MAC Address

MAC Address is also known as the Physical Address.
This address is assigned to a NIC (Network Interface Card) during its manufacture by the manufacturing company  itself.

Its 48 bit address represented in Hexa decimal.
It cannot be changed by the user.
Its Unique.

To see your PCs MAC Address.

Start> Run> cmd
c:\> ipconfig  /all

To See Ip Address

C:\>ipconfig


Eg:  Let A1B2-C3D4-E5F6 be a MAC Address

Here single digit represents 4 bits. Total 12 digits. Hence 48 bit address.
First 24 bits is called as the OUI - Organisational Unique Identifier
OUI is to represent an organistion.
Its also called as Vendor Specific or Device ID or Vendor Assigned.
So with the 24 bits we can identify the manufacturer of the NIC.

The rest 24 bits will be unique for each OUI.

Address

There are two types of Addresses

1. Internet Protocol - IP Address (Logical Address)

2. Media Access Control - MAC Address (Physical Address)


1. IP Address

    32 bit Address

    Can be changed.

    2 Types:

    1. Static IP - IP given by a user to the computer.
    2. Dynamic IP - IP attained by a PC from a DHCP Server

2. MAC Address
   
     48 bit address

     Given by the manufacturer to the NIC (Network Interface Card)

     Cannot be changed.

Frame

A frame is the PDU of layer2 datalink layer.
Frame consists of a header called Preamble just to represent its a frame.
Source MAC
Destination MAC
Data Length
Data
FCS - Frame Check Sequence. A value created by CRC (Cyclic Redundancy Check ) Algorithm

CRC is run on both Source and destination Layer 2.
If the FCS values are same in both, then the frame is accepted by the destination.
If not same it means it has errors and it it is rejected.
SOurce will heve to retransmit the frame again.

OSI Model Simplified

Source
Data flows from layer 7 to layer 1
7 - Username and password given - Data
6 - Encrypted and compressed - Formatted Data
5 - Session created with destination PC - Formatted Data
4 - Data simlified as many Segments and send in an order,
      Protocols used to represent application - Segments
3 - Source IP & Destination IP is set on packets - Packets
2 - Source MAC & Destination MAC is set on Frames - Frames
1 - Bits in the format of 0s and 1s - Bits

Destination
Data flows from layer 1 to layer 7
1 - Receives Bits -Bits
2 - Converts to Frames, Verifies the destination MAC address is correct. If not correct its rejected
3 - Frames converted to Packets and verifies the destination IP.
4 - Receives the segments in the order transmitted. If TCP used Acknowledgement send, UDP no acknowledgement.
5 - Session is ended after data transmission and reception is finished.- Formatted Data
6 - Formatted data is decrypted and decompressed.
7 - Original Data received

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.