Data Transmission across Media

Computer Networks and Internet Basics (9hrs) · Computer Fundamentals & Emerging Technologies

1. What Travels, and How

Data is transmitted as electromagnetic signals: as voltage/current in copper, as light pulses in fibre, and as radio/microwave/infrared waves in free space. The message is first converted into a signal, sent over the medium, and converted back at the receiver. What the medium determines is the achievable bandwidth, distance, error rate and cost.

2. Analog and Digital Signals

Analog signal — varies continuously over time (sine wave), described by amplitude, frequency and phase; telephone speech, radio.
Digital signal — takes only discrete levels (0 and 1); computer data, Ethernet, USB.
Bit rate (bps) = bits per second; baud rate = signal changes per second; for a signal with M levels: bit rate = baud rate × log₂ M.
A modem is needed to move between the two worlds: modulation converts digital → analog for the medium, demodulation analog → digital at the far end.

3. Transmission Modes

Mode	Direction	Example
Simplex	One direction only	Keyboard → computer, TV broadcast, radio
Half duplex	Both directions, but only one at a time	Walkie-talkie, old Ethernet hub LAN
Full duplex	Both directions simultaneously	Telephone, modern switched Ethernet, fibre links

4. Serial vs Parallel Transmission

Basis	Serial	Parallel
Bits sent	One at a time, one line	Several bits at once, several lines
Speed	Low per clock but good for long distance	High over short distance
Cost / cabling	Cheap, 1-2 wires	Expensive, many wires
Skew / crosstalk	Not a problem	Bits arrive at slightly different times
Distance	Long (USB, SATA, RS-232, fibre)	Very short (old printer ports, internal buses)
Modern trend	Universal (USB, PCIe, SATA are serial)	Largely replaced

5. Synchronous vs Asynchronous Transmission

Basis	Asynchronous	Synchronous
Framing	Each character has start and stop bits	Blocks/frames sent continuously with sync characters
Clock	No shared clock; resynchronised per character	Sender and receiver share a clock
Efficiency	Lower (2-3 extra bits per byte)	Higher (little overhead)
Speed	Low-medium	High
Example	Keyboard/COM port, UART, old modems	Ethernet, SONET, USB bulk transfers, HDLC

6. Line Coding (how bits become signal levels)

   Bits          1 0 1 1 0 0 1 0
   NRZ-L         ─┐_┌┐__┌─┐_        (1 = high, 0 = low; DC drift problem)
   Manchester    ┌┐┌┐┌─┐┌─┐        (transition in the MIDDLE of every bit
                                    carries the clock → self-synchronising)

Manchester encoding is used in classic 10 Mbps Ethernet and differential Manchester in Token Ring — the guaranteed mid-bit transition keeps the receiver's clock synchronised while encoding the data.
4B/5B and 8B/10B line codes (100 Mbps Ethernet, PCIe, SATA) add extra bits to guarantee transitions and keep DC balance.

7. Modulation Techniques (digital data over analog media)

Technique	Varies	Levels	Typical use
ASK (Amplitude Shift Keying)	Amplitude	2	Optical fibre (light on/off), simple remote links
FSK (Frequency Shift Keying)	Frequency	2	Modems, Bluetooth, caller ID
PSK (Phase Shift Keying)	Phase	2, 4, 8	Wi-Fi, satellite, 4G/5G (BPSK, QPSK)
QAM (Quadrature Amplitude Modulation)	Amplitude + phase	16, 64, 256, 1024	Cable modems, DSL, Wi-Fi 6, digital TV, 5G

Higher-order QAM (256-QAM = 8 bits per symbol) packs more bits per baud but needs a cleaner channel — noise reduces the usable order, which is exactly the trade-off Shannon's formula bounds.

8. Multiplexing — Sharing One Medium

Technique	How it shares	Example
FDM (Frequency Division)	Different carrier frequencies	Cable TV, FM broadcast, ADSL (voice + data)
TDM (Time Division)	Fixed time slots, one channel each	T1/E1 telephone lines, SDH/SONET
Statistical TDM	Time slots allotted on demand	Packet networks, statistical multiplexers
WDM / DWDM (Wavelength Division)	Different light wavelengths in one fibre	Fibre backbones, undersea cables, ISP links
CDM/CDMA (Code Division)	Different orthogonal codes, same band	3G cellular

9. Signal Impairments

Attenuation — the signal weakens with distance and frequency; needs amplifiers (analog) or repeaters (digital). Measured as gain/loss in decibels: dB = 10 × log₁₀(P₂ / P₁); a 3 dB loss means half the power, 10 dB means one tenth.
Distortion — different frequency components of a composite signal travel at slightly different speeds and arrive with changed shape (delay distortion).
Noise — thermal (random electron motion, present everywhere), crosstalk (signal from a neighbouring wire/fibre), impulse (sudden spikes from lightning, motors, switching). Noise sets the practical upper limit on data rate (Shannon's theorem).
Propagation effects in wireless — fading, multipath reflection, interference, rain attenuation (microwave).

10. Baseband vs Broadband

Baseband transmission — the entire medium capacity carries one digital signal (Ethernet LANs); needs no modulation; distance limited.
Broadband transmission — the medium is divided into multiple channels, each carrying its own modulated signal (cable TV/internet, DSL, FDM links).

11. Exam Points

Differentiate simplex, half duplex and full duplex with examples — 3-5 marks, frequent.
Compare serial and parallel transmission — 5 marks, frequent.
Differentiate synchronous and asynchronous transmission — 5 marks.
Explain ASK, FSK, PSK and QAM — 5 marks; state where QAM is used.
Explain multiplexing: FDM, TDM, statistical TDM and WDM with examples — 5 marks, very frequent.
Explain attenuation, distortion and noise; what is crosstalk? — 5 marks.
What is line coding? Explain NRZ and Manchester encoding with a diagram — 3-5 marks.
Differentiate baseband and broadband transmission — 3 marks.

Data Transmission across Media