Asia to Europe, for instance, often flows through the United States. Finding carriers that will route this traffic by a more direct geographic route can lower the latency impact. Enterprises not using satellites and working within a single continent will not experience latency problems due to distance.
Latency is the time required for a packet to traverse a network from source to destination.
Components of latency include :-
Propagation delay : the time it takes to traverse the distance of the transmission line; controlled by the speed of light in the media; rule-of-thumb : 20ms San Francisco to New York.
Transmission delay: the time it takes for the source to put a packet on the network. Rule-of-thumb for general network devices: < 1ms.
For H.323 this includes time to encode/decode the video
Store-and-forward delay: the cumulative length of time it takes the internetworking devices along the path to receive, process, and resend the packets. Rule-of-thumb: variable, and depends upon network load.
Rule of thumb:
A one-way delay of:
Satellite delay in the “unacceptable” range, but you will find that you get used to the delay
Related Topics - Delay - Jittering - Nating - LPR -