Wired networks: copper & fiber

How data travels over metal and glass: twisted pair and coax, optical fiber and transceivers, Ethernet frames, speeds and Power over Ethernet, how switches learn and split networks with VLANs, the access networks that reach homes (DSL, cable, fiber PON), long-haul DWDM and submarine cables, and how buildings are cabled. Builds on Networking from scratch; radio links are in Radio fundamentals.

Copper, fiber and radio compared

Twisted pair

An Ethernet cable holds four pairs of insulated copper wires (usually 22–24 AWG), each pair twisted together. A pair carries one signal by differential signaling: one wire carries +V, the other −V, and the receiver measures only the difference. Interference from outside (motors, fluorescent lights, other cables) couples into both wires almost equally, so it cancels when the receiver subtracts.

• Twisting keeps each wire equally close to any noise source on average, and makes the pair's own fields cancel, so it radiates less too.
• Crosstalk is one pair picking up its neighbor's signal: NEXT (near-end, at the sender's end), FEXT (far end) and alien crosstalk (from the next cable in a bundle, the limit for 10GBASE-T). Each pair in a cable twists at a different rate so pairs don't line up.
• Gigabit and faster Ethernet send and receive on all four pairs at once, using echo cancellation to separate the two directions.
• Shielding is named by ISO/IEC 11801 as overall/pair: U/UTP (none), F/UTP (foil around all pairs), U/FTP (foil on each pair), S/FTP (braid around foiled pairs). A shield only helps when it is bonded to ground properly; most office cabling is unshielded.

Cable categories

The 100 m figure is a channel: up to 90 m of solid-core cable in the wall (the permanent link) plus up to 10 m of stranded patch cords in total.

Connectors and wiring

What everyone calls RJ45 is the 8P8C modular plug (8 positions, 8 contacts). The two wiring standards differ only in swapping the orange and green pairs; pick one and use it at both ends.

• A straight-through cable has the same standard at both ends; a crossover has A at one end and B at the other. Auto-MDIX, in nearly every modern port, swaps the pairs itself, so either works.
• A gigabit link that comes up at 100 Mb/s usually means a broken pair: 100BASE-TX needs only two pairs, gigabit needs all four.
• Keep the untwisted length at a termination under about 13 mm (half an inch) for Cat5e and better.

Coax (coaxial cable) has a center conductor, an insulating dielectric, a shield and a jacket. The signal travels between the center and the shield, which keeps interference out.

Optical fiber

An optical fiber is a strand of very pure glass: a core that carries the light, a cladding around it (125 µm across for nearly all telecom fiber) and a plastic coating (usually 250 µm). The core's glass has a slightly higher refractive index than the cladding's, so light that meets the boundary at a shallow angle reflects back completely (total internal reflection) and stays in the core for kilometers.

In multimode fiber light takes many paths (modes) of different lengths, so a pulse smears out (modal dispersion) and limits reach. Single-mode fiber is thin enough that only one mode propagates; its limits are attenuation (loss) and chromatic dispersion (colors traveling at slightly different speeds).

Example: 80 km at 0.2 dB/km is 16 dB of loss, about 1/40 of the light, before connectors and splices. Tight bends leak light; bend-insensitive fiber (ITU-T G.657) tolerates the corners in homes.

Fiber connectors and transceivers

• Polish: UPC (blue body) ends are flat; APC (green) are angled at 8° to cut back-reflection, required on PON and RF video. Never mate APC with UPC.
• Dirt is the most common cause of fiber faults: inspect and clean every end before connecting.

A transceiver (an optic, or module) is a hot-pluggable part that turns electrical signals into light and back. The switch port is just a cage; the module decides the medium, wavelength and reach.

• Reach codes in optic names: SR short reach (multimode, 850 nm), DR 500 m single-mode, LR 10 km, ER 40 km, ZR 80 km and beyond (coherent 400ZR and 800ZR modules for DWDM links between data centers). BiDi optics use one fiber with two wavelengths.
• DAC (direct-attach copper) cables are twinax with modules fixed on both ends, for a few meters inside a rack; AOC (active optical cables) do the same over fiber for longer runs.
• Most optics report temperature, laser bias, transmit and receive power (DOM): ethtool -m.

Ethernet frames and MAC addresses

Ethernet (IEEE 802.3) is the link layer of almost every wired LAN, and Wi-Fi copies its addressing. Data travels in frames:

• The preamble (alternating 1s and 0s) lets the receiver lock its clock; the SFD (start frame delimiter) marks where the frame begins.
• EtherType names the payload: 0x0800 IPv4, 0x86DD IPv6, 0x0806 ARP, 0x8100 a VLAN tag.
• The FCS is a CRC-32; a frame that fails it is dropped. Short payloads are padded so frames are at least 64 bytes, a rule left from shared coax, where a sender had to still be transmitting when a collision echoed back.
• Jumbo frames (MTU around 9000) are a common but non-standard extension; every device on the path must agree.

A MAC address is 48 bits, written as six hex bytes (3c:22:fb:12:34:56). The first three bytes are usually the maker's OUI (organizationally unique identifier). Two bits in the first byte matter: the lowest is 1 for multicast (01:00:5e:… for IPv4 multicast, 33:33:… for IPv6), the next is 1 for a locally administered address, which phones use for random per-network MACs (first byte ending in 2, 6, A or E). ff:ff:ff:ff:ff:ff is broadcast: every host on the LAN.

Ethernet speeds

A name like 10GBASE-SR reads as speed (10 Gb/s), BASEband signaling (one signal on the medium), then the medium: T twisted pair, SR short-reach multimode, LR long-reach single-mode, CR copper cable (DAC), KR backplane. A trailing number counts lanes (SR4 = 4 fiber pairs).

• Half duplex shares the medium and uses CSMA/CD; full duplex sends both ways at once with no collisions. Every switch port today runs full duplex; 10 Gb/s and faster have no half-duplex mode.
• Autonegotiation lets the two ends advertise their speeds and duplex modes over link pulses and pick the best common one; 1000BASE-T requires it. Forcing one end and leaving the other on auto causes a duplex mismatch: a working but very slow link with late collisions and CRC errors. Leave both on auto.

Power over Ethernet

PoE sends DC power over the same cable. The switch (PSE, power sourcing equipment) first detects a valid PD (powered device) signature and agrees a power class with it, so a non-PoE laptop is never powered.

• The gap between the two columns is what 100 m of cable can lose as heat. The voltage is about 44–57 V DC.
• Phones take a few watts, Wi-Fi 6/7 access points often need PoE+ or Type 3, and pan-tilt cameras with heaters or LED lighting use Type 3 or 4.
• A switch's total PoE budget is often less than its port count × the maximum per port.
• "Passive PoE" (a fixed 24 V or 48 V with no detection) from some vendors is not 802.3 and can damage devices that don't expect it.

Switches

A switch forwards frames between its ports. It learns where each host is by watching the source address of every frame it receives, and stores that in its MAC table (also called the CAM table). Frames to an unknown address, and broadcasts, are flooded to every other port.

• Store-and-forward switches receive the whole frame and check its FCS before sending it on; cut-through switches start forwarding once they have read the destination, which saves microseconds in data centers but passes on corrupted frames.
• A broadcast domain is everyone who hears a broadcast: by default, every port of every switch connected together. Large ones fill with ARP and discovery chatter; split them with VLANs and routers.
• VLANs (802.1Q) split one physical switch into several logical ones. An access port belongs to one VLAN and sends untagged frames to its host; a trunk port carries many VLANs between switches, each frame marked with a 4-byte tag holding a 12-bit VLAN ID (1–4094 usable). The untagged VLAN on a trunk is the native VLAN. Traffic between VLANs must pass through a router or a layer-3 switch.
• Loops are fatal to plain Ethernet: frames have no hop limit, so a broadcast circles forever (a broadcast storm). The Spanning Tree Protocol (802.1D) elects a root bridge (lowest bridge ID) and blocks redundant links until one is needed. Classic STP takes 30–50 s to recover; RSTP (802.1w) usually takes a second or less. Large data centers avoid it with routed leaf–spine designs (Building networks).
• Link aggregation (LACP, IEEE 802.1AX, formerly 802.3ad) bundles several ports into one logical link for capacity and redundancy. Frames are spread by a hash of their addresses and ports, so one flow still uses only one member link.
• LLDP (802.1AB) lets devices announce their name and port to neighbors, which makes cabling easier to trace.

Access networks: DSL, cable and fiber to the home

The access network (the "last mile") connects homes and businesses to an operator.

• DSL reuses the copper telephone pair, sending data on frequencies above voice; the DSLAM in the exchange or cabinet terminates it. Speed drops steeply with distance.
• Cable (hybrid fiber-coax, HFC) runs fiber from the headend's CMTS to a neighborhood node, then coax to a few hundred homes that share its channels, which is why evenings are slower.
• Fiber to the home (FTTH) usually uses a PON (passive optical network): one fiber from the operator's OLT (optical line terminal) is split by an unpowered splitter into 32 or 64 fibers, one to each home's ONT (optical network terminal), up to about 20 km away.

The OLT measures each ONT's distance (ranging) so their bursts land exactly in their slots, and grants slot sizes by demand (dynamic bandwidth allocation). 25G and 50G PON are in early deployment.

Long haul: DWDM and submarine cables

Wavelength-division multiplexing sends many colors (wavelengths) of light down one fiber, each an independent channel. DWDM (dense WDM) packs channels on the ITU-T G.694.1 grid, 100 GHz or 50 GHz apart (0.8 or 0.4 nm), or in flexible slices.

• The C band holds about 40 channels at 100 GHz spacing or 80 or more at 50 GHz; adding the L band roughly doubles that. With coherent optics, which encode bits in the phase, amplitude and polarization of the light and decode with a DSP, one wavelength carries 100 Gb/s to 800 Gb/s or more, and one fiber pair carries tens of terabits per second.
• An EDFA (erbium-doped fiber amplifier) boosts every C-band channel at once; on land they sit about every 80–100 km. A ROADM (reconfigurable optical add-drop multiplexer) drops and adds individual wavelengths at a node without converting the rest.
• CWDM (coarse WDM, ITU-T G.694.2) uses 18 wavelengths 20 nm apart from about 1270 to 1610 nm: cheaper, unamplified, for metro and campus links.

Submarine cables carry almost all intercontinental internet traffic; satellites carry a small share. TeleGeography counted 570 systems in service and 81 planned in February 2025.

Systems are designed for about 25 years of service. Many new cables are funded by content companies; Google's Dunant cable (2021, Virginia to France) has 12 fiber pairs and about 250 Tb/s of design capacity.

Structured cabling

Structured cabling is the standard way to wire a building (TIA-568 in North America, ISO/IEC 11801 elsewhere): permanent cable runs from patch panels in telecom rooms to wall outlets, and short patch cords that are easy to change.

building/├── entrance/               # ISP fiber, demarcation point├── equipment-room/         # router, firewall, core switch│   └── rack-42U/│       ├── patch-panel-1/  # 24 ports to floor 1 outlets│       └── access-switch/  # PoE for phones and APs└── floor-1/    ├── desk-101/           # 2 × Cat6A outlets    └── ceiling-ap-1/       # 1 × Cat6A, PoE+

• Run two drops per desk and Cat6A to every access-point location; recabling costs far more than cable.
• Keep data cables away from power cables and fluorescent fittings, and leave service slack.
• Avoid copper-clad aluminum (CCA) cable: it doesn't meet the standards and heats up under PoE.

Try it

ethtool eth0                 # speed, duplex, autoneg, link
ip -s link show eth0         # bytes, errors, drops
ethtool -S eth0 | grep -i err   # NIC error counters
sudo ethtool -m eth0         # optic type, light levels
sudo ethtool --cable-test eth0  # pair faults (some PHYs)
bridge fdb show              # Linux bridge MAC table
lldpctl                      # switch and port you are on

Make a VLAN interface and watch tagged frames (lldpctl comes with lldpd):

sudo ip link add link eth0 name eth0.10 type vlan id 10
sudo ip link set eth0.10 up
sudo tcpdump -i eth0 -nn -e vlan   # shows "vlan 10, p 0"

Rising CRC or "rx_errors" counters point at a bad cable, connector or duplex mismatch; light levels far below the optic's receive range point at dirt, a bend or a broken fiber.

References

• IEEE 802.3 Ethernet Working Group (opens in a new tab): every Ethernet speed, medium, PoE and the 802.3dj project
• Ethernet Alliance: overview of 802.3bt (opens in a new tab): PoE types, classes and power levels
• ITU-T G.652 (opens in a new tab): standard single-mode fiber characteristics
• ITU-T G.694.1 (opens in a new tab): the DWDM frequency grid
• ITU-T G.984.1 (opens in a new tab) and G.9807.1 (opens in a new tab): GPON and XGS-PON
• ITU-T G.993.2 (opens in a new tab): VDSL2
• Fiber Optic Association technical reference (opens in a new tab): fiber types, connectors, testing and installation practice
• TeleGeography Submarine Cable Map (opens in a new tab): every submarine cable, its landings and owners
• ICPC cable data (opens in a new tab): International Cable Protection Committee resources on submarine cables
• ethtool manual (opens in a new tab): link settings, statistics, module and cable diagnostics on Linux