Top Questions and Answers on OSI Model

• List the layers of OSI?

From top to bottom, OSI layers are,

Application, Presentation, Session, Transport, Network, Data Link and Physical.

• What are the responsibilities of Data Link Layer?

Framing, Error detection, CRC and Physical Addressing is the task of DLL.

• What are the responsibilities of Network Layer?

Routing, IP Addressing and Path determination are the main responsibilities of Network Layer.

• What are the responsibilities of Transport Layer?

Transport Layer has a lot of function. Most important being,

1. Multiplexing and De-Multiplexing
2. Segmentation and Re-assembly
3. Flow Control
4. Error Correction
5. Connection Establishment
6. Sequencing
7. Acknowledgement
8. 3 way Handshake

• Routers work at which OSI layer?

Network Layer

• Switches work at which OSI layer?

Layer 2 and Some Switches can operate at Layer 3 and above

• What is a Window in networking terms?

Window is the amount of segments sent by TCP between two acknowledgements.

• What is the role of the LLC sublayer in datalink layer?

Logical Link Control provides error detection, using Ethernet trailer field frame check sequence (FCS).

• What is the function of the Application Layer in networking?

Application Layer is responsible for providing a user interface in between user and Network with the help of applications like web browsers.

• What is the difference between TCP and UDP?

Following are differences in TCP and UDP,

• TCP stands for “Transmission Control Protocol” UDP stands for “User datagram Protocol”.
• TCP is connection oriented protocol while UDP is connectionless protocol.
• TCP is more reliable than UDP.
• UDP is faster for data sending than TCP.
• UDP makes error checking but no reporting but TCP checks for errors and performs reporting.
• TCP provides guaranteed Delivery of Data but UDP has no guarantee.
• Header size of TCP is 20 bytes while that of UDP is 8 bytes.
• TCP has acknowledgement segments but UDP has no acknowledgement.
• TCP is used for application that require high reliability but less time critical whereas UDP is used for application that are time sensitive but require less reliability.

• What is the port no of DNS and Telnet?

DNS = 53, Telnet = 23

• Which service use both TCP and UDP ?

DNS uses both TCP and UDP

• What is the port no of SMTP and POP3?

POP3 = 110; SMTP = 25

• In which layer term “Frames” is used ?

Frames are PDU of Data Link Layer

• In which layer term “Packets” is used ?

Packets are PDU of Network Layer

• In which layer term “Segments” is used ?

Segments are used at Transport Layer

• Give some example for protocols work at Application layer ?

Application Layer Protocols are  HTTP, HTTPs, Telnet, SSH, DNS, FTP, TFTP, DHCP, RIP

• What is CRC? Which layer CRC works ?

Cyclic Redundancy Check is used to detect the errors in network. It works at Data Link Layer (LLC Sub Layer).

• What is the purpose of the Data Link?

Data Link Layer is responsible for Framing, Error Detection and Physical Addressing

• Which one is reliable – TCP or UDP ?

TCP is reliable.

• What is the port number of ftp (data) and ftp?

FTP port number 20 (Data); 21 for Control

• Which layer provides logical addressing that routers will use for path determination ?

Network Layer

• Which layer specifies voltage, wire speed, and pinout cables and moves bits between devices ?

Physical

• Which layer combines bits into bytes and bytes into frames, uses MAC addressing, and provide error detection ?

Data Link Layer

• Which layer is responsible for keeping the data from different applications separate on the network ?

Session layer.

• Which layer segments and resembles data into a data stream ?

Transport layer.

• Which layer provides the physical transmission sof the data and handless error notification, network topology, and flow control ?

Data Link Layer

• Which Layer manages device addressing, tracks the location of devices on the network, and determine the best way to move data ?

Network layer.

• How Data breaks down on each layer from top to bottom ?

Encapsulation occurs in following format

• MAC address works on which layer ? What are the differences of MAC sublayer and LLC sublayer?

MAC works at DATA LINK LAYER. Media Access Control provides physical addressing while Logical Link Control provides error detection, using Ethernet trailer field frame check sequence (FCS). It is 4 bytes field. When a sending device sends a data it put the data in a mathematical algorithm and it gets a product, sending device puts the product in FCS. When a receiving device receive a data it also put the data in same mathematical algorithm and get a product. If both products are same, Frame is accepted or else discarded.

• Which layer is responsible for converting data packets from the Data Link layer into electrical signals ?

Physical Layer

• At which layer is routing implemented, enabling connections and path selection between two end systems. ?

Network Layer

• Which layer defines how data is formatted, presented, encoded,and converted for use on the network ?

Presentation Layer

• Which layer is responsible for creating, managing and terminating sessions between applications ?

Session Layer

• DNS uses which protocol? Why?

DNS uses both TCP and UDP

• Which layer is closer to the user?

From sender point of view, Application Layer is closest and from Receiver point of view Pgysical Layer is closest.

• Differentiate between forward lookup and reverse lookup in DNS?

Forward Lookup: Name to IP resolution

Reverse Lookup: IP to Name resolution;

• What is IPSec?

IPSec provides data security at the IP Packet Level.

• What is the way to establish a TCP connection?

TCP Connection is established using three-way Handshake.

• What is the difference between flow control and error control?

Error Controls the process of detecting and correcting both the bit and packet level error. While flow control is a mechanism to ensure the efficient delivery of Data. Flow control is agreeing on the minimum amount of data that a receiver can handle at a time.

Questions on OSI Model with answers-

             Questions on OSI Model with answers-

OSI Reference Model

Q1- What does the acronym OSI stand for ?
Ans- Open System Interconnection. The OSI reference model is used as a guide for teaching and understanding the way networks operate.

Q2- What are the layers of the OSI model?
Ans- Application, Presentation, Session, Transport, Network, DataLink, Physical. An easy way to remember the order of the layers is: A-ll  P-eople  S-eem  T-o  N-eed D-ata  P-rocessing .

Q3- At which layer of the OSI model is a path decision made based upon an IP address ?
Ans- Network. At the network layer, data is transported in the form of a packet.

Q4- Which of the following takes place at the presentation layer ?
Ans- Encryption. The presentation layer encrypts data to be sent across the network, and decrypts it so that it can be accessed by the application layer on the receiving end.

Q5- Which is NOT a function of the Application layer ?
Ans- Windowing. Quality of Service and user authentication are also handled by the application layer.

Q6- Which layer of the OSI model contains the LLC sublayer and the MAC sublayer ?
Ans- Data Link Layer. The LLC and MAC sublayers allow for different layer 2 protocols to be used, such as Ethernet, Token Ring and FDDI.

Q7- How can data be transported in different layers ?
Ans- Data is transported in bits at the Physical layer . A bit is simply a binary digit. Either a 0 or a 1 . Data is transported in segments at the Data Link layer . At the data link layer, user data is encapsulated in a frame.

Q8- What is the function of Network Layer ?
Ans- The network layer establishes the route between the sender and receiver across switching points, which are typically routers. The most ubiquitous example of this layer is the IP protocol in TCP/IP. IPX, SNA and AppleTalk are other examples of routable protocols, which means that they include a network address and a station address in their addressing system. This layer is also the switching function of the dial-up telephone system. If all stations are contained within a single network segment, then the routing capability in this layer is not required.

Q9- The link for a switch connection is isolated from other data traffic except for what type of messages?
Ans- Following are the types of messages-
i. Multicast Messages – Message are sent to a specific group of hosts on the network.
ii. Broadcast Messages – Message is sent to all devices connected to the LAN.

Q10- Why are the layers of the OSI model important to the network administrator?
Ans- The Layers of OSI Model are important for the network administrator in the following manner-
i. To troubleshoot network problems by verifying functionality of each layer.
ii. In many cases it requires the network administrator to isolate at what layer the network problem   occurs.

OSI model FOR CCC EXAM

                        OSI model FOR CCC EXAM 

1. OSI stands for

a) open system interconnection
b) operating system interface
c) optical service implementation
d) none of the mentioned

Answer: a

2. The OSI model has _______ layers.
a) 4
b) 5
c) 6
d) 7

Answer: d

3. TCP/IP model does not have ______ layer but OSI model have this layer.
a) session layer
b) presentation layer
c) application layer
d) both (a) and (b)

Answer: d

4. Which layer links the network support layers and user support layers
a) session layer
b) data link layer
c) transport layer
d) network layer

Answer: c
Explanation: Physical, data link and network layers are network support layers and session, presentation and application layers are user support layers.

5. Which address is used in an internet employing the TCP/IP protocols?
a) physical address and logical address
b) port address
c) specific address
d) all of the mentioned

Answer: d

6. TCP/IP model was developed _____ the OSI model.
a) prior to
b) after
c) simultaneous to
d) none of the mentioned

Answer: a

7. Which layer is responsible for process to process delivery?
a) network layer
b) transport layer
c) session layer
d) data link layer

Answer: b

8. Which address identifies a process on a host?
a) physical address
b) logical address
c) port address
d) specific address

Answer: c

9. Which layer provides the services to user?
a) application layer
b) session layer
c) presentation layer
d) none of the mentioned

10. Transmission data rate is decided by
a) network layer
b) physical layer
c) data link layer
d) transport layer

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