--- title: "Full duplex on a single twisted pair" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:twisted-pair-full-duplex-converter" source: https://www.bctengineer.com/q/semester_8/telecommunications/twisted-pair-full-duplex-converter site: BCT Engineering --- # Full duplex on a single twisted pair Telecommunications · Chapter 2: Transmission Media - Asked in: 2076 Asoj, 2+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/twisted-pair-full-duplex-converter - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question How does a single twisted pair cable maintain a full duplex connection in the local loop? Describe the need and operating mechanism of the two-wire to four-wire converter with the help of a neatly labeled diagram. ## Short answer The local loop uses hybrid circuits to separate directions by impedance balancing. A transformer feeds the receive signal to the 2-wire line and cancels it from the transmit path when the balance network matches line impedance. Speech from the line is divided to the transmit port, enabling full duplex without frequency or time division. ## Answer #### Full duplex on a single twisted pair In the local loop both directions of speech travel on the **same two wires at the same time**. This is possible because the signals are added on the pair and **separated by hybrid circuits** at each end: an **anti-sidetone/hybrid network in the telephone set** and a **hybrid (2W/4W converter) in the exchange line card**. Each hybrid passes the outgoing signal to the line and the incoming signal to the receiver while largely blocking the outgoing signal from its own receive path (balanced bridge). Since the two directions are kept apart by balance, not by frequency or time, the loop works as full duplex. #### Need for the two-wire to four-wire converter - Amplifiers, repeaters, multiplexers and PCM codecs carry signals in one direction only, so trunk transmission is **four-wire** (separate go and return paths). - The converter (hybrid) joins the two-wire loop to the four-wire trunk, separating the two directions, and avoids echo and **singing** caused by signal returning round the four-wire loop. #### Operating mechanism ```text 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How can both directions of speech use the same twisted pair at the same time? A. Each direction uses a different frequency band B. Each direction uses alternate time slots C. Signals add on the pair and hybrids at each end separate the directions by balance D. The pair is split into two one-way halves Answer: C. Hybrids (in the set and line card) separate directions by bridge balance, not by frequency or time. (Key point: Full duplex on one pair) 2. Why is a 2W/4W converter needed at the trunk? A. Trunk equipment such as amplifiers and multiplexers is one-way, needing separate paths B. To increase subscriber loop voltage C. To convert digital to analogue D. To remove thermal noise Answer: A. The hybrid joins the two-wire loop to the four-wire trunk and keeps the directions apart. (Key point: Need for converter) 3. Why does a signal from the 4-wire receive pair not appear on the transmit pair of a balanced hybrid? A. The amplifier on the receive pair blocks it B. Its two halves flow in opposite senses through the windings coupled to the transmit pair, so their fluxes cancel when Z_B = Z_L C. It is filtered out because it uses a different frequency band D. The transmit pair is shorted by the balance network Answer: B. The signal splits equally into Z_L and Z_B; with Z_B = Z_L the opposing magnetic effects cancel in the transmit winding. (Key point: Why no leakage) 4. When does a four-wire circuit with hybrids at both ends sing (oscillate)? A. When the line is longer than 100 km B. When the balance return loss is very large C. When Z_B is exactly equal to Z_L D. When the loop gain round the four-wire circuit reaches or exceeds unity (0 dB) Answer: D. Leakage lets signal circulate round the loop; if loop gain is at least unity the loop oscillates and sings. (Key point: Singing condition) ## Related in Telecommunications - [Noise and two-wire to four-wire converter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anoise-two-four-wire-converter) — Telecommunications, chapter 2 - [Radio signal in twisted pair and coaxial cable](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aradio-signal-twisted-coaxial) — Telecommunications, chapter 2 - [Guided vs unguided media; 2-wire to 4-wire](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aguided-unguided-two-four-wire) — Telecommunications, chapter 2, asked 2× - [Two-wire vs four-wire communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atwo-vs-four-wire) — Telecommunications, chapter 2 - [Talker and listener echo in four-wire circuit](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afour-wire-echo-numerical) — Telecommunications, chapter 2 - [Subscriber loop and cable hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asubscriber-loop-cable-hierarchy) — Telecommunications, chapter 1 ## Related in other subjects - [Series connection of two full converters](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aseries-full-converters-waveforms) — Power Electronics, chapter 2, asked 2× - [Single-phase full converter, 10 A, α = 30°](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Afull-converter-10a-30deg) — Power Electronics, chapter 2, asked 4× - [Why wires are twisted in twisted pair](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atwisted-pair-why-twist) — Data Communication, chapter 4 - [Optical fiber advantages over coax and twisted pair](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afiber-advantages) — Data Communication, chapter 4, asked 3×