The Physical Layer

2012. Knight Capital Group rolled out a new trading algorithm. Forty-five minutes later the firm was down 440 million USD. The culprit: an 8-millisecond latency gap between servers. The physical layer killed the company faster than a kill-switch could fire.

• **HFT trading**: Virtu Financial runs direct optical lines between exchanges - every microsecond of latency is worth millions
• **Data centers**: Cat8 copper for 30-meter intra-rack runs vs single-mode fiber between buildings - a critical architectural choice
• **5G and industry**: 1-5 ms latency enables robot control and remote surgery - impossible on 4G's 50 ms

Claude Shannon and the channel limit

In 1948, Shannon published a mathematical proof of the upper bound on information rate over any physical channel in the Bell System Technical Journal. C = B log2(1 + S/N) is not empirical - it is a theorem. Bell Labs engineers spent 20 years trying to beat it and failed. Today 5G standards push as close to the Shannon limit as possible: 256-QAM at 27 dB SNR delivers 8 bits/symbol, hitting 99% of the theoretical maximum.

Предварительные знания

• OSI Model: 7 Layers

Copper cables: twisted pair

**Twisted pair** is the backbone of local networks worldwide. Eight copper conductors twisted into pairs inside one sheath. This cable wires up 90% of corporate networks on the planet.

Why twisted? The twist kills electromagnetic interference: noise hits both wires equally, and they cancel out at the receiver. More twists per meter, better noise rejection.

**UTP vs STP**: UTP (Unshielded) - no shield, cheaper, used in 90% of networks. STP (Shielded) - foil shielding, for industrial environments with strong interference.

The 100-meter limit is physics: signal attenuates. Anything longer demands repeaters or a jump to fiber. Google and Facebook design data centers so no server rack sits more than 30 meters from a ToR switch.

Fiber optics: light instead of electricity

**Fiber optics** sends data as pulses of light through glass or plastic strands. Light shrugs off electromagnetic interference, generates no crosstalk, and travels tens of kilometers without amplification. A single fiber cable carries 100 Tbps in transatlantic systems shipping in 2023.

Transatlantic cables are fiber bundles laid across the ocean floor. EDFA amplifiers boost the signal every 50-100 km. One cable can push petabits per second.

Fiber connectors: **LC** (small, common in servers), **SC** (square, common in telecom), **ST** (bayonet mount, legacy). Cleanliness rules everything - a single dust speck on the end face blocks the signal. Field engineers carry dedicated cleaning kits for exactly this reason.

Wireless transmission: Wi-Fi and more

**Wireless communication** moves data through air on electromagnetic waves. Wi-Fi, Bluetooth, 4G/5G, satellite internet - each runs on its own frequency band and protocol. Every band is a trade between speed, range, and ability to punch through walls.

**Wi-Fi channels**: 2.4 GHz has 14 channels (3 non-overlapping: 1, 6, 11). 5 GHz has more channels and less interference from neighbors. 6 GHz (Wi-Fi 6E) - a clean band, but few devices support it yet.

The wireless medium is **shared**: every device in range hears every other one. Collision and security problems follow - hence encryption (WPA3).

**Half-duplex**: Wi-Fi cannot transmit and receive on the same frequency at the same time. Wired Ethernet runs full duplex; Wi-Fi cannot. A hard physical constraint: a device's own outgoing signal drowns out anything inbound.

Signal encoding: how to transmit bits

Bits (0 and 1) need a physical representation. **Encoding** picks how: voltage level, signal transition, wave phase, or some combination. The encoding scheme dictates the channel's speed and reliability.

Heavier schemes solve that. **Manchester** encoding puts a transition in the middle of every bit: upward = 0, downward = 1. **4B/5B** swaps 4 bits for 5, killing off the "bad" combinations that have no transitions.

**PAM-4** (Pulse Amplitude Modulation 4-level) - 4 signal levels, transmits 2 bits per clock cycle. Used in 400G Ethernet. Requires quality cables and receivers with good SNR.

Bandwidth and latency

Two key network parameters: **bandwidth** - bits per second the channel can move, and **latency** - time for a packet to reach its destination. Mixing them up is a textbook system design blunder.

**Shannon's Law**: maximum channel capacity = B x log2(1 + S/N), where B is the bandwidth and S/N is the signal-to-noise ratio. A physical limit that no protocol can overcome.

Latency breaks down into: **propagation delay** (speed of light in the medium), **transmission delay** (time to clock out the bits), **processing delay** (router work), **queuing delay** (sitting in a buffer). Propagation is unbeatable - physics sets the floor.

Key ideas

• **Twisted pair** (UTP/STP) - the backbone of local networks, up to 100 m, Cat5e-Cat8 categories
• **Fiber optics** - light through glass, up to 100+ km, single-mode (longer range) and multi-mode (cheaper)
• **Wireless communication** - half-duplex, shared medium, requires encryption
• **Bandwidth vs Latency** - bandwidth matters for large files; latency matters for interactive applications
• **Shannon's Law** - physical upper bound on channel capacity, a proven theorem

What's next

The physical layer transmits bits. But how do devices know who the data is meant for?

• Data Link Layer — MAC addresses and Ethernet - addressing within a local network
• Switches — The devices that connect everything together

Вопросы для размышления

• Why do data centers typically use fiber optics between racks instead of twisted pair?
• Geostationary satellites orbit at 36,000 km. Calculate the minimum round-trip latency imposed by the speed of light alone.
• Why does Starlink use low orbit rather than geostationary?

Связанные уроки

• net-02-osi-overview — OSI model provides context for the physical layer
• net-04-data-link — Physical layer is the foundation data link sits on
• net-67-latency-numbers — Concrete latency numbers across transmission media
• net-15-tcp-basics — TCP lives above physics - the contrast matters
• arch-08-memory-hierarchy — Same speed/distance/cost trade-off logic
• net-59-troubleshooting — Network diagnostics always starts at the physical layer
• bt-03-serialization