Inside this Article:
Before we delve into the ISO-OSI model, let's understand its significance and purpose.
- The ISO-OSI model, developed by the International Organization for Standardization (ISO) in 1984, is the cornerstone architectural model for inter-computer communications.
- Functioning as a descriptive network scheme, this model ensures enhanced compatibility and interoperability across diverse network technologies.
- It serves as a guide, illustrating the journey of information or data as it traverses from one application program through a network medium to another application program on a different network.
- The model intelligently breaks down the complex task of moving information between computers over a network medium into seven more manageable and modular problems.
By adopting the ISO-OSI model, we achieve a structured approach to understanding and resolving the intricacies of inter-computer communication within a network.
Seven Layers of OSI Model
The OSI Model is like a seven-story building; each floor has its own job. Remember the order with phrases like "Please Do Not Take Seriously Peculiar Analogies" or "All People Seem to Need Data Processing."
1. Layered Structure:
- Think of the OSI Model as a building with seven floors. Each floor (layer) has a specific job in making networks work.
- The bottom four floors (1-4) use both hardware and software and deal with how data moves. The top three floors (5-7) mainly use software and focus on helping applications.
2. Communication between Layers:
- Imagine messages going down through the floors. Each floor adds something to the message.
- When a device sends a message, it asks for help from the floor below. The receiving device takes away the added stuff from each floor.
- Floors at the same level in different devices talk to each other, like neighbors. This is called peer-to-peer communication, helped by interfaces.
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Peer-to-Peer Communication |
3. Encapsulation and De-capsulation:
- Imagine adding a cover (header) before and after a message at the sender – that's encapsulation.
- The receiver takes off the covers (header and trailer) – that's de-capsulation.
- The data link layer, like a post office, adds the back cover (trailer) during encapsulation.
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Functions and Responsibilities of OSI Layers
The specific functions and responsibilities of each layer are listed as follows:
1. Physical Layer
The physical layer coordinates the functions required to transmit a bit stream over a physical medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.
The physical layer is concerned with the following functions:
- Physical characteristics of interfaces and media: The physical layer defines the characteristics of the interface between devices and the transmission media, including its type.
- Data rate: The physical layer defines the transmission rate, the number of bits sent each
- Representation of the bits: The physical layer data consists of a stream of bits without any interpretation. To be transmitted, bits must be encoded into signals –electrical or optical-. The physical layer defines the type of encoding.
- Line configuration: the physical layer is concerned with the connection of devices to the medium.
- Physical topology: The Physical layer is responsible for defining the physical layout called topology of the underlying network such as star, ring, bus, mesh, and tree topology.
- Transmission Mode: The physical layer is also responsible for defining the transmission mode such as simplex, half duplex, and full duplex mode.
2. Data Link Layer
The data link layer transforms the physical layer, a raw transmission facility, into a reliable link and is responsible for node-to-node delivery.
The data link layer performs the following functions.
- Framing: The data link layer divides the stream of bits received from the network layer into data units called frames.
- Physical addressing: If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the physical address of the sender and receiver of the frame.
- Flow Control: If the rate at which the data are absorbed by the receiver is less than the rate produced by the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.
- Error control: The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. Error control is normally achieved through a trailer to the end of the frame.
- Access Control: When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any time.
The main responsibility of the data link layer is hop to hop-to-hop transmission of frames.
3. Network Layer
The Network layer is responsible for the source-to-destination delivery of a packet possible across multiple networks.
The network layer is responsible for the following functions.
- Logical addressing: The physical addressing implemented by the data link layer handles the addressing problem locally.
- The network layer adds a header to the packet coming from the upper layer, among other things, including the logical address of the sender and receiver.
- Routing: When independent networks or links are connected to create an internetwork (a network of networks) or a large network, the connecting devices (called routers or gateways) route or switch the packets to their final destination.
The main responsibility of the Network Layer is the transmission of packets from source to destination.
4. Transport Layer
The transport layer is responsible for process-to-process delivery of the entire message. The network layer oversees the host-to-destination delivery of individual packets; it does not recognize any relationship between those packets.
The specific functions of the transport layer include:
- Port addressing: The computer often runs several processes (running programs) at the same time. Process-to-process delivery means delivery from a specific process on one computer to a specific process on the other.
- Process-to-process delivery: The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.
- Segmentation and reassembly: A message is divided into transmittable segments, each having a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arrival at the destination.
- Connection control: The transport layer can be either connectionless or connection-oriented
- Flow control: the transport layer performs a flow control end to end. The data link layer performs flow control across a single link.
- Error control: the transport layer performs error control end to end. The data link layer performs control across a single link.
The main responsibility of the transport layer is process to process delivery of the entire message.
5. Session Layer
The session layer allows two applications on separate computers to set up use and terminate a connection called a session. The session layer is the network dialog controller.
The specific functions of the session layer are:
- Dialog control: Allows communication either in half-duplex or full-duplex mode.
- Synchronization: Add checkpoints to the stream of data for acknowledgment and synchronization between the sender and the receiver.
The main responsibility of the session layer is dialog control and synchronization
6. Presentation Layer
The presentation layer is concerned with the syntax and semantics of the information exchanged between the two systems.
It carries out functions like:
- Protocol conversion
- Data translation
- Encryption and decryption
- Compression and
- Decompression
The main responsibility of the Presentation layer is translation, compression, and encryption
7. Application Layer
The application layer enables the user to access the network services.
It provides user interfaces and support for network services such as:
- Electronic email
- Remote login using TELNET
- File transfer through FTP
- Web services through WWW
- Directory services and so on.
- X500 is a directory service used to provide information and access to distributed objects
- X400 is a service that provides the basis for mail storage and forwarding
- FTAM (File transfer, access, and management) provides access to files stored on remote computers and a mechanism for transferring and managing them locally.
The main responsibility of the application layer is to provide access to network resources. |
Summary of OSI Layers functions
We use the pictorial presentation that summarizes the functions performed by various layers of communication between two users over the transmission medium. The following figure shows the main functions of each layer of the OSI reference model.
