network+ review guide 3.8
Identify components of wiring distribution
Common computer networks have many more components than first
meets the eye, that is, unless you are the involved in the initial installation
of the network! In that case, you may be involved in purchasing and installing
the components that will connect the computers throughout your organization's
building. You might also be involved in verifying that all the network
components have been installed properly and tested. In the following sections,
I'll identify each of these components and the process of verifying their proper
installation.
IDF
Often, a wire frame, called an intermediate
distribution frame (IDF), is located in an equipment or telecommunications
room. It is connected to the MDF (discussed next) and is used to provide greater
flexibility in regard to the distribution of the communications lines to the
building. In other words, it's just one more place where a network administrator
or telephone administrator can change their mind and redirect the signal. It is
typically a sturdy metal rack that is designed to hold the bulk of cables that
are coming from all over the building. The same frame might also hold networking
equipment such as routers, switches, and backup drives.
MDF
The main distribution frame (MDF) is a wiring point that is
generally used as a reference point for telephone lines. It is installed in the
building as part of the prewiring, and the internal lines are connected to it.
Then all that's left is to connect the external (telephone company) lines to the
other side, and the circuit is complete. These often have protection devices for
lightning or other electrical spikes. In addition, they are often used as a
central testing point.
Demarc
The point at which the operational control or ownership changes
from your company to a service provider is referred to as a demarcation point,
or demarc. This is often at the MDF in relation to
telephone connections and the CSU/DSU (discussed later) in regard to WAN
connections. When troubleshooting, network administrators will often test for
connectivity on both sides of the demarc to determine whether the problem is
internal or external.
Demarc extension
If your offices are in a building with many other tenants, you
then have another section of wiring with which to contend. Most office buildings
have a central location where the service provider terminates fiber and copper
connections. From there, it's either up to the building owner or up to the
individual tenants to get the communications cables to the office suites. The
length of copper or fiber that is after the demarc but still not up to your
office is referred to as a demarc extension. Many cabling
companies specialize in installing and upgrading these extensions. If you are
considering office space, you should verify that these connections are solid and
offer you the connectivity and bandwidth you need.
Smart jack
A special network interface is often used between the service
provider's network and the internal network. The device provides for code and
protocol conversion, making the signal from the service provider usable by the
devices on the internal network, such as the CSU/DSU. This device, called a network interface device or smart jack,
often also serves as the demarcation point between the inside wiring and the
outside wiring. It often contains the connections and electronic testing
equipment to perform local loopback tests and other types of troubleshooting.
Figure 3.28 shows an example of a modern smart jack.
CSU/DSU
The channel service unit/data service unit (CSU/DSU) is a
digital interface device used to connect a router to a digital circuit, such as
a T1 or T3 line. On one side of the device is the WAN, and on the other side is
the entrance to the LAN. The CSU/DSU is actually two devices in one, each with a
specific purpose. The CSU provides the termination of signal, connection
integrity, and line monitoring. The DSU converts the T-carrier line frames into
frames that the LAN can interpret. You can generally lease a CSU/DSU from your
service provider, or you can buy one of your own. In either case, a CSU/DSU is
an indispensable piece of equipment in a network with digital circuit
connections.
Exam Essentials
Know the components of wiring distribution
You should know the purpose for IDFs, MDFs, and other network equipment. You
should understand the purpose of CSU/DSU. Finally, you should know that a smart
jack at the demarc is often used to isolate whether a network problem is
internal or external.
Understand the importance of verifying wiring
installation and termination You should realize that a network wiring job
in a large building can be a complex undertaking. You should understand that
many people may be involved and that their relative skill levels will vary
greatly. Because of this, a competent network wiring installation will involve
extensive testing and documentation.