3 Network Cabling and Physical Media

How cabling carries network signals

Network cabling carries signals between computers, switches, routers, access points, and other devices. A reliable link depends on matching cable performance and distance to the network interfaces, connectors, installation environment, and applicable standards. Cable and terminations must work together: a high-category cable cannot compensate for a poorly terminated or damaged connection.

Copper cabling

Most wired Ethernet LANs use four pairs of insulated copper wires twisted together. The twists help reduce electromagnetic interference and crosstalk between pairs. UTP has no additional conductive shield; shielded variants add foil or braid to reduce interference, but require compatible shielded components and correct bonding and grounding.

A typical Ethernet twisted-pair cable has an eight-position modular plug, commonly called RJ-45 and more precisely called an 8P8C connector. Copper links can also carry electrical power using Power over Ethernet (PoE), when the network equipment, cable, and installation support it.

Coaxial cable has a central conductor surrounded by insulation and a conductive shield. It is used in applications such as cable broadband and some radio-frequency systems. Coaxial Ethernet was used in older LANs, but modern Ethernet LANs generally use twisted pair or fiber instead.

Cable categories, distance, and jacket ratings

A cable category specifies transmission performance; it is not itself an Ethernet speed. Actual speed and distance depend on the complete channel, installation quality, and equipment at each end.

Common Ethernet guidance includes:

• Cat 5e: A typical bandwidth rating of $100\,\text{MHz}$; supports $1\,\text{Gb/s}$ to $100\,\text{m}$. Speeds of $2.5$ or $5\,\text{Gb/s}$ may work on suitable installed cabling and equipment.

• Cat 6: A typical bandwidth rating of $250\,\text{MHz}$; supports $1\,\text{Gb/s}$ to $100\,\text{m}$. It may support $10\,\text{Gb/s}$ over shorter distances, often up to about $55\,\text{m}$, depending on installation and interference.

• Cat 6A: A typical bandwidth rating of $500\,\text{MHz}$; supports $10\,\text{Gb/s}$ to $100\,\text{m}$. It is a common choice for new 10GBASE-T links requiring the full distance.

• Cat 8: A typical bandwidth rating of $2{,}000\,\text{MHz}$; designed for short, high-speed data-center links. Its $25$ or $40\,\text{Gb/s}$ channels are limited to $30\,\text{m}$.

For conventional structured copper cabling, the maximum channel length is generally $100\,\text{m}$: up to $90\,\text{m}$ of installed horizontal cable plus a combined allowance of up to $10\,\text{m}$ for patch cords. Check the applicable standard and equipment specifications for the actual link. Cat 6's reach at $10\,\text{Gb/s}$ is not guaranteed at one fixed distance; crosstalk and installation conditions matter.

Cables also have jacket ratings that indicate where they may be installed. Riser cable is intended for vertical runs between floors, while plenum cable is intended for certain air-handling spaces. Use the rating required by local codes and the installation location. Do not assume that cable suitable for indoor use is also suitable for outdoor or direct-burial use.

Fiber-optic media

Fiber carries information as pulses of light through glass or plastic, rather than as electrical signals through copper. It does not conduct electricity and is inherently resistant to electromagnetic interference. Fiber is useful for long links, high bandwidth, and connections between buildings, but requires compatible optical interfaces and careful handling of connectors and bend limits.

• Multimode fiber has a larger core and carries multiple light paths. Premises types include OM1, OM2, OM3, OM4, and OM5. Supported speed and distance depend on the fiber type and the exact optical Ethernet standard.

• Single-mode fiber has a much smaller core and is used with compatible optics for longer-distance links, including campus and telecommunications connections. Its achievable reach depends on the optical transceivers and link design.

A short equipment-room link may use multimode fiber with optics designed for that fiber, while a longer building-to-building link may call for single-mode fiber and matching optics. Do not select by fiber type alone: verify the transceiver's supported fiber, wavelength, speed, and distance, and ensure the link's total optical loss is within its budget.

Copper wiring schemes

T568A and T568B are two accepted wiring schemes for terminating four-pair twisted-pair cable. They differ only in the positions of the orange and green pairs. Pin numbers are read from $1$ through $8$ at the plug contacts.

The conductor order for T568A is: pin $1$, white/green; pin $2$, green; pin $3$, white/orange; pin $4$, blue; pin $5$, white/blue; pin $6$, orange; pin $7$, white/brown; and pin $8$, brown.

The conductor order for T568B is: pin $1$, white/orange; pin $2$, orange; pin $3$, white/green; pin $4$, blue; pin $5$, white/blue; pin $6$, green; pin $7$, white/brown; and pin $8$, brown.

A straight-through cable uses the same scheme at both ends: A–A or B–B. An A–B cable is a traditional crossover cable. Many modern Ethernet ports support auto-MDI-X and can adjust for a crossover automatically, but consistent straight-through wiring remains the normal choice for structured cabling. Use the same chosen scheme throughout an installation unless extending a system that already uses the other scheme.

Connectors and compatibility

Common fiber connector styles include LC, a small form-factor connector often used in duplex pairs; SC, a push-pull connector; ST, a bayonet connector; and MPO, a multi-fiber connector. The cable connector must match the equipment or patch-panel port, or a compatible patch cord or adapter must be used.

Duplex fiber links also require correct transmit-to-receive polarity. Match the connector type, fiber type, and optical interface. Do not mate incompatible connector finishes such as UPC/PC and APC. Connector colors can help identify fiber types, but should not replace checking labels and specifications.

Choosing and verifying a cable link

Use a systematic check before installing or ordering a cable:

1. Identify the endpoints and interfaces. Confirm the Ethernet speed, copper or optical port, connector type, and, for fiber, the transceiver requirements at both ends.

2. Measure the route. Include patch cords and connection points. If a standard copper Ethernet channel would exceed $100\,\text{m}$, consider fiber or a suitable intermediate network device rather than assuming a higher cable category will extend the link.

3. Select the required performance. For example, a $10\,\text{Gb/s}$ copper channel that must reach $100\,\text{m}$ calls for Cat 6A cabling. Cat 6 may support $10\,\text{Gb/s}$ only over a shorter, installation-dependent distance.

4. Check the environment. Consider electrical interference, indoor or outdoor exposure, moisture, temperature, fire rating, and whether shielding, armor, or a particular jacket rating is needed. Fiber is a strong option when electrical isolation or immunity to electromagnetic interference is important.

5. Verify terminations and test the finished link. Use compatible connectors and follow the selected pinout. Test installed cabling to confirm continuity and performance. For fiber, also verify connector cleanliness, polarity, and optical loss.

A $1\,\text{Gb/s}$ connection from a desktop to a nearby switch can typically use a tested Cat 5e or better copper channel with matching 8P8C terminations. For a $10\,\text{Gb/s}$ connection between devices $80\,\text{m}$ apart, do not rely on Cat 6; use Cat 6A copper or choose a compatible fiber link, depending on the equipment and environment.