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Media types, four-wire system and hybrid

Short answer

Guided media include twisted pair, coax, fibre, waveguide; unguided include ground/sky/space waves, microwave, satellite, infrared. A four-wire system uses separate GO and RETURN paths, with a hybrid at each end. The hybrid’s balancing network matches line impedance, coupling signals between two-wire and four-wire while isolating receive from send to prevent echo and singing.

List the types of transmission media. Briefly explain four-wire transmission system and operation of hybrid.

Types of transmission media

  1. Guided (wired) media
    • Open-wire lines
    • Twisted pair cable (UTP/STP)
    • Coaxial cable
    • Optical fibre cable (single-mode, multimode)
  2. Unguided (wireless) media
    • Radio waves (ground wave, sky wave, broadcast)
    • Terrestrial microwave (line of sight)
    • Satellite links
    • Infrared and light-wave (free-space optics)

Four-wire transmission system

In a four-wire system each direction of transmission has its own pair: one pair (GO) carries speech from A to B and the other pair (RETURN) carries speech from B to A. Subscriber lines are two-wire (both directions on one pair), but long trunk circuits need one-way amplifiers and multiplexing equipment, which work in one direction only, so trunks are four-wire.

 Sub A                                         Sub B
 2-wire  +-----+   GO pair  ->[amp]->  +-----+ 2-wire
 o-------| HYB |=======================| HYB |-------o
         |  A  |=======================|  B  |
         +-----+  <-[amp]<-  RET pair  +-----+
              4-wire trunk / carrier

Features:

  • Gain can be inserted separately in each direction, so long distances are possible.
  • Required for FDM/TDM carrier and radio links, which are inherently one-way.
  • Needs a hybrid (2W/4W termination) at each end to connect the two-wire subscriber loops.
  • Risk of echo and singing if the hybrids are not balanced.

Operation of hybrid

A hybrid is a three-winding bridge transformer with a balancing network ZBZ_B that matches the two-wire line impedance ZLZ_L.

  1. Transmit: speech from the 2-wire line is coupled to the 4-wire GO (send) pair.
  2. Receive: speech from the 4-wire RETURN pair is fed to the centre tap and splits equally between ZLZ_L and ZBZ_B; the subscriber hears it.
  3. Isolation: the two receive currents produce opposite fluxes in the send winding, so when ZB=ZLZ_B = Z_L they cancel and nothing from the RETURN pair reaches the GO pair. This high transhybrid loss prevents echo and singing.
  4. Each through path loses about 3 dB: received power splits between the line and the balancing network, and power from the line splits between the GO and RETURN ports (the part reaching the RETURN amplifier output is absorbed).

✓ Answer checked by Claude Opus 5.5 (high effort)

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