What is the OSI reference model ?

[syeda amna]

what is osi refrence model?
what is the purpose of osi model?
what is the purpose of application layer?

 

[ckshivaram]

Re: osi refrence model..

**broken link removed**

---------- Post added at 21:05 ---------- Previous post was at 21:03 ----------

**broken link removed**

 

[sagar474]

Re: osi refrence model..

you can understand if you try to understand this YouTube - Computer Networking Tutorial - 5 - OSI Model Data Link Layer

 

[veenashree89]

Re: osi refrence model..

Open Systems Interconnection ( OSI ) is a standard reference model for communication between two end users in a network. The model is used in developing products and understanding networks.

The application layer ...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions

 

[syeda amna]

Re: osi refrence model..

thanks for your replies. veenashree and ckshivaram.
i m still confused. can you explain the significance of this layered model.

 

[ckshivaram]

the application layer provides services for an application program to ensure that effective communication with another application program in a network is possible. The application layer is not the application itself that is doing the communication. It is a service layer that provides these services:

•Makes sure that the other party is identified and can be reached
•If appropriate, authenticates either the message sender or receiver or both
•Makes sure that necessary communication resources exist (for example, is there a modem in the sender's computer?)
•Ensures agreement at both ends about error recovery procedures, data integrity, and privacy
•Determines protocol and data syntax rules at the application level
It may be convenient to think of the application layer as the high-level set-up services for the application program or an interactive user

---------- Post added at 22:39 ---------- Previous post was at 22:33 ----------

The Physical layer defines all the electrical and physical specifications for devices.
This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices. The major functions and services performed by the physical layer are:
Establishment and termination of a connection to a communications medium.

The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. This is the layer at which the bridges and switches operate.
The best known example of this is Ethernet.
Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks.

The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer.
The best known example of a layer 3 protocol is the Internet Protocol (IP).

The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable and cost-effective data transfer. This means that the transport layer can keep track of the packets and retransmit those that fail.
The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP).

The Session layer controls the dialogues (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application.

The Presentation layer transforms data to provide a standard interface for the Application layer.

The Application layer provides a means for the user to access information on the network through an application. This layer is the main interface for the user(s) to interact with the application and therefore the network
Some examples of application layer protocols include Telnet, File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and Hypertext Transfer Protocol (HTTP).

---------- Post added at 22:41 ---------- Previous post was at 22:39 ----------

HowStuffWorks "How OSI Works"

---------- Post added at 22:52 ---------- Previous post was at 22:41 ----------

importance of OSI model:

the osi model bascially just shows that a computer should be divided into differnt sections.. this way every layer is independent... so if i wanted to chage my ethernet cord to a fibre optic cord i should be able to with out all my software stuffing up... if all layers where interdependent upgrading or changeing sections would be next to impossible and computers need to adapt and change..


The OSI Model is a model that illustrates how data communications should take place. It segregates the process into seven groups, called layers. Into these layers are integrated the protocol standards developed by the ISO and other standards organization, including the Institute of Electrical and Electronic Engineers (IEEE), American National Standards Institute (ANSI), and the International Telecommunications Union (ITU), formerly known as the CCITT (Comite Consultatif Internationale de Telegraphique et Telephone). The OSI Model affirms what protocols and standards should be used at each layer. It is modular, each layer of the OSI Model functions with the one above and below it.

i have given the best shot of my answers..

if you still have doubts and not clear ,, then maybe others can help you out who are more knowledgeable and good in explaination

 

[veenashree89]

Re: osi refrence model..

thanks for your replies. veenashree and ckshivaram.
i m still confused. can you explain the significance of this layered model.

during communication between two networks, it under go several steps...those steps are explianed in the form of layers...
in a very simple way..if you want ti post a letter,you will write the data..from address,destination address, and it under goes different steps untill it reaches destination...
similarly in networks...it under go several layers(steps) from physical layer to application layers
each layer perform specific function


OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host. The seven layers are:

Layer 7: The application layer ...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.)

Layer 6: The presentation layer ...This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer.

Layer 5: The session layer ...This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination.

Layer 4: The transport layer ...This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer.

Layer 3: The network layer ...This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding.

Layer 2: The data-link layer ...This layer provides synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management.

Layer 1: The physical layer ...This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.

 

[syeda amna]

thanks veenasheer89.

 

[veenashree89]

thanks veenasheer89.

ok first consider locally For example i have data word "hello" and i want to send it to another running program simulating the OSI model. That word ill go trough the all layers and they will add their headers and stuff like OSI does and then other running program will receive it.

Basically TCP/ip protocol has to be programmed i think but may be experineced person can get it better

then i can manually change the bits to check whether error control works ok or not

 

[cubanflyer]

A useful tip in remembering the OSI structure I use is

All People Should Try New Diet Pepsi

All - Application
People -Presentation
Should - Session
Try - Transport
New -Network
Diet - Data
Pepsi - Physical

 

[saif haider]

if we use conversion of Ethernet into E1 (2MB stream) then under what layer this conversion will come. also equipment that is used for transmission and reception of E1 stream will lay under which layer.

 

[bluehole]

**broken link removed**
is a nice little description,
even this one too. Getting to Know the OSI Model for the CCNA Exam - For Dummies

 

[navenmou]

once go through this site
The World's First Free Cisco Lab - Firewall.cx