Switches, VLANs & QoS
Segmenting and prioritising AV traffic.
The network switch is to networked AV what the matrix switcher was to HDMI: the box everything plugs into. Understanding what it does — and what it can be configured to do — is the difference between an AV system that sits happily on a corporate network and one that IT wants off it.
What a switch does
A switch connects devices on the same local network. As traffic arrives it notes which MAC address is behind each port, building a table. From then on it sends each frame only out of the port where its destination lives, instead of to everyone. Each port is full duplex — it sends and receives at full speed at the same time — so devices on different ports don't compete for bandwidth unless they share a link.
The links that are shared are the uplinks between switches. Eight decoders each pulling a stream through one 1 Gb uplink share that 1 Gb, so uplinks are where bandwidth runs out first.
| Type | What you can configure | Use in AV |
|---|---|---|
| Unmanaged | Nothing: plug and play | A few control devices; never for AV-over-IP or networked audio |
| Managed | VLANs, QoS, multicast, PoE, port settings, monitoring | Anything carrying audio or video streams |
- Unmanaged
- What you can configureNothing: plug and play
- Use in AVA few control devices; never for AV-over-IP or networked audio
- Managed
- What you can configureVLANs, QoS, multicast, PoE, port settings, monitoring
- Use in AVAnything carrying audio or video streams
VLANs: several networks on one switch
A VLAN (virtual LAN) divides one physical switch into several separate networks. Devices in VLAN 20 can't see VLAN 10's traffic — including its broadcasts, the “anyone out there?” messages that every device on a network must listen to. Each VLAN is usually its own subnet.
- An access port belongs to one VLAN; the device plugged into it doesn't know VLANs exist. This is how most AV devices connect.
- A trunk port carries many VLANs between switches, labelling each frame with its VLAN number (an IEEE 802.1Q tag).
- Traffic between VLANs has to pass through a router or layer-3 switch, where IT applies firewall rules — for example, letting the control processor reach the displays but not the internet.
QoS: putting time-critical traffic first
When more traffic arrives for a port than it can send, the switch queues it. A file download doesn't notice a few milliseconds' wait; a live audio stream or a clock-sync message does. Quality of Service (QoS) lets the switch send some traffic first.
Devices mark each packet with a priority value called DSCP (Differentiated Services Code Point, 0–63). The switch is configured to map DSCP values to queues and to serve the high-priority queue first. Audinate, for example, publishes these markings for Dante audio networks:
| Dante traffic | DSCP | Priority |
|---|---|---|
| PTP clock synchronisation (time-critical) | 56 (CS7) | Highest |
| Audio and other PTP messages | 46 (EF) | High |
| Everything else | 0 (best effort) | Normal |
- PTP clock synchronisation (time-critical)
- DSCP56 (CS7)
- PriorityHighest
- Audio and other PTP messages
- DSCP46 (EF)
- PriorityHigh
- Everything else
- DSCP0 (best effort)
- PriorityNormal
QoS only matters when there is congestion, but congestion happens in bursts too short to see on a utilisation graph — so configure it before the problem appears. Follow each manufacturer's network guide; their DSCP values differ.
Power over Ethernet
Many AV devices — ceiling microphones, touch panels, cameras, small encoders and speakers — take their power from the switch over the network cable. The IEEE PoE standards set how much a switch port can supply:
| Standard | Max power at the switch port | Common AV loads |
|---|---|---|
| 802.3af (PoE) | 15.4 W | Microphones, small touch panels, IP phones |
| 802.3at (PoE+) | 30 W | Larger touch panels, PTZ cameras, encoders |
| 802.3bt (PoE++), Type 3 | 60 W | Powered speakers, larger cameras |
| 802.3bt (PoE++), Type 4 | 90 W | Small displays, high-power devices |
- 802.3af (PoE)
- Max power at the switch port15.4 W
- Common AV loadsMicrophones, small touch panels, IP phones
- 802.3at (PoE+)
- Max power at the switch port30 W
- Common AV loadsLarger touch panels, PTZ cameras, encoders
- 802.3bt (PoE++), Type 3
- Max power at the switch port60 W
- Common AV loadsPowered speakers, larger cameras
- 802.3bt (PoE++), Type 4
- Max power at the switch port90 W
- Common AV loadsSmall displays, high-power devices
Some of that power is lost in the cable, so the device receives slightly less. Check the switch's total PoE budget as well as its per-port rating: a 24-port switch might not be able to supply full power to every port at once.
Key takeaways
- 01Switches learn MAC addresses and forward each frame only to its destination port; uplinks are the shared bottleneck.
- 02VLANs split one switch into separate networks and broadcast domains; access ports carry one VLAN, trunks carry many.
- 03Traffic between VLANs passes through a router or layer-3 switch, where firewall rules apply.
- 04QoS uses DSCP markings to send time-critical traffic, such as clock sync and audio, first.
- 05Check both the per-port PoE class and the switch's total PoE budget.
Check your understanding
Q1A touch panel on VLAN 10 can't discover a control processor on VLAN 30, though both are on the same switch. Why?
Q2What does a trunk port do?
Q3Dante audio drops out briefly when large file transfers run on the same network. What is the first thing to check?