Planning Structured Cabling: Identifying Potential Wiring Closets

TIA/EIA-568-A specifies that when using an Ethernet star topology, every device that is part of the network must be connected to the hub by a run of horizontal cabling.  

The central point of the star topology, where the hub is located, is called the wiring closet. It helps to think of the hub as the center point of a circle which has lines of horizontal cabling radiating from it, like spokes from the center of a wheel.

In order to determine the location of a wiring closet, begin by drawing a floor plan of the building (approximately to scale), and adding to it all of the devices that will be connected to the network. As you do this, remember that computers are not the only devices that you will want to connect to the network; there are also printers and file servers to consider.

When you have completed this process, you should have a floor plan that is similar to the one shown in the Figure .

Horizontal Cabling System Structure

The horizontal cabling system extends from the telecommunications outlet in the work area to the horizontal cross-connect in the telecommunications closet. It includes the telecommunications outlet, an optional consolidation pointer transition point connector (horizontal cable, and the mechanical terminations and patch cords or jumpers) that comprise the horizontal cross-connect.

Some points specified for the horizontal cabling subsystem include:

·         Recognized Horizontal Cables:

·         4-pair100 Ω UTP 

·         2 fiber (duplex) 62.5/125 µm or multimode optical fiber (note: 50/125 µm multimode fiber will be allowed in ANSI/TIA/EIA-568-B)

Note: ISO/IEC 11801are 120 Ω UTP and 50/125 µm multimode optical fiber.

·         Multipair and multi-unit cables are allowed, provided that they satisfy the hybrid bundled cable requirements of TIA/EIA-568-A-3.

·         Grounding must conform to applicable building codes, as well as ANSI/TIA/EIA-697.

·         A minimum of two telecommunication outlets are required for each individual work area.

·         First outlet: 100 Ω UTP (Cat 5e recommended).

·         Second outlet: 100 Ω UTP (Cat 5e recommended).

·         Two-fiber multimode optical fiber either 62.5/125 µm or 50/ 125 µm.

·         One transition point (TP) is allowed between different forms of the same cable type (i.e. where undercarpet cable connects to round cable).

Note: The definition provided for a “transition point” on ISO/IEC 11801 broader than '568-A. It includes transitions to under carpet cabling as well as consolidations point connections. 

·         50 Ω coax and 150 Ω STP-a cabling is not recommended for new installations.

·         Additional outlets may be provided. These outlets are in addition to and may not replace the minimum requirements of the standard.

·         Bridged taps and splices are not allowed for copper-based horizontal cabling. (Splices are allowed for fiber.)

Note: In ISO/IEC 11801, the equivalent cabling element to the horizontal cross-connect (HC) is called the floor distributor (FD).

·         Application specific components shall not be installed as part of the horizontal cabling. When needed, they must be placed external to the telecommunications outlet or horizontal cross-connect (eg. Splitters, baluns).

·         The proximity of horizontal cabling to sources of electromagnetic interference (EMI) shall be taken into account.

Planning Structured Cabling: Identifying Potential Wiring Closets

A good way to start looking for a potential wiring closet location is to identify secure locations that are close to the POP. The selected location can serve as either the sole wiring closet, or as the MDF, if IDFs are required. The POP is where telecommunications services, provided by the telephone company, connect to the building's communication facilities. It is essential that the hub be located near it, in order to facilitate wide area networking and connection to the Internet.

In the floor plan graphic, five potential locations for wiring closets have been selected. They are marked on the graphic as A, B, C, D, and E.

Planning Structured Cabling: Identifying Potential Wiring Closets

After you have drawn in all of the devices that are to be connected to your network (floor plan), the next step is to determine how many wiring closets you will need to serve the area covered by the network. You will use your site map to do this.

Use your compass to draw circles that represent a radius of 50 m. from each of the potential hub locations. Each of the network devices that you drew on your floor plan should fall within one such circle. However, if each horizontal cabling run can only be 90 m. in length, can you think of a reason why circles with a radius of only 50 m. would be used?

After you have drawn the circles, look at the floor plan again. Are there any potential hub locations whose catchment areas substantially overlap? If so, you could probably eliminate one of the hub locations. Are there any potential hub locations whose catchment areas can contain all of the devices that are to be connected to the network? If so, then one of them could probably serve as the wiring closet for the entire building. If you will need more than one hub in order to provide adequate coverage for all of the devices that will be connected to the network, check to see if one of them is closer to the POP than the other(s). If so, you will probably want to select it to serve as the MDF.

Planning Structured Cabling: Identifying Potential Wiring Closets

Use the floor plan provided in this lesson. Notice that there are five potential locations for wiring closets indicated on the floor plan - A, B, C, D, and E. Using the scale indicated on the floor plan, set the compass so that it will mark a circle that equals 50 m in diameter. Mark circles for each of the potential wiring closet sites. Then answer the following questions:

1. Do any of the circles overlap?
2. Can any of the potential wiring closet locations be eliminated?
3. Do any of the circles provide coverage for all of the devices that will be connected to the network?
4. Which of the potential wiring closet locations seems to be the best?
5. Are there any circles where only a few of the devices fall outside the catchment area?
6. Which potential wiring closet is closest to the POP?
7. Based on your findings, list the three best possible locations for wiring closets.
8. Based on your findings, how many wiring closets do you believe will be required for this network?
9. What are the advantages and disadvantages of each of the potential wiring close locations shown on the floor plan?