A distribution frame is a physical part of the network that acts as a passive cable termination. This is where cables are punched down and it consists of patch panels and punch-down blocks. The distribution frame is a central concentrator for the cabling system, for example, cables that come from the data center and go to the user blocks. Distribution frames can come in multiple forms.
The main distribution frame (MDF) is the central part of the network and it is usually placed in the data center. This is where all the data from the DC devices go before it is distributed to other network areas. The MDF connects the internal network to the WAN so it is also the exit point for any external network.
Intermediate distribution frames (IDFs) are extensions of MDFs, and they are placed at strategic distribution points in the network infrastructure.
Demarcation points are network spots that connect the inside infrastructure to the outside world. They are usually found in central locations in the network.
Sometimes the box that creates the demarcation point in the network is in fact a specialized and intelligent device. They are usually referred to as smart jacks but they are also called:
- Network interface devices (NIDs)
- Network interface units
- Telephone network interfaces
The channel service unit/data service unit (CSU/DSU) is a device that is positioned between the circuit connection provided by the WAN provider and the router on the other side. When the signal arrives from the digital WAN connection, it is converted into a signal the router can understand.
The CSU/DSU device, which is a single box, contains two logical elements that provide different services:
- The CSU faces the service provider side
- The DSU provides data for the CPE (router)