CTS AV Networking and AV over IP — Questions and Answers
Question 1: Which network quality-of-service (QoS) mechanism is most commonly used to prioritize AV over IP traffic on a managed Ethernet switch?
- Spanning Tree Protocol (STP)
- Differentiated Services Code Point (DSCP) marking (Correct answer)
- Network Address Translation (NAT)
- Link Aggregation Control Protocol (LACP)
Correct answer: Differentiated Services Code Point (DSCP) marking
DSCP marking tags IP packets with a priority value, allowing switches and routers to queue and forward time-sensitive AV traffic (audio, video, control) before less-critical data. DSCP is the standard QoS tool for Dante, AES67, and other AV over IP protocols. STP manages loop prevention, NAT handles address translation, and LACP bonds multiple links.
Question 2: What is the primary advantage of using VLANs when deploying an AV over IP system alongside a corporate IT network?
- VLANs increase the physical bandwidth of each port
- VLANs eliminate the need for a managed switch
- VLANs segment AV multicast traffic so it does not flood the corporate network (Correct answer)
- VLANs automatically assign IP addresses to AV devices
Correct answer: VLANs segment AV multicast traffic so it does not flood the corporate network
AV over IP protocols like Dante use multicast, which by default floods all ports on a switch. Placing AV devices on a dedicated VLAN containing the multicast traffic prevents it from saturating corporate workstations and servers. VLANs do not increase bandwidth or replace DHCP.
Question 3: Dante, the popular AV over IP protocol, uses which transport method to carry audio across a network?
- RS-232 serial over Ethernet tunneling
- AES3 digital audio encoded in HDMI
- UDP/IP multicast and unicast packets (Correct answer)
- Fiber Channel storage protocol
Correct answer: UDP/IP multicast and unicast packets
Dante transmits audio as UDP/IP packets using both multicast (for one-to-many flows) and unicast (for point-to-point flows). UDP is chosen over TCP because its low-latency, connectionless delivery is better suited to real-time audio than TCP's retransmission overhead.
Question 4: An AV technician needs to ensure that network switches support AV over IP multicast without flooding all ports. Which switch feature should be enabled?
- Port mirroring
- IGMP snooping (Correct answer)
- MAC address filtering
- Jumbo frames
Correct answer: IGMP snooping
IGMP snooping allows a switch to listen to IGMP membership messages and only forward multicast traffic to ports that have devices subscribed to that multicast group. Without it, multicast frames are sent to every port, wasting bandwidth. Port mirroring duplicates traffic for monitoring; MAC filtering controls access; jumbo frames increase packet size.
Question 5: What does the term 'latency' mean in the context of AV over IP audio systems, and why is it critical?
- The bit depth of the audio signal; higher latency means higher fidelity
- The delay from audio input to output; excessive latency causes lip sync issues and performer disorientation (Correct answer)
- The number of audio channels the network can carry simultaneously
- The maximum cable length between switches on a Dante network
Correct answer: The delay from audio input to output; excessive latency causes lip sync issues and performer disorientation
Latency is the time delay between when audio enters the network and when it exits at the destination. In AV systems, excessive latency causes visible lip sync errors (audio lagging behind video) and can disorient performers monitoring themselves through in-ear monitors. Dante systems are configurable for latencies as low as 0.25 ms on a Gigabit LAN.
Question 6: Which IP address range is reserved for IPv4 link-local addresses, commonly used by Dante devices when no DHCP server is present?
- 10.0.0.0 – 10.255.255.255
- 172.16.0.0 – 172.31.255.255
- 169.254.0.0 – 169.254.255.255 (Correct answer)
- 192.168.0.0 – 192.168.255.255
Correct answer: 169.254.0.0 – 169.254.255.255
The 169.254.x.x range is reserved for IPv4 link-local (APIPA) addresses, automatically assigned when a device cannot reach a DHCP server. Dante devices fall back to this range, allowing them to communicate on a local segment even without infrastructure. The other ranges are private unicast ranges (RFC 1918) that require DHCP or static assignment.
CTS AV Networking and AV over IP
Which network quality-of-service (QoS) mechanism is most commonly used to prioritize AV over IP traffic on a managed Ethernet switch?