Types of OSI model

 * THE SEVEN LAYERS OF OSI MODEL OF MODEL ARE :- 

1. Physical Layer 

2. Data Link Layer 

3. Network Layer 

4. Transport Layer 

5. Session Layer 

6. Presentation Layer 

7. Application Layer 

1. Physical Layer :- 

This is the lowest layer in the model. 

This layer is responsible for activating, maintaining and deactivating a physical circuit between two end systems. 

It specifies the mechanical (physical) and electrical characteristics of interfaces and connections composing the network. 

It deals with mechanical, procedural, electrical and functional characteristics of transmission media. Thus it can be think of as a hardware layer. 

The layer exists as chips, network adapters (PCB's) and cables . 

The actual data transfer takes place between physical layer of sender and receiver stations. 

The physical layer is simply responsible for sending bits (bits are binary 1's and 0's of a digital communication) from one computer to another. 

The physical layer is not connected with the meaning of bits :instead it deal with the physical connection to the network and with transmission and reception of the signal. 

This level defines physical and electrical details, such as what will represent 1 or 0,how many pins a network connector have, how data will be synchronised and what the network adapter may or may not transmit the data. 

''passive hubs'', simple active Hub, terminators, couples, cables, connectors, repeater, multiplexes, transmitter and Tran receives are devices associated with physical layer. 

The following are the items are addressed at the physical layer :

Network connection types, including multipoint and point to point connection. 

Physical topologies, which are physical layouts of the networks such as bus, start or ring. 

Analog and digital signaling, which include several methods for encoding data in analog and digital signals. 

Bit synchronization, which deals with synchronization between sender and receiver. 

Baseband and broadband transmission, which are different methods for using media bandwidth. 

Multiplexing which involves combining several data channels into one. 

Termination, which prevents signals from reflecting back through the cable and causing signal and packets errors. It also indicates the last in a network segment. 

2. Data Link Layer :- 

This layer is responsible for transfer of data over the channel. 

At this stage of processing the electrical impulses enter or leave the transmission media or cable. 

The electrical representation of Data message bit patterns, tokens, frames, encoding methods etc. are known to this layer only. 

This layer is responsible for establishing an error free communications path between systems over the physical channel. 

It provides synchronization and identity of bits. 

It also ensures safe arrival of data at receiving end. 

It also provides flow control and prevents overburdening of receiver with too much data at any instant. 

It detects the transmission errors and provides mechanism for recovery of the lost erroneous or duplicated data. 

It splits data packet into data frame which are transmitted by sender. Acknowledgements are transmitted by receivers for data frames. Error detections and error correction are performed at this layer. 

Two sub layers of this are media accessed control (MAC) and logical link control (LLC). 

The accepts packets from the network Layer and packages the information into data units called frames to be presented to physical layer for transmission. 

The data link layer adds control information, such as frame type, routing and segmentation information to data being sent. 

This layer provides for error free transfer of frames from one computer to another. 

A cyclic redundancy check (CRC) added to the data frame can detect damaged frames and the data link layer in the receiving computer can request that the information be present. 

The data link layer can also detect when frames are lost and request that those frames be sent again. 

''Bridges, intelligent hubs and network interface cards are devices typically associated with data link layer ''. 

The data link layer splits into two sublayers. 

Logical Link Control (LLC), which controls establishes and maintains links between the communicating devices. 

Media Access control (MAC), which controls the way multiple devices share the same media channel. 

The logical link control sub - layer provides service access points (SAP) that other computers can refer to and use to transfer information from the logical link control sub-layer to upper ISO layer. 

The media access control sub-layer, the lower of the two sub layers, provides for shared access to the network adapter and communication directly with network interface cards. 

Network Interface Cards have a unique 12 hexadecimal MAC address assigned. The MAC addresses are used to establish the logical link between computers on the same LAN. 

3. Network Layer :- 

The network Layer establishes a logical connection between sender and user by providing a logical path between them. 

This layer switches and routes message packets as necessary to get them to their destination. The layer is responsible for addressing and delivering message packets. 

The layer is rich in functions. 

Network Layer makes routing decisions and forwards the packets for devices that are father away than a single link. 

A link connects two networks devices and is implemented by the data link layer. 

Two devices connects by a link communicate directly with each other and not through a direct third device. 

In larger networks there may be intermediate systems between the two systems, and the network Layer make possible for the transport layer and the layer above it to send packets without being concerned about whether the end systems is immediately adjacent or several hops away. 

The network layer translates logical network address into physical machine addresses. 

This layer also determines the quality of service and route a message will take if there are several ways a message can get to its destination. 

The network layer also may break large packets into small chunks if the packet is larger than the largest data frame the data link layer accept. 

The network reassembles the chunks into packets at receiving end. 

Intermediate systems that perform only routing and relaying functions and do not provide an environment for executing user program can implement just the first three OSI network layers. 

The protocol at this layer choose a specific route through an inter network to avoid the excess traffic caused by sending data over network and segments that do not need to access to it '' routers and gateways operate in Network layer ''. 

The network layer serves to support communication between logically separate networks. This layer is concerned with following. 

Addressing, including logical network addresses and services addresses. 

Circuit, message and packet switching. 

Connection services including network layer flow control, network layer error control, and packet sequence control. 

Gateway services. 

4. Transport Layer :- 

This layer provides control standards for communication session for enabling processes to exchange data reliably and in a sequential manner. 

It provides interfacing between data communications network and upper three layers. 

It gives the user option in obtaining certain levels of quality and cost from the network layer. 

It isolates the user from physical and functional aspects of the packet network. 

It controls the sequencing of the message components and regulates in bound traffic flow during processing of more than one packets at a time. 

At the same time if a duplicate packet arrives the layer recognizes it and discards it. 

The transport layer ensures that packets are delivered error free in sequence and with no losses or duplication. 

The transport layer breaks large messages from the session layer into packets to be sent to the destinations computer and reassembles packets into messages to be presented to the session layer. 

The transport layer typically sends an acknowledgement to the originator for the message received. 

5. Session Layer :- 

This layer provides establishing maintaining and terminating a session or dialogue between two end systems. 

It enables the applications running at two stations to Co-ordinate into a single session. 

It creates the session, manages the packets sent back and forth during the session and terminates the session. 

The dialogue type /session type is also specified by it whether simplex, duplex or half duplex. 

It also regulates the direction of messages flow. It also defines synchronization points for intermediate checks and recovery for file transfers. 

The session Layer allows applications on separate computers to share a connection called a session. 

This layer provides services such as name up and Security to allow two programs to find each other and establish the communication link. 

The session Layer also provides for data synchronization and check pointing so that in the event of network failure, only the data sent after the point of failure need be resent. 

This layer also the control the dialog between two processes determining who can transmit and who can receive at what point during the communication. 

6. Presentation Layer :- 

This layer converts data received from application layer into machine 's native internal numeric format and encoded transmitted data into displayable form for output. It performs code conversions text compressions, security encryption etc. On the messages. 

The layer consists of many tables of syntax and it's functions are limited. 

The presentation layer translates data between the formats the network require and the formats the computers expects. 

The presentation does the protocol conversion, data translation, compression and encryption, character set conversation and the interpretation of graphics commands. 

The network redirector operates at this layer. 

The network redirector is what makes the files on a file server visible to the client computer. 

The network redirector also makes remote printers as though they are attached to the local computer. 

The network redirector in an important part of the networking. 

7. Application Layer :-

This is the topmost layer of OSI architecture. It is the layer seen by the application program and hence is user oriented layer which provides services to support end user of network directory. 

A message to be sent across network enters the OSI model at this point and then travels down ward unto physical layer then through transmission line unto physical layer of destination & then upwards up to application layer of receiver end system. 

The layer is concerned with semantics of data. 

The layer supports virtual terminals and virtual file concepts. 

This also provides layer is the topmost layer of the OSI model and it provides services that directly support user applications such as database access, email and file transfer. 

It also allow application to communicate with application on other computer. 

When a programmer writes an application program that use network services, this is the layer the application program will access. 

Among these seven layers application Layer, presentation layers, session Layer transport Layer are application oriented layers, which network layers data link layer and physical layer are network dependent layers. 

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