Simplex, Half-Duplex & Full-Duplex Communication
Exam-focused notes on transmission modes for SSC, Banking (IBPS/SBI), Railway, UPSC, NIELIT, JAIIB/CAIIB and IT officer papers. Learn definitions, real-life examples, networking use, comparison tables, and practice with interactive MCQs.
What is Duplex / Transmission Mode?
How data is allowed to flow between two devices
In data communication, transmission mode (often called duplex mode) describes the direction of data flow between sender and receiver.
1. Simplex Communication
One-way street of data
In simplex mode, data flows in one direction only: Sender → Receiver. The receiver cannot reply on that channel.
Key features
- Unidirectional communication
- Only one device transmits
- Other device only receives
- Simple and low-cost design
Examples (exam favourites)
- Television broadcast
- Radio broadcast
- Public address (PA) system
- Keyboard → computer (common MCQ key)
- One-way sensor/camera stream (teaching example)
Exam hot points
- Definition: one direction only
- Receiver cannot send back
- TV / radio = classic answers
Not simplex
- Phone call (full-duplex)
- Walkie-talkie (half-duplex)
- Normal two-way chat
2. Half-Duplex Communication
Two-way, but take turns
In half-duplex mode, data can flow in both directions, but only one direction at a time. When A talks, B listens — then they can reverse.
Key features
- Bidirectional, but not simultaneous
- Needs turn-taking / protocol (e.g., push-to-talk)
- At one moment, channel used one way
- Moderate efficiency vs full-duplex
Examples
- Walkie-talkies / push-to-talk radios
- Older shared hub Ethernet (CSMA/CD)
- Some wireless links in half mode
Exam hot points
- Both ways, one at a time
- Walkie-talkie = standard example
- Collisions possible on shared half-duplex media
Analogy
- Narrow bridge: traffic can go either way
- But not both ways at once
3. Full-Duplex Communication
Talk and listen together
In full-duplex mode, data flows in both directions at the same time. Both devices can send and receive simultaneously.
Key features
- Simultaneous two-way communication
- Highest efficiency for interactive use
- Faster effective throughput both ways
- Needs capability for concurrent send/receive
Examples
- Telephone / mobile calls
- Modern switched Ethernet
- Video calls / interactive chat systems
Exam hot points
- Keyword: simultaneously
- Phone call = classic example
- Modern LAN switch links = full-duplex
Analogy
- Two-lane road: traffic both ways at once
- Or a normal phone conversation
Comparison Table (Revise First)
| Feature | Simplex | Half-Duplex | Full-Duplex |
|---|---|---|---|
| Data flow | One direction only | Both directions, one at a time | Both directions simultaneously |
| Communication | One-way | Two-way alternating | Two-way concurrent |
| Can receiver reply? | No | Yes (later / turn-based) | Yes (at the same time) |
| Speed / efficiency | Lowest for interaction | Moderate | Highest |
| Classic examples | TV, radio, PA | Walkie-talkie, hub Ethernet | Phone, modern Ethernet |
| Exam keywords | Unidirectional / one-way | Alternate / one at a time | Simultaneous / concurrent |
Duplex in Computer Networks
How this appears in LAN questions
Hub / shared Ethernet
Classic shared medium behaves as half-duplex. Devices take turns; collisions can occur (CSMA/CD era).
Switch port ↔ PC
Modern point-to-point link is usually full-duplex. Separate send/receive paths; no CSMA/CD collisions on that link.
Duplex mismatch (practical exam note)
If one side is set to full-duplex and the other to half-duplex, the link often shows errors, late collisions, and poor speed. Auto-negotiation usually sets speed + duplex correctly.
One-Glance Quick Revision
Simplex
One-way only · TV / radio
Half-duplex
Two-way turn-based · walkie-talkie
Full-duplex
Two-way same time · phone
Keyword: unidirectional
→ Simplex
Keyword: one at a time
→ Half-duplex
Keyword: simultaneous
→ Full-duplex
Keyboard→PC (exam key)
Often marked Simplex
Modern Ethernet
Full-duplex
Hub Ethernet
Half-duplex shared
Most interactive
Full-duplex
No reply path
Simplex
Needs turn control
Half-duplex
Practice MCQs — Transmission Modes
Tap any option to reveal the correct answer and a short explanation.
Exam Tips
Hunt the keyword first
Unidirectional / only one direction → Simplex. One at a time / alternate → Half. Simultaneous → Full.
Memorise three examples
TV = Simplex. Walkie-talkie = Half. Phone = Full. This trio solves most MCQs.
Don’t confuse with topology
Duplex is about direction of flow, not star/bus/ring layout.
Ethernet clue
Hub/shared → half-duplex idea. Switch link → full-duplex idea.
Keyboard MCQ
Many Indian exam keys mark keyboard→computer as simplex. Follow the standard key unless the paper is highly technical.
Order of capability
Simplex < Half-duplex < Full-duplex for two-way interaction richness.
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Related notes
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