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.

Backbone Cabling

The next subsystem of structured cabling is called backbone cabling. (Backbone cabling is also sometimes called vertical cabling, cross-connect cabling, riser cabling, or intercloset cabling.) Backbone cabling is necessary to connect entrance facilities, equipment rooms, and telecommunications rooms and enclosures. Refer to Figure 2.7 later in the chapter to see backbone cabling that connects an equipment room with telecommunications rooms. Backbone cabling consists of not only the cables that connect the telecommunications rooms, equipment rooms, and building entrances but also the cross-connect cables, mechanical terminations, or patch cords used for backbone-to-backbone cross-connection.

Permanent Link vs. Channel Link

ANSI/TIA-568-C defines two basic link types commonly used in the cabling industry with respect to testing: the permanent link and the channel link.

The permanent link contains only the cabling found in the walls (horizontal cabling), one transition point, the telecommunications outlet, and one cross-connect or patch panel. It is assumed to be the permanent portion of the cabling infrastructure. The permanent link is illustrated here:

The channel link includes the basic link, as well as installed equipment, patch cords, and the cross-connect jumper cable; however, the channel does not include phones, PBX equipment, hubs, or network interface cards. Two possible channel link configurations are shown here; one is the channel link for a 10Base-T Ethernet workstation, and one is for a telephone.

KEY TERM: cross-connect

A cross-connect is a facility or location within the cabling system that permits the termination of cable elements and their intereconnection or cross-connection by jumpers, termination blocks, and/or cables to another cabling element (another cable or patch panel).