What's the Difference Between Backbone and Horizontal Cabling?
Written by Dave Harris, trueCABLE Technical Specialist, BICSI INST1 Certified
Networks can be pretty big sometimes
Most of us are at least somewhat familiar with the networks in our homes. A cable from the service provider is brought into the building and plugs into a modem or, more likely, a router. The router is usually a self-contained unit that also features a built-in Ethernet switch, so a small number of devices can connect to it with Ethernet cables. These days, the router also usually contains a wireless access point, so more devices can connect to the internet via Wi-Fi. So all of the network devices in your home connect to the router (or switch) which in turn connects to the outside world.
So that’s the way that a typical home network is put together. It has a router and/or switch in the middle and devices at various locations around it connected by cables or Wi-Fi. Have you ever wondered about how a larger network is put together? Like a network in a large office building or a college campus or a world-wide corporation? No? Well, we’re going to talk about it anyway.
That description of a router and/or switch in the middle connected to devices at various locations around it can be thought of as the layout, or the arrangement, or maybe the map of the network. In the language of “Networkese,” we call it the network topology.
One of the first-used and simplest network topologies is called a bus topology. Figure 1 is a simple diagram of a bus topology. That’s where all of the devices in the network share a single cable, with a terminator at each end of the cable. Can you imagine the logjam that would occur with dozens of devices sharing a single network cable? Almost nobody uses a bus topology anymore.
Figure 1. Arrangement of network devices in a bus topology
Another example of a network topology is called a ring topology. This is where the first device in the network is connected to the next device, then from the second device to the third, and so on. This chain continues until the final device is reached and then it is connected back to the first device, resulting in a circular or “ring” arrangement (Figure 2). I think you can probably see the problem here. If one device spits up, the whole network goes down. That might be manageable for a very small network, but for a large network? Mayhem. So, with the exception of network switch stacks, ring topologies have fallen into disuse. And don’t worry about switch stacks. I have to mention them so that an army of network engineers aren’t compelled to send me corrections, when they have so much more important stuff to do.
Figure 2. Arrangement of network devices in a ring topology
So if those network topologies are bad, is there anything left over that’s good? Of course! It’s the one we described at the beginning. The one you probably have in your home. There is a device that controls the network at the logical (if not the physical) center, with the other network devices cabled to it. This arrangement is called a star topology. A simple diagram of a star topology is shown in Figure 3.
Figure 3. Arrangement of network devices in a star topology
Structured Cabling Systems
While browsing through our Cable Academy, you might have come across one of our blogs with the title, Maximum Ethernet Cable Length. If you have read that blog (no reason why you should have; it wasn’t assigned or anything), you would have learned that the maximum length of cable between a network switch and a device is 328 feet (100 meters). As you know, there are many buildings and campuses that are bigger than that. Think schools, warehouses, airports, amusement parks–you know, big places. They have to have many switches located in different zones in order to cover all that geography. Sticking with simple diagrams, such a network looks like what we see in Figure 4.
The network in Figure 4 is perhaps the simplest example of what’s known as a structured cabling system (SCS). Actually, no one would call this a structured cabling system because it would have to include all of the connecting hardware, mounting hardware, blueprints, testing procedures and results, and much more. But it suits our purposes here because that red line connecting two switches symbolizes the function of a backbone cable. And those blue lines connecting the switches to the devices symbolize the function of horizontal cables.
Figure 4. Extremely oversimplified diagram of a structured cabling system
Okay, wait a minute. So that big long red horizontal line is not a horizontal cable? And all those blue lines that aren’t horizontal are horizontal cables? That’s supposed to make sense? Well, if we change either our viewing position or the page’s orientation so that the page is in the horizontal plane, then they’re all horizontal. But we’re not talking about geometry here. The term horizontal refers to function, not direction.
It bears repeating that Figure 3 is offered as a simple logical diagram. Network switches are available with many more than eight ports to connect to, which can result in much larger networks. In these scenarios, cables are not routed from the switch directly to the devices, as they often are in our homes, or as shown in Figures 3 and 4.
In larger networks, cables are routed through walls and ceilings terminating at jacks permanently installed in the walls. Then the devices connect to the wall jacks through short patch cables. The other end of the cable is permanently installed into a fixture called a patch panel. The path to the network switch is then completed by installing a short patch cable between the patch panel and the access switch. The portion of a channel that is permanently installed between the patch panel and a wall jack is known as a permanent link. The cables that make up the permanent links are the horizontal cables. And all of the permanent links together comprise the horizontal cabling within the structured cabling system.
In Figure 5, the horizontal cabling is again shown in blue. These are the cables that are permanently connected to a patch panel in a telecommunications room on one end and to wall or ceiling jacks on the other end. Then those permanent links are connected to an access switch in the same telecommunications room. And the access switch is connected to a core switch or a distribution switch, probably in a whole different room. But that wouldn’t be a horizontal cable, would it? No, a cable connecting two switches in two different rooms is almost always a backbone cable.
Figure 5. Horizontal, intrabuilding backbone, and entrance cabling within a structured cabling system
Earlier, we characterized a backbone cable as one that connects two switches. That is certainly true, but as you might expect, there is a little more to it than that. A backbone cable is part of a backbone system which in turn is part of the structured cabling system.
In Figure 5, the backbone cabling is shown in white. Since all of these backbone cables are contained within the same building, they are called intrabuilding backbone cables. Each backbone cable leads from the access switch (or switches) in a telecommunications room to another room where they all connect to another switch. Note that even though these backbone cables are running mostly parallel to each other, they all connect to the same switch in the logical center. Therefore, this is still an example of a star topology.
If this is the only building in the network, the central switch is most likely a core switch, which is connected to an entrance cable from the service provider. If there are other buildings in the network, the central switch in this building is most likely a distribution (or aggregation) switch, which is in turn connected to a core switch in another building. In that case, the entrance cable coming up through the floor isn’t from the service provider, it’s an interbuilding backbone cable. A diagram of a campus network with interbuilding backbone cabling is shown in Figure 6.
Figure 6. Interbuilding backbone cabling
The backbone pathway shown in Figure 6 includes provisions for using some of the conduit more than once, so that the cabling maintains a star topology, even though the conduit doesn’t.
These days, the cables comprising a backbone system are usually fiber optic cables. This is not necessarily because of network speeds, even though fiber optic cables can usually operate at greater speeds than copper cables. The backbone cables, like the horizontal cables, are limited to the speed allowed by the service provider, when connecting outside the local network. When operating within the local network, speeds are only limited by the capabilities of the backbone infrastructure made up of the cables and switches. So within the network, speeds of 10gbps and even higher are routine.
Speed is important, but the big payoff for using fiber optics for the backbone is provided by their significant bandwidth advantage over copper. All of those horizontal cables normally feed just one or two devices each, but the backbone cables have to feed all of the horizontal cables at once. Fiber provides a much wider path.
If it seems like you have just read a lot of words like “probably” and “most likely,” it’s true; you have. Networks are designed individually with the needs of the organizations that depend on them given first priority. Structured cabling systems are also designed individually in order to provide for the specific needs of the prescribed network, and the unique aspects of geography and construction at each site. So every installation is different. That’s why there are people who spend their entire lives perfecting their network and cable plant design skills. All of those “probablys” and “likelys” represent my feeble attempts to avoid crucifixion by them in print. Seriously, though, I hear they’re actually very intelligent and hard-working professionals. Forgiving, too.
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