COMPUTER NETWORKS (R23)
UNIT-1
What Is the Internet?
The Internet is a global network of interconnected computers
and devices that communicate using standardized protocols,
most notably TCP/IP. It connects end systems (like your phone
or laptop) via routers and communication links.
The Internet provides:
Communication services (e.g., email, messaging)
Information sharing (e.g., websites, cloud storage)
Remote access (e.g., remote servers, IoT devices)
The Network Edge
What it is:
The network edge refers to the part of the Internet where end
systems (also called hosts) reside — basically, the devices you
and I use (like smart phones, laptops, servers).
Key components:
End systems: Devices that send/receive data (clients and
servers)
Access networks: The infrastructure that connects end
systems to the rest of the Internet (e.g., Wi-Fi, 5G, DSL,
fiber)
Edge routers: Routers located at the boundary between the
access network and the core network
The Network Core
What it is:
The network core is the heart of the Internet — a mesh of
routers and high-speed links that move data between different
parts of the Internet.
Two main functions:
Routing: Determining the best path for data to travel
Forwarding: Moving packets from one router to the next
Network Core Characteristics:
Contains packet switches (routers)
Often uses fiber optic and high-capacity infrastructure
Managed by Internet Service Providers (ISPs) and
backbone providers
Packet-Switched Networks
How communication happens:
Data is broken into packets, each of which is sent independently
across the network. At the destination, packets are reassembled
into the original message.
This contrasts with circuit switching (like old telephone
systems), where a dedicated path is established for the duration
of a call.
Benefits:
Efficient use of network resources
Supports many users at once
Scalability
Delay, Loss, and Throughput
These are performance metrics in packet-switched networks:
1. Delay (Latency)
The time it takes for a packet to travel from source to
destination. Includes:
Processing delay: Time to examine the packet header
Queuing delay: Time waiting in a router’s buffer
Transmission delay: Time to push packet onto the link
Propagation delay: Time for bits to travel over the link
2. Loss
Packets may be dropped if a router’s buffer is full. Causes
include:
Network congestion
Link or hardware failures
Lost packets may be retransmitted (e.g., by TCP).
3. Throughput
The rate at which data is successfully delivered over the
network, usually measured in bits per second (bps).
Instantaneous throughput: Rate at a given moment
Average throughput: Long-term data transfer rate
Summary Table
Concept Description
Global system of connected networks using
Internet
TCP/IP
End systems and access networks (clients,
Network Edge
servers)
Network Core Routers and links that forward data
Packet Data sent in discrete packets without a fixed
Switching path
Delay Time for data to reach its destination
Loss Packets dropped due to congestion or errors
Throughput Rate of successful data delivery
OSI (Open Systems Interconnection) reference model
The OSI (Open Systems Interconnection) reference model is a
conceptual framework that breaks down network
communication into seven layers,
The 7 Layers Explained
1. Physical (Layer 1): Transfers raw bits (1s and 0s)
over cables, fiber, or radio (e.g., USB, Ethernet cables,
Wi-Fi).
2. Data Link (Layer 2): Manages node-to-node
frames and MAC addresses. Handles error
detection/recovery on a local network (Ethernet,
Wi-Fi).
3. Network (Layer 3): Routes packets between
different networks using IP addressing (IPv4/IPv6,
ICMP). Think of routers forwarding data across the
internet
4. Transport (Layer 4): Provides reliable (TCP) or
fast (UDP) delivery, managing segmentation, flow
control, and error handling.
5. Session (Layer 5): Sets up, maintains, and tears
down communication sessions. Useful for login
sessions, remote desktop, or Zoom calls
6. Presentation (Layer 6): Handles data formatting,
compression, and encryption (e.g., TLS/SSL, JPEG,
MP4 decoding)
7. Application (Layer 7) :Directly interfaces with
user applications. Think HTTP for websites,
SMTP/IMAP/POP3 for email, and APIs used by apps.
Real World Network Example: Web Browsing
App Layer (7): Your browser issues an HTTPS request
Presentation (6): Encrypts the request with TLS
Session (5): Establishes a session with Google
Transport (4): Sends encrypted data using TCP for
reliability
Network (3): Routes packets via IP across networks
Data Link (2): Frames data on your local network (e.g.,
through your router/switch)
Physical (1): Sends bits via cables or Wi-Fi to the ISP
Other Practical Scenarios
Email (SMTP/IMAP/POP3) executes mostly at Layer 7 .
Ethernet LANs operate primarily on Layer 1 and 2:
cables and MAC addressing BusinessMole.
IP packet routing across the internet occurs at Layer 3 .
Streaming video or VoIP often uses UDP (Layer 4) for
speed, while TCP (Layer 4) ensures complete downloads
Why It Matters
Troubleshooting: Layered troubleshooting helps pinpoint
issues quickly—for instance, slow speeds could be physical
(Layer 1), routing (Layer 3), or application-level.
Standardized design: Each layer is modular—network
engineers and software developers can work independently
without interfering across layers Security control: Layer-
specific safeguards—like MAC-level filtering (Layer 2),
IPsec (Layer 3), TLS (Layer 6)—help build robust
networks.
Interoperability: Adherence to standard protocols ensures
devices from different vendors can communicate
seamlessly
Quick Reference Summary
Layer Key Function Real-World Protocols/Devices
Bit transmission over
1 Ethernet cables, Wi-Fi, NICs
medium
Layer Key Function Real-World Protocols/Devices
Local frame delivery,
2 Ethernet, ARP, Wi-Fi
error control
Routing, logical
3 IP, ICMP, routers
addressing
End-to-end data
4 TCP, UDP
delivery
5 Session control SSL handshake, RPC, RDP
Formatting,
6 encryption, JPEG, MP4, TLS/SSL
compression
User-facing network
7 HTTP, SMTP, FTP, DNS
services
The OSI model is not an exact implementation, but a
framework that helps understand how modern networks and
protocols (like the TCP/IP stack) operate. Let me know if you’d
like to dive into TCP/IP mapping, deeper packet-level details, or
practical troubleshooting tips!
Guided Transmission Media and Wireless Transmission
Two Main types of communication channels used in
networking:
1. Guided Transmission Media (Wired Media)
Definition:
Guided transmission media use physical paths like cables to
transmit data signals. The signal is directed and confined within
a solid medium.
Types:
Type Description
Consists of two insulated copper
Twisted Pair Cable wires twisted together. Used in
telephone and LAN cables.
Contains a central conductor,
insulating layer, metallic shield,
Coaxial Cable
and outer casing. Used in cable
TV and broadband.
Uses light signals to transmit data
through thin glass or plastic
Fiber Optic Cable
fibers. Offers high speed and
bandwidth.
Advantages:
High data security
Stable connection
Less interference
Disadvantages:
Cost of installation
Physical maintenance
Limited flexibility (not mobile)
2. Wireless Transmission Media
Definition:
Wireless transmission does not use any physical medium.
Instead, data is transmitted through air using electromagnetic
waves.
Types:
Type Description
Used in AM/FM radio, television,
Radio Waves mobile phones. Can travel long
distances and penetrate walls.
Line-of-sight transmission. Used
Microwaves in satellite communication and
cellular networks.
Short-range, line-of-sight
communication. Used in TV
Infrared
remotes and short-distance data
transfers.
Short-range wireless technology
Bluetooth / Wi-Fi for data exchange between
devices and local area networks.
Advantages:
Supports mobility
Easier and cheaper to install
Flexible access (e.g., Wi-Fi anywhere)
Disadvantages:
Prone to interference (weather, obstacles)
Less secure
Lower data transmission rates (compared to fiber optics)
Summary Table:
Feature Guided Media Wireless Media
Medium Physical (cables) Air/Electromagnetic waves
Security Higher Lower
Installation cost High Lower
Flexibility Less High
Very High (e.g.
Speed Variable
fiber)
Packet Switching vs. Circuit Switching are two fundamental
methods of data transmission in networks, each suited to
different types of communication. Here's a comparison of their
applications:
Applications of Packet Switching
Packet Switching breaks data into packets that are sent
independently and reassembled at the destination. It's used in
networks where efficiency, scalability, and resilience are
prioritized.
Internet and Web Applications
Web browsing (HTTP/HTTPS)
Email (SMTP, IMAP, POP3)
File transfers (FTP, SFTP)
Cloud services and SaaS applications
Voice over IP (VoIP)
Applications like Skype, Zoom, Google Meet, and
WhatsApp calls rely on packet switching to deliver voice
and video data efficiently.
Online Gaming
Real-time data transfer with low latency is managed well
with packet switching.
Streaming Services
Video and music streaming (e.g., YouTube, Netflix,
Spotify) adapt well to variable data rates supported by
packet switching.
Data Communication in LANs/WANs
Corporate networks and wide area networks use packet
switching to manage data flow between computers and
servers efficiently.
Applications of Circuit Switching
Circuit Switching establishes a dedicated communication path
between two endpoints for the duration of the session. It's
optimal where consistent, real-time data transmission is crucial.
Traditional Telephone Networks (PSTN)
Landline voice calls use circuit switching to ensure a
constant connection and fixed bandwidth during the call.
ISDN Networks
Integrated Services Digital Network used circuit switching
for carrying both voice and data in early digital
communication systems.
Emergency Communication Systems
Systems requiring guaranteed bandwidth and real-time
performance (e.g., 911 call systems) may use circuit-
switched infrastructure.
Legacy Telecommunication Infrastructure
Older PBX (Private Branch Exchange) systems in
businesses often used circuit-switched connections for
internal and external calls.
Summary Table
Feature Packet Switching Circuit Switching
Key Use Case Data networks Voice communication
Feature Packet Switching Circuit Switching
(Internet) (Traditional telephony)
Resource
Shared, dynamic Dedicated path
Allocation
High (better Low (reserved path
Efficiency
bandwidth utilization) even during silence)
Lower (pay per data Higher (pay per
Cost
sent) time/distance)
Scalability Highly scalable Less scalable
With QoS
Real-Time Native (constant
mechanisms (VoIP,
Suitability bandwidth)
video calls)
