Ethernet Protocol (IEEE 802.3)
The definitive guide to Ethernet standards, frame structures, cabling types, and physical layer troubleshooting.
1. The Ethernet Frame (Layer 2)
Ethernet frames encapsulate Layer 3 packets. The structure below represents the standard Ethernet II (DIX) frame used in modern TCP/IP networks.
- Preamble (7B) & SFD (1B): Alternating 1s and 0s to synchronize receiver clocks. Ends with
10101011. Not visible in Wireshark (stripped by NIC). - EtherType (2B): Identifies the upper-layer protocol.
0x0800: IPv40x0806: ARP0x86DD: IPv60x8100: 802.1Q VLAN Tag (inserted after Src MAC if present).
- Payload: Minimum 46 bytes (padding added if smaller), Maximum 1500 bytes (MTU).
- FCS (4B): Frame Check Sequence. A 32-bit CRC. If the calculation fails, the frame is discarded silently.
2. Physical Layer (Layer 1) & Cabling
Understanding the physical medium is critical for troubleshooting "flapping" links or error counters.
Twisted Pair Standards (Copper)
| Category | Speed | Max Frequency | Use Case |
|---|---|---|---|
| Cat 5e | 1 Gbps (1000BASE-T) | 100 MHz | Standard legacy deployments. |
| Cat 6 | 10 Gbps (up to 55m) | 250 MHz | Modern standard for office drops. |
| Cat 6a | 10 Gbps (100m) | 500 MHz | Data centers, high-speed uplinks. |
Fiber Optic Standards
| Type | Mode | Core Size | Distance (approx) | Common Standard |
|---|---|---|---|---|
| OM3 / OM4 | Multi-Mode (MMF) | 50 microns | 300m - 550m | 10GBASE-SR (Short Reach) |
| OS1 / OS2 | Single-Mode (SMF) | 9 microns | 10km - 80km+ | 10GBASE-LR (Long Reach) |
Never mix Multi-Mode fiber (Aqua cable) with Single-Mode transceivers (Blue latch), or vice versa. The physics won't work.
SR = Short Reach (Multi-mode). LR = Long Reach (Single-mode).
3. Auto-Negotiation & Duplex
Modern Gigabit/10G links rely on Auto-Negotiation to exchange capabilities (Speed, Duplex, Pause Frames) using Fast Link Pulses (FLP).
The Duplex Mismatch Problem
A mismatch occurs when one side is Hard-coded (Full Duplex) and the other is Auto.
- Auto Side: Fails to see FLP negotiation. Falls back to default: Half Duplex (for 10/100) or fails entirely (Gigabit requires Auto).
- Result:
- Side A (Full): Sends packets anytime.
- Side B (Half): Listens. If it receives while transmitting, it thinks a Collision occurred. It stops and jams the signal.
- Symptoms: Low throughput,
Late Collisioncounters incrementing, CRC errors.
4. Power over Ethernet (PoE)
PoE delivers DC power to devices (Phones, APs, Cameras) over the same copper cable used for data.
| Standard | Name | Power at Source | Power at Device | Devices |
|---|---|---|---|---|
| 802.3af | PoE | 15.4 W | 12.95 W | VoIP Phones, Basic IP Cameras |
| 802.3at | PoE+ | 30 W | 25.5 W | WiFi 5/6 APs, PTZ Cameras |
| 802.3bt | PoE++ (4PPoE) | 60 W / 100 W | 51 W / 71 W | Laptops, Digital Signage, High-perf APs |
References
- IEEE 802.3 Standard - Ethernet physical and data link layer specifications.
- Cisco Guide to Ethernet Duplex and Speed Negotiation - Troubleshooting auto-negotiation failures.