Unit I : Introduction of Computer

Network
Introduction of Computer Network,
Models, OSI Model, TCP-IP Model
Evolution of computer networks
 DATA COMMUNICATIONS
• When we communicate, we are sharing
information. This sharing can be local or
remote. The term telecommunication,
which includes telephony, telegraph, and
television, means communication at a
distance.
• Data communications are the
exchange of data between two devices via
some form of transmission media.
 Five components of data
communication
A data communications system has five components
Data Representation: Information today comes in
different forms such as text, numbers, images, audio,
and video.
Data Flow: Communication between two devices can
be simplex, half-duplex, or full-duplex
 NETWORKS

• A network is the interconnection of a set of

devices capable of communication. In this
definition, a device can be a host such as a
large computer, desktop, laptop, workstation,
cellular phone, or security system. A device in
this definition can also be a connecting device
such as a router a switch, a modem that
changes the form of data, and so on.
 Computer Network Models
Layered Architecture
• The main aim of the layered architecture is to divide the design into small pieces.
• Each lower layer adds its services to the higher layer to provide a full set of services to manage
communications and run the applications.
• It provides modularity and clear interfaces, i.e., provides interaction between subsystems.
• It ensures the independence between layers by providing the services from lower to higher layer
without defining how the services are implemented. Therefore, any modification in a layer will not
affect the other layers.
• The number of layers, functions, contents of each layer will vary from network to network.
However, the purpose of each layer is to provide the service from lower to a higher layer and hiding
the details from the layers of how the services are implemented.
• The basic elements of layered architecture are services, protocols, and interfaces.
– Service: It is a set of actions that a layer provides to the higher layer.
– Protocol: It defines a set of rules that a layer uses to exchange the information with peer entity. These rules
mainly concern about both the contents and order of the messages used.
– Interface: It is a way through which the message is transferred from one layer to another layer.
• In a layer n architecture, layer n on one machine will have a communication with the layer n on
another machine and the rules used in a conversation are known as a layer-n protocol
 Computer Network Models
• In case of layered architecture, no data is transferred
from layer n of one machine to layer n of another
machine. Instead, each layer passes the data to the
layer immediately just below it, until the lowest layer
is reached.
• Below layer 1 is the physical medium through which
the actual communication takes place.
• In a layered architecture, unmanageable tasks are
divided into several small and manageable tasks.
• The data is passed from the upper layer to lower layer
through an interface. A Layered architecture provides
a clean-cut interface so that minimum information is
shared among different layers. It also ensures that the
implementation of one layer can be easily replaced by
another implementation.
• A set of layers and protocols is known as network
architecture.
 Why do we require Layered
architecture?
• Divide-and-conquer approach: Divide-and-conquer
approach makes a design process in such a way that the
unmanageable tasks are divided into small and manageable
tasks. In short, we can say that this approach reduces the
complexity of the design.
• Modularity: Layered architecture is more modular.
Modularity provides the independence of layers, which is
easier to understand and implement.
• Easy to modify: It ensures the independence of layers so
that implementation in one layer can be changed without
affecting other layers.
• Easy to test: Each layer of the layered architecture can be
analyzed and tested individually.
 What is OSI Model?

• The OSI Model is a logical and conceptual

model that defines network communication
used by systems open to interconnection and
communication with other systems. The Open
System Interconnection (OSI Model) also
defines a logical network and effectively
describes computer packet transfer by using
various layers of protocols.
 Characteristics of OSI Model

• A layer should only be created where the definite levels of

abstraction are needed.
• The function of each layer should be selected as per the
internationally standardized protocols.
• The number of layers should be large so that separate
functions should not be put in the same layer. At the same
time, it should be small enough so that architecture doesn't
become very complicated.
• In the OSI model, each layer relies on the next lower layer
to perform primitive functions. Every level should able to
provide services to the next higher layer
• Changes made in one layer should not need changes in
other lavers.
 Why of OSI Model?

• Helps you to understand communication over

a network
• Troubleshooting is easier by separating
functions into different network layers.
• Helps you to understand new technologies as
they are developed.
• Allows you to compare primary functional
relationships on various network layers.
 History of OSI Model
• Here are essential landmarks from the history of OSI
model:
• In the late 1970s, the ISO conducted a program to
develop general standards and methods of networking.
• In 1973, an Experimental Packet Switched System in
the UK identified the requirement for defining the
higher-level protocols.
• In the year 1983, OSI model was initially intended to be
a detailed specification of actual interfaces.
• In 1984, the OSI architecture was formally adopted by
ISO as an international standard
OSI Model
OSI Model
 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.

* Hub, Repeater, Modem, Cables are Physical Layer devices.

** Network Layer, Data Link Layer and Physical Layer are also
known as Lower Layers or Hardware Layers.
 Physical layer
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.
• Bit synchronization: The physical layer provides the synchronization
of the bits by providing a clock. This clock controls both sender and
receiver thus providing synchronization at bit level.
• Bit rate control: The Physical layer also defines the transmission
rate i.e. the number of bits sent per second.
Physical layer
 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.
• It contains two sub-layers:
– Logical Link Control Layer
• It is responsible for transferring the packets to the Network layer of the receiver that is
receiving.
• It identifies the address of the network layer protocol from the header.
• It also provides flow control.
– Media Access Control Layer
• A Media access control layer is a link between the Logical Link Control layer and the
network's physical layer.
• It is used for transferring the packets over the network.
 Data-Link Layer
Functions of the Data-link layer
• Framing: The data link layer translates the physical's raw bit stream into packets known as Frames. The
Data link layer adds the header and trailer to the frame. The header which is added to the frame contains
the hardware destination and source address.

• Physical Addressing: The Data link layer adds a header to the frame that contains a destination address.
The frame is transmitted to the destination address mentioned in the header.
• Flow Control: Flow control is the main functionality of the Data-link layer. It is the technique through
which the constant data rate is maintained on both the sides so that no data get corrupted. It ensures that
the transmitting station such as a server with higher processing speed does not exceed the receiving
station, with lower processing speed.
• Error Control: Error control is achieved by adding a calculated value CRC (Cyclic Redundancy Check) that is
placed to the Data link layer's trailer which is added to the message frame before it is sent to the physical
layer. If any error seems to occurr, then the receiver sends the acknowledgment for the retransmission of
the corrupted frames.
• 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.
• * Packet in Data Link layer is referred as Frame.
** Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines.
*** Switch & Bridge are Data Link Layer devices.
Data-Link Layer
 Network Layer
• It is a layer 3 that manages device addressing, tracks the location of
devices on the network.
• It determines the best path to move data from source to the
destination based on the network conditions, the priority of
service, and other factors.
• The Data link layer is responsible for routing and forwarding the
packets.
• Routers are the layer 3 devices, they are specified in this layer and
used to provide the routing services within an internetwork.
• The protocols used to route the network traffic are known as
Network layer protocols. Examples of protocols are IP and Ipv6.

* Segment in Network layer is referred as Packet.

 Network Layer
Functions of Network Layer:
• Internetworking: An internetworking is the main responsibility of
the network layer. It provides a logical connection between
different devices.
• Addressing: A Network layer adds the source and destination
address to the header of the frame. Addressing is used to identify
the device on the internet.
• Routing: Routing is the major component of the network layer, and
it determines the best optimal path out of the multiple paths from
source to the destination.
• Packetizing: A Network Layer receives the packets from the upper
layer and converts them into packets. This process is known as
Packetizing. It is achieved by internet protocol (IP).
Network Layer
 Transport Layer
• The Transport layer is a Layer 4 ensures that messages are transmitted in the order
in which they are sent and there is no duplication of data.
• The main responsibility of the transport layer is to transfer the data completely.
• It receives the data from the upper layer and converts them into smaller units
known as segments.
• This layer can be termed as an end-to-end layer as it provides a point-to-point
connection between source and destination to deliver the data reliably.
• The two protocols used in this layer are:
• Transmission Control Protocol
– It is a standard protocol that allows the systems to communicate over the internet.
– It establishes and maintains a connection between hosts.
– When data is sent over the TCP connection, then the TCP protocol divides the data into
smaller units known as segments. Each segment travels over the internet using multiple
routes, and they arrive in different orders at the destination. The transmission control protocol
reorders the packets in the correct order at the receiving end.
• User Datagram Protocol
– User Datagram Protocol is a transport layer protocol.
– It is an unreliable transport protocol as in this case receiver does not send any
acknowledgment when the packet is received, the sender does not wait for any
acknowledgment. Therefore, this makes a protocol unreliable.
 Transport Layer
Functions of Transport Layer:
• Service-point addressing: Computers run several programs simultaneously due to this
reason, the transmission of data from source to the destination not only from one computer
to another computer but also from one process to another process. The transport layer adds
the header that contains the address known as a service-point address or port address. The
responsibility of the network layer is to transmit the data from one computer to another
computer and the responsibility of the transport layer is to transmit the message to the
correct process.
• Segmentation and reassembly: When the transport layer receives the message from the
upper layer, it divides the message into multiple segments, and each segment is assigned
with a sequence number that uniquely identifies each segment. When the message has
arrived at the destination, then the transport layer reassembles the message based on their
sequence numbers.
• Connection control: Transport layer provides two services Connection-oriented service and
connectionless service. A connectionless service treats each segment as an individual packet,
and they all travel in different routes to reach the destination. A connection-oriented service
makes a connection with the transport layer at the destination machine before delivering the
packets. In connection-oriented service, all the packets travel in the single route.
• Flow control: The transport layer also responsible for flow control but it is performed end-to-
end rather than across a single link.
• Error control: The transport layer is also responsible for Error control. Error control is
performed end-to-end rather than across the single link. The sender transport layer ensures
that message reach at the destination without any error.
Transport Layer
 Session Layer
• It is a layer 3 in the OSI model.
• The Session layer is used to establish, maintain and synchronizes
the interaction between communicating devices.

Functions of Session layer:

• Dialog control: Session layer acts as a dialog controller that creates
a dialog between two processes or we can say that it allows the
communication between two processes which can be either half-
duplex or full-duplex.
• Synchronization: Session layer adds some checkpoints when
transmitting the data in a sequence. If some error occurs in the
middle of the transmission of data, then the transmission will take
place again from the checkpoint. This process is known as
Synchronization and recovery.
Session Layer
 Presentation Layer
• A Presentation layer is mainly concerned with the syntax and semantics of the information
exchanged between the two systems.
• It acts as a data translator for a network.
• This layer is a part of the operating system that converts the data from one presentation format to
another format.
• The Presentation layer is also known as the syntax layer.

Functions of Presentation layer:

• Translation: The processes in two systems exchange the information in the form of character
strings, numbers and so on. Different computers use different encoding methods, the presentation
layer handles the interoperability between the different encoding methods. It converts the data
from sender-dependent format into a common format and changes the common format into
receiver-dependent format at the receiving end.
• Encryption: Encryption is needed to maintain privacy. Encryption is a process of converting the
sender-transmitted information into another form and sends the resulting message over the
network.
• Compression: Data compression is a process of compressing the data, i.e., it reduces the number of
bits to be transmitted. Data compression is very important in multimedia such as text, audio, video.
Presentation Layer
 Application Layer
• An application layer serves as a window for users and application
processes to access network service.
• It handles issues such as network transparency, resource allocation, etc.
• An application layer is not an application, but it performs the application
layer functions.
• This layer provides the network services to the end-users.

Functions of Application layer:

• File transfer, access, and management (FTAM): An application layer allows
a user to access the files in a remote computer, to retrieve the files from a
computer and to manage the files in a remote computer.
• Mail services: An application layer provides the facility for email
forwarding and storage.
• Directory services: An application provides the distributed database
sources and is used to provide that global information about various
objects.
Application Layer
 TCP/IP model
• The TCP/IP model was developed prior to the OSI model.
• The TCP/IP model is not exactly similar to the OSI model.
• The TCP/IP model consists of five layers: the application
layer, transport layer, network layer, data link layer and
physical layer.
• The first four layers provide physical standards, network
interface, internetworking, and transport functions that
correspond to the first four layers of the OSI model and
these four layers are represented in TCP/IP model by a
single layer called the application layer.
• TCP/IP is a hierarchical protocol made up of interactive
modules, and each of them provides specific functionality.
Functions of TCP/IP layers
 Network Access Layer
• A network layer is the lowest layer of the TCP/IP model.
• A network layer is the combination of the Physical layer and
Data Link layer defined in the OSI reference model.
• It defines how the data should be sent physically through
the network.
• This layer is mainly responsible for the transmission of the
data between two devices on the same network.
• The functions carried out by this layer are encapsulating
the IP datagram into frames transmitted by the network
and mapping of IP addresses into physical addresses.
• The protocols used by this layer are ethernet, token ring,
FDDI, X.25, frame relay.
 Internet Layer
• An internet layer is the second layer of the TCP/IP model.
• An internet layer is also known as the network layer.
• The main responsibility of the internet layer is to send the packets from any network, and they arrive at
the destination irrespective of the route they take.
• Following are the protocols used in this layer are:
• IP Protocol: IP protocol is used in this layer, and it is the most significant part of the entire TCP/IP suite.
• Following are the responsibilities of this protocol:
• IP Addressing: This protocol implements logical host addresses known as IP addresses. The IP addresses
are used by the internet and higher layers to identify the device and to provide internetwork routing.
• Host-to-host communication: It determines the path through which the data is to be transmitted.
• Data Encapsulation and Formatting: An IP protocol accepts the data from the transport layer protocol. An
IP protocol ensures that the data is sent and received securely, it encapsulates the data into message
known as IP datagram.
• Fragmentation and Reassembly: The limit imposed on the size of the IP datagram by data link layer
protocol is known as Maximum Transmission unit (MTU). If the size of IP datagram is greater than the MTU
unit, then the IP protocol splits the datagram into smaller units so that they can travel over the local
network. Fragmentation can be done by the sender or intermediate router. At the receiver side, all the
fragments are reassembled to form an original message.
• Routing: When IP datagram is sent over the same local network such as LAN, MAN, WAN, it is known as
direct delivery. When source and destination are on the distant network, then the IP datagram is sent
indirectly. This can be accomplished by routing the IP datagram through various devices such as routers.
 Transport Layer
• The transport layer is responsible for the reliability, flow control, and correction of
data which is being sent over the network.
• The two protocols used in the transport layer are User Datagram protocol and
Transmission control protocol.
• User Datagram Protocol (UDP)
– It provides connectionless service and end-to-end delivery of transmission.
– It is an unreliable protocol as it discovers the errors but not specify the error.
– User Datagram Protocol discovers the error, and ICMP protocol reports the error to the sender
that user datagram has been damaged.
– UDP consists of the following fields:
Source port address: The source port address is the address of the application program that
has created the message.
Destination port address: The destination port address is the address of the application
program that receives the message.
Total length: It defines the total number of bytes of the user datagram in bytes.
Checksum: The checksum is a 16-bit field used in error detection.
– UDP does not specify which packet is lost. UDP contains only checksum; it does not contain
any ID of a data segment.
 Transport Layer
• Transmission Control Protocol (TCP)
– It provides a full transport layer services to applications.
– It creates a virtual circuit between the sender and receiver, and
it is active for the duration of the transmission.
– TCP is a reliable protocol as it detects the error and retransmits
the damaged frames. Therefore, it ensures all the segments
must be received and acknowledged before the transmission is
considered to be completed and a virtual circuit is discarded.
– At the sending end, TCP divides the whole message into smaller
units known as segment, and each segment contains a sequence
number which is required for reordering the frames to form an
original message.
– At the receiving end, TCP collects all the segments and reorders
them based on sequence numbers.
 Application Layer
• An application layer is the topmost layer in the TCP/IP model.
• It is responsible for handling high-level protocols, issues of representation.
• This layer allows the user to interact with the application.
• When one application layer protocol wants to communicate with another application layer, it forwards its data to
the transport layer.
• There is an ambiguity occurs in the application layer. Every application cannot be placed inside the application
layer except those who interact with the communication system. For example: text editor cannot be considered in
application layer while web browser using HTTP protocol to interact with the network where HTTP protocol is an
application layer protocol.
• Following are the main protocols used in the application layer:
• HTTP: HTTP stands for Hypertext transfer protocol. This protocol allows us to access the data over the world wide
web. It transfers the data in the form of plain text, audio, video. It is known as a Hypertext transfer protocol as it
has the efficiency to use in a hypertext environment where there are rapid jumps from one document to another.
• SNMP: SNMP stands for Simple Network Management Protocol. It is a framework used for managing the devices
on the internet by using the TCP/IP protocol suite.
• SMTP: SMTP stands for Simple mail transfer protocol. The TCP/IP protocol that supports the e-mail is known as a
Simple mail transfer protocol. This protocol is used to send the data to another e-mail address.
• DNS: DNS stands for Domain Name System. An IP address is used to identify the connection of a host to the
internet uniquely. But, people prefer to use the names instead of addresses. Therefore, the system that maps the
name to the address is known as Domain Name System.
• TELNET: It is an abbreviation for Terminal Network. It establishes the connection between the local computer and
remote computer in such a way that the local terminal appears to be a terminal at the remote system.
• FTP: FTP stands for File Transfer Protocol. FTP is a standard internet protocol used for transmitting the files from
one computer to another computer.