The document presents an overview of the OSI (Open Systems Interconnection) reference model, which is a 7-layer architecture developed by the International Organization for Standardization (ISO) for standardizing communication protocols. Each layer of the model - from the application layer to the physical layer - has specific functions, such as data formatting, session management, and error control, facilitating the transmission of data across networks. It emphasizes how these layers work collaboratively to ensure efficient and secure data exchange between open systems.

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Computer Networks
LECTURE -1
OSI Reference Model
Mr. Abdul Rais
Assistant Professor, CSE-AIML
LIET, Hyderabad
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 OSI stands for Open Systems Interconnection.
 It has been developed by ISO – ‘International
Organization for Standardization‘,
 It is a 7-layer architecture
 Work collaboratively to transmit the data from one
node to another across the globe.
INTRODUCTION
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 Developed by the International Standards Organization (ISO) as a first step
toward international standardization of the protocols used in the various
layers.
 The model is so called because it deals with connecting open systems-that is,
systems that are open for communication with other systems.
INTRODUCTION
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LAYERED ARCHITECTURE
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APPLICATION LAYER
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 At the very top of the OSI Reference Model stack is the Application layer provide
the user interface
 The Functions of the Application Layer are
• Virtual Terminals of the network
• Control the access and manage the file transfer
• Mail Services
• Directory Services
• With the help of protocols such as FTP, TELNET, SMTP etc.
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 The data from the application layer is extracted here and manipulated as per
the required format to transmit over the network.
 The Functions of the Presentation Layer are
 Translation of number systems
 Encryption/ Decryption of data for security
 Compression to reduce the data bits
With the help of protocols such as HTTP, HTML, MPEG, NCP etc.
PRESENTATION LAYER
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 Session Layer is responsible for the establishment of connection, maintenance
of sessions, and authentication, and also ensures security.
 The Functions of the Session Layer are
 Session establishment, maintenance, and termination:
 Synchronization
 Dialog Control
With the help of protocols such as RPC, RTCP, PAP etc.
SESSION LAYER
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 Transport Layer is responsible for the End to End Delivery of data between
applications
 The Functions of the Transport Layer
 Identifying services
 Multiplexing and Demultiplexing
 Segmentation
 Sequencing and Reassembly
 Error correction
 Flow Control
With the help of protocols such as TCP, UDP, SCTP etc.
TRANSPORT LAYER
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 The network layer works for the transmission of data in the form of Packets
from one host to the other located in different networks.
 It also takes care of packet routing i.e. selection of the shortest path to
transmit the packet.
 The sender & receiver’s IP addresses are placed in the header by the network
layer.
NETWORK LAYER
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 The Functions of the Network Layer
 Routing
 Logical Addressing
With the help of protocols such as ARP, RARP, ICMP, IGMP etc.
NETWORK LAYER
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 Responsible for end to end delivery of data between the devices on LAN
segement
 When a packet arrives in a network, it is the responsibility of the DLL to
transmit it to the Host using its MAC address forming Frames.
DATA LINK LAYER
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 The Functions of the Data Link Layer
 Framing
 Physical Addressing
 Synchronization
 Error Control & Flow Control
 Multi-Access
With the help of protocols such as SLIP, PPP, LAP etc.
DATA LINK LAYER
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 The lowest layer of the OSI reference model
 Responsible for the actual physical connection between the devices.
 Contains information in the form of bits.
 Transmits individual bits from one node to the next on given media
PHYSICAL LAYER
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 The Functions of the Physical Layer
 Representation of bits
 Physical topologies
 Transmission mode
 Line configuration
 Date rate
With the help of protocols such as DSL, ISDN, USB etc.
PHYSICAL LAYER
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