Dark Fiber: What is it? and How Does it Work?

Dark fiber serves as a robust backbone for present and future demands of Internet connectivity, cloud computing, and 5G technology, with indefeasible right of use (IRU) contracts ensuring long-term network access for carriers, hyperscalers, and large enterprises alike.

Dark fiber is unused fiber optic cable infrastructure that has been laid but is not currently in use. It is called “dark” as no light, carrying data, is passing through. Organizations can lease dark fiber to create private, high-speed networks without building their own infrastructure.

Delve deeper into the world of connectivity as we explore the intriguing concept of dark fiber. Discover how it functions, its comparison to lit fiber, as well as the unique aspects of dark fiber network topology and segments. Learn about the operational and financial implications of an indefeasible right of use (IRU) versus short-term lease, pricing for metro and long-haul networks, and the many advantages dark fiber can offer.

What is Dark Fiber?

Dark fiber refers to unused optical fiber cables that are pre-installed in underground or aerial networks but are not currently active or transmitting data, lacking the necessary hardware and software components to support operational services. The unused fibers are unlit – or “dark”.

Telecommunications companies, internet service providers (ISPs), and independent fiber providers have laid out dark fiber to serve future network expansions and demands. By leasing or purchasing dark fiber, organizations can establish their own dedicated, private networks, enabling them to control the capacity, speed, and security of their data transmissions. Moreover, this is accomplished without the need to share bandwidth with other network users or be subject to the policies of a traditional internet service provider (ISP).

How Dark Fiber Works

Dark fiber enables organizations to lease or purchase unused optical fiber cables within existing telecommunications networks. The number of individual glass strands in a particular fiber optic cable can vary widely:

• Low-Count Cables: these cables typically contain 24, 36, 48, 72, or 144 strands of fiber
• High-Count Cables: these cables generally contain 288, 432, 864, 1,728, or 3,456 strands of fiber

Dark fiber is usually leased in pairs, often in quantities such as two, four, or eight strands, from a typical fiber optic cable that contains between 144 and 432 strands. Leasing dark fiber in pairs facilitates bidirectional communication between two locations. In a dark fiber pair, one fiber is designated for transmitting data, while the other is used for receiving data.

Once an organization gains access to dark fiber, they can light it up by installing their own optical equipment at each end of the fiber, utilizing transmitters to convert electrical signals into optical (light) signals and receivers to convert the optical (light) signals back into electrical signals.

Dark Fiber vs Lit Fiber

The main differences between dark fiber and lit fiber reside in the service, control, equipment, capacity, cost, configuration, and privacy that each type of network offers.

Comparison	Dark Fiber	Lit Fiber
Definition	Unused fiber optic cable	Fiber optic cable in active use
Service	Infrastructure only	Infrastructure plus service
Control	Lessee has full control	Provider controls the network
Equipment	Lessee provides equipment	Provider supplies equipment
Capacity	Virtually unlimited bandwidth	Provider predetermines bandwidth
Cost	High initial, low operational	Lower initial, recurring operational
Configuration	Highly customizable	Standardized, less flexible
Privacy	Greater; control over data flow	Depends on provider’s policies

• Dark Fiber: represents unused, unlit, or inactive optical fiber cables that have been installed but are not currently transmitting any data. They are considered “dark” because they lack the light signals used to transmit data over optical fibers. Organizations use dark fiber to establish their own private networks, giving them full control over the capacity, speed, and security of their data transmissions
• Lit Fiber: refers to optical fiber cables that are active and currently transmitting data. These fibers are considered “lit” because they carry light signals at specific wavelengths, which represent the data being transmitted. In a lit fiber network, the bandwidth is typically shared among multiple users, and the network is managed by a service provider

READ MORE: Top 200 Dark Fiber and Lit Fiber Providers in the World

Demand Drivers

Demand drivers for the dark fiber market include factors such as the:

• Surge in IP traffic due to increased user demand, particularly for video streaming
• Increased enterprise traffic driven by cloud computing and data networking activities
• Hyperscale data centers and the proliferation of cloud services
• Growing number of Internet of Things (IoT) devices
• Cell tower backhaul in support of 5G which utilizes dark fiber as an alternative to traditional lit services
• Emergence of new wireless network infrastructure such as small cells and distributed antenna systems (DAS), which require fiber optic connectivity

Dark Fiber Networks – Topology and Segments

To fully grasp dark fiber networks, it is essential to understand their diverse topologies and the distinct segments that comprise this digital infrastructure.

Network Topology

Dark fiber networks can be installed in various topologies including point-to-point and ring architectures, depending on the specific connectivity needs of an organization.

• Point-to-Point: dark fiber networks utilize a point-to-point topology to establish a direct, high-bandwidth connection between two locations, which is particularly beneficial for data centers, large enterprises, or central offices of telecommunications providers. However, this setup lacks inherent redundancy as communication can be lost if this single connection fails
• Ring: dark fiber networks employ a ring topology where fiber optic cables are arranged in a closed loop configuration. This design provides redundancy and fault tolerance by allowing data traffic to travel in both directions, so that if a disruption occurs in one part of the ring, the data can be automatically rerouted through the alternative path. This ensures continuous network functionality and minimizes downtime in case of failures or disruptions

Network Segments

Dark fiber is used in various network segments, including last-mile networks, metro networks, long-haul networks, subsea cables, and backhaul networks.

Last-Mile Networks

Last-mile networks, also known as access networks, connect end users to the local network provider. Dark fiber is utilized by internet service providers (ISPs) and businesses to manage their network configuration and add optical equipment. This enables them to establish a proprietary network and deliver high-speed, reliable internet access to their customers. Additionally, dark fiber serves as an expandable network infrastructure, allowing carriers to quickly increase capacity when needed.

Metro Networks

Metro networks, also known as intra-city networks, provide connectivity across a city or urban area, linking various local networks. Dark fiber typically provides connectivity between on-net buildings, such as data centers, located in the same metropolitan area. For instance, dark fiber could create a dedicated, private connection between major carrier hotels in the same market, such as 60 Hudson Street in New York City to 755 and 800 Secaucus Road in Secaucus, New Jersey.

Metro dark fiber networks often use a ring network topology. This architecture creates redundant paths for data to travel, which increases network reliability and resilience. If a single point of the ring fails or is disrupted, data can still be transmitted via the other direction of the ring.

In particular, cloud service providers (CSPs) utilize metro dark fiber networks to support the reliability of their data centers within the same availability zone (AZ). For example, inter-AZ connections utilize metro networks to establish communication between different data centers within the same AZ.

Long-Haul Networks

Long-haul networks, also known as inter-city networks, establish high-capacity, reliable connections over vast distances, typically between cities or countries. Dark fiber commonly provides connectivity between on-net buildings located in different large markets, such as New York and Chicago.

Additionally, dark fiber is used in long-haul networks to connect on-net buildings in small and medium-sized markets to major hubs, such as cloud data centers, carrier hotels, or mobile switching centers, in larger markets. For instance, dark fiber could create a dedicated, private connection between Columbus, Ohio, where Amazon Web Services (AWS) and Google Cloud have established regions, and Ashburn, Virginia, which hosts the largest concentration of data centers in the United States.

More broadly, the Internet backbone is essentially a long-haul network that is an integral part of the global telecommunications network. It consists of multiple high-speed fiber optic networks, typically owned by network service providers, that are interconnected.

Dark fiber plays a crucial role in Internet backbone networks by providing reliable connections that can handle vast amounts of data traffic. For example, Cogent Communications (NASDAQ: CCOI), considered an Internet backbone provider, carries approximately 20% of all Internet traffic. To establish this position, Cogent has a network backbone based on indefeasible right of use (IRU) contracts from 319 different dark fiber suppliers around the world. Cogent takes this dark fiber capacity and activates it to create usable IP bandwidth.

Subsea Cables

Subsea cables, also known as submarine cables or undersea cables, utilize dark fiber to transmit vast amounts of data across continents, beneath bodies of water. These fibers are typically owned and managed by consortia of international telecommunications companies and hyperscalers, such as cloud service providers (CSPs) and over-the-top (OTT) media service companies. These organizations utilize dark fiber in their subsea cable networks to manage their own data transmission capacity, ensuring that future demands for high-speed, reliable international communication can be met.

As an example, the 2Africa subsea cable system, which is designed to encircle the continent of Africa, represents a collaboration between hyperscalers, such as Meta Platforms (formerly Facebook), and telecommunications companies including MTN GlobalConnect, Orange, and Vodafone. The 2Africa subsea cable is designed to incorporate up to 16 fiber pairs, with certain dark or ‘unused’ fiber pairs reserved for future expansion.

To this end, when subsea cables are designed and manufactured, they typically include more fiber strands than are initially needed for data transmission. This excess capacity is considered dark fiber. As data traffic increases over time and more bandwidth is required, network operators can “light up” the dark fiber strands to expand the capacity of the cable without the need to lay additional cables.

READ MORE: Submarine Cables – the Invisible Fiber Link Enabling the Internet

Backhaul Networks

Dark fiber is utilized in backhaul networks to transport data from local networks or distribution points to long-haul networks or central network nodes, essentially serving as the link between the “last-mile” and the broader internet. Carriers and internet service providers (ISPs) often use dark fiber for backhaul to manage their own network infrastructure, ensuring high-speed, reliable data transmission. This use of dark fiber also allows for future scalability, because additional capacity can be added by lighting more fibers as data demands grow.

Since dark fiber networks are dedicated and private, they can deliver faster and more reliable data transmission than shared lit networks. Consequently, wireless carriers like Verizon increasingly choose to establish dark fiber backhaul solutions for their cell towers, as opposed to the more common lit fiber-based Ethernet connections.

READ MORE: What is Backhaul? Wired vs Wireless, Fiber vs Ethernet

Dark Fiber IRU vs Lease

When considering dark fiber leasing options, organizations typically choose between two primary alternatives: long-term indefeasible rights of use (IRUs) for exclusive, lasting access, and more flexible short-term leases for temporary or evolving connectivity requirements.

Indefeasible Rights of Use (IRUs)

An indefeasible right of use (IRU) is the most common leasing option for dark fiber, providing long-term, exclusive access to a portion of a fiber optic network’s capacity. Under an IRU agreement, the dark fiber provider grants the lessee – typically a carrier, data center operator, or large enterprise – the right to use a specified number of dark fiber strands for a predetermined period, often ranging from 10 to 25 years or more. In this setup, the customer makes an upfront lump sum payment to cover the entire term of the IRU and pays annually for maintenance costs, which are typically calculated based on fiber route miles.

IRUs offer several benefits to lessees, including:

1. Cost-Effectiveness: IRUs can be more cost-effective over the long-term, compared to shorter-term leases or purchasing and deploying an organization’s own fiber optic infrastructure. By securing long-term access to dark fiber capacity, organizations can avoid the high upfront costs associated with building their own networks, while the maintenance of the physical fiber optic network is typically performed by the fiber owner or provider
2. Control: IRUs grant the lessee exclusive access to the leased dark fiber strands, providing greater control over network capacity, speed, and security. Lessees can install and manage their own optical equipment, allowing them to tailor their network performance according to their specific needs and requirements
3. Scalability: with an IRU, lessees can easily scale their network capacity as their business grows and connectivity needs evolve. The long-term nature of IRUs ensures that organizations have access to the necessary fiber optic capacity without worrying about renegotiating leases or facing capacity limitations

Short-Term Leases

Short-term leases present an alternative leasing option for dark fiber, offering temporary, non-exclusive access to a portion of a fiber optic network’s capacity. Unlike indefeasible rights of use (IRUs), which grant long-term rights to use specific fiber strands, short-term leases offer more flexibility for organizations and do not require a large upfront payment. However, pricing over the term of the lease tends to be higher than that of the IRU pricing model over the same period.

Short-term leases typically range from a few months to a few years, and they may be renewable or extendable based on the agreement between the lessee and the dark fiber provider. This leasing option has several advantages:

• Flexibility: short-term leases provide organizations with the flexibility to adjust their network capacity and performance according to their changing needs. This is particularly useful for businesses with fluctuating bandwidth requirements, or those requiring temporary connectivity solutions for events or projects
• Lower Initial Investment: with short-term leases, organizations can avoid both the high upfront costs associated with building their own fiber optic networks and entering into long-term IRU agreements, which require an upfront cash payment. This option allows organizations to test the benefits of dark fiber connectivity without making significant financial commitments
• Faster Deployment: short-term leases can often be deployed more quickly than long-term agreements, as they typically involve leasing existing, unused fiber strands. This enables organizations to rapidly establish connectivity and take advantage of dark fiber’s high capacity and low latency. Once the short-term lease has been agreed, dark fiber capacity can be installed in less than 30 days

Dark Fiber Pricing

Dark fiber lease pricing is influenced by several factors, including the uniqueness of the route, the number of strands or fiber pairs leased, the depth or end location of the network, the contract term, as well as maintenance and support services.

The primary component of dark fiber lease pricing is the monthly recurring charge, which often includes automatic annual price escalators of CPI or 2% to 4%. These escalators are designed to help to protect the provider against inflation and changes in operating costs. In addition to this, the overall cost of dark fiber to the lessee includes an installation charge and either annual or monthly expenses related to operations and maintenance (O&M) services. These services involve the upkeep, monitoring, and repair of the physical fiber optic cable.

Below, we detail representative dark fiber pricing models for both metro and long-haul networks in the United States.

Metro Networks

Dark fiber lease pricing in U.S. metro networks typically involves a recurring charge of approximately $2,000 per month for shorter, local routes, and $5,000 to more than $10,000 per month for longer, suburban routes. This is in addition to a one-time installation charge of approximately $5,000.

Long-Haul Networks

Dark fiber lease pricing in U.S. long-haul networks typically involves a recurring charge of approximately $7,500 to more than $15,000 per month. This is in addition to a one-time installation charge of approximately $5,000.

Advantages of Dark Fiber

Dark fiber offers several advantages, such as scalability and flexibility, low latency, enhanced security, and robust customization, making it an ideal solution for organizations looking for efficient and secure data transmission.

Scalability and Flexibility

Dark fiber offers virtually unlimited bandwidth and speed potential, as its data transmission capacity is limited only by the optical equipment used to light up the fiber, the transmission technology implemented, and the quality of the fiber optic cables used. For instance, high-speed data rates such as 800G (800 gigabits per second) and even terabits per second (Tbps) can be achieved on dark fiber, offering substantial capacity for organizations to accommodate their evolving connectivity needs.

As data demands increase, organizations can easily upgrade their optical equipment to support higher bandwidth and data transmission rates without the need to modify the underlying fiber infrastructure. Crucially, with dark fiber, these equipment and bandwidth upgrades can be conducted without incurring any additional monthly lease costs associated with the underlying dark fiber network.

Latency

Dark fiber is hard spliced end-to-end, meaning that the individual strands of fiber are fused together without any active carrier equipment, such as amplifiers or repeaters, interrupting the connection. Moreover, dark fiber provides a direct pathway for data transmission, due to the absence of intermediary stops or nodes.

Since this design eliminates the need for data to navigate through multiple routers or switches (as it does in traditional internet services), there is less delay in transmitting information from one point to another via dark fiber. This directness of connection leads to lower latency times, often measured in microseconds.

Low latency is particularly important for businesses and applications that rely on real-time data processing and communication. For instance, in the financial services sector, high-frequency trading (HFT) relies heavily on rapid data exchange. Even a millisecond of delay can result in significant financial implications in such a context.

Security

Dark fiber networks, being dedicated and private, offer superior security compared to shared network solutions, as they give organizations complete control over their network infrastructure. By establishing a private network using dark fiber, organizations can enforce their own security measures and protocols to safeguard sensitive data from interception or unauthorized access. Furthermore, a strand of unlit fiber optic cable is entirely separate from the provider’s primary network, ensuring that the customer is the only one with visibility into the data traversing the network.

Customization

Dark fiber offers control to organizations, enabling them to customize their network and run multiple protocols, such as Internet Protocol (IP), Multiprotocol Label Switching (MPLS), or Ethernet, over a single fiber strand using their own equipment. For instance, organizations can utilize dark fiber to operate 10 gigabit per second (Gbps) Ethernet or multiple DWDM (Dense Wavelength Division Multiplexing) wavelengths by connecting the appropriate hardware at both ends of the service.

By installing and managing their own optical equipment, organizations can adjust their network’s capacity, speed, and latency to meet their unique demands. This customization allows organizations to optimize their network infrastructure for various applications, including high-speed internet, video streaming, Voice over IP (VoIP), Internet of Things (IoT), artificial intelligence (AI) & machine learning (ML) capabilities, and disaster recovery.