CS-324 Computer Networks.pdf

CS 352 Computer Networks
Faculty: Ms. Diksha Goyal

UNIT 1
 OSI & TCP/IP MODELS
 NETWORK TOPOLOGIES
 NETWORKING TYPES
 TRANSMISSION MEDIA
 NETWORKIN DEVICES
 WLAN
 MULTIPLEXING

Contents -
 Representation of data and its flow, network protocols and standards
 OSI & TCP/IP MODEL
 What is Network Topology?
• Network Topology Types
○ Bus Topology
○ Ring Topology
○ Star Topology
○ Tree Topology
○ Mesh Topology
○ Hybrid Topology
• Networking Types
○ Local Area Network (LAN)
○ Wide Area Network (WAN)
○ Metropolitan Area Network
(MAN)
○ Personal Area Network (PAN)

 Transmissions Media
• Guided Media
o Twisted pair cable
o Co-axial Cable
o Fiber Optic Cable
• Unguided Media
o Radio Waves
o Antenna
o Microwave
o Infrared
 Networking Devices
o Hub
o Switch
o Router
o Bridge
o Gateway
o Modem
o Repeater
o Access Point
 PSTN
 WLAN
 MULTIPLEXING

Representation of data
and its flow, network
protocols and standards

Data Representation
Data is collection of raw facts which is processed to deduce
information. There may be different forms in which data may be
represented. Some of the forms of data used in communication
are as follows;
1 : text
2 : numbers
3 : images
4 : audio
5 : video

Data can be represented by using different
forms as shown in figure

Text
Text includes combinations of alphabet in small case as well
as upper case. It is stored as pattern of bits.
In data communication , text is represented as a bit pattern
Unicode : 32 bits
ascii_ - first 127 characters in Unicode.
4

Numbers
Numbers include combination of digits from 0 to 9. it is stored as a
pattern of bits . prevalent encoding system : asci, Unicode.
Images
An image is worth a thousand words‖ is a very famous saying. In computers
images are digitally stored.
A Pixel is the smallest element of an image. To put it in simple terms, a picture or
image is a matrix of pixel elements.
The pixels are represented in the form of bits. Depending upon the type of image
(black n white or color ) each pixel would require different number of bits to
represent the value of a pixel.
The size of an image depends upon the number of pixels (also called resolution)
and the bit pattern used to indicate the value of each pixel.

Audio
Audio refers to the recording or broadcasting of sound or
music. Audio is by nature different from text, numbers , or
images.
It is continuous, not discrete. Even when we use a
microphone to change voice or music to an electric signal,
we create a continuous signal
Video
video refers to broadcasting of data in form of
picture or movie.

Data flow
Two devices communicate with each other by
sending and receiving data. The data can flow
between the two devices in the following ways.
1: simplex
2: half duplex
3: full duplex

Simplex
In simplex ,communication is unidirectional only one of the device sends
the data and the other one only receives the data. Example in the below
diagram : a CPU send data while a monitor only receives data.

Half duplex
In half duplex both the stations can transmit as well as receives but not at
the same time.
When one device is sending other can only receives and vice versa(as
shown below in figure)
Example: walkie - talkie

Full duplex
In full duplex mode, both stations can transmit and receives at the same
time.
Example: mobile.

Protocols
A protocol is basically a synonym for the rule. In computer networks,
basically,
Communications occurs between entities in different systems. An entity is
anything that is capable od sending or receiving information . any two
entities cannot simply send bitstreams to each other and expect to be
understood.
A protocol is a set of rules that mainly govern data communications. The
protocol mainly defines what is communicated, how it is communicated,
and when it is communicated.

Key elements of a protocol
The key elements of a protocol are as given below:
syntax This term mainly refers to the structure or format of the
data which simply means the order in which data is presented. For
example, A simple protocol might expect the first 8 bits of data to
be the address of the sender, then the second 8 bits to be the
address of the receiver, and then the rest of the stream to be the
message itself.

Key elements of a protocol
Semantics This term mainly refers to the meaning of each section
of bits. How does a particular pattern to be interpreted, and On the
basis of interpretation what action is to be taken? For example,
does an address identify the route to be taken or the final
destination of the message?
Timing This term mainly refers to two characteristics: At what time
the data should be sent and how fast data can be sent. For
example, if a sender produces data at 100 Mbps but the receiver
can process data at only 1 Mbps, the transmission will overload the
receiver and there will be some data loss.

Standard
Standards are essential in creating and maintaining an open and
competitive market for equipment manufacturers and in
guaranteeing national and international interoperability of data
and telecommunications technology and processes. Standards
provide guidelines to manufacturers, vendors, government
agencies, and other service providers to ensure the kind of
interconnectivity necessary in today's marketplace and in
international communications. Data communication standards fall
into two categories: de facto (meaning "by fact" or "by
convention") and de jure (meaning "by law" or "by regulation").

Standards are of two types :
De Facto Standard.
De Jure Standard.

De Facto Standard : The meaning of the work ” De Facto ” is ” By Fact
” or “By Convention”.
These are the standard s that have not been approved by any
Organization , but have been adopted as Standards because of it’s
widespread use. Also , sometimes these standards are often established
by Manufacturers.
For example : Apple and Google are two companies which
established their own rules on their products which are different . Also
they use some same standard rules for manufacturing for their products.
De facto standard

What is the OSI Model?
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.
OSI model was developed by the International Organization for Standardization (ISO) in
1984.
it is now considered as an architectural model for the inter-computer communications.
OSI model divides the whole task into seven smaller and manageable tasks. Each layer is
assigned a particular task.
OSI consists of seven layers, and each layer performs a particular network function.
OSI Model

Different layers of OSI model-
There are the seven OSI layers :

Physical layer:
The lowest layer of the OSI reference model is the physical layer.
It is responsible for the actual physical connection between the devices.
The physical layer contains information in the form of bits.
It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer
will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will
put the frame back together.

The functions of the physical layer are :
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 topologies: Physical layer specifies the way in which the different, devices/nodes are arranged
in a network i.e. bus, star or mesh topology.
Transmission mode: Physical layer also defines the way in which the data flows between the two
connected devices. The various transmission modes possible are: Simplex, half-duplex and full-duplex.
* Hub, Repeater, Modem, Cables are Physical Layer devices.

The data link layer is responsible for the node to node delivery of the message.
The main function of this layer is to make sure data transfer is error-free from one node to another, over
the physical layer. When a packet arrives in a network, it is the responsibility of DLL to transmit it to the
Host using its MAC address.
Data Link Layer is divided into two sub layers :
Logical Link Control (LLC)
Media Access Control (MAC)
The packet received from Network layer is further divided into frames depending on the frame size of
NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header.
The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the
wire asking “Who has that IP address?” and the destination host will reply with its MAC address

The functions of the data Link layer are -
Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits
that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the
beginning and end of the frame.
Physical addressing: After creating frames, Data link layer adds physical addresses (MAC address) of
sender and/or receiver in the header of each frame.
Error control: Data link layer provides the mechanism of error control in which it detects and retransmits
damaged or lost frames.
Flow Control: The data rate must be constant on both sides else the data may get corrupted thus , flow
control coordinates that amount of data that can be sent before receiving acknowledgement.
Access control: When a single communication channel is shared by multiple devices, MAC sub-layer of
data link layer helps to determine which device has control over the channel at a given time.

Network Layer:
Network layer works for the transmission of data from one host to the other located in different networks.
It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number
of routes available.
The sender & receiver’s IP address are placed in the header by the network layer.

The functions of the Network layer are -
Routing: The network layer protocols determine which route is suitable from source to destination. This
function of network layer is known as routing.
Logical Addressing: In order to identify each device on internetwork uniquely, network layer defines an
addressing scheme. The sender & receiver’s IP address are placed in the header by network layer. Such
an address distinguishes each device uniquely and universally.
**Segment in Network layer is referred as Packet.
** Network layer is implemented by networking devices such as routers

Transport Layer :
Transport layer provides services to application layer and takes services from network layer.
The data in the transport layer is referred to as Segments.
It is responsible for the End to End Delivery of the complete message.
The transport layer also provides the acknowledgement of the successful data transmission and re-transmits
the data if an error is found.

• At sender’s side:
Transport layer receives the formatted data from the upper layers, performs Segmentation and also
implements Flow & Error control to ensure proper data transmission. It also adds Source and Destination
port number in its header and forwards the segmented data to the Network Layer.
• At receiver’s side:
Transport Layer reads the port number from its header and forwards the Data which it has received to the
respective application. It also performs sequencing and reassembling of the segmented data.

The functions of the transport layer are :
Segmentation and Reassembly: This layer accepts the message from the (session) layer , breaks the
message into smaller units . Each of the segment produced has a header associated with it. The
transport layer at the destination station reassembles the message.
Service Point Addressing: In order to deliver the message to correct process, transport layer header
includes a type of address called service point address or port address. Thus by specifying this address,
transport layer makes sure that the message is delivered to the correct process
** Transport layer is operated by the Operating System. It is a part of the OS and
communicates with the Application Layer by making system calls.
Transport Layer is called as Heart of OSI model

The services provided by the transport layer :
Connection Oriented Service: It is a three-phase process which include
– Connection Establishment
– Data Transfer
– Termination / disconnection
In this type of transmission, the receiving device sends an acknowledgement, back to the source after a
packet or group of packet is received. This type of transmission is reliable and secure.
Connection less service: It is a one-phase process and includes Data Transfer. In this type of transmission,
the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication
between devices. Connection-oriented service is more reliable than connectionless Service.
* Data in the Transport Layer is called as Segments

This layer is responsible for establishment of connection, maintenance of sessions, authentication and also
ensures security.
The functions of the session layer are :
Session establishment, maintenance and termination: The layer allows the two processes to establish,
use and terminate a connection.
Synchronization : This layer allows a process to add checkpoints which are considered as synchronization
points into the data. These synchronization point help to identify the error so that the data is re-
synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
Dialog Controller : The session layer allows two systems to start communication with each other in half-
duplex or full-duplex.
The functions of the Session Layer are:

Presentation Layer:
Presentation layer is also called the Translation layer.
The data from the application layer is extracted here and manipulated as per the required format to transmit
over the network.

The functions of the presentation layer are :
Translation : For example, ASCII to EBCDIC.
Encryption/ Decryption : Data encryption translates the data into another form or code. The encrypted data is
known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as
well as decrypting data.
Compression: Reduces the number of bits that need to be transmitted on the network.

Application Layer:
At the very top of the OSI Reference Model stack of layers, we find Application layer which is implemented by
the network applications.
These applications produce the data, which has to be transferred over the network.
This layer also serves as a window for the application services to access the network and for displaying the
received information to the user.
Ex: Application – Browsers, Skype Messenger etc.
**Application Layer is also called as Desktop Layer.

The functions of the Application layer are :
Network Virtual Terminal
FTAM-File transfer access and management
Mail Services
Directory Services
OSI model acts as a reference model and is not implemented in
the Internet because of its late invention. Current model being
used is the TCP/IP model.

TCP/IP Model
TCP/IP model was designed and developed by Department of Defense (DoD) in 1960s and is based on
standard protocols.
It stands for Transmission Control Protocol/Internet Protocol.
The TCP/IP model is a concise version of the OSI model. It contains four layers, unlike seven layers in the
OSI model.

Network Access Layer :
This layer corresponds to the combination of Data Link Layer and Physical Layer of the OSI model.
It looks out for hardware addressing and the protocols present in this layer allows for the physical
transmission of data.
We just talked about ARP being a protocol of Internet layer, but there is a conflict about declaring it as a
protocol of Internet Layer or Network access layer.
It is described as residing in layer 3, being encapsulated by layer 2 protocols

Internet Layer :
This layer parallels the functions of OSI’s Network layer. It defines the protocols which are responsible for
logical transmission of data over the entire network.
The main protocols residing at this layer are :
IP – stands for Internet Protocol and it is responsible for delivering packets from the source host to the
destination host by looking at the IP addresses in the packet headers. IP has 2 versions:
IPv4 and IPv6. IPv4 is the one that most of the websites are using currently. But IPv6 is growing as the
number of IPv4 addresses are limited in number when compared to the number of users.
ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is
responsible for providing hosts with information about network problems.
ARP – stands for Address Resolution Protocol. Its job is to find the hardware address of a host from a
known IP address. ARP has several types: Reverse ARP, Proxy ARP, Gratuitous ARP and Inverse ARP

Host-to-Host Layer :
This layer is analogous to the transport layer of the OSI model.
It is responsible for end-to-end communication and error-free delivery of data.
It shields the upper-layer applications from the complexities of data.
The two main protocols present in this layer are :
Transmission Control Protocol (TCP) – It is known to provide reliable and error-free communication between
end systems. It performs sequencing and segmentation of data. It also has acknowledgment feature and
controls the flow of the data through flow control mechanism. It is a very effective protocol but has a lot of
overhead due to such features. Increased overhead leads to increased cost.
User Datagram Protocol (UDP) – On the other hand does not provide any such features. It is the go-to
protocol if your application does not require reliable transport as it is very cost-effective. Unlike TCP, which is
connection-oriented protocol, UDP is connectionless

Application Layer:
An application layer is the topmost layer in the TCP/IP mode
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

What is Network Topology?
Network topology refers to how various nodes, devices, and connections on your network
are physically or logically arranged in relation to each other. The way a network is
arranged can make or break network functionality, connectivity, and protection from
downtime.
Categories -
Physical Network Topology
The physical network topology refers
to the actual connections (wires,
cables, etc.) of how the network is
arranged. Setup, maintenance, and
provisioning tasks require insight into
the physical network.
Logical Network Topology
Logical network topology is a little
more abstract and strategic,
referring to the conceptual
understanding of how and why the
network is arranged the way it is,
and how data moves through it.

Explanation -
● Physical topology explains the
arrangement of different nodes.
● Logical topology reflects the
communication of data between
various nodes.
● For example, in the picture, the
dotted line shows how PC-A is
connected to the server through the
switch.
Also, the data from PC-A will have
to pass through two switches & two
routers to reach PC-D.
This data flow is what logical
topology is concerned about.

Bus Topology
• The bus topology is designed in such a way that all the
stations are connected through a single cable known as a
backbone cable.
• Each node is either connected to the backbone cable by
drop cable or directly connected to the backbone cable.
• When a node wants to send a message over the
network, it puts a message over the network. All the
stations available in the network will receive the message
whether it has been addressed or not.
• It is alternatively known as Line Topology.
BUS TOPOLOGY -

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Ring Topology
Single Ring Topology
Dual - Ring Topology
• It is a type of network topology where each node is exactly connected to
two other nodes, thus forming a single continuous path for transmission.
• In a ring network, packets of data travel from one device to the next until
they reach their destination. Message transmission takes place with the help
of TOKEN.
• A number of repeaters are connected with large number of nodes.
• Single Ring Topology - Most ring topologies allow packets to travel only in
one direction, called a Unidirectional/Single Ring Topology.
• Dual Ring Topology - Others permit data to move in either direction,
called bidirectional/ Dual-Ring Topology.

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PROS
• Only one station on the network is
permitted to send data at a time, which
greatly reduces the risk of packet
collisions.
• Additional workstations can be added
without impacting performance of the
network.
• These are cost-effective and
inexpensive to install
CONS
• All data being transferred over the
network must pass through each
workstation on the network, which can
make it slower than a star topology.
• The entire network will be impacted if
one workstation shuts down.
PROS OF DUAL-RING
• Very efficient - because each node has
two connections, information can be sent
both clockwise and counterclockwise
along the network.
• Dual ring topologies offer a little extra
security, too: if one ring fails within a
node, the other ring is still able to send
data.

Tree Topology -
• A tree topology is a special type of structure where many
connected elements are arranged like branches of tree.
• It has a parent-child hierarchy to how the nodes are
connected.
• This topology integrates various star topologies together in
a single bus, so it is known as a Star Bus topology.

PROS
• Tree topology is mainly used to provide
broadband transmission, i.e., signals are sent
over long distances without being attenuated.
• It provides high scalability as leaf nodes can
add more nodes in the hierarchical chain.
• Other nodes in network are not affected, if
one of their nodes get damaged.
• It provides easy maintenance and fault
identification.
CONS
• Large cabling is required as
compared to star and bus
topology.
• On the failure of a hub, the
entire network fails.
• Tree network is very difficult to
configure than other network
topologies.

Mesh Topology
• Mesh technology is an arrangement of the network in which
computers are interconnected with each other through various
redundant connections.
• There are multiple paths from one computer to another computer.
• It does not contain the switch, hub or any central computer which
acts as a central point of communication.
Types –
•Full Mesh Topology - Every computer in the network has a
connection to each of the other computers in that network.
•Partially – Connected Mesh Topology - At least two of the
computers in the network have connections to multiple other
computers in that network. If one of the primary computers or
connections in the network fails, the rest of the network continues to
operate normally.

PROS
CONS
• Very reliable - if any link breakdown will not affect the
communication between connected computers.
• Manages high amounts of traffic, because multiple
devices can transmit data simultaneously.
• The complex degree of interconnectivity between
nodes makes the network resistant to failure.
• Adding new devices would not disrupt the
communication between other devices.
• The cost to implement is higher than other
network topologies, making it a less desirable
option.
• Each interconnection between nodes requires
a cable and configuration once deployed, so it
can also be time-consuming to set up.
• The chance of redundant connections is high,
which adds to the high costs and potential for
reduced efficiency.

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Hybrid Topology -
• Hybrid Topology is an integration of two or more
different topologies to form a resultant topology.
• For example, if there exist a ring topology in one branch
of ICICI bank and bus topology in another branch of ICICI
bank, connecting these two topologies will result in Hybrid
topology.
• NOTE - If similar topologies are connected with each
other will not result in Hybrid topology.

Local Area Network (LANs) -
• A local area network (LAN) is a computer network that interconnects computers within a limited
area such as a residence, school, laboratory, university campus, office etc.
• Range covered - 1km to 10km
• Cables used - Unshielded Twisted Pair (Ethernet Cables)
• Examples - Wifi & Ethernet
In the past, all nodes were connected through a common
cable, which meant that a packet sent from one host to
another was received by all hosts. The intended
recipient kept the packet; the others dropped the
packet.
Today, most LANs use a smart connecting switch, which
is able to recognize the destination address of the
packet and guide the packet to its destination without
sending it to all other hosts.

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Metropolitan Area Network (MANs)
• A metropolitan area network (MAN) is a
computer network that connects computers within
a single large city, multiple cities and towns, or any
given large area with multiple buildings.
• A MAN is larger than a local area network (LAN)
but smaller than a wide area network (WAN).
• Range covered - 10km to 50km.
• Cables used - Fiber Optic Cables & Twisted Pair
Cables.
• Examples - Cable TV network, Telephone
networks providing high-speed DSL lines.

Wide Area Network (WANs) -
• WANs are used to connect LANs & MANs and other
types of networks together so that users and
computers in one location can communicate with users
and computers in other locations.
• WANs are often used by large businesses to connect
their office networks; each office typically has its own
local area network, or LAN, and these LANs connect
via a WAN.
• Range Covered - More than 50 kms.
• Cables Used – Fibre optic cables
• Example - Internet

Personal Area Network
• Personal Area Network (PAN) is a the computer network that connects computers/devices within the range of
an individual person. It typically involves a computer, phone, tablet, printer, PDA (Personal Digital Assistant)
etc.
• Range covered - less than 10 meters.
Types of PAN -
Wireless PAN –
WPAN is connected
through signals such as
infrared, Bluetooth and
ultra wideband etc.
Wired PAN –
Wired PAN is
connected through
cables/wires such
as USB.

NETWORKING TRANSMISSION MEDIA-
• The transmission medium can be defined as a pathway that can transmit
information from a sender to a receiver.
• Transmission media are located below the physical layer and are controlled by
the physical layer.
• Transmission media are also called communication channels.
• Transmission media are of two types −
Guided Transmission Medium
Unguided Transmission Medium

Guided Transmission Medium-
Guided transmission media are also called bounded
media or wired media. They comprise cables or wires
through which data is transmitted. They are called guided
since they provide a physical conduit from the sender device
to the receiver device. The signal traveling through these
media are bounded by the physical limits of the medium.
The most popular guided media are −
• Twisted pair cable
• Coaxial cable
• Fiber optics

Twisted Pair -
Issues:
(1) Interference due to unwanted electrical coupling of two copper
(2) Interference due to unwanted electrical coupling between the
neighboring twisted pairs
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. Installation of the
twisted pair cable is easy, and it is a lightweight cable. 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.
The degree of reduction in noise interference is determined by the number of turns per
foot. Increasing the number of turns per foot decreases noise interference.

Twisted Pair
Applications
o Most commonly used medium
o Telephone network
o Between house and local exchange
(subscriber loop)
o Within buildings
o To private branch exchange (PBX)
o For local area networks (LAN)
o 10Mbps or 100Mbps

Advantages of Twisted pair cable :
•It are often wont to carry both analog and digital data.
•It’s relatively easy to implement and terminate.
•It is the smallest amount expensive media of transmission for brief distances.
•If portion of a twisted pair cable is broken it doesn’t effect the whole network.
•Less vulnerable to electrical interference caused by nearby equipment or wires.
•It cause interference themselves.
•Best performance in short distances.
•High-cost performance
Disadvantages of Twisted pair cable :
•It result signal distortion in a very effective manner.
•Attenuation is very high.
•It supports 10 mbps upto a distance of 100 meters on a 10BASE-T which are considered to be
low bandwidth.
•It provides poor security and is relatively easy to tap.
•As they a thin so can be easily breakable.
•Low durability (must be maintained regularly).
•Susceptible to electromagnetic interference (EMI).

• Unshielded Twisted Pair (UTP)
—Ordinary telephone wire
—Less expensive
—Weak immunity against noise and interference
—Suffers from external EM interference
• Shielded Twisted Pair (STP)
—An extra metallic sheath on each pair
—Relatively more expensive
—Provide better performance than UTP
• Increased Data rate
• Increased Bandwidth
Unshielded and Shielded TP

Coaxial Cable
Architecture:
•Coaxial cable is very commonly used transmission media, for
example, TV wire is usually a coaxial cable.
•The name of the cable is coaxial as it contains two conductors
parallel to each other.
•It has a higher frequency as compared to Twisted pair cable.
•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.
•The middle core is responsible for the data transferring whereas
the copper mesh prevents from the EMI(Electromagnetic
interference).

• Television (TV) signals distribution
• Ariel to TV
• Cable TV
• Long distance telephone transmission
• Can carry 10,000 voice calls simultaneously
• Being replaced by fiber optic
• Short distance computer systems links
• Local area networks (LAN)
• Metropolitan area network (MAN)
Coaxial Cable
Applications

Advantages Of Coaxial cable:
•The data can be transmitted at high speed.
•It has better shielding as compared to twisted pair
cable.
•It provides higher bandwidth
.
Disadvantages Of Coaxial cable:
•It is more expensive as compared to twisted pair
cable.
•If any fault occurs in the cable causes the failure in
the entire network.

Fibre Optic
•Fibre optic cable is a cable that uses electrical signals for communication.
•Fibre optic is a cable that holds the optical fibres coated in plastic that are used to send the data by pulses of light.
•The plastic coating protects the optical fibres from heat, cold, electromagnetic interference from other types of wiring.
•Fibre optics provide faster data transmission than copper wires.
Diagrammatic representation of fibre optic cable:

Basic elements of Fibre optic cable:
•Core:
The optical fibre consists of a narrow strand of glass or plastic known as
a core. A core is a light transmission area of the fibre. The more the area
of the core, the more light will be transmitted into the fibre.
•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 fibre.
•Jacket:
•The protective coating consisting of plastic is known as a jacket. The
main purpose of a jacket is to preserve the fibre strength, absorb shock
and extra fibre protection.

Unguided Transmission Medium
Unguided transmission media are also called wireless media. They transport data in the
form of electromagnetic waves that do not require any cables for transmission. These
media are bounded by geographical boundaries. These type of communication is
commonly referred to as wireless communications.
Unguided signals can travel in three ways −
•Ground propagation
•Sky propagation
•Line – of – sight propagation
The commonly used unguided transmissions are −
•Radio transmission
•Microwave transmission
•Infrared transmission
•Light transmission

Propagation methods
Unguided signals travels from the source to destination in several ways it is
known as propagation.
They are three types:
▪ Ground propagation
▪ Sky propagation
▪ Line-of-Sight Propagation

▪ Omnidirectional Antenna
▪ Frequencies between 3 KHz and 1 GHz.
▪ Used for multicasts(multiple way) communications, such as
radio and television, and paging system.
▪ Radio waves can penetrate buildings easily, so that widely use
for indoors & outdoors communication.
Unguided Media – Radio Waves

An Antenna is a structure that is generally a metallic object may be a wire or group of wires,
used to convert high frequency current into electromagnetic waves.
Antenna are two types:
• Transmission antenna
▪ Transmit radio frequency from transmitter
▪ Radio frequency then
Convert to electromagnetic energy by antenna
▪ Then, radiate into surrounding environment
• Reception antenna
▪ Electromagnetic energy get in antenna
▪ Then Antenna convert radio frequency to electrical energy
▪ Then, Goes to receiver
same antenna can be used for both purposes
Antennas

Microwaves are ideal when large areas need to be covered and there are no obstacles in the path
Microwaves

Micro waves Transmission
• Microwaves are unidirectional
• Micro waves electromagnetic waves having frequency between 1 GHZ
and 300 GHZ.
• There are two types of micro waves data communication system
: terrestrial and satellite
• Micro waves are widely used for one to one communication
between sender and receiver,
example: cellular phone, satellite networks and in wireless
LANs(wifi), WiMAX,GPS

▪ Frequencies between 300 GHz to 400 THz.
▪ Used for short-range communication
▪ Example: Night Vision Camera,Remote control, File sharing
between two phones, Communication between a PC and peripheral
device,
INFRARED

What are network devices?
Network Devices are components used to connect computers or
other electronics devices together so that they can share files or
resources like printers or fax machine

Types of Networking Devices -
•Hub
•Switch
•Router
•Bridge
•Gateway
•Modem
•Repeater
•Access Point

HUB
• A hub is a physical layer networking device which is used to
connect multiple devices in a network. They are generally
used to connect computers in a LAN.
• A hub has many ports in it. A computer which intends to be
connected to the network is plugged in to one of these ports.
When a data frame arrives at a port, it is broadcast to every
other port, without considering whether it is destined for a
particular destination or not.

Types of Hub
1 Active hub
These hubs regenerate our signals as well as amplify the signal.
Active hubs need electricity to work.
2 Passive hub
Talking about passive hubs, it simply distributes the signal coming
from the previous ports. Passive hub neither regenerates any signal nor
amplifies, therefore it does not require electricity to work.
3 Intelligence hub
This helps the administrator to monitor network traffic, and you can
configure each port on it individually, also known as a manageable hub.

Switch
• A switch is a data link layer networking
device which connects devices in a
network and uses packet switching to
send and receive data over the network.
• Like a hub, a switch also has many ports,
to which computers are plugged in.
However, when a data frame arrives at
any port of a network switch, it examines
the destination address and sends the
frame to the corresponding device(s).
Thus, it supports both unicast and
multicast communications.

Router
• A router is a device like a switch that routes data packets
based on their IP addresses. Router is mainly a Network
Layer device. Routers normally connect LANs and WANs
together and have a dynamically updating routing table
based on which they make decisions on routing the data
packets. Router divide broadcast domains of hosts
connected through it.
• There are wired, wireless, core, edge and virtual routers
available. Routers usually select the best route to direct the
packets to reach faster. The best example is mail carrier.

Bridge
• Bridges are used to connect two or more hosts or
network segments together. The basic role of bridges in
network architecture is storing and forwarding frames
between the different segments that the bridge connects.
They use hardware Media Access Control (MAC)
addresses for transferring frames.
• Bridges can also be used to connect two physical LANs
into a larger logical LAN.
• Bridges work only at the Physical and Data Link layers of
the OSI model.
• Bridges are like hubs in many respects, including the fact
that they connect LAN components with identical
protocols. However, bridges filter incoming data packets,
known as frames, for addresses before they are
forwarded.

GATEWAY
• Gateways normally work at the Transport and
Session layers of the OSI model. At the
Transport layer and above, there are numerous
protocols and standards from different vendors;
gateways are used to deal with them.
Gateways provide translation between
networking technologies such as Open System
Interconnection (OSI) and Transmission
Control Protocol/Internet Protocol (TCP/IP).
• Gateways perform all of the functions of routers
and more. In fact, a router with added
translation functionality is a gateway.

Modem
• Modems (modulators-demodulators) are used to transmit digital signals over
analog telephone lines. Thus, digital signals are converted by the modem
into analog signals of different frequencies and transmitted to a modem at
the receiving location. The receiving modem performs the reverse
transformation and provides a digital output to a device connected to a
modem, usually a computer.
• A modem works as a Modulator and Demodulator both; that is; it modulates
and demodulates the signal between the binary data or digital data of a
computer and therefore the analog signal of a telephone line.

Repeater
A repeater is an electronic device that amplifies the signal it
receives. You can think of repeater as a device which receives a
signal and retransmits it at a higher level or higher power so that
the signal can cover longer distances, more than 100 meters for
standard LAN cables. Repeaters work on the Physical layer.

Access point
A wireless access point (WAP) is a networking
device that allows wireless-capable devices to
connect to a wired network. It is simpler and
easier to install WAPs to connect all the
computers or devices in your network than to
use wires and cables.

PSTN
(Public Switched
Telephone Network)

What is PSTN?
● The Public Switched Telephone Network(PSTN), also known as Plain Old Telephone
Service(POTS), is the wired phone system over which landline telephone calls are
made.
● The PSTN relies on circuit switching. To connect one phone to another, the phone call
is routed through numerous switches operating on a local, regional and national or
international level.
● These network of telephone lines are owned by both governments as well as
commercial organizations.

Circuit Switching Technique
Now here comes the circuit switching technique on which PSTN relies on:-Communication via circuit switching implies that
there is a dedicated communication path between the two stations. The path is connected through a sequence of links between
network nodes.
CIRCUIT ESTABLISHMENT: To establish an end-to-end connection before any transfer of data. Some segments of the circuit
may be a dedicated link, while some other segments may be shared.
DATA TRANSFER: Transfer of data is from the source to the destination. The data may be analog or digital, depending on the
nature of network. The connection is generally full-duplex.
CIRCUIT DISCONNECT: Terminate connection at the end of data transfer. Signals must be propagated to deallocate the
dedicated resources.

HISTORY
● It has evolved from the invention of telephone by Alexander Graham Bell.
● In early days phone calls traveled as analog signals across copper wire. Every
phone call needed its own dedicated copper wire connecting the two phones.
● The operators sat at a switchboard, literally connecting one piece of copper wire
to another so that the call could travel across town or across the country.
● Long-distance calls were comparatively expensive, because you were renting the
use of very long piece of copper wire every time you made a call.
● Present telephone signals are tightly coupled with WANs(Wide Area
networks) and are used for both data and voice communications.

Levels in PSTN
The switching centers used for switching are organized in different levels, namely:-
● Regional offices(Level 1)
● Section offices(Level 2)
● Primary offices(Level 3)
● Toll offices(Level 4)
● End offices(Level 5)
Level 1 is at the highest level and
Level 5 is the lowest level.
Figure:Basic organization of a Public Switched Telephone Network(PSTN)

EXPLANATION
● Subscribers or the customers are directly connected to these end offices. And each office is
connected directly to a number of offices at a level below and mostly a single office at higher
level. Subscriber Telephones are connected, through Local Loops to end offices(or central
offices).
● A small town may have only one end office, but large cities have several end offices.
● Many end offices are connected to one Toll office, which are connected to primary offices.
● Several primary offices are connected to a Section office, which normally serves more than one
state.
● All regional offices are connected using mesh topology. Accessing the switching station at the
end offices is accomplished through dialing.
● In the past, telephone featured rotary or pulse dialing, in which digital signals were sent to the
end office for each dialed digit.
● This type of dialing was prone to errors due to inconsistency in humans during dialing.

Cont…..
● Presently, dialing is accomplished by Touch-Tone technique.
● In this method the user sends a small burst of frequency called dual tone, because it is a
combination of two frequencies.
● This combination of frequencies sent depends on the row and column of the pressed pad.
● The connections are multiplexed when have to send to a switching office, which is one level
up. For example, Different connections will be multiplexed when they are to be forwarded
from an end office to Toll office. Figure shows a typical medium distance telephone circuit.
Figure:Typical medium distance telephone circuit

Structure Of The Telephone System
● Shortly after Alexander Graham Bell patented the phone in 1876 (just hours before its
competitor, Elisha Gray), his new invention became indispensable.
● As new inventions came and according to comfortability , there were many structures but
mainly 3 were there which are -
1. Fully Interconnected Network
2.Centralized switch
3.Two-level hierarchy

1. Fully Interconnected Network
● The initial market was the sale of handsets in pairs.
● It was up to the customer to connect a single cable between them.
● If the owner of the phone wanted to speak with other owners of the phone, separate
cables had to be connected to all the homes.
● During the year, the city was covered with cables that passed over homes and trees
in wild confusion.
● Once it became clear that the model to connect all phones on all the rest of the
phone, shown in the following figure.

2. Centralized switch
● The company sent a telegram to each home or office of each client.
● To make a call, the customer had to start the phone so that a call signal sounded in
the telephone company’s office to catch the operator’s attention.
● He then had to manually connect the caller to the caller using a connection cable.
● In short, to connect it. The model called Single Switching Office Model is shown in the
following figure.

3. Two-level hierarchy
● Very quickly, Bell switching centers appeared everywhere and people wanted to
make long-distance calls between cities.
● The Bell System has started to connect switching centers.
● The initial problem quickly returned: the connection of each wire center to any
other wire-connected switching center quickly became out of control, so second-
level switching centers were invented.
● After a while, it took several second-level offices, as shown in the following figure.

The Local Loop: Modems, ADSL, and Fiber
● The local loop is often called the “last mile”, although it can be several
kilometers long. It has been carrying analog information for over 100 years
and will likely continue to do so for many years due to the high cost of digital
conversion.
● Telephone modems transmit digital data between computers over a narrow
channel provided by the telephone network for voice calls. They were once
widely used, but have been largely supplanted by broadband technologies such
as ADSL. reuse the local loop to send digital data from the customer to the final
office, where it is transmitted to the Internet.
● The following local loops are often considered important:

1.Telephone Modems
● Telephone modems are used to transfer bits between two computers on a
voice phone line, rather than for a conversation, which usually occupies the
line.
● The main difficulty is that the voice telephone line is limited to 3,100 Hz,
which is enough for a conversation. This bandwidth is more than four orders
of magnitude less than the bandwidth used for Ethernet or 802.11 (WiFi). As
expected, data rates for telephone modems are also four orders of magnitude
lower than those for Ethernet and 802.11.
● Logically, a modem is inserted between the computer (digital) and the
telephone system (analog), as shown in the following figure:
Figure: The use of both analog and digital transmission for a computer-to-computer call.

2.Digital Subscriber Lines
● Initially, there were several high-speed incrustations under the general name
xDSL (digital subscriber line) for several x’s.
● Services with more bandwidth than standard telephone services are sometimes
called broadband, although this term is a marketing concept rather than a
specific technical concept.
● Unfortunately, the capacity of the local loop decreases quite rapidly with the
distance from the end office, as the signal deteriorates along the cable. It also
depends on the thickness and overall quality of the twisted pair.
● The graph of potential bandwidth versus distance is shown in the following
figure. In this figure, it is assumed that all other factors are optimal (new cables,
modest packages, etc.).

3.Fiber To The Home
● In general, fiber houses are combined so that only one fiber reaches the final office per
group of 100 houses.
● In the downward direction, the optical dividers divide the final desktop signal so that it
reaches all the homes. Encryption is necessary for security if only one house can decode a
signal.
● Upward, the optical adders combine the house signals into a single signal, which is
received in the last office. This architecture is called PON (passive optical network) and is
shown in the following figure.
● In general, for all downlink channels, the total wavelength is used for transmission in the
downstream direction and another wavelength is used for transmission in the upstream
direction.
Figure: Passive optical network for Fiber to The Home.

Trunks and Multiplexing
● The heart of the telephone network carries digital and non-analog information.
● This requires scanning in the end office for transmission over long-distance lines.
● Long-distance lines make thousands, if not millions, of calls at the same time.
● This exchange in trunks and multiplexing is important for achieving economies of
connecting
scale, as the installation and maintenance of a high-capacity backbone cost
two
essentially the same amount as the low-capacity mainline
switching offices.
● This is achieved by using some trunks and multiplexing versions.
1. Digitizing Voice Signals
2. Time Division Multiplexing
3. SONET/SDH
4. Wavelength Division Multiplexing

1.Digitizing Voice Signals
● Digitizing Voice Signals at the beginning of the development of
the telephone network, the kernel handled voice calls as analog
information.
● Wavelength Division Multiplexing methods have been used for
many years to multiplex voice channels of 4,000 Hz (composed of
3,100 Hz plus guard bands) into larger and larger blocks.

2.Time Division Multiplexing
● PCM-based Time Division Multiplexing is used to transmit multiple voice calls over
trunks by sending a sample of each call every 125 μsec.
● When digital transmission became a viable technology, the ITU (then called
CCITT) could not agree on an international standard for the MIC.
● As a result, various incompatible systems are being used in different countries
around the world.

3.SONET/SDH
● In 1985, Bell-core, RBOC’s research department, began working on a standard called
SONET/SDH (Synchronous Optical Network). Design SONET/SDH pursued four main
objectives.
● First of all, SONET/SDH had to ensure the interaction of the different operators. To
achieve this goal, it was necessary to define a common signaling standard for
wavelength, duration, frame structure, and other problems.
● Secondly, means are needed to integrate digital EE systems. The European Union,
Europe, and Japan, all based on 64 kbps PCM channels, but combined in different
(and incompatible) ways.
● Third, SONET/SDH was supposed to provide a way to multiplex several digital
channels. At the time of SONET’s design, the fastest digital operator, widely used in
the United States, was T3 at 44,736 Mbit / s. T4 has been defined, but little has been
used and nothing has been defined above the speed of T4. Part of SONET’s mission
was to continue the hierarchy up to gigabits / s and up. A standard method of
multiplexing slower channels in a SONET/SDH channel was also needed.
● Fourth, SONNET/SDH must support the operations, administration, and maintenance
(OAM) required to administer the network. The previous systems are not very good
and can’t do this very well.

4.Wavelength Division Multiplexing
● Frequency division multiplexing, as well as Time Division Multiplexing, is used to exploit
the enormous bandwidth of optical fiber channels. This is called Wavelength Division
Multiplexing (WDM ).
● Here, four fibers are combined in an optical adder, each having energy having a different
wavelength.
● Four beams are combined into a common fiber for transmission to a remote
destination.At the opposite end, the beam is divided into as many fibers as there were at
the entrance.
● Each output fiber contains a short core specially designed to filter all wavelengths minus
one.The resulting signals can be sent to the destination or can be combined in different
ways for additional multiplex transport.

Wireless Local Area
Network (WLAN)

INTRODUCTION
WLAN stands for Wireless Local Area Network or Local Area Wireless
Network (LAWN) .
Norman Abramson, a professor at the University of Hawaii, developed
the world’s first wireless computer communication network , ALOHA
net (operational in 1971).

INTRODUCTION
WLAN is a wireless computer network that link two or more devices
(using spectrum or OFDM radio) with in a limited area such as a
home, school, computer laboratory.
WLAN is marketed under the Wi-Fi brand name.
Wireless LAN’s have become popular in home due to ease of
installation and use.

ADVANTAGES OF WLAN
● Installation flexibility.
● Few transmitters/receivers for multiple of users.
● No cable to pull.
● Reduced cost-of-ownership.
● Mobile devices are less expensive than computer workstations
● No need to build wiring closets.

ADVANTAGES OF WLAN
● Mobility
• Access to real time information.
• Provides service opportunities.
• Promotes flexibility.
• Support Productivity.

DISADVANTAGES OF WLAN
● Cost
● Wireless networks card cost 4 times more than wired network card.
● The access points are more expensive than hubs and wires.
● Environmental Conditions
• Constrained by building, trees and terrain.
• Susceptible to weather and solar activity.
● Less capacity : Slower bandwidth.

TYPES OF WLAN
1) INFRASTRUCTURE
•Most Wi-Fi networks are deployed in infrastructure mode.
• In infrastructure mode, a base station acts as a wireless access point hub, and nodes
communicate through the hub. The hub usually, but not always, has a wired
or fiber network connection, and may have permanent wireless connections to other nodes.
• Wireless access points are usually fixed, and provide service to their client nodes within range.
• Wireless clients, such as laptops, smart phones etc. connect to the access point to join the
network.

TYPES OF WLAN
2) Peer to Peer
• Two PCs equipped with wireless adapter cards can be set up as an
independent network whenever they are within range of one another.
• A peer-to-peer network allows wireless devices to directly
communicate with each other.

TYPES OF WLAN
3) BRIDGE
• A bridge can be used to connect networks, typically of different types.
• A wireless Ethernet bridge allows the connection of devices on a wired
Ethernet network to a wireless network.
• The bridge acts as the connection point to the Wireless LAN.

TYPES OF WLAN
4)Wireless distribution system
• A Wireless Distribution System enables the wireless
interconnection of access points in an IEEE 802.11 network.

Wireless LAN Components
Wireless LANs consist of components similar to traditional Ethernet-wired LANs. In
fact, wireless LAN protocols are similar to Ethernet and comply with the same form
factors. The big difference, however, is that wireless LANs don't require wires.
1. User Devices
Users of wireless LANs operate a multitude of devices, such as PCs, laptops, and
PDAs. The use of wireless LANs to network stationary PCs is beneficial because of
limited needs for wiring. Laptops and PDAs, however, are commonly equipped with
wireless LAN connectivity because of their portable nature. User devices might
consist of specialized hardware as well.

2) Radio NICs
A major part of a wireless LAN includes a radio NIC that operates within the computer device
and provides wireless connectivity. A wireless LAN radio NIC, sometimes referred to as a
radio card, often implements the 802.11 standard. The cards generally implement one
particular physical layer, such as 802.11a or 802.11b/g. As a result, the radio card must utilize
a version of the standard that is compatible with the wireless LAN. Wireless LAN radio cards
that implement multiple versions of the standard and provide better interoperability are
becoming more common.
Radio cards come in a variety of form factors, including: ISA, PCI, PC card, mini-PCI, and CF.
PCs generally utilize ISA and PCI cards; but PDAs and laptops use PC cards, mini-PCI, and CF
adapters.

3) Access Points
An access point contains a radio card that communicates with
individual user devices on the wireless LAN, as well as a wired NIC that
interfaces to a distribution system, such as Ethernet. System software
within the access point bridges together the wireless LAN and
distribution sides of the access point. The system software
differentiates access points by providing varying degrees of
management, installation, and security functions. Figure 5-1 shows an
example of access-point hardware.

4) Routers
By definition, a router transfers packets between networks. The router chooses the next
best link to send packets on to get closer to the destination. Routers use Internet Protocol
(IP) packet headers and routing tables, as well as internal protocols, to determine the best
path for each packet.
A wireless LAN router adds a built-in access point function to a multiport Ethernet router.
This combines multiple Ethernet networks with wireless connections. A typical wireless
LAN router includes four Ethernet ports, an 802.11 access point, and sometimes a parallel
port so it can be a print server. This gives wireless users the same ability as wired users to
send and receive packets over multiple networks

5) Repeaters
Access points, which require interconnecting cabling, generally play a dominant
role for providing coverage in most wireless LAN deployments. Wireless
repeaters, however, are a way to extend the range of an existing wireless LAN
instead of adding more access points. There are few standalone wireless LAN
repeaters on the market, but some access points have a built-in repeater
mode.

6) Antennae
Most antennae for wireless LANs are omnidirectional and have low gain. Nearly
all access points, routers, and repeaters come standard with omnidirectional
antenna. Omnidirectional antenna satisfy most coverage requirements; however,
consider the use of optional directive antennae to cover a long, narrow area. In
some cases, the antenna is integrated within a radio card or access point and
there is no choice to make. If a need exists to use a directive antenna (higher gain),
ensure that the radio card or access point has an external antenna connector.

• Multiplexing to refer to the combination of information streams from multiple sources for
transmission over a shared medium.
• Multiplexer is a mechanism that implements the concept
• Demultiplexing to refer to the separation of a combination back into separate information
streams.
• Demultiplexer to refer to a mechanism that implements the concept.
• Figure illustrates the concept
• each sender communicates with a single receiver
• all pairs share a single transmission medium
• multiplexor combines information from the senders for transmission in such a way that the
demultiplexor can separate the information for receivers
MULTIPLEXING:

Need of Multiplexing :-
• Transmitting two or more signals simultaneously can be accomplished by setting up one
transmitter- receiver pair for each channel, but this is an expensive approach.
• A single cable or radio link can handle multiple signals simultaneously using a technique
known as multiplexing.
• Multiplexing permits hundreds or even thousands of signals to be combined and transmitted
over a single medium.
• Cost savings can be gained by using a single channel to send multiple information
signals.

The Basic Types of Multiplexing -
There are four basic approaches to multiplexing that each have a set of variations and
implementations
• Frequency Division Multiplexing (FDM)
• Wavelength Division Multiplexing (WDM)
• Time Division Multiplexing (TDM)
• Code Division Multiplexing (CDM)
•TDM and FDM are widely used
•WDM is a form of FDM used for optical fiber
•CDM is a mathematical approach used in cell phone mechanisms

Time Division Multiplexing (TDM) -
• Usually used with digital signals or analog signals carrying digital data
• Data from various sources are carried in repetitive frames
• Each frame consists of of a set of time slots
• Each source is assigned one or more time slots per frame.

Time Division Multiplexing -
Definition: Time Division Multiplexing (TDM) is the time interleaving of samples from several
sources so that the information from these sources can be transmitted serially over a single
communication channel.
At the Transmitter
• Simultaneous transmission of several signals on a time-sharing basis.
• Each signal occupies its own distinct time slot, for the duration of the transmission.
• Slots may be permanently assigned on demand.
At the Receiver
• Decommutator (sampler) has to be synchronized with the incoming waveform
Frame Synchronization
• Applications of TDM: Digital Telephony, Data communications, Satellite Access, Cellular radio.

TDM -
Composition of one frame of a multiplexed PAM signal incorporating four voice- signals
and a synchronizing pulse.

Synchronous TDM
• TDM is a broad concept that appears in many forms
• It is widely used throughout the Internet
• Figure shows items being sent in a round-robin fashion
• Most TDMs work this way
• No gap occurs between bits if a communication system uses synchronous transmission.
• When TDM is applied to synchronous networks, no gap occurs between items; the result is known as
Synchronous TDM
•Figure illustrates how synchronous TDM works for a system of four senders.

•Data rate of medium exceeds data rate of digital signal to be transmitted
•Multiple digital signals interleaved in time
•May be at bit level of blocks
•Time slots preassigned to sources and fixed
•Time slots allocated even if no data
•Time slots do not have to be evenly distributed amongst sources
Synchronous Time Division Multiplexing

• Telephone systems use synchronous TDM to multiplex digital streams
from multiple phone calls
• They use the acronym TDM to refer to the specific form of TDM used to
multiplex digital telephone calls
• The phone system TDM includes an interesting technique to ensure that a
demultiplexer stays synchronised with the multiplexer
• Why is synchronisation needed?
•observe that a synchronous TDM sends one slot after another without
any indication of the output to which a given slot occurs
•A demultiplexer cannot tell where a slot begins– a slight difference in the
clocks used to time bits can cause a demultiplexer to misinterpret the bit
stream.

•To prevent misinterpretation, the version of TDM used in the phone system
includes an extra framing channel as input
•Instead of taking a complete slot, framing inserts a single bit in the stream on
each round
•A demultiplexer extracts data from the framing channel and checks for
alternating 0 and 1 bits
•If an error causes a demultiplexer to lose a bit
•it is highly likely that the framing check will detect the error and allow the
transmission to be restarted
•Figure illustrates the use of framing bits

Framing Used in the Telephone System Version of TDM -

The Problem with Synchronous TDM: Unfilled Slots
• Synchronous TDM works well if each source produces data at a uniform, fixed rate equal to 1/N of the
capacity of the shared medium
• Many sources generate data in bursts, with idle time between bursts
• To understand why, consider the example in Figure

• Sources on the left produce data items at random the synchronous multiplexor leaves a
slot unfilled if the corresponding source has not produced an item by the time the slot
must be sent.
• In practice, a slot cannot be empty because the underlying system must continue
to transmit data
• the slot is assigned a value (such as zero)
• and an extra bit is set to indicate that the value is invalid

How can a multiplexing system make better use of a shared medium?
•One technique to increase the overall data rate is known as statistical TDM or
statistical multiplexing or Asynchronous TDM
•Some literature uses the term asynchronous TDM
•The technique is straightforward:
•select items for transmission in a round-robin fashion
•but instead of leaving a slot unfilled, skip any source that does not have data
ready
•By eliminating unused slots
• statistical TDM takes less time to send the same amount of data
•Figure illustrates how a statistical TDM system sends the data from Figure in only
8 slots instead of 12
Asynchronous TDM

• Statistical multiplexing incurs extra overhead shown below:
•Consider demultiplexing:
• In a synchronous TDM system a demultiplexer knows that every
N slot corresponds to a given receiver
• In a statistical multiplexing system, the data in a given slot can
correspond to any receiver.
• Each slot must contain the identification of the receiver to which
the data is being sent
• Output data rate less than aggregate input rates
• May cause problems during peak periods
• Buffer inputs
• Keep buffer size to minimum to reduce delay

4
Synchronous TDM vs. Statistical TDM

Advantages of TDM :
• Full available channel bandwidth can be utilized for each channel.
• TDM circuitry is not very complex.
• The problem of crosstalk is not severe.
• Only one carrier in the medium at any time
• Throughput high even for many users.

Disadvantages of TDM :
• Synchronization is essential for proper operation.
• Requires A/D conversions at high rate.
• Requires larger bandwidth.
• Probability of error or Bit Error Rate

UNIT 2
 ERROR DETECTION AND CORRECTION –
FUNDAMENTALS, BLOCK CODING , HAMMING
DISTANCE , CRC
 FLOW CONTROL PROTOCOLS ,STOP AND
WAIT, GO BACK N ARQ, SELECTIVE REPEAT
ARQ, SLIDING WINDOW, PIGGYBACKING
 MULTIPLE ACCESS PROTOCOL- ALOHA, CSMA,
CSMA/CA AND CSMA/CD

Error detection and
Correction –
Fundamentals, Block
coding , Hamming
distance , CRC

Fundamentals
 Networks must be able to transfer data from one device to another with
complete accuracy.
 Data can be corrupted during transmission.
 For reliable communication, errors must be detected and corrected.
 Error detection and correction are implemented either at the data link layer
or the transport layer of the OSI model.

Fundamentals - Single Bit error
 This error occurs when only one bit in the data unit has changed
(ex : ASCII STX - ASCII LF)

Fundamentals – Multi Bit error
 This error occurs when two or more non-consecutive bits in the data unit
have changed(ex : ASCII B - ASCII LF)

Fundamentals – Burst error
 Burst error means that 2 or more consecutive bits in the data unit have
changed

Fundamentals – Redundancy
 The central concept in detecting or correcting errors is redundancy. To be
able to detect or correct errors, we need to send some extra bits with our
data.
 These redundant bits are added by the sender and removed by the
receiver. Their presence allows the receiver to detect or correct corrupted
bits.

Fundamentals – Redundancy
Types :-
They are four types of redundancy checks that are used in data
communications.
 vertical redundancy check (VRC)
 longitudinal redundancy check (LRC)
 cyclic redundancy check (CRC)
 checksum

Block Coding
 In block coding, we divide our message into blocks, each of k bits, called
datawords.
 We add r redundant bits to each block to make the length n = k + r. The
resulting n-bit blocks are called codewords.

Block Coding – Error detection
 Enough redundancy is added to detect an error.
 The receiver knows an error occurred but does not know which
bit(s) is(are) in error.
 Has less overhead than error correction

Block Coding – Error Correction
In error detection, the receiver needs to know only that the received
codeword is invalid
In error correction the receiver needs to find (or guess) the original
codeword sent.
We can say that we need more redundant bits for error correction than
for error detection.

Block Coding – Error Correction
Fig -Structure of encoder and decoder in error correction

Error detection : - Methods
Fig – error detection methods

Error detection Method :– Parity check
 A parity bit is added to every data unit so that the total number of
1s(including the parity bit) becomes even for even-parity check or
odd for odd-parity check
 1 . Simple parity check

Error detection Method :– Simple Parity check
In this Blocks of data from the sender are subjected to a check bit or parity bit
 In this a parity of 1 is added to the block if it contains odd number of 1’s, and 0 is
added if it contains even number of 1’s .
This scheme makes the total number of 1’s even, that is why it is called even parity
checking

Example :-
Suppose the sender wants to send the word world. In ASCII the five
characters are coded as
1110111 1101111 1110010 1101100 1100100
The following shows the actual bits sent
11101110 11011110 11100100 11011000 11001001
Note -A simple parity-check code can detect an odd number of errors.

Fig:- Encoder and decoder for simple parity-check code

Error detection Method :– 2D Parity check
 Parity check bits are calculated for each row, which is equivalent to a simple parity
check bit.
 Parity check bits are also calculated for all columns, then both are sent along with the
data.
 At the receiving end these are compared with the parity bits calculated on the
received data.

Error detection Methods :– 2D Parity check
Example :-
Suppose the following block is sent:
10101001 00111001 11011101 11100111 10101010
However, it is hit by a burst noise of length 8, and some bits are corrupted.
10100011 10001001 11011101 11100111 10101010
When the receiver checks the parity bits, some of the bits do not follow the even-
parity rule and the whole block is discarded.
10100011 10001001 11011101 11100111 10101010

Error detection Methods :– Cyclic redundancy check (CRC)
 Crc is based in binary division
 Given a k-bit frame or message, the transmitter generates an n-bit sequence,
known as a frame check sequence (FCS), so that the resulting frame, consisting of
(k+n) bits, is exactly divisible by some predetermined number.
 At the destination, the incoming data unit is divided by the same number. If at this
step there is no remainder, the data unit is assumed to be correct and is therefore
accepted.

CRC Generator :-
 crc generator uses modular-2 division
Fig :- Binary Division
in a CRC Generator

CRC checker :-
 crc checker uses modular-2 division
Fig :- Binary Division
in a CRC Checker

Note :- CRC generator(divisor) is most often represented not as a string of
1s and 0s, but as an algebraic polynomial. For example-

Error detection Methods :– CHECK SUM
Checksum used by the higher layer protocols
It is based on the concept of redundancy(VRC, LRC, CRC)
Fig :- Checksum
Generator

 In checksum the data is divided into k segments each of m bits.
 The sections are added together using 1’s complement to get the sum for the senders.
 In the sum is complemented to get the checksum.
 The checksum segment is sent with the data segments.
 All received sections are added using 1’s
complement to get the sum for the receiver
 The sum is complemented. If the result is zero, the
received data is accepted; otherwise discarded

Example :-

References
 Book - Data Communications and Networking By Behrouz A.Forouzan
 Book –Computer Networks, 5th Edition Andrew S. Tanenbaum, rije University, Amsterdam,
 Link –https://www.geeksforgeeks.org/category/computer-subject/computer-networks/
 Link - https://www.ijirem.org/search.php?searchtext=computer%20networks&type=All

Need of Error Correction :
> There are numerous reasons such as noise , cross-talk etc. due to
which data gets corrupted during transmission.
> Most of the applications would not function expectedly(properly) if
they receive erroneous data from the transmitter.
> For error-free data processing , error correction techniques are used
to detect errors in transmitted messages and reconstructs the
original error-free data.
.

Error Correction Techniques :
The error correction techniques are of two types :-
> Single bit error correction : method of correcting single bit errors.
> Burst error correction : method of correcting burst errors in data
sequence.
In most of the communication networks and various digital systems ,
Hamming Distance Code technique is widely used for error correction.

Hamming Code ( for Error Correction ):
This error-correcting code technique is developed by developed by R.W
Hamming.
It is a set of error correcting codes which not only identifies the error bits in
whole data sequence but also corrects it.
For understanding the mechanism of Hamming code, the knowledge of
redundancy bits is needed..
> What are redundancy bits?
>> By the definition it is “The difference between number of bits of the actual
data sequence and the transmitted bits”.
Formally redundancy bits are extra binary bits that are added to info. carrying bits
to ensure that no bits were lost during the data transfer.

How the Hamming code
actually corrects the errors?
> Hamming code uses relation between redundancy bits and data bits.
> In Hamming code, the redundancy bits are placed at certain calculated
positions in order to eliminate errors.
And the distance between the two redundancy bits is called “Hamming
distance”.

Mechanism of Hamming Code :
To understand the mechanism of hamming code error correction , let’s go through
the following stages :
> Detection of parity bits: The number of parity bits to be added depends upon
the number of information bits of the transmitted data.
Number of parity bits will be calculated by the following relation..
2P >= n + P +1.
where n=number of data bits.
P=number of parity bits.

> Position of parity bits: After calculating the number of parity bits we need to
know the position of parity bits to be placed.
The ’P’ parity bits placed at bit positions of powers of 2, i.e. 1, 2, 4, 8, 16 etc.
These parity bits are referred as P1 (at position 1), P2 (at position 2), P3 (at
position 4), P4 (at position 8) and so on.

> Constructing a Bit Location Table: In Hamming code, we must find the value of
the parity bits to assign them a bit value.

Hamming code Example:
Let’s understand error corrections through an example:
Problem : We have to encode the data 1101 in even parity, by using Hamming code.
Soln. To solve we will go through the following steps:
Step 1 : Calculate the required number of parity bits.
Let P = 2, then
2P = 22 = 4 and n + P + 1 = 4 + 2 + 1 = 7.
2 parity bits are not sufficient for 4 bit data.
So let’s try P = 3, then 2P = 23 = 8 and n + P + 1 = 4 + 3 + 1 = 8
Therefore 3 parity bits are sufficient for 4 bit data.
The total bits in the code word are 4 + 3 = 7

Step 2 : Constructing bit location table

Step 3 : Determine the parity bits:
For P1 : 3, 5 and 7 bits are having three 1’s so for even parity, P1 = 1.
For P2 : 3, 6 and 7 bits are having two 1’s so for even parity, P2 = 0.
For P3 : 5, 6 and 7 bits are having two 1’s so for even parity, P3 = 0.
By inserting the parity bits code word formed is 1100101.
If the code word has all zeros , then there is no error in Hamming code.

References
Book - Data Communications and Networking By Behrouz A.Forouzan
Book – Networks, 5th Edition Andrew S. Tanenbaum, rije University, Amsterdam,
Link –https://www.geeksforgeeks.org/category/computer-subject/computer-networks/
Link - https://www.ijirem.org/search.php?searchtext=computer%20networks&type=All

FLOW CONTROL PROTOCOLS
Stop and wait
Go back N ARQ
Selective repeat ARQ
Sliding window
Piggybacking

FLOW CONTROL
● Flow control is design in Data Link Layer.
● Flow control is the process of managing the rate of data transmission between two nodes to
prevent a fast sender from overwhelming a slow receiver.
● There are two ways to control the flow of data:
1. Stop and Wait Protocol
2. Sliding Window Protocol

Stop and Wait Protocol
● It is the simplest flow control method. In this, the sender will send one frame at
a time to the receiver. Until then, the sender will stop and wait for the
acknowledgment from the receiver. When the sender gets the acknowledgment
then it will send the next data packet to the receiver and wait for the
acknowledgment again and this process will continue.
● There are four types of times while sending frames:
1. Transmission time
2. Propagation time
3. Queuing time
4. Processing time

Transmission time
● Time taken by the sender to send all the packet onto the outer going link is called
Transmission time/delay. It is denoted by (Tt).
This is calculated by dividing the data size/Length(L) which has to be sent by the
bandwidth(BW) of the link.
Tt = L / BW
Propagation time
● Time taken by the last bit of the frame to reach from one side to the other side is called
propagation time/delay. It is calculated by dividing the distance between the sender
and receiver by the wave propagation speed / velocity.
It is denoted by (Tp).
Tp = d / v
where d = distance between sender and receiver, s = wave propagation velocity

Queuing time:
Queuing delay is the sum of the delays encountered by a packet between the time of insertion into the
network and the time of delivery to the address.
This time will be very small.
It is denoted by Tq.
Processing time:
Processing time is also very small and when packet is in processing to ack,That time is called processing
time.
It is denoted by Tp.

Total Time
● The propagation delay for sending the data frame and the acknowledgment frame is the same
as distance and speed will remain the same for both frames. Hence, the total time required to
send a frame is
● Total time= Tt(Transmission Delay) + Tp(Propagation Delay for data frame) + Tp(Propagation
Delay for acknowledgment frame)
:Here Queuing time and processing time will be 0.
Total time=Tt+2Tp
The sender is doing work only for Tt time and for the rest 2Tp time the sender is waiting for the
acknowledgment.
We can find efficiency also
Efficiency = Useful Time/ Total Time
η=Tt / (Tt+2Tp) =1/(1+2(Tp/Tt))
η=1/1+2a
Where a=Tp/Tt.

Throughput
● Throughput refers to the amount of data that enters and goes through a
system. It is defines number of bits per second.
● if Tt+2Tp –L
1sec=L/ Tt+2Tp
Multiply And Divide by B.W. in L then
L/(BW)*BW/Tt+2Tp
=Tt*BW/(Tt+2Tp)
= η*BW

Advantages of Stop and Wait Protocol
1.It is very simple to implement.
Disadvantages of Stop and Wait Protocol
1.We can send only one packet at a time.
2.If the distance between the sender and the receiver is large then the propagation delay would be
more than the transmission delay. Hence, efficiency would become very low.
3.After every transmission, the sender has to wait for the acknowledgment and this time will
increase the total transmission time.

Example –
Tt=1msec
Tp=1msec
η will be
a=Tt/Tp= 1/1=1
η=1/1+2a
1/1+2(1)
=1/3
=.3333*100
=33.33%

Data Packet Lost in Stop And Wait
● Problems :
● 1. Lost Data: Deadlock condition
● Get Rid from this we need to Retransmit the packet If packet not come in the
sufficient time period called timeout timer.
● S and W + TOT

2. Lost Acknowledgement: Replicate packet problem
Get rid from this we need sequence number.
S and W + TOT + Sequence Number.

3. Delayed Acknowledgement/Data: Missing data packet problem.
After timeout on sender side, a long delayed acknowledgement might be
wrongly considered as acknowledgement of some other recent packet.

Sliding Window Protocol
● The sliding window is a technique for sending multiple frames at a time. It controls
the data packets between the two devices where reliable and gradual delivery of data
frames is needed. It is also used in TCP.
● Each frame has sent from the sequence number. The sequence numbers are used to
find the missing data in the receiver end. The purpose of the sliding window
technique is to avoid duplicate data, so it uses the sequence number.
● Types of Sliding Window Protocol:
1. Go-Back-N ARQ
2. Selective Repeat ARQ

Go-Back-N ARQ
● Go-Back-N ARQ protocol is also known as Go-Back-N Automatic Repeat Request.
It is a data link layer protocol that uses a sliding window method. In this, if any
frame is corrupted or lost, all subsequent frames have to be sent again.
● The size of the sender window is N in this protocol. For example, Go-Back-8, the
size of the sender window, will be 8. The receiver window size is always 1.
● If the receiver receives a corrupted frame, it cancels it. The receiver does not
accept a corrupted frame. When the timer expires, the sender sends the correct
frame again. The design of the Go-Back-N ARQ protocol is shown below.

Example: In Go–back 3 flow control protocol every 6th packet is lost. If we
have to send 11 packets. How many transmissions will be needed ?
-In Go back N, if we don’t receive acknowledgement for a packet, whole window of that
packet is sent again. As a packet is received window is slided.
Here, window size is 3. Initially window will contain 1,2,3 then as acknowledgement of 1 is
received window slides so 4 is transmitted. Now,when 4th packet’s acknowledgement is
received 7th packet is sent and when 5th packet’s acknowledgement is received 8th packet
is sent. Now, as acknowledgement of 6 is not received so the window of 6 i.e. 6,7,8 packets
are retransmitted.Now the 6th packet from there is 9, so 9,10 will be retransmitted.
1 2 3 4 5 6 7 8 6 7 8 9 10 11 9 10 11 .
Hence total 17 transmissions are needed.

The three main characteristic features of GBN are:
1. Sender Window Size (WS)
It is N itself. If we say the protocol is GB10, then Ws = 10. N should be always
greater than
1 in order to implement pipelining. For N = 1, it reduces to Stop and Wait
protocol.
Efficiency of GBN =N/(1+2a)
where a=Tp/Tt
If B is the bandwidth of the channel, then
Throughput and Effective Bandwidth is
=Efficiency* Bandwidth
=N/(1=2a)*BW

2.Receiver Window Size (WR): WR is Always 1 in GBN.
*we will explain with a help of example. Consider the diagram given below. We have sender
window size of 4. Assume that we have lots of sequence numbers just for the sake of
explanation. Now the sender has sent the packets 0, 1, 2 and 3. After acknowledging the packets
0 and 1, receiver is now expecting packet 2 and sender window has also slided to further
transmit the packets 4 and 5. Now suppose the packet 2 is lost in the network, Receiver will
discard all the packets which sender has transmitted after packet 2 as it is expecting sequence
number of 2. On the sender side for every packet send there is a time out timer which will expire
for packet number 2. Now from the last transmitted packet 5 sender will go back to the packet
number 2 in the current window and transmit all the packets till packet number 5. That’s why it
is called Go Back N. Go back means sender has to go back N places from the last transmitted
packet in the unacknowledged window and not from the point where the packet is lost.

3.Acknowledgements:
There are 2 kinds of acknowledgements namely:
A. Cumulative Ack: One acknowledgement is used for many packets. The main advantage is
traffic is less. A disadvantage is less reliability as if one ack is the loss that would mean that
all the packets sent are lost.
B. Independent Ack: If every packet is going to get acknowledgement independently.
Reliability is high here but a disadvantage is that traffic is also high since for every packet
we are receiving independent ack.

GBN uses Cumulative Acknowledgement. At the receiver side, it starts a acknowledgement timer
whenever receiver receives any packet which is fixed and when it expires, it is going to send a
cumulative Ack for the number of packets received in that interval of timer. If receiver has received
N packets, then the Acknowledgement number will be N+1. Important point is Acknowledgement
timer will not start after the expiry of first timer but after receiver has received a packet.
Time out timer at the sender side should be greater than Acknowledgement timer.
Minimum sequence numbers required in GBN = N + 1

Selective Repeat ARQ
● Selective Repeat ARQ is also known as the Selective Repeat Automatic Repeat Request. It is a
data link layer protocol that uses a sliding window method. The Go-back-N ARQ protocol
works well if it has fewer errors. But if there is a lot of error in the frame, lots of bandwidth
loss in sending the frames again. So, we use the Selective Repeat ARQ protocol. In this
protocol, the size of the sender window is always equal to the size of the receiver window.
The size of the sliding window is always greater than 1.
● If the receiver receives a corrupt frame, it does not directly discard it. It sends a negative
acknowledgment to the sender. The sender sends that frame again as soon as on the
receiving negative acknowledgment. There is no waiting for any time-out to send that frame.
The design of the Selective Repeat ARQ protocol is shown below.

Efficiency of SR Protocol
● Efficiency = Sender Window Size in Protocol / (1 + 2a)
Efficiency of SR Protocol = N / (1 + 2a)
In SR protocol, sender window size is always same as receiver window size
● The receiver responses either with the positive acknowledgement(ACK) or with the
negative acknowledgement(NACK) where positive acknowledgement means that the
receiver is ready to receive the transmission and negative acknowledgement means that
the receiver is unable to accept the transmission.

Piggybacking
● Piggybacking is a method of attaching acknowledgment to the outgoing data packet in
reliable full-duplex data transmission.
● Working: The concept of piggybacking is explained as follows:
● Consider a two-way transmission between host A and host B. When host A sends a data
frame to B, then B does not send the acknowledgment of the frame sent immediately.
The acknowledgment is delayed until the next data frame of host B is available for
transmission. The delayed acknowledgment is then attached to the outgoing data frame
of B. This process of delaying acknowledgment so that it can be attached to the outgoing
frame is called piggybacking.

● Now, as we are communicating between the host A and host B, three conditions can arise:
● When the host has both data and the acknowledgment to send, then it will attach the data
along with the acknowledgment. In the above diagram, the host B will attach the data
frame along with the acknowledgment of the last frame received from host A.
● When the host does not have any data to send then it will send only the acknowledgment.
In the above diagram, when host A does not have any data frame to send. So, it will only
send the acknowledgment of the last frame received.
● When the host has only data to send then it will send the data along with the
acknowledgment of the last frame received. The duplicate acknowledgment will be
discarded by the receiver and the data would be accepted.

Reason for Piggybacking
● Communications are mostly full – duplex in nature, i.e. data transmission occurs in both
directions. A method to achieve full – duplex communication is to consider both the
communication as a pair of simplex communication. Each link comprises a forward channel for
sending data and a reverse channel for sending acknowledgments.
● However, in the above arrangement, traffic load doubles for each data unit that is
transmitted. Half of all data transmission comprise of transmission of acknowledgments.
● So, a solution that provides better utilization of bandwidth is piggybacking. Here, sending of
acknowledgment is delayed until the next data frame is available for transmission. The
acknowledgment is then hooked onto the outgoing data frame. The data frame consists of
an ack field. The size of the ack field is only a few bits, while an acknowledgment frame
comprises of several bytes. Thus, a substantial gain is obtained in reducing bandwidth
requirement.

● Advantages :
Improves the efficiency, better use of available channel bandwidth.
● Disadvantages :
The receiver can jam the service if it has nothing to send. This can be solved by enabling a
counter ( Receiver timeout ) when a data frame is received. If the count ends and there is
no data frame to send, the receiver will send an ACK control frame. The sender also adds
a counter (Emitter timeout), if the counter ends without receiving confirmation, the
sender assumes packet loss , and sends the frame again.

● Conclusion
There is a dispute as to whether this is a legal or illegal activity, but piggybacking is still a dark side
of Wi-Fi. Cyber-terrorist attacks in India are a clear reminder that we cannot control incidents
occurring anywhere in the world or control unsecured Wi-Fi networks. So it is the responsibility of
the owner and administrator to secure their wireless connection.

Multiple access protocol-
ALOHA, CSMA, CSMA/CA
and CSMA/CD

Data Link Layer
• The data link layer is used in a computer network to transmit the
data between two devices or nodes. It divides the layer into parts
such as data link control and the multiple access resolution/protocol.
The upper layer has the responsibility to flow control and the error
control in the data link layer, and hence it is termed as logical of
data link control. Whereas the lower sub-layer is used to handle and
reduce the collision or multiple access on a channel. Hence it is
termed as media access control or the multiple access resolutions.

What is a multiple access protocol?
• When a sender and receiver have a dedicated link to transmit data packets, the data link
control is enough to handle the channel. Suppose there is no dedicated path to
communicate or transfer the data between two devices. In that case, multiple stations
access the channel and simultaneously transmits the data over the channel. It may
create collision and cross talk. Hence, the multiple access protocol is required to reduce
the collision and avoid crosstalk between the channels.
For example, suppose that there is a classroom full of students. When a teacher asks a
question, all the students (small channels) in the class start answering the question at
the same time (transferring the data simultaneously). All the students respond at the
same time due to which data is overlap or data lost. Therefore it is the responsibility of
a teacher (multiple access protocol) to manage the students and make them one answer.

Random Access Protocol
In this, all stations have same superiority that is no station has more
priority than another station. Any station can send data depending on
medium’s state( idle or busy). It has two features:
• There is no fixed time for sending data
• There is no fixed sequence of stations sending data
• Following are the different methods of random-access protocols for
broadcasting frames on the channel.
• Aloha
• CSMA
• CSMA/CD
• CSMA/CA

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