Saturday, June 25, 2011

Basic Requirements for Backbone Cabling

Backbone cabling includes:
  • Cabling between equipment rooms and building entrance facilities
  • In a campus environment, cabling between buildings' entrance facilities
  • Vertical connections between floors
ANSI/TIA-568-C.1 specifies additional design requirements for backbone cabling, some of which carry certain stipulations, as follows:
  • Grounding should meet the requirements as defined in J-STD-607-A, the Commercial Building Grounding and Bonding Requirements for Telecommunications.
  • The pathways and spaces to support backbone cabling shall be designed and installed in accordance with the requirements of TIA-569-B. Care must be taken when running backbone cables to avoid sources of EMI or radio frequency interference.
  • No more than two hierarchical levels of cross-connects are allowed, and the topology of backbone cable will be a hierarchical star topology. (A hierarchical star topology is one in which all cables lead from their termination points back to a central location.) Each horizontal cross-connect should be connected directly to a main cross-connect or to an intermediate cross-connect that then connects to a main cross-connect. No more than one cross-connect can exist between a main cross-connect and a horizontal cross-connect. Figure 1 shows multiple levels of equipment rooms and telecommunications rooms.
    Figure 1: Star topology of equipment room and telecommunication rooms connected via backbone cabling
  • Centralized optical fiber cabling is designed as an alternative to the optical cross-connection located in the telecommunications room or telecommunications enclosure when deploying recognized optical fiber to the work area from a centralized cross-connect. 
  • The length of the cord used to connect telecommunications equipment directly to the main or intermediate cross-connect should not exceed 30 meters (98).
  • Unlike horizontal cabling, backbone cabling lengths are dependent on the application and on the specific media chosen. (See ANSI/TIA-568-C.0 Annex D.) For optical fiber, this can be as high as 10,000 meters depending on the application! However, distances of  550 meters are more likely inside a building. This distance is for uninterrupted lengths of cable between the main cross-connect and intermediate or horizontal cross-connect.
  • Bridge taps or splices are not allowed.
  • Cables with more than four pairs may be used as long as they meet additional performance requirements such as for power-sum crosstalk. These requirements are specified in the standard. Currently, only Category 5e cables are allowed to have more than four pairs.

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