Guided and unguided transmission media

Transmission media are the physical paths that carry signals between transmitter and receiver. They are classed as guided (wired) and unguided (wireless).

Guided media

1. Twisted pair cable

  ~~~/\/\/\/\/\/\/\~~~   two insulated copper
  ~~~\/\/\/\/\/\/\/~~~   wires twisted together

• Two insulated copper wires twisted to reduce crosstalk and noise pickup; UTP and STP types.
• Used for telephone subscriber loops, DSL and LAN (Cat-5/6).
• Cheap, easy to install; attenuation high at high frequency, limited distance/bandwidth.

2. Coaxial cable

  | outer jacket | braid (shield) | insulator | core |
       ( ( ( ( ●  center conductor ) ) ) )

• Central copper conductor, dielectric insulator, outer braided conductor and jacket.
• Wider bandwidth and better shielding than twisted pair; used in cable TV, old long-distance FDM trunks, antenna feeders.

3. Optical fibre

  cladding  ───────────────────────
  core      ═══> light by total ═══>
            internal reflection
  cladding  ───────────────────────

• Glass core with higher refractive index than the cladding; light is guided by total internal reflection.
• Single-mode and multimode types; very low loss (~0.2 dB/km at 1550 nm), huge bandwidth, immune to EMI, light and secure.
• Used in backbone, undersea cables, FTTH.

Unguided media

1. Radio waves (HF/VHF/UHF): omnidirectional antennas; ground wave, sky wave (HF via ionosphere), line-of-sight; used in broadcasting, mobile, walkie-talkies.

2. Terrestrial microwave (1–40 GHz):

  [Dish]))) ─ line of sight ─ (((([Dish]
   tower    repeater every 40-60 km  tower

Highly directional parabolic dishes on towers; used for long-distance trunks and mobile backhaul; affected by rain and obstacles.

3. Satellite:

              [Satellite]
             ^          \
   uplink   /            v  downlink
 [Earth stn A]         [Earth stn B]

Geostationary satellite (36,000 km) acts as a repeater in the sky; wide coverage; delay about 250 ms one way.

4. Infrared and light-wave: short range, line of sight (remote controls, IrDA, free-space optics).