Dark Fiber vs Lit Fiber for Industrial Networks

Understanding Dark and Lit Fiber

When industrial organizations need fiber connectivity between facilities that are too far apart for self-owned cable, they typically choose between two service models: dark fiber and lit fiber. Dark fiber means leasing or purchasing rights to specific fiber strands in an existing cable plant. The fibers are "dark" because no light is passing through them. The customer provides all active electronic equipment (switches, transceivers, DWDM systems) and is responsible for end-to-end network operation. Lit fiber means purchasing a managed bandwidth service where the provider installs, maintains, and monitors all equipment, delivering a specified data rate (1 Gbps, 10 Gbps, etc.) as a service.

Dark Fiber: Full Control

How It Works

In a dark fiber arrangement, the customer leases individual fiber strands (typically in pairs) from a fiber provider for a term of 5-20 years. The provider is responsible only for the physical fiber plant (cable, splice enclosures, conduit) and ensuring the fiber meets specified attenuation standards. The customer installs their own optical equipment at each end, including switches, routers, DWDM multiplexers, or direct fiber transceivers. The customer has complete control over the bandwidth, protocols, encryption, and network management.

Dark Fiber Advantages

• Unlimited bandwidth: Upgrade from 1 Gbps to 100 Gbps by changing only the end equipment, not the fiber lease
• Full protocol control: Run any protocol or service including SCADA, voice, video, and IT traffic with custom QoS and VLAN configurations
• Security: No shared infrastructure with other customers. Complete control over encryption and access
• Cost predictability: Fixed monthly lease cost regardless of bandwidth utilization. No per-Mbps charges
• DWDM capability: Install wavelength-division multiplexing to create dozens of virtual fiber pairs on a single physical pair

Dark Fiber Challenges

• Upfront equipment cost: Customer must purchase and install all optical networking equipment
• Operational responsibility: Customer is responsible for monitoring, troubleshooting, and maintaining the active network
• Technical expertise: Requires in-house or contracted staff with fiber networking skills
• Physical maintenance: While the provider maintains the physical fiber, coordination for splicing, repairs, and testing adds complexity

Lit Fiber: Managed Service

How It Works

A lit fiber service (also called a wavelength service, Ethernet private line, or managed bandwidth service) delivers a specified data rate between two or more customer locations. The provider owns and operates all equipment including fiber cable, optical transceivers, switches, amplifiers, and network management systems. The customer receives a standard Ethernet handoff (typically an RJ45 or SFP port on a provider-managed device at each location) and pays a monthly recurring charge based on bandwidth and distance.

Lit Fiber Advantages

• No equipment investment: Provider supplies and maintains all optical networking equipment
• Managed service: Provider monitors the circuit 24/7 and dispatches repair crews for outages
• SLA guarantees: Service level agreements specify availability (typically 99.9-99.99%), latency, and repair time
• Scalable: Bandwidth upgrades are a service order, not an equipment purchase
• No specialized staff: Customer does not need fiber networking expertise

Lit Fiber Challenges

• Recurring cost: Monthly charges that increase with bandwidth. A 10 Gbps lit service may cost 5-10x more per month than a dark fiber lease
• Limited control: Customer cannot customize optical transport parameters, add DWDM wavelengths, or run non-standard protocols
• Shared infrastructure: Provider equipment may carry multiple customers' traffic on shared platforms
• Provider dependency: Repair response time, upgrade scheduling, and service changes depend on the provider's processes

Cost Comparison

The economic crossover between dark and lit fiber depends on bandwidth requirements and contract term. At lower bandwidths (1 Gbps or less), lit fiber is typically cheaper because the customer avoids equipment purchases. As bandwidth increases to 10 Gbps and above, dark fiber becomes increasingly attractive because the equipment cost is amortized over years of use while lit fiber pricing scales linearly with bandwidth. For a typical 10-year analysis, dark fiber with customer-owned equipment is often 40-60% cheaper than equivalent lit fiber service at 10 Gbps.

SCADA-Specific Considerations

For industrial SCADA networks, several factors favor dark fiber:

• SCADA traffic requires specific QoS and VLAN configurations that may not be available on lit services
• Critical infrastructure networks benefit from complete isolation from other customers' traffic
• Regulatory requirements (NERC CIP for electric utilities, TSA for pipelines) may mandate dedicated communication infrastructure
• Long equipment lifecycles (15-20 years for SCADA) align well with long-term dark fiber leases

However, lit fiber may be preferred when only one or two remote sites need connectivity, when the organization lacks fiber networking expertise, or when short-term needs (1-3 years) do not justify equipment investment.

NFM Consulting Advisory Services

NFM Consulting helps industrial and utility clients evaluate dark fiber versus lit fiber options for their SCADA and communication networks. We perform total cost of ownership analyses over the expected network lifecycle, assess regulatory requirements, evaluate available fiber providers in the project area, and recommend the approach that delivers the best combination of performance, reliability, control, and cost for each client's specific situation.