ITCloud Center: October 2018

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.

What is a network?

A group or system of interconnected people or things???

What is a Computer Network ?

A computer network is a set of computers connected together for the purpose of sharing resources

A computer network is a group of computer systems and other computing hardware devices that are linked together through communication channels to facilitate communication and resource-sharing among a wide range of users