| Category | Max Speed | Max Distance | Typical Use |
|---|---|---|---|
| Cat5 | 100 Mbps | 100 m | Legacy — avoid on new installs |
| Cat5e | 1 Gbps | 100 m | Common desktop/access cabling |
| Cat6 | 1 Gbps (10 Gbps to 55m) | 100 m / 55 m@10G | Standard modern install |
| Cat6a | 10 Gbps | 100 m | Data center, AP uplinks, PoE++ |
| Cat7 / Cat7a | 10 Gbps+ | 100 m | Shielded, high-EMI environments |
| Cat8 | 25/40 Gbps | 30 m | Server / switch short-reach links |
Unshielded Twisted Pair — 4 twisted pairs, no shielding. Cheapest, most common in offices.
Shielded / Foiled Twisted Pair — foil or braid around pairs, reduces EMI. Needs grounding.
Solid core = fixed runs, punch-down (better signal, less flexible). Stranded = patch cables (flexible, short runs).
Same standard both ends (B–B or A–A). Use for PC ↔ Switch, Switch ↔ Router — unlike-device links.
One end A, one end B. Historically for PC ↔ PC, Switch ↔ Switch — like-device links.
Virtually all modern NICs/switches auto-detect and flip TX/RX internally — crossover cables are rarely needed today.
| Standard | Name | Max Power (at PSE) | Power at Device |
|---|---|---|---|
| 802.3af | PoE | 15.4 W | ~12.95 W |
| 802.3at | PoE+ | 30 W | ~25.5 W |
| 802.3bt Type 3 | PoE++ | 60 W | ~51 W |
| 802.3bt Type 4 | PoE++ | 100 W | ~71 W |
Power riding on the data pairs: pins 1/2 and 3/6 (used by 10/100 Mbps links).
Power on the spare pairs: pins 4/5 and 7/8 (common on older PoE gear).
Switch (PSE) applies a small detection voltage first — only energizes full power once it detects a valid PD resistance signature.
Both ends exchange Fast Link Pulses (FLP) advertising supported speed/duplex; highest common capability wins.
Separate TX/RX pairs — send and receive simultaneously. Standard on switched links today.
Shared medium, one direction at a time — legacy hubs/collision domains. Rare on modern switch ports.
| Port LED | Meaning |
|---|---|
| Off | No link — cable, port, or far-end down |
| Solid amber/orange | Link up, often 10/100 Mbps (vendor-dependent) |
| Solid green | Link up at gigabit or higher |
| Blinking | Traffic activity on an established link |
NIC and switch port sense a valid electrical termination on the pairs (link presence). Idle/energy-detect signaling confirms a cable is actually connected before negotiation begins.
Layer 1 · electricalBoth sides send Fast Link Pulse bursts encoding supported speeds, duplex modes, and (for gigabit+) master/slave clocking and pair polarity. They agree on the best mutually supported combination.
IEEE 802.3 clause 28The switch port (or NIC) electrically swaps its TX/RX pair mapping if needed, so a straight-through cable works regardless of which pairs each end expects.
Layer 1 · line signalingBits are now converted to voltage/signal patterns matching the negotiated speed — e.g. Manchester (legacy 10M), MLT-3 (100M), PAM-5 (1000M) — and continuously exchanged as idle symbols even with no data, which is how link state stays "up".
Layer 1 · encodingWhen the PC has data to send, its NIC wraps it in a frame: destination MAC, source MAC (the NIC's own burned-in or configured address), EtherType, payload, and a trailing FCS checksum for error detection.
Layer 2 · framingThe NIC's PHY chip converts the frame's bits into the electrical line code from step 4 and drives them onto the twisted pairs toward the switch port.
Layer 1 → Layer 2 boundaryThe switch port's PHY recovers clocking from the signal, decodes it back into bits, reassembles the frame, and checks the FCS to confirm it arrived intact.
Layer 1 · receptionThe switch reads the frame's source MAC and records it against the ingress port in its MAC address table (CAM table) — so it now knows "this MAC lives out this port."
Layer 2 · switchingThe switch looks up the destination MAC. Known → forwarded out that single port only. Unknown/broadcast/multicast → flooded out all ports in the same VLAN except the one it arrived on.
Layer 2 · forwarding| Symptom | Likely Cause |
|---|---|
| No link light at all | Bad cable, wrong pinout, port shutdown, dead port/NIC |
| Link up, but very slow | Cable category too low, damaged pair, excessive length |
| Intermittent link drops | Loose connector, cable near EMI source (power, fluorescent) |
| Duplex mismatch symptoms | Late collisions, high CRC errors — one side forced, other auto |
| PoE device not powering | PSE budget exceeded, wrong PoE class, cable too long/thin |
| Check | How |
|---|---|
| Cable continuity | Cable tester — verify all 8 pins map correctly |
| Port errors/counters | show interface |
| Negotiated speed/duplex | show interface status |
| PoE power draw | show power inline |
| Physical swap test | Swap known-good cable/port to isolate fault side |