What is QoS (Quality of Service)?

QoS (Quality of Service) is a set of techniques for managing network resources to prioritize certain types of traffic over others, ensuring critical applications receive the bandwidth and low latency they need. Without QoS, all traffic is treated equally (best-effort) — a large file download could degrade VoIP call quality. QoS components: Classification and Marking — identify traffic type (IP Precedence, DSCP values in the IP header). Queuing — prioritize packets (FIFO, Weighted Fair Queuing, Low Latency Queuing). Traffic Shaping/Policing — control bandwidth usage. Congestion Avoidance (WRED — Weighted Random Early Detection). DSCP (Differentiated Services Code Point) marks packets with a 6-bit value (0-63) in the IP header: EF (Expedited Forwarding, 46) for VoIP, AF for business data, CS0 for best-effort. QoS is critical for unified communications, video conferencing, and cloud applications over shared WAN links.