Ethernet: The Local Network Standard

The Problem Ethernet Solved

In the early 1970s, computers were isolated machines. Bob Metcalfe at Xerox PARC realized that if multiple computers could share a common cable, they could communicate and share resources like printers and storage. His breakthrough was the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.

How CSMA/CD Works

On a shared Ethernet network (now called a "collision domain"), all computers compete for the same cable. Before transmitting, a computer "listens" to see if the cable is busy (Carrier Sense). If two computers transmit simultaneously, the voltages collide (Collision Detection). Both computers then send a 32-bit jam signal to ensure all stations detect the collision, then wait a random backoff time using a truncated binary exponential backoff algorithm. After 16 collisions, the frame is discarded. On a 10 Mbps Ethernet with a minimum frame size of 64 bytes, the maximum cable length is limited to approximately 2,500 meters to ensure a collision can be detected before transmission completes. The slot time is 51.2 µs (10 Mbps × 512 bit-times).

CSMA/CD efficiency degrades rapidly as utilization increases. At 30% utilization, throughput is reasonable; at 70%, collisions dominate and effective throughput collapses. This drove the transition from shared hubs to switched full-duplex Ethernet in the late 1990s, where CSMA/CD is no longer needed because each port is its own collision domain.

Evolution of Ethernet Media

• 10BASE5 (1983): Thick yellow coax cable (RG-8/U), 500m max segment, vampire taps, AUI connector. Maximum 100 stations per segment. NAP (Network Access Point) drops every 2.5m.
• 10BASE2 (1983): Thin RG-58 coax (Thinnet), BNC T-connectors, 185m max segment, 30 stations per segment. Cheaper and easier than thicknet but less reliable — one bad connector brought down the entire bus.
• 10BASE-T (1990): Unshielded twisted pair (Cat3/Cat5), RJ45 connectors, 100m max. Star topology with hubs (repeating hubs). Uses pairs 2-3 (pins 1,2,3,6). 10 Mbps with CSMA/CD.
• 100BASE-TX (1995): Fast Ethernet, Cat5 (4 pairs, 2 used), 100 Mbps. Uses MLT-3 encoding. 100m max. 4B/5B encoding at 125 MBaud. Auto-negotiation (IEEE 802.3u).
• 1000BASE-T (1999): Gigabit Ethernet, Cat5e/Cat6, all 4 pairs bidirectional. Uses PAM-5 (5-level pulse amplitude modulation) at 125 MBaud per pair = 1 Gbps. 100m max. Requires all 4 pairs — incompatible with older Cat5 cabling.
• 10GBASE-T (2006): 10 Gbps, Cat6a/Cat7, 100m max. Uses PAM-16 encoding. Auto-negotiation 1G/10G. Requires sophisticated DSP to handle alien crosstalk (AXT) from adjacent cables.

Cable categories for Ethernet: Cat5 (100 MHz, 1 Gbps to 100m), Cat5e (100 MHz, 1 Gbps with reduced crosstalk), Cat6 (250 MHz, 1 Gbps or 10 Gbps to 55m), Cat6a (500 MHz, 10 Gbps to 100m), Cat7 (600 MHz, 10 Gbps with shielded connectors), Cat8 (2 GHz, 25/40 Gbps to 30m for data centers).

The Switched Ethernet Revolution

The introduction of Ethernet switches in the 1990s transformed LANs. Instead of a shared cable, each device gets its own dedicated connection to the switch. Collisions became extinct, and full-duplex communication doubled throughput. A 24-port Gigabit switch provides 24 × 2 Gbps (full-duplex) = 48 Gbps of aggregate switching capacity. Modern switches use store-and-forwardswitching (reads entire frame before forwarding, checks CRC) orcut-through switching (forwards after reading destination MAC, lower latency).

VLANs (Virtual LANs, IEEE 802.1Q) partition a single physical switch into multiple logical networks. Each VLAN is a separate broadcast domain. A 4-byte 802.1Q tag is inserted into the Ethernet frame header (between source MAC and EtherType) containing a 12-bit VLAN ID (4096 VLANs) and 3-bit priority (CoS). VLANs enable network segmentation for security, traffic isolation, and QoS without physical rewiring. Trunk ports carry multiple VLANs; access ports belong to one VLAN.

Ethernet Today

Ethernet dominates local networks—homes, offices, data centers, and industrial facilities. While Wi-Fi handles client connections, Ethernet remains the backbone for its reliability, security, and ever-increasing speeds.

High-Speed Standards

• 25GbE (2016): 25 Gbps per lane. Single-lane SFP28 optics. Replaced 10GbE in hyperscale data centers. IEEE 802.3by.
• 40GbE / 100GbE (2017): 40G uses 4 × 10G lanes (QSFP+); 100G uses 4 × 25G lanes (QSFP28). Used for switch-to-switch links. IEEE 802.3ba.
• 200GbE / 400GbE (2017): 400G uses 8 × 50G lanes (QSFP-DD) or 4 × 100G. IEEE 802.3bs. Backbone links in cloud data centers.
• 800GbE (2020): 8 × 100G lanes. IEEE 802.3df. Uses 100G/lane PAM-4 modulation (4-level pulse amplitude modulation, 2 bits per symbol).
• 1.6 TbE (2025): 16 × 100G lanes. Emerging for next-gen AI/ML cluster interconnects.

Power over Ethernet (PoE)

IEEE 802.3af (2003, 15.4W), 802.3at (2009, 25.5W), 802.3bt (2018, 71.3W Type 3, 90W Type 4). Delivers DC power alongside data on the same Cat5e/Cat6 cable. Used for IP phones, WiFi access points, security cameras, and IoT devices. All 4 pairs carry power in 802.3bt (Type 4 uses all 8 wires at 0.6A per pair).

Time-Sensitive Networking (TSN)

IEEE 802.1 TSN extensions enable deterministic, low-latency Ethernet for industrial automation, audio/video (AVB), and automotive. Key standards: 802.1AS (time synchronization, sub-µs), 802.1Qbv (scheduled traffic with gate control lists), 802.1CB (frame replication and elimination for redundancy).

Automotive Ethernet

100BASE-T1 (802.3bw) and 1000BASE-T1 (802.3bp) use a single unshielded twisted pair (UTP) instead of the 4 pairs in standard Ethernet. 100BASE-T1 runs at 100 Mbps over a single pair up to 15m (in-car wiring). Used for ADAS sensors, infotainment, and backbone networks in vehicles.