Data Communication · Lecture 1

Introduction to Data Communications

How two machines agree to talk: components, data flow, networks, topologies, and layered models

Md Mahbub E Noor · Assistant Professor, Department of CSE, University of Barishal

Course: Data Communication · 2nd Year, 2nd Semester

Where today fits, and what you will learn

Define data communication and its four effectiveness characteristics
Identify the five components of every communication system
Distinguish simplex, half-duplex and full-duplex data flow
Compare network topologies and calculate their link requirements
Explain protocols, standards, and the OSI and TCP/IP layer models

The 14-lecture roadmap

01 Introduction

02 Data & Signals I

03 Data & Signals II

04 Transmission Media

05 Analog → Digital

06 Digital → Digital

07 Digital → Analog

08 Analog → Analog

09 Multiplexing

10 Switching

11 Framing & Errors

12 Link Control

13 Multiple Access

14 Wireless Basics

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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What is data communication?

Data communication: the exchange of data between two devices over a transmission medium (wire, fibre or air)
Tele means far: telecommunication is communication at a distance
Data: information represented in a form agreed on by sender and receiver
A system is effective only if it meets four characteristics (right)

Delivery

Data reaches the correct destination, and only that destination

Accuracy

Data arrives unaltered; corrupted data is unusable

Timeliness

Data arrives in time; late audio or video is useless

Jitter

Delay between packets stays even, not uneven

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Five components of a communication system

Message: the information sent (text, numbers, images, audio, video)
Sender and receiver: the devices at each end
Transmission medium: the physical path, such as twisted pair, fibre or radio waves
Protocol: the rules both sides agree on; without it, devices are connected but cannot communicate

⑤ Protocol · Rule 1 · Rule 2 · … · Rule n, agreed by both sides

② Sender

computer, phone, camera

④ Medium

cable · fibre · air

③ Receiver

server, phone, printer

① Message: the text “Hi”

01001000 01101001

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Representing data as bits

All data is ultimately a pattern of bits (0s and 1s)
Text: each character has a code; ASCII for English, Unicode for every script, including Bengali
Numbers: converted directly to binary
Images: a grid of pixels; each pixel stored as bits (1 bit black-and-white, 24 bits full colour)
Audio and video: continuous signals that must be sampled and digitised (Lecture 5)

Everything becomes bits

Data	Code or rule	Bits
Letter ‘A’	ASCII code 65	0100 0001
Bengali ‘ক’	Unicode U+0995, UTF-8	3 bytes: E0 A6 95
Number 13	Binary integer	0000 1101
One orange pixel	RGB (255, 128, 0)	24 bits
1 s of phone voice	8,000 samples × 8 bits	64,000 bits

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Checkpoint 1 · 3 minutes

Check your understanding

Q1

During a video call, packets arrive 30 ms, then 120 ms, then 40 ms apart. Which characteristic is violated?

Q2

Two laptops are joined by a working cable, yet nothing they exchange is understood. Which component is missing?

Q3

How many bits does the word “Hello” need in 8-bit ASCII?

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Data flow: simplex, half-duplex, full-duplex

Simplex: one direction only; one device always sends, the other only receives
Half-duplex: both directions, but one at a time; the sender gets the full channel
Full-duplex: both directions simultaneously; the channel capacity is shared between the two directions
Choice depends on the application: a sensor reading needs simplex, a conversation needs duplex

Simplex: keyboard → computer

Device A

Device B

one way, always

Half-duplex: walkie-talkie

Device A

Device B

one direction at a time

Full-duplex: phone call

Device A

Device B

both directions at once

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Networks and the three criteria they must meet

Network: a set of devices (nodes) connected by communication links
Performance: measured mainly by throughput and delay (latency)
Reliability: how often it fails, how long recovery takes, how robust it is in a disaster
Security: protecting data from unauthorised access, damage and loss

The highway analogy

Term	Meaning	On a highway
Bandwidth	Maximum capacity of a link	Number of lanes
Throughput	Data actually delivered per second	Cars that actually pass per minute
Latency	Time for data to travel end to end	Travel time from city A to city B

More lanes do not make the trip shorter: bandwidth and latency are different quantities.

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Type of connection: point-to-point and multipoint

Link: the communication pathway that carries data between devices
Point-to-point: a dedicated link between exactly two devices; all its capacity serves that pair
Multipoint (multidrop): more than two devices share one link
Sharing happens in space (at the same time) or in time (taking turns)

Point-to-point: a dedicated link

Device A

Device B

Multipoint: one shared link

Station 1

Station 2

Station 3

Station 4

All stations share the capacity, at the same time or by taking turns

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Physical topology: four basic layouts

Topology: the geometric arrangement of all links and nodes
Mesh: every device has a dedicated link to every other device
Star: every device links only to a central hub or switch
Bus: all devices tap into one long backbone cable
Ring: each device links to its two neighbours; signals travel around the loop

Mesh · n(n−1)/2 links

Star · n links to a hub

Bus · one shared backbone

Ring · n links in a loop

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

10 / 26

Comparing the four topologies

Topology	Cabling for n devices	Strength	Weakness	Seen today in
Mesh	n(n−1)/2 links; n−1 ports per device	Robust, private, no shared traffic	Cabling and ports grow very fast	Internet backbone (partial mesh)
Star	n links; 1 port per device, n on the hub	Cheap, easy to install; one bad link affects one device	Hub is a single point of failure	Home and office Ethernet, Wi-Fi
Bus	1 backbone + n drop lines	Least cabling	One break stops everyone; faults hard to find	Early coaxial Ethernet (10BASE5)
Ring	n links; 2 ports per device	Easy to install and isolate faults	One break can stop a one-way ring	Token Ring, metro fibre rings

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Try it: how many cables for n devices?

Interactive

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Worked example: cabling an office of 10 computers

Problem: connect n = 10 computers in a full mesh. How many links? How many ports per computer?
Each computer links to the other n − 1 = 9, so it needs 9 ports
Each duplex link joins two devices, so count it once: n(n − 1)/2 = 10 × 9 / 2 = 45 links
Same office as a star: 10 links, one port per computer, plus a 10-port switch

Full mesh

45

links

9 ports on every computer

Star

10

links

1 port per computer plus one 10-port switch

Double the office to 20 devices: the mesh needs 190 links, the star only 20.

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Checkpoint 2 · 3 minutes

Check your understanding

Q1

Six devices form a full mesh. How many links are needed, and how many ports does each device need?

Q2

In which topology does a single cut in the main cable stop every device from communicating?

Q3

Name the data-flow mode of a TV broadcast and of a walkie-talkie conversation.

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Lecture 1: Introduction to Data Communications

University of Barishal

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Network types by scale: LAN, MAN, WAN

LAN: privately owned; links devices in one office, building or campus
MAN: covers a town or city
WAN: spans a country, continent or the world; usually run by a carrier
WANs are point-to-point (one link joining two devices) or switched (many links joined by switches)
Internetwork (an internet): two or more networks connected together

Growing in scale

LAN

Office, building or campus · up to a few km

MAN

Town or city · e.g., a city-wide cable or fibre network

WAN

Country, continent or world · run by carriers

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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The Internet: a network of networks

Backbones: very high-capacity networks of large carriers, joined at peering points
Provider networks: smaller ISPs that buy capacity from backbones and serve customers
Customer networks: homes, campuses and companies at the edge
Access: dial-up, DSL, cable, wireless (Wi-Fi, 4G, 5G) or a direct leased line

Backbone ISP

Provider ISP

Home

Campus

Office

Home

Mobile

peering

Customer networks at the edge; providers in the middle; backbones at the core

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Protocols: the rules of communication

A protocol defines what is communicated, how it is communicated, and when
Connected devices without a shared protocol are like two people with no common language
Every protocol has three key elements: syntax, semantics and timing
Protocols you already use: HTTP, TCP, IP, Ethernet, Wi-Fi (IEEE 802.11)

Syntax

≈ grammar

The structure or format of the data

Ethernet: first 6 bytes = destination address, next 6 = source

Semantics

≈ vocabulary

The meaning of each section of bits

Does this field mean “data”, “acknowledge” or “resend”?

Timing

≈ turn-taking

When data is sent and how fast

A 100 Mbps sender can overwhelm a 1 Mbps receiver

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

17 / 26

Standards: why any device works with any other

Standards guarantee interoperability across manufacturers and countries
De facto: adopted through widespread use, without formal approval
De jure: formally approved by a recognised standards body
Internet standards start as Internet Drafts and are published as RFCs (Requests for Comments)

Who writes the rules

Organisation	Known for
ISO	The OSI reference model
ITU-T	Telephony and telecom standards
IEEE	802.3 Ethernet, 802.11 Wi-Fi
ANSI	US national standards; ASCII
EIA / TIA	Cabling and physical interfaces (e.g., Cat 6, RS-232)
IETF	Internet protocols, published as RFCs

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Why layers? The postal letter analogy

Complex communication is split into layers; each does one job and serves the layer above
Each layer on the sender has a matching peer layer on the receiver (dashed lines)
Layers are independent: change the carrier from truck to plane and the letter is unchanged
Computer networks are organised the same way, as the next slide shows

Sender

Receiver

Higher: write the letter, drop it in a mailbox

Higher: pick up the letter and read it

Middle: post office collects and sorts it

Middle: post office sorts and delivers it

Lower: carrier moves it by truck or plane

Lower: carrier brings it to the local office

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

19 / 26

OSI and TCP/IP: two layered models

Application: services for the user, such as web, email and file transfer
Transport: process-to-process delivery (TCP, UDP)
Network: host-to-host delivery and routing across networks (IP)
Data link: hop-to-hop delivery of frames over one link; Lectures 11 to 13
Physical: moving individual bits as signals over a medium; most of this course

OSI (7 layers)

7 · Application

6 · Presentation

5 · Session

4 · Transport

3 · Network

2 · Data link

1 · Physical

TCP/IP (5 layers)

Application

Transport

Network

Data link

Physical

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Encapsulation: each layer adds its own header

Going down, each layer wraps the data from above with its own header (H)
The data link layer also adds a trailer (T), used for error detection (Lecture 11)
The physical layer sends the whole frame as a stream of bits
Going up, each layer removes its header: decapsulation
Unit names: message → segment → datagram → frame → bits

Application
message

Data

Transport
segment

Data

Network
datagram

Data

Data link
frame

Data

Physical
bits

0110 1011 0010 …

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Try it: wrap a message layer by layer

Interactive

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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See it in action: a message's journey

Follow one message, “Hi”, from a laptop to a phone
Spot the five components and the moment text becomes bits
Watch headers being added on the way down and removed on the way up
When a “Pause & predict” card appears, answer before the video moves on

Click the video to play or pause · Press → for full screen

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Checkpoint 3 · 3 minutes

Check your understanding

Q1

Which layer is responsible for hop-to-hop delivery of frames across one link?

Q2

A university links every building on its own campus. Is this a LAN, MAN or WAN?

Q3

“The first 6 bytes are the destination address.” Is this syntax, semantics or timing?

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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Key takeaways

Data communication is judged by delivery, accuracy, timeliness and jitter
Every system has five components: message, sender, receiver, medium and protocol
Data flows as simplex, half-duplex or full-duplex
Topology sets cost and robustness: a full mesh needs n(n−1)/2 links, a star needs n
Layered models (OSI, TCP/IP) split the work; this course lives in the bottom two layers

Next lecture

Lecture 2 · Data and Signals I

Think about before next class: when your phone sends a photo over Wi-Fi, what physically travels through the air? It is not 0s and 1s.

Reading: Forouzan, Data Communications and Networking, Chapters 1 and 2

Md Mahbub E Noor

Lecture 1: Introduction to Data Communications

University of Barishal

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