OSI Model from scratch

What is OSI ?

OSI (Open Systems Interconnection) Model is a set of rules that explains how different computer systems communicate over a network. It was published in 1984

Why we need OSI ?

• Computer connected to each other by LAN network using Rj45 cable and NIC with MAC (Media Access Control) address. But two computers with different operating systme such as MacOS and Window can talk to each other, Yeah we need the rule for them, OSI is comming !!

• It has 7 layers in total, representing for protocol that makes application or services work correctly in a network

7. Application

• Application layer which is implemented by the network applications, some applications that familiar with like Google, Firefox, Email,...
• Some service and protocol corresponding:

1. File Transfer: FTP
2. Web Browser: HTTP/s
3. Email: SMTP
4. Virtual Terminal: Telnet

6. Presentation

• Received data from Application in form of character and number, this layer need to convert that to binary number for machine understanding
• Functions of the Presentation Layer

1. Translation: For example, ASCII to EBCDIC.
2. Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext, and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
3. Compression: Reduces the number of bits that need to be transmitted on the network.

5. Session

• Responsible for the establishment of connections, management of connections, terminations of sessions between two devices. It also provides authentication and security. Protocols used in the Session Layer are NetBIOS, PPTP.
• Functions of the Session Layer

1. Session Establishment, Maintenance, and Termination : The layer allows the two processes to establish, use, and terminate a connection.
2. Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely, and data loss is avoided.
3. Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full duplex.

4. Transport

• The data in the transport layer is referred to as Segments. It is responsible for the end-to-end delivery of the complete message. The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.
• Functions of the Transport Layer

1. Segmentation and Reassembly: This layer accepts the message from the (session) layer and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.
2. Flow And Error Control: change transmission rate (bandwith) for sending data, no data get lost. Automatic Repeat Request to resubmit corrupted data using checksum to check update segment

• Protocols used in Transport Layer are TCP (connection-oriented transmission - create session 3-way handsake - guarantees delivery of data if packet can't send it will resend it , UDP (connectionless transmission - These protocols simply allow data to be transferred without any link among processes. Some Of data packets may also be lost during transmission), NetBIOS, PPTP.

3. Network

• Received segment from Transport Layer, data unit in Network layer called packet
• Functions of the Network Layer

1. Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.
2. Logical Addressing: To identify each device inter-network uniquely, the network layer defines an addressing scheme. The sender and receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

2. Data Link Layer (DLL)

• When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address. Packet in the Data Link layer is referred to as Frame
• Functions of the Data Link Layer

1. Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
2. Physical Addressing: After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.
3. Error Control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
4. Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.
5. Access Control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

1. Physical Layer

• It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits
• Functions of the Physical Layer

1. Bit Synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.

2.Bit Rate Control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.

1. Physical Topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus topology, star topology, or mesh topology.
2. Transmission Mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full duplex.

Resource:
https://www.youtube.com/watch?v=uwoD5YsGACg
https://www.youtube.com/watch?v=vv4y_uOneC0&t=644s
https://www.geeksforgeeks.org/open-systems-interconnection-model-osi/