QoS (Quality of Service) is a collective term for technologies that assign priority levels to specific network traffic to guarantee a certain level of communication quality. In telephone communications, it is used to process voice packets with higher priority than web browsing or file downloads. Without proper QoS configuration, VoIP calls on a network will suffer from audio dropouts, delay, and echo.
QoS is implemented through three main techniques. First, packet prioritization (marking): a 6-bit DSCP (Differentiated Services Code Point) field assigns high priority (EF: Expedited Forwarding, value 46) to voice packets, which routers and switches read to forward them preferentially. Second, bandwidth control: reserving a fixed amount of bandwidth for voice traffic so that it is not encroached upon even when other traffic increases. Third, queuing control: creating a dedicated queue for voice packets within routers and processing them before other packets, with LLQ (Low Latency Queuing) being the common approach.
Target QoS values for VoIP calls are latency (one-way delay) under 150 ms, jitter (delay variation) under 30 ms, and packet loss rate under 1%. Exceeding these thresholds causes perceptible quality degradation. Packet loss in particular directly causes audio dropouts, making it the most critical metric. Required bandwidth per call depends on the codec: approximately 87 kbps for G.711 and 32 kbps for G.729.
In enterprise networks, the standard design assigns traffic classes to voice, video, and data, with bandwidth and priority configured per class. When deploying cloud PBX, QoS must be considered not only for the internal LAN but also for the internet connection. For ISP-provided lines, options include selecting a bandwidth-guaranteed service or deploying SD-WAN to dynamically manage quality across multiple connections.