2. Objectives
• Understand OSI model
• Explain the principle applied to arrive at seven layers.
• Explain the advantage of dividing network into layers.
• Explain the characteristics of the OSI layers.
• Understand how communication done between
systems using the OSI model.
• Explain different networking devices used in the OSI
layer.
• Understand TCP/IP stack.
• Explain connectionless and connection oriented
services.
• Understand Windowing
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3. Introduction
OSI reference model:
• Describes how information from a software
application in one computer moves through a network
medium to a software application in anther computer.
• Conceptual model composed of seven layers, each
specifying particular network functions. Each layer is
reasonably self-contained so that the functions
assigned to each layer can be implemented
independently.
• Developed by the International Standards
Organization (ISO) as a first step toward international
standardization of the protocols used in various layers
in 1983.
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4. Introduction contd..
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 is needed.
• Each layer should perform a well-defined function.
• The function of each layer should be chosen with an
eye toward defining internationally standardized
protocols.
• The layer boundaries should be chosen to minimize the
information flow across the interfaces.
• The number of layers should be large enough that
distinct functions need not be thrown together in the
same layer out of necessity, and small enough that the
architecture does not become unwieldy.
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5. Introduction contd..
The seven layers of the OSI model are numbered from
bottom to top as follows:
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data link
1 Physical
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6. Characteristics of the OSI Layers
The main characteristics of the seven layers OSI model are:
• Each layer performs a defined subset of functions
for the overall communication process
• To perform more primitive functions
• To hide the details of the lower layer functions
• Each layer provides services to the next higher
layer
• Modifications within a layer do not require
modifications of the other layers
• Dividing the communication functions into separate
layers facilitates the management of the
communication process.
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7. Characteristics of the OSI Layers contd..
The seven layers of the OSI reference model can be divided
into two categories:
• Upper Layers: Deal with application issues and
generally implemented only in software.
• Lower Layers: Handle data transport issues.
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8. Advantages of Dividing Network into
Layers
• Divides the interrelated aspects of network operation
into less complex elements.
• Defines standard interfaces for plug-and-play
compatibility and multi-vendor integration.
• Enables engineers to specialize design and promote
symmetry in the different internetwork modular functions
so that they interoperate.
• Prevents changes in one area from affecting other
areas, so each area can evolve more quickly
• Divides the complexity of internetworking into discrete,
more easily learned operation subsets.
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9. The Seven OSI Reference Model Layers
Physical Layer (Layer 1)
• The physical layer is concerned with transmission of
unstructured bit stream over physical link; involves such
parameters as signal voltage swing and bit duration.
• It accepts frames of data from the upper layer, the Data
Link Layer (Layer 2) and transmits the data serially, one
bit at a time.
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10. The Seven OSI Reference Model Layers contd..
Data Link Layer (Layer 2)
• The data link layer provides reliable transfer of data
across the physical link. It sends blocks of data (frames)
with the necessary synchronization, error control and
flow control.
• It exchanges a "frame" with Data Link Layer on another
node.
• The Data Link Layer provides services to the Network
Layer above. It receives the data from layer 3 and adds
the necessary control information, and sends it to the
layer 1 as a frame. It also verifies the frame received
from layer 1 and ensures the acknowledgement and
sends the data up to the layer 3.
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11. Data Link Layer (Layer 2) contd..
The Data Link layer contains two sub-layers:
• Logical Link Control (LLC): This sublayer is
responsible for identifying Network layer protocols and
then encapsulating them. An LLC header tells the Data
Link layer what to do with a packet once a frame is
received.
• Media Access Control (MAC): This defines how
packets are placed on the media. Contention media
access is “first cum first served” access where everyone
shares the same bandwidth. Physical addressing is
defined here, as well as logical topologies.
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12. The Seven OSI Reference Model Layers contd..
Network Layer (Layer 3)
The Network layer manages device addressing, tracks the
location of devices on the network and determines the best
way to move data.
Routers (layer 3 devices) are specified at the Network layer
and provide the routing services within an internetwork.
Two types of packets are used at the Network layer:
• Data Packets: It is used to transport user data through
the internetwork.
• Route Update Packets: It is used to update neighboring
routers about the networks connected to all routers
within the internetwork.
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13. The Seven OSI Reference Model Layers contd..
Transport Layer (Layer 4)
The transport layer accepts data from the session layer and
segments the data for transport across the network. This
layer is responsible for making sure that the data is
delivered error-free and in the proper sequence.
Flow Control: Data integrity is ensured at the Transport
layer by maintaining flow control and by allowing users to
request reliable data transport between systems.
Multiplexing enables data from several applications to be
transmitted onto a single physical link
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14. Transport Layer (Layer 4) contd..
Reliable data transport employs a connection-oriented
communications session between systems, and the
protocols involved ensure that the following will be
achieved:
• The segments delivered are acknowledged back to the
sender upon their reception.
• Any segments not acknowledged are retransmitted.
• Segments are sequenced back into their proper order
upon arrival at their destination.
• A manageable data flow is maintained in order to avoid
congestion, overloading, and data loss.
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15. The Seven OSI Reference Model Layers contd..
Session Layer (Layer 5)
The main function of the OSI model's session layer is to
control "sessions", which are logical connections between
network devices.
This layer provides dialogue control between devices, or
nodes. It coordinates communication between systems and
serves to organize their communication by offering three
different modes:
• Simplex
• Half duplex
• Full duplex
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16. The Seven OSI Reference Model Layers contd..
Presentation Layer (Layer 6)
It presents data to the Application layer. It's basically a
translator and provides coding and conversion functions.
By providing translation services, the Presentation layer
ensures that data transferred from the Application layer of
one system can be read by the Application layer of another
host.
Tasks like data compression, decompression, encryption
and decryption are associated with this layer.
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17. Presentation Layer (Layer 6) contd..
Some Presentation layer standards are involved in
multimedia operations. The following serve to direct graphic
and visual image presentation:
• JPEG: The Joint Photographic Experts Group brings
these photo standards.
• MIDI: The Musical Instrument Digital Interface is used
for digitized music.
• MPEG: The Moving Pictures Experts Group's standard
for the compression and coding of motion video for CD's
is very popular.
• QuickTime: This is for use with Machintosh or Power
PC programs, it manages audio and video applications.
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18. The Seven OSI Reference Model Layers contd..
Application Layer (Layer 7)
The application layer is the OSI layer closest to the end
user, which means that both the OSI application layer and
the user interact directly with the software application.
Application layer functions typically include identifying
communication partners, determining resource availability
and synchronizing communication.
Some examples of application layer implementations
include Telnet, File Transfer Protocol (FTP) and Simple Mail
Transfer Protocol (SMTP).
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19. Communication between Systems
using the OSI Model
Information being transferred from a software application in
one computer system to a software application in another
must pass through the OSI layers.
A given layer in the OSI model generally communicates with
three other OSI layers: System X System Y
• layer directly above it Application Application
• layer directly below it Presentation Presentation
• its peer layer in other Session Session
networked computer Transport Transport
systems Network Network
Data link Data link
Physical Physical
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20. Communication between Systems using the OSI Model contd..
How data is transmitted using the OSI model
System X System Y
Sending Receiving
Process Process
Data
Application Application
Application Protocol AH Data
Layer Layer
Presentation Presentation
Layer Presentation Protocol PH Data Unit Layer
Session Layer Session Layer
Session Protocol SH Data Unit
Transport Layer Transport Layer
Transport Protocol TH Data Unit
Network Layer Network Network Layer
NH Data Unit
Protocol
Data link Layer DH Data Unit DT Data link Layer
Physical Layer Physical Layer
Bits
Network
Actual data transmission path
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21. Communication between Systems using the OSI Model contd..
Encapsulation
As data travels down the layer stack at the sending system,
each layer adds a header to the front of it and gives the
resulting item to the lower layer. Some layer also adds a trailer
to the rear of the data unit. This addition of a header/trailer to
the higher layer data unit is called encapsulation.
De-encapsulation
As data travels upward the layer stack at the receiving system,
the layer reads the header from its peer layer, strips it off, and
passes the remaining information unit to the next highest layer.
This is known as de-encapsulation.
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22. More on Encapsulation
There are five steps of encapsulation:
1. User information is converted into data.
2. Data is converted into segments for transport
across the network.
3. Segments are converted into packets.
4. Packets and datagrams are converted into
frames and the Data Link header is added.
5. The data in the frames is converted into bits for
transmission over the physical media.
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23. More on Encapsulation contd..
The following table describes data encapsulation by OSI
layer:
OSI Layer Encapsulation
Transport Segment
Network Packet
Data Link Frame
Physical Bits
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24. Networking Devices to their OSI Layer
Physical Layer: Repeater, Hub, Network interface card (NIC
card)
• Repeater: A repeater is a network device. The main
function of repeater is to regenerate a single to allow it
to travel greater distances along a network.
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25. Networking Devices to their OSI Layer contd..
• HUB: It operates at the Physical layer of the OSI
model. HUB does not have any processing power, it's
just a box where you plug cables.
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26. Networking Devices to their OSI Layer contd..
• NIC: Puts the data into packets and transmits packet
onto the network. Every NIC card has a unique address
burnt into the card. This address is in a flat
hexadecimal number.
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27. Networking Devices to their OSI Layer contd..
Data Link Layer: Switches, Bridges
• Switch: Switch acts like a hub, but unlike the hub it
examines the destination MAC (Media Access Control)
addresses of the packet to decide where the packet
should be forwarded.
• Bridge: Bridges segments a network and reduce
network traffic by examining the source and destination
hardware address of the packet.
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28. Networking Devices to their OSI Layer contd..
Network Layer: Routers
• Routers: Devices at the Network layer are concerned
with two main things – network addressing and routing.
Routers reduce broadcast storms because they don't
route broadcast packets.
Cisco 1600 and 2500 Series Router
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29. Differences between Data Link and
Network Addresses
Data Link addresses (Data-link layer) are for the most part
referred to as flat address space unique addresses.
We mostly relate them to the term physical or hardware
addressing.
Example Address: 00-AA-BB-CC-DD-EE (MAC Address)
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30. Differences between Data Link and Network Addresses contd..
Network Addresses (Network Layer) are logical addresses
that are used for path selection, route determination and
selection.
Example Address: 192.168.100.2 where 192.168.100 is the
network and 2 is the individual host ID
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31. OSI Model and TCP/IP Stack
OSI Model Layers TCP/IP Protocol Architecture Layers TCP/IP Protocol Suite
Application Layer
FTP
HTTP
Presentation Layer Application Layer SMTP
SNMP
TELNET
Session Layer
Transport Layer Host-to-Host Transport Layer TCP UDP
Network Layer Internet Layer IP
ARP RARP ICMP
Data link Layer Ethernet
Fast Ethernet
Network Interface Layer Token Ring
Physical Layer FDDI
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32. OSI Model and TCP/IP Stack contd..
TCP/IP Protocol Suite
TCP/IP makes it possible for two computers, which are part
of different networks (connected by routers or gateways) to
exchange data.
Different protocols of TCP/IP suite:
• Application protocols occupy the highest protocol
layers and provide specific services.
• TCP (Transfer Control Protocol) and UDP (User
Datagram Protocol) facilitate the transmission of data
streams between applications running on different hosts.
• IP (Internet Protocol), a lower-level protocol than TCP
or UDP, governs the transmission of data packets
throughout a computer network.
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33. TCP/IP Protocol Suite contd..
Application Protocols:
• FTP (File Transfer Protocol)
• HTTP (Hypertext Transfer Protocol)
• SMTP (Simple Mail Transfer Protocol)
• SNMP (Simple Network Management Protocol)
• NNTP (Network News Transfer Protocol)
• Telnet
• WAP (Wireless Application Protocol)
Network layer protocols, that are less visible but play
equally important roles in TCP/IP networks, include:
• ARP (Address Resolution Protocol)
• RARP (Reverse Address Resolution Protocol)
• ICMP (Internet Control Message Protocol)
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34. Connection-Oriented and Connectionless
Services
Connection-oriented means that a connection (a virtual
link) must be established before data can be exchanged. In
connection-oriented service the source first informs the
network it wishes to start a conversation with destination,
the network sends it's request to the destination that
accepts or rejects the request. If the destination refuses,
connection fails, otherwise connection is established.
The communication proceeds through three well-defined
phases:
• Connection establishment
• Data transfer
• Connection release
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35. Connection-Oriented and Connectionless Services contd..
A typical reliable session taking place between sending and
receiving systems.
Sender Receiver
Synchronize
Negotiate Connection
Synchronize
Acknowledge
Connection Established
Data Transfer
(Send segments)
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36. Connection-Oriented and Connectionless Services contd..
Connectionless is the opposite of connection-oriented.
The sender does not establish a connection before it sends
data, it just sends without guaranteeing delivery. Packets
sent between two hosts may take different routes. UDP is
an example of a connectionless transport protocol.
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37. Windowing
The quantity of data segments (measured in bytes) the
transmitting machine is allowed to send without receiving an
acknowledgment for them is called a window.
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