3.6 REFERENCE MODELS

 In this section, we will discuss two reference models:

3.6.1 OSI (Open System Interconnection) Model

The OSI model is an abstract description for layered communications and computer network protocol design open system means that it can communicate with any other system that follows the specified standards, formats and semantics. PROTOCOLS give the rules that specify how the different parties may communicate.

In its most basic form, it divides network architecture into seven layers which from top to bottom are the Application, Presentation, Session, Transport, Network, DataLink, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model (Figure 7).

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. On each layer an instance provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides errorfree communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

Conceptionally two instances at one layer are connected by a horizontal protocol connection on that layer.

The following are the layers of OSI model:


In transmission side data flows from layer 7 to layer 1, then to cabling or suitable medium. When data reaches the reception side it flows from layer 1 to layer 7.

Application Layer

This layer is the layer for user interaction. We mush have an application software for dealing with the data.

Presentation Layer

It converts the data into suitable format. It does tasks like compression, decompression, encryption and decryption.

Session Layer

This layer manages connections between different application layers.

Transport Layer

This layer converts data into segments and reassembles the data stream. TCP and UDP are the protocols used in this layer. In this layer, data is converted into so called segments.

Network Layer

This layer translates logical address into physical address. This layer also fixes the route for data path. Router works in this layer. In this layer data is to called a packet.

DataLink Layer

This layer provides physical identification of a device using MAC Address. It adds source and destination address to packets and convert them into frames.

Physical Layer

This layer provides the functional requirements for activating a physical link. In this layer data is carried from one device to another.

Now, we can better understand the OSI layer with an example. Consider that we have to send a word document to a different network or through internet. The following are the process that will take place:

1) In the APPLICATION LAYER, the user can edit the file by using application softwares like Microsoft word or Word Viewer etc.

2) In the PRESENTATION LAYER, user can compress the word file by using WINRAR or WINZIP and convert the data into different format ie. zip or .rar. He can also convert the word document into different formats.

3) In the SESSION LAYER, the particular file has to be integrated with browser for attaching it to email or likewise clients.

4) In the TRANSPORT LAYER, data is converted to segments. Source IP and destination IP are added to each packets. Frame checks and parity bits are also added in this layer

5) In the NETWORK LAYER, the data is handed over to a router. The router calculates the best path for data transmission

6) In the DATALINK LAYER, a transmission errors are handled and also flow of data is regulated so that receivers are not swamped by fast senders.

7) In the PHYSICAL LAYER, frames are transmitted as bits through media such as network cable, Optic fibre etc.

3.6.2 TCP/IP Model (Transmission Control Protocol / Internet Protocol)

Just like the OSI model, the TCP/IP model has many layers which are described below:

Host to Host Network: In fact TCP/IP model does not specify this layer. But it basically combines functionally of physical and data link layers. Starting at the bottom, the Physical layer is what deals with hardware (wires, cables, satellite links, NICs, etc.). Utilizing the existing Physical layer, TCP/IP does not define its own, thus letting the layer be compatible with all network suites. This layer also encodes and transmits data over network communications media in the form of bits which are received by the Physical layer of the destination device. Often combined with this layer is the Data link layer which is responsible for moving packets from the network layer onto different hosts. Depending on the connection type, IP packets are transmitted using various methods. Dialup modems transmit IP packets using PPP while broadband users transmit using PPoE.


Internet Layer: This layer routes and delivers data across similar networks or completely different networks. The Network layer is responsible for end to end packet delivery while maintaining routing, flow control, and error control functions. An example of this layer is the actual Internet Protocol (IP) or the Internet Protocol Security (IPSec).

Transport Layer: The Network layer can be thought of the actual vehicle which transports information. This layer categorizes end to end message transmissions or connecting applications as either  Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). TCP is a connectionoriented protocol which is considered to provide a reliable byte stream. Some characteristics of TCP are traffic congestion control, data arrives in order, data has minimal error, and duplicate data is discarded.

The top layer of the TCP/IP model is the Application layer which is used for network communication. Ports are used by programs to transfer data through this layer. For example, the File Transfer Protocol uses port 21 while the Hypertext Transfer Protocol uses port 80.

TCP/IP has many benefits. TCP/IP enables crossplatform networking which is useful in this dayinage. This suite also has superior failure recovery and the ability to add networks without interrupting existing services. The reliability of TCP/IP is also a huge benefit to using this protocol. The fact that if one part of the network goes down, other parts are still able to function is what has set TCP/IP above other networking protocols. TCP/IP is also easily expandable which allows for the unprecedented rate of growth which the Internet possesses.


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