Module 3 Cheat Sheet — The Physical Network
Three signal carriers
| Medium | Carries | Practical limits | Choose it when |
|---|---|---|---|
| Copper (UTP) | Voltage | 100 m per run; EMI/crosstalk-prone | In-building desk-to-switch runs; you want PoE |
| Fiber | Light | MM: hundreds of m · SM: kilometers | Between floors/buildings, high rates, noisy plants |
| Radio (Wi-Fi) | Radio waves | Shared air: interference, walls, eavesdropping | Mobility, or where cable cannot go |
Every medium fights the same enemies: attenuation (signal fades), interference (EMI + crosstalk) and its own bandwidth ceiling.
UTP categories that matter in 2026
| Category | Status | Sensible use |
|---|---|---|
| Cat5e | Huge installed base | 1G everywhere; 2.5G/5GBASE-T was designed for it |
| Cat6 / 6a | New-install default | 1–10G; 6a carries 10G the full 100 m |
| Cat8 | Data-center niche only | Short switch-to-server hops |
100 m total channel budget per copper run — patch cords included. Cat5 is obsolete for new installs; coax survives only via DOCSIS and MoCA, never as LAN cabling.
Fiber and connectors
- Multimode (MM) — wider core, cheaper optics, short reach (in-building).
- Single-mode (SM) — hair-thin core, laser optics, kilometers (campus/metro/WAN).
- Connectors: LC dominates · SC/ST legacy (old racks) · MPO = multi-fiber push-on for high-density parallel optics (40/100/400G).
PoE — copper's superpower
| Standard | Power | Typically feeds |
|---|---|---|
| 802.3af (PoE) | 15.4 W | VoIP phones, basic cameras |
| 802.3at (PoE+) | 30 W | Wi-Fi APs, PTZ cameras |
| 802.3bt (PoE++) | up to 90 W | Wi-Fi 6/7 APs, displays, laptops, LED lighting |
One cable = data + power — a first-class reason copper persists.
Device evolution ladder
Repeater (regenerated a clean signal — did not merely amplify, which would boost the noise too) → hub (multiplied to every port) → bridge (learned MACs) → switch (a multiport learning bridge — the only survivor in the LAN; everything before it is history). Routers connect networks and stop broadcasts. Firewalls filter traffic — today's term is NGFW (Module 7). A home "router" = switch + router + AP + firewall + DHCP in one box.
Inside the switch
- Learns which MAC lives on which port by reading source addresses.
- Forwards frames only where they belong; floods unknown destinations and broadcasts.
- Every port = its own collision domain (micro-segment) — collisions are gone.
VLANs in three lines
- A VLAN = a broadcast domain drawn by admin decision, not by cabling.
- Between switches, frames carry an 802.1Q tag naming their VLAN.
- Traffic between VLANs must be routed — the switch alone won't cross the line.
STP (awareness level)
Redundant links between switches create loops, and looping frames never die. Spanning Tree Protocol blocks the redundant path but keeps it as failover; the deployed flavor today is RSTP.
CSMA: then and now
CSMA/CD (collision detection) ruled shared coax/hub Ethernet — historical on today's switched full-duplex links. CSMA/CA (collision avoidance) lives on in every Wi-Fi channel: shared air still can't detect collisions mid-flight.
Network Essentials · Module 3 · Turning Point Academy — backbone: Al-Doori, Network Essentials, Ch. 3, 4 & 7.