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Chapter 2: The OSI Reference Model
Core Idea
Network modeling allows you to plan network communications by level; the model may not
correspond to an exact network implementation.
Instructor Focus
The focus of this chapter is on modeling network communications. Emphasize the difference
between models and actual communications on a network. The OSI model is the most
comprehensive and stable network model but it does not have an exact implementation (layers
Network+ Exam Objective for this Chapter/Lesson
x 1.2 Explain devices, applications, protocols, and services at their appropriate OSI layers
Terminology
Make an effort to define the following terms for the designated areas. After defining these,
calling on the students for repeat definitions as you progress through the slide deck will help
them retain the terminology.
OSI Model:
OSI reference model
Physical layer
Data link layer
Protocols:
TCP
UDP
Protocol data unit (PDU)
TCP/IP Stack:
Network interface layer
Internet layer
Transport layer (TCP/IP stack)
Application layer
Focus Questions
What is the purpose of a network model?
What are the layers of the OSI model?
What are the characteristics of each layer of the OSI model?
Icebreaker Discussion
Choose one or both of the following:
1. Why is a model of network communication important?
2. What is the purpose of a protocol?
You may need to define the term protocol for your students if they have not yet covered the
chapter as you present. Similar to network modeling, a protocol describes a common standard
Icebreaker Activity
1. In your own words and terms, describe the different levels of communication that are needed for
one machine to understand a message from another machine. This should start with the physical
connection of the machines and end with the applications being used by an end user on the
machine.
The purpose of this activity is to get the students thinking about how they would model
communication on a network. The OSI model is the standard reference model, so anyone who
Chapter 2: Network Simulator Activity
OSI Model Layer and Network Devices
OSI Model Layer Functions
Main Activities
1. Building the OSI Model
For this activity, construct the OSI model layer by layer alongside a diagram of the two devices
on a network. This will allow you to show what is added for the communication of the devices
along with the description of the layer. The slide deck provides the model, but you might want
to redraw it beside the diagram of the communication nodes for emphasis.
Layer 1: Physical Layer
For this layer, you will connect the two devices on the network. This will be the physical
connection of the media. Indicate the type of media (UTP is common) and the signal transfer
(voltage modulation):
Layer 3: Network layer
Layer 4: Transport layer
This layer adds transport protocols to encapsulate information in transit.
Layer 1: UTP with
Voltage Modulation
Layer 2: MAC Address
01:23:45:67:89:ab
Layer 2: MAC Address
01:23:45:ab:98:76
Layer 2: MAC Address
01:23:45:67:89:ab
Layer 2: MAC Address
01:23:45:ab:98:76
Layer 3: IP Address
173.72.79.93
Layer 3: IP Address
173.72.79.99
Layer 4: TCP
Layer 5: Session layer
This layer adds session information to manage communication between devices.
Layer 6: Presentation layer
The presentation layer organizes information internally for the device (such as XML translation)
and provides encryption services.
The application layer transforms data into the context of the application, such as a result sent to
a user via a GUI.
Layer 5: Session Data
Layer 5: Session Data
Layer 6: XML
Layer 2: MAC Address
Layer 2: MAC Address
Layer 3: IP Address
173.72.79.93
Layer 3: IP Address
173.72.79.99
Layer 4: TCP
Layer 5: Session Data
Layer 6: XML
Layer 7: Firefox
2. The IP Packet
For this activity, diagram and review the components of an IP packet, as shown here. To extend
3. TCP Segments Versus UDP Segments
This activity explores the structure and communications of the TCP protocol. You will show how
the transmissions are constructed, specifically with the inclusion of ports and
acknowledgements as well as covering the TCP flags. An example TCP segment is shown in the
Chapter 2: Network Simulator Activity
Matching Activity: TCPIP Protocols and Their Functions
Quiz Questions
1. What are the main differences between the OSI reference model and the TCP/IP stack model?
A number of differences can be identified, but primarily OSI allows for the description of more
granular communications, whereas the TCP/IP stack focuses on the existing protocols. OSI
2. What are the main differences between TCP and UDP?
The main difference between UDP and TCP is the acknowledgment of receipt of the message.
UDP is faster and connectionless, so the recipient is not guaranteed to have received the packet.
3. Why is the OSI model useful when it does not model network communication exactly?
Even though it does not equate precisely to the current implementation of network
technologies, the OSI model allows for a granular look at the different levels of transmission to
4. Briefly describe the different methods for synchronizing communication between two devices.
Synchronous: In a synchronous transmission, the internal clocks of both devices are
synchronized before transmission begins.
5. Briefly explain the types of switching on a network.
Circuit switching: This form of connection occupies the entire media for the exchange of
messages between two devices; other devices wishing to communicate must wait in queue.
Discussion Topic: Many students, and even working networking administrators do not fully
comprehend why there are two competing networking models. Though they have been treated as
competing models, they are not. Clearing up the difference between the OSI and TCPIP models can help
students recast them in a more accurate light and yield better understanding.
Explain that all protocols depend on programming. All physical devices ultimately are controlled by
programming chips, whether they be BIOS, or RAM based programs. OSI represents functions that
must be utilized in all networks. The programs of different networking protocols may differ in their
execution, but they are identical in function.
x Programmers use the OSI model to help them write programs that implement OSI’s networking
functions.
Figure 2.18
