Quality-of-Service (QoS) – the performance, reliability and availability levels of a network connection. To achieve a certain quality, network providers must have visibility into the bit rate, delay, jitter and rate in which packets are dropped in their network. They can then engineer the traffic (traffic shaping) flowing through their network to achieve a certain service quality.
Service level guarantees mean a provider has promised and a user expects a certain QoS for a particular application or service. If the network is strained and threatening the QoS, providers may prioritize some traffic over another or reserve resources for those applications and services that have a guaranteed QoS.