Open Systems Interconnection (OSI) Model

Introduction

Let Suppose when we connect the two or more computers together for communication , if the all computers are using same protocols then they would be communicate to each other. but if the communication protocols are different all of commuters then communication would not be there.Then ISO (International Organization for Standardization) has decided to make this Model that is a standard for protocols which makes for Networking. All Networking Protocols makers have to follow this OSI model standard, at the time of making protocols.So We can say that Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems.

• It divides the communications processes into seven layers.
•  Each layer both performs specific functions to support the layers above it and offers services to the layers below it.
•  The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process.
  All Seven layers are given below:-

1.  Application Layer ------------ Layer 7

2.  Presentation Layer-------------Layer 6

3.  Session Layer ------------------Layer 5

4.  Transport Layer----------------Layer 4

5.  Network Layer-----------------Layer 3

6.  Data Link Layer---------------Layer 2

7.  Physical Layer-----------------Layer 1

Without OSI & With OSI

The main benefits of the OSI model include the following:

1. Helps users understand the big picture of networking and understand how hardware and software elements function together and understand new technologies as they are developed
2. Makes troubleshooting easiest, Because of every Layer Define the Data flow status in the Network.
3. Any Vendor can explain the hardware properties in better manner.

 All Layers Brief Explanation is Following Below:-

Layer-7 

This Layer provide a network interface to users, so that they could reach to the Data and can direct access. (exp. FTP, Telnet, HTTP, etc.)

Layer-6

This layer is responsible for presenting the data in the required format which may include:
Code Formatting, Encryption, Compression

Layer-5

This layer establishes, manages, and terminates sessions between two communicating hosts.Creates Virtual Circuit,Coordinates communication between system, Organize their communication by offering three different modes

1. Simplex
2. Half Duplex
3. Full Duplex

Layer-4

This layer breaks up the data from the sending host and then reassembles it in the receiver.It also is used to insure reliable data transport across the network.Can be reliable or unreliable.Sequencing, Acknowledgment, if error found in transmission then do Retransmission again, and

Flow Control

PDU - Segments

Exp. TCP, UDP,

Layer-3

Sometimes referred to as the “Cisco Layer”.End to End Delivery, Provide logical addressing that routers use for path determination, Segments are encapsulated. Internetwork Communication, Packet forwarding to destination, Packet Filtering, Makes “Best Path Determination” according to dynamic protocol ,Fragmentation.

PDU – Packets
Router works on this layer. and IP is also layer 3 protocol.


Layer-2

Performs Physical Addressing, This layer provides reliable transit of data across a physical link.Combines bits into bytes and bytes into frames ,Access to media using MAC address, Error detection, not correction, LLC ( Logical Link Control ) performs Link establishment and MAC Performs Access method.
PDU - Frames
Switch works on this layer.
Layer-1

This is the physical media through which the data, represented as electronic signals, is sent  from the source host to the destination host.,Move bits between devices,Encoding
PDU - Bits
Hub, Connector, work on layer 1.

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Network Topologies and NIC

 Network Topologies

• Network topology defines the structure of the network. 
• One part of the topology definition is the physical topology, which is the actual layout of the wire or media.
• The other part is the logical topology,which defines how the media is accessed by the hosts for sending data. 
• When you are cabling up your computers and networking devices, various types of topologies can be used. A topology defines how the devices are connected
  

  Network Interface Card

 A network interface card (NIC) is a printed circuit board that provides network communication capabilities to and from a personal computer.   Also called a LAN adapter.

IEEE

IEEE Stands for Institute of Electrical and Electronics Engineers. IEEE is the world’s largest professional association dedicated to advancing technological innovation and excellence for the benefit of humanity.IEEE and its members inspire a global community through IEEE's highly cited publications, conferences, technology standards, and professional and educational activities.

IEEE is a leading developer of international standards that underpin many of today's telecommunications, information technology and power generation products and services. Often the central source for standardization in a broad range of emerging technologies, the IEEE Standards Association has a portfolio of more than 1,500 standards and projects under development. This includes the prominent IEEE 802 standards for wireless networking.

Reference  

Institute of Electrical and Electronics Engineers (IEEE) 802 Standards

• IEEE 802.1: Standards related to network management.
• 
  IEEE 802.2: General standard for the data link layer in the OSI Reference Model. The IEEE divides this layer into two sublayer -- the logical link control (LLC) layer and the media access control (MAC) layer.
• 
  IEEE 802.3: Defines the MAC layer for bus networks that use CSMA/CD. This is the basis of the Ethernet standard.
• 
  IEEE 802.4: Defines the MAC layer for bus networks that use a token-passing mechanism (token bus networks).
• 
  IEEE 802.5: Defines the MAC layer for token-ring networks.
• 
  IEEE 802.6: Standard for Metropolitan Area Networks (MANs)