Dark Fiber vs. Lit Fiber: What’s the Difference?

Many Brightlio customers are interested in fiber internet services for their connectivity needs. However, understanding the difference between dark fiber and lit fiber services can be confusing.

One of the questions we are asked most often by customers interested in fiber connectivity is:  What’s the difference between dark fiber and lit fiber? 

This article explains why fiber is a popular medium for network connectivity and the differences between dark and lit fiber services.  To learn more, or for pricing for fiber connectivity services, contact us.

Why Fiber for Internet Connectivity? 

Why use fiber for internet connectivity?  In a word:  Speed. 

If you want to transmit lots of bandwidth as fast as possible and over long distances, fiber is your medium.  Fiber transmits data over glass fiber-optic cables using laser lights.  Due to the properties of light, fiber transmits data as fast as physically possible – At the speed of light. Also, fiber allows data to travel over exceptionally long distances with almost no cross-talk or electromagnetic interference. 

These properties make fiber a fantastic transport medium for customers looking to procure network connectivity services. 

Dark Fiber vs Lit Fiber 

Fiber is purchased as either “dark” or “lit”.  Dark fiber is fiber that is not in use, meaning no light is currently traversing the dark fiber cable.  Customers buy or lease dark fiber for their exclusive use.  Often, customers are responsible for providing and maintaining the hardware to transmit data across the dark fiber service. 

Dark fiber allows customers to pass almost unlimited amounts of data across the fiber optic cables, with no congestion from other users.  Customers using dark fiber services can leverage traditional Ethernet switches and routers with long-range optics.  Alternatively, customers can leverage Dense Wavelength Division Multiplexing (DWDM) equipment. DWDM splits light into various wavelengths allowing for up to 400 Gbps of capacity across a single fiber strand. 

Businesses can use dark fiber for point-to-point connections between offices and colocation facilities. The dark fiber network architecture is often designed in a ring configuration for greater redundancy.  Also, dark fiber networks are secure, as no other users are on the network. 

Dark fiber pricing is expensive to deploy and maintain, especially over long distances.  Crown Castle, Lumen, and Zayo are examples to dark fiber providers through which Brightlio can offer dark fiber services.

Depending on your location, local municipalities or utilities may offer dark fiber services.  City of Pasadena, CA and City of Palo Alto, CA are both examples of municipalities offering dark fiber services for lease.  These offerings attract business to the community and may be less expensive than offerings from private companies.

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Lit Fiber

In contrast to dark fiber, lit fiber is a fiber optic cable that is currently in use.  The network equipment is managed by a network provider.  The network provider typically has many customers leveraging the same fiber network, thus allowing customers with more limited budgets access to the performance and distance benefits of fiber.  The service is typically sold with a Service Level Agreement (SLA) guaranteeing specific uptime and capacity.  AT&T Business Fiber is an example of a lit fiber business internet service that Brightlio offers to its clients. 

The drawback to lit fiber services is that many other customers are on the network, which sometimes leads to network congestion.  Reading the details of the lit fiber service agreement is important, to understand what capacity you are guaranteed, and what the impacts of network congestion might be on your service. 

Customers that need guaranteed capacity should compare dark fiber with services built around committed bandwidth. Our dedicated internet vs. broadband comparison explains how those options differ when predictable throughput and service guarantees matter.

Fiber and SD-WAN

Adding SD-WAN services, which allow you to combine dark or lit fiber with lower-cost broadband and LTE services, are another option to improve performance and reliability. When integrating SD-WAN services, both dark fiber and lit fiber can be leveraged effectively based on the specific needs of the business. With dark fiber, businesses gain extensive control over their network infrastructure, enabling a high degree of scalability.

This is particularly beneficial for data-heavy applications or rapidly growing businesses. When paired with SD-WAN, the ability to segment networks using dark fiber further enhances security, making it a robust solution for businesses with stringent data protection requirements.

Conversely, using lit fiber with SD-WAN can provide a more managed solution, with the fiber provider taking responsibility for the lighting equipment. This reduces upfront costs and technical complexity, making it a viable option for smaller enterprises or those with limited in-house network expertise.

The combination of lit fiber and SD-WAN can improve bandwidth utilization and give businesses more control over how traffic moves between circuits. If you are weighing that architecture, our SD-WAN advantages and tradeoffs breakdown explains where the model works well and where it can add complexity.

Brightlio Delivers Dark and Lit Fiber Connectivity

Brightlio’s business connectivity services help companies compare dark and lit fiber based on route availability, capacity, and budget. Brightlio provides quotes and solution designs at no cost.

Are fiber services not a fit for your needs?  No worries.  Brightlio offers a wealth of network connectivity solutions from leading global providers.  We also offer unified communications, SD-WAN, public cloud solutions, and advisory services. Contact Brightlio today and start on a quote or solution design. 

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