Computer Network Models – Data Communication

  • August 9, 2021
  • Aashish Mishra
  1. Networking engineering is a complicated task, which involves software,firmware, chip level engineering, hardware, and electric pulses. To ease network engineering, the whole networking concept is divided into multiple layers.
  2. Each layer is involved in some particular task and is independent of all other layers.
  3. But as a whole, almost all networking tasks depend on all of these layers. Layers share data between them and they depend on each other only to take input and send output.

Layered Task

In layered architecture of Network Model, one whole network process is divided into small tasks. Each small task is then assigned to a particular layer which works dedicatedly to process the task only. Every layer does only specific work. In layered communication system, one layer of a host deals with the task done by or to be done by its peer layer at the same level on the remote host. If the task is initiated by the-top most layer, it is passed on to the layer below it for further processing. The lower layer does the same thing, it processes the task and passes on to lower layer. If the task is initiated by lower most layer, then the reverse path is taken.

OSI Model

Open System Interconnect is an open standard for all communication systems. OSI model is established by International Standard Organization (ISO). This model has seven layers:

  1. Application Layer
  2. Presentation Layer
  3. Session Layer
  4. Transport Layer
  5. Network Layer
  6. Data Link Layer
  7. Physical Layer

 

Physical Layer

  • This layer defines the hardware, cabling wiring, power output, pulse rate etc.
  • The lowest layer of the OSI reference model is the physical layer.
  • It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next.
  • When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.

The functions of the physical layer are :

  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 bit level.
  2. Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
  3. Physical topologies: Physical layer specifies the way in which the different, devices/nodes are arranged in a network i.e. bus, star or mesh topolgy.
  4. Transmission mode: Physical layer also defines the way in which the data flows between the two connected devices. The various transmission modes possible are: Simplex, half-duplex and full-duplex.

Hardware Used: Repeater, Hub, Modem, Cables etc Data Unit: Bit Stream

Data Link Layer

  • The data link layer is responsible for the node to node delivery of the message.
  • The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of DLL to transmit it to the Host using its MAC address.
  • Data Link Layer is divided into two sub layers :

1. Logical Link Control (LLC) 2. Media Access Control (MAC)

The functions of the data Link layer are :

  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, Data link layer adds physical addresses (MAC address) of sender and/or receiver in the header of each frame.
  3. Error control: 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 that amount of data that can be sent before receiving acknowledgement.
  5. Access control: When a single communication channel is shared by multiple devices, MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time.

Hardware Used: Bridges and Switches Data Unit: Frames

Network Layer

  • Network layer works for the transmission of data from one host to the other located in different networks.
  • It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available.
  • The sender & receiver’s IP address are placed in the header by the network layer.

 

The functions of the Network layer are :

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

Hardware Used: Routers
Data Unit: Packets

Transport Layer

  • The transport layer is responsible for source to destination(end to end)
    delivery of the entre message.
  • Network Layer does nor recognise any relationship between the packets
    delivered.
  • Network layer treats each packet independently, as though each
    packet belong to a separate message, whether or not it does.
  • The transport layer ensures that the whole message arrives intact and that
    too in order.


The functions of the transport layer are :

  1.  Segmentation and Reassembly: This layer accepts the message from the
    (session) layer , breaks the message into smaller units . Each of the
    segment produced has a header associated with it. The transport layer at
    the destination station reassembles the message.
  2.  Service Point Addressing: In order to deliver the message to correct
    process, transport layer header includes a type of address called service
    point address or port address. Thus by specifying this address, transport
    layer makes sure that the message is delivered to the correct process.

Hardware Used: Transport Gateway
Data Unit: Segments


Session Layer


This layer is responsible for establishment of connection, maintenance of
sessions, authentication and also ensures security.


The functions of the session layer are :

  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 which are
    considered as synchronization points into the data. These synchronization
    point 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.

Data Unit: Data

Presentation Layer

  • Presentation layer is also called the Translation layer.
  • The data from the application layer is extracted here and manipulated as
    per the required format to transmit over the network.

 

The functions of the presentation layer are :

  1. Translation : For example, ASCII to EBCDIC.
  2. Encryption/ Decryption : Data encryption translates the data into anotherform or code. The encrypted data is known as the cipher text 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.

Data Unit: Data


Application Layer

  • This layer is responsible for providing interface to the application user.
  • This layer encompasses protocols which directly interact with the user.
  • This layer also serves as a window for the application services to access
    the network and for displaying the received information to the user.
    Ex: Application – Browsers, Skype Messenger etc.

The functions of the Application layer are :

  1.  Network Virtual Terminal
  2. FTAM (File transfer access and management).
  3. Mail Services.
  4. Directory Services.

Hardware Used: Application Gateway
Data Unit: Data

 

Internet Model


  • Internet uses TCP/IP protocol suite, also known as Internet suite.
  • This defines Internet Model which contains four layered architecture.
  • OSI Model is general communication model but Internet Model is what the
    internet uses for all its communication. The internet is independent of its
    underlying network architecture so is its Model.

This model has the following layers:

  1. Application Layer: This layer defines the protocol which enables user to
    interact with the network.For example, FTP, HTTP etc.
  2. Transport Layer: This layer defines how data should flow between hosts.
    Major protocol at this layer is Transmission Control Protocol (TCP).This layer ensures data delivered between hosts is in-order and is responsible for end-to-end delivery.
  3. Internet Layer: Internet Protocol (IP) works on this layer. This layer
    facilitates host addressing and recognition. This layer defines routing.
  4. Link Layer: This layer provides mechanism of sending and receiving actual data.Unlike its OSI Model counterpart, this layer is independent of
    underlying network architecture and hardware.