OSI MODEL
OPEN SYSTEMS INTERCONNECTION MODEL
WHAT IS OSI MODEL
• The Open Systems Interconnection (OSI) model describes seven
layers that computer systems use to communicate over a network. It was
the first standard model for network communications, adopted by all major computer
and telecommunication companies in the early 1980s
• The modern Internet is not based on OSI, but on the simpler TCP/IP model.
However, the OSI 7-layer model is still widely used, as it helps visualize and
communicate how networks operate, and helps isolate and troubleshoot networking
problems.
• OSI was introduced in 1983 by representatives of the major computer and telecom
companies and was adopted by ISO as an international standard in 1984.
What are the seven layers of the OSI Model ?
7. Application Layer - Human-computer interaction layer, where applications
can access the network services
6. Presentation Layer - Ensures the data is in a usable format and is where data
encryption occurs
5. Session Layer - Maintains connections and is responsible for controlling
ports and sessions
4. Transport Layer - Transmits data using transmission protocols including TCP
end UDP
3. Network Layer - Decides which physical path the data will take
2. Data Link Layer - Defines the format of data on the network
1. Physical Layer - Transmit raw bit stream over the physical medium
LAYER 7: APPLICATION LAYER
LAYER
• Level at which applications access network services
- Represents service that directly support software applications for the file transfers,
database access, and electronic mail etc.
• Responsible for the protocols and data manipulation that the software relies on to
present meaningful data to the user
LAYER 6: PRESENTATION LAYER
LAYER
• Related to representation of transmitted data
- Translates different data representations from the Application layer into uniform standard
format
• Providing services for secure efficient data transmission
- responsible for translation, encryption, and compression of data.
LAYER 5: SESSION LAYER LAYER
• Allows two applications on different my computers to establish, use and end a session.
- e.g., file transfer, remote login
• Establishes dialog control
- Regulates which side transmits side when and how long it transmits.
• Performs taken management and synchronization
• Responsible for opening and closing communication between the two devices.
LAYER 4: TRANSPORT LAYER
• LAYER
Manages transmission packets
- Repackages long messages, when necessary, into small packets for transmission
- Reassembles packets in correct order to get the original message
• Handles error recognition and recovery
- Transport layer at receiving acknowledges packet delivery
- Resends missing packets
• Responsible for end-to-end communication between the two devices.
LAYER 3: NETWORK LAYER
• Manages addressing/routing of date within the subnet
LAYER
- Addresses messages and translates logical addresses and names into physical addresses
- Determines the route from the source to the destination computer
- Manages traffic problems such as switching routine and controlling the congestion of data packets
• Routing can be:
- Based on static tables
- Determine at start of each session
- Individually determined for each packets, reflecting the current network load
• Responsible for facilitating data transfer between two different networks.
LAYER 2: DATA LINK LAYER
LAYER
• Packages raw bits from the Physical layer into frames (logical, structured packets
from data).
• Provides reliable transmission of frames
- It waits for an acknowledgement from the receiving computer
- Retransmits frames for which acknowledgement not received
• Responsible for flow control and error control in intra-network communication
LAYER 1: PHYSICAL LAYER
• Transmit
LAYERbits from one computer to another
• Regulates the transmission of a stream of bits over a physical medium
• Defines how the cable is attached to the network adapter and what transmission
technique is used to send data over the cable. Deals with issue like
- The definition of 0 and 1, e.g., how many volts represent a 1 and how long a bit lasts ?
- Weather the channel is simplex or duplex ?
- How many pins a connector has, and what the function of each pin is?
HOW DATA FLOWS THROUGH THE OSI MODEL
• For human-readable information to be transferred over a
network from one device to another, the data must travel
down the seven layers of the OSI Model on the sending device
and then travel up the seven layers on the receiving end.
FOR EXAMPLE:
When a sender clicks “Send” on an email application, the message is sent to the presentation layer using a defined
protocol (SMTP for outgoing email). The presentation layer compresses the data and sends the message to the session
layer, which opens a session for communication between the sender’s device and the outgoing server.
The message is sent to the transport layer where data is segmented, and then the network layer breaks the segments
into packets. Then, the packets are sent from the network layer to the data link layer, where packets are further broken
down into frames. The frames are sent to the physical layer where data is converted to bitstreams of ones and zeros
and transferred across a medium such as wireless connections or cables.
When the message reaches the recipient, the process is reversed. Data is sent from the physical layer to the
application layer, where data is converted from the bitstream ones and zeros to the message available in the
recipient’s email client. When a message is sent back to the sender, the process is repeated, and communication flows
down to layer 1 from layer 7 and back up the OSI Model when it reaches the recipient’s device.
Advantages:
• It is a layered model, and each layer is independent. Thus, changes in one layer don’t affect the other layers.
• It divides the complex function into smaller parts.
• It is a generic model as it has the flexibility to adapt to many protocols.
• It supports both connection-oriented as well as connectionless services.
• It is more secure and adaptable than having all services bundled in a single layer.
Disadvantages:
• It is purely a theoretical model and ignores the availability of resources and technologies. Hence, its practical
implementation is somewhat restricted.
• It is very complex. The initial implementation was cumbersome, slow, and costly.
• The layers are interdependent, they can’t operate parallelly as they have to wait for the data/ packets from
the predecessor layer.
THE END
