BCS Written ICT cheat sheet

Number systems: binary base 2, octal base 8, hexadecimal base 16; each hex digit equals 4 bits, each octal digit equals 3 bits.

Conversion rule: divide by the target base repeatedly, collect remainders bottom-up for integers; multiply fraction by base repeatedly for fractional parts.

Two's complement: invert all bits of the magnitude and add 1; an n-bit signed range is -2^(n-1) to 2^(n-1)-1.

Memory hierarchy by speed and cost: registers > cache (L1/L2/L3) > RAM > SSD/HDD > optical/tape; capacity grows as speed falls.

CPU cycle: fetch, decode, execute, write-back; clock speed alone does not determine performance, since IPC and cache also matter.

Logical gates: AND, OR, NOT, NAND, NOR, XOR; NAND and NOR are universal gates because either alone can build all others.

Addressing modes: immediate (operand in instruction), direct (address in instruction), indirect (address points to address), indexed (base plus index register).

Storage units: 1 byte = 8 bits; 1 KiB = 1024 bytes; 1 MiB = 1024 KiB; 1 GiB = 1024 MiB; disk vendors often use decimal powers of 1000.

Von Neumann architecture shares one memory for instructions and data (von Neumann bottleneck); Harvard architecture uses separate instruction and data memories.

Process states: new, ready, running, waiting/blocked, terminated; a process moves to ready on I/O completion and to running only when the scheduler dispatches it.

Scheduling metrics: turnaround = completion - arrival; waiting = turnaround - burst; response = first dispatch - arrival.

SJF gives minimum average waiting time but needs burst prediction; Round Robin with quantum q bounds response time and favors interactivity.

Deadlock's four Coffman conditions: mutual exclusion, hold and wait, no preemption, circular wait; breaking any one prevents deadlock.

Banker's algorithm is deadlock avoidance: grant a request only if the resulting state remains safe (some completion order exists).

Paging splits logical memory into fixed pages and physical memory into frames; a page table maps page number to frame number, with TLB caching entries.

Thrashing occurs when multiprogramming exceeds available frames, so page faults dominate; working-set model limits resident pages per process.

File allocation: contiguous (fast, fragments), linked (no external fragmentation, poor random access), indexed (fast random access, index overhead).

Translators: compiler translates the whole source before execution; interpreter executes statement by statement; assembler converts assembly to machine code; linker resolves external symbols; loader places the program in memory.

OSI layers bottom-up: Physical, Data Link, Network, Transport, Session, Presentation, Application; TCP/IP merges the top three into Application.

Encapsulation order: data, segment (TCP) or datagram (UDP), packet (IP), frame (data link), bits (physical).

TCP is connection-oriented and reliable (sequencing, ACK, retransmission, flow and congestion control); UDP is connectionless and best-effort.

IPv4 address = 32 bits in four octets; IPv6 = 128 bits. Subnet hosts = 2^h - 2, where h is the number of host bits.

Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; loopback is 127.0.0.0/8; APIPA 169.254.0.0/16 signals failed DHCP.

CSMA/CD detects collisions on shared Ethernet; CSMA/CA avoids them in wireless LANs using ACK frames and random backoff.

DNS resolves names to IP addresses; DHCP assigns IP configuration automatically; ARP maps IP to MAC within a local network; ICMP carries diagnostics such as ping.

Nyquist theorem: maximum bit rate = 2B log2(L) for a noiseless channel of bandwidth B with L signal levels; Shannon capacity = B log2(1 + S/N).

Transmission media: twisted pair (up to about 100 m for Ethernet), coaxial cable, fiber optic (highest bandwidth, immune to EMI), and wireless radio.

Three-schema architecture: external (views), conceptual (logical design), internal (physical storage); mappings provide data independence.

ACID: Atomicity (all or nothing), Consistency (constraints preserved), Isolation (concurrent transactions do not interfere), Durability (committed changes persist).

Normalization: 1NF removes repeating groups; 2NF removes partial dependencies on part of a composite key; 3NF removes transitive dependencies; BCNF requires every determinant to be a candidate key.

Relational algebra operators: selection (sigma), projection (pi), union, difference, Cartesian product, join; SQL SELECT corresponds to projection and WHERE to selection.

SQL clauses run logically as FROM, WHERE, GROUP BY, HAVING, SELECT, ORDER BY; WHERE filters rows, HAVING filters groups.

Keys: superkey identifies tuples; candidate key is minimal; primary key is chosen and NOT NULL; foreign key references another relation's candidate key.

ER model: entity, attribute, relationship; cardinality 1:1, 1:N, M:N; an M:N relationship becomes a junction table with both foreign keys.

Indexes speed reads but slow writes and consume space; a clustered index fixes physical row order, a secondary index does not.

Concurrency control uses two-phase locking (growing then shrinking phase) or timestamp ordering; a schedule is serializable if equivalent to some serial order.

Complexity order: O(1) < O(log n) < O(n) < O(n log n) < O(n^2) < O(2^n) < O(n!); drop constants and lower terms when stating Big-O.

Binary search requires a sorted array and runs in O(log n); linear search runs in O(n); merge sort is O(n log n) worst case but needs O(n) extra space.

Quicksort averages O(n log n) and is in-place but degrades to O(n^2) on already sorted input with a poor pivot; randomized pivot avoids this.

Stack is LIFO (push/pop), queue is FIFO (enqueue/dequeue); applications: expression evaluation and recursion (stack), scheduling and buffering (queue).

Linked list insertion or deletion at a known node is O(1); array random access is O(1) while linked list access is O(n).

Binary search tree: inorder traversal yields sorted order; search, insert, delete are O(h) where h is height, O(log n) when balanced, O(n) when skewed.

Hash table average lookup is O(1); collisions are handled by chaining or open addressing (linear probing causes clustering).

Graph traversal: BFS uses a queue and finds shortest paths in unweighted graphs; DFS uses a stack or recursion and detects cycles.

Dijkstra finds single-source shortest paths with non-negative weights in O((V+E) log V) using a heap; it fails on negative edges, where Bellman-Ford applies.

SDLC phases: requirement analysis, design, implementation, testing, deployment, maintenance; maintenance typically consumes the largest share of lifecycle cost.

Waterfall is sequential with frozen requirements; iterative/incremental and Agile deliver working increments in short sprints with continuous feedback.

Requirement types: functional (what the system does), non-functional (performance, security, usability), and constraints (platform, regulatory).

DFD symbols: process (circle), data store (open rectangle), external entity (rectangle), data flow (arrow); levelled DFDs must be balanced between parent and child.

ER diagram: rectangle = entity, ellipse = attribute, diamond = relationship, underlined attribute = primary key; cardinality is shown on relationship lines.

UML: use case diagram captures actors and goals; class diagram shows structure; sequence diagram shows time-ordered messages; activity diagram models workflow.

Coupling should be low and cohesion high: content coupling is worst, data coupling best; functional cohesion is strongest, coincidental weakest.

Testing levels: unit, integration, system, acceptance; black-box uses equivalence partitioning and boundary value analysis, white-box uses statement, branch and path coverage.

COCOMO estimates effort as a function of size in KLOC with multiplicative cost drivers; function point analysis measures size from inputs, outputs, queries, files and interfaces.

CIA triad: confidentiality (encryption, access control), integrity (hashing, checksums), availability (redundancy, backups, DDoS protection).

Symmetric ciphers (AES, DES) use one shared key and are fast; asymmetric ciphers (RSA, ECC) use public/private key pairs and enable digital signatures and key exchange.

Hashing (SHA-256, MD5) is one-way and detects tampering; MD5 and SHA-1 are deprecated for security use because collisions have been demonstrated.

Digital signature: sign with sender's private key, verify with sender's public key; it provides authentication, integrity and non-repudiation, not confidentiality.

TLS handshake establishes a session key using asymmetric cryptography, then encrypts traffic symmetrically; HTTPS is HTTP over TLS, usually on port 443.

Malware classes: virus (attaches to files), worm (self-propagating over networks), trojan (disguised payload), ransomware (encrypts data for extortion), spyware (covert collection).

Attack types: phishing (deceptive messages), SQL injection (unsanitized input), XSS (injected script in pages), MITM (interception), DoS/DDoS (resource exhaustion).

Defense in depth: firewalls (packet filtering, stateful, application proxy), IDS (detects) versus IPS (blocks), DMZ for public servers, least privilege, patching, and multi-factor authentication.

Bangladesh legal frame: ICT Act 2006 (amended 2013) and Digital Security Act 2018 address cybercrime; the Cyber Security Act 2023 replaced the DSA, and the Personal Data Protection Act is pending.

HTTP is stateless request-response; methods GET (retrieve, idempotent), POST (submit), PUT (replace), DELETE, PATCH; status codes 200 OK, 301 Moved, 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 500 Server Error.

HTTP versus HTTPS: HTTPS adds TLS on port 443 (HTTP uses 80), giving confidentiality and integrity plus server authentication via certificates.

HTML structures content, CSS styles presentation, JavaScript adds client-side behavior; separation of concerns keeps the three in separate files.

Client-side versus server-side: browser code can be viewed and altered by the user, so validation and authorization must be repeated on the server.

DOM is the browser's tree representation of a page; JavaScript manipulates it, and events such as click and submit trigger handlers.

Cookies are small key-value records stored per domain and sent with matching requests; sessions store state server-side and reference it by a session ID.

REST: resources identified by URIs, stateless requests, standard HTTP verbs, JSON/XML representations; SOAP is an XML envelope protocol with a WSDL contract.

DNS resolution order: browser cache, OS cache and hosts file, recursive resolver, then root, TLD and authoritative servers.

URL anatomy: scheme://host:port/path?query#fragment; e.g. https://example.gov.bd:443/page?id=5#top.

E-government stages (Gartner): presence, interaction, transaction, transformation; each stage raises integration and citizen value.

G2C, G2B, G2G, G2E denote government-to-citizen, business, government and employee service relationships.

Digital Bangladesh (Vision 2021) pillars: human resource development, connecting citizens, digital government, and promotion of the ICT industry.

Smart Bangladesh (Vision 2041) rests on four pillars: Smart Citizen, Smart Government, Smart Society, and Smart Economy.

Key Bangladeshi initiatives: a2i (Access to Information), Union Digital Centers, National Web Portal, e-Payment through mobile financial services, and the Bangladesh National Digital Architecture.

Education technology models: LMS hosts courses and assessments; MOOCs are open online courses at scale; blended learning mixes face-to-face with online delivery; flipped classroom moves lectures to pre-class video.

Digital divide dimensions: access (infrastructure and devices), skills (digital literacy), and usage (meaningful application); access alone does not close the gap.

ICT4D evaluation uses indicators such as teledensity, internet penetration, e-service uptake, and cost and time saved per transaction.

Open data and interoperability: machine-readable formats, open APIs, unique citizen IDs and shared registries reduce duplication across agencies.

Cloud service models: IaaS (virtual machines, storage), PaaS (runtime and middleware), SaaS (ready applications); deployment types are public, private, community and hybrid.

Cloud characteristics (NIST): on-demand self-service, broad network access, resource pooling, rapid elasticity, measured service.

AI and machine learning: supervised learning uses labeled data, unsupervised finds structure in unlabeled data, reinforcement learning learns from reward signals.

Blockchain: distributed ledger of hash-linked blocks, consensus (proof of work or stake), immutability and decentralization; smart contracts execute code on-chain.

IoT architecture layers: perception (sensors/actuators), network (connectivity), processing (edge/cloud analytics), application (user services); constraints are power, bandwidth and security.

Big data characteristics: volume, velocity, variety, veracity, value; MapReduce and Spark process data in parallel across nodes.

Project management: WBS decomposes scope; critical path is the longest dependent path and sets minimum duration; float is slack on non-critical activities.

Earned value: CV = EV - AC; SV = EV - PV; CPI = EV/AC; SPI = EV/PV; CPI below 1 means over budget, SPI below 1 means behind schedule.

Policy instruments: National ICT Policy, Bangladesh Computer Council, BTRC regulation, Digital Security/Data Protection law, and standards such as ISO 27001 for information security management.