Outside plant

Outside plant
Serving Area Interface
Serving Area Interface

In telecommunication, the term outside plant has the following meanings:

The CATV industry divides its fixed assets between head end or inside plant, and outside plant. The electrical power industry also uses the term outside plant to refer to electric power distribution systems.



A Main Distribution Frame showing 110 punch block and twisted pair wiring
Distribution Frame
Damage to outside plant, 1913

Network connections between devices such as computers, printers, and phones require a physical infrastructure to carry and process signals. Typically, this infrastructure will consist of:

  • Cables from wall outlets and jacks run to a communications closets, sometimes referred to as station cable.
  • Cables connecting one communications closet to another, sometimes referred to as riser cable.
  • Racks containing telecommunications hardware, such as switches, routers, and repeaters.
  • Cables connecting one building to another.
  • Exterior communications cabinets containing hardware outside of buildings.
  • Radio transceivers used inside or outside buildings, such as wireless access points, and hardware associated with them, such as antennas and towers.

The portion of this infrastructure contained within a building is the inside plant, and the portion of this infrastructure connecting buildings or facilities is the outside plant. Where these two plants meet in a given structure is the demarcation point.

Outside plant cabling, whether copper or fiber, is generally installed as aerial cable between poles, in an underground conduit system, or by direct burial.[2] Hardware associated with the outside plant must be either protected from the elements (for example, distribution frames are generally protected by a street side cabinet) or constructed with materials suitable for exposure to the elements. Installation of the outside plant elements often require construction of significant physical infrastructure, such as underground vaults.[3] In older large installations, cabling is sometimes protected by air pressure systems designed to prevent water infiltration. While this is not a modern approach, the cost of replacement of the older cabling with sealed cabling is often prohibitively expensive.[4] The cabling used in the outside plant must also be protected from electrical disturbances caused by lightning or voltage surges due to electrical shorts or induction.[5]

Example: Copper access network

In civilian telecommunications, the copper access network (also known as the local loop) providing basic telephone or DSL services typically consists of the following elements:[6]

  • In-house wiring that connects customer premises equipment to the demarcation point, usually in residential installations contained in a weather protected box.
  • One or more twisted pairs, called a drop wire. The drop wires typically connect to a splice case, located in line for aerial cables, or in a small weather protected case for underground wiring, where the local cabling is connected to a secondary feeder line. These cables contain fifty or more twisted pairs.
  • Secondary feeder lines run to a streetside cabinet containing a distribution frame called a Serving Area Interface (SAI).
  • The SAI is connected to the main distribution frame, located at a Telephone exchange or other switching facility, by one or more primary feeder lines which contain hundreds of copper twisted pairs. An SAI may also contain a Digital subscriber line access multiplexer (DSLAM) supporting DSL service.

Active equipment (such as a POTS or DSL line circuit) can then be connected to the line in order to provide service, but this is not considered part of outside plant.

Protecting Equipment in the Outside Plant

The environment can play a large role in the quality and lifespan of equipment used in the outside plant. It is critical that environmental testing criteria as well as design and performance requirements be defined for this type of equipment.

There are generally four operating environments or classes covering all outside plant (OSP) applications, including wireless facilities.

  • Class 1: Equipment in a Controlled Environment
  • Class 2: Protected Equipment in Outside Environments
  • Class 3: Protected Equipment in Severe Outside Environments
  • Class 4: Products in an Unprotected Environment

Electronic equipment located in one or more of these environmental class locations are designed to withstand various environmental operating conditions resulting from climatic conditions that may include rain, snow, sleet, high winds, ice, salt spray, and sand storms. Since outside temperatures can possibly range from -40°C (-40°F) to 46°C (115°F), with varying degrees of solar loading, along with humidity levels ranging from below 10% up to 100%, significant environmental stresses within the enclosure or facility can be produced.

Telcordia GR-3108, Generic Requirements for Network Equipment in the Outside Plant (OSP) contains the most recent industry data regarding each Class described above. It also discusses what is currently happening in ATIS and Underwriters Laboratories (UL)

The document also includes

  • environmental criteria such as operating temperatures, humidity, particulate contamination, pollution exposure, and heat dissipation
  • mechanical criteria such as structural requirements, packaging, susceptibility to vibration, earthquake, and handling
  • electrical protection and safety including protection from lightning surges, AC power induction and faults, and Electromagnetic Interference (EMI), and DC power influences

See also

Further reading


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