1

IUNIT FIVE
NETWORKING STANDARDS AND THE OSI MODEL
After reading this unit, you will be able to:

ƒ Understand Basics Of Standards

ƒ Types Of Standards
ƒ Understand Basic Concept of Open and Close systems
ƒ Identify organizations that set standards for networking
ƒ Understand OSI Reference Model
ƒ Function OF Each Layer Of the Model
ƒ Describe specific networking services within each layer of the OSI
ƒ Explain how two systems communicate through the OSI Model

UNIT 5
NETWORKING STANDARDS AND THE OSI MODEL

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 2

STANDARDS
Standards are documented agreements containing technical specifications or other precise
criteria that guide how a particular product and service should be designed.
Because of the wide variety of hardware and software in use today, standards are
especially important in the world of networking. Without standards , you could not
design a network because one piece of hardware might not work properly with another.
Likewise, one software program might not be able to communicate with another. For
example, if one manufacturer designed a network cable with a 1-centimeter –wide plug
and another company manufactured a wall plate with a 0.8 –centimeter –wide opening
you would not be able to insert the cable into the wall plate.

Types of standards
The network industry uses two types of standards: de facto standards and de jure
standards. To understand the concept of open systems architecture, you must be familiar
with the concepts of de facto and de jure standards.

De facto standards arise through widespread commercial and educational use. These
standards often are proprietary and usually remain unpublished and unavailable to outside
vendors. Unpublished and unavailable standards are known as closed system standards.
Published and accessible standards, on the other hand, are known as open system
standards. Through the introduction of the OSI model, which is discussed later in this
chapter, and the growing acceptance of the concept of interoperability, many closed
proprietary systems (such as IBM’s Systems Network Architecture) have started to
migrate toward open system standards. Certainly, de facto standards are not always
closed system standards—examples of proprietary open system standards include
Novell’s NetWare network operating system and Microsoft’s Windows.

The second type of standards, de jure standards, are nonproprietary, which means that no
single company creates them or owns the rights to them. De jure standards are developed
with the intent of enhancing connectivity and interoperability by making specifications
public so that independent manufacturers can build to such specifications. TCP/IP is an
example of a nonproprietary de jure standard.

Concept of Close and Open systems

What Is an Open System?

There are many definitions of open systems, and a single, concise definition that
everyone is happy with is far from being accepted. For most people, an open system is
best loosely defined as one for which the architecture is not a secret. The description of
the architecture has been published or is readily (quickly and without difficulty) available
to anyone who wants to build

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 3

Products for a hardware or software platform. This definition of an open system applies
equally well to hardware and software. When more than a single vendor begins producing
products for a platform, customers
have a choice. You don't particularly like Nocrash Software's network monitoring
software? No problem, because FaultFree Software's product runs on the Nocrash
hardware, and you like its fancy interface much better. You need a more colorful
graphical front-end to your Whiz bang PC than the one Whizbang provides? Download
one from Super Software through the Internet, and it works perfectly. The primary idea,
of course, is a move away from proprietary platforms to one that is multivendor.
A decade ago, open systems were virtually nonexistent. Each hardware manufacturer had
a product line, and you were practically bound to that manufacturer for all your software
and hardware needs. Some companies took advantage of the captive market, charging
outrageous(very shocking and unacceptable) prices or forcing unwanted configurations
on their customers. The groundswell(the sudden increase of a particular feeling among a
group of people) of resentment(a feeling of anger or unhappiness about sth that you think
is unfair) grew to the point that customers began forcing the issue.

The lack of choice in software and hardware purchases is why several dedicated
minicomputer and mainframe companies either went bankrupt or had to accept open
system principles: their customers got fed up with relying on a single vendor. A good
example of a company that made the adaptation is Digital Equipment Corporation (DEC).
They moved from a proprietary operating system on their VMS minicomputers to a
UNIX standard open operating system. By doing that, they kept their customers happy,
and they sold more machines. That's one of the primary reasons DEC is still in business
today.
UNIX is a classic example of an open software platform. UNIX has been around for 30
years. The source code for the UNIX operating system was made available to anyone
who wanted it, almost from the start. UNIX's source code is well understood and easy to
work with, the result of 30 years of development and improvement. UNIX can be ported
to run on practically any hardware platform, eliminating all proprietary dependencies.
The attraction of UNIX is not the operating system's features themselves but simply that
a UNIX user can run software from other UNIX platforms, that files are compatible from
one UNIX system to another (except for disk formats), and that a wide variety of vendors
sell products for UNIX.
The growth of UNIX pushed the large hardware manufacturers to the open systems
principle, resulting in most manufacturers licensing the right to produce a UNIX version
for their own hardware. This step let customers combine different hardware systems into
larger networks, all running UNIX and working together. Users could move between
machines almost transparently, ignorant of the actual hardware platform they were on.
Open systems, originally of prime importance only to the largest corporations and
governments, is now a key element in even the smallest company's computer strategy.
Although UNIX is a copyrighted work now owned by X/Open, the details of the
operating system have been published and are readily available to any developer who
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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 4

wants to produce applications or hardware that work with the operating system. UNIX is
unique in this respect.
The term open system networking means many things, depending on whom you ask. In its
broadest definition, open system networking refers to a network based on a well-known
and understood protocol (such as TCP/IP) that has its standards published and readily
available to anyone who wants to use them. Open system networking also refers to the
process of networking open systems (machaine-specific hardware and software) using a
network protocol.

NETWORKING STANDARDS ORGANIZATIONS

The following making standards organizations are responsible for establishing the future
of networking:

ANSI
ANSI (American National Standards Institute) is an organization composed of more
than 1000 representatives from industry and government who together determine
standards for electronics industry in addition to other fields, such as chemical and nuclear
engineering, health and safety, and construction.
You can purchase ANSI standards documents online from ANSI’s Web
site(www.ansi.org) or find them at a university or public library.

EIA
EIA (Electronic Industries Alliance) is a trade (the activity of buying and selling or of
exchanging goods or services between people or countries) organization composed of
representatives from electronics manufacturing firms across the United States. EIA began
as the Radio Manufacturers Association (RMA) in 1924; over time, it evolved to include
manufacturers of televisions, semiconductors, computers, and networking devices.

IEEE
The IEEE (Institute of Electrical and Electronic Engineers), or “I-triple-E, is an
international society composed of engineering professionals. Its goals are to promote
development and education in the electrical engineering and computer science fields.

ISO (International Organization for Standardization) is collection of standards

organizations representing 130 countries; its headquarter is located in Geneva,
Switzerland. ISO develops and publishes standards and coordinates the activities of all
national standardization bodies. In 1977, the ISO initiated efforts to design a
communication standard based on the open systems architecture theory from which
computer networks would be designed. This model came to be known as the Open
Systems Interconnection (OSI) model. The International Standards Organization (ISO)—
whose name is derived from the Greek prefix iso, meaning “same” or equal or standard.
You might assume it should be called “IOS”, but ISO is not meant to be an acronym.

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 5

The OSI Reference Model

In the early 1980 s, ISO began work on a universal set of specifications that would enable
computer platforms across the world to communicate openly. (Working on open system,
so ISO made TCP/IP , and all other software and services opened for every one). After
that organization created a helpful model for understanding computer-to-computer
communications. This model, called the Open Systems Interconnection (OSI) Model,
divides networking architecture into seven layers: Physical, Data Link, Network,
Transport, Session, Presentation, and Application.

Note: The combination of a network’s building blocks is often described as its

“architecture”. The use of the term “architecture” in the networking filed reflects the fact
that, like a building, a network contains many distinct but integrated elements: the
cabling, servers, protocols, clients, applications, NICs, and so on. A professional involved
in network design is sometimes called a network architect.

It has since become the most widely accepted model for understanding network
communications.

The OSI model is not a physical thing, but it is simply a conceptual frame work that is
developed to better understand the complex interactions taking place among the various
devices on a network. This model explains the communication process between different
devices in a network. The OSI model defines which task need to be done and which
device and protocols will handle that task at each of the seven layers 0f the OSI model.
Every thing you will learn about networking can be associated with a layer of this
model, however, so you should know not only the names of the layers, but also
their functions and the way in which the layers interact. Figure 1.2 depicts the
OSI Model and its layers.

Figure 1.2 The OSI model is composed of seven layers.

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 6

Note:

You should learn the names and the order of the seven OSI layers for the Networking
Essentials exam. The following two phrases help you remember the First letters of the
layers:

All People Seem To Need Data Processing (top down)

Please Do Not Throw Sausage Pizza Away (bottom up)

Choose one, depending on whether you are most comfortable working from the
top of the model down or from the bottom up.

Need for OSI Model

Having a model in mind will help you understand how the pieces of different
things fit together. In the field of chemistry, for example, even though you can’t
see a water molecule, you can represent it with a simple drawing of two
hydrogen atoms and one oxygen atom. Similarly, in the field o networking, even
though you can’t see the communication that occurs between two nodes on a
network, you can use a model to depict how the communication takes place. The
model commonly used to describe network communication is called the open
systems Interconnection (OSI) model.

Why it is called a Reference Model?

It is called a reference model because we have to consider at least two sides for
communication i.e. one will be the speaker & other will be listener or one will be
sender & the other will be receiver.

Why it is called open Model?

If noticed the alphabet O in OSI stands for OPEN. This model is open because OSI
is platform independent which means any media, any vendors devices, any thing
of any party can be used; hence applying no restrictions for communication
between two devices. This model is absolutely open for every one.

Why seven layers approach is used for OSI Model?

In developing Open System Interconnection Model a layered approach is used.
Layered approach:
Two main benefits of using layered approach are:
Errors become specific to one layer means that scope of error is reduced i.e.
Trouble shooting an error becomes easy because that error will have information
which will tell us at which level the error is occurred. An error occurred in
communication can easily be rectified e.g. when we are dialing an internet
connection and if we do not plug in telephone line into modem; by dialing we
--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 7

receive an error that “no dialing tone”. This error with this description clearly shows
a physical layer problem. similarly if we provide a wrong user name or password
an error showing that password is wrong which shows that this problem is related
to another level not to physical layer.
But if only one layer approach was used then any error occurred at any place will
have only on description i.e. “ERROR”. This makes us unable to rectify the
problem. These will unable us to troubleshoot the problem because the problem
can be anywhere either due to not plugging in telephone cable or not providing
correct user name & password or server at the other end is down. So a layered
approach is used.
Development & enhancement etc becomes specific & easy because now we have
to deal functions specific to a layer. By using layered approach we are confining
different functions involved in communication to a specific layer. Actually all the
layers are connected with each other with logical static sockets so if some one
wants to replace a layer with something else he just remove one layer and insert
the corresponding layer of his own. it will not effect functions of other layers. It
made programmers life easy because for one change they do not have to
reconfigure the whole model

Why it is called interconnection Model?

People sometimes think of a network as a single, local cabling system that
enables any device on the network to communicate directly with any other device
on the same network. A network by this definition, however, has no connections
other remote networks.
An internetwork consists of multiple independent networks that are connected
and can share remote resources. These logically separate but physically
connected networks can be dissimilar in type. The device that connects the
independent networks together may need a degree of “intelligence” because it
may need to determine when packets will stay on the local network or when they
will be forwarded to a remote network.
So OSI model is not only used to explain communication process between two devices
which are connected to the same network , OSI model is also use to explain
communication process between two devices that are connected with different networks.
So that is why OSI Model is called Interconnection model , OSI model also explain
communication process between inter connected networks.

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 8

Seven layers 0f the OSI model.

Physical layer

The physical layer is the lowest, or first, layer of the OSI Model. This layer contains
physical networking medium, such as cabling, connectors, and repeaters. Protocols at the
physical layer generate and detect voltage so as to transmit and receive signals carrying
data. When you install a NIC in your desktop PC, you are establishing the foundation that
allows the computer to be networked. (See Figure 1.2).

In a nut shell, the physical layer is simply responsible for sending bits (0, 1) of digital
communication.
This level defines physical and electrical details , such as what cables (coaxial, twisted
pair, fiber optics) are used,. How many pins a network connecter will have and when the
network adaptor (NIC) may or may not transmit the data.
Physical layer is responsible to carry the signals for all the higher layers. The
communication process will not more exist if the physical layer has any fault. For
example, severed wire (to cut into two pieces), affect the physical layer. Similarly, if you
insert a NIC card but fail to seat it deeply enough in the computer’s circuit board, your
computer will experience network problems at the physical layer. Devices that operate at
--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 9

the physical layer include repeaters and hubs, terminators, cables& cabling, connectors,
transmitters, receivers are devices associated with physical layer. NICs operate at both
the physical layer and at the Data Link layer.

The Data Link Layer

The Data-Link layer provides for the flow of data over a single physical link from one
device to another. It accepts packets from the Network layer and packages the
information into data units called frames; these frames are presented to the Physical layer
for transmission. The Data-Link layer adds control information, such as frame type, to the
data being sent. This layer also provides for the error-free transfer of frames from one
computer to another. A cyclic redundancy check (CRC) added to the data frame can
detect damaged frames, and the Data-Link layer in the receiving computer can request
that the CRC information be present so that it can check incoming frames for errors. The
Data-Link layer can also detect when frames are lost and request that those frames be
sent again. Figure 1.3 shows how the Data-Link layer establishes an error-free connection
between two devices
A frame is a structured package for moving data that include sender’s and receiver’s
network addresses, and error checking and control information. The addresses tell the
network where to deliver the frame, whereas the error checking and control information
ensure that the frame arrives without any problems. A shown in the figure 1.3

Destination Source Control Data checking information Error checking

Address Address information information
Figure 1.3 Simplified Data Frame
.

The Data-Link layer is split into two sub layers:

The Logical Link Control (LLC) layer establishes and maintains the logical
communication links between the communicating devices.
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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 10

The Media Access Control (MAC) layer acts like an airport control tower—it controls
the way multiple devices share the same media channel in the same way that a control
tower regulates the flow of air traffic into and out of an airport.

Figure 1.4 illustrates the division of the Data-Link layer into the LLC and MAC layers.

To fully understand the function of the Data Link layer, pretend (to behave in a particular
way, in order to make other people believe some thing that is not true) for a moment that
computers communicate as humans do. You might be in large classroom full of noisy
students (full of noise) and need to ask the teacher a question. Your teacher’s name is Ms.
Jones. To get your message through, you might say, “Ms. Jones? Can you explain more
about the effects of railroads on commerce in the mid-nineteenth century? In this
example, you are the sender (in a busy network) and you have addressed your recipient (a
person who receives sth), Ms. Jones, just as the data link layer addresses another
computer on the network. In addition, you have formatted your thought as a question, just
as the Data Link layer formats data into frames that can be interpreted by receiving
computers.
When happens if the room is so noisy that Ms. Jones hears only part of your question?
For example, she might receive “on commerce in the late-nineteenth century? This kind
of error can happen in network communications as well (because of electrical
interference or wiring problems). The data Link layer’s job is to find out that information
has been dropped and ask the first computer to retransmit its message –just as in a
classroom setting Ms. Jones might say, “I didn’t hear you. Can you repeat the question?
The data Link Layer accomplishes this task through a process called error checking.

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 11

In general, the sender’s data Link layer waits for acknowledgment from the receiver that
data was received correctly. If the sender does not get this acknowledgment, its data link
layer gives instruction to retransmit the information. The data Link layer does not try to
figure out what went wrong in the transmission. Similarly, as in a busy classroom, Ms.
Jones will probably say, “pardon” me?” rather than, “It sound as if you might have a
question about railroads, and I heard only the last part of it, which dealt with commerce,
so I assume you are asking about commerce and railroads; is that correct?” obviously, the
former method is more efficient for both the sender and the receiver.

Another communication mishap (a small accident or piece of bad luck that does not have
serious results) that might occur in a noisy classroom or on a busy network is a glut (to
supply or provide sth with too much of sth) of communication requests. For example, at
the end of class, 20 students might ask Ms.Jones 20 different questions at once. Of
course, she can’t pay attention to all of them simultaneously. She will probably say, “One
student at a time, please, “then point to one student who asked a question. This situation
is analogous to what the Data Link Layer does for the physical layer. One node on a
network ( a server, for example) may receive multiple requests that include many frames
of Data each. The data Link layer controls the flow of this information, allowing the NIC
to process data without error.
Connectivity devices, such as bridges and switches, work in the data Link Layer, because
they decode frames and use the frame information to transmit data to its correct recipient.

In a nut shell, The Data link layer provides for the flow of data over a single link from
one device to another. It accepts data packets from the network layer and present it to the
physical layer for transmission.
This layer provides for the error free transfer of data packets from one computer to
another. The data link layer can also detect when data packets are lost and request that
those frames be sent again.

Network layer
As you learned in the preceding section, the Data Link layer deals with communication
between devices on the same network. Physical device addresses are used to identify data
frames, and each device is responsible for monitoring the network and receiving frames
Address to that device.
The Network layer handles communication with devices on logically separate networks
that are connected to form internetworks. Because internetworks can be large and can be
constructed of different types of networks, the Network layer utilizes routing algorithms
that guide packets from their source to their destination networks. Within the Network
layer, each network in the internetwork is assigned a network address that is used to route
packets. The Network layer manages the process of addressing and delivering packet on
complex networks.

--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 12

Figure 1.5 shows how the Network layer moves packets across multiple links in a
network.

In a nut shell, The network layer makes routing decisions and forwards packets for
devices that are further away than a single link ( A link connects two network devices and
is implemented by the data link layer).
The network layer decides which physical path or way the data should take base on
network conditions, priority of the service and the factors. The network layer performs
several important function that enable data to arrive at its destination.
The protocol which works in this layer is called IP (Internet Protocol). This protocol is
used to address, route and handle the packets of data. Routers and gateways are the
devices that operate on the network layer.

The primary function of the Network layer, the third layer in the OSI Model, is to
translate network addresses into their physical counterparts and decide how to route data
from the sender to the receiver. For example, a computer might have a network address
of 200.100.100.2 (if it’s using the TCP/IP protocol stack) and a physical address of
006097E97F3.

In the classroom example, this addressing scheme is like saying that “Ms.Jones “and
“U.S.” Citizen with Social Security number 123-45-6789 “are the same person. Within
the confines of your classroom, however, there is only one Ms. Jones, so you can be
certain the correct person will respond when you say, “Ms. Jones?”

Since the Network Layer handles routing, Routers ---the devices that connect different
networks and intelligently direct data—belong in the Network layer. In networking, the
term “to route “means to direct data based on addressing, and availability of path.

--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 13

Transport Layer
The Transport Layer is primarily responsible for ensuring that data are transferred from point
A to point B (which may or may not be on the same network) reliably, in the correct
sequence, and without errors. The transport layer may be considered the most important layer
in the OSI Model because without it, data could not be verified or interpreted by the
recipients. Transport protocols also handle flow control, or the method of gauging the
appropriate rate of transmission based on how fast the recipient can accept data.

In addition, Transport Layer services break long packets into the maximum size that the type
of network in use can handle. For example, Ethernet networks cannot accept packets larger
than 1500 bytes. Segmentation refers to the process of decreasing the size of the data
units when moving data from part of a network that can handle larger data units to
part of a network that can handle only smaller data units. This process is just like the
process of breaking down words into recognizable syllables that a small child uses when
learning to read.
Reassembly is the process of reconstructing the segmented data units. When a child
understands the separate syllables, he can combine them into a word---that is, reassemble
the parts into a whole.
So when the sending node’s Transport Layer services divide its data into smaller pieces,
they assign a sequence number to each piece, so that the data can be reassembled in the
correct order by the receiving node’s transport layer services. This process is called
sequencing. (as in Figure 1.6).

To understand how sequencing works, consider the classroom example again. Suppose
you asked the question, “Ms.Jones? What is meant by computer network and OSI
Model?” but that the words arrived at Ms.Jones’s ear as “what is meant by computer

--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 14

network Ms.Jones? What is meant by OSI Model? “. On a network, the Transport layer
would recognize this disorder and rearrange the data pieces so that they make sense.

In addition, the Transport Layer sends an acknowledgment (ACK) to notify the sender
that data were received correctly. If the data contained errors, the Transport Layer would
request that the sender retransmit the data. Also, if the data weren’t acknowledged within
a given time period, the sender’s Transport layer would consider the data lost and
retransmit them.
The protocol or service that works in the Transport layer is TCP (Transmission Control
Protocol) of the TCP/IP protocol stack.

The Session Layer:

The session Layer is responsible fro establishing and maintaining communication
between two nodes on the network. The term session refers to a connection for data
exchange between two parties.
Among the session Layer’s functions are establishing and keeping alive communications
link for the duration of the session, synchronizing the dialog between the two nodes,
determining whether communications have been cut off, and if so , figuring out where to
restart transmission.
When you dial your Internet service provider (ISP) to connect to the Internet, the session
Layer services at your ISP’s server, and on your PC client, negotiate the connection. If
your phone line is accidentally pulled out of the wall jack, the session Layer on your end
will detect the loss of a connection and initiate attempts to reconnect.

In addition, the Session Layer also sets the terms of communication by deciding which
node will communicate first and how long a node can communicate. In this, sense, the
Session layer acts as a judge in a debate competition. For example, if you were a member
of a debate team and had two minutes to state your opening arguments, the judge might
signal you after one and a half minutes that you have only 30 seconds remaining. If you
tried to interrupt a member of the opposing debate team, he would tell you to wait your
turn. (see Figure 1.7).

--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 15

Finally, the Session Layer monitors the identification of session (Connection)

participants, ensuring that only the authorized nodes can access the session (connection).

Presentation layer
The Presentation layer deals with the syntax, or grammatical rules, needed for
communication between two computers. The Presentation layer converts system-specific
data from the Application layer into a common, machine-independent format that will
support a more standardized design for lower protocol layers.
The Presentation layer also attends to other details of data formatting, such as data
encryption and data compression.
Note:
The name “Presentation layer” has caused considerable confusion in the industry because
some people mistakenly believe that this layer presents data to the user. However, the
name has nothing to do with displaying data. Instead, this function is performed by
Application running above the Application layer. Presentation layer is so named because
it presents a uniform data format to the Application layer. As a matter of fact, this layer is
not commonly implemented because applications typically perform most presentation
layer functions.
On the receiving end, the Presentation layer converts the machine-independent data from
the network into the format required for the local system. Figure 1.8 shows the
Presentation layer’s role in the protocol stack.

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 16

In a nut shell, the presentation Layer translates data between the formats the network
requires and the formats the computer expects. The presentation Layer also manages data
encryption and decryption, systems passwords. For example, if you look up your
bank account status on the Internet, due to presentation layer you are using a secure
connection, and your account data will be encrypted before they are transmitted. On your
end of the network, the presentation layer will decrypt the data as they are received.

Character code translation. Different computers use different binary schemes for
representing character sets. For instance: ASCII, the American Standard Code for
Information Interchange, is used to represent English characters on all microcomputers
and most minicomputers , EBCDIC, the Extended Binary Coded Decimal Interchange
Code, is used to represent English characters on IBM mainframes.

Application layer:
The Application layer is the topmost layer of the OSI model, and it provides services that
directly support user applications, such as database access, e-mail, and file transfers. It
also allows applications to communicate with applications on other computers as though
they were on the same computer. When a programmer writes an application program that

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 17

uses network services, this is the layer the application program will access. For example,
Internet Explorer uses the Application layer to make its requests for files and web pages;
the application layer then passes those requests down the stack, with each succeeding
layer doing its job (as in Figure 1.9).

A common misunderstanding is that the Application layer is responsible for running user
applications such as word processors. This is not the case. The Application layer,
however, does provide an interface whereby applications can communicate with the
network. For example, if you are running Microsoft Word on a network and choose to
open a file, your request for that data is transferred from Microsoft Word to the network
by the Application Layer.
The part of Microsoft Word that handles this request is its application program interface
(API). An API is a sub-program that allows a program to interact with the operating
system. APIs belong to the Application Layer of the OSI Model. An example of an API
used in a network environment is Microsoft message Queuing (MSMQ). MSMQ stores

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 18

messages sent between nodes in queues and then forwards them to their destinations
based on when the link to the recipient becomes available.

COMMUNICATION BETWEEN TWO SYSTEMS IN THE CONTEXT OF OSI MODEL

When a message is sent from one machine to another, it travels down the layers on one
machine and then up the layers on the other machine, as shown in Figure 1.10.
As the message travels down the first stack, each layer it passes through (except the
Physical layer) adds a header. These headers contain pieces of control information that
are read and processed by the corresponding layer on the receiving stack. As the message
travels up the stack of the other machine, each layer removes the header added by its peer
layer and uses the information it finds to figure out what to do with the message contents
(see Figure 1.11).

An exemplary process to trace through the OSI Model layers is the retrieval of a message
file from the mail server. Once you log in to the network and starts your mail program,
you can choose to pick up your mail. At that point, the application Layer recognizes your
choice and formulates a request for data from a remote node (in this case, the mail
server). The Application layer transfers the request to the presentation layer.

The presentation layer first determines whether and how it should format or encrypt the
data request received from the Application layer. After it has made that determination, it
adds any translation or codes required to implement that formatting and then passes your
request on to the Session layer.

At the Session layer picks up your formatted request and assigns control information that
ensure a physical session between source and destination computer. The Session layer the
passes your data to the transport layer.

At the Transport layer, your data and the control information it has accumulated thus far
are broken down into manageable chunks of data and prepared to be packaged in frames
at the Data Link Layer. If the data is too large to fit in one frame, the Transport layer
subdivides it into several smaller blocks and assigns sequence identifiers to each block.
This layer passes the data blocks, one at a time, to the Network layer.

The Network layer adds addressing information to the data it receives from the Transport
Layer, so that subsequent layers will know the source and destination of the data. It then
passes the data blocks, with their addressing identifications, to the Data Link Layer.

At the Data Link layer, the data blocks are packaged into individual frames. As you have
learned, a frame is a structured format for transmitting small blocks of data. Using frames
reduces the possibility of lost data or errors on the network, because each frame has its
own built-in error check. This error checking algorithm, also known as the Frame Check
Sequence (FCS), is inserted at the end of the frame by the Data Link Layer. In addition ,
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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 19

the Data Link Layer adds a header to the frame that incorporates destination and source
addresses assigned by the Network Layer. The Data Link Layer then passes the frames to
the physical layer.

Finally, your request for your mail message hits the NIC at the physical layer. The
physical layer does not interpret the frame or add information to the frame; it simply
delivers the data to the cabling and sends it across the network. Once the data arrives at
the physical layer of the remote system, the mail server’s Data Link layer begins to
unravel your request, reversing the process just described, until it responds to your
request with its own transmission, beginning from its Application layer. Figure 2-3 shows
how data is transferred from your system to the mail server, then back to your system
through the OSI Model.

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By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 20

Figure 2-3 Data transfer between two systems

--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT
 21

One of the most common and useful analogies used to describe the networking model is
to imagine the process a letter goes through to get to its destination. Figure 2.12 shows a
sample of the process.

--------------------------------------------------------------------------------------
By:
Noor Ul Wahab Khattak
(M.Sc Computer Science)
Lecturer in college of Engineering, Department of Computer Engineering, EIT