QoS (Quality of Service) collectively denotes technologies and methods for assuring a defined level of service quality in a data network.
Prioritization is a core function of QoS, but QoS is much more than simply giving packets different priorities.
In fact, QoS is about ensuring that different properties of the packet transmission meet pre-defined criteria like packet loss rate, delay, delay variation called jitter and error rate.
A QoS-enabled router in the transmission path must enforce the defined QoS through appropriate queueing strategies like priority queueing, round robin and weighed fair queueing.
Additional algorithms like random early discard improve a router's performance in case of congestion.
Commonly used QoS protocols are DSCP / TOS (DiffServ) for IP layer QoS, MPLS and 802.1p (VLAN) for layer 2 QoS and RSVP for IntServ based scenarios.
Active queue management algorithms (AQM) are employed to avoid congestion in routers along the transmission path.