OSI MODEL

 What is OSI?

Open Systems Interconnection or OSI model was first proposed by

International Standards Organization (ISO) to solve the multiple problems

faced in telecommunication.  

It is a conceptual model of telecommunication networking where the whole process is divided into seven effective layers. It synchronizes the whole process and helps one understand the interoperation of the diverse communication system under the standard protocol. The seven layers we are talking about are abstraction layers and each layer is connected to either one or two layers depending on its position. The interlinked layers serve one another to forward the communication process.

Understanding the seven layers of OSI 

Brief description of each of OSI Layers.

    Layer 1: Physical Layer:             

This layer is the primary layer of the OSI model and consists of connecting devices such as Ethernet cable, Token ring, hubs, repeaters and electrical specification for data connection. This layer is responsible for connecting the device to physical transmission medium such as copper or optical cable or radio frequency or pulses of infrared. When operating at the physical layer, the data is transmitted through electric voltage, radio frequency or pulses.

Layer 2: Data Link Layer:

Data link layer encode and decode data packet into bits and using its transmission protocol it handles the errors in the physical layer. It also controls flow and synchronization of the frames and at the same time it manages controlling access of the physical layer network devices.

There are two sub layers in Layer 2,

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

While MAC determines how the computer in the network will have access to the LLC layer, the LLC layer works in frame synchronization

 Layer 3: Network Layer:

The Network.layer adds the concept of routing above the Data Link layer. When data arrives at the Network layer, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination. If the data has reached the final destination, this Layer 3 formats the data into packets delivered up to the Transport layer.

In layer 3 logical paths is created for data transmission through network devices such as switch and routing devices. This works for transmitting data from one node to another.

Addressing, routing and forwarding are the three major functions at this layer. To support routing, the Network layer maintains logical addresses such as IP addresses for devices on the network. The Network layer also manages the mapping between these logical addresses and physical addresses. In IP networking, this mapping is accomplished through the Address Resolution Protocol (ARP).

Layer 4: Transport Layer:

The transport layer is to be delivered the entire message from source to destination, and receiving data without any errors. Different transport protocols may support a range of optional capabilities including error recovery, flow control, and support for re-transmission.
In the fourth layer, variable length data sequence has to be transferred from source to destination host through one or more network. This Transport Layer delivers data across network connections.

The supported protocols at this Layer are TCP (Transmission Control Protocol) UDP (User Datagram Protocol) and SPX (Sequential Packet Exchange — Novell Netware Operating System).

Layer 5: Sessions layer:

In the session layer, communication or connection between computers is controlled in a managed environment. It has the sole capacity of managing and terminating connection and helping in establishing connections between local and remote applications.

At Layer 5, it is built to support multiple types of connections that can be created dynamically and run over individual networks. Dynamically created connections that can run over individual networks can be controlled' through the layer.

Layer 6: Presentation Layer:

The Presentation layer is the simplest in function of any piece of the OSI model. At Layer 6, it handles syntax processing of message data such as format conversions and encryption / decryption needed to support the Application layer above it.

This Presentation layer is the most simplified layer in the OSI model where data segregation is done based on the file type. Data can be classified in 4 types like ASCII — text format, Audio format, Picture format and Video format. Encryption to formats such as GIF, ASCII, PICT, JPEG, and much more are done to make it suitable for the application layer. The major job of this layer is to translate between application and network format.

Layer 7: Application Layer:

This is the last layer of the OSI system and this layer represents the data that is understood by the end user. The Application layer supplies network services to user applications. Network services are typically protocols that work with user's data For Example, in a Web browser application, the Application layer protocol "HTTP (Hyper text transfer protocol)"

In this layer the user interacts with the software application and Communicating component is implemented. In this layer communication partners are identified along with quality of service.

This Layer 7 provides data to the Presentation layer.

PDU in OSI model

         In telecommunication the term protocol data unit (PDU) has the

         following meaning

In a OSI layered model, a unit of data which is specified in a protocol of a given layer and which consists of protocol-control information and possibly user data Of that layer.

The PDU for each layer of the OSI model is listed below.

1. Physical layer — raw bits (Is or Os) transmitted physically via the hardware
2. Data Link layer — a frame (or series of bits)
3. Network layer — a packet that contains the source and destination address
4. Transport layer — a segment that includes a TCP header and data
5. Session layer — the data passed to the network connection
6. Presentation layer — the data formatted for presentation
7. Application layer — the data received or transmitted by a software application

As you can see, the protocol data unit changes between the seven different layers. The resulting information that is transferred from the application layer to the physical layer (and vice versa) is not altered, but the data undergoes a transformation in the process. The PDU defines the state of the data as it moves from one layer to the next.