OSI Reference Model
And TCP/IP Reference Model
In order for a computer to send information to
another computer, and for that computer to
receive and understand the information, there
has to exist a set of rules or standards for this
These standards ensure that varying devices
and products can communicate with each
other over any network. This set of standards
is called Network protocols.
The Network functions are separated in the
form of Layers.
2Sep-13 BBC & BIOM III
What is Layering?
r A technique to organize a networked
system into a succession of logically
distinct entities, such that the service
provided by one entity is solely based on
the service provided by the previous (lower
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Layered Tasks Example
Sender, Receiver and Carrier
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Why divide network into layers???
Provides compatibility - It standardizes
network components to allow multiple-
vendor development and support.
It allows different types of network hardware
and software to communicate with each
Accelerates evolution of technology - It
prevents changes in one layer from affecting
the other layers, so that they can develop
more quickly. 5Sep-13 BBC & BIOM III
ISO (International Standards
It released a set fo specifications that describe
network architecture dissimilar devices.
In 1984 the ISO released set of specifications
called Open Systems Interconnection (OSI)
The OSI model has become an international
standard for networking.
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Mainly, it provides a way to understand how
an internetwork operates.
It also serves as a guideline or framework for
creating and implementing network
standards, devices, and internetworking
Helps in troubleshooting network problems
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OSI model consists of 7 layers and each layer provides some
services to the layer above it.
The principles that were applied to arrive at the seven layers
are as follows:
A layer should be created where a different level of
abstraction (idea, concept) is needed.
Each layer should provide some well defined functions.
Each layer should work independently from other layers.
Layer boundaries should minimize data flow across those
The right number of layers should be there. Don't put too
many functions together in a layer, and not too many layers
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OSI Layers cont’d…
The three upper layers (Application Layers)
define how the applications within the end
stations will communicate with each other
and with users.
The four lower layers (Data Flow Layers)
define ways for end stations to establish
connections to each other in order to
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Description of each layer
It is Layer 1, the bottom layer of the OSI model. It
specifies how data is processed into bits and
physically transferred over medium, such as cables.
The physical layer defines the
electrical, mechanical, procedural, and functional
specifications for activating, maintaining, and
deactivating the physical link between
communicating network systems. Physical layer
specifications define characteristics such as voltage
levels, timing of voltage changes, physical data
rates, maximum transmission distances, and physical
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Data Link Layer
It is Layer 2, and sends data frames from the
Network layer to the Physical layer.
On the receiving end, it packages raw bits from
the Physical layer into data frames.
It combines bits into bytes and bytes into
The data link layer is concerned with physical
(as opposed to logical) addressing, network
topology, network access, error notification
(acknowledgement) and retransmission, error
recovery (CRC check), ordered delivery of
frames, and flow control. 12Sep-13 BBC & BIOM III
Layer 3, the Network layer, is responsible for providing
end-to-end delivery of Packets.
To accomplish this the network layer defines logical
addresses to identify the endpoint of the network.
It also determines the route from the source to the
It determines which path the data should take based on
network conditions, priority of service, and other factors.
It also manages traffic problems on the network, such as
packet switching, routing, and controlling the congestion of
If the destination computer cannot receive a data chunk as
large as the source computer sends, the Network layer on
the source compensates by breaking the large data chunk
into smaller units.
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On the destination end, the Network layer reassembles the
data into the original large chunk.
Internet Protocol (IP) and Internetwork Packet Exchange
(IPX) are examples of network layer protocols.
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Layer 4, the transport layer, is responsible for breaking the
data into segments and establishing an end-to-end logical
connection between machines.
The transport layer ensures that packets are delivered error
free, in sequence, and without losses or duplications.
At the sending computer, this layer repackages messages,
dividing long messages into several packets and collecting
small packets together in one package. This process ensures
that packets are transmitted efficiently over the network.
At the receiving computer, the transport layer opens the
packets, reassembles the original messages, and, typically,
sends an acknowledgement that the message was received.
If a duplicate packet arrives, this layer will recognize the
duplicate and discard it. Protocols include TCP n UDP
15Sep-13 BBC & BIOM III
Layer 5, the Session layer defines how to start,
control and end the conversation (called session)
between two applications running on different
It provides synchronization between user tasks by
placing checkpoints in the data stream.
This way, if the network fails, only the data after
the last checkpoint has to be retransmitted.
This layer also implements dialog control between
communicating processes by regulating which side
transmits, when, for how long, and so on.
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The main purpose of this layer is defining the data formats.
It is concerned with how data is converted and formatted for
The Presentation Layer provides a variety of coding and
conversion functions that are applied to the application layer
data. These functions ensure that information sent from the
application layer of one system will be readable by the
application layer of another system
The Presentation layer is also responsible for protocol
conversion, encrypting the data, and managing data
The presentation layer also manages data compression to
reduce the number of bits that need to be transmitted.
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Layer 7, the topmost layer of the OSI model, is the
The Application Layer is where the user interacts with the
This layer relates to the services that directly support
applications, such as software for file transfers, database
access, and E-mail.
A message to be sent across the network enters the OSI
model at this point and exits the OSI model’s application
layer on the receiving computer.
Protocols at this layer identify communication partners,
determine resource availability, and synchronize
communication. For example, a word processing
application is serviced by file transfer services at this layer.
Application layer protocols; Http, telnet, SNMP,SMTP etc
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Figure 2.15 Summary of layers
Summary of Layers
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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
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
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.
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TCP/IP reference model layers
The TCP/IP model has four layers:
a) Application layer,
b) Transport layer,
c) Internet layer, and
d) Host to Network layer.
NB: 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. 21Sep-13 BBC & BIOM III
TCP/IP Application layer
-Combines the functions of the OSI application,
presentation, and session layers.
The TCP/IP combines all application related issues into one
layer, and assumes this data is properly packaged for the
Protocols involved in this layer: HTTP, FTP, SMTP, DNS,
POP3, TFTP etc.
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TCP/IP Transport layer
This layer works almost similar to the Transport layer of
Deals with the quality-of-service, issues of reliability, flow
control, and error correction.
It has two major protocols TCP and UDP
The transmission control protocol (TCP), provides excellent
and flexible ways to create reliable, well-flowing, low-error
network communications. TCP is a connection-oriented
Note: 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.
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TCP/IP Transport layer cont’d……
•The second protocol in this layer is
User Datagram Protocol (UDP), its an
unreliable, connectionless protocol for
the applications that do not want TCP’s
sequencing and flow control and where
quick delivery is more important than
accurate delivery like speech and video
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TCP/IP Internet(Network) layer
The purpose of the Internet layer is to send source
packets from any network on the internet work 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.
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TCP/IP Network Access layer
Provides the same functionality as the physical,
the data link and network layers in the OSI
It includes the LAN and WAN technology
The OSI model is proven to be very useful for
understanding computer networks.
However the OSI Protocols have not become
very popular. The opposite is the case for
TCP/IP reference model. The model is
practically non-existing, but two of its protocols
TCP and IP are widely used in the networks. 26Sep-13
BBC & BIOM III
Associated TCP/IP Protocols & Services
HTTP This protocol, the core of the World Wide Web, facilitates
retrieval and transfer of hypertext (mixed media) documents.
Stands for the HyperText Transfer protocol
Telnet A remote terminal emulation protocol that enables clients to
log on to remote hosts on the network.
SNMP Used to remotely manage network devices. Stands for the
Simple Network Management Protocol.
DNS Provides meaningful names like achilles.mycorp.com for
computers to replace numerical addresses like 126.96.36.199.
Stands for the Domain Name System.
SLIP (Serial Line Internet Protocol) and PPP (Point to Point
Protocol) encapsulate the IP packets so that they can be sent
over a dial up phone connection to an access provider’s
Sep-13 27BBC & BIOM III
Address Resolution Protocol (ARP)
The first system knows that the
second is located somewhere
on the network but does not
know its exact location on the
The source will broadcast an
ARP request to find the MAC
address of the intended
destinations MAC address.
The signal sent is a broadcast
message and all the devices in
the LAN will hear it.
Only the destination device will
respond to the ARP request.
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Simple Mail Transfer Protocol (SMTP) is a protocol for
sending e-mail messages between servers. The messages
can then be retrieved with an e-mail client using either Post
Office Protocol (POP) or Internet Message Access Protocol
Post Office Protocol version 3 (POP3) is a common mail
service protocol that is used by ISPs that provide Internet
and e-mail service to home customers.
FTP is a general-purpose protocol that can be used to copy
all types of files from one computer to another. FTP makes
use of the TCP reliable transport services to establish a
logical connection between the systems. FTP is one of the
most heavily used protocols on the Internet.
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Comparing the OSI Model and
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Similarities of the OSI and TCP/IP
Both have layers.
Both have application layers, though they
include very different services.
Both have comparable transport and network
Packet-switched, not circuit-switched,
technology is assumed.
Networking professionals need to know both
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Differences of the OSI and TCP/IP
TCP/IP combines the presentation and session
layer 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
TCP/IP transport layer using UDP does not
always guarantee reliable delivery of packets as
the transport layer in the OSI model does.
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TCP/IP protocols are considered to be
standards around which the internet has
The OSI model consists of 7 layers whereas
the TCP/IP only has 4 layers.
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