User guide

4.7 Quality of Service

Quality of Service (QoS) is the ability of a network to provide improved

(better) service to selected network traffic. One of the main challenges of

implementing VoIP, especially in developing regions, is to ensure that

bandwidth for telephone calls will always be available no matter how busy the

Internet connection is.

When designing a VoIP network you should try to optimized the available bandwidth, control jitter, and

minimize latency.

4.7.1 Latency

Latency is a synonym for delay, and measures how much time it takes for a

packet of data to get from one designated point to another.

To improve the quality of your VoIP conversations, try to minimize the “delay” by giving priority to the

voice traffic. Giving priority to the voice traffic means that you allow the voice packets to jump to a

better position in the queue of packages to be transmitted.

If the communication requires the use of a satellite, a latency of ~300 ms is unavoidable. In order to

minimize the delay, you need to pay special attention to your routers and switches along the route.

Conversations are still feasible if the connection requires more than one satellite hop, but be ready to

wait at least one second without talking for the other party to answer. A rule of the thumb for minimizing

latency is to place your PBX in the least congested or saturated part of the network.

4.7.2 Jitter

In VoIP, jitter is the variation in the time between packets arriving caused by

network congestion, timing drift, or route changes.

A jitter buffer can be used to handle jitter and decrease its negative effects. The basic idea of a jitter

buffer is to improve the quality of the phone call by deliberately delaying the playback of the

conversation, thus ensuring that the “lazy” packets have arrived.

24. Special attention needs to be taken if the VoIP connections run in a wireless network such as the ones based on IEEE

802.11b/g/a, to ensure traffic prioritization for VoIP.

Page 18 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008