Solution

1. Waterfall Model (Classic / Linear Sequential)

Definition: Oldest SDLC, proposed by Winston Royce (1970). Phases in linear cascade where each phase must complete before next begins. Output of one is input to next. No overlap.

Phases:
Requirements → System & Software Design → Implementation (Coding) → Testing (Verification) → Deployment → Maintenance

Requirements
     ↓
  Design
     ↓
Implementation
     ↓
 Verification
     ↓
 Maintenance

Characteristics: Simple, document-driven, easy to manage. Each phase has review & gate.

2. When to Prefer Spiral over Waterfall

Spiral is risk-driven, iterative, incremental (Barry Boehm, 1986). Four quadrants per loop: Objective setting, Risk analysis, Development, Planning.

Prefer Spiral when:

Condition	Why Waterfall fails
High risk / unknown requirements	Waterfall assumes frozen requirements
Large, complex, innovative projects	Spiral prototypes & risk mitigation each loop
Requirements change frequently	Waterfall costly to change later
Need early risk detection (technical/feasibility)	Waterfall discovers risk late in testing
Long-term projects needing incremental releases	Spiral gives successive prototypes
Budget / cost uncertainty	Spiral evaluates cost-risk each cycle

Example Situations:

• New OS / AI product where feasibility unknown
• Defence / aerospace where failure cost high
• Startup product with evolving customer needs

Conclusion: Use Waterfall for small, well-understood, stable-requirement projects (e.g., payroll system). Use Spiral for large, risky, complex projects.