DESCO Assistant Engineer (CSE) Exam: 10.09.2022 (BUET)প্রশ্ন ও সমাধান

Cyberspace is the virtual, interconnected environment created by computer networks, the internet, and digital communication systems, where information is created, stored, exchanged, and processed electronically — it has no physical boundary and spans the globe via interconnected TCP/IP-based networks.

Common threats in cyberspace:

Threat	Description
Malware	Malicious software (virus, worm, trojan, ransomware) that damages or takes control of a system
Phishing	Fraudulent messages tricking users into revealing credentials or installing malware
Denial of Service (DoS/DDoS)	Overwhelming a service with traffic to make it unavailable to legitimate users
Man-in-the-Middle (MITM)	Attacker secretly intercepts/alters communication between two parties
SQL Injection	Malicious SQL inserted via input fields to manipulate/steal database data
Password/Brute-force Attacks	Repeated guessing or cracking of credentials to gain unauthorized access
Identity Theft	Stealing personal data to impersonate a victim for fraud

graph LR
    U[User/Organization] -->|connects via| C[Cyberspace / Internet]
    C --> M[Malware]
    C --> P[Phishing]
    C --> D[DDoS]
    C --> Mi[MITM]
    C --> S[SQL Injection]

Exam takeaway: cyberspace = the global, borderless digital environment built on interconnected networks; its major threats span malware, phishing, DoS/DDoS, MITM, SQL injection, and credential attacks — defended via firewalls, encryption, patching, and user awareness.

The DoD (Department of Defense) model — the conceptual ancestor of today's TCP/IP model — condenses OSI's 7 layers into 4 practical layers.

graph TD
    PA[Process/Application] --- O7[Application]
    PA --- O6[Presentation]
    PA --- O5[Session]
    HH[Host-to-Host] --- O4[Transport]
    IN[Internet] --- O3[Network]
    NA[Network Access] --- O2[Data Link]
    NA --- O1[Physical]

DoD Layer	Equivalent OSI Layer(s)	Key protocols
Process/Application	Application + Presentation + Session	HTTP, FTP, SMTP, DNS
Host-to-Host	Transport	TCP, UDP
Internet	Network	IP, ICMP
Network Access	Data Link + Physical	Ethernet, ARP

Key differences:

• OSI is a theoretical, protocol-independent reference model built for standardization; DoD/TCP-IP is a practical model built around the protocols actually used on the internet.
• OSI keeps Session and Presentation as separate layers; DoD merges them into a single Application layer since real protocols usually handle these functions themselves.
• OSI was designed before its protocols existed (top-down); TCP/IP was built from already-working protocols (bottom-up).

Exam takeaway: DoD's 4 layers (Process/Application, Host-to-Host, Internet, Network Access) map onto OSI's 7 by merging Application+Presentation+Session and merging Data Link+Physical — DoD/TCP-IP is the model actually implemented; OSI is the theoretical teaching reference.