The document provides an introduction to computer networks and network models. It discusses the following key points in 3 sentences: Data communication involves the exchange of data between two devices via transmission mediums like wired or wireless connections. The five basic components of a data communication system are the message, sender, receiver, transmission medium, and protocols. Network models like OSI and TCP/IP reference models organize networks into layers, with each layer performing specified functions and following protocols to communicate between devices.

1. INTRODUCTION TO
NETWORKS AND
NETWORKS MODELS.
-By Pooja Bahirat.
Chapter No 1

2. DATA COMMUNICATION
▪ Data communications means exchange of data between two devices via some form of
transmission medium. This medium can be wired or wireless.
▪ A communication system is made up of hardware(physical equipment) and software(programs).
2.1 Components of Data Communications:
A data communication system has five basic components:
1. Message : Data to be communicated is called the message. It may include text,image,audio and
video.
2. Sender : Device that send the message is called as sender. It could be a computer, sensor , router
etc.
3. Receiver : The device which receives the message is called as receiver.
4. Transmission Medium : It is a path between the sender and receiver which carries the message.
5. Protocol : It is a set of rules that governs data communication.

3. Figure – Components of Data Communication System

4. DATA PRESENTATION:
▪ The data types must be presented in binary format so that sender can send it over the
medium and the receiver can receive and interpret it correctly.
▪ These types are:
1. Text : Text symbols are represented with a sequence of bits 0 or 1. Each sequence is
called a code, and the process is called coding. Two coding standards are Unicode &
ASCII (American Standard Code For Information Interchange).
2. Numbers: Data is often in the numeric format. Numbers are directly converted to binary
and sent in a message.
3. Images : Images are represented as a matrix of pixels (picture element). Pixels is
represented using specific number of bits. Ex : JPGE , GIF etc.
4. Audio : Audio is a continuous signal which represents sound or music. several standards
have been developed for audio representation like WAV, MP3 etc.
5. Video : video represents moving pictures or images. It requires maximum storage because
it contains several images and may have audio components.

5. COMPUTER NETWORKS.
▪ A network is a set of devices which are connected by communication link. The
communication links between nodes can be wired or wireless.
▪ A computer network is a collection of autonomous computers interconnected by a single
technology. Two computers are said to be interconnected if they are able to exchange
information.

6. 3.1 NETWORK
CRITERIA
1. Performance : Performance is the defined as the rate of transferring error free data.
The performance depends on several factors including type of communication medium,
numbers of users , network hardware and software etc.
2. Reliability : Network reliability is measured by the frequency of failure, the time takes a
link to recover from the failure, and the network’s robustness in the situation of an
extreme failure.
3. Security : Security is the protection of Hardware, Software and Data from
unauthorized access. Restricted physical access to computers, password
protection. Anti-Virus monitoring programs to defend against computer viruses are a
security measure.
3.2 Computer Network Applications:
1. Business Applications.
2. Home Applications.
3. Mobile Users.

7. 3.3 Network Types:
Computer networks are of three types:
1. Local Area Network (LAN).
2. Wide Area Network (WAN).
3. Internetwork.
1. Local Area Network (LAN) :
▪ Local Area Network is a group of computers connected to each other in a small area such
as building, office.
▪ LAN is used for connecting two or more personal computers through a communication
medium such as twisted pair, coaxial cable, etc.
▪ It is less costly as it is built with inexpensive hardware such as hubs, network adapters, and
ethernet cables.
▪ The data is transferred at an extremely faster rate in Local Area Network.
▪ Local Area Network provides higher security.

8. 2. Wide Area Network (WAN) :
▪ A WAN spans a large geographical area, often a country continent or the whole world.
Types of WAN’s :
1. Point-to-point WAN : A point-to-point WAN is a network that connects two communicating
devices through a transmission media (cable or air).
1. Switched WAN : A switched WAN is a combination of several point-to-point WAN’s that
are connected by switches.

9. 3. Internetwork :
▪ When two or more networks are connected, it forms an internetwork or internet. A collection
of interconnected networks is called an internetwork or internet.
▪ They are connected through device like routers.
The following diagram shows several LAN’s and WAN connected to form an internetwork.

10. 3.4 Switching :
▪ The mechanism by which the exchange of information between different computer networks and
network segments happens is commonly referred to as Switching.
Types of Switching :
1. Circuit Switched
2. Packet Switched.
1. Circuit Switched : A network consists of a set of switches that are connected by the physical links
commonly known as Circuit-Switched Network.
▪ Whenever one device communicates with another device then a dedicated communication path is
established between them over the network.
▪ The Circuit Switching technique is mainly used in the public telephone network for voice
communication as well as for data communication.

11. 2. Packet switched networks :
▪ Packet Switching is a technique of switching in which the message is usually divided into smaller
pieces that are known as packets.
▪ Each packet is having source and destination by which they find the route. A unique number is
given to each packet in order to identify them at the receiving end.
3. The Internet :
▪ The internet is a connection of multiple networks. The internet is an interconnection of thousands of
networks which interact using common communication protocol (TCP/IP).

12. 3.6 Accessing the Internet :
▪ The type of Internet access you choose will determine the type of modem you need.
▪ Dial-up access uses a telephone modem,
▪ DSL service uses a DSL modem, cable access uses a cable modem, and satellite service
uses a satellite adapter.
▪ when you sign a contract, which helps ensure that you have the right type of modem.
However, if you would prefer to shop for a better or less expensive modem, you can choose
to buy one separately.
4. Network Software :
▪ Network software is an equally important component of computer networks.
▪ Two important terms associated with network software are :
1. Protocols.
2. Standard.
1. Protocols : In Order to make communication successful between devices , some rules and
procedures should be agreed upon at the sending and receiving ends of the system.
Such rules and procedures are called as Protocols . Different types of protocols are used
for different types of communication.

13. ▪ A protocols has three key elements :
1. Syntax : The structure or format of data is the syntax of the data. It specifies the order in
which data is presented.
For example the data bits also have to be sent in some format so that the receiver can
correctly understand the message. The message must have parts like source address ,
destination address , message , controls information etc.
2. Semantics : Semantics refers to meaning of the data.
3. Timing : This specifies when data should be sent and how fast it should be sent so that
the receiver can receive it properly and process it.
2. Standards :
▪ Standards are the set of rules for data communication that are needed for exchange of
information among devices. It is important to follow Standards which are created by
various Standard Organization like IEEE , ISO , ANSI etc.
▪ Types of Standards :
▪ Standards are of two types :
▪ De Facto Standard.
▪ De Jure Standard.

14. i. De Facto Standard : The meaning of the work ” De Facto ” is ” By Fact ” or
“By Convention”.
These are the standard s that have not been approved by any Organization ,
but have been adopted as Standards because of it’s widespread use. Also ,
sometimes these standards are often established by Manufacturers.
▪ For example : Apple and Google are two companies which established their
own rules on their products which are different . Also they use some same
standard rules for manufacturing for their products.
ii. De Jure Standard : The meaning of the word “De Jure” is “By Law” or
“By Regulations” .
Thus , these are the standards that have been approved by officially
recognized body like ANSI , ISO , IEEE etc. These are the standard which are
important to follow if it is required or needed.
▪ For example : All the data communication standard protocols like SMTP , TCP , IP
, UDP etc. are important to follow the same when we needed them.

15. 4.1 PROTOCOL HIERARCHIES:
▪ Most networks are organized as a stack of layers, one on the top of another. The number of
layers and their names vary from network to network. Each layer has a specified
function and follows to specified protocols. Thus we obtain a stack of protocols.
▪ The following figure illustrates a four-layer network −

16. ▪ The above figure represents communication between Device A and Device B. The data stream
from one device to the other is not sent directly but has to pass through a number of layers.
▪ The dotted arrows show virtual communication between peer layers, while the solid arrows
represent the physical communications between the adjacent layers.
▪ Let us consider a situation where Device A wants to send a message to Device B. As soon as a
data stream reaches a layer, it performs some specified functions on it and passes it to the
layer below. This continues until the data stream reaches the lowest layer.
▪ Advantages :
1. The layers generally reduce complexity of communication between networks.
2. It does not require overall knowledge and understanding of network.
4.2 Design Issues for the layers :
1. Addressing : Every layer needs a mechanism to identify senders and receivers. To identify
the machine some forms of addressing is needed.
2. Types of data transfer : data transfer is simplex ; in other cases it may be half duplex or full
duplex.
3. Error control : Its an important issue because physical communication circuits are not
perfect. Many error detecting and error correcting codes are available. Both sending and
receiving ends are must agree to use any one code.

17. 4. Numbering : In some communication channels, the message are not delivered in the same order
that they are sent. So , some provision have to made so that the receiver can put them in order.
5. Flow Control : Flow control ensures that the sender and receiver are sending and receiving at the
same speed. Therefore, some form of feedback from the receiver is needed.
6. Multiplexing and de-multiplexing : If the data has to be transmitted on transmission media
separately, it is inconvenient or expensive to setup separate connection for each pair of
communicating processes. So, multiplexing is needed in the physical layer at sender end and de-
multiplexing is need at the receiver end.
7. Routing: When there are multiple paths between source and destination, only one route must be
chosen. This decision is made on the basis of several routing algorithms, which chooses optimized
route to the destination.
4.3 Types of services :
There are two types of services offered by layers. These are : Connection Oriented Services and
Connectionless Services.
Some application require connection oriented service(email sending using SMTP , file transfer
using FTP , web pages access using HTTP etc.) while other use connectionless services.

18. 1. Connection Oriented Services :
✔There is a sequence of operation to be followed by the users of connection oriented service.
These are:
✔Connection is established.
✔Information is sent.
✔Connection is released.
✔In connection oriented service we have to establish a connection before starting the
communication. When connection is established, we send the message or the information and
then we release the connection.
✔Connection oriented service is more reliable than connectionless service. We can send
the message in connection oriented service if there is an error at the receivers end.
Example of connection oriented is TCP (Transmission Control Protocol) protocol.
Advantages of connection oriented services :
1. Provides guarantee and reliable delivery of data.
2. All data units arrive in the same order as they are sent.
3. Suitable for large volume of data transfer , all packet need not have source and destinations
address.

19. Disadvantages of connection oriented services :
✔This allocation of resources is mandatory before communication.
✔The speed of connection is slower.
✔Transmission protocol is very complex.
2. Connection Less Services :
▪ It is similar to the postal services, as it carries the full address where the message (letter) is to
be carried. Each message is routed independently from source to destination. The order of
messages sent can be different from the order received.
▪ In connectionless the data is transferred in one direction from source to destination without
checking the destination is still there or not or if it prepared to accept the message.
Authentication is not needed in this. An example of a Connectionless service is UDP (User
Datagram Protocol) protocol.
Advantages :
1. Fast delivery of data.
2. Transmission protocol is very simple.
Disadvantages :
1. Data packets may arrive out of order.
2. Each packets requires the source and destination address.

20. 5. Networks Models :
A network model is a conceptual structure which allows better understanding of how
networking tasks are performed.
The two important network models are:
1. OSI Reference Model (Open System Interconnection).
2. TCP/IP Reference Model (Transmission Control Protocol / Internet Protocol).
5.1 OSI Reference Model :
▪ OSI stands for Open System Interconnect. It is an open standard for establishing
communication between systems. We will go over this model in more depth.
▪ The OSI Model is divided into seven distinct layers which are shown below :

22. 1. Physical layer :
▪ The main functionality of the physical layer is to transmit the individual bits from one node
to another node.
▪ It is the lowest layer of the OSI model.
▪ It establishes, maintains and deactivates the physical connection.
▪ It specifies the mechanical, electrical and procedural network interface specifications.
Functions of a Physical layer:
▪ Line Configuration: It defines the way how two or more devices can be connected physically.
▪ Data Transmission: It defines the transmission mode whether it is simplex, half-duplex or full-
duplex mode between the two devices on the network.
▪ Topology: It defines the way how network devices are arranged.
▪ Signals: It determines the type of the signal used for transmitting the information.

23. 2. Data Link Layer :
▪ This layer is responsible for the error-free transfer of data frames. It defines the format of the
data on the network.
▪ It provides a reliable and efficient communication between two or more devices. It is mainly
responsible for the unique identification of each device that resides on a local network.
i. Logical Link Control Layer (LLC): which establishes and maintains link between
communicating services.
ii. Media Access Control Layer (MAC ) : which controls the way multiple devices share the
same communication channel.
The main functions of the data link layers are :
1. Framing : The stream of bits is divided into logical units called frames.
2. Error control : Provides mechanisms to detect or correct errors.
3. Flow control : Ensures that a fast sender does not overcome a slow receiver.
4. Physical addressing : Identifies a machine in the network using its physical address.
5. Multiple access control : When two or more devices are connected to the same
communication channel, then the data link layer protocols are used to determine which
device has control over the link at a given time.

24. 3. Network Layer :
▪ The network layer works for the transmission of data from one host to the other
located in different networks. It also takes care of packet routing i.e. selection of the
shortest path to transmit the packet, from the number of routes available. The
sender & receiver’s IP addresses are placed in the header by the network layer.
The functions of the Network layer are :
1. Routing: The network layer protocols determine which route is suitable from source
to destination. This function of the network layer is known as routing.
2. Logical Addressing: The sender & receiver’s IP addresses are placed in the header
by the network layer. Such an address distinguishes each device uniquely and
universally.
▪ * Segment in Network layer is referred to as Packet.
4. Transport Layer :
▪ This is an end-to-end layer, which ensures process-to-process delivery of data from
the source application to the destination application.
▪ The main functions of the transport layer are :
1. Port addressing : the transport layer has to identify the application in the host
machine for whom the data is indented. It does this by an address called Port
address.

25. 2. Segmentation and reassembly: A large message may be split up if needed. This layers
takes care of sequencing and reassembly at the destination.
3. Flow and Error control : Flow control is meant only for the transmission of data from
sender to receiver. Error control is meant for the transmission of error free data from sender to
receiver.
5. Session Layer :
▪ The Session Layer is the 5th layer in the Open System Interconnection (OSI) model. This layer
allows users on different machines to establish active communications sessions between them.
It is responsible for establishing, maintaining, synchronizing, terminating sessions between end-
user applications.
▪ The main functions of the session layer are as follows −
1. dialog controller : It allows the systems to communicate in either half-duplex or full-duplex
mode of communication.
2. Synchronization : It adds synchronization points or checkpoints in data streams for long
communications. This ensures that data streams up to the checkpoints are successfully
received and acknowledged. In case of any failures, only the streams after the checkpoints
have to be re-transmitted.

26. 6. Presentation Layer :
▪ The presentation layer (Layer 6) ensures that the message is presented to the upper layer in a
standardized format. It deals with the syntax and the semantics of the messages.
The main functions of the presentation layer are as follows −
▪ It is responsible for data encryption and decryption of sensitive data before they are transmitted
over common channels.
▪ It is also responsible for data compression. Data compression is done at the source to reduce
the number of bits to be transmitted. It reduces the storage space and increases the file transfer
rate. It is particularly useful for transmission of large multimedia files.
7. Application Layer :
▪ This layer provides a means for user applications to access the network. It contains a variety of
services commonly needed by users like transfer, e-mail , remote terminal access , access to
world wide web, etc.
Layers 1 to 3 are called Lower layers.
Layers 4 to 7 are called Upper layers.

27. 5.2 TCP/IP Reference Model :
▪ TCP/IP Reference Model is a four-layered suite of communication protocols. It was developed
by the DoD (Department of Defence) in the 1960s.
▪ It is named after the two main protocols that are used in the model, namely, TCP and IP. TCP
stands for Transmission Control Protocol and IP stands for Internet Protocol.
▪ The four layers in the TCP/IP protocol suite are −
▪ Host-to- Network Layer −It is the lowest layer that is concerned with the physical transmission
of data. TCP/IP does not specifically define any protocol here but supports all the standard
protocols.
▪ Internet Layer −It defines the protocols for logical transmission of data over the network. The
main protocol in this layer is Internet Protocol (IP) and it is supported by the protocols ICMP,
IGMP, RARP, and ARP.
▪ Transport Layer − It is responsible for error-free end-to-end delivery of data. The protocols
defined here are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
▪ Application Layer − This is the topmost layer and defines the interface of host programs with
the transport layer services. This layer includes all high-level protocols like Telnet, DNS, HTTP,
FTP, SMTP, etc.
▪ The following diagram shows the layers and the protocols in each of the layers −

29. 5.3 COMPARISON BETWEEN OSI AND TCP/IP MODEL.