Explain 7 layers of OSI model ? What are the benifits of OSI model ? Does every networking device need to have all the layers ?

October 28, 2007 · Filed Under Networks, Placement Questions 



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Explain 7 layers of OSI Reference model

This is the first questions that will be asked by CISCO and any other networking related Company. Infact this question was asked to me by 6 companies CISCO,Adobe,Infosys, Wipro,Motorola and IBM. How you explain OSI model determines how much you know about it so always use big terms and concerend protocols with every layer.

OSI is “Open Systems Interconnection” is a layered, abstract description for communications and computer network protocol design. Try to remember all of them by APSTNDP or any such name of your own.
7 Layers of OSI
Layer 7: 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, applications which use File Transfer Protocol (FTP), applications which use Simple Mail Transfer Protocol (SMTP) and applications which use Hypertext Transfer Protocol (HTTP). Applications built to utilize a protocol, such as FTP, should not be confused with the protocols themselves, which often reside at the session layer.
Layer 6: Presentation layer

The Presentation layer transforms data to provide a standard interface for the Application layer. MIME encoding, data compression, data encryption and similar manipulation of the presentation is done at this layer to present the data as a service or protocol developer sees fit. Examples: converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of, e.g., XML.
Layer 5: Session layer

The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either full-duplex or half-duplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for “graceful close” of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocols suite.
Layer 4: Transport layer

The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable data transfer. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. 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 transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets. Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatching and classification of mail and parcels sent.
Layer 3: Network layer

I have seen lot’s of people getting confused between What Transport and Network layer Do. Try to remember in this way. They both are very tightly glued together. If Transport layer is a car than Network layer is its internal parts which makes the car work. Network layer deals with all the complex networking details but and Transport layer resides on it.

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 Network layer performs network routing functions, and might also perform segmentation/desegmentation, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible (also existing at layer 3 (or IP) are routers). This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical. The best known example of a layer 3 protocol is the Internet Protocol (IP). Perhaps it’s easier to visualize this layer as the actual Air Mail or Consolidated Carrier that transfers the mail from Point A to Point B.
Layer 2: Data link layer

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. 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. On IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this layer may be split into a Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer. It arranges bits from physical layer into logical chunks of data, known as frames.

This is the layer at which the bridges and switches operate. Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding. Other protocols may be imposed on the data frames to create tunnels and logically separated layer 2 forwarding domain.
Layer 1: Physical layer

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 is the establishment and termination of a connection to a communications medium.

Try to keep this image in your memory as sometimes an image can help you more in recalling than reading so much content.

osi.jpg

Second Question on OSI - What are the benifits of OSI model ?
You can say a lot on this without knowing anything. Some of the advantages of OSI is that it gives clear seperation and functionality of a complete network. Each protocol and device can be understood in terms of its corresponding layer. And ofcourse, being layered its easy to distinguish between various parts of network and we can easily categorize them. Find some better point of yours as well

Another Question - Does every networking device need to have all the layers
NO, A networking device only need the layer and the subsequent below layers to work properly. CISCO ask this question.

Remember the Golden Equation - You will be exponentially harming your chances of not getting selected by answering this whole OSI question poorly.

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