--- title: "Telephone office structure and distribution plant" subject: "Telecommunications" chapter: 1 key: "semester_8/telecommunications:telephone-office-distribution-plant" source: https://www.bctengineer.com/q/semester_8/telecommunications/telephone-office-distribution-plant site: BCT Engineering --- # Telephone office structure and distribution plant Telecommunications · Chapter 1: Telecommunication Networks - Asked in: 2071 Chaitra, 10 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/telephone-office-distribution-plant - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1 ## Question With a neat sketch of technical structure of a telephone office, explain the distribution plant. ## Short answer A telephone office contains MDF, line equipment, switching network, control, trunk equipment, and power plant; outside plant starts at MDF. Distribution plant uses feeder cables to cabinets, distribution cables to DPs, and drop wires to subscribers, with flexibility at cross-connect points. ## Answer #### Technical structure of a telephone office A telephone office (local exchange) has the switching and control equipment inside the building and the **line (outside) plant** outside it. Inside, all lines pass through the MDF. ```text TELEPHONE OFFICE (EXCHANGE) ┌──────────────────────────────────────────┐ │ Power Control Switching Trunk │─ trunks to │ plant (SPC) ─ network ─ equipment │ other offices │ -48 V │ │ │ │ Line equipment (BORSCHT) │ │ │ │ │ Main Distribution Frame (MDF) │ └────────────────┬─────────────────────────┘ Feeder (main) cables in ducts │ ┌────────┴────────┐ [Cabinet/PCP] [Cabinet/PCP] │ │ │ Distribution cables (aerial/underground) [DP] [DP] [DP] │ │ │ │ Drop wires to subscribers' telephones ``` **Inside equipment:** MDF (termination, protection, cross-connection), subscriber line units, switching network, control processors, signalling units, trunk equipment, charging/billing, O&M terminals and the −48 V power plant with batteries. #### Distribution plant The **distribution plant** is the part of the outside plant that carries subscriber pairs from the exchange to individual premises. It is designed in a hierarchy so that cable sizes reduce as they approach subscribers: 1. **Main/feeder cable:** large cables (600–2400 pairs) leave the MDF through cable vaults and underground ducts to the serving areas. 2. **Primary cross-connection point (PCP) or cabinet/pillar:** a street cabinet where feeder pairs are jumpered to distribution pairs. It gives **flexibility**: spare feeder pairs can be used wherever demand grows, so fewer total pairs are needed. 3. **Distribution cables:** smaller cables (25–200 pairs), underground or aerial on poles, run along streets. 4. **Distribution point (DP):** a terminal box on a pole, building wall or inside a building, serving 10–20 subscribers. 5. **Drop (service) wire:** a one- or two-pair wire from the DP to the subscriber's premises, ending in a protector and socket. **Design considerations:** - Forecast demand (5–20 years) to size cables and leave spare pairs. - Limit loop resistance and attenuation (gauge selection, loading coils on long loops) for good transmission and signalling. - Use ducts and manholes for easy expansion and repair; aerial cables in rural areas for low cost. - Protect against lightning, moisture (pressurised or jelly-filled cables) and physical damage. - Today the same structure is used for fibre access (FTTC/FTTH) with splitters at cabinets and DPs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the role of the MDF in a telephone office? A. Terminates outside cables, provides protection and cross-connects them to line equipment B. Stores the program of the exchange C. Generates -48 V D. Switches speech in time slots Answer: A. All lines pass through the MDF, which provides termination, protection and cross-connection. (Key point: Role of MDF) 2. How do cable sizes change in the distribution plant towards subscribers? A. They increase B. They stay the same C. They reduce: feeder (600-2400 pairs) to distribution (25-200) to DP to drop wire D. They alternate Answer: C. The hierarchy reduces pair count as cables approach subscribers. (Key point: Cable size hierarchy) 3. What is the advantage of the primary cross-connection point? A. Spare feeder pairs can be used where demand grows, so fewer total pairs are needed B. It removes the need for DPs C. It increases loop resistance D. It performs billing Answer: A. Jumpering lets spare pairs serve growth anywhere in the serving area. (Key point: Benefit of PCP flexibility) 4. Which is a design consideration of the distribution plant? A. Forecast demand 5-20 years and leave spare pairs B. Use only aerial cables everywhere C. Ignore lightning protection D. Use one cable size for all Answer: A. Cables are sized from demand forecasts with spare pairs, with ducts for expansion and protection from lightning and moisture. 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