Ethernet Evolution: From ALOHA to 100 Gbps

While the previous page surveyed Ethernet's speed evolution chronologically, this page provides an engineering-focused deep dive into each major speed tier. We'll examine the specific physical layer (PHY) technologies, encoding schemes, cable requirements, and design trade-offs that define each generation.

Understanding these details is essential for network engineers who must select appropriate media, troubleshoot physical layer issues, and design networks that balance performance, distance, and cost.

10 Mbps Ethernet established the physical layer framework that would evolve through all subsequent generations. Let's examine each 10 Mbps standard in detail:

10BASE5 (Thick Ethernet):

The original IEEE 802.3 physical layer used 50-ohm thick coaxial cable with the following specifications:

10BASE2 (Thin Ethernet/Cheapernet):

10BASE2 reduced cost and simplified installation while maintaining 10 Mbps performance:

10BASE-T (Twisted Pair Ethernet):

10BASE-T's star topology transformed Ethernet deployment:

Manchester Encoding Deep Dive:

All 10 Mbps Ethernet standards use Manchester encoding:

• Each bit period contains a transition in the middle
• '1' = low-to-high transition at bit center
• '0' = high-to-low transition at bit center
• Additional transitions at bit boundaries if consecutive bits are identical

Advantages:

• Self-clocking—receiver extracts clock from data transitions
• DC-balanced—equal positive and negative voltage times
• Simple to implement with 1970s technology

Disadvantage:

• 50% efficiency—20 Mbaud signaling for 10 Mbps data
• Limits usable cable bandwidth

Despite its inefficiency, Manchester encoding's simplicity and robustness made it ideal for Ethernet's first generation.

Fast Ethernet (IEEE 802.3u) defined three PHY standards optimized for different environments:

100BASE-TX: The Dominant Standard

100BASE-TX requires Category 5 (or higher) cabling and uses two pairs for transmission:

The 100BASE-TX Encoding Pipeline:

100BASE-TX uses a sophisticated three-stage encoding process:

Stage 1: 4B5B Encoding

• Each 4-bit group maps to a 5-bit code word
• Ensures at least 2 transitions per 5-bit group (for clock recovery)
• Example: 0000 → 11110, 1111 → 11101
• Efficiency: 80% (100 Mbps × 1.25 = 125 Mbps)

Stage 2: Scrambling (NRZI)

• Non-Return-to-Zero-Inverted encoding
• Transition on '1', no transition on '0'
• Scrambles data to break up long runs

Stage 3: MLT-3 (Multi-Level Transmit)

• Uses three voltage levels: +V, 0, -V
• Cycles through: +V → 0 → -V → 0 → +V
• Only transitions on '1' bits (from NRZI)
• Reduces fundamental frequency by 4×
• 125 Mbaud appears as 31.25 MHz signal

100BASE-T4: Legacy Infrastructure Support

100BASE-T4 enabled 100 Mbps over existing Category 3 cabling:

100BASE-T4's 8B6T encoding converts 8 data bits into 6 ternary symbols (+, 0, -). Each pair therefore carries 33.3 Mbps at only 25 Mbaud—well within Cat 3's 16 MHz bandwidth. However, the requirement for all four pairs and the lack of full-duplex support limited adoption. Cat 5 installation quickly became the default.

100BASE-FX: Fiber Optic Fast Ethernet

100BASE-FX omits MLT-3 because fiber optics easily support 125 Mbaud signaling. The 2 km full-duplex range made 100BASE-FX ideal for building-to-building connections and data center backbones, establishing fiber as the medium of choice for longer distances.

Gigabit Ethernet (IEEE 802.3z and 802.3ab) faced unique challenges: 10× faster than Fast Ethernet, it pushed the limits of copper cabling while requiring robust fiber solutions for data center deployments.

1000BASE-SX: Short Wavelength Fiber

1000BASE-LX: Long Wavelength Fiber

1000BASE-LX Mode Conditioning:

When using 1000BASE-LX with multi-mode fiber, a mode conditioning patch cord is required. The laser's narrow beam would excite only a small portion of the multi-mode fiber core, causing differential mode delay and signal degradation. The patch cord offsets the laser beam to excite more modes, spreading the signal evenly.

1000BASE-T: Gigabit over Copper

1000BASE-T (IEEE 802.3ab) achieved Gigabit speeds over standard Cat 5e cabling—a remarkable engineering achievement:

4D-PAM5 Encoding Explained:

1000BASE-T's 4D-PAM5 (4-Dimensional, Pulse Amplitude Modulation with 5 levels) is remarkably clever:

1. Five voltage levels: -2, -1, 0, +1, +2
2. Four pairs simultaneously: Creating a "4-dimensional" symbol space
3. Each pair carries 2 bits per symbol: 5 levels encode 2+ bits (2.32 bits theoretically)
4. Trellis coding: Adds redundancy for error correction (reduces to exactly 2 bits/symbol)
5. Simultaneous bidirectional: Each pair carries TX and RX using echo cancellation

Signal Processing Requirements:

1000BASE-T transceivers include sophisticated DSP:

• Echo cancellation: Removes local TX signal from RX path (same pair)
• NEXT cancellation: Removes interference from adjacent pairs
• FEXT cancellation: Removes far-end crosstalk
• Adaptive equalization: Compensates for cable frequency response
• Trellis decoding: Corrects errors using redundancy

10 Gigabit Ethernet (IEEE 802.3ae) marked a decisive break: full-duplex only, no CSMA/CD support. This freed designers from collision domain constraints, enabling optimization purely for point-to-point links.

10GbE Encoding: 64B/66B

Unlike 8B10B's 25% overhead, 64B/66B encoding adds only 3% overhead:

64B/66B Mechanics:

1. 64 data bits form a block
2. 2-bit sync header prepended (01 for data, 10 for control)
3. Block is scrambled (XOR with LFSR sequence) to ensure transitions
4. Receiver unscrambles and identifies block type from sync header

The reduced overhead is critical: at 10 Gbps, the 22.5% difference between 8B10B and 64B/66B represents 2.25 Gbps of unnecessary line rate—a significant engineering burden.

10 Gigabit Fiber Standards:

10GBASE-T: 10 Gigabit over Copper

IEEE 802.3an (2006) defined 10GBASE-T for copper twisted pair:

Short-reach copper: SFP+ DAC

For data center use, Direct Attach Copper (DAC) cables with SFP+ connectors offer a simpler alternative:

• 10GBASE-CR: Twinaxial copper, 7 meters maximum
• Lower cost: ~$30 vs. $300+ for optics
• Lower power: Passive cable uses no power; active cables under 1 watt
• Typical use: Top-of-rack switch to server connections

DAC cables dominate intra-rack connections where distance permits.

Beyond 10 Gbps, Ethernet standards increasingly target hyperscale data center requirements. Speed increases use lane parallelism and higher-order modulation.

25 Gigabit Ethernet (IEEE 802.3by, 2016):

25GbE emerged from data center demands for higher per-port bandwidth without the cost of 40GbE:

Why 25 Gbps?

25 Gbps is exactly the lane rate needed for 100G (4 × 25G) using PAM-4 modulation at practical signal-to-noise ratios. By defining 25GbE as a single-lane standard, IEEE enabled:

• Cost-effective server NICs (single lane, not quad)
• Linear scaling to 100G (simply use 4 lanes)
• Better utilization than 10G/40G steps

40 Gigabit Ethernet (IEEE 802.3ba, 2010):

40GbE was designed primarily for core network aggregation and uplinks:

100 Gigabit Ethernet (IEEE 802.3ba, 2010 and later amendments):

100GbE became the workhorse of hyperscale data centers:

PAM-4 Modulation:

At 50+ Gbps per lane, NRZ (PAM-2) signaling becomes impractical—electronics can't switch fast enough. PAM-4 (4-level modulation) encodes 2 bits per symbol:

• NRZ: 2 levels (0, 1), 1 bit/symbol → 100 Gbps needs 100 Gbaud
• PAM-4: 4 levels (00, 01, 10, 11), 2 bits/symbol → 100 Gbps needs 50 Gbaud

The trade-off: PAM-4's closer levels require ~7 dB better SNR. This is compensated by mandatory Forward Error Correction:

• KR4 FEC: Reed-Solomon RS(528,514) for ~100GBASE-CR4
• KP4 FEC: RS(544,514) for tighter error correction at higher speeds

With dozens of PHY standards across speed tiers, selecting the appropriate option requires understanding your requirements:

We've examined the technical details of each major Ethernet speed tier. Let's consolidate the key insights:

What's next:

Now that we understand the physical layer technologies at each speed tier, we'll examine the IEEE 802.3 standards body and standardization process that created these specifications.