COMPUTER NETWORKS

BCS502

Introduction to Data Communications and Networking

Bhoomika S Babu
Department of CSE(Data Science)
 Outline

• Introduction to Data Communications

• Networks and Network Types
• Network Models: Protocol Layering, TCP/IP, and OSI
• Introduction to the Physical Layer
• Transmission Media: Guided and Unguided
• Switching: Packet Switching and Types
 Textbook

• Textbook:
1. Behrouz A. Forouzan, Data Communications and Networking, 5th Edition,
Tata McGraw-Hill,2013.

• Reference Books:
1. Larry L. Peterson and Bruce S. Davie: Computer Networks – A Systems
Approach, 4th Edition, Elsevier, 2019.
2. Nader F. Mir: Computer and Communication Networks, 2nd Edition,
Pearson Education, 2015. 3. William Stallings, Data and Computer
Communication 10th Edition, Pearson Education, Inc., 2014.
 Assessment
Components Total Weightage
(Test1+Test2+Record+LabTe
st)
Theory 25 (Test1+Test2)
Lab 25(Record+Test)
Total 50

Theory:
Test1+Test2+Assignment -> 15+15+10 = 40 will be reduced to
25marks

Lab:
Record+LabTest -> 15+50(will be reduced to 10) =25 marks
 Computer Networks

A computer networks is system in which multiple

computer are connected to each other to share the
resources or information.
 Introduction to Data Communications

• Definition of Data Communications:

- Data Communications refers to the exchange of data between two devices via
some form of transmission medium.
• For data communication to occur, the communication must be part of a
communication system made up of combination of hardware(physical
equipment), and software (programs).
• Effectiveness of data communication system depends on 4 fundamental
characteristics:
1. Delivery: The system must ensure that data reaches the correct destination.
2. Accuracy: The system must deliver data without errors or alterations.
3. Timeliness: The system must deliver data promptly, especially for real-time
applications like audio and video.
4. Jitter: Jitter is the variation in packet arrival time, affecting the quality of
audio or video transmission.
Components of Data Communication System
• Components of a Data Communication System:
- Message: The data being communicated.
- Sender: The device that sends the data.
- Receiver: The device that receives the data.
- Transmission Medium: The physical path by which the message travels
from sender to receiver.
- Protocol: A set of rules that govern data communications
 Data Representation

• Information now-a-days comes from different forms such as text, numbers,

images, audio, and video.
• Here all the forms uses BIT PATTERNS.
1. Text: : Text is represented as a bit pattern—a sequence of bits (0s and 1s).
- Coding is the process of representing text symbols with bit patterns.
- The most common coding system today is **Unicode**, which uses 32 bits
to represent characters from any language. ASCII (American Standard Code
for Information Interchange), developed in the U.S., uses 7 bits (with a range
of 127 characters) and is a subset of Unicode, often referred to as Basic Latin.
- ASCII codes are fundamental for text representation in computers, with
examples including 'A' represented as 01000001 and 'a' as 01100001.
2. Number:
•Numbers are represented directly as binary numbers instead of using codes
like ASCII. Binary representation is used to facilitate mathematical operations
directly in computers.
•For instance, the decimal number 5 is represented as 101 in binary.
3. Images:
• Images are composed of tiny dots called pixels, and each pixel is
represented by a bit pattern.
•The resolution of an image refers to the number of pixels used—higher
resolution means more pixels and better image quality.
•Simple black-and-white images can be represented with 1-bit per pixel, while
grayscale images use more bits to represent different shades.
•Color images are often represented using the **RGB model**, where colors
are made by combining red, green, and blue, each with its own intensity
represented by a bit pattern. Another method is called **YCM**, in which a
color is made of a combination of three other primary colors: yellow, cyan,
and magenta.
4. Audio:
•Audio refers to sound or music, which is naturally continuous.
•Audio is captured as continuous signals, unlike the discrete nature of text or
images.
•These signals are then digitized for processing and transmission, converting
them into a series of bits.
5. Video :
•Video involves the recording or broadcasting of moving images.
• Video can be captured as a continuous stream (like a TV camera) or as a
sequence of discrete images that convey motion.
•These images are then transmitted as a series of bit patterns, similar to how
individual images are handled.
 Data Flow
Communication between two devices can be simplex, half-duplex, or full-
duplex.
1. Simplex Mode:
- Unidirectional Communication: In simplex mode, communication flows in
only one direction, similar to a one-way street.
- One Device Transmits, the Other Receives: Only one device on the link can
send data, and the other can only receive.
- Examples: Keyboards (input only) and traditional monitors (output only) are
typical simplex devices.
- Full Channel Capacity: The entire bandwidth of the channel is used for
transmission in one direction, maximizing the data transfer rate.
2. Half-Duplex Mode:
- Bidirectional Communication (One at a Time): In half-duplex mode, both
devices can transmit and receive, but not simultaneously—only one device
can send data at any given time.
- Analogy: Think of it like a one-lane road where traffic flows in both
directions, but vehicles must take turns.
- Examples: Walkie-talkies and CB radios operate in half-duplex mode, where
one person speaks while the other listens, and then they switch roles.
- Efficient Use of Channel: The full channel capacity is used by the
transmitting device, but only one direction at a time.
3. Full-Duplex Mode:
- Simultaneous Bidirectional Communication: In full-duplex mode, both
devices can send and receive data simultaneously, much like a two-way street.
- Shared Channel Capacity: The channel's capacity is either divided between
the two directions or uses separate paths for sending and receiving.
- Examples: Telephones are a common example, allowing both parties to talk
and listen at the same time without interruption.
- Continuous Communication: This mode is ideal when constant
communication in both directions is required, though the channel's capacity
must be shared.
 Networks and Network Types
• Definition of a Network:
- A network is a collection of interconnected devices that can share resources and
communicate with each other.
-A network is required to meet specific criteria to function effectively. The key
factors that determine the quality of a network are Performance, Reliability, and
Security.
1. Performance:
• Performance reflects how efficiently a network operates. It is influenced by
factors such as transit time, response time, the number of users, transmission
medium, hardware capabilities, and software efficiency.
• Transit time refers to the time taken for a message to travel between devices,
while response time is the duration between an inquiry and its response.
• Performance is often assessed through throughput (how much data can be
transmitted) and delay (how long it takes for data to reach its destination).
Although higher throughput is desirable, it can lead to increased delays due to
network congestion.
2. Reliability:
• Reliability measures how consistently a network delivers accurate data.
• It is assessed by the frequency of network failures, the time required to recover
from these failures, and the network’s ability to withstand catastrophic events.
• A reliable network minimizes disruptions and ensures continuous operation.
3. Security:
• Security in networking involves safeguarding data from unauthorized access and
damage. This includes protecting information during transmission, preventing
data corruption, and establishing protocols for recovering data in case of security
breaches.
• Effective security practices ensure that sensitive information remains confidential
and intact.
 Physical Structures (Topologies)

Type of Connection:
• In a network, devices are interconnected through links, which are pathways
that allow data transfer. These links can be visualized as lines connecting
devices. For communication to occur, devices must be connected to the
same link simultaneously. There are two main types of connections:
1. Point-to-Point:
This type of connection is a direct link between two devices, where the
entire link capacity is dedicated to them.
Examples: include a cable connecting two computers or a remote control
communicating with a television. Point-to-point connections can also be
established wirelessly, such as through microwave or satellite links.
 Physical Structures (Topologies)
2. Multipoint (Multidrop):
In a multipoint connection, a single link is shared among multiple devices.
The link’s capacity can be shared in two ways:
- Spatially Shared Connection: Multiple devices use the link at the same time.
- Timeshared Connection: Devices take turns using the link.
 Physical Topologies
Physical topology refers to the physical layout of a network, depicting how
devices (nodes) and links are arranged. It’s a geometric representation of the
relationship between all links and devices in a network. There are four basic
topologies:
 Mesh Topology
Definition:
• In a mesh topology, every device has a dedicated point-to-point link to
every other device.
• Each node communicates both direction also called as DUPLEX MODE.
• Each connection is exclusive, ensuring no traffic issues and making fault
detection easier. Here we can divide the number of links by which is,
n(n-1)/2.
• To accommodate that many links every device on the network must have
(n-1)
• I/O ports Each device requires n-1 ports.
For example: if the number of node n=5;
n(n-1)/2 = 5(5-1)/2 = 10 links/stations
Advantages:
• Dedicated Links: - Each connection can handle its own data load.
- Eliminates traffic problems caused by shared links.
• Robustness:
- System remains operational even if one link fails.
• Privacy/Security: - Messages travel on dedicated lines.
- Only the intended recipient can see the message.
• Fault Identification: - Easy to locate and isolate faults.
- Traffic can be rerouted to avoid problematic links.
Disadvantages:
• High amount of cabling and many I/O ports needed.
• Installation and reconnection are complex.
• Bulk wiring may exceed available space.
• Expensive hardware requirements.
Application:
• Used to connect telephone regional offices.
 Star Topology
Definition:
• In a star topology, each device has a dedicated point-to-point link only to a
central controller, usually called a hub. The devices are not directly linked
to one another.
• No Direct Traffic: Devices cannot communicate directly with each other.
• Data Transmission: When a device wants to communicate with another
device, it sends data to the central controller.
Here's a simple breakdown:
Device A wants to send data to Device B.
Device A sends the data to the central controller.
The central controller forwards the data to Device B.
Device B receives the data.
Advantages:
• Robustness: - Failure of a single link affects only that specific link.
- Other links remain active, enhancing network reliability.
• Easy Fault Identification and Isolation: - Faults can be easily detected and
isolated.
- The central hub can monitor and manage link problems, allowing
defective links to be bypassed.
Disadvantages:
• Central Hub Dependency: - The entire network relies on the central hub.
- If the hub fails, the whole network goes down.
• Higher Cabling Requirement: - More cabling may be needed compared to
some other topologies (e.g., ring or bus).
- Each node requires a separate connection to the central hub.
Application:
• Local-Area Networks (LANs):
– Commonly used in LANs.
– High-speed LANs often employ a star topology with a central hub.
 Bus Topology
Definition:
• Bus Topology is a multipoint network topology where a single long cable, known
as the backbone or bus, connects all devices in the network.
• Connections:
1. Drop Lines: These are individual connections running from each device to the
main cable.
2. Taps: Connectors that either splice into the main cable or puncture the cable’s
sheathing to make contact with the metallic core, allowing devices to connect to
the bus.
• Signal Transmission: As signals travel along the backbone, some energy is lost
and converted into heat, causing the signal to weaken over distance.
Advantages:
• Easy Installation:
– The backbone cable can be laid along the most efficient path.
– Nodes are connected to the backbone by drop lines of varying lengths.
– Requires less cabling compared to topologies like star or mesh.
(Redundancy is eliminated).
Disadvantages:
• Difficult Reconnection and Fault Isolation:
– Adding new devices can be challenging and may require modifications to
the backbone.
– Signal quality can degrade due to reflections at the taps.
– A break in the backbone cable affects the entire network, causing all
communication to stop.
Application:
• Early LANs:
– Bus topology was commonly used in traditional Ethernet LANs.
– Although less popular now due to its limitations, it was one of the first
topologies for local-area networks.
 Ring Topology
Definition:
• Ring Topology is a network configuration where each device has a dedicated
point-to-point connection with the two devices directly adjacent to it, forming a
circular data path.
• Signals travel in one direction around the ring, passing from device to device
until reaching their destination.
• Each device contains a repeater that regenerates and amplifies the signal.
• The repeater ensures the signal maintains strength and integrity as it travels
around the ring, causing the signal to weaken over distance.
Advantages:
• Ease of Installation and Reconfiguration:
– Devices are connected only to their immediate neighbors, making it easy to
add or remove devices.
– Fault isolation is simplified; if a device detects a problem, it can alert the
network operator to the issue and its location.
Disadvantages:
• Unidirectional Traffic:
– Unidirectional traffic can be a disadvantage. In a simple ring, a break in the
ring (such as a disabled station) can disable the entire network.
– This weakness can be solved by using a dual ring or a switch capable of
closing off the break.
Application:
• Token Ring Networks:
– Ring topology was commonly used in IBM's Token Ring LANs.
– It has become less popular due to the need for higher-speed LANs, which
favor other topologies.
 Network Types

Types of Networks:
• - LAN (Local Area Network): A network that covers a small geographical
area, such as a single building.
• - WAN (Wide Area Network): A network that covers a large geographical
area, often spanning cities or countries.
• - MAN (Metropolitan Area Network): A network that covers a city or a large
campus.
• - PAN (Personal Area Network): A network that is used for communication
among devices close to one person.
Local Area Network
• A LAN is usually privately owned and connects hosts in a single office,
building, or campus.
• It can be as simple as two PCs and a printer in a home office, or extend
throughout a company with audio and video devices.
• Host Address: Each host in a LAN has an identifier (address) that uniquely
defines it. A packet sent from one host to another carries both the source and
destination addresses.
• In the past (Traditional LANs), all hosts were connected through a common
cable. A packet sent from one host was received by all hosts, but only the
intended recipient kept the packet.
• Modern LANs: Most LANs now use a smart switch that recognizes the
destination address and sends the packet only to that host. This reduces traffic
and allows multiple communications at the same time between different host
pairs.
• There is no minimum or maximum number of hosts defined for a LAN. Today,
LANs are often connected to each other and to WANs to enable communication
over a wider area.
Wide Area Network
LAN (Local Area Network):
• Limited to small areas (office, building, campus)
• Interconnects hosts (computers, printers, etc.)
• Privately owned(Company, labs,etc.)
WAN (Wide Area Network):
• Spans large geographical areas (city, country, world)
• Interconnects devices (routers, switches)
• Usually leased from service providers

We see two distinct examples of WANs today: point-to-point WANs and switched
WANs.
1. Point-to-Point WAN
2. Switch
 Point-to-Point WAN
• Direct connection between two devices(network that connected with
transmission media, cable or air)
• Example: Leased lines connecting two offices

Switched WAN
• Multiple interconnected devices
• Backbone for global communication (like the internet)
• Example: Combination of point-to-point WANs using switches
Internetwork

An internetwork (or internet) is created when two or more networks (LANs or

WANs) are connected. (can be indicated as lowercase “i”)
• This allows communication between different locations or networks.

Example:
• An organization with offices on the East Coast and West Coast:
– Each office has its own LAN.
– A dedicated point-to-point WAN connects the two LANs, forming an
internetwork.
– This enables communication between employees at both offices.
Switching
A switched network connects multiple links through switches.
Switches forward data between networks as needed.
Two primary types: Circuit-Switched and Packet-Switched networks.

A heterogeneous network made of four WANs and three LANs

1. Circuit-Switched Network:
• Dedicated connection (circuit) between two end systems, which is called
“CIRCUIT”
• Circuit is always available, but can be made active or inactive.
• Figure below shows a very simple switched network that connects four
telephones to each end. We have used telephone sets instead of computers as an
end system because circuit switching was very common in telephone networks in
the past, although part of the telephone network today is a packet-switched
network.

Example:
• When two people are on a phone call, a dedicated line connects them until the
call ends.
2. Packet-Switched Network:
• The communication between the two ends is done in blocks of data called
packets.
• Data is sent in packets, not a continuous stream.
• Packets can be stored and forwarded by routers.
• Packets are independent and can be stored and forwarded.
• More efficient than circuit-switched networks, but may experience delays.
• A router in a packet-switched network has a queue that can store and forward the
packet.

Example:
• In a computer network, data is broken into packets and sent independently.
Routers store and forward them as needed.
 Network Models: Protocol Layering

• What is Protocol Layering?

- Protocol layering is the organization of protocols in layers to reduce
complexity in the network design.
• Importance of Layering in Networking:
- Simplifies the networking process.
- Facilitates troubleshooting and network management.
• Example of Layering:
- TCP/IP Model: A four-layer model used in the internet.
- OSI Model: A seven-layer model developed by ISO(International
Organization for Standard).
 Principles of Protocol Layering
• First Principle: Bidirectional Communication
Dual Functionality of Layers: Each layer in the protocol must be capable of
performing two opposite tasks, one for each direction of communication.
– Layer 3:
• Task 1: Listen (receive data)
• Task 2: Talk (send data)
– Layer 2:
• Task 1: Encrypt data (for sending)
• Task 2: Decrypt data (for receiving)
– Layer 1:
• Task 1: Send mail (outgoing)
• Task 2: Receive mail (incoming)
• Second Principle: Identical Objects Under Each Layer
Uniformity Across Sites: The objects managed by each layer at both
communicating sites must be identical.
– Layer 3:
• Object: Plaintext letter (same at both sites)
– Layer 2:
• Object: Ciphertext letter (same at both sites)
– Layer 1:
• Object: Piece of mail (same at both sites)

• First Principle emphasizes the need for each layer to handle tasks in both
sending and receiving modes to facilitate effective bidirectional
communication.
• Second Principle stresses that the entities being processed by each layer
must be consistent across both ends of the communication, ensuring clarity
and integrity of data exchanged.
 Logical Connections
· Layer-to-Layer Communication:
Each layer in the protocol stack can communicate directly with its
corresponding layer at the other end.
This creates a series of logical (imaginary) connections between the
layers at both communicating sites.
· Role of Logical Connections:
Understanding logical connections helps clarify the purpose and function
of each layer in the protocol.
It provides a framework for visualizing how data is encapsulated,
transmitted, and processed across layers in data communication.
· Importance in Data Communication:
The concept of logical connections simplifies the complexity of
networking by illustrating how layers interact with one another.
It aids in grasping the layered architecture, which is essential for
troubleshooting and developing networking protocols.
 TCP/IP Protocol Suite

• Overview of TCP/IP Protocol Suite:

- The TCP/IP protocol suite is the set of protocols used for communication
over the internet.
• Layers in TCP/IP:
- Application Layer: Provides network services to applications.
- Transport Layer: Ensures reliable data transfer.
- Network Layer: Handles the routing of data.
- Data Link Layer: Provides node-to-node data transfer.
- Physical Layer: Deals with the physical connection between devices.
 OSI Model
• Overview of the OSI Model:
- The OSI (Open Systems Interconnection) model is a conceptual framework that
standardizes the functions of a communication system.

• Comparison between OSI and TCP/IP:

- OSI has seven layers, while TCP/IP has four.
- OSI is a reference model, while TCP/IP is a protocol suite.

• Detailed Layers:
- Application: Interfaces with the user application.
- Presentation: Ensures data is in a usable format.
- Session: Manages sessions between applications.
- Transport: Provides reliable data transfer.
- Network: Manages routing of data.
- Data Link: Handles error detection and correction.
- Physical: Manages the physical transmission of data.
 Introduction to the Physical Layer
• What is the Physical Layer?
- The physical layer is the first and lowest layer in both the OSI and TCP/IP
models, dealing with the transmission of raw bit streams over a physical
medium.

• Functions of the Physical Layer:

- Defines the hardware means of sending and receiving data.
- Establishes and terminates connections between devices.
- Defines the electrical, mechanical, and procedural specifications for the
network.
Transmission Media:
- The physical path between transmitter and receiver.
• In Data communication, transmission medium involves free space, metallic
cables or fiber cables.
• The Evolution of Transmission Media
• Telegraph (19th Century):
First long-distance electric signal communication, slow and metallic-
dependent.
• Telephone (1869):
Voice communication over metallic wires, initially unreliable due to noise.
• Wireless Communication (1895):
Hertz’s high-frequency signals, Marconi’s transatlantic telegraph messages.
Types of Transmission media
 Transmission Media: Guided Media
• Definition:
Guided media, which are those that provide a conduit from one device
to another.
A signal traveling along any of these media is directed and contained by
the physical limits of the medium.
Twisted-pair and coaxial cable use metallic (copper) conductors that
accept and transport signals in the form of electric current.
Optical fiber is a cable that accepts and transports signals in the form of
light.

• Types of Guided Media:

1. Twisted-pair cable
2. Coaxial cable
3. Fiber-optic cable
1. Twisted-pair cable:
A twisted pair consists of two conductors (normally copper), each with
its own plastic insulation, twisted together.

• One of the wires is used to carry signals to the receiver, and the other is used
only as a ground reference.
· Issue with Noise and Crosstalk:
Both wires in a cable are subject to interference (noise) and crosstalk.
Parallel wires are affected differently by noise since one wire might be
closer to the noise source than the other.
This creates a difference in interference between the two wires at the
receiver end.
· Twisted-Pair Design:
Wires are twisted to balance the effect of noise.
In one twist, one wire is closer to the noise source, and in the next twist, the
other wire is closer.
This alternating pattern ensures that both wires are equally affected by
external noise.

· Noise Cancellation at the Receiver:

The receiver calculates the difference between the signals in the two wires.
Since the noise affects both wires equally due to the twisting, the unwanted
signals (noise) mostly cancel out.
This reduces interference and improves the quality of the received signal.

· Twists per Unit Length:

The number of twists per inch affects the quality of the cable.
More twists usually lead to better noise cancellation, improving the overall
performance of the cable.
Types of Twisted-pair Guided cable:
1. Unshielded twisted-pair (UTP)
2. Shielded twisted-pair (STD)

The most common twisted-pair cable used in communications is referred to as

unshielded twisted-pair (UTP). (The most common UTP connector is RJ45 (RJ
stands for registered jack)).
Shielded twisted-pair (STP) cable has a metal foil or braided mesh covering that
encases each pair of insulated conductors. Although metal casing improves the
quality of cable by preventing the penetration of noise or crosstalk, it is bulkier and
more expensive.

Applications: Twisted-pair cables are used in telephone lines to provide voice and
data channels.
Local-area networks, such as l0Base-T and l00Base-T, also use twisted-pair cables.
Categories of UTP:

Category Specification Data Rate (Mbps) Use

1 Unshielded twisted-pair < 0.1 Telephone

used in telephone
2 Unshielded twisted-pair originally 2 T-1 lines
used in T lines

3 Improved CAT 2 used in LANs 10 LANs

4 Improved CAT 3 used in Token 20 LANs

Ring networks
5 Cable wire is normally 24 AWG 100 LANs
with a jacket and outside sheath
5E An extension to category 5 that 125 LANs
includes extra features to minimize
the crosstalk and electromagnetic
interference
6 A new category with matched 200 LANs
components coming from the same
manufacturer. The cable must be
tested at a 200-Mbps data rate.
7 Sometimes called SSTP (shielded 600 LANs
screen twisted-pair). Each pair is
individually wrapped in a helical
metallic foil followed by a
metallic foil shield in addition to
the outside sheath. The shield
decreases the effect of crosstalk
and increases the data rate.
Performance of UTP:

One way to measure the performance of twisted-pair cable is to compare

attenuation versus frequency and distance. A twisted-pair cable can pass a wide
range of frequencies, -- with increasing frequency, the attenuation, measured in
decibels per kilometer (dB/km), sharply increases with frequencies above 100
kHz. Note that gauge is a measure of the thickness of the wire.
2. Coaxial Cable:
Coaxial cable (or coax) carries signals of higher frequency ranges than
those in twisted pair cable, in part because the two media are constructed quite
differently.
• Instead of having two wires, coaxial cable has a central core conductor
made of solid or stranded wire (usually copper).
• This core conductor is enclosed in an insulating sheath.
• The insulating sheath is then encased in an outer conductor made of metal
foil, braid, or a combination of both.
• The outer conductor acts as a shield against noise and completes the circuit.
• The outer metallic wrapping serves two primary purposes:
– Acts as a shield against noise.
– Functions as the second conductor, completing the circuit.
• This outer conductor is enclosed in an insulating sheath.
• The entire cable is further protected by a plastic cover.
Coaxial Cable Standards:
Coaxial cables are categorized by their Radio Government (RG) ratings.
Each RG number denotes a unique set of physical specifications, including the
wire gauge of the inner conductor, the thickness and type of the inner insulator,
the construction of the shield, and the size and type of the outer casing. Each
cable defined by an RG rating is adapted for a specialized function.

Category Impedance Use

RG-59 75 Ω Cable TV
RG-58 50 Ω Thin Ethernet
RG-11 50 Ω Thick Ethernet
Coaxial Cable Connectors:
To connect coaxial cable to devices, we need coaxial connectors. The
most common type of connector used today is the Bayonet Neill-Concelman
(BNC) connector.
Three popular types of these connectors: the BNC connector, the BNC T
connector, and the BNC terminator.
• BNC Connector: Connects the cable to devices (e.g., TV sets).
• BNC T Connector: Used in Ethernet networks for branching connections.
• BNC Terminator: Prevents signal reflection at the cable’s end.
Performance:
We can measure the performance of a coaxial cable. We notice in Figure
7.9 that the attenuation is much higher in coaxial cable than in twisted-pair cable.
In other words, although coaxial cable has a much higher bandwidth, the signal
weakens rapidly and requires the frequent use of repeaters.

Applications:
• Cable TV Networks (RG-59)
• Ethernet LANs:10Base2 (Thin Ethernet): Uses RG-58, BNC connectors, 10
Mbps, 185m range.
• 10Base5 (Thick Ethernet): Uses RG-11, 10 Mbps, 5000m range, specialized
connectors.
3. Fiber Optic Cable:
A fiber-optic cable is made of glass or plastic and transmits signals in
the form of light.

• The Nature of Light

Basic Principle:
– Light travels in a straight line as long as it moves through a single
uniform substance.
Transition Between Media:
– When light moves from one substance to another (of a different density),
its direction changes.
Fiber Optic Cable: Refraction and Critical Angle
When light encounters the interface between two substances, its
behavior depends on the angle of incidence (I).
If the angle is less than the critical angle, the light ray refracts, bending
towards the surface.
At the critical angle, the light bends along the interface. If the angle
exceeds the critical angle, the ray reflects back into the denser substance.
The critical angle varies for different materials, reflecting their unique
properties.
Fiber Optic Cable: Structure
Optical fibers use reflection to guide light through a channel.
A glass or plastic core is surrounded by a cladding of less dense glass or
plastic. The difference in density of the two materials must be such that a beam
of light moving through the core is reflected off the cladding instead of being
refracted into it.

Propagation:
Current technology supports two modes (multimode and single mode)
for propagating light along optical channels, each requiring fiber with different
physical characteristics.
Multimode can be implemented in two forms: step-index and graded-
index
Multimode:
Multimode is so named because multiple beams from a light source
move through the core in different paths. How these beams move within the
cable depends on the structure of the core.
Types of Multimode:
1. Multimode Step Index:
The density of the core remains constant from the center to the edges. A
beam of light moves through this constant density in a straight line until it
reaches the interface of the core and the cladding.
At the interface, there is an abrupt change due to a lower density; this
alters the angle of the beam’s motion.
The term step-index refers to the suddenness of this change, which
contributes to the distortion of the signal as it passes through the fiber.
2. Multimode Graded Index:
A second type of fiber, called multimode graded-index fiber, decreases
this distortion of the signal through the cable.
The word index here refers to the index of refraction. The index of
refraction is related to density.
A graded-index fiber, therefore, is one with varying densities.
Density is highest at the center of the core and decreases gradually to its
lowest at the edge.
In the below figure we can see the impact of this variable density on the
propagation of light beams.
Single-Mode:
Single-mode uses step-index fiber and a highly focused source of light
that limits beams to a small range of angles, all close to the horizontal.
The single-mode fiber itself is manufactured with a much smaller
diameter than that of multimode fiber, and with substantially lower density
(index of refraction).
The decrease in density results in a critical angle that is close enough to
90° to make the propagation of beams almost horizontal.
In this case, propagation of different beams is almost identical, and
delays are negligible. All the beams arrive at the destination “together” and can
be recombined with little distortion to the signal.
Fiber size:
Fiber Sizes Optical fibers are defined by the ratio of the diameter of
their core to the diameter of their cladding, both expressed in micrometers.
The common sizes are shown below:(Note that the last size listed is for
single-mode only)

Type Core (μm) Cladding (μm) Mode

50/125 50.0 125 Multimode,
graded index
62.5/125 62.5 125 Multimode,
graded index
100/125 100.0 125 Multimode,
graded index
7/125 7.0 125 Single mode
• Fiber Optics Cable Connection:
The outer jacket is made of either PVC or Teflon.
Inside the jacket are Kevlar strands to strengthen the cable.
Kevlar is a strong material used in the fabrication of
bulletproof vests. Below the Kevlar is another plastic coating to cushion the fiber.
The fiber is at the center of the cable, and it consists of cladding and core.
• Fiber Optics Cable Connectors:
There are three types of connectors for fiber-optic cables,
1. Subscriber channel (SC) connector is used for cable TV. It uses a push/pull
locking system.
2. Straight-tip (ST) connector is used for connecting cable to networking
devices. It uses a bayonet locking system and is more reliable than SC.
3. MT-RJ is a connector that is the same size as RJ45.
Performance of Optic-fiber:
• Phenomenon: The attenuation in fiber-optic cables is flatter across different
wavelengths compared to twisted-pair and coaxial cables.
Fiber-optic cables experience less signal loss, leading to fewer
repeaters being needed—one-tenth as many compared to other cable types.

Application:
Fiber-optic cable is often found in backbone networks because its wide
bandwidth is cost-effective. Today, with wavelength-division multiplexing
(WDM), we can transfer data at a rate of 1600 Gbps.
• Optical Fiber and Coaxial Cable Hybrid
Some cable TV companies use a hybrid system:
– Fiber-optic cable for the backbone.
– Coaxial cable for the last-mile connection to the user’s premises.

• Local-area networks such as 100Base-FX network (Fast Ethernet) and

1000Base-X also use fiber-optic cable.
Advantages of Optic-fiber:
• Higher bandwidth:
Fiber-optic cable supports much higher bandwidth and data rates compared to
twisted-pair and coaxial cables.
Currently, data rates and bandwidth utilization over fiber-optic cable are
limited not by the medium but by the signal generation and reception technology
available.
• Less signal attenuation:
Fiber-optic cables can transmit signals over longer distances without
significant loss. A signal can run for 50 km without requiring regeneration. We
need repeaters every 5 km for coaxial or twisted-pair cable.
• Immunity to electromagnetic interference. Electromagnetic noise cannot
affect fiber-optic cables.
• Resistance to corrosive materials:
Since fiber-optic cables are made of glass, they are more resistant to
corrosion than copper cable.
• Light weight. Fiber-optic cables are much lighter than copper cables.
• Greater immunity to tapping. Fiber-optic cables are more immune to tapping
than copper cables. Copper cables create antenna effects that can easily be
tapped.

Disadvantages:
• Installation and maintenance:
Fiber-optic cable is a relatively new technology. Its installation and
maintenance require expertise that is not yet available everywhere.
• Unidirectional light propagation:
Propagation of light is unidirectional. If we need bidirectional
communication, two fibers are needed.
• Cost:
The initial cost of fiber-optic cables and interfaces is higher compared to
other guided media like copper.
 Transmission Media: Unguided Media
(Wireless)
• Definition
- Unguided media refers to transmission media where signals are
transmitted without the use of physical means.
- This type of communication is often referred to as wireless
communication. Signals are normally broadcast through free space and thus are
available to anyone who has a device capable of receiving them.

Types of Unguided medium: Radio Wave, Microwave, Infrared.

The part of the electromagnetic spectrum, ranging from 3 kHz to 900 THz, used
for wireless communication.
- Unguided signals can travel from the source to the destination in several ways.
- Propagation methods: ground propagation, sky propagation, and line-of-sight
propagation,
• Ground Propagation:
Low-frequency radio waves travel along the Earth’s surface.
Distance: Depends on the power of the signal.
• Sky Propagation:
Higher-frequency radio waves reflect from the ionosphere back to Earth.
Advantage: Longer distances with lower power.
• Line-of-Sight Propagation:
High-frequency signals travel in straight lines.
Requires directional antennas with minimal interference from Earth's
curvature.
 Propagation Frequency Range Distance Applications

Ground Propagation 30 kHz to 300 kHz Long (follows curvature) AM Radio

Sky Propagation 3 MHz to 30 MHz Long (reflected by ions) International Broadcasting

Line-of-Sight Above 30 MHz Short (direct antenna) Satellite, TV, Cellular

• Radio Frequency Band:
- The section of the electromagnetic spectrum defined as radio waves
and microwaves is divided into eight ranges, called bands
These bands are rated from very low frequency (VLF) to extremely high
frequency (EHF).
Band Range Propagation Application

very low frequency (VLF) 3–30 kHz Ground Long-range radio

navigation
low frequency (LF) 30–300 kHz Ground Radio beacons and
navigational locators
middle frequency (MF) 300 kHz–3 MHz Sky AM radio

high frequency (HF) 3–30 MHz Sky ship/aircraft

very high frequency (VHF) 30–300 MHz Sky and line-of-sight VHF TV, FM radio

ultrahigh frequency (UHF) 300 MHz–3 GHz Line-of-sight UHF TV, cellular phones,
paging, satellite

superhigh frequency (SF) 3–30 GHz Line-of-sight Satellite

extremely high frequency 30–300 GHz Line-of-sight Radar, satellite

(EHF)
Types of Unguided Media:
1. Radio Waves:
Electromagnetic waves ranging in frequencies between 3 kHz and 1
GHz are normally called radio waves.
Radio waves, particularly those waves that propagate in the sky mode,
can travel long distances.
Radio waves are omni directional.
When an antenna transmits radio waves, they are propagated in all
directions. This means that the sending and receiving antennas do not have to
be aligned.
A sending antenna sends waves that can be received by any receiving
antenna.
The omni directional property has a disadvantage, too. The radio
waves transmitted by one antenna are susceptible to interference by another
antenna that may send signals using the same frequency or band.
• Radio Waves (Low and Medium Frequencies)
Penetration Through Walls:
– Advantage: Radio waves can pass through walls, allowing signals to be
received inside buildings.
– Disadvantage: This also means we can't limit the communication to just
inside or outside of a building, which can lead to signal interference.
Narrow Bandwidth:
– The radio wave frequency range is relatively narrow (just under 1
GHz), especially compared to microwave frequencies.
– When divided into smaller subbands, this results in lower data rates for
digital communication.
Omnidirectional Antenna:
Radio waves use omnidirectional antennas that send out signals in all
directions. Based on the wavelength, strength, and the purpose of transmission,
we can have several types of antennas.

Application:
The omnidirectional characteristics of radio waves make them useful
for multicasting, in which there is one sender but many receivers.
AM and FM radio, television, maritime radio, cordless phones, and
paging are examples of multicasting.
2. Microwaves:
Electromagnetic waves having frequencies between 1 and 300 GHz are
called microwaves.
Microwaves are unidirectional. The sending and receiving antennas need to
be aligned.
The unidirectional property has an obvious advantage. A pair of antennas
can be aligned without interfering with another pair of aligned antennas.

Characteristics of microwave propagation:

Microwave propagation is line-of-sight. Since the towers with the mounted
antennas need to be in direct sight of each other, towers that are far apart need to be
very tall. The curvature of the earth as well as other blocking obstacles do not allow
two short towers to communicate by using microwaves. Repeaters are often needed
for long-distance communication.
Very high-frequency microwaves cannot penetrate walls. This characteristic
can be a disadvantage if receivers are inside buildings.
The microwave band is relatively wide, almost 299 GHz. Therefore wider
subbands can be assigned, and a high data rate is possible.
Use of certain portions of the band requires permission from authorities.
Unidirectional Antenna:
Microwaves need unidirectional antennas that send out signals in one
direction. Two types of antennas are used for microwave communications: the
parabolic dish and the horn.

Parabolic Dish Antenna:

• Design of Parabolic Dish:
– The parabolic dish antenna is shaped like a parabola.
– Lines parallel to the axis of symmetry (line of sight) reflect off the
curved surface and converge at a single point known as the focus.
• Signal Collection:
– The dish acts like a funnel, capturing a wide range of microwave signals
and directing them to the focus.
– This design allows for more efficient signal recovery compared to a
single-point receiver, enhancing the quality of received signals.
• Outgoing Transmissions:
– Transmissions are sent out using a horn antenna aimed at the parabolic
dish.
– The microwaves hit the dish and are deflected outward, reversing the
path taken when receiving signals.
Horn Antenna
• Design and Function:
– The horn antenna resembles a giant scoop.
– Outgoing transmissions travel up a "stem" (like a handle) and are
deflected outward in narrow, parallel beams by the curved head of the
horn.
• Receiving Signals:
– The horn's scooped shape collects incoming transmissions similarly to
the parabolic dish.
– Signals are then deflected down into the stem, allowing for effective
reception.
Application:
Microwaves, due to their unidirectional properties, are very useful
when unicast (one-to-one) communication is needed between the sender and the
receiver. They are used in cellular phones, satellite networks, and wireless
LANs.
• Wireless Local Area Networks (WLAN):
Function: Microwaves are employed in Wi-Fi networks, enabling
wireless connectivity for devices like laptops, smartphones, and tablets.
Advantage: Offers high-speed internet access without the need for
physical cables, enhancing mobility and convenience.
• Remote Sensing:
Function: Microwaves are used in remote sensing technologies, such as
satellite imaging and weather forecasting. They help gather data about the
Earth's surface and atmosphere.
Advantage: Allows for the monitoring of environmental changes,
natural disasters, and resource management.
3. Infrared:
Infrared waves, with frequencies from 300 GHz to 400 THz
(wavelengths from 1 mm to 770 nm), can be used for short-range
communication.
Infrared waves, having high frequencies, cannot penetrate walls. This
advantageous characteristic prevents interference between one system and
another; a short-range communication system in one room cannot be affected by
another system in the next room.
When we use our infrared remote control, we do not interfere with the
use of the remote by our neighbors. Infrared signals useless for long-range
communication.
In addition, we cannot use infrared waves outside a building because
the sun's rays contain infrared waves that can interfere with the communication.

Applications:
Infrared signals can be used for short-range communication in a closed
area using line-of-sight propagation.
 Switching: Packet Switching
What is Switching?
- Switching refers to the process of directing data packets between devices
in a network.
In data communications, we need to send messages from one end system
to another.
If the message is going to pass through a packet-switched network, it
needs to be divided into packets of fixed or variable size.
The size of the packet is determined by the network and the governing
protocol.

• Resource Allocation in Packet Switching

– No resource allocation for packets (no reserved bandwidth).
– Resources are allocated on demand.
– Allocation follows a first-come, first-served basis.
• Packet Structure:
A packet is constructed as a unit of data routed from the origin in the
packet-switched network to its destination.
For Ex: A user sends an email to the company’s customer support. An
email can contain an attached image or PDF etc. The email is sent through the
network as a packet-based transfer. The protocol in the network layer i.e. TCP/IP
breaks the data into smaller chunks as packets and routed towards the destination.
Each packet will be numbered and routed to different routes; when it has arrived at
the destination, the packets are assembled to the original format. This reassembling
is done by TCP at the destination.
• Packets are structured as Header, Payload, and Footer.
• Waiting for Packet Processing
– Packets must wait if the network is busy.
– This can lead to delays in packet transmission.

• We can have two types of packet-switched networks: datagram networks and

virtual circuit networks.
1. Datagram Network:
In a datagram network, each packet is treated independently of all
others. Even if a packet is part of a multipacket transmission, the network
treats it as though it existed alone. Packets in this approach are referred to as
datagrams.
Datagram switching is normally done at the network layer.
 The datagram networks are sometimes referred to as connectionless
networks. The term connectionless here means that the switch (packet switch) does
not keep information about the connection state.
There are no setup or teardown phases. Each packet is treated the same by a
switch regardless of its source or destination.

Routing Table in Datagram:

In this type of network, each switch (or packet switch) has a routing table
which is based on the destination address. The routing tables are dynamic and are
updated periodically.
The destination addresses and the corresponding forwarding output ports are
recorded in the tables.
Destination Addressing in Datagram:
Every packet in a datagram network carries a header that contains, among
other information, the destination address of the packet.
When the switch receives the packet, this destination address is examined;
the routing table is consulted to find the corresponding port through which the packet
should be forwarded.
This address, unlike the address in a virtual-circuit network, remains the
same during the entire journey of the packet.

Efficiency of Datagram:
• Better efficiency compared to circuit-switched networks.
• Resources allocated only when packets are transferred.
• Example of resource reallocation when delays occur.
Delay:
• Potential for greater delay compared to virtual-circuit networks.
• Waiting times at switches contribute to delay.
• Variation in delays among packets of the same message.
Formula for total delay: Total delay=3T+3τ+w1​+w2​
2. Virtual-Circuit Networks:
A virtual-circuit network is a cross between a circuit-switched network and
a datagram network. It has some characteristics of both.
- As in a circuit-switched network, there are setup and teardown phases in
addition to the data transfer phase.
- Resources can be allocated during the setup phase, as in a circuit-switched
network, or on demand, as in a datagram network.
- As in a datagram network, data are packetized and each packet carries an
address in the header. However, the address in the header has local jurisdiction
(it defines what the next switch should be and the channel on which the packet
is being carried), not end-to-end jurisdiction. The reader may ask how the
intermediate switches know where to send the packet if there is no final
destination address carried by a packet. The answer will be clear when we
discuss virtual-circuit identifiers in the next section.
- As in a circuit-switched network, all packets follow the same path established
during the connection.
- A virtual-circuit network is normally implemented in the data-link layer, while a
circuit-switched network is implemented in the physical layer and a datagram
network in the network layer. But this may change in the future.
Addressing:
In a virtual-circuit network, two types of addressing are involved:
global and local (virtual-circuit identifier).

Definition: A global address is a unique identifier used to

distinguish a source or destination node in the scope of a network or globally
across interconnected networks (e.g., internationally). It ensures that
communication can occur between any two systems across the entire
network.
Purpose: Global addresses are used primarily during the setup phase of a
virtual circuit. Once the circuit is established, the global address is no longer
needed for individual data frames, and communication proceeds using a
simpler identifier.
Example: In an international phone network, a global address is analogous to
a phone number that uniquely identifies someone in the network.
Virtual-Circuit Identifier:
The identifier that is actually used for data transfer is called the
virtual-circuit identifier (VCI) or the label.
A VCI, unlike a global address, is a small number that has only
switch scope; it is used by a frame between two switches.
Three Phases:
As in a circuit-switched network, a source and destination need to go
through three phases in a virtual-circuit network: setup, data transfer, and
teardown.
In the setup phase, the source and destination use their global addresses
to help switches make table entries for the connection.
In the teardown phase, the source and destination inform the switches to
delete the corresponding entry.
Data transfer occurs between these two phases.

1. Data transfer:
To transfer a frame from a source to its destination, all switches need to
have a table entry for this virtual circuit.
The table, in its simplest form, has four columns. This means that the
switch holds four pieces of information for each virtual circuit that is already set
up.
2. Setup Phase:
In the setup phase, a switch creates an entry for a virtual circuit. For
example, suppose source A needs to create a virtual circuit to B. Two steps are
required: the setup request and the acknowledgment.
Step Request: A setup request frame is sent from the source to the destination.
Acknowledgment: A special frame, called the acknowledgment frame, completes
the entries in the switching tables.
Setup Request:

a. (Source A): Source A initiates a connection by sending a setup frame to Switch 1.

b. (Switch 1):
Switch 1 receives the setup frame from A through incoming port 1.
It assigns an available incoming Virtual Circuit Identifier (VCI), which is 14 in this case.
The outgoing port for reaching the destination (B) is determined to be port 3 (as per its routing
table). However, the outgoing VCI isn't assigned yet because it will be determined later.
The setup frame is then forwarded to Switch 2 via port 3.
c. (Switch 2):
Switch 2 receives the frame on its port 1 and assigns an incoming VCI of 66.
The frame is to be forwarded via port 2.
The frame is sent to Switch 3.
d. (Switch 3):
Switch 3 receives the frame on port 2 and assigns an incoming VCI of 22.
The frame will be forwarded through port 3 to the final destination (B).
e. (Destination B):
B receives the setup frame and assigns a VCI (77) to identify frames coming from A.
This VCI will be used to process incoming frames from A and distinguish them from other
possible sources.
Acknowledge:

a. Destination B to Switch 3 (Step a):

Destination B sends an acknowledgment to Switch 3, containing VCI 77.
Switch 3 uses this VCI to update its table by adding the outgoing VCI (77) for frames going to
Destination B.
b. Switch 3 to Switch 2 (Step b):
Switch 3 sends an acknowledgment to Switch 2.
The acknowledgment contains the VCI used in Switch 3’s table (VCI 22), which Switch 2 uses to
update its own table as the outgoing VCI for this connection.
c. Switch 2 to Switch 1 (Step c):
Switch 2 sends an acknowledgment to Switch 1.
The acknowledgment carries VCI 66 (from Switch 2’s table), which Switch 1 uses as the
outgoing VCI in its table.
d. Switch 1 to Source A (Step d):
Switch 1 sends the final acknowledgment to Source A.
The acknowledgment contains VCI 14 (from Switch 1’s table), which Source A uses as the
outgoing VCI for the virtual circuit.
e. Source A Handling (Step e):
After receiving the acknowledgment, Source A uses VCI 14 as the outgoing VCI for the data
frames it sends to Destination B.
Teardown Phase:
In this phase, source A, after sending all frames to B, sends a special frame
called a teardown request. Destination B responds with a teardown confirmation frame.
All switches delete the corresponding entry from their tables.

Efficiency:
- Resources are reserved for the entire duration of the communication, ensuring
consistent quality of service (QoS) with a predictable delay for each packet.
- Resources are allocated as needed during the data-transfer phase, which may result
in varying delays for packets. However, this method can be more efficient in
scenarios where constant resource usage is not required.

- In virtual-circuit switching, all packets belonging to the same source and destination
travel the same path, but the packets may arrive at the destination with different
delays if resource allocation is on demand.
Delay in Virtual Network:
In a virtual-circuit network, there is a one-time delay for setup and a one-time
delay for teardown. If resources are allocated during the setup phase, there is no wait
time for individual packets. The delay for a packet traveling through two switches in a
virtual-circuit network.
1. Transmission Time (T): This is the time required to push all of the packet’s bits onto the link. In this
case, since there are three transmission stages (through two switches/routers), the total transmission
time will be 3T3T3T.
2. Propagation Time (τ\tauτ): This is the time it takes for a signal to travel from the sender to the receiver
across a link. Here, since the packet passes through three links (between host A, two switches, and host
B), the total propagation time will be 3τ3\tau3τ.
3. Setup Delay: This delay occurs before the actual data transmission, as a virtual circuit needs to be
established. The setup delay includes transmission and propagation time in both directions, which
typically involves sending control packets to reserve resources across the network.
4. Teardown Delay: After the data transmission is complete, the virtual circuit is torn down, releasing the
resources. The teardown delay includes transmission and propagation time in one direction.
Total Delay Formula:
The total delay for the packet is the sum of the following components:
Total delay=3T+3τ+setup delay+teardown delay

• 3T: Transmission delay across three links.

 3τ: Propagation delay across three links.
 Setup delay: Time for establishing the virtual circuit, including transmission and propagation in both
directions.
 Teardown delay: Time for tearing down the circuit, including transmission and propagation in one
direction.
 Conclusion

• Recap of Key Points:

- Overview of data communications and networking concepts.
- Importance of understanding network types, models, and media.
- Introduction to switching and its types.
- Importance of Understanding Networking Basics:
- Fundamental knowledge for anyone entering the field of networking.
 References

• Textbook: Behrouz A. Forouzan, Data Communications and Networking,

5th Edition, Tata McGraw Hill, 2013.
THANK YOU