If you have gone through the previous tutorials, you must be well acquainted with subnetting, supernetting, classful addressing, classless addressing, access control methods, error control methods, flow control methods, switching techniques and topologies. From this tutorial onwards, we will study the heart of computer networks: The ISO/OSI model layer. We will go through the various layers of computer networks and how the layers are interconnected. We will also study the functions that are performed at different layers during the communication. In this tutorial, we will study the classification of various iso / osi model layer in brief.

Main Task of Computer Networks

The main task of computer network is to enable 2 processes that wish to communicate.

• If client and server are in the same host, they can communicate directly by inter-process communication.
• Computer Networks provide hardware and software so that the two processes in two different hosts can communicate like processes in the same host.

There can be two types of functions for two processes in two different hosts to communicate:

1. Mandatory Functions: These functions have to be provided to every process. Eg. Error Control, Flow Control, Access Control, Multiplexing, De-multiplexing, Addressing etc.
2. Optional Functions: Only the processes which require them get them. Eg. Encryption, Decryption, checkpointing, routing, flooding etc.

What is ISO / OSI Model?

ISO stands for International Standards organisation and OSI stands for Open System Interconnection. ISO/OSI divides various functionalities into groups and each group is assigned to a layer. Implementing a group or a layer means implementing those functionalities.

Advantages of Layering

• Divide and conquer is possible in case of layering.
• Encapsulation is possible.
• Abstraction is possible since without worrying about how a layer is implementing we can worry about what it is implementing.
• Testing is made easy by this approach since all the layers can be tested independently without worrying about the other layers.

Various Layers in ISO / OSI Model

1. Application Layer – The data at application layer is called a message.
2. Presentation Layer – The data at presentation layer is also called a message.
3. Session Layer – The data at session layer is also called a message.
4. Transport Layer – It is a very thick layer since huge no of functions are implemented here.
   The data at transport layer is called a segment for TCP (Transmission Control Protocol) and datagram for UDP (User Datagram Protocol). Gateways are used at transport layer.
5. Network Layer – This is complex layer since all the routing algorithms are implemented here. The data at this layer is referred to as packets and the device used in this layer are routers.
6. Data link Layer (DLL) – It deals with hardware and software. Bridges are used at Data link layer. The data at DLL is referred to as frame.
7. Physical Layer – It only deals with the hardware of a computer. The data is referred to as bits in physical layer. Hubs and Repeaters are used at this layer.

Note:

Application layer, Presentation layer and session layer are used for implementing user interactiveness. In a five layer model, all these layers are merged into one and called application layer.

Way to Learn All The Layers:

Please Do Not Touch Steve’s Pet Alligator.

Remembering this statement will help you remember all the layers if you remember what the underlined and bold letter stands for in case of layers.

How does ISO / OSI model Work?

Below picture clearly depicts the working of different layers of iso / osi model layer –

ISO / OSI model layer working

** All the layers add a header to the message they receive from the previous layer.
** IP Addresses at the network layer never change else the destination may be lost or source may not be known. Only physical addresses change at the data link layer.
*** Router is a network layer device and thus it has only three layers: Network layer, Data link layer, Physical Layer

Application (Layer 7):

Everything on this layer is application specific like identifying communication partners, identifying quality of service, considering user authentication and privacy or any constraints on data syntax. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation (Layer 6):

This layer provides independence from differences in data representation like encryption, decryption etc. by translating from application format to network format and vice-versa. It adds a header of its own a makes the data compatible for application layer. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session (Layer 5):

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport (Layer 4):

OSI Model, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network (Layer 3):

Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Data Link (Layer 2):

At OSI Model, Layer 2, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Physical (Layer 1):

OSI Model, Layer 1 conveys the bit stream – electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

That’s end of tutorial on brief introduction about iso / osi model layer. We will study these layers in greater detail in the upcoming tutorials. In the tutorials ahead, we will also study the network devices like hubs, repeaters, bridge, gateway, switch etc. in greater detail.