mca 4 OSI MODEL

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Physical layer (Layer 1) of the OSI model. The Physical layer is responsible for the ultimate transmission of data over network communications media. It operates with data in the form of bits that are sent from the Physical layer of the sending (source) device and received at the Physical layer of the destination device.Hubs and other repeaters are standard network devices that function at the Physical layer. Cables and connectors also are a part of the Physical layer. At the Physical layer, data are transmitted using the type of signaling supported by the physical medium:

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• electric voltages
• radio frequencies
• pulses of infrared or ordinary light

Layer 1:Physical Layer

Defines physical means of sending data over network devices

Interfaces between network medium and devices

Defines optical, electrical and mechanical characteristics

 

Layer 2:Data Link Layer

Defines procedures for operating the communication links

Frames packets

Detects and corrects packets transmit errors

Data Link layer (Layer 2) of the OSI model. When obtaining data from the Physical layer, the Data Link layer checks for physical transmission errors and packages bits into frames. The Data Link layer also manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of any various network devices to the physical medium. Because the Data Link layer is the single most complex layer in the OSI model, it is often divided into two parts, the Media Access Control sublayer and the Logical Link Control sublayer.

Network Layer

Devices which work on Network Layer mainly focus on routing. Routing may include various tasks aimed to achieve a single goal. These can be:

 

• Addressing devices and networks.
• Populating routing tables or static routes.
• Queuing incoming and outgoing data and then forwarding them according to quality of service constraints set for those packets.
• Internetworking between two different subnets.
• Delivering packets to destination with best efforts.
• Provides connection oriented and connection less mechanism.

Network Layer Features

With its standard functionalities, Layer 3 can provide various features as:

• Quality of service management
• Load balancing and link management
• Security
• Interrelation of different protocols and subnets with different schema.
• Different logical network design over the physical network design.

Layer 4:Transport Layer

Manages end-to-end message delivery in network

Provides reliable and sequential packet delivery through error recovery and flow control mechanisms

Provides connectionless oriented packet delivery

Transport layer offers peer-to-peer and end-to-end connection between two processes on remote hosts. Transport layer takes data from upper layer (i.e. Application layer) and then breaks it into smaller size segments, numbers each byte, and hands over to lower layer (Network Layer) for delivery.

Functions

• This Layer is the first one which breaks the information data, supplied by Application layer in to smaller units called segments. It numbers every byte in the segment and maintains their accounting.
• This layer ensures that data must be received in the same sequence in which it was sent.
• This layer provides end-to-end delivery of data between hosts which may or may not belong to the same subnet.
• All server processes intend to communicate over the network are equipped with well-known Transport Service Access Points (TSAPs) also known as port numbers.

Layer 5:Session Layer

Manages user sessions and dialogues

Controls establishment and termination of logic links between users

Reports upper layer errors

Session Layer (Layer 5)

The fifth layer in the OSI Reference Model is the session layer. As we proceed up the OSI layer stack from the bottom, the session layer is the first one related to the addressing, packaging and delivery of data . It is the lowest of the three upper layers, which collectively are concerned mainly with software application issues and not with the details of network and internet implementation.

The name of this layer tells you much about what it is designed to do: to allow devices to establish and manage sessions. In general terms, a session is a persistent logical linking of two software application processes, to allow them to exchange data over a prolonged period of time. In some discussions, these sessions are called dialogs; they are roughly analogous to a telephone call made between two people.

 

 Layer 6: Presentation Layer

Masks the differences of data formats between dissimilar systems

Specifies architecture-independent data transfer format

Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data

The presentation layer is the sixth layer of the OSI Reference Model protocol stack, and second from the top. It is different from the other layers in two key respects.

First, it has a much more limited and specific function than the other layers; it's actually somewhat easy to describe,! Second, it is used much less often than the other layers; in many types of connections it is not required.

The name of this layer suggests its main function as well: it deals with the presentation of data. More specifically, the presentation layer is charged with taking care of any issues that might arise where data sent from one system needs to be viewed in a different way by the other system. It also takes care of any special processing that must be done to data from the time an application tries to send it until the time it is sent over the network.

Presentation Layer Functions

Here are some of the specific types of data handling issues that the presentation layer handles:

• Translation: Networks can connect very different types of computers together: PCs, Macintoshes, UNIX systems, AS/400 servers and mainframes can all exist on the same network. These systems have many distinct characteristics and represent data in different ways; they may use different character sets for example. The presentation layer handles the job of hiding these differences between machines. 
• Compression: Compression (and decompression) may be done at the presentation layer to improve the throughput of data. (There are some who believe this is not, strictly speaking, a function of the presentation layer.) 
• Encryption: Some types of encryption (and decryption) are performed at the presentation layer. This ensures the security of the data as it travels down the protocol stack. For example, one of the most popular encryption schemes that is usually associated with the presentation layer is the Secure Sockets Layer (SSL) protocol. Not all encryption is done at layer 6, however; some encryption is often done at lower layers in the protocol stack, in technologies such as IPSec.

 

Layer 7: Application Layer

Defines interface to user processes for communication and data transfer in network

Provides standardized services such as virtual terminal, file and job transfer and operations

The Application Layer is the level of the protocol hierarchy where user-accessed network processes reside. In this text, a TCP/IP application is any network process that occurs above the Transport Layer. This includes all of the processes that users directly interact with, as well as other processes at this level that users are not necessarily aware of.

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