Telephone office structure and distribution plant

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.

          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.