The backbone cabling system provides interconnections between telecommunications closets, equipment rooms, and entrance facilities. It includes backbone cables, intermediate and main cross-connects, mechanical terminations, and patch cords or jumpers used for backbone-to-backbone cross-connections. The backbone also extends between buildings in a campus environment.

Some points specified for the backbone cabling subsystem include:

• Equipment connections to backbone cabling should be made with cable lengths of 30m (98 ft.) or less.
• The backbone cabling shall be configured in a star topology. Each horizontal cross-connect is connected directly to a main cross-connect or to an intermediate cross-connect, then to a main cross-connect.
• The backbone is limited to no more than two hierarchical levels of cross-connects (main and intermediate). No more than one cross-connect may exist between a main and a horizontal cross-connect and no more than three cross-connects may exist between any two horizontal cross-connects.
• Cross-connects for different cable types must be located in the same facilities.
• A total maximum backbone distance of 90 m (295 ft.) is specified for certain applications. This distance is for uninterrupted backbone runs. (No intermediate cross-connect).
• The proximity of backbone cabling to sources of electromagnetic interference (EMI) shall be taken into account. (Specific distances are provided in ANSI/TIA/EIA-569-A.)
  
  

• The distance between the terminations in the entrance facility and the main cross-connect shall be documented and should be made available to the service provider.
• Recognized media may be used individually or in combination, as required by the installation. Quantity of pairs and fibers needed in individual backbone runs depends on the area served. Recognized backbone cables are:

  

   

• Multipair cable is allowed, provided that it satisfies the power sum crosstalk requirements.

   

• Bridged taps are not allowed.

  Notes: In ISO/IEC 11801, the equivalent cabling elements to the main cross-connect (MC) and intermediate cross-connect (IC) are called the campus distributor (CD) and building distributor respectively.

  In addition to those listed, two alternate backbone cabling types allowed by ISO/IEC are 120 Ohm twisted-pair and 50/125 µm multimode optical fiber.

  50 Ohm coaxial cabling is recognized by '568-A, but is not recommended for new installations.