Wired vs wireless media; four-wire and hybrid

Wired vs wireless communication media

Four-wire transmission system

In a four-wire circuit, each direction of transmission uses a separate pair (or separate channel), so amplifiers, multiplexers and digital equipment, which are one-way devices, can be used. The subscriber loop, however, is two-wire (both directions on one pair). A hybrid converts between the two at each end.

  West                                       East
 2-wire  ┌──────┐   Tx pair  ──>  (amp)  ┌──────┐ 2-wire
 ────────┤Hybrid├──────────────────────> ┤Hybrid├────────
 sub A   │  H1  │ <──────────────────────┤  H2  │ sub B
         └──┬───┘   Rx pair  <──  (amp)  └──┬───┘
           Z_B                             Z_B

• Speech from A enters hybrid H1 and goes only to the transmit (go) pair, is amplified and reaches hybrid H2, which sends it to subscriber B.
• Speech from B goes through H2 to the return pair and then through H1 to A.
• Long-distance and carrier/PCM systems are four-wire; the conversion happens at the exchange or line card.

Operation of the hybrid

                    4-wire receive (from far end)
                         │  (amplifier)
                         v
              ┌──────────────────────┐
  2-wire  ────┤ T1             T2    │
  line   ─────┤  (hybrid transformer)│
  (Z_L)       │   centre-tapped      │
              │   windings           │
  Balance ────┤                      ├───> 4-wire transmit
  network Z_B └──────────────────────┘     (to far end)

Operation of the hybrid (bridge principle):

1. Receive → 2-wire: a signal arriving on the 4-wire receive pair is fed to the centre-tapped winding. It divides equally between the 2-wire line ($Z_L$) and the balance network ($Z_B$). Half the power reaches the line (3 dB loss).
2. No leakage to transmit pair: the two halves flow in opposite senses through the windings coupled to the transmit pair, so their magnetic effects cancel when $Z_B = Z_L$. Ideally no receive signal appears on the transmit side.
3. 2-wire → transmit: speech from the 2-wire line divides between the transmit and receive ports; the transmit port gets half (3 dB loss), and the part going to the receive port is absorbed by the amplifier output and does no harm.
4. Balance: in practice $Z_B$ only approximates $Z_L$ over the band, so a small signal leaks across. This is measured by the balance return loss

$$B = 20\log_{10}\left|\frac{Z_L + Z_B}{Z_L - Z_B}\right|\ \text{dB}$$

Poor balance causes echo, and if the loop gain round the four-wire circuit exceeds unity, singing (oscillation).