CN UNIT-1

Basics of Networking - Components – Direction of Data flow – Networks – Components and

Categories – Types of Connections – Topologies –Protocols and Standards – ISO / OSI model,
TCP/IP model.Physical layer - Digital transmission, Multiplexing, Transmission Media,
Switching, Circuit Switched Networks, Datagram Networks, Virtual Circuit Networks.

Data Communication is the process of exchanging data or information between two devices
over a transmission medium, such as cables or wireless signals236. It involves a
communication system made up of hardware (sender, receiver, and intermediate devices) and
software (rules or protocols for how data is exchanged)125.

Key Components of Data Communication

Message: The information to be sent (text, audio, video, etc.)

Sender: The device that sends the data
Receiver: The device that receives the data
Transmission Medium: The path (wired or wireless) through which data travels
Protocol: The set of rules that govern how data is communicated125

Fundamental Characteristics for Effective Data Communication

1. Delivery: Data must reach the correct destination and intended user.
2. Accuracy: Data should be delivered without errors.
3. Timeliness: Data (especially audio/video) should be delivered on time for real-time use.
4. Jitter: Variation in packet arrival time; too much jitter can affect timely delivery8.

Data Flow Modes

Two devices communicate with each other by sending and receiving data. The data can flow
between the two devices in the following ways.

1. Simplex 2. Half Duplex 3. Full Duplex

Simplex:
 Unidirectional: Only one device sends data; the other only receives.
Example: CPU sends data to a monitor; monitor cannot reply245.
Half Duplex:

Bidirectional, but one at a time: Both devices can send and receive, but not
simultaneously.
Example: Walkie-talkie—each person speaks in turn245.
Full Duplex:

Bidirectional and simultaneous: Both devices can send and receive at the same time.
Example: Mobile phones—both parties can talk and listen at once245.

Computer Network Components

A computer network connects devices (nodes) through communication links, enabling data
exchange26. The main components are:

Hardware Components

Server: High-powered computer that manages network resources and provides services
to other devices13.
Client: Device (like a PC or smartphone) that requests and accesses network services13.
Peer: Device that both provides and receives services in peer-to-peer networks13.
 Transmission Media: Physical or wireless channels (cables, fiber optics, radio waves) that
carry data between devices16.

Connecting Devices: Devices that link networks or computers:

Routers: Direct data between networks46.
Switches: Connect devices within a network47.
Hubs, Bridges, Repeaters, Gateways: Help extend, filter, or boost network signals47.

Software Components

Network Operating System: Installed on servers, manages network resources and user
access16.
Protocol Suite: Set of rules (like TCP/IP or OSI) that govern how devices communicate
over the network16.

Computer Network Types

Computer networks are groups of connected computers that share resources and
information.
Uses:

Communication: Email, video calls, messaging

Sharing: Printers, files, software, and data

Main Types of Computer Networks

Network Type Full Form Description Example/Use

PAN Personal Area Connects devices Phone, laptop

Network within a short range
(about 10 meters)

LAN Local Area Network Links devices in a Office network

small area (home,
office, school)

WAN Wide Area Network Connects devices Internet

over large
distances (cities,
countries)

WLAN Wireless Local Area Wireless version of Home Wi-Fi

Network LAN

CAN Campus Area Connects LANs University network

Network within a campus or
university

MAN Metropolitan Area Covers a city or City-wide network

Network large area

SAN Storage Area Connects storage Data center storage

Network devices for fast
access

SAN System-Area High-speed Server clusters

Network network for servers
and processors

POLAN Passive Optical Uses optical fiber Office with fiber

Local Area Network for efficient LANs

EPN Enterprise Private Private network Corporate network

Network built by a company

VPN Virtual Private Secure, private Remote work

Network connection over access
 public networks

HAN Home Area Connects devices Smart home

Network within a home devices

1. PAN (Personal Area Network)

A Personal Area Network connects devices within about 10 meters (sometimes up to 30

meters) for personal use, such as linking desktops, laptops, smartphones, game consoles, and
gadgets.

Wireless PAN: Uses Bluetooth or Wi-Fi for short-range, wireless connections.

Wired PAN: Uses cables like USB for direct connections.

2. LAN (Local Area Network)

A Local Area Network connects computers and devices within a small area (e.g., home,
school, office) using Ethernet or Wi-Fi. It allows sharing files, printers, and resources among
connected devices.

3. WAN (Wide Area Network)

A Wide Area Network spans large distances, connecting computers and LANs across cities,
countries, or even globally. The Internet is the most common example of a WAN.

4. MAN (Metropolitan Area Network)

A Metropolitan Area Network covers a city or large area, connecting multiple buildings or
locations. It is larger than a LAN but smaller than a WAN. Examples include city-wide networks
and cable TV networks124.

Network Topology: Basics

Definition:
Network topology is how devices (nodes) are arranged and connected in a
network156.
Types:

Physical Topology: Physical layout of devices and cables.

Logical Topology: How data actually flows between devices17.

Bus Topology
 Structure:
All devices are connected to a single main cable (backbone).
Each node connects to the backbone directly or via a drop cable.
How it works:
When a node sends a message, all nodes receive it, but only the intended recipient
processes it.
Uses CSMA (Carrier Sense Multiple Access) to manage data flow:
CSMA/CD: Detects and recovers from data collisions.
CSMA/CA: Avoids collisions by checking if the media is busy before sending.
Advantages:
Low cost and simple setup.
Easy to add devices.
Failure of one node does not affect others.
Disadvantages:
If the main cable fails, the whole network goes down.
Troubleshooting is difficult.
Adding devices slows down the network.
Signal interference and attenuation can occur.

Ring Topology

Structure:
Each device is connected to two others, forming a closed loop.
Data travels in one direction (unidirectional) around the ring.
How it works:
Uses token passing: A token (frame) circulates; only the node with the token can send
data.
Data moves from node to node until it reaches the intended recipient.
Advantages:
Easy to manage and monitor.
Inexpensive cabling.
Reliable—not dependent on a single host.
Disadvantages:
If one node fails, the entire network can be affected.
Troubleshooting is difficult.
Adding devices increases delay and slows the network.

Star Topology
 Structure:
All devices (nodes) are connected to a central hub or switch.
The central device is often called a server; connected devices are clients.
Uses coaxial or RJ-45 cables and hubs/switches for connections126.
Advantages:
Easy troubleshooting: Faults are isolated to individual connections.
Scalable: New devices can be added easily by connecting to the central hub.
Limited failure: Failure of one cable or device does not affect the whole network.
Network control: Centralized management and easy to implement changes.
Familiar and cost-effective: Uses common, inexpensive equipment.
High data speeds: Supports up to 100 Mbps (e.g., Ethernet 100BaseT).
Low data collision: Each device has a dedicated link to the hub.
Disadvantages:
Central point of failure: If the central hub fails, the whole network goes down.
Cable management: Requires a lot of cabling, which can be complex and costly.
Depends on hub capacity: Network performance is limited by the central hub’s
capabilities156.

Tree Topology

Structure:
Combines features of bus and star topologies.
Devices are connected in a hierarchical (parent-child) structure.
Top node is the root; all other nodes are descendants.
Only one path exists between any two nodes.
Advantages:
Broadband transmission: Supports long-distance signal transmission without loss.
Easily expandable: New devices can be added to the network.
Easily manageable: Network is divided into segments (star networks) for easier
management.
Easy error detection: Errors are simple to detect and correct.
Limited failure: Failure of one node does not affect the whole network.
Point-to-point wiring: Each segment has dedicated wiring.
 Disadvantages:
Difficult troubleshooting: Faults in nodes can be hard to locate.
High cost: Equipment for broadband transmission is expensive.
Main bus failure: Failure of the main bus cable damages the whole network.
Difficult reconfiguration: Adding new devices can be complicated6.

Mesh Topology

Structure:
All devices are interconnected with multiple redundant connections.
No central device (switch, hub, or server) required.
Multiple paths exist between any two devices.
Categories:

Full Mesh: Every device connects to every other device.

Partial Mesh: Only some devices connect to all others, usually those that
communicate frequently.
Cable Formula:

Number of cables = n(n−1)/2 (where n = number of nodes).

Use Cases:
Mainly used in WANs and wireless networks.
Internet is an example of mesh topology.
Advantages:
Highly reliable: Failure of one link does not disrupt the whole network.
Fast communication: Multiple paths allow quick data transfer.
Easy reconfiguration: Adding devices is simple and does not disrupt others.
Disadvantages:
High cost: Requires many devices and cables.
Difficult management: Large networks are hard to monitor and maintain.
Reduced efficiency: Redundant connections can lower overall network efficiency.

Hybrid Topology
 Structure:
Combines two or more different topologies (e.g., star and bus, ring and mesh).
Connecting similar topologies does not make a hybrid topology.
Advantages:
Reliable: Fault in one part does not affect the whole network.
Scalable: Easy to expand by adding new devices.
Flexible: Can be designed to meet specific needs.
Effective: Maximizes strengths and minimizes weaknesses of individual topologies.
Disadvantages:
Complex design: Architecture is difficult to plan and implement.
High cost: Hubs and infrastructure are expensive.
Costly infrastructure: Requires a lot of cabling and devices.

Protocol:

A set of agreed rules and procedures for successful communication between devices
in a network124.
Defines what, how, and when data is sent and received.
Key Elements of Protocols:
1. Syntax:
Defines the structure or format of the data being transmitted (e.g., header, data,
footer)3.
2. Semantics:
Explains the meaning or interpretation of each part of the data (control, error
handling, etc.)3.
3. Timing:
Determines the sequence and speed at which data is sent and received3.

Standards:

Official or widely accepted rules for data communication.

Ensure devices from different manufacturers can work together.
Created by organizations like IEEE, ISO, and ANSI.
Types of Standards:
1. De Facto Standard:
Means “by fact” or “by convention.”
 Not officially approved, but widely used because of popularity or manufacturer
adoption.
Example: Apple and Google’s own rules for their products, or widely used formats.
2. De Jure Standard:
Means “by law” or “by regulations.”
Officially approved by recognized organizations (e.g., IEEE, ISO, ANSI).
Example: Standard protocols like TCP, IP, UDP, and SMTP.

Why Use the OSI Model?

Simplifies Understanding:
Helps you understand how communication happens over a network.
Easier Troubleshooting:
Separates network functions into layers, making it easier to find and fix problems.
Supports New Technologies:
Makes it easier to understand and integrate new technologies as they emerge.
Enables Comparison:
Allows you to compare how different network layers and devices work together.

History of the OSI Model

Late 1970s:
ISO started developing general networking standards.
1973:
UK’s Experimental Packet Switched System highlighted the need for higher-level
protocol standards.
1983:
OSI model first created as a detailed specification for network interfaces.
1984:
OSI architecture officially adopted as an international standard by ISO.

7 Layers of the OSI Model

Physical Layer (Layer 1)
Purpose:
Establishes the physical connection between devices.
Transmits raw bits (0s and 1s) over a physical medium156.
Functions:
Bit Synchronization: Uses a clock to synchronize sender and receiver at the bit level.
Bit Rate Control: Sets the number of bits sent per second.
Physical Topology: Defines how devices are arranged (bus, star, mesh).
Transmission Mode: Determines data flow direction (simplex, half-duplex, full-
duplex).
Devices:
Hub, repeater, modem, cables.
Note:
Network, Data Link, and Physical Layers are called Lower or Hardware Layers.

Data Link Layer (Layer 2)

Purpose:
Ensures error-free, node-to-node delivery of data over the physical layer.
Sublayers:
Logical Link Control (LLC): Manages error checking and flow control.
 Media Access Control (MAC): Controls device access to the network.
Functions:
Framing: Organizes bits into meaningful frames.
Physical Addressing: Adds MAC addresses to frames.
Error Control: Detects and retransmits lost or damaged frames.
Flow Control: Manages data rate to prevent overload.
Access Control: Determines which device can use the channel at a time.
Addressing:
Uses ARP to find the MAC address for a given IP address.
Devices:

Switch, bridge.
Note:
Data Link Layer is handled by the NIC and device drivers.
Packets at this layer are called frames.

Network Layer (Layer 3)

Purpose:
Transmits data between hosts in different networks.
Handles routing and logical addressing.
Functions:
Routing: Selects the best path for data from source to destination.
Logical Addressing: Uses IP addresses to uniquely identify devices.
Addressing:
Adds sender and receiver IP addresses to the packet header.
Note:
Segments at this layer are called packets.

Transport Layer (Layer 4)

Purpose:
Ensures end-to-end delivery of complete messages between applications146.
Key Functions:
Segmentation & Reassembly:
Breaks messages into segments at sender, reassembles at receiver156.
Service Point Addressing:
Uses port numbers to deliver data to the correct application process126.
Flow & Error Control:
 Manages data rate and retransmits lost/corrupted data146.
Connection Management:
Connection-Oriented Service:
Establishes, maintains, and terminates a reliable connection (e.g., TCP).
Uses acknowledgments for reliable, secure data transfer.
Connectionless Service:
Faster, but less reliable (e.g., UDP).
No acknowledgment; used when speed is more important than reliability.
Data at this Layer:
Called segments.
Operations:
Operated by the operating system; communicates with application layer via system
calls.
Importance:
Known as the “heart of the OSI model” for its critical role in reliable communication.

Session Layer (Layer 5)

Purpose:
Manages sessions between processes: establishes, maintains, and terminates
connections35.
Key Functions:
Session Establishment, Maintenance, and Termination:
Controls connection setup, use, and closure.
Synchronization:
Adds checkpoints in data to allow error recovery and prevent data loss.
Dialog Control:
Manages communication mode (half-duplex or full-duplex).
Upper Layers:
Session, Presentation, and Application layers are integrated as the “Application
Layer” in TCP/IP.
Implemented by network applications; also called Upper or Software Layers.

Presentation Layer (Layer 6)

Purpose:
Also called the Translation layer, it prepares data for transmission by converting it
into a suitable format.
Key Functions:
Translation:
Converts data between different formats (e.g., ASCII to EBCDIC).
Encryption/Decryption:
 Encrypts data for security (ciphertext) and decrypts it at the receiver (plain text).
Uses a key for both encryption and decryption.
Compression:
Reduces the number of bits to be transmitted, improving network efficiency.

Application Layer (Layer 7)

Purpose:
Provides a user interface for network applications and enables user access to
network services.
Key Functions:
Network Virtual Terminal:
Allows users to log in to a remote host.
File Transfer, Access, and Management (FTAM):
Supports file transfer and management over the network.
Mail Services:
Enables email communication.
Directory Services:
Provides distributed database sources and access for global information.
Examples:
Web browsers, Skype, Messenger, email clients.
Also Called:
Desktop Layer.

OSI and TCP/IP Models:

OSI Model
Purpose:
Acts as a reference/logical model for network communication.
Divides communication into 7 distinct layers for easier understanding and
troubleshooting.
Not Used on the Internet:
Not implemented on the Internet due to its late invention.
Used mainly for educational and standardization purposes.
OSI Model in a nutshell
 TCP/IP Model
Origin:
Developed by the U.S. Department of Defense (DoD) in the 1960s.
Structure:
Consists of 4 layers:
1. Process/Application Layer (combines OSI Application, Presentation, and
Session layers)
2. Host-to-Host/Transport Layer
3. Internet Layer
4. Network Access/Link Layer
Focus:
Based on standard protocols, widely implemented on the Internet.

Key Differences Between TCP/IP and OSI Models

The diagrammatic comparison of the TCP/IP and OSI model is as follows :
 Feature TCP/IP Model OSI Model

Full Name Transmission Control Open Systems

Protocol/Internet Protocol Interconnection

Number of Layers 4 7

Reliability More reliable (real-world Less reliable (theoretical

implementation) model)

Layer Boundaries Not very strict Strict boundaries between

layers

Design Approach Horizontal (integrated Vertical (layered approach)

approach)

Session/Presentation Combined in Application Separate Session and

Layers Layer Presentation Layers

Development Order Developed protocols first, Developed model first,

then model then protocols

Transport Layer Assurance Does not guarantee packet Guarantees packet delivery
delivery

Network Layer Services Only connectionless Both connectionless and

services connection-oriented

Protocol Replaceability Protocols not easily Protocols easily replaced

replaced as technology changes

TCP/IP Model Layers

1. Network Access Layer

Corresponds to:
OSI Data Link and Physical layers.
Functions:
Handles hardware addressing and physical transmission of data.
Encapsulates IP datagrams into frames for transmission.
Maps IP addresses to physical (MAC) addresses.
Protocols:
 Ethernet, Wi-Fi, PPP, Frame Relay.
Note:
ARP is often considered part of the Internet Layer, but in practice, it operates at the
boundary of the Network Access and Internet layers.

2. Internet Layer
Corresponds to:
OSI Network layer.
Functions:
Responsible for logical transmission of data across the network.
Routes packets from source to destination using IP addresses.
Protocols:
IP (Internet Protocol):
Delivers packets using IP addresses.
Versions: IPv4 (most common), IPv6 (newer, larger address space).
ICMP (Internet Control Message Protocol):
Reports network problems and errors.
ARP (Address Resolution Protocol):
Finds hardware (MAC) address from a known IP address.
Types: Reverse ARP, Proxy ARP, Gratuitous ARP, Inverse ARP.

3. Host-to-Host/Transport Layer
Corresponds to:
OSI Transport layer.
Functions:
Provides end-to-end communication and error-free data delivery.
Shields upper-layer applications from network complexities.
Protocols:
TCP (Transmission Control Protocol):
Reliable, connection-oriented, with sequencing, acknowledgment, and flow
control.
Higher overhead due to reliability features.
UDP (User Datagram Protocol):
Connectionless, unreliable, but fast and cost-effective.
No sequencing or acknowledgment.

4. Application Layer
Corresponds to:
OSI Application, Presentation, and Session layers.
Functions:
Handles user interface and node-to-node communication.
Manages data exchange between applications.
Protocols:
HTTP/HTTPS:
 Manages web browser-server communication (HTTPS is secure).
FTP, TFTP:
File transfer protocols.
Telnet, SSH:
Remote terminal access (SSH is secure).
SMTP, SNMP:
Email and network management.
NTP:
Synchronizes clocks for accurate timekeeping (important for transactions).
DNS, DHCP, NFS, X Window, LPD:
Domain name resolution, dynamic IP assignment, network file sharing, etc.

Digital Transmission in Computer Networks

Data Representation:
Analog: Continuous signal.
Digital: Discrete binary data (0s and 1s).
Need: Computers require data in digital form for processing and transmission.
Digital-to-Digital Conversion:
Purpose: Converts digital data into digital signals for transmission.
Types:
Line Coding: Essential; converts binary data into digital signals.
Block Coding: Optional; adds redundancy for error detection/correction.

Line Coding

Definition:
Converts digital data into digital signals using voltage levels.
Types of Line Coding Schemes:

1. Uni-polar Encoding:
Single voltage level: High voltage for binary 1, no voltage for binary 0.
 Also called: Unipolar-Non-Return-to-Zero (Unipolar NRZ).

1. Polar Encoding:
Multiple voltage levels: Different voltages for binary 1 and 0.
Subtypes:
Polar NRZ: Two voltage levels (positive for 1, negative for 0).

NRZ-L: Voltage changes for different bits.

NRZ-I: Voltage changes only for binary 1.
RZ (Return to Zero):

Three voltage levels (positive, negative, zero).

Voltage returns to zero between bits for synchronization.
Manchester Encoding:
Combines RZ and NRZ-L.
Transition in the middle of the bit for synchronization.
Differential Manchester Encoding:
Combines RZ and NRZ-I.
Transition in the middle of the bit, phase change for binary 1.
1. Bipolar Encoding:
 Three voltage levels: Positive, negative, and zero.
Zero voltage for binary 0.
Alternating positive/negative for binary 1.

Block Coding
Purpose:
Adds redundant bits to ensure data accuracy (error detection/correction).
Example:
Even-parity: Adds a parity bit to make the number of 1s even.
Notation:
mB/nB: Substitutes an m-bit block with an n-bit block (n > m).
Steps:
1. Division: Divides data into blocks.
2. Substitution: Replaces each m-bit block with an n-bit block.
3. Combination: Combines the substituted blocks for transmission.
After block coding:
Data is line coded for transmission.

Analog-to-Digital Conversion: Pulse Code Modulation (PCM)

Purpose:
Converts continuous analog signals (like voice or video) into discrete digital data for
transmission over digital networks.
Process:
Uses Pulse Code Modulation (PCM), the most common method for analog-to-digital
conversion.
Steps in PCM:
1. Sampling:

The analog signal is sampled at regular intervals (every T seconds).

Sampling rate must be at least twice the highest frequency in the signal (Nyquist
Theorem).

Quantization:
 Each sample’s amplitude is approximated to the nearest discrete value.
This step converts the continuous range of amplitudes into a limited set of levels.

3 Encoding:

Each quantized value is converted into a binary code.

The digital signal is now ready for transmission or storage.
Result:
The original analog signal is transformed into a series of binary numbers (0s and 1s),
suitable for digital processing and transmission236.

Multiplexing

Definition:
Sharing a single medium or bandwidth by combining multiple signals from different
sources and transmitting them over one communication line.
Purpose:
Efficiently utilizes available bandwidth or resources.

Types of Multiplexing

1. Frequency Division Multiplexing (FDM)

Uses:
Analog signals.
Method:
 Divides the carrier bandwidth into logical channels.
Each user gets a unique frequency channel.
Channels are separated by guard bands (unused frequencies).
Advantage:
Users have exclusive access to their assigned frequency.

2. Time Division Multiplexing (TDM)

Uses:
Primarily digital signals (can also be used for analog).
Method:
Divides the shared channel into time slots.
Each user transmits data only during their assigned time slot.
Digital signals are divided into frames, each frame fits into a time slot.
Synchronization:
Multiplexer and De-multiplexer are synchronized; both switch channels at the same
time.
Advantage:
Efficient use of bandwidth for digital communication.

3. Wavelength Division Multiplexing (WDM)

Uses:
Optical signals (fiber optics).
Method:
Combines multiple optical signals (each with a different wavelength/color) into one
fiber.
Conceptually similar to FDM, but uses light instead of electrical signals.
Extension:
TDM can be applied on each wavelength to increase data capacity.
4. Code Division Multiplexing (CDM)
Uses:
Digital signals.
Method:
Transmits multiple signals over a single frequency by assigning each user a unique
code (chip).
Signals are spread using orthogonal codes and transmitted simultaneously.
Receiver uses the known code to extract the intended signal.
Advantage:
Users can use the full bandwidth all the time.

Network Switching
Definition:
Switching is the process of forwarding packets from an input (ingress) port to an
output (egress) port leading to the destination.
Categories:
Connectionless Switching:
Data is forwarded using routing tables.
No prior circuit setup is needed.
Acknowledgements are optional.
Connection-Oriented Switching:
A circuit is established before data transfer.
Data is sent over this dedicated path.
Circuit can be kept open or closed after use.

Types of Switching Techniques

1. Circuit Switching
 Process:
A dedicated communication path is set up between sender and receiver.
Data travels only over this pre-established route.
Phases:
1. Circuit Establishment:
A connection request is sent and acknowledged.
2. Data Transfer:
Data is transmitted along the established path.
3. Circuit Disconnect:
The path is terminated after data transfer.
Examples:
Traditional telephone networks (PSTN).
Features:
Circuits can be permanent or temporary.
Designed for voice applications.

2. Message Switching

Process:
The entire message is treated as a single unit and sent in one go.
Uses a store-and-forward mechanism.
Operation:
Each switch buffers the whole message until resources are available to forward it.
If resources are unavailable, the message is stored and waits.
Drawbacks:
Each switch needs enough storage for the entire message.
Slow due to storage and waiting for resources.
Not suitable for real-time or streaming applications.

3. Packet Switching
 Process:
The message is divided into smaller chunks called packets.
Each packet has a header with switching information and is sent independently.
Advantages:
Intermediate devices need less storage.
Enhances line efficiency (multiple applications can share the line).
Packets can be prioritized for quality of service.
Usage:
The internet uses packet switching.
Suitable for real-time and streaming applications.

Approaches to Packet Switching

1. Datagram Packet Switching

Description:
Each packet (called a datagram) is treated as an independent entity.
Packets contain full destination address information.
Operation:
No fixed path: Each packet may take a different route.
Routing decisions: Made dynamically by intermediate nodes.
Reassembly: Packets are reassembled in correct order at the destination.
Nature:
Connectionless: No prior setup or connection required.
Also called: Connectionless switching136.

2. Virtual Circuit Switching

Description:
A pre-planned (logical) route is established before data is sent.
Also called connection-oriented switching256.
Operation:
Setup: Call request and call accept packets establish the connection.
Fixed path: All packets follow the same route for the duration of the connection.
Data transfer: Data is sent over the established path.
Acknowledgment: Receiver sends an acknowledgment after data is received.
Termination: Clear signal is sent to end the connection.
Nature:
Connection-oriented: Requires setup and termination phases.
Logical path: Appears as a dedicated circuit to the user.

Diagrammatic Example (Virtual Circuit Switching):

Sender (A) and Receiver (B) are connected via nodes 1 and 2.
Call request and call accept establish the connection.
Data is transferred over the fixed path.
Acknowledgment is sent after data is received.
Clear signal terminates the connection.