OSI Model

Layer                                             PDU (Protocol Data Unit)

7 - Application Layer                   Data
                  
6 - Presentation Layer                 Formatted Data

5 - Session Layer                         Formatted Data

4 - Transport Layer                      Segments

3 - Network Layer                       Packets

2 - Datalink Layer                        Frames

1 - Physical Layer                        Bits


These are the7 layers of OSI Model. Data flows from layer 7 to layer 1 in the source and from layer 1 to layer 7 on the destination side.  Data is represented in each layer with each name known as PDU(Protocol Data Unit)

Application layer is the application used by the user such as Internet Explorer or Mozilla firefox or any web browser.
Consider an eg : i am logging to my mail using my user id and password.
Let the user name be "abc" and password be "123"
The username and passsword given is the data which is the PDU of layer 7 Application Layer.

The data should not be sent as clear text to prevent hacking of your accounts.
So the data is formatted by the physical layer. Hence formatted data is the PDU in layer 6.
Physical layer does the functions like compression and encryption.

The function of Session layer is to create a session with the destination. That is like,
you have a doctor and u have to fix the appointment.
Just create a virtual circuit. So here data is not changed at all. So here also PDU is formatted data.
The 3 modes are there for PCs to establish connection. Simplex. Half Duplex And Full Duplex which i have posted earlirer.
http://networkselfstudy.blogspot.com/2010/04/transmission-modes.html


Next comes the transport layer. Transport layer converts the formatted data into segments.
Hence PDU is segments.
There are two types of communication - Reliable, Unreliable.
In transport layer port numbers are used to represent the application.
Consider you have Yahoo on Internet explorer and Google on Firefox.
Your PC represent explorer with a port number and represent firefox with another port number.
Even if you open  a new tab, that will be representedby another port number.
So from the servers reply is sent to the same PCs using the port numbers, thats why you get the yahoo and google on the correct page itself.
There are two protocols for communication.
1. TCP -  Transmission Control Protocol for Reliable communication
2. UDP -  User Datagram Protocol for Un reliable Communication
This layer has the function of sending segments in an order and to receive the segments in the transmitted order itself.

Network Layer converts segments into packets.
So PDU is packets.
 In network layer logical address (IP Address) is used.
Packet forwarding and filtering is the function of this layer.
Source IP and Destination IP is set to the packets which is not changed unless the packet reaches the destination.
Layer 3 device - Router

Datalink layer which is the second layer converts packets into frames. SO PDU is frames.
In data link layer physical address (MAC Address) is used, which is changed during communication.
Layer 2 device - Switch
Frames will be explained in upcoming posts.


In physical layer frames are converted into bits. So PDU is bits.
Bits are transmitted through cables to the destination.
Device - Hub and cables.