Data Communication & Networking Basics

Data Communication &
Computer Networking
Md. Mahbub Alam
Scientific Officer
Institute of Electronics
Atomic Energy Research Establishment
Bangladesh Atomic Energy Commission

1.2
1-1 DATA COMMUNICATIONS
The term telecommunication means communication at a
distance. The word data refers to information presented in
whatever form is agreed upon by the parties creating and
using the data. Data communications are the exchange of
data between two devices via some form of transmission
medium such as a wire cable.
 Components of a data communications system
 Data Flow
Topics discussed in this section:

1.3
Figure 1.1 Components of a data communication system

1.4
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

1.5
1-2 NETWORKS
A network is a set of devices (often referred to as nodes)
connected by communication links. A node can be a
computer, printer, or any other device capable of sending
and/or receiving data generated by other nodes on the
network. A link can be a cable, air, optical fiber, or any
medium which can transport a signal carrying
information.
 Network Criteria
 Physical Structures
 Categories of Networks

1.6
Figure 1.4 Categories of topology

1.7
Figure 1.5 A fully connected mesh topology (five devices)

1.8
Figure 1.6 A star topology connecting four stations

1.9
Figure 1.7 A bus topology connecting three stations

1.10
Figure 1.8 A ring topology connecting six stations

1.11
Figure 1.9 A hybrid topology: a star backbone with three bus networks

• Local Area Networks (LANs)
– Short distances
– Designed to provide local interconnectivity
• Wide Area Networks (WANs)
– Long distances
– Provide connectivity over large areas
• Metropolitan Area Networks (MANs)
– Provide connectivity over areas such as a city, a campus
1.12
Categories of Networks

2.14
Figure 2.1 Tasks involved in sending a letter

2.15
2-2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO) is a multinational body dedicated to
worldwide agreement on international standards. An ISO
standard that covers all aspects of network
communications is the Open Systems Interconnection (OSI)
model. It was first introduced in the late 1970s.
Layered Architecture
Peer-to-Peer Processes
Encapsulation

2.16
Figure 2.2 Seven layers of the OSI model

2.17
Figure 2.15 Summary of layers

Guided Transmission Data
• Magnetic Media
• Twisted Pair
• Coaxial Cable
• Fiber Optics

Twisted Pair
(a) Category 3 UTP.
(b) Category 5 UTP.

Coaxial Cable
A coaxial cable.

Fiber Optics
(a) Three examples of a light ray from inside a silica fiber
impinging on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.

Fiber Cables
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.

Wireless Transmission
• The Electromagnetic Spectrum
• Radio Transmission
• Microwave Transmission
• Infrared and Millimeter Waves
• Lightwave Transmission

Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.

Communication Satellites
• Geostationary Satellites
• Medium-Earth Orbit Satellites
• Low-Earth Orbit Satellites
• Satellites versus Fiber

Communication Satellites
Communication satellites and some of their properties, including
altitude above the earth, round-trip delay time and number of
satellites needed for global coverage.

IP Addresses
IP address formats.

DNS – The Domain Name System
• The DNS Name Space
• Resource Records
• Name Servers

The DNS Name Space
A portion of the Internet domain name space.

Network Security
• Computer Security - generic name for the collection of tools
designed to protect data and to thwart hackers.
• Network Security - measures to protect data during their
transmission.

Security Goals
Information needs to be hidden from
• Unauthorized access (Confidentiality)
• Protected from unauthorized change
(integrity)
• Available to an authorized entity when it is
needed (availability)
Security Goals
Confidentiality Availability Integrity

Security Attack
• any action that compromises the security of
information owned by an organization
• information security is about how to prevent
attacks, or failing that, to detect attacks on
information-based systems
• often threat & attack used to mean same
thing
• have a wide range of attacks
• can focus of generic types of attacks
– passive
– active

Cryptography
 Cryptography is the study of
 Secret (crypto-) writing (-graphy)
 Concerned with developing algorithms:
 Conceal the context of some message
from all except the sender and recipient
(privacy or secrecy), and/or
 Verify the correctness of a message to
the recipient (authentication)
 Form the basis of many technological
solutions to computer and
communications security problems

Cryptography
• Basic idea: convert clear text (also
called plain text – the original message)
to ciphertext (the encrypted message)
ciphertext = encrypt(plaintext,
KE)
plaintext = decrypt(ciphertext,
KC)
• Can either make the encryption process
hidden, so that an intruder cannot know
it
• Or, can use a known technique and use
a hidden key

Some Basic Terminology
 plaintext - original message
 ciphertext - coded message
 cipher - algorithm for transforming plaintext to
ciphertext
 key - info used in cipher known only to
sender/receiver
 encipher (encrypt) - converting plaintext to
ciphertext
 decipher (decrypt) - recovering ciphertext from
plaintext
 cryptography - study of encryption
principles/methods

Type of encryption operations used
Substitution Technique
Caesar Cipher
 replaces each letter by 3rd letter on
 can define transformation as:
a b c d e f g h i j k l m n o p q r s t u v w x
y z
D E F G H I J K L M N O P Q R S T U V W X Y Z A
B C
 mathematically give each letter a number
a b c d e f g h i j k l m n o p q r s t u v w x y
z
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
25
 then have Caesar cipher as:
c = E(p) = (p + k) mod (26)
p = D(c) = (c – k) mod (26)

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