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Introduction to Computer Networks

(2019 Pattern) (310244)

Q. Answer
1 Explain Switch and Router. [5 Marks]
The Switch

A network switch is a hardware device that connects multiple devices (like

computers, printers, and servers) together on a single computer network,
specifically a Local Area Network (LAN). Its primary job is to intelligently forward
data to the correct destination device within that local network.
Analogy: Think of a switch as the mailroom clerk in a large office building. When a
letter arrives for "John Smith," the clerk doesn't just shout "LETTER FOR JOHN
SMITH!" down the hallway (that's what an old, unintelligent device called
a hub would do). Instead, the clerk knows that John Smith works in Office #302 and
delivers the letter directly to that office only.

How a Switch Works

• OSI Layer: A switch operates at Layer 2 (the Data Link Layer) of the OSI
model.
• Address Type: It works with MAC (Media Access Control) addresses. A
MAC address is a unique, permanent hardware address burned into every
network card.
• The Process:
1. Learning: When a switch is first turned on, it knows nothing. As
devices connected to it start communicating, the switch inspects the
incoming data packets (called frames) to learn the MAC address of
each device and notes which physical port it's connected to. It stores this
information in a table called a MAC address table.
2. Forwarding: When a frame arrives at a port, the switch reads
the destination MAC address in the frame's header.
3. It looks up this destination MAC address in its table.
4. The table tells the switch which port the destination device is connected
to.

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5. The switch then creates a temporary, private connection between the

source and destination ports and forwards the frame only out of the
correct port. All other devices on the switch do not see this traffic.

Key Functions and Advantages:

• Reduces Network Traffic: By sending data only to the intended recipient, it
drastically reduces unnecessary traffic on the network.
• Improves Performance: Because it creates dedicated connections, multiple
pairs of devices can communicate simultaneously without interfering with
each other (unlike a hub).
• Filters and Forwards Frames: It intelligently decides whether to forward a
frame or drop it based on the destination MAC address.
• Reduces Collisions: It creates separate "collision domains" for each port,
meaning collisions (when two devices transmit at the same time) are virtually
eliminated in a modern switched network.

Primary Use Case: Connecting devices within a single LAN. For example, in an
office, all the computers, printers, and servers would be connected to one or more
switches.

The Router

A router is a networking device that connects two or more different networks

together and forwards data packets between them. Its primary job is to determine the
best path for data to travel from its source network to its destination network.
Analogy: Think of a router as the international post office sorting center in a city.
This sorting center doesn't care about the specific street address in a foreign country.
It only needs to know which country (i.e., which network) the package is going to.
It then puts the package on the correct airplane (the best path) heading to that country.

How a Router Works

• OSI Layer: A router operates at Layer 3 (the Network Layer) of the OSI
model.
• Address Type: It works with IP (Internet Protocol) addresses. An IP address
is a logical address that identifies both the network and the specific device
(host) on that network.
• The Process:

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1. Reading the Destination: When a data packet arrives at a router, the

router reads the destination IP address in the packet's header.
2. Consulting the Routing Table: The router maintains a routing table,
which is like a map of the internet. This table lists different network
addresses and the best "next hop" (the next router to send the packet to)
to reach those networks.
3. Path Determination: The router uses its routing table to decide the
most efficient path to send the packet on its journey toward the final
destination network.
4. Forwarding: The router then forwards the packet out of the correct
interface (port) to the next router on that path.

Key Functions and Advantages:

• Connects Different Networks: This is its main purpose. It allows a local
network (LAN) to communicate with a Wide Area Network (WAN) like the
Internet.
• Determines the Best Path: Using routing protocols, routers can dynamically
find the best route for data, even if a part of the network is down.
• Acts as a Default Gateway: For all devices on a local network, the router
serves as the single exit point, or "gateway," to all other networks.
• Blocks Broadcasts: Routers do not forward broadcast traffic from one
network to another, which helps to keep networks separate and reduces
unnecessary traffic.

Primary Use Case: Connecting a home or office LAN to the Internet. Your home
"Wi-Fi router" is actually a combination device: it's a router (to talk to the internet),
a switch (for the wired Ethernet ports), and a wireless access point (for Wi-Fi).

Summary of Key Differences: Switch vs. Router

Feature Switch Router

Primary Connects devices on a single Connects different
Function network (LAN). networks together.
OSI Layer Layer 2 (Data Link Layer) Layer 3 (Network Layer)
Address Type MAC Address (Hardware IP Address (Logical Address)
Used Address)
Data Unit Frame Packet
Handled

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Decision Forwards based on destination Forwards based on destination

Making MAC address. IP address.
Table Used MAC Address Table Routing Table
Scope of Local Area Network (LAN) LAN, MAN, WAN
Operation
Broadcasts Forwards broadcasts to all Blocks broadcasts by default.
ports by default.

2 Explain Client Server and Peer to Peer Network Architecture? [5 marks]

Network Architectures:

1. Client-Server Architecture:

This is a centralized model where one or more powerful computers, called servers,
provide services and resources to other computers, called clients. The
communication is a request-response cycle. It is highly scalable and secure but has
a single point of failure (the server).

• A Server is a high-performance computer that is always on, waiting for

requests. It hosts and manages shared resources like websites, databases,
files, or applications.
• A Client is a standard computer (like your laptop or smartphone) that
--
initiates requests to a server to access a resource.
• The communication follows a request-response pattern:
• The client sends a request over the network to the server's address (e.g.,
asking for a webpage).
• The server processes this request, retrieves the necessary data (e.g., the
HTML file for the webpage), and sends a response back to the client.

Real-World Examples:
• Web Browsing: Your web browser (client) requests a webpage from a web
server.
• Email: Your email app (client) connects to a mail server to send and receive
emails.
• Online Banking: Your banking app (client) communicates with the bank's
secure server to manage your account.

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2. Peer-to-Peer (P2P) Architecture:

This is a decentralized model where all computers (peers) are equal and share
resources directly with each other without a central server. Each peer acts as
both a client and a server. It is resilient and cost-effective but harder to manage
and secure.

• Each computer (peer) on the network maintains its own data and resources.
• When a peer needs a resource (e.g., a file), it connects directly to another
peer that has the resource.
• A peer can be requesting a file from one peer while simultaneously providing
a different file to another peer. This distributed workload is the key feature of
P2P.

Real-World Example:

• BitTorrent: A popular file-sharing system where users (peers) download

pieces of a file from many other users simultaneously and upload pieces to
others.
• Cryptocurrencies (like Bitcoin): Transactions are verified and recorded on
a distributed ledger maintained by all peers in the network.

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3 Explain Bridge and Gateway and access point. [6 marks]

Explanation of Bridge and Gateway:

1. 1. Bridge:

OSI Layer: Operates at the Data Link Layer (Layer 2).

Function:
A bridge is used to connect two separate LAN segments, making them
appear as a single, larger LAN. It intelligently filters traffic based on MAC
addresses. When a data frame arrives, the bridge checks the destination
MAC address. If the destination is on the same segment, the bridge drops the
frame; if it's on the other segment, it forwards it. This reduces unnecessary
network traffic.

Use Case: Connecting two different departments' LANs within the same
building.

2. 2.Gateway:

OSI Layer: Can operate at any layer, but often at the Application Layer (Layer
7) or Network Layer (Layer 3).

Function:
A gateway is a device that connects two networks that use entirely different
protocols. It acts as a protocol translator. For example, it can connect a

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TCP/IP network to a network running a different protocol like AppleTalk or

connect a LAN to a mainframe system. It is more complex and intelligent
than a router or bridge.

Use Case: Connecting a corporate LAN to the public telephone network for VoIP
calls. Your home router also acts as a gateway to connect your LAN to your ISP's
network.
3.Access Point (AP)

• An Access Point (AP) is a device that allows wireless devices to connect to

a wired network.
• It works at the Data Link Layer (Layer 2) and acts as a bridge between
wireless and wired LANs.
• Access points extend the range of a wireless network and are essential in Wi-
Fi environments.

Key Features:
• Enables wireless connectivity.
• Converts wireless signals to wired Ethernet signals.
• Can serve multiple wireless users at the same time.
•
Example: A Wi-Fi hotspot in a café or campus where users connect to the internet
wirelessly.

List and Explain following types of networks: [6 marks]

4
LAN , MAN , PAN

Computer networks are categorized based on their geographical coverage, scale,

and purpose. The three most common types are:

i) LAN – Local Area Network

A Local Area Network (LAN) connects computers and devices within a limited
area, such as:
• A single room
• An office building
• A home

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• A school or college campus

Characteristics:
• Covers a small geographical area (usually under 1 km).
• Provides high-speed data transfer (100 Mbps to 10 Gbps).
• Low cost to set up and maintain.
• Managed and owned by a single organization or individual.

Technologies Used:
• Ethernet (Wired LAN): Uses switches and cables.
• Wi-Fi (Wireless LAN): Uses wireless routers or access points.
Applications:
• Sharing printers, files, and internet among office computers.
• Connecting student computers in a school lab.
• Multiplayer gaming or media streaming at home.

ii) MAN – Metropolitan Area Network

A Metropolitan Area Network (MAN) spans a larger area than a LAN, typically a
city or large campus. It connects multiple LANs together to form a wider
network.
Characteristics:
• Covers an area of 5 to 50 km.
• Often used to connect networks in different buildings, branches, or
institutions within the same city.
• Requires high-speed fiber optic or leased lines.
• Managed by a single ISP or network service provider.
Technologies Used:
• Fiber optic cables, DSL, leased lines, microwave links.
• Metro Ethernet, SONET, or WiMAX.
Applications:
• Cable TV network across a city.
• Government or municipal networks connecting departments.
• Universities with multiple campuses connected to a central server.

iii) PAN – Personal Area Network

A Personal Area Network (PAN) is the smallest and most localized type of
network, created around a single person, usually within a range of 1 to 10 meters.

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Characteristics:
• Connects personal electronic devices used by one person.
• Typically wireless, using Bluetooth, Infrared, or USB.
• Does not require central networking devices like switches or routers.
Technologies Used:
• Bluetooth (most common)
• Infrared (IR)
• ZigBee
• NFC (Near Field Communication)
Applications:
• Connecting a smartphone to wireless earphones or a smartwatch.
• Sending files between mobile phones using Bluetooth.
• Using a wireless keyboard/mouse with a laptop.

5 What are design issues of layers? Explain it. [6 marks]

Layered architecture is a fundamental concept in networking that divides the

complex task of communication into smaller, more manageable parts called layers.
Each layer is responsible for a specific set of functions and provides services to the
layer above it. However, designing these layers involves solving several recurring
problems, known as design issues.

Here are the key design issues that must be addressed at most layers:

1. Addressing: For communication to happen, a sender needs a way to identify

and locate a specific receiver. Each layer requires a specific addressing
mechanism.
o Data Link Layer: Uses physical addresses (MAC addresses) to
identify devices on a local network.

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o Network Layer: Uses logical addresses (IP addresses) to identify

hosts across the entire internet.
o Transport Layer: Uses port numbers to identify specific processes or
applications running on a host.
2. Error Control: Data can get corrupted during transmission due to physical
noise or interference. Layers must have mechanisms to detect and potentially
correct these errors.
o Data Link Layer: Uses techniques like CRC (Cyclic Redundancy
Check) to detect errors in a frame and may request retransmission.
o Transport Layer: Uses checksums and acknowledgments to ensure
that entire data segments arrive correctly and in order.

3. Flow Control: A fast sender can easily overwhelm a slow receiver with data,
causing data loss. Flow control is the mechanism that manages the rate of
data transmission to prevent this.
o Data Link Layer & Transport Layer: Both implement flow control
using techniques like the Sliding Window Protocol, which allows the
receiver to tell the sender how much data it is prepared to receive at
any given time.

4. Multiplexing and Demultiplexing: Multiplexing is the process of

combining multiple signals or data streams into a single channel. This allows
several conversations to share a single physical link or network connection.
Demultiplexing is the reverse process at the receiving end.
o Transport Layer: Uses port numbers to multiplex data from different
applications (e.g., web browser, email client) into a single stream to
the Network Layer. At the other end, it demultiplexes the incoming
data and sends it to the correct application.

5. Routing: This issue is specific to the Network Layer. In a large network,

there are many possible paths from a source to a destination. Routing is the
process of determining and selecting the optimal path for a packet to travel.
This involves complex routing algorithms and protocols.

Draw Manchester and differential Manchester code for the bit sequence:
6 0100110 [5 marks]

Rules:

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• Manchester Code: A mid-bit transition is always present for clocking.

o 0 = High-to-Low transition in the middle.
o 1 = Low-to-High transition in the middle.
• Differential Manchester Code: Information is in the transition at
the beginning of the bit period. A mid-bit transition is always present for
clocking.
o 0 = A transition occurs at the beginning of the period.
o 1 = No transition occurs at the beginning of the period.
Drawing:
Here is the encoding for the bit sequence 0100110:

7 Explain Star and Bus topologies

Bus Topology

• Description: In a bus topology, all network devices

(computers, printers, etc.) are connected to a single
central cable, known as the bus or backbone. Both ends
of the bus must be terminated with a special resistor (a
terminator) to prevent signal reflection.
• How it works: When a device wants to send data,
it broadcasts the message onto the bus. The message travels along the entire
length of the cable. Every other device on the bus receives the message, but
only the device whose address matches the destination address in the
message will accept and process it.
• Advantages:
o Simple and inexpensive to install.
o Requires less cable than other topologies.

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• Disadvantages:
o Single Point of Failure: If the main bus cable breaks, the entire
network fails.
o Difficult to Troubleshoot: Finding a fault on the bus can be very
difficult.
o Collisions: If two devices transmit at the same time, a collision occurs,
and the data is lost. Performance degrades quickly as more devices are
added.

Star Topology

• Description: In a star topology, every network device has a dedicated point-

to-point connection to a central network device, such as a hub or a switch.
• How it works: If a device wants to send data to another, it sends the data to
the central device. A hub will broadcast the data to all other connected
devices, while a more intelligent switch will send the data only to the
intended destination device.
• Advantages:
o Robust: If one cable fails, only
that single device is disconnected from the
network; the rest of the network
continues to function.
o Easy to Install and Manage: Adding or
removing devices is simple.
o Easy to Troubleshoot: It is easy to isolate a
faulty device or cable.
• Disadvantages:
o Central Point of Failure: If the central hub or switch fails, the entire
network goes down.
o Requires more cable than a bus topology, which can increase costs.
Diagrams:

8 Explain Ring and Mesh topologies. [6 marks]

Ring Topology

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• Description: In a ring topology, devices are connected in a circular fashion,

with each device connected to exactly two other
devices, forming a single continuous pathway for
signals.
• How it works: Data travels around the ring in one
direction. Each device acts as a repeater, regenerating
the signal and passing it along to the next device in the
ring. Often, a "token" is passed around the ring, and a
device must possess the token before it is allowed to transmit data (this is
called Token Ring).
• Advantages:
o Performs better than a bus topology under heavy network load because
there are no collisions.
o Orderly access to the network (no one can monopolize the channel).
• Disadvantages:
o Single Point of Failure: A break in the ring or the failure of a single
device can bring down the entire network.
o Changes, like adding or removing a device, disrupt the network.

Mesh Topology
• Description: In a mesh topology, devices are interconnected with many
redundant connections. In a full mesh, every single device is connected
directly to every other device. In a partial mesh, some devices are connected
to all the others, but some are only connected to those with which they
exchange the most data.
• How it works: Data can be sent via multiple paths from a source to a
destination. This allows for dynamic routing to avoid broken links or
network congestion.
• Advantages:
o Extremely Fault-Tolerant and Reliable: If one link or node fails, the
network can reroute the traffic through another path. There is no single
point of failure.
o High Security and Privacy: Point-to-point links make it harder to
eavesdrop on traffic.
• Disadvantages:
o Very Expensive: The amount of cabling and the number of network
ports required are immense.

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oComplex to Install and Manage: The wiring can be very

complicated.
Diagrams:

Explain Frequency Hopping Spread Spectrum. [6 marks]

9

Frequency Hopping Spread Spectrum (FHSS) is a wireless transmission technique

designed to be highly resistant to interference and difficult to intercept. Instead of
transmitting on a single, fixed frequency, the signal "hops" between many different
frequencies across a wide band.

How it Works:
1. Channel Division: A wide frequency band is divided into many smaller,
discrete channels.
2. Hopping Sequence: The sender and receiver agree on a specific, pseudo-
random sequence of channels to hop between. This sequence is their shared
secret.
3. Transmission: The sender transmits a small chunk of data on one frequency
channel for a very short period of time (called the "dwell time").
4. Hopping: After the dwell time expires, both the sender and receiver
simultaneously hop to the next frequency in the sequence and repeat the
process.

Key Advantages:
• Interference Avoidance: If a particular frequency is jammed or noisy, the
system is only affected for the tiny fraction of a second it dwells on that
frequency before hopping to a clear one.
• Security: An eavesdropper who does not know the hopping sequence cannot
follow the signal. To them, the transmission looks like random, low-power
bursts of noise across the spectrum.

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• Coexistence: Multiple FHSS systems can operate in the same area by simply
using different hopping patterns, which makes it unlikely they will interfere
with each other.

Primary Example: Bluetooth technology uses FHSS to allow devices like phones,
headphones, and keyboards to communicate reliably even in areas with a lot of Wi-
Fi and other 2.4 GHz radio noise.

Draw Manchester and differential Manchester code for the bit sequence:
10
01001011. [6 marks]
Rules (repeated for clarity):
• Manchester Code: 0 = High-to-Low mid-bit transition; 1 = Low-to-High
mid-bit transition.
• Differential Manchester Code: 0 = Transition at the start of the bit; 1 = No
transition at the start of the bit.
Drawing:
Here is the encoding for the bit sequence 01001011:

11 Explain TCP/IP Model. [6 marks]

The TCP/IP Model, also known as the Internet Protocol Suite, is the practical, 4-
layer networking model that forms the foundation of the modern internet. It is more
concise than the theoretical OSI model and describes the set of communication
protocols used for network communication.

The four layers are:

1. Network Access Layer (or Link Layer):

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o This is the lowest layer, combining the functions of the OSI model's
Physical and Data Link layers.
o Function: It is responsible for the physical transmission of data over
the physical medium. It handles all the hardware details of interfacing
with the physical network, such as cabling, converting bits into
electrical signals, and MAC addressing on the local network.
o Protocols: Ethernet, Wi-Fi, PPP.
2. Internet Layer:
o This layer corresponds to the OSI model's Network Layer.
o Function: Its primary job is to move packets from a source host to a
destination host across one or more networks. It handles logical
addressing (IP addresses) and routing. It provides a "best-effort"
delivery service, meaning it doesn't guarantee that packets will arrive
in order or at all.
o Protocols: IP (Internet Protocol) is the core protocol. Others include
ICMP and ARP.
3. Transport Layer:
o This layer corresponds to the OSI model's Transport Layer.
o Function: It provides process-to-process communication between
applications on different hosts. It is responsible for end-to-end data
transfer, flow control, and error correction. It offers two main
protocols:
▪ TCP (Transmission Control Protocol): A reliable,
connection-oriented protocol. It guarantees that data arrives in
the correct order and without errors by using acknowledgments
and retransmissions. Used for web browsing (HTTP), email
(SMTP), and file transfers (FTP).
▪ UDP (User Datagram Protocol): An unreliable,
connectionless protocol. It is much faster than TCP because it
does not guarantee delivery. Used for applications where speed
is more important than perfect reliability, such as video
streaming, online gaming, and DNS.
4. Application Layer:
o This is the top layer, combining the functions of the OSI model's
Session, Presentation, and Application layers.
o Function: This layer contains the protocols that network applications
use to communicate. It is the layer the user interacts with.

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o Protocols: HTTP (Web), SMTP (Email), FTP (File Transfer), DNS

(Domain Name System), SSH (Secure Shell).

12 Draw and explain OSI Model. [6 marks]

The Open Systems Interconnection (OSI) Model is a conceptual framework that

standardizes the functions of a telecommunication or computing system into seven
distinct layers. It was developed to ensure interoperability between diverse network
products. It is a reference model, meaning it provides a theoretical blueprint rather
than being a strict implementation like TCP/IP.

Here are the seven layers, from bottom to top:

Layer 1: Physical Layer

• Function: Responsible for the physical transmission of raw bits over a
communication channel. It deals with the hardware specifications of the
network, such as cables, connectors, voltages, and signal timing.
• PDU (Protocol Data Unit): Bit.
Layer 2: Data Link Layer
• Function: Takes the raw bits from the Physical Layer and organizes them
into structured units called frames. It is responsible for reliable node-to-node
delivery on the same physical network. It handles physical (MAC)
addressing, error detection (using CRC), and flow control.

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• PDU: Frame.
Layer 3: Network Layer
• Function: Responsible for routing packets from a source host to a
destination host across multiple networks. It handles logical (IP) addressing,
path determination, and forwarding.
• PDU: Packet.
Layer 4: Transport Layer
• Function: Provides reliable, end-to-end communication between processes
on different hosts. It ensures that data is delivered completely, in order, and
without errors. It handles segmentation, flow control, and error correction.
• PDU: Segment (TCP) or Datagram (UDP).
Layer 5: Session Layer
• Function: Manages, maintains, and terminates communication "sessions"
(dialogues) between applications. It handles synchronization by placing
checkpoints in the data stream, allowing a session to resume from a
checkpoint if it fails.
• PDU: Data.
Layer 6: Presentation Layer
• Function: Acts as a data translator for the network. It ensures that data sent
from the application layer of one system can be read by the application layer
of another. It handles data formatting, character set translation (e.g., ASCII to
EBCDIC), data compression, and encryption/decryption.
• PDU: Data.
Layer 7: Application Layer
• Function: The topmost layer and the one closest to the end-user. It provides
the network services that user applications (like web browsers and email
clients) use to communicate.
• PDU: Data.

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13 Explain Direct Sequence Spread Spectrum [6 marks]

Direct Sequence Spread Spectrum (DSSS) is a wireless transmission technique that

provides high resistance to interference and allows multiple users to share the same
frequency band. It works by "spreading" a narrow-band data signal across a much
wider, fixed frequency band.

How it Works:
1. Chipping Code: A special high-rate bit sequence, known as a chipping
code or spreading code, is generated. This code is unique and is known to
both the sender and the receiver.
2. Spreading Process: To transmit a single data bit (a 1 or a 0), the sender
combines it with the entire chipping code. For example, to send a 1, the
sender might transmit the original chipping code. To send a 0, it might
transmit the inverted version of the code.
3. Transmission: Because the chipping code is much longer and faster than the
original data bit, this process dramatically increases the bandwidth of the
signal, spreading its energy across a wide frequency range. The resulting
signal is very low power and resembles background noise.
4. De-spreading Process: The receiver, which knows the exact same chipping
code, multiplies the incoming wideband signal with the code. This
mathematical operation has two effects:

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o It collapses the intended signal back down to the original single data
bit.
o It takes any narrow-band interference that was received and spreads it
out over the wide band, rendering its power negligible and effectively
filtering it out.

Key Advantages:
• Excellent Interference Rejection: The de-spreading process is extremely
effective at removing narrow-band noise or jamming signals.
• Low Probability of Intercept: The transmitted signal is very weak and
looks like noise, making it hard for an unauthorized party to detect or
decode.
• Code Division Multiple Access (CDMA): Multiple users can transmit on
the same frequency band at the same time by simply using different,
orthogonal chipping codes. A receiver tuned to one code will only be able to
de-spread the signal from the sender using that specific code, ignoring all
others.

Primary Examples: Early Wi-Fi standards (802.11b) and the Global Positioning
System (GPS) rely on DSSS.

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