Physical Layer

The physical layer (Layer 1) is responsible for transmitting raw bits over a communication medium. It defines how bits are converted to physical signals (electrical, optical, or electromagnetic) and transmitted across a link.

Transmission Media

Twisted Pair Copper

Pairs of copper wires twisted together to reduce electromagnetic interference (EMI). Used for Ethernet and telephone.

Categories:

• Cat5e: 1 Gbps up to 100 m.
• Cat6: 10 Gbps up to 55 m.
• Cat6a: 10 Gbps up to 100 m.
• Cat8: 25/40 Gbps, data centers.

UTP (Unshielded Twisted Pair): standard office/home wiring.

STP (Shielded Twisted Pair): shielded for environments with high EMI.

Coaxial Cable

Central conductor surrounded by insulator and mesh shield. Higher bandwidth and noise immunity than twisted pair.

Uses: cable TV (CATV), broadband internet (DOCSIS), older Ethernet (10BASE2).

Fiber Optic

Transmits light pulses through a glass or plastic core. Very high bandwidth; immune to EMI; very low attenuation.

Single-mode (SMF): narrow core (~8 µm); laser light source; low attenuation; long-distance (up to hundreds of km). Used in WAN and metro fiber.

Multi-mode (MMF): wider core (~50 µm); LED or VCSEL source; higher attenuation; short-distance (up to ~500 m). Used in data center.

Bandwidth: 100 Gbps per wavelength; terabits per second with WDM (Wavelength Division Multiplexing, multiple wavelengths per fiber).

Wireless

Wi-Fi (IEEE 802.11): 2.4 GHz (longer range, more interference), 5 GHz (shorter range, less interference, faster), 6 GHz (Wi-Fi 6E, less congestion).

Standard	Max speed	Frequency
802.11n (Wi-Fi 4)	600 Mbps	2.4/5 GHz
802.11ac (Wi-Fi 5)	3.5 Gbps	5 GHz
802.11ax (Wi-Fi 6/6E)	9.6 Gbps	2.4/5/6 GHz
802.11be (Wi-Fi 7)	46 Gbps	2.4/5/6 GHz

Cellular: 4G LTE (100 Mbps average), 5G (sub-6 GHz: 100-900 Mbps; mmWave: 1-10 Gbps).

Bluetooth: short range (10 m typical); 2.4 GHz; low power. Used for personal devices.

Signal Encoding

Physical bits must be encoded as physical signals.

NRZ (Non-Return to Zero): 1 = high voltage, 0 = low voltage. Simple; but long runs of the same bit cause DC drift and loss of clock synchronization.

NRZI (NRZ Inverted): a transition = 1, no transition = 0 (or vice versa). Better for long runs of 1s.

Manchester encoding: each bit has a transition in the middle. 1 = high-to-low, 0 = low-to-high. Self-clocking; halves bandwidth (each bit uses two signal periods).

4B/5B encoding: encode every 4 bits as 5 bits such that no more than one leading zero and no more than two trailing zeros appear (ensures transitions). Used by Fast Ethernet.

8B/10B encoding: 8 bits -> 10 bits. Used by Gigabit Ethernet, Fibre Channel.

64B/66B encoding: more efficient (3% overhead vs. 20% for 8B/10B). Used by 10 Gbps+ Ethernet.

Modulation

For wireless and high-speed wired links, bits are modulated onto a carrier wave.

AM (Amplitude Modulation): vary signal amplitude to represent bits.

FM (Frequency Modulation): vary signal frequency.

PSK (Phase Shift Keying): vary signal phase. BPSK (1 bit/symbol), QPSK (2 bits/symbol).

QAM (Quadrature Amplitude Modulation): vary both amplitude and phase. 16-QAM (4 bits/symbol), 64-QAM (6), 256-QAM (8), 1024-QAM (10), 4096-QAM (12). Higher-order QAM requires better signal-to-noise ratio.

OFDM (Orthogonal Frequency Division Multiplexing): divide the channel into many narrow subcarriers; modulate each independently. Robust to multipath fading. Used in Wi-Fi, LTE, 5G, ADSL.

Shannon’s Theorem

The theoretical maximum data rate (channel capacity) of a noisy channel:

\[C = B \log_2\left(1 + \frac{S}{N}\right)\]

Where:

• $C$: channel capacity in bits/second.
• $B$: bandwidth in Hz.
• $S/N$: signal-to-noise ratio (linear, not dB).

Example: 1 MHz bandwidth, SNR = 1000 (30 dB):

\[C = 10^6 \times \log_2(1001) \approx 10^6 \times 10 = 10 \text{ Mbps}\]

Real systems cannot exceed Shannon capacity, but modern coding schemes (turbo codes, LDPC, polar codes) come within a fraction of a dB.

Nyquist Rate

For a noiseless channel with bandwidth $B$ Hz and $M$ signal levels:

\[\text{Max rate} = 2B \log_2 M\]

Example: 3000 Hz telephone channel with 8 signal levels: $2 \times 3000 \times 3 = 18{,}000$ bps. This is the Nyquist limit; noise further limits achievable rate to the Shannon capacity.

Multiplexing

Allow multiple signals to share a single physical medium.

FDM (Frequency Division Multiplexing): divide the frequency band into sub-bands, one per signal. Used in cable TV, radio, ADSL.

TDM (Time Division Multiplexing): each signal gets the full bandwidth for a time slot in rotation. Used in T1/E1, SONET.

WDM (Wavelength Division Multiplexing): each signal uses a different wavelength of light. DWDM (Dense WDM): 80-160 channels per fiber, each at 100 Gbps. Used in long-haul optical networks.

CDM (Code Division Multiplexing / CDMA): each user has a unique spreading code; all transmit on the same frequency simultaneously. Used in 3G cellular, GPS.

OFDMA (Orthogonal FDM with Multiple Access): assign different subcarriers to different users dynamically. Used in LTE, 5G, Wi-Fi 6.