QoS — Quality of Service

Quality of Service

For full QoS configuration specifics for Dante and AV traffic, see networking/qos-for-audio.

QoS is a set of network mechanisms that prioritize certain types of traffic over others when links become congested. Without QoS, a network switch or router treats all packets equally — a large file transfer competes equally with a real-time Dante audio stream. When the audio packets are delayed by even a few milliseconds, the audio buffer underruns and dropouts occur. QoS marks, queues, and forwards AV traffic ahead of bulk data, eliminating these dropouts.

DSCP Marking

DSCP (Differentiated Services Code Point) is the primary QoS mechanism in modern networks. A 6-bit field in the IP packet header indicates the packet's priority class. Switches and routers read this field and sort packets into priority queues.

Key DSCP values for AV:

Dante marks its own packets DSCP EF (46) — the devices do this automatically. The switch must be configured to honor this marking and schedule EF packets into the strict-priority queue ahead of all other traffic.

Switch QoS Configuration

On managed switches (Cisco Catalyst, Aruba, Netgear M series, Ubiquiti UniFi), QoS is configured at the port or globally:

1. Trust DSCP: configure each switch port to trust the DSCP markings incoming from connected devices. Without this, the switch ignores or overwrites the DSCP value.
2. Queue mapping: map DSCP EF to the highest-priority queue (typically queue 7 of 8 in Cisco, or the strict-priority queue in HP/Aruba).
3. Scheduling policy: use strict priority for the EF queue so Dante packets are always forwarded first. Use weighted fair queuing or deficit round-robin for other queues to prevent starvation.

For Dante, Audinate's recommended QoS configuration requires at minimum two queues: a strict-priority queue for DSCP EF and a best-effort queue for everything else. Eight-queue switches are preferred as they provide more granular control for mixed AV and IT environments.

Where QoS Matters Most

QoS is most critical at congestion points:

• Uplinks between access switches and distribution/core switches — aggregated traffic from many devices competes for bandwidth
• Ports with mixed Dante and bulk-data traffic — a server generating a large backup while sharing a link with Dante devices
• WAN connections — if any AV traffic traverses a WAN, QoS is essential; on-premises Ethernet rarely congests at 1 GbE with modern switches

On a dedicated AV VLAN with no IT traffic and 1 GbE links, QoS provides defense in depth but may not be strictly required for Dante audio alone. It becomes critical when AV and IT share infrastructure.

Common Pitfalls

• Switch configured for QoS but not set to trust DSCP. The Dante devices mark packets EF, but the switch ingress port is configured in "untrusted" mode and overwrites the DSCP marking to 0 (Best Effort). All Dante traffic now competes equally with bulk data. Fix: set all ports connecting Dante devices to trust dscp in the switch configuration; verify with a packet capture that the EF marking is present end-to-end.

• QoS configured on access switch but not on uplinks. The access switch prioritizes Dante correctly, but the uplink to the distribution switch has no QoS policy — all traffic is treated equally at that hop. Fix: apply QoS policy and DSCP trust on every switch in the path, not just the access layer.

• Incorrect DSCP value for AV-over-IP. NVX encoders should be marked AF41 (34), not EF (46). Marking video traffic EF may starve non-EF audio or control traffic under congestion. Fix: configure AV-over-IP sources to use AF41 and map AF41 to a high-priority (but not strict-priority) queue.

• QoS not configured on the return path. QoS is configured in one direction but not the reverse — return control and feedback packets are delayed. Fix: QoS policy must be symmetric; configure it on all ports in both directions.