Comparing Dark Fiber Vs Lit Fiber Networks

In the world of fiber optic networks, two terms often come up, dark fiber and lit fiber. Both types of fiber offer unique advantages and disadvantages, making them suitable for different applications and data networking needs. Understanding the differences between dark and lit fiber services can help businesses and organizations make informed decisions on the best WAN infrastructure. In this guide, we will explore what are dark vs lit fiber networks, the pros and cons of each, as well as common applications.

Dark Fiber Networks

Dark Fiber refers to the unused fiber optic cables that have been installed by a fiber network provider but are currently not in use. This unused fiber capacity does not carry any light signals, thus making them “dark”. Unlike lit fiber connections, dark fiber is not actively transmitting data or providing any services. Dark fiber services are leased to customers on a monthly or yearly basis, typically with larger upfront costs involved. It is the responsibility of the customer to provide and manage the optical equipment required to light the fiber for WAN applications.


  1. Scalability: Dark fiber provides unlimited scalability, allowing businesses to expand their network bandwidth according to their needs. There are no constraints by service provider limitations or installation timeframes once in place. This scalability is particularly advantageous for industries with rapidly growing data demands, such as artificial intelligence, cloud services, streaming media, and data centers.
  2. Control: Another important aspect of dark fiber is complete control over network infrastructure. This allows businesses to customize and optimize their network to meet specific requirements. Businesses that require a high level of customization for their applications or services will benefit greatly.
  3. Security: Since dark fiber is dedicated to a specific customers use only, it offers a higher level of security compared to shared or public network infrastructure. This is particularly important for industries that manage sensitive data, such as finance, healthcare, and government, where data security and privacy are paramount.
  4. Future-Proofing: Investing in dark fiber can be seen as a long-term strategy for future-proofing network infrastructure. As technology continues to advance and data demands grow, dark fiber provides a flexible and scalable foundation. It can adapt to future networking needs, without any infrastructure overhaul needed.
  5. Cost Savings: While the overall upfront costs of dark fiber can be significant, it can result in cost savings over time, especially for businesses with large data networking requirements. By investing in dark fiber infrastructure, businesses can avoid higher recurring monthly fees associated with lit fiber services, which can lead to potentially significant cost savings.


  1. Upfront Costs: Acquiring dark fiber infrastructure can involve substantial upfront costs. These include the purchase or lease of the fiber itself, optical equipment needed to light fiber, as well as technical expertise needed. These initial investments can be a barrier for businesses with limited capital or those looking for a quick network deployment solution.
  2. Maintenance and Management: While businesses have control over their dark fiber infrastructure, they are also responsible for its maintenance and management. This includes tasks such as monitoring network performance, troubleshooting issues, and upgrading equipment, which can require dedicated resources and expertise. For some agreements, this may also include maintenance and repair of the physical dark fiber network.
  3. Time to Deploy: Deploying dark fiber infrastructure may take longer compared to lit fiber solutions. This may involve coordination with multiple parties, including fiber providers, equipment vendors, construction teams, and network engineers. A fiber buildout may also be needed to bring dark fiber infrastructure to a customer, further increasing the deployment timeframe.
  4. Technical Expertise: Managing dark fiber infrastructure requires a high level of technical expertise in networking and fiber optic technologies. Businesses may need to invest in training or hire specialized personnel to effectively manage and maintain their dark fiber networks.

Lit Fiber Networks

Lit fiber refers to fiber-optic cables that actively transmit data using light signals. These cables are equipped with optical equipment on both ends to send and receive data signals, thus making them “lit” and active for services. Lit fiber networks are the foundation for all telecommunication services, which are managed and maintained by providers. Lit fiber is available for various configurations of data services as well as internet connectivity. Pricing is based on the service sold, bandwidth speed, and term of contract.


  1. Cost-Effective: Lit fiber solutions often require no or low upfront costs since the fiber infrastructure is already in place and managed by the service provider. This can be advantageous for businesses with limited capital or those looking for a more predictable monthly cost structure.
  2. Quick Deployment: The service can be installed relatively quickly since the fiber network is already active. This is beneficial for businesses that need to expand their network capacity rapidly to meet growing demands or new opportunities.
  3. Managed Service: With lit fiber, the service provider is responsible for managing and maintaining the fiber network infrastructure. This offloads the burden of network management from the customer, allowing them to focus on their business and applications.
  4. Predictable Performance: Since bandwidth is pre-configured and comes with service level agreements (SLA), there is a predictable level of performance. This can be crucial for applications that require consistent and reliable data networks.
  5. Flexibility: Lit fiber services can be provisioned for many data services as well as Internet connectivity. This also includes flexible contract terms, bandwidth speeds, and applications.


  1. Limited Scalability: Lit fiber solutions may have limited scalability when compared to dark fiber for high bandwidth applications. Businesses can be constrained by the bandwidth options offered by the service provider. This may limit the ability to scale up quickly in response to changing network demands.
  2. Less Control: With lit fiber, businesses have less control over their network infrastructure. They rely on the service provider for management, configuration, upgrades, and repairs. This may not align with the specific network needs or timeframes.
  3. Security Concerns: Since lit fiber can have shared network infrastructure, there may be security concerns associated with data privacy and potential vulnerabilities. Businesses will need to trust the service provider’s security measures to protect their data.

How to Buy Dark Fiber vs Lit Fiber Services

When considering dark fiber networks, you typically have two options, a lease or an Indefeasible Right of Use (IRU) agreement. A lease is usually what most organizations choose since it is typically a monthly payment with multiple-year contract terms. Since the fiber provider maintains the physical network infrastructure, it makes it the easiest and most common way to purchase dark fiber. The other option is an IRU, which is a long-term agreement, usually lasting 10 years or longer, with a large upfront capex payment. With an IRU, the ownership and maintenance of the fiber infrastructure are typically transferred to the customer during this timeframe. This requires additional technical expertise and expenses, making this a less common way to purchase dark fiber. Pricing is based on the number of strands needed and route miles.

On the other hand, lit fiber services are sold exclusively as a lease or monthly payment. Contract terms can be as short as a year, with multi-year terms available. Lit fiber service can be provisioned for various data services including Wavelengths, EPL, MPLS, IP VPN, and SD-WAN. Fiber Internet connectivity is the most common configuration with either a shared or dedicated fiber connection. Lit fiber services are sold and provisioned based on the type of service, bandwidth speed, and in some cases route miles.

Choosing the Right Fiber Network

When choosing between lit fiber vs dark fiber it depends on several factors, such as scalability needs, budget constraints, control requirements, and the level of technical expertise available. Dark fiber offers unparalleled scalability and control but requires significant upfront investments and ongoing maintenance. It is ideal for larger organizations with very large bandwidth requirements and deep IT resources. For most businesses, dark fiber is not the appropriate fiber solution due to these factors.

Lit fiber, on the other hand, provides a cost-effective, managed network solution, with a quicker deployment timeframe. However, there may be some limitations in scalability and network control, which can be a negative aspect. Lit fiber services are ideal for businesses of all sizes with various bandwidth requirements and needs. Businesses should take the time to carefully evaluate their specific needs and weigh the pros and cons of each option. The decision to purchase dark fiber vs lit fiber must align with these needs as well as long-term data networking goals. Having an outside IT consulting firm to assist with advice and exploring fiber options is recommended.