OSI Model
- The OSI, or Open Systems Interconnection, is a reference model outlining how information moves from one software application in a computer to another via a physical medium.
- Comprising seven layers, each layer within the OSI model serves a distinct network function.
- Developed by the International Organization for Standardization (ISO) in 1984, the OSI model serves as an architectural framework for inter-computer communications.
- The OSI model divides the overall task into seven smaller, more manageable tasks, with each layer assigned a specific function.
- Each layer operates independently and is self-contained, enabling it to perform its designated task autonomously.
Characteristics of OSI Model
- The OSI model is divided into two layers: the upper layers and the lower layers.
- The upper layers of the OSI model primarily handle application-related matters and are exclusively implemented in software. The application layer, positioned closest to the end user, facilitates interaction with software applications. Each upper layer is positioned directly above another layer.
- On the other hand, the lower layers of the OSI model address data transport issues. Both the data link layer and the physical layer are implemented using a combination of hardware and software. The physical layer, situated at the bottom of the OSI model, is nearest to the physical medium. Its primary responsibility involves transmitting information onto the physical medium.
7 Layers of IOS model
The OSI model is divided into seven distinct layers, each with its own
specific functions. Here are the seven layers
- Physical Layer
- Data Link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
Physical Layer
- Transmits individual bits from one node to another.
- Functions as the lowest layer of the OSI model.
- Establishes, maintains, and deactivates the physical connection.
- Specifies the mechanical, electrical, and procedural network interface specifications.
Function of Physical Layer
- Line Configuration: Defines how two or more devices are physically connected.
- Data Transmission: Specifies the transmission mode (simplex, half-duplex, or full-duplex) between devices on the network.
- Topology: Describes the arrangement of network devices.
- Signals: Determines the type of signals used for information transmission.
Data Link Layer
- This layer ensures the error-free transfer of data frames.
- It defines the format of the data on the network.
- Provides reliable and efficient communication between two or more devices.
- Mainly responsible for the unique identification of each device on a local network.
It consists of two sub-layers:
Logical Link Control (LLC) Layer:
- Responsible for transferring packets to the Network layer of the receiving device.
- Identifies the address of the network layer protocol from the header.
- Provides flow control.
Media Access Control (MAC) Layer:
- Acts as a link between the Logical Link Control layer and the network's physical layer.
- Transfers packets over the network.
Function of Data Link Layer
- Framing: The Data Link layer converts the raw bit stream from the Physical layer into packets known as Frames. It adds a header and trailer to each frame, with the header containing hardware destination and source addresses.
- Physical Addressing: A header added by the Data Link layer contains the destination address, directing the frame to its intended recipient.
- Flow Control: Flow control is a key function of the Data Link layer, ensuring a consistent data rate on both ends to prevent data corruption. It regulates data transmission so that a faster transmitting station, like a server, doesn't overwhelm a slower receiving station.
- Error Control: Error control involves appending a calculated value known as CRC (Cyclic Redundancy Check) to the trailer of the frame by the Data Link layer before it's sent to the Physical layer. If errors occur, the receiver requests retransmission of corrupted frames through acknowledgment.
- Access Control: When multiple devices share the same communication channel, Data Link layer protocols determine which device has control over the link at any given time.
Network Layer
- Manages device addressing and tracks device locations within the network.
- Determines the best data path based on network conditions, service priority, and other factors.
- Responsible for routing and forwarding packets.
- Layer 3 devices, like routers, are specified within this layer to provide routing services.
- Protocols used for routing network traffic, such as IP and IPv6, are categorized as Network layer protocols.
Function of Network Layer
- Internetworking: Handled primarily by the Network layer, internetworking establishes logical connections between diverse devices.
- Addressing: The Network layer augments the frame header with source and destination addresses, crucial for device identification on the internet.
- Routing: A fundamental function of the Network layer, routing selects the most efficient path among multiple options from source to destination.
- Packetizing: The Network Layer accepts packets from the upper layer and transforms them into packets, a process known as Packetizing, facilitated by the Internet Protocol (IP).
Transport Layer
- The Transport layer, positioned at Layer 4, guarantees the sequential and non-duplicated transmission of messages.
- Its core responsibility lies in the complete transfer of data.
- It receives data from the upper layer and segments it into smaller units, termed segments.
- Functioning as an end-to-end layer, it establishes a reliable point-to-point connection between the source and destination for data delivery.
Two protocols commonly utilized within this layer are:
Transmission Control Protocol (TCP):
- TCP is a standardized protocol facilitating communication between systems over the internet.
- It establishes and manages connections between hosts.
- Data sent via TCP is segmented into smaller units called segments. These segments traverse the internet through various routes and may arrive at the destination in different orders. TCP reassembles the segments in the correct sequence at the receiving end.
User Datagram Protocol (UDP):
- UDP is a transport layer protocol.
- It operates as an unreliable transport protocol, meaning the receiver doesn't acknowledge packet receipt, and the sender doesn't wait for acknowledgment. This characteristic renders the protocol unreliable.
Function of Network Layer
- Service-point Addressing: The Transport layer adds a header containing a service-point address or port address, enabling the transmission of data not just between computers but also between processes running on those computers. While the Network layer transmits data between computers, the Transport layer ensures messages reach the correct processes.
- Segmentation and Reassembly: Upon receiving a message from the upper layer, the Transport layer segments it into multiple segments, each assigned a unique sequence number. Upon reaching the destination, the Transport layer reassembles the message based on these sequence numbers.
- Connection Control: The Transport layer offers two services: connection-oriented and connectionless. Connectionless service treats each segment as an independent packet, traversing different routes to the destination. Connection-oriented service establishes a connection with the Transport layer at the destination before delivering packets, ensuring all packets follow a single route.
- Flow Control: Besides, the Transport layer manages flow control, albeit on an end-to-end basis rather than within a single link.
- Error Control: Additionally, the Transport layer handles error control, ensuring error-free message delivery end-to-end, rather than just across a single link. The sender's Transport layer guarantees message integrity throughout transmission to the destination.
Session Layer
- The Session layer resides at Layer 5 of the OSI model.
- Its primary function is to establish, maintain, and synchronize interactions between communicating devices.
Function of Session Layer
- Dialog Control: The Session layer functions as a dialog controller, facilitating communication between two processes, which can operate in either half-duplex or full-duplex mode.
- Synchronization: In data transmission, the Session layer introduces checkpoints to ensure sequence integrity. If errors interrupt data transmission, the process initiates again from the nearest checkpoint, a mechanism known as Synchronization and Recovery.
Presentation Layer
- The Presentation layer focuses on the syntax and semantics of the information exchanged between two systems.
- It serves as a data translator within a network.
- This layer, part of the operating system, converts data from one presentation format to another.
- The Presentation layer is also referred to as the syntax layer.
Function of Presentation Layer
- Translation: The Presentation layer manages the exchange of information between processes in two systems, which may use different encoding methods for character strings, numbers, and other data types. It ensures interoperability by converting data from a sender-dependent format to a common format and then converting it again to a receiver-dependent format at the destination.
- Encryption: To maintain privacy, the Presentation layer performs encryption, which involves converting the sender's information into another form before transmitting the resulting message over the network.
- Compression: Data compression reduces the number of bits to be transmitted, making it a crucial process for multimedia data such as text, audio, and video.
Application Layer
- The Application layer acts as a window for users and application processes to access network services.
- It addresses issues like network transparency and resource allocation.
- Although not an application itself, the Application layer performs essential functions to support applications.
- This layer delivers network services directly to end-users.
Function of Application Layer
- File Transfer, Access, and Management (FTAM): The Application layer enables users to access, retrieve, and manage files on a remote computer.
- Mail Services: The Application layer facilitates email forwarding and storage.
- Directory Services: The Application layer offers distributed database sources, providing global information about various objects.
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