The document explains the OSI (Open System Interconnection) model, which facilitates communication between computers in a network by defining seven layers, each with specific functions. These layers include the application, presentation, session, transport, network, data link, and physical layers, which handle tasks such as data encoding, reliable transmission, routing, and error detection. Each layer contributes to the overall effectiveness of networking protocols and data communication between different systems.

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2. What is OSI layer?
• OSI stands for Open System Interconnection which defines how one computer
communicates with another computer in a network. For example, one computer
with IOS and another computer with window OS then how does these computers
communicates? In order to accomplish successful communication between
different computers, networks or different architectures OSI layer was introduced
by ISO (international Organization for Standardization) in 1984. ISO layer consists of
sever layers which are explained below in detail.

3. Application Layer
1. Application layer: Application layer is the bundles of protocols
that is used by network applications to work correctly in a
network or Internet. For example, s SMTP/POP to send and
receive message, FTP to send message, HTTP/s to serf internet
etc.

4. Presentation Layer

5. Presentation Layer
2.Presentation layer: just below the application layer there is presentation layer.
a. Presentation layer receive data from application which is in the form of characters and
numbers. These characters and number are translated into machine understandable form
for example ASCII to EBCDIC.
b. Before data transmission number of bits is reduced, the process of reduction bits is called
code reduction which improves the speed of data transmission.
c. After the code deduction, the message is encrypted at the sender side (Encryption is the
process of encoding message so the message will be meaningful to the unauthorized
person) and decrypted at the received side (reverse process of encryption). This is done
by protocol SSL (secure socket layer).

6. Session Layer

7. Session Layer
3.Session Layer: session layer does have its helpers’ APIs and NetBIOS which helps to setting up and
management of connection for effective data communication.
a. Authentication and Authorization: Just before creating session between server and computer,
Server performs authentication (authentication is the process of identifying user based on user
name and password), Once entered username and password is matched session is created
between server and computer. After authentication authorization (authorization is the process of
giving access of resources to the user computer based on their identity) is performed. Both
Authentication and Authorization is done by Session layer.
b. Session management: Session layer keep track the files that are being downloading from web
server. for example. A web page consists text, audio etc. these texts, audio files stores separately in
web server. When a user make request for web page each of these files download separately then
session layer keep track which files belong to which either text file or audio file and track where
does it goes? Hence session layer helps in session management.

8. Transport layer

9. Transport Layer
4.Transport Layer: just below the session layer is transport layer, it receives the data from the session
layer. IT performs the following operations for the reliability of communication:
a. Segmentation: Transport layer brake the data in small data items called segment. While
creating segment it adds source and destination port number and serial number. Source and
destination port number helps to direct the data in correct application and serial number
reassembles data items in correct order during data transmission. This process called
segmentation.
b. Flow control: transport layer controls amount of data that is being transmitted. For example,
if the processing speed of server is 10mbps and user computer has 5mbps. If server sends
8mbps data then transport layer tells server to slow down the speed to avoid data loss
similarly, if server send data at 3 mbps then transport layer tells server to increase speed.

10. Transport layer
c. Error Control: When transmitted data doesn’t reach at the destination then the transport layer
sends Automatic Repeat Request to retransmit lost data. Transport layer also add group of bits
called checksum to find out corrupted data during data transmission.
d.Connection oriented transmission and connection less transition: Transport layer also performs
two types of service Connection oriented transmission (TCP) and connection less transmission
(UDP). UDP (User Datagram Protocol) is faster than TCP (transmission control protocol) because UDP
doesn’t need to give feedback whether data has been received or not. TCP is used when whole data
transmission is most and UDP is used where there is no matter whether data has been received or
not. Online. streaming videos, audios, telephone communication is done by UDP and sending mail
searching web information is done by TCP.

11. Network

12. Network Number
5.Network Layer: It work for transmission of received data from one computer to another located in different network
. Just below the transport layer is network layer it receives the data segment from transport layer and performs the
following operation:
a. Logical addressing: The received data segment is added with source and destination IP address to transmit
data in correct destination. This process is called logical addressing. In this layer data segment is converted in
data packets.
b. Determining Best path: There is number of paths to transmit data. Network layer find the best path to
transmit data using different protocol like BGP(Border Gateway Protocol), IS-IS(Intermediate system to
intermediate System protocol) or Open Shortest Path First Protocol(OSPF).
c. Routing: Routing is the process of sending data packets from source to the correct destination. For this
Network layer use IP address and subnet marks.
• Device: router,IP

13. Data Link Layer

14. Data Link Layer
6.Datalink Layer: Ensures error free transfer of data frames between two nodes on the same network.
a. Framing: Data link layer is the software embedded into the Network Interface Card(NIC).It allows to access
media to the upper layer of OSI. The technique is called framing. During framing data packets is added with
source and destination MAC address along with tail called physical address.
b. Handles MAC: It also controls how data is placed and received from media the technique is called Media
access control. For example, several device may be connected with a common cable, when two or more device
sends data at a same time then the message can be meaningless to the recipients so Datalink layer check the
shared media and when the media becomes free data transmission takes place. This is called Carrier Sense
Multiple Access (CSMA).
c. Error Detecting and correction: Tail of each frame contains bits which are used to detect error at the received
frame due to certain limitation of media.
d. Flow control:
• Device : MAC Protocol, switch.

15. Physical layer

16. Physical layer
7.Physical layer: physical layer converts sequence of binary bits into electric signal and
transmit through local media. It can be electric signal in case of Lan cables, light in case of
optics fiber cable and radio signals in the case of air. So types of signal converted depends
upon types of cable used.
Key Responsibilities:
• Encoding and signalling of binary data (0s and 1s) into electrical signals, light signals, or
radio waves.
• Physical connection between devices (e.g., cables, switches, or connectors).
• Transmission speed and medium (e.g., copper, fiber, wireless).
• Voltage levels, timing, and data rates.
For examples. Ethernet cables, fiber optics, hubs, repeaters.

17. Layer Name Key Function Example Protocols & Devices
7 Application Layer End-user services, data exchange HTTP, FTP, DNS, Email
6 Presentation Layer Data encoding, encryption, compression JPEG, SSL/TLS, ASCII
5 Session Layer Session management, synchronization NetBIOS, RPC
4 Transport Layer Reliable communication, error control TCP, UDP
3 Network Layer Routing, logical addressing IP, Routers, ICMP
2 Data Link Layer Error detection, frame transfer Ethernet, Wi-Fi, Switches
1 Physical Layer Transmission of raw bits over a medium Ethernet cables, Wi-Fi

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