1.3 Physical Interfaces and Cabling

Copper: unshielded twisted pair (UTP)

UTP is the dominant LAN cabling. It uses four twisted pairs of copper terminated in RJ-45 connectors. Twisting cancels crosstalk and electromagnetic interference (EMI). Every UTP link segment is limited to 100 meters regardless of category, this is a hard exam number.

UTP category cheat sheet

Copper is susceptible to EMI and has a hard 100 m reach, so it is used for the last 100 meters to the desk, not for long backbone runs.

Fiber optics

Fiber transmits light, immune to EMI, supports far longer distances, and offers higher bandwidth. Two families:

Single-mode fiber (SMF)

• Core ~8-10 microns, a single light path.
• Driven by a laser, typically yellow jacket.
• Reaches tens of km to 100+ km; used for WAN and campus backbone.

Multimode fiber (MMF)

• Core 50 or 62.5 microns, many light paths, causing modal dispersion.
• Driven by LED or VCSEL, often aqua (OM3/OM4) jacket.
• Reaches up to roughly 400-550 m at 10 Gbps (OM4); cheaper for in-building runs.

Connectors and transceivers

Common fiber connectors include LC, SC, and ST; copper uses RJ-45. Modular optics use SFP, SFP+, and QSFP transceivers so one switch port can use copper or various fiber types.

Power over Ethernet (PoE)

PoE delivers DC power and data over the same Ethernet cable, removing the need for a separate power supply for APs, IP phones, and cameras.

Some power is lost over the cable, so the device receives slightly less than the source figure.

Straight-through vs crossover

• Straight-through connects unlike devices: PC to switch, switch to router.
• Crossover connects like devices: switch to switch, router to router, PC to PC.
• Mnemonic: routers and PCs are "alike" (both end hosts) for this purpose; switches and hubs are "alike."
• Modern switches use auto-MDIX to detect and adjust automatically, so the wrong cable type usually still works on current gear, but the exam tests the manual rule.

Worked example

A 40 km link between two buildings exceeds copper (100 m) and multimode (~550 m), so you must choose single-mode fiber with a laser transceiver. A 90 m run from a switch to a desktop is fine on Cat 6/6a UTP.

Copper Ethernet standards by speed

Match the copper standard to its speed and cabling, a frequent exam pairing:

Notice the distance never changes, the medium's reach is fixed at 100 m for all twisted-pair Ethernet. The category and standard only raise speed.

Pinouts and the T568 standard

UTP terminations follow the T568A or T568B wiring standard, both define which colored wire lands on which RJ-45 pin. A straight-through cable uses the same standard on both ends (B-to-B or A-to-A). A crossover uses T568A on one end and T568B on the other, which swaps the transmit and receive pairs. Gigabit and 10G use all four pairs, but the legacy 10/100 transmit-on-1,2 and receive-on-3,6 mapping is the classic teaching model behind the crossover concept.

When to use which media

• Desk-to-switch and most in-building horizontal runs: Cat 6/6a UTP (cheap, easy, within 100 m).
• Switch-to-switch backbone within a building: multimode fiber (immune to EMI, higher speed over a few hundred meters).
• Building-to-building or WAN spans of kilometers: single-mode fiber (laser, long reach).
• Powering APs/phones/cameras without separate adapters: PoE-capable switch ports matched to the device's PoE class.

Common traps

• All UTP categories share the same 100 m limit, higher categories raise speed, not distance.
• Multimode cannot reach tens of km, do not pick it for WAN distances.
• Auto-MDIX does not change the exam's straight-through/crossover rule.
• A device receives slightly less PoE wattage than the source rating because of cable loss, so size the switch's PoE budget with headroom.