Generic Cabling Architecture

Both building-cabling standards TIA/EIA 568-B.1-2001 and ISO/IEC 11801:2002 define generic cabling for use in commercial buildings and between commercial buildings in a campus setting. These two standards pertain to data and telephone cabling, but not to wiring for power, HVAC, or building control systems.

Generic cabling uses a star-wiring topology (Figure 7.1). In the star topology, cables fan out from a single location to individual work areas.

Figure 7.1. TIA/EIA and ISO/IEC use slightly different terms to describe hierarchical building cabling.

Cables running between equipment rooms and from equipment rooms to telecommunications rooms are called backbone cables . [61] In the generic cabling plan, these cables establish hierarchical connections between telecommunications closets. Backbone cables are also called vertical cables in reference to how they run between floors in tall buildings.

[61] This description of the generic wiring architecture uses TIA/EIA terminology.

Cables running from the telecommunications rooms to the work areas are called horizontal cables . The word horizontal refers to the way users normally install such cables, running them along floors and ceilings. Horizontal cables are used for both computer and telephone communications.

There are more choices for backbone cables than for horizontal cables. For example, backbone cables often employ fiber, whereas horizontal cables usually do not. The plethora of choices makes it difficult for customers to choose the right backbone cabling for the future, and it makes it difficult for system manufacturers to decide what to support.

Fortunately, most users do not view backbone cabling as a permanent choice. Unlike horizontal cabling, typical installations do not have very many backbone cables. Backbone cables run only between closets, whereas horizontal cables run to every work area. Furthermore, backbone cables terminate in telecommunications and equipment rooms, not in the work area where they have to look pretty. Both factors make backbone cables easier to replace than horizontal cables. The difference in effort required to replace backbone cabling compared to horizontal cabling may be a factor of a hundred or more.

A weird backbone cabling requirement is a sales obstacle to be overcome .

A weird horizontal cabling requirement is a wooden stake in the heart of your project .

Horizontal cabling is the most widely deployed, highest-volume element of the building-cabling architecture. Horizontal cabling components are available at rock-bottom prices from a wide variety of sources. If you design your system to work on standard horizontal cabling, you will find a ready market of customers who already have this cabling installed and a large pool of trained technicians who know how to install and maintain it. Standard horizontal cabling is the best choice for most interconnections at lengths from 3 m to 100 m.

While the generic cabling standards leave lots of leeway in the definition of backbone cabling, they leave very little slack in the definition of the horizontal connections. This is important because there are so many more horizontal connections in a building than backbone connections.

A horizontal connection consists of no more than 100 m of cable, independent of cable type. The standards permit no bridges, taps, or Y connections in a horizontal cable. Each horizontal cable must be a dedicated, point-to-point connection. Two horizontal cables serve each work area (Figure 7.2).

Figure 7.2. Two horizontal cables serve each work area.

TIA/EIA 568-B.1-2001 and ISO/IEC 11801:2002 both specify (or point to specifications for) all the cables, connectors, jumpers , and cords necessary for interconnecting equipment. These two standards then roll up the entire signal-to-noise budget for various combinations of interconnections, showing how the cable, connector, jumper , and cord specifications each relate to the whole budget.

The beauty of these cable standards is the way they break each interconnection into three levels of testable specifications: a cable , a permanent link , and a channel (Figure 7.3).

Figure 7.3. Cable manufacturers care about cable specs , installers care about permanent link specs, and system manufacturers care about channel specs. Generic cabling standards roll together all three differing views.

Cable ” One continuous piece of balanced transmission media. People who manufacture cables are interested only in this portion of the specifications.

Permanent link ” Extends from the work area telecommunications outlet to the permanent termination point of the horizontal cabling in the telecommunications room. It may include a short section of cabling with an intermediate transition point designed to facilitate layout of open -air offices using under-carpet cabling or cabling built into cubicle walls. People who install building cabling are expected to properly install and test all permanent links.

Channel ” An entire connection, starting at the end of the telecommunications equipment cable and completing at the bitter end of the work area cable. People who use building cabling or who manufacture equipment that interfaces to building cabling care about the performance of the whole channel.

TIA/EIA and ISO/IEC cabling standards differ , but less and less as time passes . The most important point is that the horizontal channels for both standards include support for the most popular cable type ”category 5e 100- W balanced cabling. Some of the remaining differences are called out in Table 7.1

Table 7.1. Popular Cables Recognized for Use as Horizontal Cabling

[1] NOTE (1) ”Category 3 cables are still recognized by both standards but no longer widely available in the market. Category 5 and 5e cables perform better and cost no more.

[2] NOTE (2) ”150- W STP-A is still recognized by both standards but no longer widely available in the market. It may be dropped from future versions of the generic wiring standards.

[3] NOTE (3) ”Pairs of 100- W balanced cable or strands of fiber.

POINTS TO REMEMBER

• Horizontal cabling is the most widely deployed, highest-volume element of the building-cabling architecture.
• New buildings in North America provide two outlets in every work area, with four-pair, 100- W UTP, category 5 or better cabling to both outlets.
• Backbone cables are mostly a mix of category 5 cables, multimode fiber (62.5- m m or 50- m m), and some single-mode fiber.
• A weird backbone cabling requirement is a sales obstacle to be overcome. A weird horizontal cabling requirement is a wooden stake in the heart of your project.