Communication Theory – Data Communication Model & Foundational Concepts

Simple Data-Communication Model

• Canonical block diagram: Source → Transmitter → Communication System (medium) → Receiver → Destination.
• Purpose: move information from its point of origin to its point of use with minimum loss, delay and error.

Source

• Origin of the data or message.
• Typical entities: computer, workstation, server, smartphone, PDA, peripheral or any communications equipment.

Transmitter

• Accepts raw data from the source and converts it into a form suitable for the chosen transmission medium.
• Operations may include: line coding, modulation, multiplexing, power scaling, pulse shaping, error-control coding, etc.

Communication System / Transmission Medium

• Physical path over which the signal propagates.
• Guided (wired) media: twisted-pair cable, coaxial cable, optical fibre.
• Unguided (wireless) media: air / vacuum supporting broadcast radio, terrestrial microwave, satellite microwave, infrared.
• Can span LAN, MAN, WAN, PAN, or other network scopes.
• When available bandwidth is high, the same medium can support simultaneous transmission of multiple independent signals.
  • Aggregate signal expressed as $s<em>1(t),\,s</em>2(t),\dots$.

Receiver

• Accepts the signal from the medium, performs the inverse operations of the transmitter and reconstructs an approximation of the original data $d'(t)$.
• Steps: amplification, filtering, demodulation/decoding, error detection & correction, clock recovery, buffering, delivery to destination.

Destination

• Final consumer of the information: host computer, storage device, display, peripheral, mobile terminal, etc.

Data, Signals & Their Properties

Data ($d(t)$)

• Entity that conveys meaning under mutually agreed conventions between sender & receiver.
• Types:
  • Analog data: continuous in value and time (e.g., voice, music).
  • Digital data: discrete symbols from a finite alphabet (e.g., ASCII text, sensor samples).

Signal ($s(t)$)

• Electric, electromagnetic or optical representation of data suitable for propagation over a medium.
• Types:
  • Analog signal: continuous amplitude (e.g., sinusoid).
  • Digital signal: discrete amplitude levels (e.g., NRZ waveform).

Natural vs. Synthetic Signals

• Natural: human voice, bird chirps, fragrances, biological chemical signals, bee dances.
• Synthetic: voltages on a wire, EM waves from antennas, light pulses in fibre, etc.

Dimensionality

• 1-D (time), 2-D (images), M-D (video, spatial-temporal fields).

Continuous–Discrete Taxonomy

• Continuous-time analog, discrete-time analog, quantised digital.
• Reminder: “All continuous-time signals are analog, but not all analog signals are continuous-time.”

Converting Data ↔︎ Signal

Purpose: match the form of information to the constraints of the medium (bandwidth, noise, channel impairments).

Selection criteria: medium type, available bandwidth, channel SNR, regulatory constraints, implementation cost.

Multiplexing (Sharing High Bandwidth Media)

• Goal: carry several independent data streams concurrently over a single physical path.
• Techniques:
  • Frequency-Division Multiplexing (FDM): allocate disjoint frequency bands.
  • Wavelength-Division Multiplexing (WDM): optical analogue of FDM using distinct light wavelengths.
  • Time-Division Multiplexing (TDM)
  • Synchronous TDM: fixed time slots pre-assigned.
  • Asynchronous (Statistical) TDM: slots dynamically assigned to active channels.

Error Detection and Correction

• After propagation we have $s'(t)\;\to\;d'(t)$. Compare $d'(t)$ with transmitted $d(t)$.
• If different → error has occurred.

Error Types

• Single-bit error.
• Burst error (two or more consecutive bits corrupted).

Detection Techniques

• Parity check (even/odd).
• Two-dimensional parity.
• Checksum.
• Cyclic Redundancy Check (CRC).

Error-Correcting Codes

• Add redundancy enabling the receiver to identify & correct certain error patterns without retransmission (e.g., Hamming, Reed-Solomon).

Flow Control & Automatic Repeat reQuest (ARQ)

• Flow control maintains a pace that the receiver can handle.

Techniques

• Stop-and-Wait Flow Control: sender transmits one frame, waits for acknowledgement.
• Sliding Window Flow Control: allow multiple outstanding frames, controlled by window size.

ARQ (Backward Error Control)

• Stop-and-Wait ARQ.
• Go-Back-N ARQ (retransmit from error onward).
• Selective Repeat ARQ (retransmit only erroneous frames).

Data Link Control (DLC)

• Ensures reliable, ordered, framed data transfer over a single physical link.

Key Components

• Frame Synchronisation: mark frame boundaries.
• Flow Control: regulate sender’s rate.
• Error Control: detection, retransmission/correction.
• Link Management: establish, maintain, release the link.

High-Level Data Link Control (HDLC)

• ISO standard DLC protocol.
• Features:
  • Types of stations: Primary, Secondary, Combined.
  • Data-transfer modes: Normal Response, Asynchronous Response, Asynchronous Balanced, etc.

Core Mathematical Notation Recap

• Data as a function of time: $d(t)$ (input), $d'(t)$ (reconstructed).
• Signal on medium: $s(t)$ (transmitted), $s'(t)$ (received).
• Multiple simultaneous signals: ${s<em>k(t)}</em>{k=1}^N$.

Key Insights & Real-World Context

• Living organisms’ survival hinges on correct signal processing – biological analogy emphasises the fundamental nature of communications engineering.
• Modern networks exploit layered conversion (data ↔︎ signal) and multiplexing to scale from voice links to global Internet backbones.
• Robust DLC with flow/error control underpins higher-layer protocols; failures here propagate upward, affecting application performance.