There are two types of networking models available: OSI reference Model and the TCP/IP Network Model for the design of computer network system. In this post we shall look at these models.

OSI (Open System Interconnection) Networking Model

An open system is a model that allows any two different systems to communicate regardless of their underlying architecture.  The purpose of the OSI model is to open communication between different devices without requiring changes to the logic of the underlying hardware and software.

The OSI model is not a protocol, it is a model for understanding and designing a network architecture that is inter- operable, flexible and robust. The OSI model has a seven-layered architecture.  These are:

  • Application layer
  • Presentation layer
  • Session layer
  • Transport Layer
  • Network layer
  • Data link layer
  • Physical Layer

Physical layer: 

The physical layer is concerned with sending raw bits between the source and destination nodes over a physical medium. The source and destination nodes have to agree on a number of factors. 
  • Signal encoding: how are the bits 0 and 1 to be represented?
  • Medium: what is the medium used and its properties?
  • Bit synchronization: is the transmission synchronous or asynchronous?
  • Transmission type: whether the transmission is serial or parallel?
  • Transmission mode: is the transmission simplex, half-duplex or full duplex?
  • Topology: what is the network topology i.e. star, mesh, ring or bus?

Data link layer:

The data link layer is responsible for transmitting a group of bits between the adjacent nodes. The group of bits is known as frame. The network layer passes a data unit to the data link layer and data link layer adds the header information to this data unit. The data link layer performs the following functions:   

Addressing:

Headers and trailers are added containing the physical addresses of the adjacent nodes and removed on a successful delivery.

Framing:

Grouping of/bits received from the network layer into manageable units called frame

Flow control:

To regulate the amount of data that can be sent to the receiver.

Media access control (MAC):

Who decide who can send data, when and how much.

Synchronization:

This layer also contains bits to synchronize the timing to know the bit interval to recognize the bit correctly.

Error control:

It incorporates the CRC to ensure the correctness of the frame. Node to node 

delivery:

It is also responsible for error-free delivery of the entire frame/packet to the next adjacent node. 

Network layer:

The network layer is responsible for routing a packet within the subnet that is,  from source to destination nodes across multiple nodes in the same network or across multiple networks. This layer also ensures the successful delivery of a packet to the destination node. The network layer performs the following functions:

Routing:

To find the optimal route 

Congestion control:

Which is based on two approaches (i) Increase on the resources (ii) Decrease the word.

Transport layer:

This layer is the first end-to-end layer. Header of the transport layer contains information that helps send the message to the corresponding layer at the
destination node. The message is broken into packets and may travel through a number of intermediate nodes. This layer takes care of error control and flow control both at the source and destination for the entire message. The responsibilities of the transport layer are: 
  • Host-to-host message delivery 
  • Flow Control 
  • Segmentation and reassembly 

Session layer:

The main functions of this layer are to establish, maintain and synchronize the interaction between two communication hosts. It makes sure that once a session is established it must be closed gracefully. It also checks and establishes connections between the hosts of two different users. The session layer also decides whether both users can send as well as receive data at the same time or whether only one host can send and the other can receive. The responsibilities of session layer are:
    Sessions and sub sessions: this layer divides a session into sub session for avoiding retransmission of entire message by adding the checkpoint feature.
    Synchronization: this layer decides the order in which data needs to be passed to the transport layer.
     Dialog control: this layer also decides which user application sends data and at what point of time and whether the communication is simplex, half duplex or full duplex.
     Session closure: this layer ensures that the session between the hosts is closed gracefully. 

Presentation layer: 

When two hosts are communicating with each other they might use different coding standards and character sets for representing data internally. This layer is responsible for taking care of such differences. This layer is responsible for:
  • Data encryption and decryption for security.
  • Compression.
  •  Translation.

Application layer: 

It is the topmost layer in the OSI model, which enables the user to access the network. This layer provides user interface for network applications such as remote login, World Wide Web and FTP. The responsibilities of the application layer are:
  • File access and transfer 
  • Mail services 
  • Remote lo-gin 
  • World Wide Web

TCP/IP Networking Model: 

TCP/IP is an acronym for Transmission Control Protocol / Internet Protocol. TCP/ IP is a collection of protocols, applications and services. TCP/IP protocol were developed prior to the OSI model therefore its layers do not match with the OSI model.

The TCP/IP protocol suit is made of the five layers: Physical, data link, network, transport & application. The first four layers provide physical standards network interface, inter-networking and transport mechanism whereas the last layer comprises of the functionalities of the three topmost layers in the OSI model.