OSI Model

OSI stands for Open Systems Interconnection. It has been developed by ISO International Organization for Standardization in the year 1984.

In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

1.Physical Layer

2.Data Link Layer(DDL) 2.Data Link Layer

1.Physical Layer

It is the lowest layer of the OSI model.
It is responsible for the actual physical connection between the devices.
This layer converts the digital bits into electrical, optically or via radio waves.

Some Functions of Physical layer

Data Transmission-The various transmission modes possible are Simplex, full-duplex and half-duplex.

Line Configuration-It helps in providing Physical Medium and Interface decisions

Signals-It determines the type of the signal used for transmitting the information.

Topology-It helps in Physical Topology (Mesh, Star, Bus, Ring) decisions (Topology through which we can connect the devices with each other)

Physical Layer devices-are Modem, Hub, Repeater and Cables.

2.Data Link Layer(DDL) 2.Data Link Layer

The data link layer is responsible for the node-to-node delivery of the message.
The data link provides a efficient & reliable communication between two or more devices.
Defines the format of the data on the network.
It is the responsibility of the data link layer to transmit it to the Host using its MAC address.
It contains two sub-layers

Logical Link Control Layer(LLC)

LLC is responsible for transferring the packets to the Network layer of the receiver that is receiving.
It also provides flow control, responsible for multiplexing, acknowledgement and even error-checking functions of DDL

Media Access Control Layer(MAC)

A MAC (Media Access Control) layer is a link between the LLC(Logical Link Control) layer and the network's physical layer.
Used for transferring the packets over the network.

Some Functions of Data-Link Layer

Framing-Framing is a function of the data link layer. it is adds a header to the frame that contains a destination address. The frame is transmitted to the destination address mentioned in the header.

Flow Control-Flow control is the main functionality of the data-link layer. This prevents a fast sender from overwhelming a slower receiver by using techniques like sliding window protocols.

Physical Addressing-These addresses are used to identify the source and destination devices within the same local network. The Data Link Layer assigns unique addresses.

Error Control-It checks for errors that might occur during transmission using techniques like checksums or cyclic redundancy checks (CRC). If errors are detected, the frame can be retransmitted.

Access Control-When two or more devices are connected to the same communication channel, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

Switch & Bridge are Data Link Layer devices

3.Network Layer

The Network Layer deals with logical addressing, routing, and forwarding of data packets between different networks.
It also takes care of packet routing.
It's layer is responsible for routing and forwarding the packets.
It is used to route the network traffic are known as Network layer protocols.

Some Functions of Network Layer

Logical Addressing-A network layer adds the source and destination address to the header of the frame. Addressing is used to identify the device over the internet. these addresses are hierarchical and provide a way to uniquely identify devices across different networks.
Routing-The Network Layer is responsible for determining the best path that data packets. This involves analysing network topology, congestion, and other factors to make efficient routing decisions.

4.Transport Layer

The main responsibility of the transport layer is to transfer the data as a whole.
it is ensures that messages are transmitted in the order in which they are sent and there is no duplication of data.
It is responsible for determining the optimal path for data packets to travel from the source to the destination across interconnected networks.
The Internet Protocol (IP) operates at this layer.
The transport layer is called as Heart of the OSI model.
Protocol Use : TCP, UDP NetBIOS, PPTP

The two protocols used in this layer are

Transmission Control Protocol (TCP)

TCP is a connection-oriented protocol that offers reliable, ordered, and error-checked data delivery.

User Datagram Protocol (UDP)

UDP is a connectionless protocol that provides a lightweight way of sending data between devices without the overhead of establishing a connection and ensuring reliable delivery.

Some Functions of Transport Layer

Segmentation and Reassembly-When the transport layer receives a message from the upper layer, it divides the message into several segments, and each segment is assigned a sequence number that uniquely identifies each segment. When the message reaches the destination, the transport layer reassembles the message based on their sequence numbers.
Service Point Addressing-Computers run many programs simultaneously, so the transfer of data from source to destination is not only from one computer to another but also from one process to another. The transport layer adds a header that includes the address known as the service-point address or port address. The responsibility of the network layer is to transmit data from one computer to another and the responsibility of the transport layer is to transmit the message to the right process.

5.Session Layer

The session layer is used to establish, maintain, and synchronous interaction between communicating devices.

Some Functions of Session Layer

Dialog Controller-The session layer allows the two systems to begin communication with each other in half-duplex or full-duplex.

Synchronization-The session layer adds some checkpoints when transmitting the data in a sequence. If some error occurs in the middle of the data transmission, then the transmission will take place again from the checkpoint. This process is known as synchronization and recovery.

6.Presentation Layer

A presentation layer is primarily concerned with the syntax and semantics of the information exchanged between two systems.
its layer also handles the encryption and decryption that the application layer requires.
It deals with data format conversion, encryption, and data compression.

Some Functions of Presentation Layer

Data Translation-data format conversions between the sender's and receiver's systems For example, it could translate between ASCII and EBCDIC character encoding.

Encryption/ Decryption-Data encryption translates data into another form or code.

Protocol Use : JPEG, MPEG, GIF

7.Application Layer

Application layer serves as a window for users and application processes to access network services.
Web browsers and other internet-connected apps, like file transfer, email, remote access, and more.
protocols at the application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP.

Some Functions of Application Layer

File transfer, access, and management (FTAM)-File transfer access and management :This application allows a user to

access file in a remote host, retrieve files in remote host and manage

Controlling files from a remote computer.

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