Quality of Service (QoS) is the measure of the overall performance of a network from a technical and precedence of service perspective. It should not be confused with quality of experience (QoE), which is the measure of overall performance from the user’s perspective.
QoS is a quantitative measurement of service performance that uses several related aspects of network behavior such as error rate, throughput, transmission delay, latency, bandwidth, jitter, packet loss and availability of service.
QoS is therefore focused on how traffic traverses the network and the many things that can happen to the packets as it travels from source to destination.
- Packet loss – routers may drop packets at congested interfaces
- Low throughput – caused by lack of sufficient bandwidth and network congestion
- Errors – packets can be corrupted due to noise and interference especially on wireless networks
- Latency – delay caused by distance or media type, for example satellite communications
- Jitter – packets from the source reach the destination with different delays
- Out of Sequence – packets arrive at the destination out-of-sequence and need reordered
Latency, jitter and out-of-sequence packets have a serious detrimental effect on real-time voice and video traffic. Good guidelines set Latency below 150ms one-way, and jitter less than 30ms.
In order to improve QoS the best practice is to overprovision the network so that bandwidth and congestion is not a problem. If that is not possible then at a minimum IP priority and precedence will need to be set giving Voice the highest priority of all data in the network. Similarly, QoS traffic policing and queuing techniques will have to be deployed on every device in the network to be effective. Consequently, as QoS requires configured on every interface, it is out with your administrative control over the internet, which renders it effectively redundant outside your own network.