Introduction to Networking

TN 122
Lecture 3 – Overview of OSI Layered
Model
Introduction to OSI Model

⚫ Networking evolved from the basic

principle of moving data from one
computer to another.
⚫ However, there were no standards in
place.
⚫ This meant that one network
manufacturer implemented a different
means of data transfer than another.
Introduction to OSI Model

⚫ In 1984, a group known as the

International Organization for
Standardization (ISO) created a model
called the Open Systems Interconnect
(OSI).
⚫ This model defined guidelines for
interoperability between network
manufacturers.
Introduction to OSI Model

⚫ A company could now mix and match

network devices and protocols from
various manufacturers in its own network
without being locked into using a single
vendor.
⚫ It also had a great side effect:
Competition meant lower prices.
Introduction to OSI Model

⚫ Although the OSI model defined a set of

standards, it is important to note that it is
merely a model.
⚫ Many other models exist in the networking
industry; however, understanding a single
model gives us the capability of understanding
other models in the future.
⚫ The OSI model is the most widely taught as
the foundation for this knowledge.
Why Layered Model

⚫ We use the concept of layers in our daily

life. As an example, let us consider two
friends who communicate through postal
mail. The process of sending a letter to a
friend would be complex if there were no
services available from the post office.
Why Layered Model
Why Layered Model

⚫ By using a layered model, we can

categorize the procedures that are
necessary to transmit data across a
network.
⚫ Imagine that we are developers and we
are about to create a new protocol for
communication across a network.
Why Layered Model

⚫ A protocol is a set of guidelines or rules

of communication.
⚫ A protocol, is more than just the words
of computers.
⚫ It also includes the timing and the same
dictionary so that at any time, both
computers using the same protocol have
an exact, complete understanding of
each other.
Why Layered Model

⚫ Developing a new protocol without a model would

be a tedious and time-consuming task.
⚫ We would need to reinvent the wheel by recreating
work that has already been done by other
developers.
⚫ We could save time by cutting and pasting the code,
but we would still need to do extensive testing.
⚫ Further, when we needed to update the protocol, we
would have to redesign and retest the entire
protocol.
Why Layered Model
⚫ However, if we were to use a layered design, we
could separate the processes into specific layers.
⚫ We could then design, enhance, and test each
individual layer.
⚫ As the process continued, we would have a
complete protocol based on a layered model.
⚫ When we needed to update code, we would only
have to modify the one layer that needed the
updating; the rest of the layers would not be
affected.
⚫ This allows us to enhance specific functions easier
and quicker.
Why Layered Model

⚫ Further, by using a layered model, we could then

license our protocol to other developers for use in
their own networks.
⚫ If the protocol did not work on their chosen hardware
platform, they could replace one of the layers with
their own version, thereby creating multivendor
compatibility.
⚫ If we did decide to enhance one of the layers, we
could take just that specific layer and redistribute it
to all the developers, thereby making our protocol
even better.
Why Layered Model

➢ To summarize, layered modeling allows us to

⚫ Create a protocol that can be designed and
tested in stages, which, in turn, reduces the
complexity
⚫ Enhance functionality of the protocol without
adversely affecting the other layers
⚫ Provide multivendor compatibility
⚫ Allow for easier troubleshooting by locating the
specific layer causing the problem
How Does a Model Work?

⚫ Before defining how a model works, we

must clarify one thing.
⚫ The OSI model defines what each layer
should do; it does not tell you how to do
it.
⚫ This allows developers the freedom to
choose the best method they can design.
How Does a Model Work?

⚫ The OSI model is

divided into seven
layers.
⚫ Notice that the
bottom layer is
identified as the first
layer.
How Does a Model Work?

⚫ It is important to
remember the order
of the layers in the
OSI model.
⚫ Doing so creates a
better understanding
of the network data
flow.
⚫ It is also needed to
pass an exam.
How Does a Model Work?

⚫ Each layer is separated, or encapsulated, from

each other layer; This means that each layer can
function on its own.
⚫ Each layer thinks it is talking directly to the same
layer on the remote computer through a virtual
link.
⚫ Furthermore, each layer can only communicate
with the layers above and below it.
⚫ In fact, the layer doesn’t know that any other
layers even exist.
How Does a Model Work?

⚫ For example, notice

in that the Transport
layer can
communicate only
with the Network and
the Session layers.
How Does a Model Work?

⚫ Finally, the flow of data

starts at the Application
layer of the sending
computer, flows down
the layers, across the
wire to the receiving
computer, and then
back up the layers to
the Application layer
Physical Layer
⚫ Provides physical interface for transmission of
information.
⚫ Defines rules by which bits are passed from one
system to another on a physical communication
medium.
⚫ Covers all - mechanical, electrical, functional and
procedural - aspects for physical communication.
⚫ 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.
Physical Layer
Data Link Layer
⚫ Data link layer attempts to provide reliable
communication over the physical layer interface.
⚫ The data link layer is responsible for moving
frames from one hop (node) to the next.
⚫ Breaks the outgoing data into frames and
reassemble the received frames.
⚫ Create and detect frame boundaries.
⚫ Handle errors by implementing an
acknowledgement and retransmission scheme.
⚫ Implement flow control.
Data Link Layer
Figure 2.7 Hop-to-hop delivery

2.24
Network Layer

⚫ The bottom two layers allow the

communication partners to communicate only
if they are on the same segment.
⚫ If we were to have all the computers in the
world on a single segment, there would be no
Internet.
⚫ Either there would be far too many broadcasts,
or the time for a single node to communicate
with another via a token would be ridiculously
slow.
Network Layer

⚫ The function of the Network layer is to identify

a remote network and deliver the data to it.
⚫ This allows us to have segmentation.
⚫ The Network layer enables us to send data to
any computer in the world, as long as there is
a network connection.
⚫ The device that allows us to accomplish this
spectacular feat is the router, sometimes
referred to as a Layer 3 device.
Network Layer
Figure 2.9 Source-to-destination delivery

2.28
Transport Layer

⚫ Responsible for end-to-end

communication over a network.
⚫ It provides logical communication
between application processes running
on different hosts within a layered
architecture of protocols and other
network components.
Transport Layer
⚫ Ensures that the data units are delivered error
free.
⚫ Ensures that data units are delivered in
sequence.
⚫ Ensures that there is no loss or duplication of
data units.
⚫ Provides connectionless (UDP) or connection
oriented service (TCP).
⚫ Provides for the connection management.
⚫ Multiplex multiple connection over a single
channel.
Transport Layer
Figure 2.11 Reliable process-to-process delivery of a message

2.32
Session Layer
⚫ Session layer provides mechanism for controlling
the dialogue between the two end systems. It
defines how to start, control and end conversations
(called sessions) between applications.
⚫ This layer requests for a logical connection to be
established on an end-user’s request.
⚫ Any necessary log-on or password validation is also
handled by this layer.
⚫ Session layer is also responsible for terminating the
connection.
⚫ This layer provides services like dialogue discipline
which can be full duplex or half duplex.
⚫ Session layer can also provide check-pointing
mechanism such that if a failure of some sort
occurs between checkpoints, all data can be
retransmitted from the last checkpoint.
Session Layer
Presentation Layer
⚫ Presentation layer defines the format in
which the data is to be exchanged between
the two communicating entities.
⚫ Also handles data translation, data
compression and data encryption
(cryptography).
For example, when communicating from a PC to
a mainframe, data may need to be converted
between ASCII and EBCDIC (a different
character formatting method used on many
mainframes).
Presentation Layer
Application Layer
⚫ Application layer interacts with application
programs and is the highest level of OSI
model.
⚫ Application layer contains management
functions to support distributed applications.
⚫ Examples of application layer are
applications such as file transfer, electronic
mail, remote login etc.
Summary of 7 OSI Layers
TCP/IP Protocol Suite

⚫ TCP/IP, the protocol on which the Internet is

built, is actually not a single protocol but rather
an entire suite of related protocols.
⚫ TCP is even older than Ethernet.
⚫ It was first conceived in 1969 by the
Department of Defense.
⚫ Currently, the Internet Engineering Task
Force, or IETF, manages the TCP/IP protocol
suite.
TCP/IP Protocol Suite

⚫TCP/IP is built into the UNIX operating

system and is used by the Internet,
making it the de facto standard for
transmitting data over networks.
⚫ Even network operating systems that
have their own protocols, such as
Netware, also support TCP/IP.
TCP/IP Protocol Suite

⚫ The TCP/IP model uses four layers to

perform the functions of the seven-layer
OSI model.
TCP/IP Protocol Suite
Network Interface Layer
⚫ The Network Interface layer (also called the Network
Access layer) sends TCP/IP packets on the network
medium and receives TCP/IP packets off the network
medium.
⚫ TCP/IP was designed to be independent of the
network access method, frame format, and medium.
⚫ Therefore, you can use TCP/IP to communicate
across differing network types that use LAN, Wireless
LAN and WAN technologies.
⚫ By being independent of any specific network
technology, TCP/IP can be adapted to new
technologies.
Internet Layer

⚫ The Internet layer responsibilities include

addressing, packaging, and routing
functions.
⚫ The Internet layer is analogous to the
Network layer of the OSI model.
Transport Layer
⚫ The Transport layer (also known as the Host-to-Host
Transport layer) provides the Application layer with
session and datagram communication services. The
Transport layer encompasses the responsibilities of
the OSI Transport layer.
⚫ TCP provides a one-to-one, connection-oriented,
reliable communications service.
⚫ TCP establishes connections, sequences and
acknowledges packets sent, and recovers packets lost
during transmission.
Application Layer
⚫ The Application layer allows applications to access the
services of the other layers, and it defines the
protocols that applications use to exchange data.
⚫ The Application layer contains many protocols, and
more are always being developed.
➢ The Hypertext Transfer Protocol (HTTP) transfers files
that make up pages on the World Wide Web.
➢ The File Transfer Protocol (FTP) transfers individual
files, typically for an interactive user session.
➢ The Simple Mail Transfer Protocol (SMTP) transfers
mail messages and attachments.