Picture an office with multiple floors, each of which has its own LAN, consisting of a telecommunications room and horizontal cabling that leads out to the work areas. Picture then another network running vertically through the building, connecting all of the telecommunications rooms together ... This is the backbone of the network.

The ANSI/TIA-568-C standards don't stop at the horizontal cabling and the telecommunications rooms for individual LANs. They also define the networking infrastructure for the entire enterprise, including backbone networks and services originating outside the building, such as telephone and Internet services.

Each telecommunications room contains a switch that actually connects the work area cable runs together. With the cable installation completed, you use patch cables to connect the ports in the patch panels to the ports in the switch. In the parlance of structured cabling, this switch is the horizontal cross connect; it joins all of the individual horizontal cable runs into a single LAN.

At this point in the example, what you have is a series of individual LANs, one on each floor. The object, though is to connect all of the LANs together, so that a computer on any LAN can communicate with a computer on any other LAN. You also want all of the computers on all of the LANs to be able to access a single connection to the Internet.

The backbone is another network, the primary function of which is to connect the LANs together. There are no workstations connected directly to the backbone. In some cases, you might connect servers to it, but many backbones carry nothing but intermediate traffic.

Somewhere in the building, often on the ground floor or in the basement, there is an equipment room containing another patch panel. This patch panel, called the vertical cross connect, contains one end of the vertical cable runs leading to each of the otehr telecommunications rooms in the building. These vertical cable runs can use the same or a different type of cable as the horizontal runs on each floor.

Also in the equipment room is the backbone switch that connects all of the LANs together into one internetwork. As in the telecommunications rooms, you use patch cables to connect the ports in the vertical cross connect patch panel to the ports in the backbone switch.

Because the backbone network carries all of the internetwork traffic generated by the computers on the LANs, it should be as fast and efficient as you can make it.

If, for example, you use Fast Ethernet on your horizontal network, it would be a good idea to run Gigabit Ethernet on the backbone. However now that Gigabit Ethernet is common, even on horizontal networks, you might consider moving up to 10 Gigabit Ethernet on the backbone.

Many networks also use a different medium for backbone networks. If, for example, your topmost horizontal network is 50floors above the equipment room, you will need to use fiber optic cable to span such a long distance.

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