Kuzee
Computer Networks

Transmission Media

Transmission Media

The transmission media is the physical carrier of data in a network. Each medium has its own characteristics and its own suitable applications, and choosing the right medium has a significant effect on network performance.

1. Wired Transmission Media

1.1 Twisted Pair

Structure:

  • Two insulated conductors twisted together according to a defined pattern
  • A cable usually consists of several twisted pairs
  • The outside is covered by a protective sheath

Classification:

By shielding:

  • Unshielded Twisted Pair (UTP): no shielding, low cost
  • Shielded Twisted Pair (STP): has shielding, strong resistance to interference

By performance category:

  • Cat3: 16MHz, used for telephone lines
  • Cat5: 100MHz, used for 100Mbps Ethernet
  • Cat5e: 100MHz, used for 1000Mbps Ethernet
  • Cat6: 250MHz, used for 1000Mbps Ethernet
  • Cat6a: 500MHz, used for 10Gbps Ethernet
  • Cat7: 600MHz, used for 10Gbps Ethernet

Characteristics:

  • Low cost, simple installation
  • Moderate resistance to interference
  • Limited transmission distance (within 100 metres)
  • Suitable for local area networks

Applications:

  • Ethernet connections
  • Telephone lines
  • Local area network cabling

1.2 Coaxial Cable

Structure:

  • Central conductor (copper wire)
  • Insulating layer
  • Outer conductor (shield)
  • Protective sheath

Classification:

By impedance:

  • 50Ω: used for digital transmission
  • 75Ω: used for analog transmission

By diameter:

  • Thick coax: larger diameter, longer transmission distance
  • Thin coax: smaller diameter, easier to install

Characteristics:

  • High bandwidth, strong resistance to interference
  • Relatively long transmission distance (within 500 metres)
  • Moderate cost
  • Relatively complex installation

Applications:

  • Cable television
  • Broadband access
  • Early Ethernet

1.3 Optical Fibre

Structure:

  • Core: glass with a high refractive index
  • Cladding: glass with a low refractive index
  • Protective layer: a plastic jacket

Classification:

By transmission mode:

  • Single-mode fibre: transmits only one mode, long distance
  • Multi-mode fibre: transmits several modes, shorter distance

By core diameter:

  • 50μm multi-mode fibre
  • 62.5μm multi-mode fibre
  • 9μm single-mode fibre

Characteristics:

  • Extremely high bandwidth (up to the THz range)
  • Very strong resistance to interference
  • Long transmission distance (tens of kilometres)
  • Higher cost
  • Installation requires professional equipment

Applications:

  • Backbone networks
  • Long-distance transmission
  • High-speed networks

2. Wireless Transmission Media

2.1 Radio Waves

Characteristics:

  • Long transmission distance
  • Strong penetration
  • Easily affected by interference
  • Requires a licence

Bands:

  • Low frequency (LF): 30-300kHz
  • Medium frequency (MF): 300kHz-3MHz
  • High frequency (HF): 3-30MHz
  • Very high frequency (VHF): 30-300MHz
  • Ultra high frequency (UHF): 300MHz-3GHz

Applications:

  • Broadcasting
  • Mobile communication
  • Satellite communication

2.2 Microwaves

Characteristics:

  • Long transmission distance
  • High bandwidth
  • Requires line-of-sight propagation
  • Easily affected by the weather

Classification:

  • Terrestrial microwave: point-to-point transmission
  • Satellite microwave: relayed through satellites

Applications:

  • Long-distance telephone calls
  • Television broadcasting
  • Satellite communication

2.3 Infrared

Characteristics:

  • Short transmission distance
  • Strong directionality
  • Unaffected by electromagnetic interference
  • Easily blocked by obstacles

Applications:

  • Remote controls
  • Short-range communication
  • Infrared data transfer

2.4 Laser

Characteristics:

  • Very strong directionality
  • High bandwidth
  • Long transmission distance
  • Easily affected by the weather

Applications:

  • Space communication
  • Point-to-point communication
  • Optical fibre communication

3. Performance Metrics of Transmission Media

3.1 Bandwidth

Definition: the highest frequency range a transmission medium can carry

Unit: Hz

Factors:

  • The physical properties of the transmission medium
  • The transmission distance
  • Environmental factors

3.2 Attenuation

Definition: the loss of signal power during transmission

Unit: dB/km

Factors:

  • The transmission distance
  • The transmission frequency
  • The characteristics of the medium

3.3 Delay

Definition: the time a signal needs to travel from the sending end to the receiving end

Unit: μs/km

Factors:

  • The transmission distance
  • The transmission medium
  • The signal frequency

3.4 Interference Resistance

Definition: the ability of a transmission medium to withstand external interference

Factors:

  • The shielding effectiveness
  • The transmission frequency
  • Environmental noise

4. Choosing a Transmission Medium

4.1 Factors to Consider

  1. Transmission distance

    • Short distance: twisted pair, infrared
    • Long distance: optical fibre, microwave

  2. Transmission rate

    • Low speed: twisted pair, radio waves
    • High speed: optical fibre, laser

  3. Cost

    • Low cost: twisted pair, radio waves
    • High performance: optical fibre

  4. Environmental factors

    • Indoors: twisted pair, infrared
    • Outdoors: optical fibre, microwave

  5. Security requirements

    • High security: optical fibre
    • Ordinary security: twisted pair

4.2 Common Application Scenarios

Local area networks:

  • Twisted pair: offices, homes
  • Optical fibre: data centres, backbone networks

Wide area networks:

  • Optical fibre: long-distance transmission
  • Microwave: wireless access

Mobile communication:

  • Radio waves: cellular networks
  • Microwave: links between base stations

5.1 Fibre Optic

Trends:

  • Fibre to the home (FTTH)
  • Fibre to the office (FTTO)
  • All-optical networks

Advantages:

  • High bandwidth
  • Long transmission distance
  • Strong resistance to interference

5.2 Wireless

Trends:

  • 5G mobile communication
  • WiFi 6
  • The Internet of Things

Advantages:

  • Flexible deployment
  • Low cost
  • Good mobility support

5.3 Intelligence

Trends:

  • Smart antennas
  • Adaptive transmission
  • Software-defined networking

Advantages:

  • Optimised performance
  • High resource utilisation
  • Flexible management

6. Maintaining Transmission Media

6.1 Maintaining Wired Media

Regular inspection:

  • Condition of the connectors
  • Integrity of the cables
  • Signal quality

Troubleshooting:

  • Test with a tester
  • Check the connectors
  • Replace damaged cables

6.2 Maintaining Wireless Media

Regular inspection:

  • Condition of the antennas
  • Signal strength
  • Sources of interference

Troubleshooting:

  • Adjust the direction of the antenna
  • Replace the antenna
  • Remove the sources of interference
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Physical Layer Devices

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