Compare transmission media (twisted-pair copper, coaxial copper, optical fibre, free-space radio) on bandwidth, attenuation, distance, cost, electromagnetic immunity, and installation; justify the medium choice for a given application

What this dot point is asking

A communication channel is the physical medium that carries the modulated signal between transmitter and receiver. The HSC Telecommunications Engineering module expects you to know the main transmission media, the engineering criteria that distinguish them, and the reasoning behind a medium choice for a stated application.

The answer

Transmission media split into guided (signal confined to a physical conductor: copper, optical fibre) and unguided (signal propagates through free space: radio, microwave, optical line-of-sight). Each has characteristic bandwidth, attenuation, cost, EMI behaviour, and installation profile.

Twisted-pair copper

Two insulated copper conductors twisted together (the twist cancels external interference and crosstalk between adjacent pairs).

• Bandwidth. Up to several hundred MHz for high category cables (CAT6A supports 10 GbE Ethernet to 100 m).
• Attenuation. Around 20 dB per 100 m at gigabit frequencies, more at higher frequencies. Limits useful distance.
• Cost. Lowest per metre. Connector cost low.
• EMI immunity. Moderate (twisted pair design helps; shielding adds protection).
• Installation. Easy. Field-terminated connectors.
• Used for. Local-area networks (Ethernet), telephone subscriber lines, structured cabling in buildings.

Coaxial copper

A central conductor surrounded by an insulating dielectric, an outer braided conductor (ground), and an outer insulator.

• Bandwidth. High (broadcast TV bands and beyond, GHz range for short distances).
• Attenuation. Significant at high frequencies; better than twisted pair at given frequency but worse than fibre.
• Cost. Moderate. Connectors more expensive than twisted-pair.
• EMI immunity. Excellent (the outer conductor acts as a shield).
• Installation. Slightly more involved than twisted pair.
• Used for. Cable TV, older cable internet networks (DOCSIS), short-distance RF feeds (antenna connections, test equipment).

Optical fibre

A glass or polymer fibre carrying light by total internal reflection. A core (high refractive index) surrounded by cladding (lower refractive index); the core-cladding boundary reflects the light internally if the angle of incidence exceeds the critical angle.

• Single-mode fibre. Very small core (around 9 micrometres). Carries only one propagation mode; lowest dispersion; longest distance. Used in long-haul telecoms.
• Multi-mode fibre. Larger core (around 50 or 62.5 micrometres). Carries multiple modes; higher dispersion; shorter distance but cheaper transmitters (LEDs vs lasers). Used in shorter LAN runs and within data centres.

Operating wavelengths. Three common wavelength windows aligned with glass-fibre attenuation minima: 850 nm (multi-mode, short links), 1300 nm (single-mode, medium links), 1550 nm (single-mode, long-haul; lowest attenuation).

• Bandwidth. Very high (terabits per second per fibre using wavelength-division multiplexing). Bandwidth limited primarily by the transmitter and receiver electronics, not the fibre itself.
• Attenuation. Very low. Around 0.2 dB per km at 1550 nm in modern single-mode fibre, around 0.3 to 0.4 dB per km at 1300 nm, around 2 to 3 dB per km at 850 nm. Allows tens of kilometres without amplification.
• Cost. Fibre itself is now low-cost. Connectors, transceivers, and splicing equipment add cost. Per-bit cost is the lowest of any medium at scale.
• EMI immunity. Total (the light is not affected by electromagnetic fields).
• Installation. Skilled splicing required; fragile under sharp bends.
• Used for. Long-haul terrestrial telecoms, submarine cables, fibre-to-the-premises (FTTP), data centre backbones.

Free-space radio

Electromagnetic waves propagating through air or vacuum. Sub-categories by frequency:

• HF (3 to 30 MHz). Propagates via ionospheric reflection over long distances. Used for amateur radio, some maritime and aviation.

• VHF (30 to 300 MHz). Line of sight with some diffraction. Used for FM broadcast, aviation voice (AM), maritime, two-way radio.

• UHF (300 MHz to 3 GHz). Line of sight. Used for cellular (most bands), TV broadcast, Wi-Fi (2.4 GHz band).

• SHF / microwave (3 to 30 GHz). Line of sight; substantial atmospheric absorption at higher frequencies. Used for satellite, point-to-point microwave links, Wi-Fi (5 GHz / 6 GHz bands), 5G mid-band.

• EHF / millimetre wave (30 to 300 GHz). Very high atmospheric absorption; very short range; very high bandwidth. Used for 5G mmWave, some satellite, automotive radar.

• Bandwidth. Varies by frequency. Higher bands have more bandwidth available.

• Attenuation. Free-space path loss increases with frequency squared and distance squared. Atmospheric absorption peaks at specific frequencies (water vapour around 22 GHz; oxygen around 60 GHz).

• Cost. No physical medium cost. Antenna, radio, and spectrum licence costs.

• EMI immunity. None. Radio shares the spectrum; interference is a regulatory and technical issue.

• Installation. Antennas; line-of-sight planning; no trenching required.

• Used for. Broadcast, cellular, Wi-Fi, satellite, point-to-point microwave, IoT.

Medium-selection trade-offs

A typical decision framework:

• Distance. Long-haul terrestrial or submarine: single-mode fibre. Inter-building: fibre or microwave. In-building: copper twisted pair or short fibre.
• Bandwidth needed. Multi-gigabit: fibre. Hundred-megabit to gigabit: copper twisted pair or coaxial. Lower bandwidth: any medium.
• Mobility. Mobile or roaming users: radio (cellular, Wi-Fi). Fixed users: any.
• EMI environment. Heavy industrial, power-station, near high-voltage cables: fibre (immune). Office: any.
• Cost sensitivity. Lowest-cost short link: twisted pair. Lowest cost per bit at scale: fibre.
• Installation constraints. Existing trenching, ducts, conduit dictate choice often.

Examples in context

Example 1. Submarine cable systems. Trans-oceanic data is carried almost entirely on optical fibre submarine cables. Multiple cable systems link Australia to Asia, the US and Europe (e.g. SEA-ME-WE 5, ASC, INDIGO West/Central). Single-mode fibre with optical amplifiers (erbium-doped fibre amplifiers) every around 80 to 100 km maintain signal over thousands of kilometres. Capacity per cable runs to terabits per second. The engineering case is decisive: no other medium can deliver this bandwidth over this distance.

Example 2. Australian NBN multi-technology mix. The National Broadband Network uses different transmission media for different premises: fibre to the premises (FTTP, single-mode optical fibre to the home), fibre to the node (FTTN, optical to a street cabinet then VDSL2 copper to the home), fibre to the curb (FTTC, optical closer to the home), HFC (hybrid fibre coaxial, common in older cable TV areas), fixed wireless (radio to the home from a tower) and satellite (Sky Muster). The mix illustrates how different media are economic for different deployment contexts.

Try this

Q1. Compare twisted-pair copper, optical fibre and free-space radio on EMI immunity and explain why fibre is preferred for installations near high-voltage cabling. [4 marks]

• Cue. Twisted pair: moderate EMI immunity (twist cancels some interference; shielding helps further). Free-space radio: shares spectrum, susceptible to interference and intentional jamming. Optical fibre: total EMI immunity (light is not affected by electromagnetic fields). Near high-voltage cabling, electrical noise and induced currents would degrade copper signals; fibre is unaffected and is the engineering choice.

Q2. Justify the choice of single-mode rather than multi-mode optical fibre for a 50 km link between two exchanges. [4 marks]

• Cue. Single-mode has very low dispersion (light travels in a single propagation mode) and low attenuation at 1550 nm (around 0.2 dB / km). At 50 km, single-mode loses around 10 dB, well within typical link budget. Multi-mode has higher dispersion (modes arrive at slightly different times, smearing pulses) and higher attenuation at 850 nm (around 2 to 3 dB / km), limiting useful range to a few kilometres at gigabit rates. Single-mode is the right choice for any link beyond a few kilometres.

Q3. Explain why high-frequency radio (5G mmWave at 28 GHz, for example) needs cells with shorter range than lower-frequency cellular bands. [5 marks]

• Cue. Free-space path loss scales as frequency squared at fixed distance; mmWave frequencies have much higher path loss than sub-6 GHz cellular bands. Atmospheric absorption (water vapour around 22 GHz, oxygen around 60 GHz) further reduces range at higher frequencies. mmWave does not penetrate building walls or foliage as well as lower frequencies. Combined, the practical mmWave cell radius is on the order of hundreds of metres rather than kilometres. The engineering trade is more bandwidth per cell but more cells required for area coverage.