OSI model

OSI Model

• OSI Open Systems Interconnection or OSI model was first proposed by international Standard Organization (ISO) to solve the multiple problems faced in telecommunication.
• The OSI Model is also called ISO OSI Reference Model. it is a conceptual model that has seven layers.
• 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.

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 used to establish or terminate a connection to communication medium.
• It is responsible for the actual physical connection between the devices.The physical layer contains information in the form of bits.
• 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. 

The primary differentiators would be, cost effectiveness of installation, performance, convenience and maintenance. It defines the cable or physical medium such as thinnet, thicknet, unshielded twisted pairs.(UTP)

Layer 2- Data Link Layer

• The data link layer organizes information on the system. It handles data transfer between the network and the physical layers.
• Data link layer encodes and decodes 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.
• The Data link layer receives data from the network layer. Each data unit is considered as a frame.
• These layer use these Bridge,Switch and Ethernet ATM devices and protcol.

There are two sub layers in Layer 2

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

• MCA:- This layer deals with the broadcast network and protocols. when many users require transmitting data over the media at the same time, MAC sub-layer helps determining multi-access based on portocol .
• LLC:- LLC is one of the sublayers in the data link layer which provides:-
• Flow control
• Acknowledgment
• Error notification
• Tip:- Bridges use software for address learning while Switches use a Hardware chip for address learning.

Layer 3- Network Layer

• The Network Layer determines the path and the logical IP address.
• 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 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 deviceds such as switch and routing devices. This works for transmitting data from one node to another.
• Three Major functions in this layer Addressing, routing and Forwarding. 
• 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

• 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.
• This layer ensures that the entire message reaches its destination in order and handles the flow and error control at the source-to-destination level.
• The supported protocols at this Layer are TCP (Transmission Control Protocol) UDP (User Datagram Protocol) SPX (Sequential Packet Exchange - Novell Netware operating system).
• TCP: This is a connection-oriented protocol. TCP uses a sequence numbering to identify each byte of data transmission.
• UDP: This is a connectionless transmission protocol. There is no guarantee of delivery of data from source to destination and hence has no reliability.

Layer 5- Session Layer

• In the session layer, communication or connection between computers is controlled in a managed environment.
• Since this layer defines the start and stop point for any conversion between two hosts, it is termed as a session.
• It provides either half-duplex or full-duplex service
• It controls the communication process. It provides a dialog management facility.
• 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 network. Dynamically created connections that can run over individual networks can be controlled through the layer.

Layer 6- Presentation Layer

• This layer presents data to the application layer ad is responsible for data translation and encoding. Every application sends/receives data in a specific format which is defined by the 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 massage 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, ASCLL, 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 end-user applications. 
• Network services are typically protocols that work with user's data. 
• It provides a user interface to the user to communicate and the type of service to be used for the communication
• This layer refers to the communication services to be used by the application for communicating between to devices host and Application.
• 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.