Advanced Networking & VLAN Management

1. VLAN Partitioning and 802.1Q Encapsulation On a high-level tour, a single 10Gbps fiber backbone often carries multiple disparate data streams: Console Control, Audio-over-IP (AoIP) such as Dante or Milan, and Stage Lighting (Art-Net or sACN). Operating these on a "flat" network is a catastrophic engineering risk. We utilize Virtual Local Area Networks (VLANs) to partition a single physical switch into multiple isolated logical networks.

The primary mechanism is 802.1Q Tagging. When a packet enters a "Trunk" port (a port carrying multiple VLANs), a 4-byte tag is inserted into the Ethernet frame header. This tag contains the VLAN ID (VID). This allows the switch to maintain separate Broadcast Domains. In professional audio, this is critical for preventing "Broadcast Storms." For example, Art-Net lighting protocol is notoriously "chatty" and often utilizes broadcast packets to discover fixtures. On a flat network, every audio interface would receive these lighting packets, forcing their internal CPUs to process and discard them. This "CPU Interrupt" overhead can cause sub-microsecond timing variations in the audio clock, leading to audible clicks or network "dropout" warnings. VLANs ensure the audio clock remains in its own quiet, isolated lane.

2. Quality of Service (QoS) and the DSCP Hierarchy While a 1Gbps link has massive bandwidth, it can still suffer from Jitter (Packet Delay Variation). If a technician starts a 64-channel multitrack recording, the network bandwidth might spike. If a PTP (Precision Time Protocol) clock packet is stuck in a buffer behind a large audio data packet, the "Time of Arrival" is delayed. In digital systems, 1 millisecond of jitter is equivalent to 48 samples (at 48kHz). If the jitter exceeds the receiver's buffer, the system mutes.

We mitigate this with Quality of Service (QoS). Using Differentiated Services Code Point (DSCP) tags in the Layer 3 IP header, we label packets with a priority level.

• DSCP 56 (CS7): Typically reserved for the PTP Clock. This is the highest priority.

• DSCP 46 (EF - Expedited Forwarding): Typically used for the Audio Data stream.

• DSCP 0 (BE - Best Effort): Used for control data, internet, and general traffic. The switch uses Strict Priority Queuing to ensure that if a DSCP 56 packet arrives while it is sending a DSCP 0 packet, the clock packet is moved to the "front of the line" for the very next available transmission slot.

3. The "Silent Killer": Energy Efficient Ethernet (EEE) One of the most common causes of intermittent Dante sync errors is Energy Efficient Ethernet (802.3az), also known as "Green Ethernet." This protocol is designed to save power by putting inactive ports into a "Low Power Idle" state. In a professional audio environment, where audio data is intermittent but clocking is constant, EEE may falsely assume a port is idle during quiet passages or between clock pulses. The time it takes for a port to "wake up" from its idle state (the wake-up latency) is significantly longer than the PTP timing window. This results in the "Grandmaster" clock appearing to disappear and reappear, causing total system instability. In a professional system drive rack, EEE must be disabled at the hardware/firmware level on every managed switch.

II. Practical Lab: Network Static Mapping

1. Tool: Terminal/Network Utility.

2. Location: Run from ~/Desktop/Audio_Tools/.

3. Tasks: * Use ifconfig or ipconfig to identify your current IP range and subnet mask.

   • Assign a Static IP to your laptop in the "Control" range (e.g., 192.168.1.15). Ensure the Subnet Mask (typically 255.255.255.0) matches the console.

   • Run the network_vlan_audit.sh script to verify Multicast support and check if the PTP ports (UDP 319/320) are accessible.

III. Daily Assessment (Thursday)

• Q1: What is the difference between Layer 2 (MAC) and Layer 3 (IP) in the context of Dante routing?

• Q2: Why must "Energy Efficient Ethernet" (EEE/Green Ethernet) be disabled for professional audio networks?