Once a WAN connection has made its way across the country or the world, it needs a place to terminate. In this video, you’ll learn about demarcation points, CSU/DSU connectivity, and smartjacks.
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When you’re working with a third party network provider, there needs to be some point where the hand-off is made between the provider’s network and your internal network. This is the demarcation point or the demarc, and it’s often a network interface on the outside of your building or it may be an interface location on the inside of your building. We commonly have wide area network demarcs in our businesses, but we also have demarcs on the outside of our house where the phone provider or the cable company will provide that handoff point to the rest of the devices in your home.
In your business, there’s probably a central point in your data center where all of the demarcs from all of the providers are brought in. For your home, you probably have a box on the side of the house that’s providing an RJ-11 and RJ-45 or a culex connection for your internet connectivity. On your side of the demarc, you connect your CPE or your customer premises equipment. You may hear this abbreviated as your customer prem.
If this is a traditional t1 or t3 connection, your customer premises equipment may first be a CSU/DSU. This is a channel service unit/ data service unit that sits between your router and the circuit that was just installed from the demarc. The CSU part of this device connects to the network provider side, and the DSU part of this device connects to your data terminal equipment or DTE. This is commonly a router that you might have in your environment.
This is commonly providing the conversion between the providers digital wide area network and a serial connection that can be understood by your router. Sometimes this CSU/DSU can be an external device, but you’ll often see the CSU/DSU functionality built directly into a router. So you would come directly from the demarc and connect immediately into a router where the internal DSU performs the same functions as you would have in an external device.
From the demarc we often connect with an RJ-45 light connection that’s technically an RJ-48c. It might also come in and connect through a 15 pin connector to a network interface on a CSU/DSU. There are a number of different ways to connect between the CSU/DSU and the router, but some of the more common might use a v35 connection, which is this larger blocky connection or it might be a 25 pin serial connection between the CSU/DSU and the router.
Some CSU/DSUs also include monitor jacks that allow you to connect diagnostic equipment without disrupting the connection that’s already in place. Sometimes the wide area connection at the demarc is a bit more intelligent, and instead of having just a cable handoff, we would have a smart jack. This is also commonly called a network interface unit or NIU.
The smart jack is more than just a simple cable handoff, as you can see by these pictures. These are inside the provider’s side of the communication, so you don’t usually have access to the equipment that’s inside of the smart jack. The smart jack is a way for your wide area network provider to perform some additional functions.
For example, they could set up a loopback and provide diagnostics directly from this interface at the demarc. This can also provide alarm information and reconfiguration details for the wide area network provider, so they can make a number of changes on the smart jack without having to visit your location.
Category: CompTIA Network+ N10-007