OSI Model

OSI Model

While studding for Networking it is very obvious to start with OSI Model, it is the basic building block for managing Network, You should be familiar with the OSI model because it is the most widely used method for understanding and talking about network communications. However, remember that It is only a theoretical model that defines standards for programmers and network administrators, not a model of actual physical layers.

OSI model & Its advantages:

• Divides networking tasks into logical layers for easier comprehension
• Provides a common language or reference point between network professionals
• Allows specialization of features at different levels
• Helps  in troubleshooting
• Promotes standards between networks and devices
• Developers can change features without Changing the entire approach

Limitations of the OSI model.

• OSI layers are theoretical and do not actually perform real functions.
• Industry implementations rarely have a layer-to-layer correspondence with the OSI layers.
• Different protocols perform different functions that help send or receive the overall message.
• A particular protocol implementation may not represent every OSI layer (or may spread across multiple layers).
  

Basically there is seven layers in OSI Model which could be tedious to remember, we could help you to remember the layers by adding mnemonics 

                         Layer Name ................................Mnemonic

               (Bottom to top ..................Top to bottom)

• Layer 7 Application ......................Away........................... All
• Layer 6 Presentation......................Pizza ............................People
• Layer 5 Session............................ Sausage ........................Seem
• Layer 4 Transport..........................Throw ..........................To
• Layer 3 Network........................... Not............................ ...Need
• Layer 2 Data Link......................... Do ................................Data
• Layer 1 Physical............................Please............................Processing

Have some fun and come up with your own mnemonic for the OSI model, but stick to just one

Layers of OSI Model

Function of All layers of  OSI model

In Previous section we learn the no. of layers of OSI model, and in this section we will cover the  Basic function of different layers,

As we all knows that OSI model consist of seven layers which are as:- (Top to bottom) 

• Application Layer
• Presentation Layer
• Session Layer
• Transport Layer
• Network Layer
• Data link Layer
• Physical Layer

Application Layer & it's Function:-

It is the upper most layer of the osi model and It consists of protocols that focus on process-to-process communication across an IP network and provides a firm communication interface and end-user services.

The Application layer integrates network functionality into the host operating system, and enables network services. The Application layer does not include specific applications that provide services, but rather provides the capability for services to operate on the network. These services include:

• File services--transferring, storing, and updating shared data
• Print services--enabling network printers to be shared by multiple users
• Message services--transferring data in many formats (text, audio, video) from one location to another, or from one user to another
• Application services--sharing application processing throughout the network and enabling specialized network servers to perform processing tasks
• Database services--storing, retrieving, and coordinating database information  The Application layer specifies many important network services like
•  HTTP, Telnet,FTP, TFTP, SNMP

Note: Most Application layer protocols operate at multiple layers down to the Session and even Transport layers. However, they are classified as Application layer protocols because they start at the Application layer.

Function of Presentation layer:-

Presentation layer is the second layer from top and " It is used to present data to the application layer in an accurate, well-defined and standardized format." the responsibility of The Presentation layer formats or "presents" data into a compatible form for receipt by the Application layer or the destination system. Specifically, 

the Presentation layer ensures:

• Formatting and translation of data between systems
• Converting character sets to the correct format
• Compatibility with the host
• Encapsulation of data into message envelopes by encryption and compression
• Restoration of data by decryption and decompression

The Presentation layer formats data for the Application layer. Therefore, it also sets standards for multimedia and other file formats. These include standard file formats such as:

• JPEG, BMP, TIFF, PICT
• MPEG, WMV, AVI
• ASCII, EBCDIC
• MIDI, WAV

Function of Session Layer:-

The Session layer's primary function is managing the sessions in which data is transferred. Functions at this layer may include:

•  Establishment and maintenance of communication sessions between the network hosts, 
• Management of multiple sessions (each client connection is called a session).
• Assignment of the session ID number to each session, which is then used by the Transport layer to properly route the messages.
• Dialog control--specifying how the network devices coordinate with each other (simplex, half-duplex, and full-duplex).
• Termination of communication sessions between network hosts upon completion of session.

The Session layer protocols and interfaces coordinate requests and responses between different hosts using the same application. These protocols and interfaces include:

• Network File System (NFS)
• Apple Session Protocol (ASP)
• Structured Query Language (SQL)
• Remote procedure call (RPC)
• X Window

Function of Transport Layer

Basically Transport layer is responsible for transparent transfer of data between systems or hosts, and is responsible for and flow control and end-to-end error recovery. It ensures complete data transfer, its function area such as:-

• Reliable(connection oriented i.e TCP) and unreliable (connection less i.e UDP)communications
• End-to-end flow control
• Port and socket numbers
• Segmentation, sequencing, and combination

Function of Network Layer 

Network layer provides switching and routing technologies, uses logical address for transmitting data from node to node. It is also  known as virtual circuits. Routing and forwarding are functions of this layer, in short following are the basic function of the Network layer,

• Addressing, 
• Inter-networking,
• Error handling, 
• Congestion control
• Packet sequencing.
• Logical addresses
• Path determination (identification and selection)
• Routing packets

Function of Data Link Layer:- 

This layer of OSI Model is divided into two sub layers: 

• The Media Access Control (MAC) layer and
• The Logical Link Control (LLC) layer. 

The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

• Data packets are encoded and decoded into bits 
• Handles errors in the physical layer, 
• Flow control and frame synchronization. 
• Convert bits into bytes and bytes into frames
• Manage the MAC address i.e hardware address
• Logical network topology
• Media access
• Parity and CRC
• Flow control
• Acknowledgements
• Buffering
• Windowing

Function of Physical Layer:-

Physical layer conveys the bit stream at the electrical and mechanical, send /receive electrical impulse, light or radio signal throughout the network. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

The basic function of it are as:-

• Move bits across media
• Cables, connectors, pin positions
• Electrical signals (voltage,bit synchronization)
• Physical topology (network layout)

So This is the end of this session hope you like the session, In next session we will Learn about TCP/IP, feel free to give your suggestions  and feedback.