Troubleshooting Customer Premise Equipment – CompTIA Network+ N10-006 – 4.8

Our customer premise equipment includes a number of troubleshooting features. In this video, you’ll learn about demarcation points, smartjacks, CSU/DSUs, and copper line drivers.
<< Previous: Troubleshooting WAN InterfacesNext: Company Security Policies >>

When you’re dealing with wide area networks, you’re almost always working with a third party that’s not part of your organization. Because of that, there needs to be a clear delineation between what is inside the network and your responsibility and what is outside the network or the wide area network that is the responsibility of the wide area network provider. We call this line in the sand the demarcation point. This is the place where both sides meet and you’re able to make a very clear determination of what is your responsibility and what is the provider’s responsibility.

You have these demarcation points not only at an office or an organization that’s very large, but you also have them at home. There has to be a determination of where your cable company connects to your home, or your telephone company connects to your home. If it’s in a building, it’s usually in a central point. All the demarcations may be coming into a single room, and then you’re feeding off to your local networks from that single room. Sometimes, it’s a simple RJ45 jack on the wall. That’s a simple demarcation point, but it’s certainly speaks to where you can separate the wide area network on the outside from the wide area network connectivity on the inside.

This is also the place where you commonly connect your CPE, that stands for customer premises equipment. That means that your router, and your CSU/DSU, and anything else that is your responsibility is the customer premises equipment. The wide area network providers often refer to this as the customer prem. The network connection that creates that demarcation between the inside and the outside is often referred to as a network interface unit or NIU. You might also hear this referred to as a network interface device or a telephone network interface.

Sometimes, you have more than just a simple jack on the wall. This might be a bit of electronics that add some additional capabilities called a smartjack. This is something like a circuit board in a chassis, very similar to the picture that you see here. It’s a power device. There are lights on the device that can usually give you an idea of the status of the wide area network connection, and because it is a piece of electronics, there’s usually diagnostics built into this device as well. You can turn on a loopback from the smartjack or get other network statistics directly from the smartjack rather than relying on a loopback that’s coming from the central office.

The smartjack is usually under the control of the wide area network provider, and you can see, in this case, that it is locked up with a screen in front of it, so that the customer does not have direct access to the device. But you can still look through the clear window there and see that there are status lights and messages that you can get from the blinking displays that are on the front of the smartjack. The device that usually connects you to the wide area network provider is the CSU/DSU. This stands for channel service unit and data service unit.

The CSU is what connects to the network provider. It then converts the network provider’s communication to something that your router can understand on the inside using the DSU. This is something that is usually connecting to the data terminal equipment or the DTE side of the wide area network. This might be an external physical device similar to the one that you see here, or it may be an adapter card that’s integrated into the router that you’re using to connect to the wide area network.

The wide area connection from the demarc is usually an RJ-45 looking connector, but it’s wired a little bit differently than our Ethernet. So you may see it referred to as an RJ-48c because of the wiring that it uses. From the CSU/DSU to the router is usually a serial connection. It may be using one of these big square v.35 connectors or it might be a 9 pin or 25 pin serial cable connection that we call RS-232. Sometimes, the CSU/DSU will have jacks on it that you can connect to with a special monitor jack cable, and then connect to a protocol analyzer to see exactly what might be happening across that wide area network.

With wide area networks we’re going over very long distances, so the wide area network provider is usually going to have some type of signal regeneration or extender built into the lines. This is not something that you might normally see, but it’s certainly something that the provider’s going to use on their side. You often see this on serial links. You could also use this on copper Ethernet. It just depends on the type of media converter or media extension device that you’re using.

This is often a powered device because it is regenerating a signal between one link and another, and it’s sometimes useful when you’re performing testing after the installation. And although these devices provide an important capability for extending our networks over very long distances, they can occasionally have problems. So if you need to troubleshoot one of these line drivers or line extenders, you need to test after you install it, make sure that you’re able to send traffic through without any errors.

You should also check if you’re not getting any signal across this link to make sure that the device is powered up, because it does rely on power to regenerate that signal. And occasionally, a bad power supply or a noisy power supply will also introduce errors onto the network media as well, so you want to be sure that every component within this line driver is working optimally.