Multicast, QoS and timing: why AV-over-IP is different

Series: Connecting AV and IT networks without losing control

When a laptop has internet access, email works and files are reachable, we quickly say: “the network works”. For office IT, that is often a reasonable first conclusion. For AV-over-IP, it is not enough.

AV traffic is real-time. Audio and video do not only have to arrive; they have to arrive on time. Many AV protocols also use technologies that are less critical in standard office environments, such as multicast, PTP timing and QoS.

That is why a network that works perfectly for laptops and printers can still create problems for Dante, AES67, NDI, Q-SYS, SDVoE or ST 2110.

Unicast and multicast

With unicast, one source sends traffic to one receiver. If you have three receivers, you may also create three separate streams. This is easy to understand, but it can consume a lot of bandwidth.

With multicast, one source sends a stream to a multicast group. Only receivers that join that group should receive the stream. This is efficient, but only when the network understands which ports need which multicast groups.

This is where IGMP Snooping becomes important.

IGMP Snooping allows a switch to listen to multicast membership messages and forward multicast traffic only to the ports where receivers are present. NDI documents that multicast without IGMP querying and snooping can be treated like broadcast traffic, which means it may be forwarded to all ports. For successful NDI multicast, NDI describes IGMP as required.

The role of the IGMP Querier

IGMP Snooping needs up-to-date information. That information is maintained by the IGMP Querier. The Querier periodically asks: which devices are still members of which multicast groups?

In an AV VLAN that uses multicast, you should know exactly which device acts as the Querier. Often this is the core switch or Layer 3 switch in the AV network.

With NETGEAR AV switches, this concern has been addressed by NETGEAR IGMP Plus™. Q-SYS warns that incorrect use of the Snooping Querier Address field can result in multiple switches announcing the same querier IP address, which can lead to multicast problems and instability. Querier election is based on the lowest IP address among participating switches.

In other words: simply enabling multicast is not enough. You need to know where the Querier runs, which VLANs use multicast, and whether IGMP Snooping works correctly.

QoS: not all traffic is equal

QoS stands for Quality of Service. It allows certain traffic to receive priority over other traffic. In an AV network this matters, because audio, clocking and control are often more sensitive than ordinary data.

A file transfer can usually wait a few milliseconds. A clock packet for audio or a real-time video stream should not.

Q-SYS describes Q-LAN as the network protocol used for audio and video distribution, device discovery, synchronization, control and management. Depending on the network, switches may need to be optimized with QoS and other management functions to support real-time media properly.

QoS is not a magic speed button. It is a way to create predictability when multiple types of traffic share the same infrastructure.

Timing: audio and video must stay synchronized

In professional audio and video, timing is critical. Dante, for example, uses IEEE 1588 Precision Time Protocol, or PTP, to synchronize Dante devices over the network with a leader clock. Audinate describes this as a method for sample-accurate time alignment.

With SMPTE ST 2110, timing is even more fundamental. ST 2110 describes the transport, synchronization and description of separate audio, video and ancillary data streams over IP for professional real-time media. Streams can run as unicast or multicast flows over the network fabric and are timed individually.

This explains why AV networks are more sensitive to switch configuration than standard IT networks. A network can have IP connectivity and still be unsuitable for real-time media.

Practical example

Imagine a meeting room with Dante for audio, NDI for cameras and Q-SYS for control. Everything is connected to the same switch, but without proper VLANs, IGMP Snooping and QoS.

At first it seems to work. Devices receive IP addresses and sometimes appear in the tools. But when multiple streams become active, problems start: audio briefly drops out, cameras appear and disappear, and touch panel control feels slow.

The cause is not necessarily a lack of bandwidth. It is often a combination of multicast flooding, no clear Querier, incorrect priorities and insufficient segmentation.

Practical advice

Start with the design, not with the outage.

At minimum, check:

- Which protocols use multicast?
- Which VLANs are required?
- Where does the IGMP Querier run?
- Is IGMP Snooping enabled?
- Which QoS profiles are required?
- Which uplinks carry which streams?
- Is timing or PTP required?
- Does media stay local inside the AV network?

NETGEAR Engage and AV profiles can help apply these settings consistently, but the design itself still needs to be correct.

Conclusion

AV-over-IP uses standard network technology, but it places higher demands on predictability. Multicast, QoS and timing determine whether an installation runs reliably.

In the next blog, we look at another reason why AV and IT are often separated: security, management and responsibility.