DEPARTMENT OF ARTIFICIAL INTEELLIGENCE & DATA SCIENCE (AI &

DS)
Course:B.Tech Sem-VI
Subject: CN Subject Code: BCS603
 [Computer Network Notes-Unit-1]

Data Communication Transmission medium could be guided (with wires) or

Data communications refers to the transmission of this unguided (without wires), for example, twisted pair cable,
digital data between two or more computers and a fiber optic cable, radio waves, microwaves, etc.
computer network or data network is a 5. Set of rules (Protocol) :
telecommunications network that allows computers to To govern data communications, various sets of
exchange data. The physical connection between rules had been already designed by the designers
networked computing devices is established using either of the communication systems, which represent a
cable media or wireless media. The best-known computer kind of agreement between communicating
network is the Internet. devices.
Data Communication System Components:
There are mainly five components of a data A typical example of a data communication system
communication system: is sending an e-mail.
1. Message
CHARACTERISTICS:
2. Sender
 Delivery: Data must be delivered to the correct
3. Receiver
destination and must be received by the intended
4. Transmission Medium
receiver only.
5. Set of rules (Protocol)
 Accuracy: Data delivered must be accurate. Any
All above mentioned elements are described below: alteration to the data during transmission renders it
useless.
 Timeliness: Data must be delivered within the ideal
time else it is useless. In case of audio and video, the
data must be delivered as they are produced; in the
same ordered they are produced without significant
delay. This kind of delivery is called real-time
transmission.
 Jitter: It is the variation in the arrival time of audio
or video packets or unevenly delays in the delivery of
the audio or video packets.

DATA REPRESENTATION: Information is

Figure – Components of Data Communication represented in various forms such as text, numbers,
System images, audio and video.
1. Message :
 Text: In data communication, it is represented as a bit
A message could be in any form, it may be in pattern, a sequence of bits (0s and 1s). Characters can
form of a text file, an audio file, a video file, etc. be represented in Unicode by using the American
2. Sender : Standard Code for Information Interchange (ASCII).
It is simple a device that sends data message. The  Numbers: They are also represented by bit patterns.
device could be in form of a computer, mobile, To simplify mathematical operations the number is
telephone, laptop, video camera, or a workstation, directly converted to a binary number.
etc.  Images: They are also represented by bit patterns. In
3. Receiver : its simplest form, an image is composed of a matrix
It is destination where finally message sent by of pixels, where each pixel is a small dot. Resolution
source has arrived. Same as sender, receiver can determines the size of the pixel.
 Audio: It refers to the recording or broadcasting of
also be in form of a computer, telephone mobile,
sound or music. Audio is by nature different from
workstation, etc.
text, numbers, and images. It is not discrete, but
4. Transmission Medium: continuous.
In entire process of data communication,

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 [Computer Network Notes-Unit-1]

 Video: It refers to the recording or broadcasting of a

picture or a movie. financial and communications services currently
provided by the telephone companies and
computer networks.
Applications of Computer Networks?
 Information Services:- Network information

 Marketing and sales: – Computer networks services include bulletin boards and data banks.

are widely used in both marketing sales firms. A World Wide Web site offering the technical

These are used by marketing professionals to specifications for a new product is an

collect, exchange, and analyzes data relating to information service.

customer requirements and product  Electronic Messaging:– Electronic mail (e-

development cycles. mail) is the most widely used network
application.
 Manufacturing: – Now days, computer  Electronic Data Interchange (EDI):– EDI
networks are used in a several aspects of permits business information to be transferred
manufacturing, including the manufacturing without using paper.
process itself. Two applications which use a  Directory services: – By using directory
network to provide necessary services are services, it is possible to store the last of files in
computer-assisted manufacturing (CAM) and a central location to speed worldwide search
computer –assisted designing (CAD) both of operations.
which permit multiple users to work on a  Cellular Telephone: – In the past, two parties
project simultaneously. desiring to use the services of the telephone
 Financial Services: – In Present, Financial company had to be linked by a fixed physical
services are completely dependent on computer connection. But, in present cellular network
networks. Main applications are credit history make it possible to maintain wireless phone
searches, foreign exchange and investment connections even while travelling over large
services, and Electronic Funds Transfer (EFT) distances.
Internet
that permits a user to transfer money without Service Provider (ISP) –

going into bank. ISP is a company which provides internet connection to end
 Teleconferencing: – With The help of user, but there are basically three levels of ISP. There are
3 levels of Internet Service Provider (ISP): Tier-1 ISP,
teleconferencing conferences are possible to
Tier-2 ISP, and Tier-3 ISP.
occur without the participants being in the same
place. Applications include simple text
conferencing, voice conferencing, and video
conferencing.
 Cable Television:-Future Services provided by
cable television network can include video on
request, as well as the same information,

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 [Computer Network Notes-Unit-1]
o

 Tier-1 ISP:

These ISPs are at the top of the hierarchy and they

have a global reach they do not pay for any internet
traffic through their network instead lower-tier ISPs
have to pay a cost for passing their traffic from one
geo location to another which is not under the reach
of those ISPs.
Examples: BSNL, Hathway etc
 Tier-2 ISP:

These ISPs are service provider who connects

between tier 1 and tier 3 ISPs. They have regional or
country reach and they behave just like Tier-1 ISP for
Tier-3 ISPs.
Examples: Vodafone,
 Tier-3 ISP:

These ISPs are closest to the end users and helps

them to connect to the internet by charging some
money. These ISPs work on purchasing model. These
ISPs have to pay some cost to Tier-2 ISPs based on
traffic generated.
Examples:
Examples of Tier-3 ISPs: Tikona

OSI Model
o OSI stands for Open System Interconnection is a
reference model that describes how information from
a software application in one computer moves through a
physical medium to the software application in another
computer.
o OSI consists of seven layers, and each layer performs a
particular network function.
o OSI model was developed by the International
Organization for Standardization (ISO) in 1984, and it is
now considered as an architectural model for the inter-
computer communications.
o OSI model divides the whole task into seven smaller and
manageable tasks. Each layer is assigned a particular task.

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Each layer is self-contained, so that task assigned to each layer can be
performed independently.
Characteristics of OSI Model:

Functions of the OSI Layers

There are the seven OSI layers. Each layer has different functions.
Lists of seven layers are given below:
1. Physical Layer
2. Data-Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Physical layer

o The main functionality of the physical layer is to transmit

the individual bits from one node to another node.
o It is the lowest layer of the OSI model.

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o It establishes, maintains and deactivates the physical o It also provides flow control.
connection. o Media Access Control Layer
o It specifies the mechanical, electrical and procedural o A Media access control layer is a link
network interface specifications. between the Logical Link Control layer
and the network's physical layer.
o It is used for transferring the packets over
the network.

Functions of the Data-link layer

o 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.

Functions of a Physical layer: o Physical Addressing: The Data link layer adds a header
o Line Configuration: It defines the way how two or more to the frame that contains a destination address. The frame
devices can be connected physically. is transmitted to the destination address mentioned in the
o Data Transmission: It defines the transmission mode header.
whether it is simplex, half-duplex or full-duplex mode o Flow Control: Flow control is the main functionality of
between the two devices on the network. the Data-link layer. It is the technique through which the
o Topology: It defines the way how network devices are constant data rate is maintained on both the sides so that
arranged. no data get corrupted. It ensures that the transmitting
o Signals: It determines the type of the signal used for station such as a server with higher processing speed does
transmitting the information. not exceed the receiving station, with lower processing
speed.
o Error Control: Error control is achieved by adding a
Data-Link Layer 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 occur, then the receiver sends the
acknowledgment for the retransmission of the corrupted
frames.
o 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.

Network Layer

o This layer is responsible for the error-free transfer of data

frames.
o It defines the format of the data on the network.
o It provides a reliable and efficient communication
between two or more devices.
o It is mainly responsible for the unique identification of
each device that resides on a local network.
o It contains two sub-layers: o It is a layer 3 that manages device addressing, tracks the
o Logical Link Control Layer location of devices on the network.
o It is responsible for transferring the o It determines the best path to move data from source to the
packets to the Network layer of the destination based on the network conditions, the priority
receiver that is receiving. of service, and other factors.
o It identifies the address of the network o The Data link layer is responsible for routing and
layer protocol from the header. forwarding the packets.

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o Routers are the layer 3 devices, they are specified in this

layer and used to provide the routing services within an
internetwork.
o The protocols used to route the network traffic are known
as Network layer protocols. Examples of protocols are IP
and Ipv6.
Functions of Network Layer:
o Internetworking: An internetworking is the main
responsibility of the network layer. It provides a logical
connection between different devices.
o 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.
o 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.
o 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).

Transport Layer

o 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.
o The main responsibility of the transport layer is to transfer
the data completely.

o It receives the data from the upper layer and converts them
into smaller units known as segments.
o 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:
o Transmission Control Protocol
o It is a standard protocol that allows the systems to
communicate over the internet.
o It establishes and maintains a connection between
hosts.
o When data is sent over the TCP connection, then
the TCP protocol divides the data into smaller
units known as segments. Each segment travels

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over the internet using multiple routes, and they arrive in different
orders at the destination.
o User Datagram Protocol
o User Datagram Protocol is a transport layer
protocol.
o 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.
Functions of Transport Layer:
o 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.
o 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.
o 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.
o Flow control: The transport layer also responsible for
flow control but it is performed end-to-end rather than
across a single link.
o 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.

Session Layer
o It is a layer 3 in the OSI model.
o The Session layer is used to establish, maintain and
synchronizes the interaction between communicating
devices.

Functions of Session layer:

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o Dialog control: Session layer acts as a dialog controller o An application layer serves as a window for users and
that creates a dialog between two processes or we can say application processes to access network service.
that it allows the communication between two processes o It handles issues such as network transparency, resource
which can be either half-duplex or full-duplex. allocation, etc.
o Synchronization: Session layer adds some checkpoints o An application layer is not an application, but it performs
when transmitting the data in a sequence. the application layer functions.
o This layer provides the network services to the end-users.
Presentation Layer Functions of Application layer:
o 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.
o Mail services: An application layer provides the facility
for email forwarding and storage.
o Directory services: An application provides the distributed
database sources and is used to provide that global
information about various objects.

Organization of the Internet

There is a great deal of control and organization within

o A Presentation layer is mainly concerned with the syntax cyberspace and the Internet. While there are many
and semantics of the information exchanged between the hyperbolic and somewhat utopian views of cyberspace as
two systems.
o It acts as a data translator for a network.
being some entity separate to the real world, suggests that
o This layer is a part of the operating system that converts the Internet, and the networks and technologies through
the data from one presentation format to another format. which it operates, are controlled and organized at four
o The Presentation layer is also known as the syntax layer. particular levels (see figure below):
Functions of Presentation layer: - Computer and network hardware
o Translation: The processes in two systems exchange the - Internet access and Internet service providers (ISPs)
information in the form of character strings, numbers and - Navigation within the Internet
so on. Different computers use different encoding - The use of the Internet through forums and communities.
methods, the presentation layer handles the
interoperability between the different encoding methods.
o 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.
o 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.
Application Layer

TCP/IP model
o The TCP/IP model was developed prior to the OSI
model.
o The TCP/IP model is not exactly similar to the OSI
model.
o The TCP/IP model consists of five layers: the
application layer, transport layer, network layer,
data link layer and physical layer.

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o The first four layers provide physical standards,

network interface, internetworking, and transport IP Protocol: IP protocol is used in this layer, and it is the most
functions that correspond to the first four layers of significant part of the entire TCP/IP suite.
the OSI model and these four layers are Following are the responsibilities of this protocol:
represented in TCP/IP model by a single layer o IP Addressing: This protocol implements logical host
called the application layer. addresses known as IP addresses. The IP addresses are
o TCP/IP is a hierarchical protocol made up of used by the internet and higher layers to identify the
device and to provide internetwork routing.
interactive modules, and each of them provides
o Host-to-host communication: It determines the path
specific functionality. through which the data is to be transmitted.
Functions of TCP/IP layers: o 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.
o 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.
o Routing: When IP datagram is sent over the same local
network such as LAN, MAN, WAN, it is known as direct
delivery.

ARP Protocol
Network Access Layer o ARP stands for Address Resolution Protocol.
o A network layer is the lowest layer of the TCP/IP o ARP is a network layer protocol which is used to find the
physical address from the IP address.
model.
o The two terms are mainly associated with the ARP
o A network layer is the combination of the Physical Protocol:
layer and Data Link layer defined in the OSI o ARP request: When a sender wants to know the
reference model. physical address of the device, it broadcasts the
o It defines how the data should be sent physically ARP request to the network.
through the network. o ARP reply: Every device attached to the network
o This layer is mainly responsible for the will accept the ARP request and process the
request, but only recipient recognize the IP
transmission of the data between two devices on
address and send back its physical address in the
the same network. form of ARP reply.
o The protocols used by this layer are Ethernet,ICMP Protocol
token ring, FDDI, X.25, frame relay. o ICMP stands for Internet Control Message Protocol.
o It is a mechanism used by the hosts or routers to send
Internet Layer notifications regarding datagram problems back to the
sender.
o A datagram travels from router-to-router until it reaches
o An internet layer is the second layer of the TCP/IP its destination.
model. o An ICMP protocol mainly uses two terms:
o An internet layer is also known as the network o ICMP Test: ICMP Test is used to test whether
layer. the destination is reachable or not.
o The main responsibility of the internet layer is to o ICMP Reply: ICMP Reply is used to check
send the packets from any network, and they arrive whether the destination device is responding or
not.
at the destination irrespective of the route they
o The core responsibility of the ICMP protocol is to report
take. the problems, not correct them. The responsibility of the
correction lies with the sender.
o ICMP can send the messages only to the source, but not to
Following are the protocols used in this layer are: the intermediate routers because the IP datagram carries
the addresses of the source and destination but not of the
router that it is passed to.
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o At the receiving end, TCP collects all the

segments and reorders them based on sequence
Transport Layer numbers.

The transport layer is responsible for the reliability, flow control,

and correction of data which is being sent over the network. Application Layer
The two protocols used in the transport layer are User Datagram o An application layer is the topmost layer in the TCP/IP
protocol and Transmission control protocol. model.
o User Datagram Protocol (UDP) o It is responsible for handling high-level protocols, issues
o It provides connectionless service and end-to-end of representation.
delivery of transmission. o This layer allows the user to interact with the application.
o It is an unreliable protocol as it discovers the o There is an ambiguity occurs in the application layer. For
errors but not specify the error. example: text editor cannot be considered in application
o User Datagram Protocol discovers the error, and layer while web browser using HTTP protocol to interact
ICMP protocol reports the error to the sender that with the network where HTTP protocol is an application
user datagram has been damaged. layer protocol.
o UDP consists of the following fields:
Main protocols used in the application layer:
Source port address: The source port address is
the address of the application program that has
o HTTP: HTTP stands for Hypertext transfer protocol. This
created the message.
protocol allows us to access the data over the World Wide
Web. It transfers the data in the form of plain text, audio,
Destination port address: The destination port
video.
address is the address of the application program
o SNMP: SNMP stands for Simple Network Management
that receives the message.
Protocol. It is a framework used for managing the devices
on the internet by using the TCP/IP protocol suite.
Total length: It defines the total number of bytes
o SMTP: SMTP stands for Simple mail transfer protocol.
of the user datagram in bytes.
The TCP/IP protocol that supports the e-mail is known as
a Simple mail transfer protocol. This protocol is used to
Checksum: The checksum is a 16-bit field used
send the data to another e-mail address.
in error detection.
o DNS: DNS stands for Domain Name System. An IP
o UDP does not specify which packet is lost. UDP
address is used to identify the connection of a host to the
contains only checksum; it does not contain any
internet uniquely.
ID of a data segment.
o 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.
o FTP: FTP stands for File Transfer Protocol. FTP is a
standard internet protocol used for transmitting the files
from one computer to another computer.

o Transmission Control Protocol (TCP)

o It provides a full transport layer services to
applications.
o It creates a virtual circuit between the sender and
receiver, and it is active for the duration of the
transmission.
o TCP is a reliable protocol as it detects the error
and retransmits the damaged frames.
o 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.

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Moderate data speeds: Coaxial or twisted pair cables are mainly used
in bus-based networks that support upto 10 Mbps.
o Familiar technology: Bus topology is a familiar
What is Topology? technology as the installation and troubleshooting
techniques are well known, and hardware components are
Topology defines the structure of the network of how all the easily available.
components are interconnected to each other. There are two types o Limited failure: A failure in one node will not have any
of topology: physical and logical topology. effect on other nodes.
Physical topology is the geometric representation of all the nodesDisadvantages of Bus topology:
in a network. o Extensive cabling: A bus topology is quite simpler, but
still it requires a lot of cabling.
o Difficult troubleshooting: It requires specialized test
equipment to determine the cable faults. If any fault
occurs in the cable, then it would disrupt the
communication for all the nodes.
o Signal interference: If two nodes send the messages
simultaneously, then the signals of both the nodes collide
with each other.
o Reconfiguration difficult: Adding new devices to the
network would slow down the network.
o Attenuation: Attenuation is a loss of signal leads to
communication issues. Repeaters are used to regenerate
the signal.

Bus Topology Ring Topology

The bus topology is designed in such a way that all the

o
stations are connected through a single cable known as a
backbone cable. o Ring topology is like a bus topology, but with connected
o Each node is either connected to the backbone cable by ends.
drop cable or directly connected to the backbone cable. o The node that receives the message from the previous
o When a node wants to send a message over the network, it computer will retransmit to the next node.
puts a message over the network. All the stations available o The data flows in one direction, i.e., it is unidirectional.
in the network will receive the message whether it has o The data in a ring topology flow in a clockwise direction.
been addressed or not. o The most common access method of the ring topology
o The bus topology is mainly used in 802.3 (Ethernet) and is token passing.
802.4 standard networks. o Token passing: It is a network access method in
o The configuration of a bus topology is quite simpler as which token is passed from one node to another
compared to other topologies. node.
o The backbone cable is considered as a "single o Token: It is a frame that circulates around the
lane" through which the message is broadcast to all the network.
stations. Working of Token passing
o The most common access method of the bus topologies o A token move around the network, and it is passed from
is CSMA (Carrier Sense Multiple Access). computer to computer until it reaches the destination.
Advantages of Bus topology: o The sender modifies the token by putting the address
o Low-cost cable: In bus topology, nodes are directly along with the data.
connected to the cable without passing through a hub. o In a ring topology, a token is used as a carrier.
Therefore, the initial cost of installation is low.
o
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kilometers of cable. In a star topology, all the stations are

connected to the centralized network. Therefore, the
network administrator has to go to the single station to
troubleshoot the problem.
Advantages of Ring topology: o Network control: Complex network control features can
o Network Management: Faulty devices can be removed be easily implemented in the star topology. Any changes
from the network without bringing the network down. made in the star topology are automatically
o Product availability: Many hardware and software tools accommodated.
for network operation and monitoring are available. o Limited failure: As each station is connected to the
o Cost: Twisted pair cabling is inexpensive and easily central hub with its own cable, therefore failure in one
available. Therefore, the installation cost is very low. cable will not affect the entire network.
o Reliable: It is a more reliable network because the o Familiar technology: Star topology is a familiar
communication system is not dependent on the single host technology as its tools are cost-effective.
computer. o Easily expandable: It is easily expandable as new stations
Disadvantages of Ring topology: can be added to the open ports on the hub.
o Difficult troubleshooting: It requires specialized test o Cost effective: Star topology networks are cost-effective
equipment to determine the cable faults. If any fault as it uses inexpensive coaxial cable.
occurs in the cable, then it would disrupt the o High data speeds: It supports a bandwidth of approx
communication for all the nodes. 100Mbps. Ethernet 100BaseT is one of the most popular
o Failure: The breakdown in one station leads to the failure Star topology networks.
of the overall network. Disadvantages of Star topology
o Reconfiguration difficult: Adding new devices to the o A Central point of failure: If the central hub or switch
network would slow down the network. goes down, then all the connected nodes will not be able
o Delay: Communication delay is directly proportional to to communicate with each other.
the number of nodes. Adding new devices increases the o Cable: Sometimes cable routing becomes difficult when a
communication delay. significant amount of routing is required.
Star Topology
Tree topology
o Tree topology combines the characteristics of bus
topology and star topology.
o A tree topology is a type of structure in which all the
computers are connected with each other in hierarchical
fashion.
o The top-most node in tree topology is known as a root
node, and all other nodes are the descendants of the root
node.

oStar topology is an arrangement of the network in which

every node is connected to the central hub, switch or a
central computer.
o The central computer is known as a server, and the
peripheral devices attached to the server are known
as clients.
o Coaxial cable or RJ-45 cables are used to connect the
computers. o There is only one path exists between two nodes for the
o Hubs or Switches are mainly used as connection devices data transmission. Thus, it forms a parent-child hierarchy.
in a physical star topology. Advantages of Tree topology
o Star topology is the most popular topology in network o Support for broadband transmission: Tree topology is
implementation. mainly used to provide broadband transmission, i.e.,
Advantages of Star topology signals are sent over long distances without being
o Efficient troubleshooting: Troubleshooting is quite attenuated.
efficient in a star topology as compared to bus topology. o Easily expandable: We can add the new device to the
In a bus topology, the manager has to inspect the existing network. Therefore, we can say that tree topology
is easily expandable.

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o Easily manageable: In tree topology, the whole network

is divided into segments known as star networks which
can be easily managed and maintained.
o Error detection: Error detection and error correction are
very easy in a tree topology.
o Limited failure: The breakdown in one station does not
affect the entire network.
o Point-to-point wiring: It has point-to-point wiring for
individual segments.
o Full Mesh Topology: In a full mesh topology, each
Disadvantages of Tree topology computer is connected to all the computers available in the
o Difficult troubleshooting: If any fault occurs in the node, network.
then it becomes difficult to troubleshoot the problem. o Partial Mesh Topology: In a partial mesh topology, not
o High cost: Devices required for broadband transmission all but certain computers are connected to those computers
are very costly. with which they communicate frequently.
o Failure: A tree topology mainly relies on main bus cableAdvantages of Mesh topology:
and failure in main bus cable will damage the overall
o Reliable: The mesh topology networks are very reliable as
network.
if any link breakdown will not affect the communication
o Reconfiguration difficult: If new devices are added, then between connected computers.
it becomes difficult to reconfigure.
o Fast Communication: Communication is very fast
between the nodes.
Mesh topology o Easier Reconfiguration: Adding new devices would not
disrupt the communication between other devices.
Disadvantages of Mesh topology
o Cost: A mesh topology contains a large number of
connected devices such as a router and more transmission
media than other topologies.
o Management: Mesh topology networks are very large
and very difficult to maintain and manage. If the network
is not monitored carefully, then the communication link
failure goes undetected.
o Efficiency: In this topology, redundant connections are
high that reduces the efficiency of the network.

o Mesh technology is an arrangement of the network in

which computers are interconnected with each other Hybrid Topology
through various redundant connections.
o There are multiple paths from one computer to another
computer.
o It does not contain the switch, hub or any central computer
which acts as a central point of communication.
o The Internet is an example of the mesh topology.
o Mesh topology is mainly used for WAN implementations
where communication failures are a critical concern.
o Mesh topology is mainly used for wireless networks.
o Mesh topology can be formed by using the formula:
Number of cables = (n*(n-1))/2;
Where n is the number of nodes that represents the network.
Mesh topology is divided into two categories:
o Fully connected mesh topology
o Partially connected mesh topology

o The combination of various different topologies is known

as Hybrid topology.
o A Hybrid topology is a connection between different links

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o When two or more different topologies are combined o Transmission impairment: When the received signal is
together is termed as Hybrid topology and if similar not identical to the transmitted one due to the transmission
topologies are connected with each other will not result in impairment. The quality of the signals will get destroyed
Hybrid topology. due to transmission impairment.
Advantages of Hybrid Topology o Interference: Interference is defined as the process of
o Reliable: If a fault occurs in any part of the network will disrupting a signal when it travels over a communication
not affect the functioning of the rest of the network. medium on the addition of some unwanted signal.
o Scalable: Size of the network can be easily expanded by
adding new devices without affecting the functionality of Causes of Transmission Impairment:
the existing network.
o Flexible: This topology is very flexible as it can be
designed according to the requirements of the
organization.
o Effective: Hybrid topology is very effective as it can be
designed in such a way that the strength of the network is
maximized and weakness of the network is minimized. o Attenuation: Attenuation means the loss of energy, i.e.,
Disadvantages of Hybrid topology the strength of the signal decreases with increasing the
distance which causes the loss of energy.
o Complex design: The major drawback of the Hybrid
topology is the design of the Hybrid network. It is very o Distortion: Distortion occurs when there is a change in
difficult to design the architecture of the Hybrid network. the shape of the signal. This type of distortion is examined
o Costly Hub: The Hubs used in the Hybrid topology are from different signals having different frequencies. Each
very expensive as these hubs are different from usual frequency component has its own propagation speed, so
Hubs used in other topologies. they reach at a different time which leads to the delay
o Costly infrastructure: The infrastructure cost is very distortion.
high as a hybrid network requires a lot of cabling, network o Noise: When data is travelled over a transmission
devices, etc. medium, some unwanted signal is added to it which
creates the noise.
Classification of Transmission
What is Transmission media? Media:
o Transmission media is a communication channel that
carries the information from the sender to the receiver.
Data is transmitted through the electromagnetic signals.
o The main functionality of the transmission media is to
carry the information in the form of bits through LAN
(Local Area Network).
o It is a physical path between transmitter and receiver in
data communication.
o In a copper-based network, the bits in the form of
electrical signals.
o In a fiber based network, the bits in the form of light
pulses.
o The electrical signals can be sent through the copper wire,
fiber optics, atmosphere, water, and vacuum.
o Transmission media is of two types is wired media and
wireless media. In wired media, medium characteristics
are more important whereas, in wireless media, signal
characteristics are more important.
o Different transmission media have different properties o Guided Transmission Media
such as bandwidth, delay, cost and ease of installation and o UnGuided Transmission Media
maintenance.
o The transmission media is available in the lowest layer of
the OSI reference model, i.e., Physical layer.
Guided Media
Some Effecting factors: It is defined as the physical medium through which the signals are
o Bandwidth: All the factors are remaining constant, the transmitted. It is also known as Bounded media.
greater the bandwidth of a medium, the higher the data
transmission rate of a signal.

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Twisted pair:
Twisted pair is a physical media made up of a pair of cables
twisted with each other. A twisted pair cable is cheap as compared
to other transmission media. The frequency range for twisted pair
cable is from 0 to 3.5KHz.
A twisted pair consists of two insulated copper wires arranged in a
regular spiral pattern.
.

Types of Twisted pair:

Coaxial Cable
o Coaxial cable is very commonly used transmission media,
for example, TV wire is usually a coaxial cable.
o The name of the cable is coaxial as it contains two
conductors parallel to each other.
o It has a higher frequency as compared to twisted pair
cable.
o The inner conductor of the coaxial cable is made up of
copper, and the outer conductor is made up of copper
mesh. The middle core is made up of non-conductive
cover that separates the inner conductor from the outer
conductor.
o The middle core is responsible for the data transferring
whereas the copper mesh prevents from the EMI
(Electromagnetic interference).

Coaxial cable is of two types:

1. Baseband transmission: It is defined as the process of
transmitting a single signal at high speed.
2. Broadband transmission: It is defined as the process of
transmitting multiple signals simultaneously.
Fiber Optic
o Fiber optic cable is a cable that uses electrical signals for
communication.
o Fiber optic is a cable that holds the optical fibers coated in
plastic that are used to send the data by pulses of light.
o The plastic coating protects the optical fibers from heat,
cold, electromagnetic interference from other types of
wiring.
o Fiber optics provide faster data transmission than copper
wires.

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Diagrammatic representation of fiber optic cable:

Basic elements of Fiber optic cable:

o Core: The optical fiber consists of a narrow strand of
glass or plastic known as a core. A core is a light
transmission area of the fiber. The more the area of the
core, the lighter will be transmitted into the fiber.
o Cladding: The concentric layer of glass is known as
cladding. The main functionality of the cladding is to
provide the lower refractive index at the core interface as
to cause the reflection within the core so that the light
waves are transmitted through the fiber.
o Jacket: The protective coating consisting of plastic is
known as a jacket. The main purpose of a jacket is to
preserve the fiber strength, absorb shock and extra fiber
protection.
Following are the advantages of fiber optic cable over copper:
o Greater Bandwidth: The fiber optic cable provides
more bandwidth as compared copper. Therefore, the fiber
optic carries more data as compared to copper cable.
o Faster speed: Fiber optic cable carries the data in the
form of light. This allows the fiber optic cable to carry
the signals at a higher speed.
o Longer distances: The fiber optic cable carries the data
at a longer distance as compared to copper cable.
o Better reliability: The fiber optic cable is more reliable
than the copper cable as it is immune to any temperature
changes while it can cause obstruct in the connectivity of
copper cable.
o Thinner and Sturdier: Fiber optic cable is thinner and
lighter in weight so it can withstand more pull pressure
than copper cable.

Un-Guided Transmission
o An unguided transmission transmits the electromagnetic
waves without using any physical medium. Therefore it
is also known as wireless transmission.
o In unguided media, air is the media through which the
electromagnetic energy can flow easily.
Unguided transmission is broadly classified into three categories:
Radio waves
o Radio waves are the electromagnetic waves that are
transmitted in all the directions of free space.
o Radio waves are omnidirectional, i.e., the signals are
propagated in all the directions.
o The range in frequencies of radio waves is from 3Khz to
1 khz.
o In the case of radio waves, the sending and receiving
antenna are not aligned, i.e., the wave sent by the sending
antenna can be received by any receiving antenna.

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o An example of the radio wave is FM radio. tower for a longer distance.

Applications Of Radio waves:

o A Radio wave is useful for multicasting when there is one
sender and many receivers.
o An FM radio, television, cordless phones are examples of
a radio wave.
Advantages Of Radio transmission:
o Radio transmission is mainly used for wide area networks
and mobile cellular phones.
o Radio waves cover a large area, and they can penetrate the
walls.
o Radio transmission provides a higher transmission rate.
Microwaves

Microwaves are of two types:

o Terrestrial microwave
o Satellite microwave communication.
Terrestrial Microwave Transmission
o Terrestrial Microwave transmission is a technology that
transmits the focused beam of a radio signal from one
ground-based microwave transmission antenna to another.
o Microwaves are the electromagnetic waves having the
frequency in the range from 1GHz to 1000 GHz.
o Microwaves are unidirectional as the sending and
receiving antenna is to be aligned, i.e., the waves sent by
the sending antenna are narrowly focussed.
o In this case, antennas are mounted on the towers to send a
beam to another antenna which is km away.
o It works on the line of sight transmission, i.e., the
antennas mounted on the towers are the direct sight of
each other.
Characteristics of Microwave:
o Frequency range: The frequency range of terrestrial
microwave is from 4-6 GHz to 21-23 GHz.
o Bandwidth: It supports the bandwidth from 1 to 10 Mbps.
o Short distance: It is inexpensive for short distance.
o Long distance: It is expensive as it requires a higher

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o Attenuation: Attenuation means loss of signal. It is
affected by environmental conditions and antenna size.
Advantages Of Microwave:
o Microwave transmission is cheaper than using cables.
o It is free from land acquisition as it does not require any
land for the installation of cables.
o Microwave transmission provides an easy
communication in terrains as the installation of cable in
terrain is quite a difficult task.
o Communication over oceans can be achieved by using
microwave transmission.
Disadvantages of Microwave transmission:
o Eavesdropping: An eavesdropping creates insecure
communication. Any malicious user can catch the signal
in the air by using its own antenna.
o Out of phase signal: A signal can be moved out of phase
by using microwave transmission.
o Susceptible to weather condition: A microwave
transmission is susceptible to weather condition. This
means that any environmental change such as rain, wind
can distort the signal.
o Bandwidth limited: Allocation of bandwidth is limited
in the case of microwave transmission.
Satellite Microwave Communication
o A satellite is a physical object that revolves around
the earth at a known height.
o Satellite communication is more reliable nowadays as
it offers more flexibility than cable and fiber optic
systems.
o We can communicate with any point on the globe by
using satellite communication.
How Does Satellite work?
The satellite accepts the signal that is transmitted from the earth
station, and it amplifies the signal. The amplified signal is
retransmitted to another earth station.
Advantages Of Satellite Microwave Communication:
o The coverage area of a satellite microwave is more than
the terrestrial microwave.
o The transmission cost of the satellite is independent of
the distance from the centre of the coverage area.
o Satellite communication is used in mobile and
wireless communication applications.
o It is easy to install.
o It is used in a wide variety of applications such as
weather forecasting, radio/TV signal broadcasting,
mobile communication, etc.
Disadvantages Of Satellite Microwave Communication:
o Satellite designing and development requires more time
and higher cost.
o The Satellite needs to be monitored and controlled on
regular periods so that it remains in orbit.
o The life of the satellite is about 12-15 years. Due to this
reason, another launch of the satellite has to be planned
before it becomes non-functional.
Infrared
o An infrared transmission is a wireless technology used
for communication over short ranges.

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o The frequency of the infrared in the range from 300 GHz

to 400 THz.
o It is used for short-range communication such as data
transfer between two cell phones, TV remote operation,
data transfer between a computer and cell phone resides in
the same closed area.
Characteristics Of Infrared:
o It supports high bandwidth, and hence the data rate will be
very high.
o Infrared waves cannot penetrate the walls. Therefore, the
infrared communication in one room cannot be interrupted
by the nearby rooms.
o An infrared communication provides better security with
minimum interference.
o Infrared communication is unreliable outside the building
because the sun rays will interfere with the infrared
waves.

Encoding is the process of converting the data or a given

sequence of characters, symbols, alphabets etc., into a
specified format, for the secured transmission of
data. Decoding is the reverse process of encoding which
is to extract the information from the converted format.

Data Encoding
Encoding is the process of using various patterns of voltage
or current levels to represent 1s and 0s of the digital
signals on the transmission link.
The common types of line encoding are Unipolar, Polar,
Bipolar, and Manchester.

Encoding Techniques
The data encoding technique is divided into the following
types, depending upon the type of data conversion.
 Analog data to Analog signals − The modulation
techniques such as Amplitude Modulation,
Frequency Modulation and Phase Modulation of
analog signals, fall under this category.
 Analog data to Digital signals − This process can
be termed as digitization, which is done by Pulse
Code Modulation PCMPCM. Hence, it is
nothing but digital modulation. As we have
already discussed, sampling and quantization are
the important factors in this. Delta Modulation
gives a better output than PCM.
 Digital data to Analog signals − The modulation
techniques such as Amplitude Shift
Keying ASKASK, Frequency Shift
Keying FSKFSK, Phase Shift Keying PSKPSK,
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etc., fall under this category. These will be discussed in

subsequent chapters.
 Digital data to Digital signals − These are in
this section. There are several ways to map
digital data to digital signals. Some of them are
−

Non Return to Zero NRZNRZ

NRZ Codes has 1 for High voltage level and 0 for Low
voltage level. The main behavior of NRZ codes is that
the voltage level remains constant during bit interval.
The end or start of a bit will not be indicated and it will
maintain the same voltage state, if the value of the
previous bit and the value of the present bit are same.
The following figure explains the concept of NRZ coding.

If the above example is considered, as there is a long

sequence of constant voltage level and the clock
synchronization may be lost due to the absence of bit
interval, it becomes difficult for the receiver to
differentiate between 0 and 1.

Bi-phase Encoding
The signal level is checked twice for every bit time, both
initially and in the middle. Hence, the clock rate is
double the data transfer rate and thus the modulation rate
is also doubled. The clock is taken from the signal itself.
The bandwidth required for this coding is greater.
There are two types of Bi-phase Encoding.
 Bi-phase Manchester
 Differential Manchester

Bi-phase Manchester
In this type of coding, the transition is done at the middle of
the bit-interval. The transition for the resultant pulse is

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from High to Low in the middle of the interval, for the

input bit 1. While the transition is from Low to High for
the input bit 0.

Differential Manchester
In this type of coding, there always occurs a transition in
the middle of the bit interval. If there occurs a transition
at the beginning of the bit interval, then the input bit is 0.
If no transition occurs at the beginning of the bit interval,
then the input bit is 1. Unipolar
o Digital transmission system sends the voltage pulses over
the medium link such as wire or cable.

o In most types of encoding, one voltage level represents 0,

and another voltage level represents 1.

o The polarity of each pulse determines whether it is

positive or negative.

o This type of encoding is known as Unipolar encoding as it

uses only one polarity.

Digital Transmission
Data can be represented either in analog or digital form. The
computers used the digital form to store the information.
Therefore, the data needs to be converted in digital form so that it
can be used by a computer.

DIGITAL-TO-DIGITAL
CONVERSION Unipolar encoding has two problems that make this scheme less
desirable:
Digital-to-digital encoding is the representation of digital
information by a digital signal. When binary 1s and 0s generated o DC Component
by the computer are translated into a sequence of voltage pulses
o Synchronization
that can be propagated over a wire, this process is known as
digital-to-digital encoding.
Polar
o Polar encoding is an encoding scheme that uses two
voltage levels: one is positive, and another is negative.

o By using two voltage levels, an average voltage level is

reduced, and the DC component problem of unipolar
Digital-to-digital encoding is divided into three categories: encoding scheme is alleviated.
o Unipolar Encoding

o Polar Encoding

o Bipolar Encoding

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NRZ Disadvantage of RZ:

o NRZ stands for Non-return zero. It performs two signal changes to encode one bit that acquires
more bandwidth.
o In NRZ encoding, the level of the signal can be
represented either positive or negative. Biphase
The two most common methods used in NRZ are: o Biphase is an encoding scheme in which signal changes at
the middle of the bit interval but does not return to zero.
NRZ-L: In NRZ-L encoding, the level of the signal depends on the
type of the bit that it represents. If a bit is 0 or 1, then their Biphase encoding is implemented in two different ways:
voltages will be positive and negative respectively. Therefore, we
can say that the level of the signal is dependent on the state of the Manchester
bit.
o It changes the signal at the middle of the bit interval but
NRZ-I: NRZ-I is an inversion of the voltage level that represents 1 does not return to zero for synchronization.
bit. In the NRZ-I encoding scheme, a transition occurs between the
positive and negative voltage that represents 1 bit. In this scheme, o In Manchester encoding, a negative-to-positive transition
0 bit represents no change and 1 bit represents a change in voltage represents binary 1, and positive-to-negative transition
level. represents 0.

o Manchester has the same level of synchronization as RZ

scheme except that it has two levels of amplitude.

Differential Manchester

o It changes the signal at the middle of the bit interval for

synchronization, but the presence or absence of the
transition at the beginning of the interval determines the
bit. A transition means binary 0 and no transition means
binary 1.

o In Manchester Encoding scheme, two signal changes

represent 0 and one signal change represent 1.

RZ
o RZ stands for Return to zero.

o There must be a signal change for each bit to achieve

synchronization. However, to change with every bit, we
need to have three values: positive, negative and zero.

o RZ is an encoding scheme that provides three values,

positive voltage represents 1, the negative voltage
represents 0, and zero voltage represents none.

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Bipolar o B8ZS ensures synchronization of a long string of 0s by

providing force artificial signal changes called violations,
o Bipolar encoding scheme represents three voltage levels: within 0 string pattern.
positive, negative, and zero.
o When eight 0 occurs, then B8ZS implements some
o In Bipolar encoding scheme, zero level represents binary changes in 0s string pattern based on the polarity of the
0, and binary 1 is represented by alternating positive and previous 1 bit.
negative voltages.
o If the polarity of the previous 1 bit is positive, the eight 0s
o If the first 1 bit is represented by positive amplitude, then will be encoded as zero, zero, zero, positive, negative,
the second 1 bit is represented by negative voltage, third 1 zero, negative, positive.
bit is represented by the positive amplitude and so on.
This alternation can also occur even when the 1bits are not
o If the polarity of previous 1 bit is negative, then the eight
consecutive.
0s will be encoded as zero, zero, zero, negative, positive,
Bipolar can be classified as: zero, positive, negative.

HDB3
o HDB3 stands for High-Density Bipolar 3.

o HDB3 technique was first adopted in Europe and Japan.

o HDB3 technique is designed to provide the

synchronization of a long sequence of 0s bits.

o In the HDB3 technique, the pattern of violation is based

on the polarity of the previous bit.

AMI If the number of 1s bits since the last substitution is odd.

o AMI stands for alternate mark inversion where mark

work comes from telegraphy which means 1. So, it can be
redefined as alternate 1 inversion.

o In Bipolar AMI encoding scheme, 0 bit is represented by

zero level and 1 bit is represented by alternating positive
and negative voltages.

Advantage:

o DC component is zero.

o Sequence of 1s bits are synchronized.

Disadvantage:

o This encoding scheme does not ensure the synchronization

of a long string of 0s bits.

B8ZS
o B8ZS stands for Bipolar 8-Zero Substitution.

o This technique is adopted in North America to provide

synchronization of a long sequence of 0s bits.

o In most of the cases, the functionality of B8ZS is similar

to the bipolar AMI, but the only difference is that it
provides the synchronization when a long sequence of 0s
bits occur. If the number of 1s bits is even, then the violation is made on the
place of the first and fourth consecutive 0s. If the polarity of the

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previous bit is positive, then violations are negative, and if the

polarity of the previous bit is negative, then violations are positive.

If the number of 1s bits since the last substitution is even.

ANALOG-TO-DIGITAL
CONVERSION
o When an analog signal is digitalized, this is called anPCM
analog-to-digital conversion.
o PCM stands for Pulse Code Modulation.
o Suppose human sends a voice in the form of an analog
signal, we need to digitalize the analog signal which is o PCM technique is used to modify the pulses created by
less prone to noise. It requires a reduction in the number PAM to form a digital signal. To achieve this, PCM
of values in an analog message so that they can be quantizes PAM pulses. Quantization is a process of
represented in the digital stream. assigning integral values in a specific range to sampled
instances.
o In analog-to-digital conversion, the information contained
in a continuous wave form is converted in digital pulses. o PCM is made of four separate processes: PAM,
quantization, binary encoding, and digital-to-digital
encoding.
Techniques for Analog-To-Digital
Conversion
PAM
o PAM stands for pulse amplitude modulation.

o PAM is a technique used in analog-to-digital conversion.

o PAM technique takes an analog signal, samples it, and

generates a series of digital pulses based on the result of
sampling where sampling means measuring the amplitude
of a signal at equal intervals.

o PAM technique is not useful in data communication as it

translates the original wave form into pulses, but these
pulses are not digital. To make them digital, PAM
technique is modified to PCM technique.

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What is Multiplexing?
Multiplexing is a technique used to combine and send the multiple
data streams over a single medium. The process of combining the
data streams is known as multiplexing and hardware used for
Frequency-division Multiplexing
multiplexing is known as a multiplexer. (FDM)
Multiplexing is achieved by using a device called Multiplexer
o It is an analog technique.
(MUX) that combines n input lines to generate a single output line.
Multiplexing follows many-to-one, i.e., n input lines and one o Frequency Division Multiplexing is a technique in which
output line. the available bandwidth of a single transmission medium
is subdivided into several channels.
De-multiplexing is achieved by using a device called De-
multiplexer (DEMUX) available at the receiving end. DEMUX
separates a signal into its component signals (one input and n
outputs). Therefore, we can say that de-multiplexing follows the
one-to-many approach.

Concept of Multiplexing
o The carriers which are used for modulating the signals are
known as sub-carriers. They are represented as f1,f2..fn.
o FDM is mainly used in radio broadcasts and TV networks.

o The 'n' input lines are transmitted through a multiplexer

and multiplexer combines the signals to form a composite
signal.
o The composite signal is passed through a Demultiplexer
and demultiplexer separates a signal to component signals
and transfers them to their respective destinations.

Advantages of Multiplexing:
o More than one signal can be sent over a single medium. Advantages Of FDM:
o The bandwidth of a medium can be utilized effectively.
o FDM is used for analog signals.

Multiplexing Techniques o FDM process is very simple and easy modulation.

o A Large number of signals can be sent through an FDM
simultaneously.
Multiplexing techniques can be classified as:
o It does not require any synchronization between sender
and receiver.

Wavelength Division Multiplexing

(WDM)
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o In TDM, the signal is transmitted in the form of frames.

Frames contain a cycle of time slots in which each frame
contains one or more slots dedicated to each user.
o It can be used to multiplex both digital and analog signals
but mainly used to multiplex digital signals.

There are two types of TDM:

o Wavelength Division Multiplexing is same as FDM o Synchronous TDM

except that the optical signals are transmitted through the o Asynchronous TDM
fiber optic cable.
o WDM is used on fiber optics to increase the capacity of a Synchronous TDM
single fiber.
o It is used to utilize the high data rate capability of fiber o A Synchronous TDM is a technique in which time slot is
optic cable. preassigned to every device.
o It is an analog multiplexing technique. o In Synchronous TDM, each device is given some time slot
o Optical signals from different source are combined to irrespective of the fact that the device contains the data or
form a wider band of light with the help of multiplexer. not.
o At the receiving end, demultiplexer separates the signals o If the device does not have any data, then the slot will
to transmit them to their respective destinations. remain empty.
o Multiplexing and Demultiplexing can be achieved by
using a prism. Asynchronous TDM
o Prism can perform a role of multiplexer by combining the
various optical signals to form a composite signal, and the o An asynchronous TDM is also known as Statistical TDM.
composite signal is transmitted through a fiber optical
cable. o An asynchronous TDM is a technique in which time slots
are not fixed as in the case of Synchronous TDM. Time
o Prism also performs a reverse operation, i.e., slots are allocated to only those devices which have the
demultiplexing the signal. data to send. Therefore, we can say that Asynchronous
Time Division multiplexor transmits only the data from
active workstations.
o An asynchronous TDM technique dynamically allocates
the time slots to the devices.
o In Asynchronous TDM, total speed of the input lines can
be greater than the capacity of the channel.

o In Asynchronous TDM, each slot contains an address part

Time Division Multiplexing that identifies the source of the data.

o It is a digital technique.
o In Frequency Division Multiplexing Technique, all signals
operate at the same time with different frequency, but in o The difference between Asynchronous TDM and
case of Time Division Multiplexing technique, all signals Synchronous TDM is that many slots in Synchronous
operate at the same frequency with different time. TDM are unutilized, but in Asynchronous TDM, slots are
o In Time Division Multiplexing technique, the total time fully utilized. This leads to the smaller transmission time
available in the channel is distributed among different and efficient utilization of the capacity of the channel.
users. Therefore, each user is allocated with different time o In Synchronous TDM, if there are n sending devices, then
interval known as a Time slot at which data is to be there are n time slots. In Asynchronous TDM, if there are
transmitted by the sender. n sending devices, then there are m time slots where m is
o A user takes control of the channel for a fixed amount of less than n (m<n).
time. o The number of slots in a frame depends on the statistical
o In Time Division Multiplexing technique, data is not analysis of the number of input lines.
transmitted simultaneously rather the data is transmitted
one-by-one. Concept Of Asynchronous TDM

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o The layer 2 switches are used for transmitting the data on

the data link layer, and it also performs error checking on
transmitted and received frames.
o The layer 2 switches forward the packets with the help of
MAC address.
o Different modes are used for forwarding the packets
known as Switching modes.
o In switching mode, Different parts of a frame are
recognized. The frame consists of several parts such as
preamble, destination MAC address, source MAC address,
Switching user's data, FCS.

o When a user accesses the internet or another computer

network outside their immediate location, messages are
sent through the network of transmission media. This
technique of transferring the information from one
computer network to another network is known
as switching.
o Switching in a computer network is achieved by using
switches. A switch is a small hardware device which is
used to join multiple computers together with one local
area network (LAN).
o Network switches operate at layer 2 (Data link layer) in
the OSI model. There are three types of switching modes:

Why is Switching Concept required? o Store-and-forward

o Cut-through
Switching concept is developed because of the following reasons: o Fragment-free

o Bandwidth: It is defined as the maximum transfer rate of

a cable. It is a very critical and expensive resource.
Therefore, switching techniques are used for the effective
utilization of the bandwidth of a network.
o Collision: Collision is the effect that occurs when more
than one device transmits the message over the same
physical media, and they collide with each other. To
overcome this problem, switching technology is
implemented so that packets do not collide with each Store-and-forward
other.

Advantages of Switching:

o Switch increases the bandwidth of the network.

o It reduces the workload on individual PCs as it sends the
information to only that device which has been addressed.
o Store-and-forward is a technique in which the
o It increases the overall performance of the network by intermediate nodes store the received frame and then
reducing the traffic on the network. check for errors before forwarding the packets to the next
o There will be less frame collision as switch creates the node.
collision domain for each connection.
o The layer 2 switch waits until the entire frame has
received. On receiving the entire frame, switch store the
Switching Modes frame into the switch buffer memory. This process is
known as storing the frame.

[ By- Prof. Sushree Sasmita Dash] Page 22

 [Computer Network Notes-Unit-1]

o When the frame is stored, then the frame is checked for It combines the speed of Cut-through Switching with the error checking
the errors. If any error found, the message is discarded functionality.
otherwise the message is forwarded to the next node. This o This technique checks the 64 bytes of the ethernet frame
process is known as forwarding the frame. where addressing information is available.
o CRC (Cyclic Redundancy Check) technique is o A collision is detected within 64 bytes of the frame, the
implemented that uses a number of bits to check for the frames which are collided will not be forwarded further.
errors on the received frame.
o The store-and-forward technique ensures a high level of
security as the destination network will not be affected byDifferences b/w Store-and-forward and Cut-
the corrupted frames. through Switching.
o Store-and-forward switches are highly reliable as it does
not forward the collided frames.
Store-and-forward Cut-through Switching
Cut-through Switching Switching

Store-and-forward Cut-through Switching

is a
Switching is a technique technique that checks
the
that waits until the entire first 6 bytes following
the
o Cut-through switching is a technique in which the switch frame is received. preamble to identify
the
forwards the packets after the destination address has been destination address.
identified without waiting for the entire frame to be
received.
It performs error checking It does not perform
o Once the frame is received, it checks the first six bytes of any
the frame following the preamble, the switch checks the functionality. If any error error checking. The
destination in the switching table to determine the frame
outgoing interface port, and forwards the frame to the is found in the frame, the with or without errors
destination. will
frame will be discarded be forwarded.
o It has low latency rate as the switch does not wait for the
otherwise forwarded to
entire frame to be received before sending the packets to
the next node.
the destination.
o It has no error checking technique. Therefore, the errors It has high latency rate as It has low latency rate
can be sent with or without errors to the receiver. as it
o A Cut-through switching technique has low wait time as it waits for the entire checks only six bytes of
it forwards the packets as soon as it identifies the the
destination MAC address. frame to be received frame to
determine the
o In this technique, collision is not detected, if frames have before forwarding to the destination address.
collided will also be forwarded.
next node.

Fragment-free Switching It is highly reliable as it It is less

reliable as
forwards only error-free compared to Store-
and-
packets. forward technique
as it
forwards error prone
packets
as well.
o A Fragment-free switching is an advanced technique of
the Cut-through Switching.
It has a high wait time as It has low wait time as
o A Fragment-free switching is a technique that reads atleast cut-
64 bytes of a frame before forwarding to the next node to it waits for the entire through switches do
provide the error-free transmission. not
o frame to be received store the whole frame
Classification Of Switching Techniques or
[ By- Prof. Sushree Sasmita Dash] Page 23
 [Computer Network Notes-Unit-1]
before taking any packets.
forwarding decisions.

Switching techniques
In large networks, there can be multiple paths from sender to
receiver. The switching technique will decide the best route for
data transmission.

Switching technique is used to connect the systems for making

one-to-one communication.

[ By- Prof. Sushree Sasmita Dash] Page 24

 [Computer Network Notes-Unit-1]

o Space Division Switching is a circuit switching

technology in which a single transmission path is
accomplished in a switch by using a physically separate
set of crosspoints.
o Space Division Switching can be achieved by using
crossbar switch. A crossbar switch is a metallic crosspoint
or semiconductor gate that can be enabled or disabled by a
control unit.
o The Crossbar switch is made by using the semiconductor.
For example, Xilinx crossbar switch using FPGAs.
o Space Division Switching has high speed, high capacity,
and nonblocking switches.

Circuit Switching Space Division Switches can be categorized in two ways:

oCircuit switching is a switching technique that establishes o Crossbar Switch

a dedicated path between sender and receiver. o Multistage Switch
o In the Circuit Switching Technique, once the connection is
established then the dedicated path will remain to existCrossbar Switch
until the connection is terminated.
o Circuit switching in a network operates in a similar way as The Crossbar switch is a switch that has n input lines and n output
the telephone works. lines. The crossbar switch has n2 intersection points known
o A complete end-to-end path must exist before the as crosspoints.
communication takes place.
o In case of circuit switching technique, when any userDisadvantage of Crossbar switch:
wants to send the data, voice, video, a request signal is
sent to the receiver then the receiver sends back the The number of crosspoints increases as the number of stations is
acknowledgment to ensure the availability of the increased. Therefore, it becomes very expensive for a large switch.
dedicated path. After receiving the acknowledgment, The solution to this is to use a multistage switch.
dedicated path transfers the data.
o Circuit switching is used in public telephone network. It isMultistage Switch
used for voice transmission.
o Fixed data can be transferred at a time in circuit switching o Multistage Switch is made by splitting the crossbar switch
technology. into the smaller units and then interconnecting them.
o It reduces the number of crosspoints.
Communication through circuit switching has 3 phases: o If one path fails, then there will be an availability of
another path.
o Circuit establishment
o Data transfer Advantages Of Circuit Switching:
o Circuit Disconnect
o In the case of Circuit Switching technique, the
communication channel is dedicated.
o It has fixed bandwidth.

Disadvantages Of Circuit Switching:

o Once the dedicated path is established, the only delay

occurs in the speed of data transmission.
o It takes a long time to establish a connection approx 10
seconds during which no data can be transmitted.
Circuit Switching can use either of the two technologies:
o It is more expensive than other switching techniques as a
dedicated path is required for each connection.
Space Division Switches:

[ By- Prof. Sushree Sasmita Dash] Page 25

 [Computer Network Notes-Unit-1]

o It is inefficient to use because once the path is established

and no data is transferred, then the capacity of the path is
wasted.
o In this case, the connection is dedicated therefore no other
data can be transferred even if the channel is free.

Message Switching
o Message Switching is a switching technique in which a
message is transferred as a complete unit and routed
through intermediate nodes at which it is stored and
forwarded.
o In Message Switching technique, there is no establishment
of a dedicated path between the sender and receiver.
o The destination address is appended to the message.
Message Switching provides a dynamic routing as the
message is routed through the intermediate nodes based on
the information available in the message.
o Message switches are programmed in such a way so that
they can provide the most efficient routes.
o Each and every node stores the entire message and then
forward it to the next node. This type of network is known
as store and forward network.
o Message switching treats each message as an independent
entity.

Advantages Of Message Switching

o Data channels are shared among the communicating

devices that improve the efficiency of using available
bandwidth.
o Traffic congestion can be reduced because the message is
temporarily stored in the nodes.
o Message priority can be used to manage the network.
o The size of the message which is sent over the network
can be varied. Therefore, it supports the data of unlimited
size.

Disadvantages Of Message Switching

o The message switches must be equipped with sufficient

storage to enable them to store the messages until the
message is forwarded.
o The Long delay can occur due to the storing and
forwarding facility provided by the message switching
technique.

[ By- Prof. Sushree Sasmita Dash] Page 26

 [Computer Network Notes-Unit-1]

Packet Switching

o The packet switching is a switching technique in which

the message is sent in one go, but it is divided into
smaller pieces, and they are sent individually.
o The message splits into smaller pieces known as packets
and packets are given a unique number to identify their
order at the receiving end.
o Every packet contains some information in its headers
such as source address, destination address and sequence
number.
o Packets will travel across the network, taking the shortest
path as possible.
o All the packets are reassembled at the receiving end in
correct order.
o If any packet is missing or corrupted, then the message
will be sent to resend the message.
o If the correct order of the packets is reached, then the
acknowledgment message will be sent.

Approaches Of Packet Switching:

There are two approaches to Packet Switching:

Datagram Packet switching:

o It is a packet switching technology in which packet is

known as a datagram, is considered as an independent
entity. Each packet contains the information about the
destination and switch uses this information to forward
the packet to the correct destination.
o The packets are reassembled at the receiving end in
correct order.
o In Datagram Packet Switching technique, the path is not
fixed.
o Intermediate nodes take the routing decisions to forward
the packets.
o Datagram Packet Switching is also known as
connectionless switching.

Virtual Circuit Switching

o Virtual Circuit Switching is also known as connection-

oriented switching.

[ By- Prof. Sushree Sasmita Dash] Page 27

 [Computer Network Notes-Unit-1]

o In the case of Virtual circuit switching, a preplanned route o Cost-effective: In packet switching technique, switching
is established before the messages are sent. devices do not require massive secondary storage to store
o Call request and call accept packets are used to establish the packets, so cost is minimized to some extent.
the connection between sender and receiver. Therefore, we can say that the packet switching technique
o In this case, the path is fixed for the duration of a logical is a cost-effective technique.
connection. o Reliable: If any node is busy, then the packets can be
rerouted. This ensures that the Packet Switching technique
Let's understand the concept of virtual circuit switching provides reliable communication.
through a diagram: o Efficient: Packet Switching is an efficient technique. It
does not require any established path prior to the
transmission, and many users can use the same
communication channel simultaneously, hence makes use
of available bandwidth very efficiently.

o In the above diagram, A and B are the sender and receiver

respectively. 1 and 2 are the nodes.
o Call request and call accept packets are used to establish a
connection between the sender and receiver.
o When a route is established, data will be transferred.
o After transmission of data, an acknowledgment signal is
sent by the receiver that the message has been received.
o If the user wants to terminate the connection, a clear
signal is sent for the termination.

Differences b/w Datagram approach and

Virtual Circuit approach

Datagram approach Virtual Circuit

approach

Node takes routing decisions to Node does not take any

forward the packets. routing decision.

Congestion cannot occur as all the Congestion can occur when the
packets travel in different node is busy, and it does
directions. not allow other packets to
pass through.

It is more flexible as all the packets It is not very flexible.

are treated as an independent
entity.

Advantages Of Packet Switching:

[ By- Prof. Sushree Sasmita Dash] Page 28