1. Introduction
  2. Cable Plant
    1. Residential
      1. Local Loop
      2. Network Interface Device (NID)
      3. Inside Wiring
      4. Telephone Jack
    2. Business
      1. Demarcation Point ('demarc')
        1. 4-wire facility (U.S.)
          1. Smart Jack
          2. Loopback Testing
        2. DSX-1 (non-U.S.)
      2. CSU/DSU
    3. Trunk Lines
      1. Basics
        1. Purpose and Function
        2. Physical Specifications
        3. Time Division Multiplexing
      2. Types
        1. Copper
        2. Fiber
    4. Private Lines
  3. Regenerators
  4. Load Coils
  5. Digital Packet Assembler Disassemblers
  6. Telephone poles
  7. Conduits
  8. Cabinets and Junction Boxes


The telephone system came into existence during America's Industrial Revolution. The government gave tax breaks to the manufacturing businesses to encourage them to build more plants and upgrade the capacity of those plants. The phone company wanted to take advantage of the same tax breaks and argued that they 'manufactured' phone service from their physical plant--the wiring outside the central office. It is from this categorization of outside wiring as physical plant facilities that the phone company's physical plant gets its name. The term physical plant covers all facilities, wiring and equipment owned by the phone company. However, later developments split this generic term into Central Office and Outside Plant groupings of telecommunications equipment as they met different performance standards. These groupings were recommended by Bellcore (now Telcordia) as part of the NEBS recommendations documents.

Cable Plant

The cable plant is all cabling and wiring outside the central offices and includes regenerators, load coils and other devices used to sustain and transmit a signal from the central office to a remote location. This wiring may run to another central office or to a residence.

Residential Wiring

Residential wiring includes all wiring and cables that run between a central office and one or more residences. Because of U.S. Federal tarrif laws, residential and commercial services are billed and taxed at different rates. This is the primary reason why the phone company makes a distinction between commercial and residential service. The residential wiring includes the local loop and the network interface device.

While not strictly part of the residential wiring maintained by the phone company, the telephone wiring inside a residence is also needed to provide service. For a fee, most phone companies will make repairs to this inside wiring.

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