OSI Model and its Layers

OSI Model and its Layers

In this tutorial, you are going to learn about OSI Model and its Layers. In Computer Networks the OSI Model and its Layers is included.

OSI Model and its Layers

The Open System Interconnection (OSI) model is a reference tool for understanding data communication between any two networked system. It divides the communication processes into 7 layers. Each layer performs specific functions to support the layers above it and uses services of the layers below it. Each layer represents a different level of abstraction and layers boundaries are well defined.

Benefits of OSI Model

It helps users understand the big picture of networking. It helps users understand how hardware and software elements function together. OSI model makes troubleshooting easier by separating networks into manageable pieces. The OSI model provides a Common language to explain components and their functionality.

Physical Layer

The physical layer coordinates the functions required to transmit a bit stream over a physical medium. It deals with the mechanical and electrical specifications of interface and transmission medium. It also defines the Procedures and functions that physical devices and interfaces have to perform for transmission to occur.

Functions of Physical Layer

There are several functions physical layer. Physical layer defines characteristics of the interface between the devices and the transmission medium.

  • It defines the type of transmission medium.
  • It defines the transmission rate i.e., the number of bits sent each second,
  • It performs synchronisation of sender and receiver clocks.
  • It is concerned with the connection of devices to the medium. (1) Point-to-point configuration- Two devices are connected together through a dedicated link. (2) Multipoint configuration- A link is shared between several devices
  • It is concerned with the physical topology.
  • It defines the direction of transmission i.e., transmission mode (simplex, half duplex or duplex).
  • It transmits bit stream over the communication channel. Hardware Used – Repeater and Hub. Data Unit – Bit stream.

Data Link Layer

The data link layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for Node-to-Node delivery. It makes the physical layer appear error free to the upper layer (i.e., network layer).

Functions of Data Link Layer

Data link layer is responsible for :

  • Framing (i.e., division of stream of bits received from network layer into manageable data units called frames/or segmentation of upper layer datagrams (also called packets) into frames).
  • Flow Control (i.e., to manage communication between a high speed transmitter with the low speed receiver).
  • Error Control (i.e., adding mechanism to detect and retransmit damaged or lost frames and to prevent duplication of frames. To achieve error control, a trailer is added at the end of a frame).
  • Access Control (i.e., to determine which device has control over the link at any given time, if two or more devices are connected to the same link.
  • Physical Addressing (i.e., adding a header to the frame to define the physical address of the sender (source address) and/or receiver (destination address) of the frame.)
  • Hardware Used – Bridges and switches.
  • Data Unit – Frames
  • Protocol Used – Simplex protocol, stop and wait protocol, gliding window, HDLC (High Level Data Link Control), SDLC, NDP, ISDN, ARP, PSL, OSPF, NDP.

Network Layer

Network layer is responsible for source to destination delivery of a packet possibly across multiple networks (links). If the two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between networks, there is often a need of the network layer to accomplish source to destination delivery.

Functions of the Network Layer

Network layer responsibilities include:

  • Logical Addressing The physical addressing implemented by the data link layer handles the addressing problem locally (i.e. , if the devices are in the same network). If packet passes the network boundary, we need another addressing system to distinguish the source and destination systems.
  • Routing Independent networks or links are connected together with the help of routers or gateways. Routers route the packets to their final destination. Network layer is responsible for providing routing mechanism.
  • Hardware Used – Routers
  • Data Units – Packets
  • Protocols Used IP (Internet, Protocol), NAT (Network Address Translation), ARP (Address Resolution Protocol), ICMP (Internet control Message Protocol), BGP(Border Gateway Protocol), RARP (Reverse Address Resolution Protocol), DHCP (Dynamic Host Configuration Protocol), BOOTp, OSPF.

Transport Layer

The transport layer is responsible for source to destination (end-to-end) delivery of the entire message. Network layer does not recognise any  relationship between the packets delivered. Network layer treats each Packet independently, as though each packet belonged to a separate message, whether or not it does. The transport layer ensures that the whole message arrives intact and in order.

Functions of Transport Layer

Responsibilities of transport layer includes:

  • Service Point Addressing The transport layer header must include a type of address called service point address (or part address). Segmentation and Reassembly A message is divided into transmittable segments, each segment containing a sequence number. Flow Control Flow control at this layer is performed end to end rather than across a single link.
  • Error Control This layer performs an end to end error control by ensuring that the entire message at the receiving transport layer without error (damage, loss or duplication). Error correction is usually achieved through retransmission.
  • Connection Control Transport layer can deliver the segments using either connection oriented or connectionless approach.
  • Hardware Used – Transport Gateway
  • Data Unit – Segments
  • Protocol Used – TCP (Transmission Control Protocol) for connection oriented approach and UDP (User Datagram Protocol) for connectionless approach.

Session Layer

The session layer is the network dialog controller. It establishes, maintains and synchronises the interaction between communicating systems. It also plays an important role in keeping applications data separate.

Functions of Session Layer

Specific responsibilities of the session layer include the following:

  • Dialog Control Session layer allows the communication between two processes to take place either in half duplex or full duplex. It allows applications functioning on devices to establish, manage and terminate a dialog through a network.
  • Synchronization – The session layer allows a process to add checkpoints (synchronization points) into a stream of data. For example, if a system is sending a file of 2000 pages, we can insert check points after every 100 pages to ensure that each 100 page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, retransmission begins at page 501, pages 1 to 500 need not to be retransmitted.

Presentation Layer

This layer is responsible for how an application formats data to be sent out onto the network. This layer basically allows an application to read (or understand) the message.

Functions of Presentation Layer

Specific responsibilities of this layer include the following

  • Translation – Different systems use different encoding systems, so the presentation layer provides interoperability between these different encoding methods. This layer at the sender end changes the information from sender dependent format into a common format. The presentation layer at receiver end changes the common format into its receiver dependent format.
  • Encryption and Decryption – This layer provides encryption and decryption mechanism to assure privacy to carry sensitive information. Encryption means the sender transforms the original information to another form and at the receiver end, decryption mechanism reverses the new form of data into its original form.
  • Compression This layer uses a compression mechanism to reduce the number of bits to be transmitted. Data compression becomes important in the transmission of multimedia such as text, audio and video.

Application Layer

This layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer shared database management and other types of distributed information services.

Functions of Application Layer

Specific services provided by the application layer include the following:

  • Network Virtual Terminal A network virtual terminal is a software version of a physical terminal and allows a user to logon to a remote host. To do so, the application creates a software emulation of a terminal at the remote host.
  • File Transfer, Access and Management This application allows a user to access files, retrieve files, manage files or control files in a remote computer.
  • Mail Services Electronic messaging (i.e., e-mail storage and forwarding) is provided by this application.
  • Directory Services This application provides distributed database sources and access for global information about various objects and services.

This article on OSI Model and its Layers is contributed by Amal Kumar (Delhi University). OSI Model and its Layers is a topic in Computer Networks. If you like TheCode11, then do follow us on Facebook, Twitter and Instagram.

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