OSI Reference Model

        What is OSI Reference Model?

       Open System Interconnection Model

         A conceptual model that characterizes and standardizes the communication                functions of a telecommunication or computing system without regard to its                   underlying internal structure and technology

• Open-System means allow the communication between all kind of systems worldwide between different Standards and Technologies

• Interconnection means the physical linking of a carrier's network with equipment or facilities not belonging to that network

       History:

• Rapid growth of computer networks caused compatibility problems

• ISO recognized the problem and released the OSI model in 1984

Why we need to understand OSI Model?

• To Understand how applications can communicate over a network

• To help network administrators by dividing large data exchange process in smaller segments. Smaller segments are easier to understand, manage and troubleshoot

• Standardization of network components allows multiple-vendor development.

• It allows different types of network hardware and software to communicate.
• To help Students and Professionals to understand communication functions
• Troubleshooting Problems …

       Encapsulation in PDUs

• Data exists at each layer contained within a unit called a Protocol Data Unit (PDU).

• PDU’s are referred two ways: N-PDU, and by special names.
• The process by which data moves between PDU types is called Encapsulation 
• PDU move through interfaces between layers using Service Access Points (SAP)

          OSI Model and TCP/IP Protocol Suite Comparison 

1.Physical Layer Functions: 

• Coveys the bit stream through the network at the electrical and mechanical level.
• Defines physical means of moving data over network devices.
• Interfaces between network medium and devices.
• Defines optical, electric and mechanical characteristics, voltage levels, timing of voltage changes, physical data rates, transmission distances & physical connections.

2.Data Link Layer Functions : 

• Takes a string of bits and delivers it across a link.
• Conveys the bit stream through the network at the electrical & mechanical level. (i.e.layer1)
• Turns packets into raw bits and bits into packets.
• Framing & error detection.

3.Network Layer Functions: 

• Translate logical network address and names to their physical address. (e.g. Device name to MAC address)
• Responsible for: Addressing, determining routes for sending, managing network problems such as packet switching, data congestion & routing.
• Breaks the data into smaller units and assembles data.
• Shields higher layer from details of how the data gets to its destination.

4.Transport Layer Functions: 

• Divide streams of data into chunks or packets.
• Reassembles the message from packets.
• Provide error-checking to guarantee error-free data delivery, with no losses or duplications.
• Provide acknowledgement of successful transmissions.
• Request retransmission if some packets don’t arrive error-free.
• Provides flow control & error handling.

5.Session Layer Functions: 

• Establishes, maintains and ends sessions across the network.
• Responsible for identification so only the designated parties can participate in the session.
• Provides synchronization services by planning check points in the data stream.
• If session fails, only data after the most recent check point need be transmitted.
• Manages who can transmit data at a certain time and for how long.

6.Presentation Layer Functions: 

• Translates from applications to network format and vice verse.
• All different formats from all sources are made into a common uniform format that the rest of the OSI can understand.
• Responsible for protocol conversion, character conversion. expanding graphics data encryption / decryption, commands & data compression.
• Sets standards for different systems to provide seamless communication from multiple protocol stacks.

7.Presentation Layer Functions: 

• Used for applications specially written to run over the network.
• Allows access to network services that support applications.
• Directly represents the services that directly support user applications. (e.g. file transfer and email)
• What the user sees or does.