When connecting high-density enterprise campuses, private clouds, or synchronous data centre clusters (such as Interxion London to Equinix Slough), standard point-to-point Ethernet circuits reach their architectural limits. As aggregate bandwidth requirements exceed 40Gbps and latency budgets tighten below 2 milliseconds, infrastructure leaders face two optical options: leasing unlit Dark Fibre to light via Dense Wavelength Division Multiplexing (DWDM), or procuring Carrier-Managed Optical Wavelengths (100G/400G).
The 400G ZR/ZR+ Revolution: Simplifying Dark Fibre
Historically, lighting dark fibre required deploying dedicated, high-maintenance optical transport chasses (transponders, muxponders, and optical line systems) occupying full equipment racks.
The arrival of 400G ZR and OpenZR+ coherent optical transceivers (packaged in standard QSFP-DD or OSFP form factors) has transformed this architecture. Enterprise switch and router ports (from Arista, Cisco, and Juniper) can now output coherent DWDM wavelengths directly into passive optical multiplexers, transmitting up to 120km (ZR) or 400km+ (ZR+ with inline amplification) without standalone transponder chasses.
Architectural Comparison Matrix
The comparison below evaluates self-lit dark fibre against managed optical wavelengths and high-capacity carrier Ethernet across critical operational criteria:
| Interconnect Criterion | Self-Lit Dark Fibre (DWDM) | Managed Wavelengths (100G/400G) | Point-to-Point Carrier Ethernet |
|---|---|---|---|
| Operational Demarcation | Physical unlit fibre pair; enterprise procures and manages transceivers, OLS, and filters | Carrier optical transponder handoff (QSFP28/QSFP-DD client-side grey optics) | Carrier switch port (MEF E-Line) with hardware QoS, buffers, and policers |
| Propagation Latency & Jitter | True line-rate optical propagation (~4.9 µs/km in G.652 glass); zero queuing jitter | Near-identical (~5.0 µs/km); OTN (G.709) wrapper adds sub-microsecond framing overhead | Subject to switch packet store-and-forward queuing and buffer congestion |
| Throughput & Channel Scalability | Virtually unconstrained: expand to 64+ channels of 400G/800G per pair via C-band DWDM | Granular scaling in dedicated 100Gbps or 400Gbps increments per carrier port | Hard ceiling based on carrier switch port interfaces (typically 10Gbps or 100Gbps) |
| Coherent Optic Power Draw | Requires high-power 400G ZR/ZR+ optics (15–20W per port); mandates high-airflow switch chassis | Standard short-reach client transceivers (SR4/LR4 at <4.5W per port) in switch ports | Standard client-side optics (<3.5W per port) |
| Fault Diagnostic Ownership | Enterprise owns OTDR trace analysis, optical power budgets (dB loss), and chromatic dispersion | Carrier NOC monitors Pre-FEC Bit Error Rates (BER) and OSNR with strict MTTR SLAs | Carrier monitors standard Layer 2/3 OAM (IEEE 802.1ag / Y.1731) |
| Suitability Verdict | Exceptional | Recommended | Limited |
Resilience & Physical Diversity (RO2)
Whether deploying Dark Fibre or Managed Wavelengths, high-capacity interconnects require physical path separation. A service-level failure often stems from physical disruptions (e.g., civil construction strikes on trunk routes).
To guarantee uptime, enterprise interconnects must be provisioned with full Resilience Option 2 (RO2) architecture:
- Dual Building Entries: Two distinct lead-in boreholes and intake rooms at both terminating premises.
- Diverse Exchange Routing: Path A and Path B routed through entirely independent Openreach serving exchanges or alt-net fibre nodes (e.g., Neos Networks, Colt, or Virgin Media Business).
- Zero Shared Trenching: Verified radial separation between fibre paths to prevent single-point dig disruptions. Review our guide to Excess Construction Charges (ECCs) for civil dig cost structures.
Designing a High-Capacity Campus Interconnect?
ManagedByUs plans, surveys, and provisions dark fibre pairs and dedicated 100G/400G optical wavelengths across the UK. We handle carrier route trace audits, optical loss budgets, and RO2 civil engineering validation. Model your payload with our Bandwidth Sizing Tool or consult directly with an optical engineer.