QoS for Audio Networks

Quality of Service (QoS) is a set of networking techniques that prioritize certain types of traffic—audio, video, control data—over less time-sensitive traffic like web browsing. Without QoS, a user syncing a large file to the cloud can starve audio packets of bandwidth, causing dropouts and clicks. With proper QoS, real-time streams get guaranteed bandwidth and lowest-priority delivery.

Why QoS Matters for AV

Unlike office email or file transfer, audio and video streaming can't tolerate random delays or packet loss. A single dropped audio packet creates an audible click. Sustained congestion causes buffer underruns, leading to loud dropouts and system instability.

QoS solves this by:

• Prioritizing AV traffic over other network users
• Reserving bandwidth for critical flows
• Managing congestion during peak periods
• Guaranteeing low latency for time-sensitive streams

Without QoS, AV systems on shared networks are unreliable. With it, a single managed switch can safely handle office traffic and professional audio simultaneously.

QoS Mechanisms

Packet Marking — AV devices mark outgoing packets with priority tags (DSCP codes, 802.1p priority). Switches read these tags and process accordingly.

Priority Queuing — Switches maintain separate output queues for different priority levels. High-priority packets are serviced first, low-priority packets wait.

Rate Limiting/Shaping — Bandwidth is allocated per queue. AV queue gets, say, 80% of available bandwidth; office queue gets 20%. Overflow is queued or dropped.

Congestion Management — During overload, low-priority traffic is dropped first; high-priority traffic passes through.

Admission Control — The network can reject new flows if bandwidth is already allocated. This prevents oversubscription.

Practical QoS Configuration

Most managed Ethernet switches have QoS menus. Key configuration steps:

1. Enable QoS on the switch
2. Define traffic classes — typically:
   • Class 0: AV streams (highest priority)
   • Class 1: Control/signaling (medium)
   • Class 2: Office traffic (lowest)
3. Map DSCP codes to classes (audio packets marked DSCP 46 map to Class 0)
4. Set queue weights — allocate 80% bandwidth to AV, 20% to office
5. Enable 802.1p priorities if using tagged traffic on vlan-configuration-for-av

Each manufacturer (Cisco, Arista, Juniper, Netgear) has different menus, but the concepts are identical.

DSCP and Priority Tagging

DSCP (Differentiated Services Code Point) is a field in the IP packet header. AV devices set DSCP values:

• DSCP 46 (EF, Expedited Forwarding) — audio/video streams
• DSCP 26-30 — control and signaling
• DSCP 0 — office, best-effort traffic

Switches read DSCP and apply corresponding QoS treatment.

802.1p is Layer 2 priority, useful if VLAN'd AV traffic doesn't cross routers. It's simpler than DSCP for internal AV networks.

dante and AES67 QoS

dante devices automatically mark traffic with DSCP 46. Dante Controller shows if QoS is configured; if the switch doesn't honor the marking, Dante warns the user.

aes67 devices similarly set DSCP 46 or can be configured to use other priority levels. Check manufacturer documentation.

The key: AV equipment marks its traffic; the network must respect those marks.

Bandwidth Planning

Calculate peak load to size QoS allocations:

• One dante flow (64 channels, 48 kHz) = ~288 Mbps
• Video encoder (H.264, 5 Mbps bitrate) = 5 Mbps
• Control/signaling (DSP, RTUs) = <1 Mbps

If your AV VLAN will carry two Dante flows (576 Mbps), allocate at least 700 Mbps to the AV class to leave headroom. On a 1 Gbps link, this uses 70% for AV, 30% for office—reasonable for a mixed-use facility.

Common QoS Mistakes

Not Enabling QoS — Configured priority marks are worthless if QoS is disabled on the switch.

Wrong DSCP Codes — AV device sets DSCP 46, but switch is configured to recognize DSCP 32. Traffic gets wrong priority.

Inadequate Bandwidth Allocation — Allocating only 50% to AV on a busy network invites congestion.

Forgetting Egress — QoS on ingress ports matters, but egress (outgoing) queuing is where real bottlenecks occur.

No Monitoring — Without switch statistics or packet capture tools, you can't verify QoS is working.

Verifying QoS is Working

Use switch management tools or CLI to:

1. Check that QoS is enabled
2. View queue depths and drop counters
3. Confirm DSCP-to-queue mapping
4. Monitor packet loss and latency on the AV VLAN

Many integrators use a simple test: play audio over Dante while a user on the office VLAN downloads a large file. If audio is unaffected, QoS is likely working. If audio clicks, QoS is either disabled or misconfigured.

Integration with VLANs

VLANs and QoS work together. vlan-configuration-for-av isolates AV traffic to its own broadcast domain; QoS then prioritizes that domain within the physical switch infrastructure. This combination is powerful: isolated broadcast, guaranteed bandwidth, no interference.

Common Pitfalls

• QoS Only Works If Configured End-to-End — Configuring QoS on one switch but not others (especially at network boundaries) means priority markings are lost, defeating the purpose.
• DSCP Markings Stripped at Network Boundaries — When traffic crosses from a VLAN to a routed network or to a different administrative domain, DSCP markings may be stripped. Plan for this and configure QoS on both sides of the boundary.
• Not Monitoring Egress Traffic — Integrators often configure ingress QoS but forget that output (egress) ports are where bottlenecks occur. Monitor and configure queue depths on both ingress and egress.
• Inadequate Bandwidth Allocation to AV Class — Allocating only 50% of switch bandwidth to AV on a mixed network invites congestion during peak office usage. Plan conservatively; allocate 70% or more to AV if the network is shared.