Identify the components of wiring distribution. This chapter covers in detail the different wiring distribution components, including distribution frames, demarcation points and extensions, smart jacks, and the CSU/DSU.
Distribution Frames
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. Distribution frames are the central concentrator for the cabling system, for example, cables that come from the data center and go to the user blocks, and they can come in multiple forms:
- Free-standing (vertical blocks that are installed on the floor)
- Installed on the wall
Distribution frames can also come in multiple types, based on the media type and purpose:
- Distribution frames for copper
- Distribution frames for fiber
- Distribution frames for data
- Distribution frames for voice
In large environments, distribution frames are placed in a dedicated room because they are a critical part of the network, as all the data flows in the network go through the distribution frames. Distribution frames for specific signal types have special names:
- Audio distribution frame (ADF)
- Digital distribution frame (DDF)
- Optical fiber distribution frame (ODF)
- Video distribution frame (VDF)
The main distribution frame (MDF), shown in Figure 20.1 below, 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 the exit point for any external network.
Figure 20.1 – Main Distribution Frame
The MDF is a very good test point because all the network data passes through this point. You can test connections in both directions, either from the MDF to the DC equipment or from the MDF to the user areas. You can also test WAN connections here to find out whether a problem is within the network or on the ISP side.
Intermediate distribution frames (IDFs), shown in Figure 20.2 below, are extensions of MDFs and they are placed at strategic distribution points in the network infrastructure, for example:
- In another room
- On another floor
- In other buildings in the campus
Figure 20.2 – Intermediate Distribution Frame
IDFs are the points that connect the uplinks from the MDF with the workgroup switches (connecting end-users) or other local resources (printers, etc.). IDF systems are often connected to the MDF with high-speed fiber connections. IDFs are generally used in medium to large organizations where users are split over large geographical areas. Small networks usually have a single MDF that aggregates all the connections.
Demarcation Points
Demarcation points are network spots that connect the inside infrastructure to the outside world. They are usually found in central locations of the network and they can connect the following:
- A WAN connection
- An Internet connection
Demarcation points are present not only in enterprise environments but also in small networks or even residential locations (e.g., spots that connect the TV, the telephone, and the ISP).
In enterprise environments, demarcation points are usually placed in the main data center, as they need to have a close connection to the main network devices. This can be something like a simple box with RJ45 or fiber connectors that can aggregate WAN links.
The demarcation point at the customer location is usually referred to as customer premises equipment (CPE) and it directly connects to this equipment. The ISP assumes responsibility for the connection up until it reaches the demarcation point box (i.e., the CPE). Everything that happens after that point falls under the customer’s responsibility.
Smart Jacks
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 (see Figure 20.3 below) but they are also called:
- Network interface devices (NIDs)
- Network interface units
- Telephone network interfaces
Figure 20.3 – Smart Jacks
The smart jack is more than just a simple interface. It can be in the form of a circuit card in a chassis and it is often useful for the WAN or ISP so it can check the status of the connection. The service provider can run remote diagnostics and tests on the WAN connection by sending signals to the smart jack to detect any problems with sending traffic from their side to the customer side.
Smart jacks also include alarm indicators and sensors so the service provider can remotely check the status of the demarcation point.
CSU/DSU
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)
The CSU/DSU can be a physical device placed between the demarcation point and the router or it can be integrated directly into a router (in this case, the demarcation point can be directly connected to the router). An example of a CSU/DSU device is shown in Figure 20.4 below:
Figure 20.4 – CSU/DSU Device
From the demarcation point, you can plug in the CSU/DSU device with a standard RJ45 cable but the wiring used inside it is called RJ-48c, which uses the same connector as Ethernet links but with a different set of pins. The connection from the CSU/DSU device to the router is usually a serial link (e.g., RS-232), which converts the digital signal coming from the WAN side into a “router-friendly” signal.
The CSU/DSU device can also have a number of diagnostic jacks that can be used with dedicated equipment to monitor the WAN signal’s health and status.
Summary
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)
Wikipedia structured cabling guide.
Pass your Network+ using 101 Labs – CompTIA Network+.
