Fiber, short for fiber-optic cable, is a transmission medium made of hair-thin strands of glass that carry data as pulses of light. Compared with copper, it can carry far more bandwidth over far longer distances and is not affected by electrical interference. Fiber is the usual physical layer under business internet access, private Ethernet circuits, cloud connections and links between data centers. When a provider says “fiber,” though, it could be selling any of several very different services over that glass.
At a glance
- Fiber is a cable, not a service; internet access, Ethernet, wavelengths and dark fiber can all run over it.
- Single-mode fiber carries signals for long distances and is standard for carrier services; multimode fiber suits short runs inside buildings.
- Whether your building is already connected (“on-net”) largely decides price and lead time.
- Two fiber circuits are only as resilient as their physical paths; shared conduits and entrances can defeat redundancy.
- Fiber isn’t a security control; encrypt sensitive traffic over it.
What problem it solves
Cloud applications, video meetings, backups and data replication need connections that are fast, steady and able to grow. Copper telephone lines lose speed quickly with distance, and shared cable networks vary with neighborhood demand. Fiber carries high, symmetrical speeds over long distances with low latency and jitter, and the same glass can usually be upgraded to much higher speeds by changing the equipment at each end.
The buying problem is that “fiber” describes the medium, not what you get. A shared fiber broadband plan, a dedicated internet access circuit and a leased dark fiber pair all run over glass, but they differ in commitments, price and who manages the equipment.
How it works
A fiber strand has a glass core surrounded by cladding that keeps light inside the core. A laser or LED at one end turns data into light, and a receiver at the other turns it back. There are two main types:
- Single-mode fiber has a very small core and is used with lasers, usually at 1310 or 1550 nm. It carries signals for many kilometers and is the norm for metro and long-haul carrier services.
- Multimode fiber has a larger core and is cheaper to use over short distances, typically within a building, campus or data center.
Services delivered over fiber include:
- Fiber broadband. Often delivered over a shared passive optical network such as GPON as fiber to the premises, usually best-effort.
- Dedicated internet access (DIA). A business internet circuit with a committed rate and an SLA.
- Ethernet services. Point-to-point or multipoint private connections between sites.
- Wavelengths. A dedicated optical channel at a fixed speed, often carried with DWDM.
- Dark fiber. Unlit strands you lease and light with your own equipment.
At the hand-off, the provider’s equipment connects to yours over a copper Ethernet port or an optical transceiver on a fiber connector, usually LC. The optic type, wavelength and reach must match on both sides.
When it matters for buyers
- Moving or opening an office. Ask early whether the building is on-net. If not, the provider may need permits, digging and landlord approval, and construction can take months. Temporary broadband or fixed wireless can bridge the gap.
- Designing for resilience. Two circuits can share a conduit, a building entrance or a provider facility. Ask for route maps and diverse-entrance commitments, and see path diversity for what to verify.
- Choosing the service type. Match it to the need: DIA for internet, private Ethernet or wavelengths between sites, dark fiber where you want full control and can run the optics yourself.
- Data center and cloud links. In colocation facilities, the provider hands off in a meet-me room and you order a cross-connect to your cage, which adds to the monthly cost.
- Understanding the market. Who owns the conduits and rights-of-way limits which providers can actually reach your building. Our podcast How Your Internet Access Is Controlled by a Handful of Companies covers why.
Our dedicated internet access and dark fiber pages cover the two most common ways businesses buy fiber.
Questions to ask vendors
- Is our building on-net today? If not, what construction is needed, who handles permits and landlord approval, and what is the realistic lead time?
- Which service is this exactly: shared broadband, dedicated internet, Ethernet, wavelength or dark fiber?
- What is the hand-off: copper or optical, which speed, which optic type and connector?
- Can you show the route, including the building entrance, conduit and the facilities it passes through? Will you commit to diversity from our other circuit in writing?
- What are the SLA terms for availability, repair time and, where offered, latency and packet loss?
- Can we upgrade speed mid-term on the same fiber, and what does it cost?
- What acceptance tests and light-level readings will you provide at turn-up?
How it differs from lit fiber and dark fiber
“Fiber” is the physical cable. Lit fiber is a service where the provider runs the equipment that sends light over the cable, so you buy a managed connection at a set speed, such as internet access, Ethernet or a wavelength. Dark fiber is the cable leased with no provider equipment, so you supply the optics and manage capacity yourself. When a quote says “fiber,” ask which of these it is, because commitments, costs and responsibilities differ sharply between them.
Problems this helps with
- Our building has no fiber and the quote includes months of construction
- Two circuits from two providers went down in the same outage
- Not knowing which fiber service type to order
- Copper or cable service that can't keep up with cloud and video traffic
