Reference

Every term, one sentence each.

62 terms covering the physical layer, access networks, protocols, backbone routing, wireless, performance, CDN, and infrastructure — defined plainly. Use the letter index to jump.

A4 terms

ADSL2+: Asymmetric Digital Subscriber Line 2+. Runs internet over phone-line copper at up to 24 Mbps down and 3.5 Mbps up — only at short distances from the central office.
Anycast: A routing trick where many servers in different places share the same IP address. BGP automatically delivers your request to the nearest one. Backbone of how CDNs work.
AS: Autonomous System — a single network with its own external routing policy. Every ISP, hyperscaler, or major enterprise has a globally-unique AS number (e.g., Cloudflare is AS13335).
Attenuation: Signal loss as it travels through a medium. Measured in dB per kilometer. Fiber attenuates roughly 0.20 dB/km at 1550 nm — astonishingly low.

B5 terms

Beamforming: Steering a radio beam toward a specific direction by coordinating many antenna elements. Used in 5G MIMO arrays and Starlink phased-array dishes.
Bend-insensitive fiber: ITU-T G.657.A2 single-mode fiber that survives 10 mm bend radius without losing signal. What lets installers staple drop cable around baseboards.
BGP: Border Gateway Protocol (RFC 4271). The protocol by which networks tell each other which IP prefixes they can reach. The internet's routing nervous system.
BNG: Broadband Network Gateway. The ISP-side router that authenticates subscribers, assigns IP addresses, and enforces traffic policies. Often colocated with the OLT.
Bufferbloat: Excessive queuing latency caused by oversized buffers in modems and routers. Endemic to older cable modems — pings can climb from 30 ms to 1+ second under load.

C3 terms

Cell sector: One directional segment of a cellular tower's coverage. A 5G base station typically has 3 sectors covering 120° each. Capacity is per-sector, not per-tower.
Central office CO: The local building where an ISP houses its OLT, BNG, aggregation routers, batteries, and generators. Usually a windowless concrete box you've driven past without noticing.
CMTS: Cable Modem Termination System. The cable industry's equivalent of a PON OLT — terminates DOCSIS connections from neighborhood nodes at the headend.

D8 terms

Deprioritization: QoS-class demotion of certain traffic when the network is congested. T-Mobile Home Internet customers get deprioritized below mobile phone users above 1.2 TB/month.
DFZ: Default-Free Zone. The set of BGP routers carrying the full global routing table — roughly 950k IPv4 + 200k IPv6 prefixes — with no default route.
DMT: Discrete Multi-Tone modulation. Splits DSL's frequency band into hundreds of narrow sub-carriers, each independently modulated. The whole reason DSL works at all.
DOCSIS: Data Over Cable Service Interface Specification. The standard for cable internet — 3.0, 3.1, and 4.0 are the meaningful generations as of 2026.
Drop cable: The fiber that runs from a street splitter cabinet to your home's external network interface. Modern drops use bend-insensitive G.657.A2 fiber.
DSL: Digital Subscriber Line. A family of technologies that delivers internet over copper telephone wire. Being retired across the US and UK.
DSLAM: DSL Access Multiplexer. The ISP-side equipment that aggregates many copper pairs into IP backhaul. Lives in central offices or street cabinets (for FTTC).
DWDM: Dense Wavelength Division Multiplexing. Carries many wavelengths of light on the same fiber — typically 96 channels in the C-band — each at 400 Gbps or 800 Gbps.

E2 terms

EDFA: Erbium-Doped Fiber Amplifier. Boosts all wavelengths in a fiber simultaneously without converting light back to electricity. Used every 60–100 km on long-haul fiber.
Ethernet: The wired-LAN standard. Fiber ONTs hand off Ethernet to your router. Gigabit (1000BASE-T) is standard; 2.5GbE and 10GbE are increasingly common for multi-gig plans.

F3 terms

FTTC: Fiber To The Cabinet. Fiber runs to a street-side cabinet; copper VDSL2 covers the last few hundred meters to homes. Hybrid architecture.
FTTH: Fiber To The Home. The fiber connection runs all the way to your residence — no copper or coax in the last mile. Also written FTTP.
FWA: Fixed Wireless Access. Internet delivered to a home via cellular technology (T-Mobile Home Internet, Verizon 5G Home, AT&T Internet Air).

G4 terms

GEM port GEM: GPON Encapsulation Method port. A logical channel inside a GPON tree assigned to a specific subscriber. AES-128-CTR encryption is applied per GEM port.
GEO orbit GEO: Geostationary Earth Orbit, 35,786 km up. The satellite appears fixed in the sky, but signal-travel adds 600+ ms of unavoidable latency.
GGC: Google Global Cache. Caching appliances Google ships to ISPs and embeds in their networks — serves YouTube, Google Search, Maps tiles locally instead of from a distant origin.
GPON: Gigabit Passive Optical Network (ITU-T G.984). 2.488 Gbps downstream / 1.244 Gbps upstream shared across a tree of 32–64 homes. The dominant residential fiber tech.

H2 terms

Headend: Cable industry term for the central facility where the CMTS lives and where the cable plant terminates. Equivalent to a telco central office.
HFC: Hybrid Fiber-Coax. The cable industry's architecture: fiber from headend to a neighborhood node, coaxial cable to homes from there.

I1 term

IXP: Internet Exchange Point. A physical building (e.g., DE-CIX, AMS-IX, Equinix Ashburn) where many networks bring their cables to swap traffic directly.

J1 term

Jitter: Variability in latency. If pings vary from 30 to 200 ms, jitter is 170 ms — a video-call disaster. Fiber typically holds jitter below 1 ms.

L2 terms

Latency: Time for a packet to round-trip. Measured in milliseconds. Fiber typically 5–15 ms; cable 15–30 ms idle, much higher loaded; LEO satellite 20–40 ms; GEO satellite 600+ ms.
LEO orbit LEO: Low Earth Orbit, ~160–2,000 km up. Starlink lives here. Low latency, but satellites move fast and need handoffs every few minutes.

M1 term

mmWave: Millimeter wave — radio frequencies above ~24 GHz. Massive bandwidth but blocked by walls, leaves, and even humid air. Used as a 5G overlay in dense urban pockets.

O5 terms

OCA: Open Connect Appliance. Netflix's caching server — ships them to ISPs, embeds in their networks. Serves over 95% of Netflix traffic from inside the ISP.
OFDM: Orthogonal Frequency Division Multiplexing. Splits a wide channel into thousands of narrow sub-carriers. Used in DOCSIS 3.1 downstream, LTE/5G, Wi-Fi.
OFDMA: Orthogonal Frequency Division Multiple Access. Like OFDM but lets multiple users share the channel simultaneously by assigning each a subset of sub-carriers.
OLT: Optical Line Terminal. The ISP-side end of a passive optical network — lives in the central office and terminates PON trees on SFP+ optics.
ONT: Optical Network Terminal. The small white box on your wall that converts fiber's light pulses into Ethernet your router can use. Also called an ONU.

P6 terms

Peering: Two networks agreeing to exchange traffic directly, usually at no cost. The settlement-free peering done at IXPs is what makes the modern internet economically viable.
Phased array: An antenna with many small elements whose individual timings can be adjusted to electronically steer a beam — no moving parts. Starlink's dish has 1,280 of them.
PLC splitter PLC: Planar Lightwave Circuit splitter. A passive piece of fused glass that divides one fiber's light into many identical copies (typically 1:32 or 1:64). Zero power, decades of life.
PNI: Private Network Interconnect. A dedicated cross-connect between an ISP and a hyperscaler (Google, Meta, Amazon) — typically 100GE+ — bypassing the public IXP fabric.
PON: Passive Optical Network. Fiber architecture where everything between the OLT and the ONT is unpowered — including the splitters that broadcast one fiber's signal to many homes.
PSTN: Public Switched Telephone Network. The legacy analog telephone system. Being retired across the US and UK as carriers move to IP-based voice.

Q1 term

QUIC: A transport protocol built on UDP that powers HTTP/3. Eliminates head-of-line blocking, supports 0-RTT session resumption, and migrates connections across networks.

R2 terms

Rain fade: Loss of satellite signal during heavy precipitation. Ka-band (HughesNet/Viasat) is hit hardest; Ku-band (Starlink) is moderately affected; sub-6 GHz is immune.
RPKI: Resource Public Key Infrastructure. A cryptographic layer over BGP that lets networks verify which ASes are authorized to originate which IP prefixes. Defends against route hijacks.

S5 terms

SC/APC connector: The square fiber connector with a green-tipped ferrule used on most residential ONTs. The 'APC' angled-polish prevents reflected light from causing laser noise.
Single-mode fiber: Fiber with a 9 µm core that propagates light in a single transverse mode. Used for almost all telecom fiber — long distances, high bandwidth.
Splice: A permanent joint between two fibers. Fusion splicing melts the ends together with an arc; mechanical splicing aligns them with index-matching gel.
Submarine cable: An undersea fiber cable carrying intercontinental internet traffic. About 552 are active in 2026, with 8–24 fiber pairs each and EDFAs every 60–80 km.
Symmetric: Upload speed equal to download speed. Fiber does this natively (XGS-PON: 10/10 Gbps); cable usually doesn't (DOCSIS 3.1 download is 30–50× upload).

T3 terms

TDMA: Time Division Multiple Access. Used in PON upstream: the OLT assigns precise time slots so 32 ONTs can transmit on the same fiber without colliding.
Total internal reflection: When light hits the boundary between two materials at a shallow angle and the second has lower refractive index, all of it reflects back. The whole reason fiber works.
Transit: Paid upstream connectivity. A smaller network pays a larger one (typically a tier-1) to deliver traffic to the rest of the internet. Costs $0.30–$2/Mbps committed in 2026.

V2 terms

VDSL2: Very-high-bitrate DSL 2 (ITU-T G.993.2). Reaches ~100/40 Mbps within 300 m of a fiber-fed street cabinet. The DSL flavor most commonly deployed today.
Vectoring: Noise cancellation between copper pairs in the same bundle. Lets VDSL2 push higher rates by mathematically eliminating crosstalk across many lines.

W1 term

Wavelength: The color of light. GPON uses 1490 nm down / 1310 nm up; XGS-PON uses 1577 / 1270 nm. Different wavelengths don't interfere — they share the same fiber.

X1 term

XGS-PON: Ten-Gigabit Symmetric PON (ITU-T G.9807.1). 10 Gbps in both directions over the same fiber and splitters as legacy GPON. The modern fiber-to-the-home standard.