Two-wire vs four-wire communication

Two-wire communication carries both directions of speech on a single pair of wires; four-wire communication uses two separate pairs (or two separate channels), one for each direction.

Difference between two-wire and four-wire communication

Four-wire communication

In long-distance circuits the signal must be amplified many times. Amplifiers are one-way devices, so the two directions are separated: a GO path from A to B and a RETURN path from B to A. Subscriber lines remain two-wire, so a hybrid (four-wire termination set) at each end converts 2-wire to 4-wire.

 Sub A                                         Sub B
         +-----+  GO  ->[A1]->[A2]->  +-----+
 o-------| HYB |=======================| HYB |-------o
 2-wire  |  A  |=======================|  B  | 2-wire
     Z_B-+-----+  <-[A3]<-[A4]<- RET  +-----+-Z_B

Working:

1. Speech from subscriber A enters hybrid A and is sent on the GO pair. Amplifiers along the route make up for line loss.
2. At B the hybrid passes the GO signal to subscriber B's two-wire line.
3. B's reply travels on the RETURN pair the same way to A.
4. In each hybrid the balancing network $Z_B$ copies the 2-wire line impedance, so the incoming RETURN signal is not passed back into the GO pair (high transhybrid loss).

Equivalent four-wire: on carrier and radio systems there are no physical pairs; the two directions use different frequency bands (FDM) or time slots/channels (TDM) — still treated as four-wire.

Advantages:

• Gain can be set separately for each direction and the circuit can be made very long with low net loss.
• Directly suits multiplexing, digital transmission and radio/satellite links.
• Four-wire switching in trunk exchanges avoids repeated 2W/4W conversions and the loss and echo they cause.

Problems and remedies:

• Echo: imperfect hybrid balance reflects some received speech back to the talker. Long-delay circuits use echo suppressors or cancellers.
• Singing: if total loop gain $G_1 + G_2 - (L_A + L_B) \geq 0$ dB the loop oscillates. Good balancing and a net-loss margin prevent it.
• Cost: twice the conductors and the hybrids add cost, justified only on long routes.