ISO/OSI Reference Model

Let's discuss ISO/OSI Reference Model in Detail

• The OSI Model may be thought of as a universal computer networking language.
• It's based on the idea of breaking down a communication system into seven abstract layers that are piled on top of each other.
• The current Internet is built on the TCP/IP paradigm rather than the OSI model. The OSI 7-layer model, on the other hand, is still extensively used since it helps to see and describe how networks work, as well as isolate and diagnose networking issues.
• Network communication is divided into seven tiers according to the OSI Model. These layers are helpful in detecting network problems.

Physical layer

Physical Layer

• The physical layer's primary duty is to transport individual bits from one node to another.
• It is the OSI model's lowest layer.
• The physical connection is established, maintained, and deactivated by it.
• It specifies network interface mechanical, electrical, and procedural specifications.

Physical layer functions include:

1. Line Configuration: This specifies how two or more devices can be physically connected.
2. Data Transmission: It specifies whether the network's two devices are communicating in simplex, half-duplex, or full-duplex mode.
3. Topology: Topology refers to the arrangement of network devices.
4. Signals: It specifies the sort of signal that is utilized to transfer data.

Data-Link Layer

Data Link Layer

• This layer is in charge of ensuring that data frames are sent without errors.
• It specifies the data format on the network.
• It allows two or more devices to communicate reliably and efficiently.
• It is primarily responsible for each device on a local network's unique identifier.
• It is divided into two layers:

Logical Link Control Layer

1. It is in charge of transmitting packets to the Network layer of the receiving device.
2. It extracts the network layer protocol's address from the header.
3. It also regulates the flow of information.

Media Access Control Layer

1. A Media Access Control Layer connects the Logical Link Control layer to the physical layer of the network.
2. It is used to send and receive packets via a network.

Basic Functions of the Data-link layer

Framing: 

• The data connection layer converts the raw bit stream from the physical into packets known as frames. 
• The header and trailer are added to the frame by the Data link layer. The hardware destination and source addresses are included in the frame header.

Header

Physical Addressing: 

• The Data Link Layer adds a destination address to the frame's header. The frame is sent to the specified destination address in the header.
• Flow Control: The Data-link layer's primary function is flow control. It is a technique for maintaining a consistent transmission rate on both sides in order to prevent data corruption. 
• It guarantees that the broadcasting station, such as a server, does not outperform the receiving station, which has a slower processor.

Error control: 

• It is done by adding a computed value CRC (Cyclic Redundancy Check) to the Data Link Layer's trailer, which is then appended to the message frame before it is delivered to the physical layer. 
• If an error appears, the receiver sends an acknowledgement for the corrupted frames to be retransmitted.

Access Control: 

• When two or more devices are linked to the same communication channel, data link layer protocols are used to determine which device has control of the channel at any particular time.

Network Layer

Network Layer

• It's a layer 3 that handles device addressing and keeps track of where devices are on the network.
• Based on network conditions, service priority, it determines the best path to move data from source to destination.
• The Data Link Layer is in charge of packet routing and forwarding.
• Routers are layer 3 devices that offer routing services inside an internetwork. They are specified at this layer and are utilized to deliver these services.
• Network layer protocols are the protocols that are used to route network traffic. IP and IPv6 are two examples of protocols.

Basic Functions of Network Layer:

Internetworking: 

• The network layer's primary task is internetworking. It establishes a logical link between many devices.

Addressing: 

• The source and destination addresses are added to the frame header by a network layer. 
• On the internet, addressing is used to identify the particular device.

Routing: 

• Routing is a significant component of the network layer that identifies the best optimum path from the source to the destination among many paths.

Packetizing: 

• The packets from the higher layer are received by a Network Layer, which turns them into packets. 
• Packetizing is the term for this procedure. The internet protocol is responsible for this (IP).

Transport Layer

Transport Layer

• The Transport layer is a Layer 4 that guarantees that messages are sent in the order they are received and that data is not duplicated.
• The transport layer's primary job is to finish the data transmission.
• It gets data from the higher layer and transforms it into segments, which are smaller units.
• This layer is known as an end-to-end layer because it establishes a reliable point-to-point link between the source and the destination.

The following are the two protocols used in this layer:

Transmission Control Protocol

• It's a standard protocol that allows computers to connect with one another across the internet.
• It is responsible for establishing and maintaining a connection between hosts.
• The TCP protocol breaks data into smaller pieces known as segments when it is transferred across a TCP connection. 
• Each segment goes through the internet via several routes and arrives at the destination in a varied order. 
• At the receiving end, the transmission control protocol reorders the packets in the right sequence.

User Datagram Protocol

• A transport layer protocol is the User Datagram Protocol.
• It is an unstable transport protocol because when the packet is received, the receiver does not transmit any acknowledgement, and the sender does not wait for any acknowledgment. As a result, a protocol becomes unreliable.

Basic Functions of Transport Layer:

Service-point addressing: 

• Because computers execute several programs at the same time, data must be sent from source to destination not just from one machine to another, but also from one process to another. 
• The transport layer inserts a header with the service-point address, also known as a port address. 
• The network layer's job is to send data from one computer to another, while the transport layer's job is to get the information to the right process.

Segmentation and reassembly: 

• When the transport layer gets a message from the top layer, it separates it into numerous segments and assigns a sequence number to each segment that uniquely identifies it. 
• The transport layer reassembles the message based on its sequence numbers after it has arrived at its destination.

Controlling connections: 

• The transport layer offers two functions. There are two types of services: connection-oriented and connectionless. 
• Each part of a connectionless service is treated as a separate packet, and they all take distinct paths to get to their destination. 
• Before delivering packets, a connection-oriented service establishes a connection with the transport layer at the target computer. 
• All packets in a connection-oriented service travel via a single path.

Flow control: 

• It is likewise provided by the transport layer, however it is done end-to-end rather than over a single connection.

Error control: 

• It is also the responsibility of the transport layer. Error control is carried out from beginning to finish, rather than across a single connection. 
• The sender transit layer ensures that the message arrives at its intended location without mistake.

Session Layer

Session Layer

• In the OSI model, it is a layer 3.
• The Session layer is responsible for establishing, maintaining, and synchronizing communication between devices.

Session layer's functions are as follows:

 Dialogue Controlling: 

• The session layer is a dialogue controller that produces a dialogue between two processes, or more precisely, it facilitates communication between two processes that can be half-duplex or full-duplex.

Synchronization: 

• When transferring data in a series, the session layer inserts certain checkpoints. 
• If a data transmission fault happens in the midst of the process, the transmission will be restarted from the checkpoint. Synchronization and respite is the term for this procedure.

Presentation Layer

Presentation Layer

• The syntax and semantics of the information transferred between the two systems are the primary concerns of the Presentation layer.
• It serves as a network's data translator.
• This layer is a component of the operating system that transforms data between different display formats.
• The syntax layer is another name for the presentation layer.

Presentation layer's functions are as follows:

Translation: 

• Information is sent between two systems in the form of character strings, integers, and so on. 
• Different computers employ different encoding methods, and the presentation layer is in charge of ensuring compatibility between them. 
• It translates data from a sender-dependent format to a common format, which it then converts back to a sender-dependent format at the receiving end.

Encryption: 

• To guarantee privacy, encryption is required. 
• Encryption is the process of changing information sent by the sender into a different format and sending the resultant message through the network.

Compression: 

• Data compression is the process of compressing data, or reducing the number of bits that must be transferred. 
• In multimedia such as text, music, and video, data compression is critical.

Application Layer

Application Layer

• Users and application processes can access network services through an application layer, which acts as a window.
• It deals with concerns like network transparency, resource allocation, and so on.
• Although an application layer is not an application, it performs the duties of the application layer.
• This layer offers end-users with network services.

Application layer functions include:

File transfer, access, and management (FTAM): 

• A user can access files on a distant computer, recover files from a computer, and manage files on a remote computer using an application layer.

Email services: 

• Email services are provided via an application layer that allows for email forwarding and storage.

Directory services: 

• A directory service is a program that offers access to distributed database sources and is used to deliver global information about diverse objects.