Wiring Concepts and Terms

Before you start

Objectives: learn about wiring, common tools and passive networking equipment.

Prerequisites: no prerequisites.

Key terms: block, wire, pair, connect, demarcation, cable, punch, wiring, point, panel, building, connector

Typical Network

Central Office

Image 218.1 – Central Office – CO

The cloud represents the network that we’re connecting to. It might be the phone system or it might be the Internet. Connected to the Internet/phone system is a Central Office, sometimes called the CO, or in the case of a telephone, the Local Exchange Carrier or LEC. In case of the Internet it is called the Internet Service Provider or ISP. This is an office that connects to the main network.

When we contract for service with the LEC, they will install a line to our customer site. The place where the line enters our building is called the demarc, or the Demarcation Point. The demarcation point separates the portion of the network that is the responsibility of the LEC from the part of the network that is our responsibility. The demarc is also called the Minimum Point Of Entry (MPOE) or the End User Point Of Termination (EU-POT). Typically, the LEC or other service provider is responsible for everything from the demarc into the main network, and we are responsible for everything on the other side of the demarc. The demarc is typically a connection just inside of our building. It’s usually on the main floor or in a basement area.

Demarcation Point

Image 218.2 – Demarcation Point

The first entry point within the building is typically called a Main Distribution Frame (MDF). MDF is the main wiring closet within the building. Within the MDF, wires connect the MDF with wiring closets on all floors, and these are often called IDFs or Intermediate Distribution Frames. The IDFs are typically located directly above the MDF, so that we can have a straight line running up to the IDF, on each floor. A vertical cross connect connects the MDF with the IDFs on each floor. It’s vertical because that’s the direction that the cable runs.


Image 218.3 – MDF and IDF

Depending on our implementation, we may also have additional wiring closets on the same floor. In that case, we have two or more IDFs and they are connected with horizontal cross connects.

More IDFs On The Same Floor

Image 218.4 – More IDFs On The Same Floor

This would be the case if we own the whole building. Now, let’s say that our business is located on the first floor. We’ve contracted with an ISP to get some Internet service and they install a line to the demarc in the bottom floor of the building. In this case, we actually need to get the wiring from the demarc up to the first floor into our wiring closet. To do this we need to install a demarc extension. The demarc extension is simply a line that goes from the demarc in one location to another location. The demarc extension typically consists of a single wire bundle that attaches to the existing demarc and supplies a termination point to a different location. If we left the demarc in the bottom floor, other businesses at other floors might have access to our demarc. Typically, the service provider does not install the demarc extension unless we pay them extra, or we can contract with another service to do that. In some cases we can even do that ourselves.

Wiring Closet

Within a wiring closet we’ll typically have Switches, Routers and other equipment. We may connect our computers to the Patch Panels or even directly to the Switches and run UTP or other cable out to our computers.

Vertical Connections

For the vertical cross connects and the horizontal cross connects we don’t necessarily want to string multiple CAT5 cables (although many implementations use CAT5 cables). In this case there’s a special cable that we can use. There’s a 25 pair cable, which holds 25 twisted pair sets (50 wires), or we can also get 100 pair cable (200 wires). And there’s other cable bundles as well. In each case these are similar to CAT5 cable, except for they have more pairs of copper wiring.

25 pair cable

25 pair cables consist of 25 pairs of copper wires in a single bundle (containing a total of 50 wires). 25 pair cables are often used for telephone installations that have multiple telephone lines, to replace multiple Cat3/5/5e/6 cables in a single bundle. They are also used for horizontal and vertical cross connects between the MDF and IDFs.

25 Pair Cable

Image 218.5 – 25 Pair Cable

Individual wires within the 25 pair cable use the color coding scheme. A total of 10 colors are used in two different groups:

  • Group 1 colors are white, red, black, yellow, and violet.
  • Group 2 colors are blue, orange, green, brown, and slate.

There are 5 wires of each color. Every colored wire in Group 1 is paired with each color in Group 2. For example, we will have the following pairs for the white wires:

  • White with blue
  • White with orange
  • White with green
  • White with brown
  • White with slate

Instead of using solid colors, some schemes use striped wires to uniquely identify each wire and its matching wire. For example, the white wire with a blue stripe is twisted with the blue wire with a white stripe, and the red wire with the orange stripe is twisted with the orange wire with the red stripe. An RJ-21 connector is used to connect 25 pair cable to other wiring devices, or you can manually connect each wire to the necessary location.

RJ21 Male Female

Image 218.6 – RJ21 Male Female

100 pair cable

100 pair cable consists of 100 pairs of copper wires in a single bundle (containing 200 wires). 100 pair wires use the same coloring scheme as 25 pair wires, repeated 4 times. Each bundle of 25 wires is often wrapped together with a colored nylon string to help separate wires of the same color.

Punch Down Block

To connect 25 pair cable to a single CAT5 cable we use Punch Down Blocks. That’s a block with metal connectors that we can use to connect the individual pairs of wires together. So we may have a 25 pair cable on one side of the Punch Down Block that connects to multiple CAT5 cables on the other side.

Punch Down Block Example

Image 218.7 – Punch Down Block Example

66 block

A 66 block is a Punch Down Block used for connecting individual copper wires together. The 66 block has 25 rows of four metal pins. Pushing a wire into the pin pierces the plastic sheath on the wire, making contact with the metal pin. There are two different 66 block configurations:

  • With the 25 pair block (also called a non-split block), all 4 pins are bonded (electrically connected). Use the 25 pair block to connect a single wire with up to 3 other wires.
  • With the 50 pair block (also called a split block), each set of 2 pins in a row are bonded. The left pin is connected with the middle left pin, while the right pin is connected with the middle right pin. Use the 50 pair block to connect a single wire to one other wire.

With a 50 pair block, we can use a bridge clip to connect the left two pins with the right two pins. Adding or removing the bridge clip is an easy way to connect wires within the row for testing purposes. 66 blocks are used primarily for telephone applications. When used for data applications be sure to purchase 66 blocks rated for Cat5. When inserting wires in the block, place both wires in a pair through the same slot to preserve the twists as much as possible.

110 block

A 110 block is a Punch Down Block used for connecting individual wires together. The 110 block comes in various sizes for connecting pairs of wires (for example 50, 100, or 300 pair). It has rows of plastic slots. Each plastic slot connects two wires together. To connect two wires, place the first wire in the plastic slot on the 110 block, then insert a connecting block over the wire and slot. The connecting block has metal connectors that pierce the plastic cable sheath. Then place the second wire into the slot on the connecting block.

C-4 connectors connect four pairs of wires; C-5 connectors connect five pairs of wires. When connecting data wires on a 110 block, we typically connect wires in the following order:

  • White wire with a blue stripe, followed by the solid blue wire.
  • White wire with an orange stripe, followed by the solid orange wire.
  • White wire with a green stripe, followed by the solid green wire.
  • White wire with a brown stripe, followed by the solid brown wire.

We can use BLOG term (BLue-Orange-Green) to remember the wire order, and remember to start with the white striped wire first. 110 blocks can be used for both telephone and data, and are better suited for Cat5 installations. When using for Cat5 wiring, be sure to preserve the twists in each wire pair to within one-half of an inch of the connecting block.

Patch Panel

From the Punch Down Block we can take multiple UTP cables and connect them to the Patch Panel. A Patch Panel includes multiple RJ-45 connectors on the one side, with bare wire connectors on the back. A Patch Panel is a device that typically connects individual stranded wires into female RJ-45 connectors. For example, we might connect 4 pairs of wires from a Punch Down Block to a port on the Patch Panel. Then we can connect drop cables (cables with RJ-45 connectors) to the patch panel on one end and a computer, hub, switch, or router on the other end.

Patch Panel

Image 218.8 – Patch Panel

Punch Down Tool

We will use a Punch Down Tool to insert wires into 66 or 110 blocks. The Punch Down Tool pushes the wire into the block and cuts off the excess wire. We have to be sure to position the blade on the side of the clip towards the end of the wire. The blade for a 66 block is straight, while the blade for a 110 block has a notch in the blade.

Punch Down Tool

Image 218.9 – Punch Down Tool


When we contract for phone or other services, the service provider installs a line to our building to the Demarcation Point, or the Demarc. The service provider is responsible for everything up to the Demarc, and we are responsible for everything within our building past the Demarc. The Demarc is typically installed into the MDF, which is on the main floor, and the MDF connects with vertical cross connects to the IDFs on each floor. A Demarc Extension extends the Demarc from the entry point to another location. 25 pair and 100 pair cable is used to connect the MDFs and the IDFs together. Punch Down Blocks are used to connect the individual strands within the 25 or 100 pair cables to other devices, such as a Patch Panel, or to the CAT5 cables which then connect to other devices.