QoS stands for quality of service. Reserving bandwidth for certain transmissions on a network and guaranteeing quality of transmission speed makes it possible to prevent the occurrence of communication delays and stoppages. QoS guarantee refers to control mechanisms that guarantee priority transmissions/receivals on network-connected devices, priority relays on relay devices like routers, and so on at various locations in networks. It is said QoS guarantee is necessary for communications under a strict delay requirement (such as telephony and video streaming delivery via the internet).
