Highlights
Since the CM4 supports Auto-MDIX, you can route the TX pins to the RX pins on the transformer without any issues.
Ethernet
A family of wired networking technologies (IEEE 802.3) that has dominated local area networks since the mid-1980s. It defines how bits are framed, addressed, and physically transmitted across copper, fiber, and (historically) coaxial cabling. Modern Ethernet spans speeds from 10 Mb/s to 800 Gb/s on the same fundamental frame format, which is the main reason it has outlasted every competing LAN technology (Token Ring, FDDI, ATM, ARCnet).
How it works
Each Ethernet device has a globally unique MAC address (48 bits, e.g. a4:5e:60:01:23:cd). Data is packaged into frames of 64–1518 bytes (1522 with an 802.1Q VLAN tag) containing source MAC, destination MAC, an EtherType (which higher-level protocol — IPv4, IPv6, ARP, etc.), a payload, and a 32-bit CRC for error detection.
Originally Ethernet was a shared bus with CSMA/CD (Carrier Sense Multiple Access with Collision Detection) — devices listened, transmitted when the line was clear, and backed off if two collided. Modern Ethernet is fully switched and full-duplex: each device has a dedicated link to a switch port, so collisions are impossible and CSMA/CD is effectively obsolete.
Speed tiers
| Standard | Year | Common name | Speed | Typical media |
|---|---|---|---|---|
| 10BASE5 / 10BASE2 | 1983 / 1985 | Thicknet / Thinnet | 10 Mb/s | Coaxial bus |
| IEEE 802.3i — 10BASE-T | 1990 | Ethernet | 10 Mb/s | Cat3 twisted pair |
| IEEE 802.3u — 100BASE-TX | 1995 | Fast Ethernet | 100 Mb/s | Cat5 |
| IEEE 802.3ab — 1000BASE-T | 1999 | Gigabit Ethernet | 1 Gb/s | Cat5e |
| IEEE 802.3an — 10GBASE-T | 2006 | 10 GbE | 10 Gb/s | Cat6a / Cat7 |
| IEEE 802.3bz — 2.5/5GBASE-T | 2016 | Multi-Gig | 2.5 / 5 Gb/s | Cat5e / Cat6 (re-uses existing plant) |
| IEEE 802.3ba — 40/100GBASE | 2010 | 40 / 100 GbE | 40 / 100 Gb/s | Cat8, twinax, fiber |
| IEEE 802.3bs — 200/400GBASE | 2017 | 200 / 400 GbE | 200 / 400 Gb/s | Fiber, twinax |
| IEEE 802.3df — 800GBASE | 2022 | 800 GbE | 800 Gb/s | Fiber |
| 1.6TBASE (in dev) | ~2026 | 1.6 TbE | 1.6 Tb/s | Fiber |
Maximum segment length on copper is 100 m for almost everything from 10BASE-T through 10GBASE-T. Fiber variants (-SR, -LR, -ER, -ZR) extend that to hundreds of meters, kilometers, or tens of kilometers depending on optics.
Twisted-pair cable categories
| Cat | Bandwidth | Supports |
|---|---|---|
| Cat3 | 16 MHz | 10BASE-T (legacy) |
| Cat5 | 100 MHz | 100BASE-TX |
| Cat5e | 100 MHz | 1000BASE-T |
| Cat6 | 250 MHz | 1 Gb/s @ 100 m, 10 Gb/s @ ≤55 m |
| Cat6a | 500 MHz | 10 Gb/s @ 100 m |
| Cat7 / Cat7a | 600 / 1000 MHz | 10 Gb/s, shielded; rare in practice |
| Cat8 | 2000 MHz | 25/40 Gb/s @ 30 m (data center top-of-rack) |
Variants worth knowing
-T— twisted pair copper (most LAN gear).-SR/-LR/-ER/-ZR— fiber, short/long/extended/very-long range.-CR/ DAC (twinax) — short copper for server-to-switch in racks.- AVB / TSN (Time-Sensitive Networking) — extensions for deterministic real-time delivery; used in pro audio/video and increasingly in industrial and automotive.
- Industrial Ethernet — PROFINET, EtherCAT, EtherNet/IP — Ethernet wire and frames with deterministic timing for factory floors.
- Automotive Ethernet — single-pair variants like 100BASE-T1 and 1000BASE-T1 used in modern vehicles for ADAS and infotainment.
- **PoE (Power over Ethernet) — power-and-data over the same cable.
Quick history
- 1973 — Bob Metcalfe writes the founding Ethernet memo at Xerox, inspired in part by the ALOHAnet radio protocol.
- 1980 — DEC, Intel, and Xerox publish the DIX standard: 10 Mb/s over coax. The “Ethernet II” frame format from this draft is still what runs on every link today.
- 1983 — IEEE ratifies 802.3, the open standard. Token Ring and ARCnet compete for the next decade.
- 1990 — 10BASE-T (twisted pair) replaces coax. Star topology with hubs becomes the norm.
- 1995 — Fast Ethernet (100 Mb/s).
- 1999 — Gigabit Ethernet over copper. Switched full-duplex makes hubs obsolete.
- 2002 — 10 GbE arrives, initially fiber-only; copper variant follows in 2006.
- 2010s — 40 / 100 / 200 / 400 GbE roll out across data centers as the cloud era scales.
- 2022 — IEEE 802.3df defines 800 GbE; 1.6 TbE is in active development.