Communication Compact

ece-1109 ECE-1109 Introduction to ECE

1. Communication Systems Fundamentals

Definition & Components Communication is the exchange of information (Transmission + Reception + Processing).

• 5 Basic Components: Data Source $\to$ Transmitter $\to$ Transmission Medium $\to$ Receiver $\to$ Destination.
• Modem: Modulates digital to analog (for transmission) and demodulates analog to digital (for reception).

Transmission Modes

Mode	Direction	Example
Simplex	One-way only	TV Broadcasting, Keyboard to CPU
Half-Duplex	Both ways, but one at a time	Walkie-Talkie, Police Radio
Full-Duplex	Both ways simultaneously	Mobile Phones, Telephone Network

Data Transmission Methods

• Asynchronous: Data sent as bytes/chars with Start & Stop bits. Gaps exist between data units (Low efficiency).
• Synchronous: Data sent in blocks/frames. Requires clock synchronization. No gaps (High efficiency).
• Isochronous: Real-time data (Audio/Video). Guarantees data arrives at a fixed rate with no jitter (delay difference).

The OSI Model (Bottom-Up)

Layer #	Layer Name	Function	Examples/Protocols
7	Application	User interface, network services	HTTP, FTP, SMTP, DNS
6	Presentation	Data format, encryption, compression	JPEG, MPEG, SSL/TLS
5	Session	Establishes, manages sessions	NetBIOS, RPC
4	Transport	End-to-end connections, reliability	TCP, UDP
3	Network	Routing, logical addressing	IP, ICMP, IGMP
2	Data Link	Frame transmission, error detection	Ethernet, PPP, MAC
1	Physical	Physical, Data link, Network, Transport, Session, Presentation, Application	Cables, hubs, signals

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2. RF Spectrum & Modulation

Why Modulate?

1. Antenna Height: Height must be $\lambda =v/f$. Higher frequency = smaller antenna.
2. Multiplexing: Prevents signal mixing.
3. Range: High frequencies travel further.

Bandwidth Requirements

• AM (Amplitude Mod): $BW=2f_m$
• FM (Frequency Mod): $BW=10f_m$
• PM (Phase Mod): $BW=6f_m$
• Digital: $BW\ge \text{Bit Rate}/2$

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3. Transmission Media

Guided Media (Wired)

• Twisted Pair:
  • UTP (Unshielded): Cheap, easy, susceptible to noise (Telephone/LAN).
  • STP (Shielded): Metal braid reduces noise, expensive, heavy.
• Coaxial Cable: Better bandwidth (500MHz), less noise than twisted pair. Used for Cable TV.
• Optical Fiber:
  • Uses light (LED/Laser) and Total Internal Reflection.
  • Advantages: High capacity (Gbps), lightweight, immune to EM interference, long repeater spacing.
  • Modes: Single-mode (long distance, straight path) vs. Multimode (Step-index or Graded-index).

Unguided Media (Wireless)

• Radio Waves: Omnidirectional (multicast), penetrate walls.
• Microwaves: Unidirectional (unicast), Line-of-Sight required, cannot penetrate walls well.
• Infrared: Short-range, cannot penetrate walls.

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4. Signal Impairments & Electronics

Types of Impairments

1. Attenuation: Loss of signal strength over distance (exponential). Measured in dB.
2. Distortion: Change in signal shape/waveform. Critical in digital data.
3. Noise:
   • Thermal: Electron agitation (Temperature dependent, cannot be eliminated).
   • Intermodulation: Frequencies mixing to create artifacts ($f_1+f_2$).
   • Crosstalk: Signal coupling between wires (NEXT = Near-End, FEXT = Far-End).
   • Impulse: Spikes from lightning/power lines (Corrupts digital data blocks).

Analog vs. Digital Systems

Feature	Analog	Digital
Noise Immunity	Poor	Good
Privacy/Security	Low	High (Encryption)
Bandwidth	Low required	High required
Power	High consumption	Low consumption

Electronic Devices

• Carbon Microphone: Sound pressure moves diaphragm $\to$ compresses carbon granules $\to$ resistance changes $\to$ current changes.
• Speaker: Converts electrical energy to mechanical (sound) via magnet and voice coil.
• Amplifiers: Increase signal strength.
  • Gain (dB): $10{\log }_{10}(P_{out}/P_{in})$ or $20{\log }_{10}(V_{out}/V_{in})$.
  • Bandwidth: Frequencies where gain $\ge 70.7\%$ of max gain.

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6. Field Effect Transistors (FET)

JFET (Junction FET)

• Control: Voltage-controlled device (Current controlled by $V_{GS}$).
• Terminals: Drain (D), Source (S), Gate (G).
• Shockley’s Equation: $I_D=I_{DSS}(1-\frac{V_{GS}}{V_P}{)}^2$
  • $V_P$ (Pinch-off Voltage): The $V_{GS}$ where $I_D$ drops to 0.
  • $I_{DSS}$: Max current when $V_{GS}=0$.

MOSFET (Metal Oxide Semiconductor FET)

• Depletion (D-MOSFET):
  • Normally ON.
  • Can operate in Depletion mode (negative Gate voltage) or Enhancement mode (positive Gate voltage).
• Enhancement (E-MOSFET):
  • Normally OFF.
  • Requires a gate voltage ($V_{GS}$) to induce a channel. Current flows only when $V_{GS}>V_{Threshold}$.