Ethernet media: copper, fiber, and transceivers

Ethernet, the standard

Ethernet is an IEEE network standard, not just a cable or a protocol: guidelines specifying cabling and signaling at OSI Layers 1 and 2. “Ethernet” and “IEEE 802.3” get used synonymously, though 802.3 usually refers to a specific part of the standard (like a frame format).

Three physical media: coax (dead, but it’s where Ethernet started as a shared cable), twisted-pair copper, and fiber.

Reading a standard name

1000BASE-T breaks down as:

• 1000: speed in Mbps (1 Gbps)
• BASE: baseband signaling, only Ethernet signals on the medium
• T: twisted pair (F, S, and L variants mean fiber)

The standards worth knowing:

• 100BASE-TX: Cat 5 UTP, two-pair. 100 m max segment, RJ-45.
• 100BASE-FX: 62.5/125-micron MMF. 400 m, duplex MIC ST connector.
• 1000BASE-T: Cat 5/5e UTP, four-pair. 100 m, RJ-45.
• 1000BASE-SX: MMF. 275 m on 62.5-micron, 550 m on 50-micron.
• 1000BASE-LX: 9-micron SMF. 5-10 km.

Signals and why distance is limited

Inside devices everything is electrical. Optical transmitters convert electrical to light and receivers convert back. Simplified: fiber sends 1 as light on and 0 as light off, while copper sends 1 as +10 V and 0 as -10 V.

Both signal types degrade with distance (attenuation), which is why segment lengths are capped.

Copper also suffers electrical noise, unwanted signal disturbance. Poor connections and patch cables are the main culprits, plus motors, fluorescent lights, high-voltage lines, lightning, even solar and cosmic noise. The twist in twisted pair exists to cancel EMI.

UTP copper

The default for short and medium runs because it’s cheap: speeds from 10 Mbps to 40 Gbps, the least expensive per node, small connectors.

Max cable length is 100 m (only 30 m at 40 Gbps). Categories: Cat 5 is 100 Mbps, Cat 5e is 1 Gbps, Cat 6/6a/7 are 10 Gbps, Cat 8 is 40 Gbps.

RJ-45 and structured cabling

• Plug: male, crimped on the cable. Viewed from the front, pins number 8 to 1, left to right.
• Jack: female, in the device, wall, or patch panel. Viewed from the front, pins number 1 to 8, left to right.
• Horizontal cabling runs wall socket to wiring closet; vertical cabling runs wiring closet to wiring closet.
• PoE: power + data on one Ethernet cable (all four pairs, Cat 5+). Feeds APs, IP cameras, VoIP phones.

Straight-through vs. crossover

• Straight-through: pins match on both ends. For unlike devices: switch-to-router, switch-to-PC, switch-to-server.
• Crossover: some wires flip so transmit lands on the far side’s receive. For like devices: switch-to-switch, router-to-router, PC-to-PC, and router-to-PC.

To identify one, hold both ends side by side, connectors facing you. Same colors in same order means straight-through.

auto-MDIX changes the stakes: modern interfaces detect the wrong cable type and fix it internally, making crossover cables mostly legacy. Old gear without auto-detection can still bite you in a lab.

The exam still tests which cable goes where, so know the like/unlike rule cold. Note that router-to-PC is crossover (both are “like” electrically), even though you rarely cable that in practice.

Fiber

A glass strand not much thicker than a hair, acting as a light pipe. Longer distances and higher bandwidth than copper, low loss, immune to EMI. Fragile.

Construction, dimensions in microns:

• Core (9 for SMF): where the light travels.
• Cladding (125, standardized so connectors fit everything): confines the light to the core.
• Buffer/coating (250): protects from scratches and moisture, nothing to do with light. A scratch can propagate and snap the fiber.

MMF vs. SMF

Same glass, same cladding diameter; the difference is core size, which changes how light travels.

• MMF: multiple light paths (“modes”) bouncing through a wider core, LED light source. Lower bandwidth, shorter distance, cheaper.
• SMF: one light path (approximately), laser source. Higher bandwidth, longer distance, pricier.

Fiber connectors

Around 70 types exist; orgs standardize on one or two. Locking mechanisms: threaded (screw), bayonet (twist-lock), push-pull (snap).

• LC: enterprise gear, the usual on SFP modules. Small form factor.
• SC: enterprise gear.
• ST: patch panels (durable).
• FC: patch panels, service providers.
• MT-RJ: two fibers (transmit + receive) in one connector, enterprise.

Duplex LC or SC means a transmit and receive pair. SFF connectors like LC are replacing SC to pack more ports per faceplate.

SFP transceivers

Hot-swappable modules that plug into switch and server ports and terminate fiber (or copper: RJ-45 SFPs exist). The speed ladder:

• SFP: 1 Gbps
• SFP+: 10 Gbps
• SFP28: 25 Gbps
• QSFP: 40 Gbps, with a 100 Gbps variant
• QSFP-DD: 400 Gbps

Hot-pluggable means no powering down the device. Handling: seat the fiber fully into the SFP, and when removing an SFP, pull the bail hook, never the cable.

QSFP breakout is common switch-to-server setup: one 40G port splits into 4x10G links to feed multiple SFP+ ports.