Skip to main content

Computer Network : ISO-OSI Reference Model

ISO-OSI Reference Model

Principal of OSI Reference Model

The OSI reference model has 7 layers. The principles that were applied to arrive at the seven layers can be briefly summarized as  follows:
  1. A layer should be created where a different abstraction is needed.
  2. Each layer should perform a well-defined function.
  3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
  4. The layer boundaries should be chosen to minimize the information flow across the interface
  5. The  number  of  layers  should   be  large  enough  that  distinct functions   need  not  be  thrown together   in the same layer out of necessity and small enough that architecture does not become unmanageble.

Feature of OSI Model:

  1. Big picture of communication over network is understandable through this OSI model.
  2. We see how hardware and software work together.
  3. We can understand new technologies as they are developed.
  4. Troubleshooting is easier by separate networks.
  5. Can be used to compare basic functional relationships on different networks.

Description of   Different Layers:

  Layer 1: The Physical Layer
  1.  It is the lowest layer of the OSI Model.
  2. It activates, maintains and deactivates the physical connection.
  3. It is responsible for transmission and reception of the unstructured raw data over network.
  4. Voltages and data rates needed for transmission is defined in the physical layer.
  5. It converts the digital/ analog bits into electrical signal or optical signals.
  6. Data encoding is also done in this layer.

Layer 2: Data Link Layer:

  1. Data link layer synchronizes the information which is to be transmitted over the physical layer.
  2. The main function of this layer is to make sure data transfer is error free from one node to another, over the physical layer.
  3. Transmitting and receiving data frames sequentially is managed by this layer.
  4. This layer sends and expects acknowledgements for frames received and sent respectively. Resending of non-acknowledgement received frames is also handled by this layer.
  5. This layer establishes a logical layer between two nodes and also manages the Frame traffic control over the network. It signals the transmitting node to stop, when the frame buffers are full.

Layer 3: The Network Layer:

  1. It routes the signal through different channels from one node to other.
  2. It acts as a network controller. It manages the Subnet traffic.
  3. It decides by which route data should take.
  4.  It divides the outgoing messages into packets and assembles the incoming packets into messages for higher levels.

Layer 4: Transport Layer:

  1. It decides if data transmission should be on parallel path or single path.
  2. Functions such as Multiplexing, Segmenting or Splitting on the data are done by this layer
  3.  It receives messages from the Session layer above it, converts the message into smaller units and passes it on to the Network layer.
  4. Transport layer can be very complex, depending upon the network requirements.

Layer 5: The Session Layer:

  1. Session layer manages and synchronize the conversation between two different applications.
  2. Transfer of data from source to destination session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely, and data loss is avoided.

Layer 6: The Presentation Layer:

  1. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.
  2. While receiving the data, presentation layer transforms the data to be ready for the application layer.
  3. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role of translator.
  4. It performs Data compression, Data encryption, Data conversion etc.

Layer 7: Application Layer:

  1. It is the topmost layer.
  2. Transferring of files disturbing the results to the user is also done in this layer. Mail services, directory services, network resource etc are services provided by application layer.
  3. This layer mainly holds application programs to act upon the received and to be sent data.

Merits of OSI reference model:

  1.  OSI model distinguishes well between the services, interfaces and protocols.
  2.  Protocols of OSI model are very well hidden.
  3.  Protocols can be replaced by new protocols as technology changes.
  4.  Supports connection-oriented services as well as connectionless service.

Demerits of OSI reference model:

  1. Model was devised before the invention of protocols.
  2. Fitting of protocols is tedious task.
  3. It is just used as a reference model.
Labels:

Comments

Post a Comment

Please do not post any spam link in comment box

Popular posts from this blog

Computer Network : Layered Architecture

Layered Architecture Protocol Hierarchy To tackle with the design complexity most of the networks are organize as a set of layers or levels. The fundamental idea of layered architecture is to divide the design into small pieces. The layering provides modularity to the network design. The main duty of each layer is to provide offer services to higher layers, and provide abstraction. The main benefits of layered architecture are modularity and clear interfaces. Five Layered Network Design Issues:  Layered architecture in computer network design  Layered architectures have several advantages. Some of them are,   Modularity and clear interface   Provide flexibility to modify network services   Ensure independence of layers   Management of network architecture is easy   Each layer can be ...
Post a Comment
Read more

Can coronavirus spread through mosquitoes?

  Can coronavirus spread through mosquitoes? Scientists have confirmed for the first time that the novel coronavirus behind the COVID-19 pandemic cannot be transmitted to people by mosquitoes , a finding that adds evidence to WHO claims that the disease is not mosquito-borne. The research, published in the journal Scientific Reports, provided the first experimental evidence on the capacity of SARS-CoV-2, the virus that causes COVID-19 disease, to infect and be transmitted by mosquitoes. "Here we provide the first experimental data to investigate the capacity of SARS-CoV-2 to infect and be transmitted by mosquitoes," the study noted. "While the World Health Organization (WHO) has definitively stated that mosquitoes cannot transmit the virus, our study is the first to provide conclusive data supporting the theory," said Stephen Higgs, a co-author of the research from Kansas State University in the US. According to the study, conducted at the university...
Post a Comment
Read more

Do Google Assistant work on KaiOS?

Yes Google Assistant can now read, translate text on KaiOS Google on Tuesday announced that now within Google Assistant, KaiOS users in India can use Google Lens to read, translate, and define words. At Google I/O in 2019, the US-based search engine giant brought a camera-based translation to Google Lens to help understand information. With Lens, one can point the camera at text one see and translate it into more than 100 languages and can even speak the words out loud in the preferred language. "We brought these Lens capabilities to Google Go, so even those on the most affordable smartphones can access them. Today we are extending this capability to the millions of Google Assistant users on KaiOS devices in India," the company said in a statement. It is currently available in English and several Indian languages, including Hindi, Bengali, Telugu, Marathi, and Tamil, and will soon be available in Kannada and Gujarati. Users can simply press the right soft key ...
Post a Comment
Read more

Topology-Mesh, Star, Bus, Ring and Hybrid

Computer Network Topology – Mesh, Star, Bus, Ring and Hybrid Geometric representation of how the computers are connected to each other is known as topology. There are five types of topology – Mesh, Star, Bus, Ring and Hybrid. Types of Topology There are five types of topology in computer networks: 1. Mesh Topology 2. Star Topology 3. Bus Topology 4. Ring Topology 5. Hybrid Topology Mesh Topology In mesh topology each device is connected to every other device on the network through a dedicated point-to-point link. When we say dedicated it means that the link only carries data for the two connected devices only. Lets say we have n devices in the network then each device must be connected with (n-1) devices of the network. Number of links in a mesh topology of n devices would be n(n-1)/2. Advantages of Mesh topology 1. No data traffic issues as there is a dedicated link between two devices which means the link is only available for those two devices. 2. Mesh topolog...
Post a Comment
Read more

BIT ORIENTED PROTOCOLS : SDLC, HDLC, BISYNC, LAP & LAPB

BIT ORIENTED PROTOCOLS A bit-oriented protocol is a communications protocol that sees the transmitted data as an opaque stream of bits with no semantics, or meaning. Control codes are defined in terms of bit sequences instead of characters. Bit oriented protocol can transfer data frames regardless of frame contents. It can also be stated as "bit stuffing" this technique allows the data frames to contain an arbitrary number of bits and allows character codes with arbitrary number of bits per character. SDLC Synchronous Data Link Control (SDLC) supports a variety of link types and topologies. It can be used with point-to-point and multipoint links, bounded and unbounded media, half-duplex and full-duplex transmission facilities, and circuit-switched and packet-switched networks. SDLC identifies two types of network nodes: primary and secondary. Primary nodes control the operation of other stations, called secondary. The primary polls the secondary in a predeter...
Post a Comment
Read more