NPCBL Junior Assistant Manager (ICT) Exam: 2022 (BUET)প্রশ্ন ও সমাধান

Cloud computing is the on-demand delivery of computing resources — servers, storage, databases, networking, software — over the internet, with pay-as-you-go pricing, instead of owning and maintaining physical infrastructure yourself.

graph LR
  U[User Devices] --> I[Internet]
  I --> C{Cloud Provider}
  C --> IaaS[IaaS: Virtual Machines, Storage]
  C --> PaaS[PaaS: Dev/Runtime Platform]
  C --> SaaS[SaaS: Ready Applications]

Advantages:

• Cost-efficiency: no upfront hardware investment; pay only for the resources actually used.
• Scalability/Elasticity: resources scale up or down instantly with demand.
• Accessibility: available from anywhere with an internet connection.
• Maintenance offloaded: the provider handles hardware upkeep, patching, and redundancy.
• Disaster recovery: data is replicated across the provider's data centers.

Disadvantages:

• Internet dependency: no connectivity means no access to the service.
• Security/privacy concerns: data resides on third-party infrastructure, raising trust and compliance issues.
• Vendor lock-in: migrating between cloud providers can be costly and complex.
• Limited control: the customer cannot control the underlying physical infrastructure.
• Recurring cost: long-term subscription costs can exceed the cost of owned hardware over time.

Exam takeaway: cloud computing trades capital expenditure (CapEx) for operational expenditure (OpEx) and elasticity, at the cost of dependency on the provider's availability, security, and pricing model.

A gate is called universal if any of the three basic gates (NOT, AND, OR) — and hence any Boolean function — can be built using only that one gate type. Both NAND and NOR have this property.

Using NAND only:

• NOT: tie both inputs of a NAND together -> NAND(A,A) = (A.A)' = A'
• AND: NAND then NOT (another NAND) -> NAND(NAND(A,B), NAND(A,B)) = A.B
• OR: NOT each input then NAND -> NAND(A', B') = (A'.B')' = A+B (De Morgan's law)

Using NOR only:

• NOT: tie both inputs together -> NOR(A,A) = (A+A)' = A'
• OR: NOR then NOT -> NOR(NOR(A,B), NOR(A,B)) = A+B
• AND: NOT each input then NOR -> NOR(A', B') = (A'+B')' = A.B (De Morgan's law)

Gate wanted	Built from NAND only	Built from NOR only
NOT	NAND(A,A)	NOR(A,A)
AND	NAND(NAND(A,B),NAND(A,B))	NOR(NOR(A,A),NOR(B,B))
OR	NAND(NAND(A,A),NAND(B,B))	NOR(NOR(A,B),NOR(A,B))

graph LR
  A1[A] --> N1[NAND]
  A1 --> N1
  N1 --> R1["A' (NOT via NAND)"]

Exam takeaway: NAND and NOR are called "universal" because any digital circuit, however complex, can be built using only NAND gates or only NOR gates — which is exactly why most real IC logic families are fabricated almost entirely from a single one of these two gate types.