CNE Unit 4 Notes


UNIT –4 osi reference model


1) State the names of 2 sub layers of Data Link Layer. (List of 2 sub layers 1 mark each)


  1. a) MAC (Media access control)
  2. b) LLC (Logical link control)


2) List any two components which works at physical layer of OSI model. (Listing of any 2 components – 1 Mark each)


  1. Repeater
  2. Hub
  3. Network Interface card (NIC)
  4. Cables
  5. Connectors


3) Describe horizontal and vertical communication. (Horizontal Comm.-2 Marks, Vertical Comm.- 2 Marks )


Horizontal communications

The horizontal communication between the different layers is logical, there is no direct communication between them.

Information included in each protocol header by the transmitting system is message that will be carried to the same protocol in the destination system.

  • For two computers to communicate over a network, the protocols used at each layer of the OSI model in the transmitting system must be duplicated at the receiving system.
  • The packet travels up through the protocol stack and each successive header is stripped off by the appropriate protocol and processed.
  • When the packet arrives as it destination, the process by which the headers are applied at the source is respected in reverse.
  • The protocol operating at the various layers communicate horizontally with their counterparts in the other system, as shown in below fig.



Vertical Communications

  • In addition to communicating horizontally with the same protocol in the other system, the header information also enables each layers to communicate with the layer above and below it.
  • The headers applied by the different protocols implemented the specific functions carried out by those protocols.
  • For Example: When a system receives a packets and passes it up through the protocol stack, the data link layer protocol header includes afield that identifies which network-layer protocol the system should use to process the packet.
  • Network –layer protocol header in tern specifies one of the transport-layer protocol and the transport-layer protocol identifies the application for which the data is ultimately destined.
  • Vertical communication makes it possible for a computer to support multiple protocol at each of the layers simultaneously.


4) Identify the following devices operate in which of OSI reference model –repeater, bridges, switches, gateways. (Each point – 1 Mark)


Repeater : Physical layer

Bridge : Data link layer

Switch : Data link layer

Gateway : All seven layers or upper 3 layers


5) Explain the function of each layer of OSI reference model with neat diagram. (2 marks for diagram, 6 marks for functions of layers)


5) Describe the seven layers of OSI model. ( list of all seven layer 1 mark, brief description of each layer 3 marks)


5) Why layered architecture is used in OSI reference model? Discuss. (Explanation-3 marks, diagram 1 mark)


5) Describe OSI model with layer structure. (Diagram – 1 Mark, Explanation – 3 Marks)



OSI model is a layered frame work for the design of network systems that allows for communication across all types of computer systems.

1) Physical layer:

  • It co-ordinates the functions required to transmit bit stream over physical medium.
  • It deals with mechanical and electrical specifications of interface and transmission medium.
  • For transmission it defines procedures and functions that devices and transmission medium has to perform.
  • Physical characteristics of interfaces and media.
  • Representation of bits:
  • Data rate(transmission rate).
  • Synchronization of bits.
  • Line configuration: Point to point or multipoint configuration should be used.

2) Data link layer:

  • It is responsible for transmitting group of bits between the adjacent nodes.
  • The group of bits is called as frame.
  • The network layer passes a data unit to the data link layer.
  • Header and trailer is added to the data unit by data link layer.
  • This data unit is passed to the physical layer.
  • Data link layer is responsible for moving frames from one node to the next.

Functions of data link layer are:

1) Framing

2) Physical addressing

3) Flow control

4) Error control

5) Media access control

6) Node to node delivery


3) Network layer:

  • It is responsible for routing the packets within the subnet i.e. from source to destination.
  • It is responsible for source e to destination delivery of individual packets across multiple networks.
  • It ensures that packet is delivered from point of origin to destination.

Functions of network layer:

1) logical addressing

2) Routing.

3) Congestion control

4) Accounting and billing

5) Address transformation

6) Source host to destination host error free delivery of packet.

4) Transport layer:

  • Responsibility of process to process delivery of message Ensure that whole message arrives in order.

Functions of Transport layer:

1) Service point addressing

2) Segmentation and reassembly

3) Connection control

4) Flow control: Flow control is performed end to end

5) Error control

5) Session layer:

  • Establishes, maintains, synchronizes the interaction among communication systems.
  • It is responsible for dialog control and synchronization.

Functions of session layer:

1) Dialog control

2) Synchronization, session and sub session

3) Session closure

6) Presentation layer:

  • It is concerned with syntax , semantics of information exchanged between the two systems.

Functions of Presentation layer:


2) encryption

3) compression


7) Application layer:

  • It enables user to access the network.
  • It provides user interfaces and support for services like email, remote file access.

Functions of Application layer:

1) network virtual terminal

2)file transfer access and management

3)mail services and directory services


6) Draw a neat labled diagram showing the OSI reference model. Explain two functions of network layer and presentation layer. (Labeled diagram -4 Marks, Any two function of n/w & presentation layer -1 Mark each)


Function of network layer:

1) Addressing: source and destination address of a packet is stamped by network layer.

2) Routing: Packets are guided by the network layer to select appropriate path till destination.


Function of Presentation layer:

1) Translation: presentation layer is responsible for converting various formats into required format of the recipient.

2) Encryption: Data encryption and decryption is done by presentation layer for security.

3) Compression and Decompression: data to be transform compressed while sending and decompress while receiving for reducing time of transmission.



7) Describe the importance/role of presentation layer in OSI model. (Any 4 importance/functions of presentation layer – 1 Mark each)


Role of presentation layer in OSI model the presentation layer makes it sure that the information is delivered in such a form that the receiving system will understand and use it.

The form and syntax (language) of the two communicating systems can be different e.g. one system is using the ASCII code for file transfer and the other one user IBM’s EBCDIC.

Under such condition the presentation layer provider the “translation” from ASCII to EBCDIC and vice versa.


The presentation layer performs the following function:

  1. It translates data between the formats the network requires and the format the computer expects.
  2. It does the protocol conversion
  3. For security and privacy purpose it carries out encryption at the transmitter and decryption at the receiver.
  4. It carries out data compression to reduce the bandwidth of the data to be transmitted.
  5. Unlike the session layer, which provides many different functions, the presentation layer has only one function.
  6. It basically functions as a pass through device. It receiver primitives from the application layer and issues duplicate primitives to the session layer below it using the Presentation Service Access point (PSAP) and Session Service Access point (SSAP)


8) Explain the services provided by Data link layer in OSI Model. (Explanation of any four services: 1 mark each)


The data link layer is the protocol layer that transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment.

The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect error ask to transmitter to retransmit the data if required for correction.

The data link layer is concerned with local delivery of frames between devices on the same LAN.

When devices attempt to use a medium simultaneously, frame collisions occur. Data-link protocols specify how devices detect and recover from such collisions, and may provide mechanisms to reduce or prevent them.

Specific Services provided by data link layer :

Framing: Adding headers & trailers to the packet received from network layer & convert it in frame.

Addressing : Source MAC Address & destination MAC address is added with frame which can guide to the frame for forwarding the frame one node to another

Flow control: Ensures about the synchronization of transmitting & receiving system which results in transmission of data without any loss

Error control: Under data link layer error detection is done on parity check basis & as far as correction is concern receiving system will request sender system to resend once again.

Media access control: The media which are in share is controlled by applying media access control methods such as CSMA/CD OR TOKEN RING TOKEN PASSING etc.


9) What are the services provided by the network layer of OSI model.

(Any 4 Services – 1 Mark each)


The services provided by the network layer of OSI model as follows

  1. To route the signals through various channels to the other end.
  2. To act as the network controller by deciding which route data should take.
  3. To divide the outgoing messages into packets and to assemble incoming packets into messages for the higher levels.
  4. The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links)
  5. Logical addressing: The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.


10) Write the name of layers that perform the following function:

i) Data encryption ii) Error detection. iii) File transfer iv) Data encoding (1 Mark each point)


10) Write the names of the layers that performs the following functions in OSI. (1 mark each point)


1) Data Encryption: Presentation layer

2) Error detection: Data link layer & Transport layer

3) File transfer: Application layer

4) Data Encoding: Presentation layer


11) Describe the functioning of ‘application layer’ in TCP/IP protocol suit. (Explanation- 4 Mark)


Function of Application layer:

The TCP/IP protocols at the application layer can take several different forms. Some protocols, such as the File Transfer Protocol (FTP), can be applications in themselves, whereas others, such as Hypertext Transfer Protocol (HTTP), provide services to applications.

The most widely-known Application layer protocols are those used for the exchange of user information:

The Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of the World Wide Web.

The File Transfer Protocol (FTP) is used for interactive file transfer.

The Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments.

Telnet, a terminal emulation protocol, is used for logging on remotely to network hosts.

Additionally, the following Application layer protocols help facilitate the use and management of TCP/IP networks:

The Domain Name System (DNS) is used to resolve a host name to an IP address.

The Routing Information Protocol (RIP) is a routing protocol that routers use to exchange routing information on an IP internetwork.

The Simple Network Management Protocol (SNMP) is used between a network management console and network devices (routers, bridges, intelligent hubs) to collect and exchange network management information.

Examples of Application layer interfaces for TCP/IP applications are Windows Sockets and NetBIOS. Windows Sockets provides a standard application programming interface (API) under Windows 2000.

NetBIOS is an industry standard interface for accessing protocol services such as sessions, datagram, and name resolution.


12) Explain the Data encapsulation in OSI Model(Diagram: 2 marks, explanation 2 marks)


12) Explain encapsulation with example. (Diagram 1 mark, explanation 1 mark, example 2 mark)



  • The protocols operating at the various layers work together to supply a unified quality of service.
  • Each protocol layer provides a service to the layers directly above and below it. The process of adding the headers and trailers to the data is called as data encapsulation.
  • A packet(header and data ) at level 7 is encapsulated in a packet at level 6.
  • The whole packet at level 6 is encapsulated in a packet at level 5, and so on.
  • In other words, the data portion of a packet at level N-1 carries the whole packet (data and header and maybe trailer) from level N.The concept is called
  • The level N-1 is not aware of which part of the encapsulated packet is data and which part is the header or trailer.
  • For level N-1, the whole packet coming from level N is treated as one integral unit.


  • Each layer in the layered architecture provides service to the layers which are directly above and below it.
  • The outgoing information will travel down through the layers to the lowest layer.
  • While moving down on the source machine, it acquires all the control information which is required to reach the destination machine.
  • The control information is in the form of headers and footers which surrounds the data received from the layer above.
  • This process of adding headers and footers to the data is called as data encapsulation.
  • The headers and footers contain control information in the individual fields.
  • It is used to make message packet reach the destination.
  • The headers and footers form the envelope which carries the message to the desired destination.
  • The figure shows the example of five layer stack for data encapsulation.
  • The fifth layer of sending machine wants to send a message M to the fifth layer of destination machine.
  • The message M is produced by layer 5 of machine 1 and given to layer 4 for transmission.
  • Layer 4 adds header H4 in front of the message and pass it to layer 3.
  • Layer 3 breaks up the incoming message into small units as M1 and M2 and pass these packets to layer 2.
  • Layer 2 adds the header as well as footer to each packet obtained from layer 3 and pass it to layer 1 for physical transmission.


13) Explain the services provided by transport layer in TCP/IP model. (Any four functions 1 mark each)


Responsibility of process to process delivery of message Ensure that whole message arrives in order.

  1. Service point addressing: –
  • Several programs run at a time on computer.
  • Delivery is not only from one computer to another but also from specific process on computer to specific process on another computer.
  • For this transport layer uses port addresses.
  • Transport layer delivers entire message to the correct process on the computer.
  1. Segmentation and reassemble: –
  • Each segment of a message contains a sequence number which is used to reassemble the message correctly upon arriving at destination and to identify and replace packets that are lost in transmission.
  1. Connection control:-
  • Logical connection is created between source and destination for the duration of complete message transfer.
  1. Flow control:-Flow control is performed end to end.
  2. Error control:-
  • Error control is performed process to process.
  • It ensures that entire message arrives at receivers transport layer without error(damage or loss or duplication).
  • Error correction is done by retransmission.



14) Which layer of OSI model packages raw data bit. Describe bit stuffing with one example. (Identification of Data link layer – 1 Mark, bit stuffing- 2 Marks, Example- 1 Mark)


Data link layer of OSI model packages raw data bit.

Bit stuffing:

Bit stuffing is the process of adding one extra 0 whenever five consecutive 1’s in the data, so that the receiver does not mistake the pattern 0111110 or a flag.

At sender side the bit is stuffed and at receiver side stuffed bit is removed.

As shown in following





15) Describe TCP/IP with neat sketch. Compare TCP/IP and OSI reference model. (TCP/IP model digram-2 Marks, explanation 2 Marks, Comparison of TCP/IP with OSI model any 4 points – 4 Marks)


TCP/IP Model Application Layer
Transport Layer
Internet Layer
Network access Layer ( Host to Network Layer )


TCP/IP Model contains following layer.

1) Network Access Layer

  • It defines characteristics of transmission media.
  • It also concern with delivery of data when two systems are attached to same network.

2) Internet Layer –

  • This layer permits host to inject packets into network and packet travels independently to destination.
  • This layer defines packet format and protocol called IP (internet Protocol)

3) Transport Layer –

  • It has TCP and UDP
  • TCP (transmission control protocol) –It is Reliable & connection oriented protocol.
  • UDP (User Datagram Protocol)- It is Unreliable & connectionless protocol.

4) Application Layer –

  • It includes virtual Terminal (TELNET), File Transfer Protocol (FTP),
  • Simple Mail Transfer Protocol (SMTP) and other protocols like HTTP, WWW,DNS.


16) Comparison of TCP/IP model with OSI model.(1 mark for each comparison)

1 It has 4 layers It has 7 layers
2 Function of presentation layer are performed by application layer It has separate presentation layer
3 Function of session layer are performed by transport layer It has separate session layer
4 Transport layer supports both connection oriented and connection less services. Transport layer support only connection oriented services.
5 It is difficult to add a new protocol in existing protocol stack. Adding a new protocol is easy.
6 Network layer supports only connectionless service Network layer support both connection oriented and connection less service
7 It is used as internet standard . It describe protocols widely used around internet. It is generic protocol independent standard. It describes general protocols
8 TCP/IP model not clearly distinguish between services, interface and protocols OSI model clearly distinguish between services, interface and protocol.
9 Draw the diagram of TCP/IP Draw the diagram of OSI model


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