Chapter 2: The OSI Model
Outline:
Chapter Review
Chapter Overview
2.1
2.2
2.3
General Model of Communication
2.1.1
Using layers to analyze problems in a flow of materials
2.1.2
Source, destination, and data packets
2.1.3
Media
2.1.4
Protocol
2.1.5
The evolution of ISO networking standards
The OSI Reference Model
2.2.1
The purpose of the OSI reference model
2.2.2
The names of the seven layers of the OSI reference model
2.2.3
Descriptions of the seven layers of the OSI reference model
2.2.4
Encapsulation
2.2.5
Names for data at each layer of the OSI model
Comparison of the OSI Model and the TCP/IP Model
2.3.1
The importance of the TCP/IP reference model
2.3.2
Names and descriptions of the layers of the TCP/IP reference model
2.3.3
TCP/IP protocol graph
2.3.4
Comparison of the OSI model and the TCP/IP model
2.3.5
Use of the OSI and the TCP/IP models in the curriculum
Chapter Summary
Chapter Quiz
2
Overview
The OSI Model
During the past two decades there has been a tremendous increase in the numbers and sizes
of networks. Many of the networks, however, were built using different implementations of
hardware and software. As a result, many of the networks were incompatible and it became
difficult for networks using different specifications to communicate with each other. To address
this problem, the International Organization for Standardization (ISO) researched many network
schemes. The ISO recognized that there was a need to create a network model that would help
network
builders
implement
networks
that
could
communicate
and
work
together
(interoperability) and therefore, released the OSI reference model in 1984.
This chapter explains how standards ensure greater compatibility and interoperability between
various types of network technologies. In this chapter, you will learn how the OSI reference
model networking scheme supports networking standards. In addition, you will see how
information or data makes its way from application programs (such as spreadsheets) through a
network medium (such as wires) to other application programs located on other computers on a
network. As you work through this chapter, you will learn about the basic functions that occur at
each layer of the OSI model, which will serve as a foundation as you begin to design, build and
troubleshoot networks.
2.1 General Model of Communication
2.1.1 Using layers to analyze problems in a flow of materials
The concept of layers will help you understand the action that occurs during communication
from one computer to another. Shown in the Figure
are questions that involve the movement
of physical objects such as highway traffic, or electronic data. This motion of objects, whether it
is physical or logical, is referred to as flow. There are many layers that help describe the details
of the flow process. Other examples of systems that flow, are the public water system, the
highway system, the postal system, and the telephone system.
Now examine the Figure
"Comparing Networks" chart. What network are you examining?
What is flowing? What are the different forms of the object that is flowing? What are the rules for
flow? Where does the flow occur? The networks listed in this chart give you more analogies to
help you understand computer networks.
Another example of how you might use the concept of layers to analyze an everyday subject is
to examine human conversation. When you create an idea that you wish to communicate to
another person, the first thing you do is choose how you want to express that idea, then you
decide how to properly communicate it, and finally, you actually deliver the idea.
Imagine a young boy seated at one end of a very long dinner table. On the other end of the
table, quite a distance away, sits the young boy's grandmother. The youngster speaks English.
The grandmother prefers to speak Spanish. The table has been set with a wonderful meal that
the grandmother has prepared. Suddenly the young boy shouts at the top of his lungs, "Hey,
you! Give me the rice!" and reaches across the table to grab it. In most places, this action is
considered quite rude. What should the young boy have done to communicate his wishes in an
acceptable manner?
To help you find the solution to this question, analyze the communication process by using
layers. First there is the idea – the young boy wants rice; then there is the representation of the
idea– spoken English (instead of Spanish); next is the method of delivery – "Hey, you"; and
finally, the medium – shouting (sound) and grabbing (physical action) across the table for the
rice.
From this group of four layers, you can see that three of them prevent the young boy from
communicating his idea in an appropriate/acceptable manner. The first layer (the idea) is
acceptable. The second layer (representation), using spoken English instead of Spanish, and
the third layer (delivery), demanding instead of a politely requesting, most definitely do not
follow acceptable social protocol. The fourth layer (medium), shouting and grabbing from the
table rather than politely requesting assistance from another person seated nearby, is
unacceptable behavior in most any social situation.
By analyzing this interaction in terms of layers you can understand more clearly some of the
problems of communication in both humans or computers, and how you might solve them.
Web Links
The OSI Reference Model
2.1.2 Source, destination, and data packets
As you learned in chapter 1, the most basic level of computer information consists of binary
digits, or bits (0s and 1s). Computers that send one or two bits of information, however, would
not be very useful, so other groupings - bytes, kilobytes, megabytes, and gigabytes - are
necessary. In order for computers to send information through a network, all communications
on a network originate at a source, then travel to a destination.
As illustrated in the Figure, the information that travels on a network is referred to as data,
packet, or data packet. A data packet is a logically grouped unit of information that moves
between computer systems. It includes the source information along with other elements that
are necessary in order to make communication possible and reliable with the destination device.
The source address in a packet specifies the identity of the computer that sends the packet.
The destination address specifies the identity of the computer that finally receives the packet.
Web Links
The OSI Reference Model
2.1.3 Media
During your study of networking, you will hear references to the word "medium". (Note: The
plural form of medium is media.) In networking, a medium is a material through which data
packets travel. It could be any of the following materials:

telephone wires

Category 5 UTP (used for 10BASE-T Ethernet)

coaxial cables (used for cable TV)

optical fibers (thin glass fibers that carry light)
There are two more types of media that are less obvious, but should nonetheless be taken into
account in network communications. First, is the atmosphere (mostly oxygen, nitrogen, and
water) that carries radio waves, microwaves, and light.
Communication without some type of wires or cables is called wireless or free-space
communication. This is possible using electromagnetic (EM) waves. EM waves, which in a
vacuum all travel at the speed of light, include power waves, radio waves, microwaves, infrared
light, visible light, ultraviolet light, x-rays, and gamma rays. EM waves travel through the
atmosphere (mostly oxygen, nitrogen, and water), but they also travel through the vacuum of
outer space (where there is virtually no matter, no molecules, no atoms).
Web Links
The OSI Reference Model
2.1.4 Protocol
In order for data packets to travel from a source to a destination on a network, it is important
that all the devices on the network speak the same language or protocol. A Protocol is a set of
rules that make communication on a network more efficient. Some common examples are as
follows:

In Congress, a form of Roberts Rules of Order makes it possible for hundreds of
representatives, who all like to talk, to take turns, and to communicate their ideas in an
orderly manner.

While driving a car, other cars (should!) signal when they wish to make a turn; if they did
not, then the roads would be chaos.

While flying an airplane, pilots obey very specific rules for communication with other
airplanes and with air traffic control.

When answering the telephone, someone says, "Hello," then the person calling says,
"Hello. This is.... "; and so it goes back and forth.
One technical definition of a data communications protocol is: a set of rules, or an agreement,
that determines the format and transmission of data. Layer n on one computer communicates
with Layer n on another computer. The rules and conventions used in this communication are
collectively known as the Layer n protocol.
Web Links
The OSI Reference Model
2.1.5 The evolution of ISO networking standards
The early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980's
saw tremendous increases in the numbers and sizes of networks. As companies realized the
money they could save and the productivity they could gain by using networking technology,
they added networks and expanded existing networks almost as rapidly as new network
technologies and products could be introduced.
By the mid-1980's, these companies began to experience growing pains from all the expansions
they had made. It became harder for networks that used different specifications and
implementations to communicate with each other. They realized that they needed to move away
from proprietary networking systems.
Proprietary systems are privately developed, owned, and controlled. In the computer industry,
proprietary is the opposite of open. Proprietary means that one or a small group of companies
controls all usage of the technology. Open means that free usage of the technology is available
to the public.
To address the problem of networks being incompatible and unable to communicate with each
other, the International Organization for Standardization (ISO) researched network schemes like
DECNET, SNA, and TCP/IP in order to find a set of rules. As a result of this research, the ISO
created a network model that would help vendors create networks that would be compatible
with, and operate with, other networks.
The process of breaking down complex communications into smaller discrete tasks could be
compared to the process of building an automobile. When taken as a whole, the design,
manufacture, and assembly of an automobile is a highly complex process. It’s unlikely that one
single person would know how to perform all the required tasks to build a car from
scratch. This is why mechanical engineers design the car, manufacturing engineers design the
molds to make the parts, and assembly technicians each assemble a part of the car.
The OSI reference model (Note: Do not confuse with ISO.), released in 1984, was the
descriptive scheme they created. It provided vendors with a set of standards that ensured
greater compatibility and interoperability between the various types of network technologies that
were produced by the many companies around the world.
Web Links
Networking Research Links and Standards
The Development of Communication Networks
2.2 The OSI Reference Model
The OSI reference model is the primary model for network communications. Although there are
other models in existence, most network vendors, today, relate their products to the OSI
reference model, especially when they want to educate users on the use of their products. They
consider it the best tool available for teaching people about sending and receiving data on a
network.
The OSI reference model allows you to view the network functions that occur at each layer.
More importantly, the OSI reference model is a framework that you can use to understand how
information travels throughout a network. In addition, you can use the OSI reference model to
visualize how information, or data packets, travels from application programs (e.g. spreadsheets,
documents, etc.), through a network medium (e.g. wires, etc.), to another application program
that is located in another computer on a network, even if the sender and receiver have different
types of network media.
In the OSI reference model, there are seven numbered layers, each of which illustrates a
particular network function. This separation of networking functions is called layering. Dividing
the network into these seven layers provides the following advantages:

It breaks network communication into smaller, simpler parts.

It standardizes network components to allow multiple-vendor development and support.

It allows different types of network hardware and software to communicate with each
other.

It prevents changes in one layer from affecting the other layers, so that they can
develop more quickly.

It breaks network communication into smaller parts to make learning it easier to
understand.
Web Links
The OSI Seven Layers Model
2.2.2 The seven layers of the OSI reference model
The problem of moving information between computers is divided into seven smaller and more
manageable problems in the OSI reference model. Each of the seven smaller problems is
represented by its own layer in the model. The seven layers of the OSI reference model are:
Layer 7: The application layer
Layer 6: The presentation layer
Layer 5: The session layer
Layer 4: The transport layer
Layer 3: The network layer
Layer 2: The data link layer
Layer 1: The physical layer
During the course of this semester, you will start your studies with Layer 1 and work your way
through the OSI model, layer by layer. By working through the layers of the OSI reference
model, you will understand how data packets travel through a network and what devices
operate at each layer as data packets travel through them. As a result, you will understand
how to troubleshoot network problems as they may occur during data packet flow. For more
information about the OSI model, visit the following site:
Web Links
The OSI Seven Layers Model
2.2.3 The functions of each layer
Each individual OSI layer has a set of functions that it must perform in order for data packets to
travel from a source to a destination on a network. Below is a brief description of each layer in
the OSI reference model as shown in the Figure.
Layer 7: The Application Layer
The application layer is the OSI layer that is closest to the user; it provides network services to
the user’s applications. It differs from the other layers in that it does not provide services to any
other OSI layer, but rather, only to applications outside the OSI model. Examples of such
applications are spreadsheet programs, word processing programs, and bank terminal
programs. The application layer establishes the availability of intended communication partners,
synchronizes and establishes agreement on procedures for error recovery and control of data
integrity. If you want to remember Layer 7 in as few words as possible, think of browsers.
Layer 6: The Presentation Layer
The presentation layer ensures that the information that the application layer of one system
sends out is readable by the application layer of another system. If necessary, the presentation
layer translates between multiple data formats by using a common format. If you want to think of
Layer 6 in as few words as possible, think of a common data format.
Layer 5: The Session Layer
As its name implies, the session layer establishes, manages, and terminates sessions between
two communicating hosts. The session layer provides its services to the presentation layer. It
also synchronizes dialogue between the two hosts' presentation layers and manages their data
exchange. In addition to session regulation, the session layer offers provisions for efficient data
transfer, class of service, and exception reporting of session layer, presentation layer, and
application layer problems. If you want to remember Layer 5 in as few words as possible, think
of dialogues and conversations.
Layer 4: The Transport Layer
The transport layer segments data from the sending host's system and reassembles the data
into a data stream on the receiving host's system. The boundary between the transport layer
and the session layer can be thought of as the boundary between application protocols and
data-flow protocols. Whereas the application, presentation, and session layers are concerned
with application issues, the lower four layers are concerned with data transport issues.
The transport layer attempts to provide a data transport service that shields the upper layers
from transport implementation details. Specifically, issues such as how reliable transport
between two hosts is accomplished is the concern of the transport layer. In providing
communication service, the transport layer establishes, maintains, and properly terminates
virtual circuits. In providing reliable service, transport error detection-and-recovery and
information flow control are used. If you want to remember Layer 4 in as few words as possible,
think of quality of service, and reliability.
Layer 3: The Network Layer
The network layer is a complex layer that provides connectivity and path selection between two
host systems that may be located on geographically separated networks. If you want to
remember Layer 3 in as few words as possible, think of path selection, routing, and addressing.
Layer 2: The Data Link Layer
The data link layer provides reliable transit of data across a physical link. In so doing, the data
link layer is concerned with physical (as opposed to logical) addressing, network topology,
network access, error notification, ordered delivery of frames, and flow control. If you want to
remember Layer 2 in as few words as possible, think of frames and media access control.
Layer 1: The Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications
for activating, maintaining, and deactivating the physical link between end systems. Such
characteristics as voltage levels, timing of voltage changes, physical data rates, maximum
transmission distances, physical connectors, and other, similar, attributes are defined by
physical layer specifications. If you want to remember Layer 1 in as few words as possible, think
of signals and media.
Web Links
The OSI Seven Layers Model
2.2.4 Encapsulation
You know that all communications on a network originate at a source, and are sent to a
destination, and that the information that is sent on a network is referred to as data or data
packets. If one computer (host A) wants to send data to another computer (host B), the data
must first be packaged by a process called encapsulation.
Encapsulation wraps data with the necessary protocol information before network transit.
Therefore, as the data packet moves down through the layers of the OSI model, it receives
headers, trailers, and other information. (Note: The word "header" means that address
information has been added.)
To see how encapsulation occurs, lets examine the manner in which data travels through the
layers as illustrated in the Figure
. Once the data is sent from the source, as depicted in the
Figure, it travels through the application layer down through the other layers. As you can see,
the packaging and flow of the data that is exchanged goes through changes as the networks
perform their services for end-users. As illustrated in the Figures, networks must perform the
following five conversion steps in order to encapsulate data:
Figure
:
1. Build the data.
As a user sends an e-mail message, its alphanumeric characters are converted to data
that can travel across the internetwork.
2. Package the data for end-to-end transport.
The data is packaged for internetwork transport. By using segments, the transport
function ensures that the message hosts at both ends of the e-mail system can reliably
communicate.
3. Append (add) the network address to the header.
The data is put into a packet or datagram that contains a network header with source
and destination logical addresses. These addresses help network devices send the
packets across the network along a chosen path.
4. Append (add) the local address to the data link header.
Each network device must put the packet into a frame. The frame allows connection to
the next directly-connected network device on the link. Each device in the chosen
network path requires framing in order for it to connect to the next device.
5. Convert to bits for transmission.
The frame must be converted into a pattern of 1s and 0s (bits) for transmission on the
medium (usually a wire). A clocking function enables the devices to distinguish these
bits as they travel across the medium. The medium on the physical internetwork can
vary along the path used. For example, the e-mail message can originate on a LAN,
cross a campus backbone, and go out a WAN link until it reaches its destination on
another remote LAN. Headers and trailers are added as data moves down through the
layers of the OSI model.
Web Links
The OSI Seven Layers Model
2.2.5 Names for data at each layer of the OSI model
In order for data packets to travel from the source to the destination, each layer of the OSI
model at the source must communicate with its peer layer at the destination. This form of
communication is referred to as Peer-to-Peer Communications. During this process, each
layer's protocol exchanges information, called protocol data units (PDUs), between peer layers .
Each layer of communication, on the source computer, communicates with a layer-specific PDU,
and with its peer layer on the destination computer as illustrated in the Figure.
Data packets on a network originate at a source and then travel to a destination. Each layer
depends on the service function of the OSI layer below it. To provide this service, the lower
layer uses encapsulation to put the PDU from the upper layer into its data field; then it adds
whatever headers and trailers the layer needs to perform its function. Next, as the data moves
down through the layers of the OSI model, additional headers and trailers are added. After
Layers 7, 6, and 5 have added their information, Layer 4 adds more information. This grouping
of data, the Layer 4 PDU, is called a segment.
The network layer, for example, provides a service to the transport layer, and the transport layer
presents data to the internetwork subsystem. The network layer has the task of moving the data
through the internetwork. It accomplishes this task by encapsulating the data and attaching a
header creating a packet (the Layer 3 PDU). The header contains information required to
complete the transfer, such as source and destination logical addresses.
The data link layer provides a service to the network layer. It encapsulates the network layer
information in a frame (the Layer 2 PDU); the frame header contains information (e.g. physical
addresses) required to complete the data link functions. The data link layer provides a service to
the network layer by encapsulating the network layer information in a frame.
The physical layer also provides a service to the data link layer. The physical layer encodes the
data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire)
at Layer 1.
2.3 Comparison of the OSI Model and the TCP/IP Model
2.3.1 The TCP/IP reference model
Although the OSI reference model is universally recognized, the historical and technical open
standard of the Internet is Transmission Control Protocol/Internet Protocol (TCP/IP). The TCP/IP
reference model and the TCP/IP protocol stack make data communication possible between
any two computers, anywhere in the world, at nearly the speed of light. The TCP/IP model has
historical importance, just like the standards that allowed the telephone, electrical power,
railroad, television, and videotape industries to flourish. To get up-to-date information on
networking models and standards, visit the following websites:
Web Links
The Internet Engineering Task Force
Introduction to the Internet Protocols
2.3.2 The layers of the TCP/IP reference model
The U.S. Department of Defense (DoD) created the TCP/IP reference model because it wanted
a network that could survive any conditions, even a nuclear war. To illustrate further, imagine a
world at war, criss-crossed by different kinds of connections - wires, microwaves, optical fibers,
and satellite links. Then imagine that you need information/data (in the form of packets) to flow,
regardless of the condition of any particular node or network on the internetwork (which in this
case may have been destroyed by the war). The DoD wants its packets to get through every
time, under any conditions, from any one point to any other point. It was this very difficult design
problem that brought about the creation of the TCP/IP model, and which has since become the
standard on which the Internet has grown.
As you read about the TCP/IP model layers, keep in mind the original intent of the Internet; it
will help explain why certain things are as they are. The TCP/IP model has four layers: the
application layer, the transport layer, the Internet layer, and the network access layer. It is
important to note that some of the layers in the TCP/IP model have the same name as layers in
the OSI model. Do not confuse the layers of the two models, because the application layer has
different functions in each model.
Application Layer
The designers of TCP/IP felt that the higher level protocols should include the session and
presentation layer details. They simply created an application layer that handles high-level
protocols, issues of representation, encoding, and dialog control. The TCP/IP combines all
application-related issues into one layer, and assures this data is properly packaged for the next
layer.
Transport Layer
The transport layer deals with the quality-of-service issues of reliability, flow control, and error
correction. One of its protocols, the transmission control protocol (TCP), provides excellent and
flexible ways to create reliable, well-flowing, low-error network communications. TCP is a
connection-oriented protocol. It dialogues between source and destination while packaging
application layer information into units called segments. Connection-oriented does not mean
that a circuit exists between the communicating computers (that would be circuit switching). It
does mean that Layer 4 segments travel back and forth between two hosts to acknowledge the
connection exists logically for some period. This is known as packet switching.
Internet Layer
The purpose of the Internet layer is to send source packets from any network on the
internetwork and have them arrive at the destination independent of the path and networks they
took to get there. The specific protocol that governs this layer is called the Internet protocol (IP).
Best path determination and packet switching occur at this layer. Think of it in terms of the
postal system. When you mail a letter, you do not know how it gets there (there are various
possible routes), but you do care that it arrives.
Network Access Layer
The name of this layer is very broad and somewhat confusing. It is also called the host-tonetwork layer. It is the layer that is concerned with all of the issues that an IP packet requires to
actually make a physical link, and then to make another physical link. It includes the LAN and
WAN technology details, and all the details in the OSI physical and data link layers.
For more TCP/IP information, visit the following Web sites:
Web Links
Introduction to the Internet Protocols
2.3.3 TCP/IP protocol graph
The diagram shown in the Figure is called a protocol graph. It illustrates some of the common
protocols that are specified by the TCP/IP reference model. At the application layer, you will see
different network tasks you may not recognize, but as a user of the Internet, probably use every
day. You will examine all of these during the course of the curriculum. These applications
include the following:

FTP - File Transfer Protocol

HTTP - Hypertext Transfer Protocol

SMTP - Simple Mail Transfer protocol

DNS - Domain Name System

TFTP - Trivial File Transfer Protocol
The TCP/IP model emphasizes maximum flexibility, at the application layer, for developers of
software. The transport layer involves two protocols - transmission control protocol (TCP) and
user datagram protocol (UDP). You will examine these, in detail, later in the CCNA curriculum.
The lowest layer, the network access layer, refers to the particular LAN or WAN technology that
is being used.
In the TCP/IP model, regardless of which application requests network services, and regardless
of which transport protocol is used, there is only one network protocol - internet protocol, or IP.
This is a deliberate design decision. IP serves as a universal protocol that allows any computer,
anywhere, to communicate at any time.
Web Links
Introduction to the Internet Protocols
2.3.4 Comparison of the OSI model and the TCP/IP mode
If you compare the OSI model and the TCP/IP model, you will notice that they have similarities
and differences. Examples include:
Lab Activity
In this lab, you will learn to relate the seven layers of the OSI model to the 4 layers of the
TCP/IP model as well as name the primary TCP/IP protocols and utilities that operate at
each layer.
---------------------------------------------------------------------------------------- ---------------------------------------
Lab 2.3.4 OSI Model and TCP/IP - Overview
Estimated time: 20 min.
Objectives:
This Lab will focus on your ability to accomplish the following tasks:

Describe the 4 layers of the TCP/IP model

Relate the seven layers of the OSI model to the 4 layers of the TCP/IP
model

Name the primary TCP/IP protocols and utilities that operate at each
layer
Background:
This lab will help you develop a better understanding of the seven layers of the
OSI model as they relate to the most popular functioning networking model in
existence, the TCP/IP model. The Internet is based on TCP/IP which has
become the standard language of networking. Although the TCP/IP model is
the most widely used, the 7 layers of the OSI model are the ones most
commonly used to describe and compare networking software and hardware
from various vendors. It is very important to know both the OSI and TCP/IP
models and be able to relate (or map) the layers of one to the other. An
understanding of the TCP/IP model and the protocols and utilities that operate
at each layer is essential when troubleshooting.
Tools / Preparation:
You may work individually or in teams. The following resources will be
required:

PC workstation with Monitor, keyboard, mouse, and power cords

Windows operating system (Win 95, 98, NT or 2000) installed on PC

NIC installed and Cat 5 patch cable with connection to the Internet

Browser software installed (Netscape Navigator 4.6.1 or higher or
Internet Explorer 5.1 or higher

Java, JavaScript, and Style Sheets (must be enabled in your browser's
preference settings)

Flash plug-in (Curriculum version 2.1 only)

Apple QuickTime, RealPlayer G2 and ShockWave Macromedia
browser plug-ins (Curriculum version 1 and 2 only)

Sample Ethernet and Token Ring NICs with different connectors (Coax,
AUI, RJ-45)

Sample Hubs, Switches and Routers
Notes:
Step 1 – The OSI model and associated TCP/IP protocol stack layer.
Task: Fill out the following charts based on your knowledge of the OSI model
and TCP/IP models.
Explanation: Your understanding of the OSI model as it relates to the TCP/IP
model will greatly increase your ability to absorb and categorize networking
information as you learn it.
1. List the 7 layers of the OSI model from the top to the bottom with the proper
name for each layer. List the TCP/IP layer number and its correct name in the
next columns. Also list the term used for the encapsulation units, the related
TCP/IP protocols / utilities and the devices that operate at each layer. NOTE:
More than one OSI layer will be related to certain TCP/IP layers.
OSI comparison with TCP/IP Protocol Stack
OSI OSI
#
TCP/IP # TCP/IP
Layer
Layer
Name
name
Encapsul.
TCP/IP
Units
At
Protocols TCP
each
TCP/IP Utilities
layer
7
6
5
4
3
2
1
----------------------------------------------------------------------------------------------------- -------------------------Similarities

both have layers

both have application layers, though they include very different services

both have comparable transport and network layers

packet-switched (not circuit-switched) technology is assumed

networking professionals need to know both
Differences

TCP/IP combines the presentation and session layer issues into its application layer

TCP/IP combines the OSI data link and physical layers into one layer

TCP/IP appears simpler because it has fewer layers

TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP
model gains credibility just because of its protocols. In contrast, typically networks aren't
built on the OSI protocol, even though the OSI model is used as a guide.
Web Links
Introduction to the Internet Protocols
2.3.5 Use of the OSI and the TCP/IP models in the curriculum
Although TCP/IP protocols are the standards with which the Internet has grown, this curriculum
will use the OSI model for the following reasons:

It is a worldwide, generic, protocol-independent standard.

It has more details, which makes it more helpful for teaching and learning.

It has more details, which can be helpful when troubleshooting.
Many networking professionals have different opinions on which model to use. You should
become familiar with both. You will use the OSI model as the microscope through which to
analyze networks, but you will also use the TCP/IP protocols throughout the curriculum.
Remember that there is a difference between a model (i.e. layers, interfaces, and protocol
specifications) and an actual protocol that is used in networking. You will use the OSI model but
the TCP/IP protocols.
You will focus on TCP as an OSI Layer 4 protocol, IP as an OSI Layer 3 protocol, and Ethernet
as a Layer 2 and Layer 1 technology. The diagram in the Figure shows that later in the course
you will examine one particular data link and physical layer technology out of the many choices
available - Ethernet. If you want a preview of Ethernet, visit the Web site below.
Lab Activity
In this lab, you will identify the characteristics of each layer as well as the terminology and
physical devices that operate at each layer.
--------------------------------------------------------------------------------------------------------Lab 2.3.5 OSI Model - Overview
Estimated time: 20 min.
Objectives:
This Lab will focus on your ability to accomplish the following tasks:

Name the seven layers of the OSI model in order using a mnemonic
(memory jogger)

Describe the characteristic, functions and keywords relating to each
layer

Describe the packaging units used to encapsulate each layer

Name several protocols and standards that operate at each layer
Background:
This lab will help you develop a better understanding of the seven layers of the
OSI model. You will identify the characteristics of each layer as well as the
terminology at each layer. The OSI model was developed by the ISO to help
provide a common framework for the development of both Local Area Networks
(LANs) and Wide Area Networks (WANs). Most network architectures and
companies do not adhere exactly to the OSI model but use it to describe their
products and compare them to others. The OSI model helps us troubleshoot
networking problems by breaking down the networking process (communication
from hosts to servers) into distinct layers where functions must occur and
identifying tools which can help to isolate the problem. An understanding of the
OSI model is essential to success in the world of networking. This lab focuses
on the Ethernet network architecture and the Internet Protocol suite (TCP/IP).
Tools / Preparation:
You may work individually or in teams. The following resources will be
required:

PC workstation with monitor, keyboard, mouse, and power cords

Windows operating system (Win 95, 98, NT or 2000) installed on PC

NIC installed and Cat 5 patch cable with connection to the Internet

Browser software installed (Netscape Navigator 4.6.1 or higher or
Internet Explorer 5.1 or higher

Java, JavaScript, and Style Sheets (must be enabled in your browser's
preference settings)

Flash plug-in (Curriculum version 2.1 only)

Apple QuickTime, RealPlayer G2 and ShockWave Macromedia
browser plug-ins (Curriculum version 1 and 2 only)
Notes:
Step 1 – The OSI model and associated TCP/IP protocol stack layer.
Task: Fill out the following charts based on your knowledge of the OSI model.
Explanation: Your understanding of the OSI model will greatly increase your
ability to absorb and categorize networking information as you learn it.
1. List the 7 layers of the OSI model from the top to the bottom. Give a
mnemonic word for each layer that can help you remember it and then list the
keywords and phrases that describe the characteristics and function of each.
Layer
#
Name
Mnemonic
Key Words and Description of Function
7
6
5
4
3
2
1
2. List the 7 layers of the OSI model and the encapsulation unit used to
describe the data grouping at each layer.
Layer #
Name
Encapsulation
Logical Grouping
Unit
or Devices
or
Components
Operate at this Layer
7
6
5
4
3
2
1
---------------------------------------------------------------------------------------------------------Web Links
Ethernet Web Site
Introduction to the Internet Protocols
that
Summary
This chapter began by describing how layers are used for general forms of communication. In
this section, you learned that data travels from a source to a destination over media and that a
protocol is a formal description of a set of rules and conventions that govern how devices on
networks exchange information.
Following the discussion on layered communication, you learned that:

The OSI reference model is a descriptive network scheme whose standards ensure
greater compatibility and interoperability between various types of network technologies.

The OSI reference model organizes network functions into seven numbered layers:
o
Layer 7 -The application layer
o
Layer 6 -The presentation layer
o
Layer 5 -The session layer
o
Layer 4 -The transport layer
o
Layer 3 -The network layer
o
Layer 2 -The data link layer
o
Layer 1 -The physical layer

Encapsulation is the process in which data is wrapped in a particular protocol header
before it is sent across the network.

During Peer-to-Peer Communications, each layer's protocol exchanges information,
called protocol data units (PDUs), between peer layers.
In the last section of the chapter, you learned about the TCP/IP model and it compares to the
OSI model. Now that you have a basic understanding of the OSI model, you will start looking
at each layer in more depth in the following chapters.