Unit 2: Physical layer

I. Orientation — Foundation of Data Transmission

The physical layer is Layer 1 of the OSI model and is responsible for transmitting raw bits across a communication medium. It converts data into electrical, optical, or electromagnetic signals and defines the mechanical, electrical, timing, and procedural characteristics of interfaces between devices.

• Core responsibility: Transfers a bit stream between directly connected nodes; it does not interpret addresses, frames, or application data.
• Physical representation: Represents binary digits through voltage levels, light pulses, or radio-frequency changes.
• Bit synchronization: Coordinates sender and receiver clocks so that the receiver can identify bit boundaries.
• Interface characteristics:
  • Mechanical: Connector shape, cable dimensions, and pin arrangement.
  • Electrical: Voltage levels, signal duration, and impedance.
  • Procedural: Rules governing activation, transmission, and deactivation.
• Transmission direction:
  • Simplex: Data moves in one direction only, as in a broadcast sensor.
  • Half-duplex: Both ends transmit, but not simultaneously, as with walkie-talkies.
  • Full-duplex: Both ends transmit simultaneously, as in modern switched Ethernet.
• Physical topology: Devices may be arranged using bus, star, ring, mesh, or hybrid connections.
• Performance measures: Bandwidth, data rate, latency, jitter, attenuation, and error rate determine link quality.
• Layer boundary: The physical layer transports bits; the data-link layer organizes those bits into frames.

II. Signals — Physical Representation of Information

A. Analog and digital signals

Analog and digital signals are the two basic forms used to represent information over a physical medium.

1. Analog signals
   • Definition: An analog signal varies continuously with time and can assume infinitely many amplitude values within a range.
   • Periodic form: A simple periodic analog signal is a sine wave described by amplitude, frequency, and phase.

s(t) = A sin(2πft + φ)

• Symbols:
  • s(t) is the instantaneous signal value at time t.
  • A is peak amplitude, commonly measured in volts.
  • f is frequency in hertz (Hz).
  • φ is phase in radians.
• Period-frequency relation:

T = 1/f

• Symbols: T is one cycle’s duration in seconds, and f is cycles per second.
• Composite signals: Voice and video contain multiple sine-wave components. Fourier analysis represents a composite periodic signal as a sum of harmonically related sine waves.
• Spectrum and bandwidth: The spectrum is the range of frequencies present; bandwidth is the difference between highest and lowest significant frequencies.
• Concrete example: A 1 kHz sine wave has a period of 1/1000 s = 1 ms.

2. Digital signals
   • Definition: A digital signal uses a finite set of discrete levels; a binary signal commonly assigns one level to 0 and another to 1.
   • Bit interval: The time required to transmit one bit is inversely related to bit rate.
   • Signal levels: A system with L distinct levels can carry more than one bit per signal element when L > 2.

Bits per signal element = log₂L

• Baseband transmission: Digital pulses are sent directly through a low-pass channel, as in an Ethernet cable.
• Broadband transmission: Digital data modulates an analog carrier so that it can pass through a band-pass channel.
• Explicit contrast: Analog signals vary continuously and are described chiefly by amplitude, frequency, and phase; digital signals use discrete levels and are described chiefly by bit intervals and transition patterns.

III. Transmission Performance — Capacity and Speed

A. Data rate

Data rate measures how much digital information a link transfers per unit time, normally in bits per second.

• Bit rate: The number of transmitted bits each second, expressed as bps, kbps, Mbps, or Gbps; network units normally use decimal multiples.
• Baud rate: The number of signal elements or symbols transmitted each second. Bit rate and baud rate are equal only when each symbol represents one bit.

Bit rate = Baud rate × Bits per symbol

• Nyquist limit: For an ideal noiseless channel, the maximum bit rate depends on bandwidth and the number of signal levels.

Rmax = 2B log₂L

• Symbols:
  • Rmax is maximum bit rate in bps.
  • B is channel bandwidth in Hz.
  • L is the number of discrete signal levels.
• Shannon capacity: For a noisy channel, theoretical capacity depends on bandwidth and signal-to-noise ratio.

C = B log₂(1 + S/N)

• Symbols:
  • C is channel capacity in bps.
  • B is bandwidth in Hz.
  • S/N is the linear signal-to-noise power ratio.
• Decibel conversion:

SNRdB = 10 log₁₀(S/N)

• Throughput: The useful rate actually achieved; protocol headers, congestion, retransmissions, and device limitations usually make throughput lower than nominal bandwidth.
• Goodput: The rate of useful application data, excluding headers and retransmitted information.
• Worked example: A noiseless 3 kHz channel using four levels has Rmax = 2 × 3000 × log₂4 = 12,000 bps.

IV. Signal Degradation — Limits on Reliable Communication

A. Transmission impairments

Transmission impairments alter a signal as it travels, reducing its strength or changing its shape.

• Attenuation: Signal power decreases with distance because energy is absorbed or dispersed by the medium. Amplifiers restore analog strength, while repeaters reconstruct digital pulses.

Gain or loss (dB) = 10 log₁₀(Pout/Pin)

• Pin and Pout are input and output powers.
• A negative result indicates loss; for example, Pout = Pin/10 gives −10 dB.
  • Distortion: Different frequency components experience unequal delay or attenuation, changing the composite signal’s shape. It is significant in channels carrying wide frequency ranges.
  • Noise: Unwanted energy becomes combined with the intended signal.
• Thermal noise: Random electron movement present in electronic components.
• Induced noise: Electromagnetic interference from motors, power lines, or radio transmitters.
• Crosstalk: A signal in one wire or channel couples into another.
• Impulse noise: Short, high-energy spikes caused by switching or lightning; these can corrupt multiple adjacent bits.
  • Delay: Propagation delay depends on distance and propagation speed.

Propagation delay = Distance / Propagation speed

• Transmission delay: The time needed to place all packet bits onto a link.

Transmission delay = Packet length / Data rate

• Jitter: Variation in packet arrival delay; real-time audio and video require buffering to compensate for it.
• Mitigation: Shielding, equalization, error detection, retransmission, forward error correction, and suitable signal power improve reliability.

V. Wired Transmission Paths — Bounded Signal Propagation

A. Guided media

Guided media carry signals through a solid physical path, giving predictable propagation and controlled coverage.

• Twisted-pair cable: Two insulated copper conductors are twisted to reduce electromagnetic interference and crosstalk.
  • UTP: Unshielded twisted pair is inexpensive and widely used for Ethernet and telephone wiring.
  • STP: Shielded twisted pair adds metallic shielding for stronger interference protection.
  • Connector: Ethernet installations commonly use an 8P8C modular connector, often called RJ-45.
  • Limitation: Copper links have greater attenuation and interference susceptibility than optical fibre.
• Coaxial cable: A central conductor is surrounded by dielectric insulation, a metallic shield, and an outer jacket.
  • Performance: Its shielding supports higher frequencies and better noise resistance than basic twisted pair.
  • Uses: Cable television, broadband access, CCTV, and radio-frequency connections.
• Optical fibre: Data travels as light through a glass or plastic core surrounded by lower-refractive-index cladding.
  • Principle: Total internal reflection confines light largely within the core.
  • Single-mode fibre: Uses a narrow core and one propagation mode, supporting long distances and high capacity.
  • Multimode fibre: Uses a wider core and multiple light paths, making it suitable for shorter links.
  • Advantages: Very high bandwidth, low attenuation, electrical isolation, low weight, and immunity to electromagnetic interference.
  • Limitations: Installation, splicing, optical transceivers, and repair require specialized equipment.
• Comparison: Twisted pair offers the lowest cost, coaxial cable provides stronger shielding, and fibre offers the greatest capacity and distance.

VI. Wireless Transmission Paths — Free-Space Propagation

A. Unguided media

Unguided media transmit electromagnetic waves through air, vacuum, or space without a continuous physical conductor.

• Radio waves: Typically radiate in many directions and can penetrate buildings, making them useful for broadcasting, mobile communication, Wi-Fi, and low-power networks.
  • Concern: Shared spectrum creates interference and requires frequency allocation and access control.
• Terrestrial microwaves: High-frequency directional waves travel mainly by line of sight between aligned antennas.
  • Uses: Point-to-point backhaul, cellular links, and building-to-building communication.
  • Constraint: Earth curvature, obstacles, and atmospheric conditions limit link distance.
• Satellite microwaves: An uplink sends signals from an earth station to a satellite, and a downlink returns them to another region.
  • GEO satellites: Orbit at about 35,786 km above the equator and appear stationary, but produce substantial propagation delay.
  • LEO satellites: Operate much closer to Earth, reducing delay while requiring moving constellations and handovers.
• Infrared: Short-range communication uses infrared light and usually does not pass through walls.
  • Uses: Remote controls, sensors, and room-confined device links.
• Propagation modes:
  • Ground wave: Follows Earth’s surface at relatively low frequencies.
  • Sky wave: Reflects or refracts through the ionosphere, enabling long-range radio.
  • Line of sight: Travels directly between antennas at higher frequencies.
• Limitations: Wireless links face interference, fading, obstruction, eavesdropping, weather effects, and spectrum constraints.
• Protection: Directional antennas, channel planning, power control, modulation, diversity, and encryption improve performance and security.

VII. Physical Connectivity Equipment — Extending and Directing Links

A. Networking devices

Networking devices connect hosts, regenerate signals, forward traffic, and provide access across network boundaries.

• Repeater: A Layer 1 device that receives a weakened digital signal, reconstructs its timing and shape, and retransmits it; it does not inspect addresses.
• Hub: A multiport repeater that copies incoming bits to every other port.
  • Limitation: All ports share bandwidth and one collision domain; hubs have largely been replaced by switches.
• Modem: A modulator-demodulator that converts digital data into a form suitable for an analog or carrier-based service and recovers the data at the destination.
• Network interface card: Connects a host to the medium and provides the physical transceiver; it also commonly performs Layer 2 framing and MAC addressing.
• Media converter: Converts one physical medium or signaling system into another, such as copper Ethernet to optical fibre.
• Wireless access point: Uses radio transceivers to connect wireless stations to a wired distribution network; it normally bridges frames rather than routing them.
• Bridge: A Layer 2 device that learns MAC addresses and selectively forwards frames between LAN segments.
• Switch: A high-port-count bridge that creates a separate collision domain per port and can support simultaneous full-duplex connections.
• Router: A Layer 3 device that examines network-layer addresses and selects paths between different IP networks.
• Gateway: Connects systems using dissimilar protocols or architectures, potentially translating data formats or application protocols.
• Operational distinction:
  • Physical-layer devices manipulate signals and bits: repeaters, hubs, transceivers, and media converters.
  • Higher-layer devices interpret structured information: switches use frames, routers use packets, and gateways may process application data.