If the 100 m catchment area of a simple star topology wiring closet cannot provide enough coverage for all the devices that need to be networked, the star topology can be extended by using repeaters. Their purpose is to avoid the problem of signal attenuation, and are called hubs. Generally speaking, when repeaters, or hubs, are used in this manner, they are located in additional wiring closets called IDFs. These IDF closets are linked by networking media to a central hub located in another wiring closet called the MDF. TIA/EIA-568-A  specifies the use of one of the following types of networking media:

• 100 Ohm UTP (four pair)
• 150 Ohm STP-A (two pair)
• 2 fiber (duplex) 62.5/125 µm optical fiber
• multimode optical fiber

The TIA/EIA recommends the use of CAT 5 UTP for horizontal cabling, when an Ethernet LAN uses a simple star topology.

The main hub of an extended star topology Ethernet LAN is  usually centrally located. This central location is so important that in a high rise building, the MDF is usually located on one of the middle floors of the building, even though the POP might be located on the first floor, or in the basement.

The main graphic illustrates where the backbone cabling and horizontal cabling would be used in an Ethernet LAN, in a multi-story building. In the Figure to the left, the backbone cabling (red lines) connects the POP to the MDF. Backbone cabling is also used to connect the MDF to the IDFs located on each floor. Horizontal cabling runs (blue lines) radiate out, from the IDFs on each floor, to the various work areas. Wherever the MDF is the only wiring closet on the floor, horizontal cabling radiate from it to the PCs on that floor.

Another example of a LAN that would probably require more than one wiring closet would be a multi-building campus. The main figure illustrates locations where backbone and horizontal cabling have been placed, in an Ethernet LAN, in just such a multi-building campus.  It shows an MDF in the center of the campus. In this instance, the POP is located inside the MDF. The backbone cabling (red lines) runs from the MDF to each of the IDFs. The IDFs (yellow boxes) are located in each of the campus buildings. In addition, the main building has an IDF, as well as an MDF, so that all computers fall within the catchment area. Horizontal cabling, running from the IDFs and MDFs to the work areas, is represented by the blue lines.

The type of cabling that TIA/EIA-568 specifies for connecting wiring closets to each other, in an Ethernet LAN extended star topology, is called backbone cabling. Sometimes - to differentiate it from horizontal cabling - you may see backbone cabling referred to as vertical cabling.

Backbone cabling consists of the following:

• backbone cabling runs
• intermediate and main cross-connects
• mechanical terminations
• patch cords used for backbone-to-backbone cross-connections
• vertical networking media between wiring closets on different floors
• networking media between the MDF and the POP
• networking media used between buildings in a multi-building campus

TIA/EIA-568-A specifies four types of networking media that can be used for backbone cabling. These include:

• 100 Ω UTP (four-pair)
• 150 Ω STP-A (two-pair)
• 62.5/125 µm multimode optical fiber
• single-mode optical fiber

Although TIA/EIA-568-A recognizes 50 Ω coaxial cable, generally, it is not recommended for new installations. It is anticipated that it will be removed as a choice the next time the standard is revised. Most installations today use the 62.5/125 µm fiber-optic cable, as a matter of course, for backbone cabling.

Planning Structured Cabling: Horizontal and Backbone Cabling

TIA/EIA-568-A requirements for backbone cabling

The topology that is used when more than one wiring closet is required is the extended star topology. Because more complex equipment is located at the most central point in an extended star topology, sometimes it is referred to as a hierarchical star topology.

In the extended star topology, there are two ways in which an IDF can be connected to the MDF. In the first, each IDF can be connected directly to the main distribution facility. In this case, because the IDF is where the horizontal cabling connects to a patch panel in the wiring closet, whose backbone cabling then connects to the hub in the MDF, the IDF is sometimes referred to as the horizontal cross-connect (HCC). The MDF is sometimes referred to as the main cross-connect (MCC) because it connects the backbone cabling of the LAN to the Internet.

A second method of connecting an IDF to the central hub uses a "first" IDF interconnected to a "second" IDF. The "second" IDF is then connected to the MDF. The IDF that connects to the work areas is called the horizontal cross-connect. The IDF which connects the horizontal cross-connect to the MDF is called the intermediate cross-connect (ICC). Note that no work areas or horizontal wiring connects to the intermediate cross-connect when this type of hierarchical star topology is used.

When the second type of connection occurs, TIA/EIA-568-A specifies that no more than one ICC can be passed through to reach the MCC.

As you have already learned, the maximum distances for cabling runs varies from one type of cable to another. For backbone cabling, the maximum distance for cabling runs can also be impacted by how the backbone cabling is to be used. To understand what this means, assume that a decision has been made to use single-mode fiber-optic cable for the backbone cabling. If the networking media is to be used to connect the HCC to the MCC, as described above, then the maximum distance for the backbone cabling run would be 3,000 m. 

At times the maximum distance of 3000m for the backbone cabling run must be split between two sections. This occurs when the back bone cabling is to be used to connect the HCC to an ICC and the ICC to the MCC. When this occurs, the maximum distance for the backbone cabling run between the HCC and the ICC is 500 m. The maximum distance for the backbone cabling run between the ICC and the MCC is 2,500 m.

The Figure lists TIA/EIA-568-A specifications for maximum distances for backbone cabling runs for each type of networking media.