Digital Backbones: Fiber Loops & Deterministic Switching
I. Theoretical Context
1. Optocore Logic and Synchronous Redundancy In the world of DiGiCo and high-end festival drive systems, the Optocore Loop is the gold standard. Unlike an Ethernet network, which is "Asynchronous" (packets arrive when they arrive), Optocore is a Synchronous network. It uses a fixed "Time Slot" for every channel. This guarantees that latency is absolute and zero-jitter.
The Loop: Data travels in both directions. If a fiber cable is cut (the "Backhoe" scenario), the system instantly "wraps" the data back the other way. This happens in less than a single sample ($<10.4 \mu s$), meaning the audio never stops.
Master Clocking: In an Optocore loop, one device is the "Clock Master." Every other device "clocks" off the incoming light stream. If the Master fails, the system must perform a "Seamless Clock Handover" to a secondary Master.
2. Milan/AVB: The Deterministic Ethernet Standard While Dante is popular, the industry (led by L-Acoustics, d&b, and Meyer) is moving toward Milan (AVB). Standard Ethernet is "Best Effort"—it will send your audio if the network isn't busy. Milan is Deterministic. It uses Credit-Based Shaping (CBS).
Bandwidth Reservation: A Milan switch "reserves" 75% of its bandwidth purely for audio. If someone plugs in a video camera and tries to flood the network, the switch will block the video data before it allows it to interfere with the audio.
Layer 2 Timing (gPTP): Unlike Dante, which uses Layer 3 PTP, Milan uses 802.1AS. This is "Hardware-Level" timing. The switch itself is a clock. This eliminates "Switch Jitter" entirely, allowing for hundreds of hops without a loss of sync.
3. Fiber Trunking and Wavelength Division Multiplexing (WDM) On a stadium show, we don't run 50 fiber cables. We use Trunking. We also use WDM (Wavelength Division Multiplexing).
Bi-Directional SFP: This allows us to send and receive data on a single strand of fiber by using two different colors of light (e.g., 1310nm for Send and 1550nm for Receive).
The Infrastructure: We use "Tactical Fiber" (military grade). An engineer must understand the Minimum Bend Radius. If you "kink" a fiber cable, you create "Micro-bends" that scatter the light, leading to "CRC Errors" or a total loss of the digital link.
II. Practical Lab: The Redundant Backbone
Tool: Managed Switch (Luminex/Netgear) / Optocore Monitor.
Tasks: * Build an Optocore loop between two consoles and two racks.
The "Cut" Test: Physically disconnect a fiber cable and monitor the "Error Count." Observe the wrap-around behavior.
Milan Configuration: Connect two Milan-enabled amplifiers. Observe the "Stream Reservation" process in the controller software.
III. Daily Assessment (Thursday)
Theory: Explain why "Synchronous" clocking (Optocore) is inherently lower-jitter than "Asynchronous" clocking (Dante).
Networking: Define "Credit-Based Shaping" and how it prevents a broadcast storm.
Fiber: What is the difference between "Single-Mode" and "Multi-Mode" fiber in terms of distance and core diameter?