Network Models: Computer Networks Class Notes

Mobiprep has created last-minute notes for all topics of Computer networks to help you with the revision of concepts for your university examinations. So let’s get started with the lecture notes on Computer networks.

1. Computer Networks - Basics

2. Network Devices

3. Network Models

4. Physical Layer

5. Network Layer

6. Transport Layer and Congestion control

7. Application Layer

8. Web Security

9. Email and IP Security

Our team has curated a list of the most important questions asked in universities such as DU, DTU, VIT, SRM, IP, Pune University, Manipal University, and many more. The questions are created from the previous year's question papers of colleges and universities.

1. what is the need of using layered architecture in networking?

2. what is meant by reference model?

3. define osi network model. explain it with a diagram.

4. what are the layers in the osi model? what are their functions?

5. mention some protocols that are used in each layer of the osi model?

6. explain tcp/ip protocol suite.

7. what are the layers in the tcp/ip model? what are their functions?

8. what is the difference between osi model and tcp/ip model?

9. what is sap, pdu, and sdu?

10. what role does interface plays in a networking?

11. what are various connection oriented and connectionless services?

12. how pdu's are used in tcp/ip model?

13. explain various types of addresses used in data transfer?

14. what do you understand by physical address and how it is different from logical address?

15. what do you understand by serial and parallel transmission?

Network Models

Question 1) What is the need of using layered architecture in networking?

Answer) In layered network architecture, several communication functions are combined together to form groups called layers. The layered architecture clearly defines the functionalities of each layer and the interaction between the layers. It helps parallel execution of different parts of an application with minimal dependency. It enables the development of loosely coupled systems. Hence, layered architecture is preferred for computer networks.

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Question 2) What is meant by reference model?

Answer) In layered network architecture, several communication functions are combined together to form groups called layers. The layered architecture clearly defines the functionalities of each layer and the interaction between the layers. It helps parallel execution of different parts of an application with minimal dependency. It enables the development of loosely coupled systems. Hence, layered architecture is preferred for computer networks.

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Question 3) Define OSI network model. explain it with a diagram.

Answer) OSI – Open System Interconnection

The OSI Model is a conceptual reference framework that has seven different layers. It is the universal standard for computer networking. It provides the standards for communication between the computer systems. The whole process of communication is divided into different tasks (or layers) which are independent of each other.

OSI Network Model

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Question 4) What are the layers in the OSI model? what are their functions?

Answer) The seven different layers in the OSI model are:

1. APPLICATION LAYER

a. The application layer provides user interface.

b. Services provided by application layer:

• Network virtual terminal

• File transfer, access and management

• Mail services

• Directory services

c. Example: E-mail

2. PRESENTATION LAYER

a. This layer is concerned with the syntax and semantics of the information exchanged between two segments

b. Responsible for

i. Translation :

• Information usually consists of character strings, number and so on.

• Information must be changed into bit streams before being transmitted. This is called translation.

ii. Compression

• Compression reduces number of bits contained in the information.

iii. Encryption

• Encryption is done to ensure privacy.

3. SESSION LAYER

• This layer acts as a network dialog controller.

• This layer is responsible for dialog control and synchronization.

• The session layer is responsible for establishment, maintenance and synchronization of interaction between communication systems.

4. TRANSPORT LAYER

a. The transport layer is responsible for process to process delivery of the entire message.

b. The transport layer ensures that the whole message arrives intact and in order at the receiver.

c. Responsibilities of transport layer:

i. Service-point addressing

ii. Segmentation and reassembly

• Message is divided into segments, with each segment containing a sequence number

• This sequence number helps in reassembly of messages correctly at the destination

iii. Connection control

iv. Flow control (between source and destination)

v. Error control

5. NETWORK LAYER

a. The network layer is responsible for host to host delivery is responsible for delivery of individual packets from the source host to the destination host.

b. It ensures that each packet gets from the point of origin to its final destination.

c. Responsibilities of network layer:

• Logical addressing (IP)

• Routing

• Fragmentation

6. DATALINK LAYER

• The data link layer is responsible for hop to hop delivery

• Responsibilities of the data link layer

• Hop to hop delivery

• Framing

• Physical addressing

• Flow control

Managing the data rate at sender and receiver at the same pace.

• Error control

Detection and retransmission of lost frames is done

It adds reliability of the data to physical layer

• Access control :

When two or more devices are connected to the same link, datalink protocols determine which device has control over the link at any point of time

Methods – CSMA-CD, Aloha, Token ring or token bus

7. PHYSICAL LAYER

• The physical layer converts the data bits into signals which can be transmitted over the medium

• Physical layer is responsible for movements of individual bits from one hop (node) to the next

• Data in physical layer consists of stream of bits (0’s and 1’s). These must be encoded into signals –- electrical or optical

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Question 5) Mention some protocols that are used in each layer of the OSI model?

1. Application Layer - HTTPS, HTTPS, SMTP, FTP

2. Presentation Layer - JPEG, JPG, PNG, GIF, MIME

3. Session Layer - SQL, NFS, ASP, RPC

4. Transport Layer - TCP, UDP

5. Network Layer - IP, ICMP, IGMP

6. Data link Layer - PPP, HDLC, Ethernet

7. Physical Layer - RJ45, ST/SC, V Series

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Question 6) Explain TCP/IP protocol suite.

Answer) The TCP/IP protocol suite is a set of hierarchical protocols which is used to provide end-to-end communication between the computer systems. It is a concise version of the OSI Model. It is a more practical model when compared to the OSI model. It consists of four layers. They are:

1. Application layer

2. Transport layer

3. Internet layer

4. Host to network layer

The following diagram shows the comparison between the TCP/IP and the OSI models.

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Question 7) What are the layers in the tcp/ip model? what are their functions?

Answer) a. Application layer

1. It is the top-most layer in the TCP/IP model

2. This layer is the combination of the application layer, presentation layer and the session layer in the OSI model.

3. The functions of the application layer are:

• Process to process communication and

• Provision of user interface

b. Transport layer

1. The functions of this layer is similar to the transport layer in the OSI model

2. The functions of the transport layer are:

• Message segmentation

• Traffic control

• Error detection and correction

• Session multiplexing and

• Message reordering

c. Internet layer

1. This layer is similar to the network layer in the OSI model

2. The functions of the internet layer are:

• Logical addressing

• Traffic control and

• Routing

d. Host to network layer

1. The data link layer and the physical layers are combined to form the host-to-network layer. The functions of the host-to-network layer are:

• Frame synchronization

• Line coding

• Error detection

• Bit synchronization

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Question 8) What is the difference between OSI model and TCP/IP model?

Answer)

OSI Model	TCP/IP Model
It is based on Layered functioning.	It is based on Protocols used
Has 7 layers	Has 4 layers
It is a theoretical model	It is a practical model
Vertical approach is followed	Horizontal Approach is followed
Multiple layers cannot work in parallel	Multiple layers can work in parallel
Provides both connection-oriented and connectionless service.	Provides connectionless service only.
Protocol replacement is not easy	Protocol replacement is easy.
Has separate Presentation layer and session layer	Does not have separate presentation layer and session layer.
​The transport layer guarantees the delivery of packets	The transport layer does not guarantee the delivery of packets

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Question 9) What is SAP, PDU, and SDU?

Answer) SAP

1. SAP – Service Access Point

2. SAP is an identifier to the buffer area in the memory.

3. It identifies the protocol handler that is used to process the incoming frame.

4. It is used to define the communication between the layers in the OSI Model.

PDU

1. PDU – Protocol Data Unit

2. PDU is the unit of data of a particular layer in OSI model

3. The OSI model has seven PDUs (for seven Layers)

4. The following are the PDUs used in each layer of the OSI Model

LAYER	PDU
Application layer	Data
Presentation layer	Data
Session layer	Data
Transport layer	Segment or datagram
Network layer	Packet
Datalink layer	Frame
Physical layer	Bits

SDU

1. SDU – Service Data Unit

2. SDU is the data unit that has been passed from an OSI layer to the sub-layer (or the layer below it)

3. The lower layer encapsulates the SDU from its upper layer to form the PDU.

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Question 10) What role does interface plays in a networking?

Answer) The interface (Network Interface Card) helps the computer to connect to the network. It provides dedicated network connection to the computer. It enables both intra-network and inter-network communications. It works in the data link layer and the physical layer. Each NIC has a MAC address which is used to uniquely identify the computer system. Hence, the NIC is a very important component for communication in the computer network.

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Question 11) What are various connection oriented and connectionless services?

Answer) Connection oriented service

In connection oriented services, the connection is established prior to the beginning of communication session. A dedicated channel is established between the sender and the receiver. The following are the steps followed in connection oriented service:

• Connection establishment

• Data transfer

• Connection termination

The connection oriented service is more reliable and secure than the connectionless service.

Connection-Oriented service

Connectionless service

In connectionless service, the connection is not established prior to the beginning of the communication session. The data is routed independently from source to destination. There is no dedicated path between the source and destination. The packets of the same data do not follow the same path. Hence, they are received out of order at the destination.

The connectionless service is unreliable and insecure. But, it is much faster and cheaper than the connection oriented service.

Connectionless Service

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Question 12) How PDU'S are used in TCP/IP model?

Answer) The steps involved in encapsulation of PDUs and SDUs are given below:

• The top-most layer (Layer 4) sends the message called ‘segment’ to layer 3. Segment is the PDU of the layer 4.

• The segments are SDUs of layer 3. These are encapsulated into Packets at layer 3 (internet layer). Then, the packets are sent to layer 2. (Packets are the PDUs of layer 3).

• Layer 2 encapsulates the packets into Ethernet Frames (PDU of layer 2)

Steps involved in encapsulation of PDUs and SDUs

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Question 13) Explain various types of addresses used in data transfer?

Answer) The different types of addresses used in data transfer are:

• IP address

The IP address is a logical address which is used to locate a computer uniquely in a network. It is assigned by the Internet Service Provider. It is a 32-bit (IPv4) or 48-bit (IPv6) address. It contains both network id and host id.

• MAC address

The MAC address is the physical address which is used to uniquely identify a hardware device in the network. The MAC address is used in the data link layer. It is assigned by the manufacturer. It is a 48-bit address.

• Port address

The port address is the logical address given to each process that uses the network. The port address is assigned by the operating system. It is a 16 bit address.

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Question 14) What do you understand by physical address and how it is different from logical address?

Answer) Physical Address

The Physical address is a unique address given to the hardware. It is also called MAC address or hardware address. The MAC address is given to the NIC card of a computer system. The MAC address works at the Data Link Layer.

The MAC address is a 48 bit address which acts as a fingerprint to the hardware device. So, totally 248 devices can be addressed using the MAC address. The address is divided into 2 parts, each of 2 hexadecimal digits. The first three parts represent the manufacturer and the next three parts represent the host.

MAC Address

Physical Address	Logical Address
It is also called MAC address	It is also called IP address.
It is a 48-bit address.	It is a 32-bit (IPv4) or 48-bit (IPv6) address
It is assigned by the manufacturer	It is assigned by the Internet Service Provider
It is unique for a Hardware Device.	It is not unique for a Hardware device. But it is unique for a network.
Cannot be changed	Can be changed.
Used by the datalink layer	Used by the network layer.
A third party device cannot find the MAC address easily.	A third party device can find the Mac address easily.
Example: 02-D0-56-F2-A5-12	Example: 192.168.1.1

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Question 15) What do you understand by serial and parallel transmission?

Answer) Serial transmission

In serial transmission, only one bit is transmitted at a time. The bits are transmitted one by one. Here, the data flows in a single line. Hence, it is slow.

Serial transmission is less expensive than parallel transmission as it involves only one line for transmission.

Serial Transmission

Parallel Transmission

In parallel transmission, many bits are transmitted at a time. The bits are transmitted parallel. Here, the data flows in multiple lines. Hence, parallel transmission is much faster than serial communication.

Parallel transmission is more expensive than serial transmission.

Parallel Transmission

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