Twisted Pair Technology Overview

Twisted Pair

• Common uses:
  • Digital signaling
  • Connections to digital data switches and PBXs within buildings (data rate: 64 kbps)
  • Local area networks supporting personal computers within buildings (data rate: 100 Mbps)
• Cost: Less expensive than coaxial cable and optical fiber.
• Transmission characteristics:
  • Can transmit both analog and digital signals.
  • Analog signals require amplifiers every 5-6 km.
  • Digital signals require repeaters every 2-3 km.
  • Limited in distance, bandwidth, and data rate compared to coaxial cable and optical fiber.
  • Strong attenuation with frequency.
  • Susceptible to interference and noise due to easy coupling with electromagnetic fields (e.g., 60 Hz energy from power lines).
  • Impulse noise easily intrudes.
• Measures to reduce impairments:
  • Shielding with metallic braid or sheathing reduces interference.
  • Twisting wire reduces low-frequency interference.
  • Different twist lengths for adjacent pairs reduce crosstalk.

Coaxial Cable

• Applications:
  • Television distribution
  • Long-distance telephone transmission
  • Short-run computer system links
  • Local area networks
• Transmission characteristics:
  • Transmits both analog and digital signals.
  • Superior frequency characteristics compared to twisted pair, enabling higher frequencies and data rates.
  • Less susceptible to interference and crosstalk than twisted pair due to shielded, concentric construction.
  • Principal constraints on performance: attenuation, thermal noise, and intermodulation noise.
  • Analog signal transmission requires amplifiers every few kilometers, with closer spacing for higher frequencies.
  • Digital signaling requires repeaters every kilometer or so, with closer spacing for higher data rates.

Optical Fiber

• Physical description:
  • Thin and flexible medium, guiding an optical ray.
  • Made of glass or plastic.
  • Ultrapure fused silica offers lowest losses, but is difficult to manufacture.
  • Multicomponent glass fibers are more economical and provide good performance.
  • Plastic fiber is even less expensive and suitable for short-haul links.
• Components:
  • Core: Inner section, consisting of glass or plastic fibers (8-50µm diameter).
  • Cladding: Surrounds the core, has different optical properties to reflect light back into the core (125µm diameter).
  • Jacket: Outermost layer, protects against moisture, abrasion, crushing, and other environmental factors.
• Applications:
  • Advantages over twisted pair and coaxial cable:
    • Greater capacity: High bandwidth and data rates (hundreds of Gbps over tens of kilometers demonstrated), compared to coaxial cable (hundreds of Mbps over 1 km) and twisted pair (few Mbps over 1 km or up to 100 Mbps to 10 Gbps over few tens of meters).
    • Smaller size and lighter weight: More compact and easier to install in confined spaces.
    • Lower attenuation: Less signal degradation over long distances.
    • Electromagnetic isolation: Not affected by external electromagnetic fields, making it resistant to interference, impulse noise, and crosstalk; also secures against eavesdropping.
    • Greater repeater spacing: Longer distances between repeaters, reducing cost and errors.
• Application Categories:
  • Long-haul trunks: High capacity routes (20,000-60,000 voice channels) averaging 1500 km in length.
  • Metropolitan trunks: Shorter routes (12 km average) with high capacity (up to 100,000 voice channels).
  • Rural exchange trunks: Connect towns and villages (40-160 km length).
  • Subscriber loops: Connect central exchange directly to subscribers.
  • Local area networks: Support hundreds to thousands of stations in buildings, with total capacity of 100 Mbps to 10 Gbps.
• Transmission characteristics:
  • Transmits signal-encoded beam of light through total internal reflection.
  • Acts as a waveguide for frequencies in the infrared and visible spectra.
  • Types of transmission:
    • Step-index multimode: Multiple angles of light rays, resulting in distortion.
    • Single-mode: Only one angle of light ray (axial ray), eliminating distortion, ideal for long distances (e.g., telephone, cable television).
    • Graded-index multimode: Varying refractive index in the core, light rays helically curve, reducing travel distance.

Explore the characteristics and uses of twisted pair cables in networking. This quiz covers aspects such as digital signaling, cost comparisons, transmission limitations, and measures to reduce impairments. Test your understanding of this fundamental technology in local area networks.