---
title: "Need for bus hierarchy"
subject: "Computer Organization and Architecture"
chapter: 1
key: "semester_5/computer_organization_and_architecture:bus-hierarchy"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/bus-hierarchy
site: BCT Engineering
---

# Need for bus hierarchy

Computer Organization and Architecture · Chapter 1: Introduction

- Asked in: 2071 Chaitra, 2 marks
- Video: [computer architecture multiple buses hierarchy & elements of buses design](https://www.youtube.com/watch?v=HZ7qAIAqCKg) — Med teacher training 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/bus-hierarchy
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-1

## Question

Why is a bus hierarchy required?

## Short answer

A single shared bus becomes a bottleneck as more devices are added, increasing propagation delay and contention. A hierarchy with local, system, and expansion buses joined by bridges lets each bus run at its optimal speed, separates fast CPU-memory traffic from slow I/O, and boosts overall performance.

## Answer

A bus hierarchy is needed because a single bus connecting many devices becomes slow and overloaded. The extra devices increase propagation delay and the contention for the bus, and slow devices hold up fast ones. By using a local bus for the CPU and cache, a system bus for memory, and expansion buses for I/O, joined by bridges, each bus can run at the speed suited to its devices, traffic is separated, and overall performance improves.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is a bus hierarchy needed?
   A. Single buses cannot carry data
   B. A single bus with many devices becomes slow and overloaded
   C. It reduces the number of devices
   D. Bridges are cheaper than buses
   Answer: B. More devices add delay and contention. (Key point: Reason)

2. Which bus connects slow devices such as keyboard?
   A. Local bus
   B. System bus
   C. Cache bus
   D. Expansion bus
   Answer: D. The expansion bus serves slow I/O. (Key point: Bus roles)

3. What is a benefit of using bridges between buses?
   A. All buses run at the same speed
   B. Each bus can run at the speed suited to its devices
   C. Traffic from all devices is mixed
   D. Arbitration becomes unnecessary
   Answer: B. Bridges separate traffic and speeds. (Key point: Benefit)

## Related in Computer Organization and Architecture

- [Bus hierarchy and bus arbitration](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Abus-hierarchy-arbitration) — Computer Organization and Architecture, chapter 1
- [Performance balance and bus design elements](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aperf-balance-bus-design) — Computer Organization and Architecture, chapter 1, asked 2×

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