LESSON-2::UNDERSTANDING THE COMMUNICATION MODEL

OSI Reference Model   

                                                            The Open System Interconnection reference model provides a means of describing how data is transmitted over a network.

                                                      Why a Layered Network Model  

Reduce Complexity:                                      7, Application

The OSI model breaks network communication in to smaller components to make learning easier.

Standardizes Interfaces:                             6, Presentation

The OSI model provides for efficient update and improvement to individual components without affecting other components or having to rewrite the entire protocol.

Facilitates modular engineering:               5, Session

The OSI model prevents changes in one layer from affecting the other layer, thus allowing for quicker development.

Ensures Interoperable technology:               4, Transport 

The OSI model allow different types of network hardware and software to communicate with one another.

Accelerates Evolution:                                      3,Network

The OSI model of standardizes network components to allow multi vendor development and support.

Simplifies teaching and learning:                2, Data Link

The OSI model breaks network communication in to smaller, simpler parts.

                   OSI Model Layers and Their Functions

layer1: the physical layer

                                           The physical layer defines the electrical, mechanical, procedural and functional specification for activating, deactivating and maintaining the physical link for transmission between end devices. Physical layer specification are defining characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distance, physical connectors and other similar attributes. 

layer2: the data link layer

The data link layer defines how data is formatted for transmission and how access to the physical media is controlled. This layer also typically includes error detection and correction to ensure reliable delivery of the data.  

layer3: the network layer

The network layer provides connectivity and path selection between two host system that may be located on geographically separated network. The growth of the internet has increased the number of users that access information from sites around the world. The network layer is the layer that manages the connectivity of these users by providing logical addressing. 

layer4: the transport layer

The transport layer defines services to segment, transfer and reassemble the data for individual communication between the end devices. For example, business users in large corporations often transfer large files from field locations to a corporate sites. Reliable delivery of the files is important, so the transport layer will break down file in to smaller segment that are less likely to incur transmission problems.
                                      The transport layer shields the upper layer from transport implementations details. Specifically, issues such as reliability of transport between two hosts are assigned to the transport layer. In providing a communication a communication service the transport layer establishes, maintain and properly terminates virtual circuit. Transport error detection and recovery, as well as information flow control, ensure reliable service. 

layer5: the session layer  

The session layer establishes, manages and terminates session between two communicating hosts. The session layer also synchronizes dialog between the presentation layers of the two hosts and manages their data exchange. For example, web server have many users, so there are many communication process open at a given time. It is important then to keep track of which user communicates on which path. In addition to session regulation, the session layer offers provision for efficient data transfer, class of service(CoS) and exception reporting of session layer and application layer problems.

layer6: the presentation layer 

The presentation layer ensures that the information that is sent at the application layer of one system is readable by the application layer of another system. For example, a PC program communicate with another computer. One PC is using extended binary coded decimal interchange code (EBCDIC) and the other one is using ASCII to represent the some characters. If necessary the presentation layer translates between multiple data formats by using a common format.

layer7: The application layer      

The application layer is the OSI layer that is closest to the user. This layer provides network service to the application of the user, such as email, file transfer and terminal emulation. The application layer differs from the other layer in that it does not provide service to any other OSI Layer. It provides services only to application outside the OSI model. The application layer establishes the availability of intended communication partner, and synchronizes and establishes aggrement on procedures for error recovery and control of data integrity.

               ENCAPSULATION & DE-ENCAPSULATION

Information that is transmitted over a network must undergo a process of  conversion at the sending end the receiving end of the communication. That conversion process is known as encapsulation and de-encapsulation of data. This topic describes the encapsulation and de-encapsulation process.

ENCAPSULATION:

                                     The information that is sent on a network is referred to as data or data packets. As application data is passed down the protocol stack on its way to be transmitted across the network media, various protocols add information to it at each level. This process is commonly known as the encapsulation process. Each layer adds a header (and a trailer, if applicable) to the data before passing it to a lower layer.The headers and trailers contain control information for the network devices and receiver to ensure proper delivery of the data and to ensure that the receiver can correctly interpret the data.

These steps shows you in which manner data travels through the layers.  

Step1: 

The user data is sent from an application to the application layer.

Step2:

The application layer adds the application layer header (Layer 7 header) to the user data. The layer 7 header and the original user data become the data that is passed down to the presentation layer.

Step3:

The presentation layer adds the presentation  layer header (Layer 6 header) to the data. The layer 6 header and the previous data become the data is passed down to the session layer.

Step4:

The session layer adds the session layer header (Layer 5 header) to the data. The Layer 5 header and the previous data become the data that is passed down to the transport layer.

Step5:

The transport layer adds the transport layer header (Layer 4 header) to the data. The layer 4 header and the previous data become the data that is passed down to the network layer.

Step6:

The network layer adds the network layer header (Layer 3 header) and the previous data become the data that is passed down to the data link layer.

Step7:

The data link layer adds the data link layer header & trailor(Layer 2 header and trailer) to the data. A layer 2 trailer is usually the frame check sequence(FCS), which is used by the receiver to detect whether the data is in error. The layer 2 header and the previous data become the data that is passed down to the physical layer.

   

LESSON-1::EXPLORING THE FUNCTION OF NETWORKING

What is a Network:-

                                   A Network is a connected collection of devices and end systems, such as computers and servers, which can communicate with each other. Networks carry data in many types of environment, including homes, small businesses and large enterprises.

                                             PHYSICAL COMPONENTS OF A NETWORK

Pc: The computers serve as a end point in  a network. They send and receive data. 

Interconnection: The interconnection consists that components which provide  a platform to a point to other point. Eg:- NIC Card, cable, connector.

Switch: Switches are the devices that provide network attachment to the end system.

Routers: Routers interconnect network and choose best path between networks.

Wireless LAN: W-LAN devices connect network devices computers and other end point  to the network without routers .

                                                       NETWORK USER APPLICATION

Email: Email is very valuable application for most network user. User can communicate information, message and files with an electronically method to same network and other also. 
Web Browser: A web browser allows access to the internet through a common interface. A web browser provides a common interface for communicating with suppliers and customers. Some famous web browser like Microsoft internet explorer, Mozilla fire fox, apple safari and google chrome.

Database: In this application servers stored data in  a centralized form on a network such  as file server, exchange server, and AD.All connected users access this information easily.   

                                            CHARACTERISTICS OF A NETWORK

Speed: Speed is the most important part in a  network.Its depend on ISP that how fast data transmitted over the network.
Security: Security is describe as a firewall of a network. Its use for save the transmit information from un-authorised used that flow on  a Network.
Topology: In networks, there are physical and logical topology. Physical is used for wiring the devices and logical is used for how data send one to another by physical.   

                                 LOGICAL VS. PHYSICAL TOPOLOGIES

Logical Topology: The logical topology of a network refers to the logical paths that the signal use to travels from one point in the network to another point. This paths defines the way in which data accesses the network media and transmits packets across it.
Physical topology: The physical topology of a network to the physical structure layout of the devices and cabling. There are three primary category of physical topologies:-
(i). Bus: Computers and other network devices were cabled together in  a line using coaxial cable. In modern bus topologies using twisted pair wiring.
(ii). Ring: Computers and other network devices connect like as to create a ring.
(iii). Star: All computers and devices are connected by a central cabling.