Complete RGPV exam-oriented notes on fundamental MAC protocols, WSN MAC design, contention-based, contention-free and hybrid protocols, routing fundamentals, challenges, taxonomy and major data-dissemination approaches.
A Medium Access Control protocol determines how multiple sensor nodes share a common wireless communication channel and when a node is allowed to transmit or receive data.
Why MAC is Needed
Many nodes share the same wireless medium.
Simultaneous transmissions may cause collisions.
Radio communication consumes significant energy.
Nodes require coordination for sleeping and waking.
Applications may require low delay or high reliability.
Sensor A ----\
Sensor B -----\ Shared Wireless Channel
Sensor C ------> [ MAC Protocol ] ----> Sink
Sensor D -----/
Sensor E ----/
Fundamental Access Methods
Random or contention-based access: Nodes compete for the channel.
Scheduled or contention-free access: Nodes receive fixed or assigned transmission opportunities.
Hybrid access: Combines contention and scheduled methods.
2. Functions of a WSN MAC Protocol 7 Marks
Channel access coordination
Collision avoidance
Frame transmission and reception
Synchronization
Sleep and wake-up scheduling
Error detection and retransmission
Energy conservation
Fair channel allocation
Support for traffic priorities
Unlike conventional wireless networks, WSN MAC protocols primarily optimize energy consumption and network lifetime.
3. Sources of Energy Waste at MAC Layer 14 Marks
Collision
When two nodes transmit simultaneously, packets may be corrupted and require retransmission.
Idle Listening
A node keeps its radio active while waiting for possible traffic, even when no packet arrives.
Overhearing
A node receives and processes a packet intended for another node.
Control-Packet Overhead
Energy is consumed by RTS, CTS, acknowledgment, synchronization and scheduling packets.
Over-Emitting
A sender transmits while the receiver is not ready or is sleeping.
Frequent State Switching
Repeated changes between sleep, idle, receive and transmit states consume energy.
Total MAC Energy ≈ Transmission + Reception + Idle Listening + Overhearing + Collision + Control Overhead
4. MAC Design Objectives for WSN 14 Marks
Energy efficiency: Reduce radio-active time and unnecessary communication.
A ----> B <---- C
A and C cannot hear each other.
Both may transmit to B simultaneously,
causing a collision at B.
Exposed-Terminal Problem
A ----> B C ----> D
C may unnecessarily avoid transmission
because it hears B, even though C → D
would not interfere with A → B.
6. Contention-Based MAC Protocols 14 Marks
In contention-based MAC protocols, nodes compete for access to the shared wireless channel using carrier sensing, random backoff or handshake mechanisms.
Working
A node senses the channel.
If the channel is free, it transmits.
If the channel is busy, it waits for a random period.
After backoff, it senses the channel again.
Acknowledgment may confirm successful reception.
Advantages
Simple and flexible
No strict global synchronization
Efficient under low traffic
Adapts to changing topology
Disadvantages
Collisions under heavy traffic
Idle listening
Unpredictable delay
Energy loss due to retransmissions
7. S-MAC Protocol 14 Marks
Sensor-MAC is a contention-based protocol that saves energy by using periodic sleep and listen schedules.
Each node follows a periodic listen-and-sleep schedule.
During the listen period, nodes use carrier sensing and RTS/CTS.
During the sleep period, the radio is turned off.
Advantages
Reduces idle listening
Uses sleep scheduling
Reduces collisions using RTS/CTS
Suitable for periodic traffic
Limitations
Fixed duty cycle may waste energy or increase delay.
Packets may wait until the next listen period.
Synchronization packets add overhead.
8. T-MAC Protocol 14 Marks
Timeout-MAC improves S-MAC by using an adaptive active period. A node sleeps early if no communication activity occurs for a specified timeout interval.
Heavy Traffic:
|------ Active Period ------| Sleep |
Light Traffic:
| Active | Sleep -------------------|
Working
A node wakes during the scheduled active period.
It remains active while communication events continue.
If no event occurs for time TA, it goes to sleep.
The active duration changes according to traffic load.
Advantages
Adaptive duty cycle
Better energy saving under variable traffic
Shorter unnecessary active periods
Disadvantages
Early sleeping problem
More complex than S-MAC
May increase delay in multi-hop traffic
9. B-MAC Protocol 7 Marks
Berkeley MAC is a lightweight contention-based protocol that uses low-power listening, clear-channel assessment and long preambles.
Working
Nodes periodically wake and sample the channel.
A sender transmits a preamble longer than the receiver's checking interval.
The receiver detects the preamble and stays awake for the data packet.
Clear-channel assessment helps avoid collisions.
Advantages
Simple and configurable
Low-power listening
No strict synchronization
Limitations
Long preambles waste energy.
Neighboring nodes may overhear.
Delay can be high.
10. Contention-Free MAC Protocols 14 Marks
Contention-free protocols assign separate time slots, frequencies or codes to nodes so that transmissions occur without competing for the medium.
Major Techniques
Time Division Multiple Access
Frequency Division Multiple Access
Code Division Multiple Access
Token-based access
Advantages
Collision-free communication
Predictable delay
Efficient under heavy traffic
Supports guaranteed channel access
Disadvantages
Requires synchronization or scheduling.
Unused slots waste bandwidth.
Schedule maintenance is difficult in dynamic networks.
Less efficient under low or irregular traffic.
11. TDMA-Based MAC 14 Marks
Time Division Multiple Access divides communication time into frames and assigns a different time slot to each node.
One TDMA Frame
| Slot A | Slot B | Slot C | Slot D |
| Node A | Node B | Node C | Node D |
Working
The sink or coordinator creates a schedule.
Each node receives a transmission slot.
A node wakes during its assigned slot.
It sleeps during the slots of other nodes.
The schedule repeats in every frame.
Advantages
No collisions
Low idle listening
Predictable channel access
Good energy efficiency under stable traffic
Limitations
Requires clock synchronization.
Slot assignment creates overhead.
Empty slots waste capacity.
Node addition or failure requires rescheduling.
12. Hybrid MAC Protocols 14 Marks
Hybrid MAC protocols combine contention-based and contention-free access to obtain flexibility under light traffic and predictable performance under heavy traffic.
General Working
Nodes may use contention to request channel resources.
The coordinator assigns scheduled slots for data transmission.
The protocol may switch between contention and TDMA modes.
Traffic load or application priority controls the selected mode.
Low Traffic
|
v
Contention-Based Access
|
Traffic Increases
|
v
Scheduled / TDMA Access
Advantages
Adapts to traffic variation.
Reduces collisions under heavy traffic.
Uses bandwidth efficiently.
Balances energy, delay and throughput.
Limitations
More complex implementation
Control and synchronization overhead
Mode-switching decisions may be difficult
13. Comparison of MAC Protocol Categories 14 Marks
Basis
Contention-Based
Contention-Free
Hybrid
Channel access
Nodes compete
Slots/resources assigned
Uses both methods
Collision
Possible
Normally avoided
Reduced
Synchronization
Low or moderate
Usually high
Moderate to high
Low traffic
Efficient
May waste slots
Efficient
High traffic
More collisions
Efficient
Efficient
Delay
Variable
Predictable
Adaptive
Complexity
Low
Moderate
High
14. Routing in Wireless Sensor Networks 14 Marks
Routing is the process of selecting energy-efficient paths for forwarding sensed data from source nodes to a sink or base station.
Source Sensor
|
v
Intermediate Node
|
v
Intermediate Node
|
v
Sink
Why Traditional Routing is Not Sufficient
Sensor nodes have limited energy.
Node count can be very large.
Nodes may not have global identifiers.
Data from nearby nodes is often redundant.
Network topology may change.
Traffic is commonly many-to-one.
Routing Objectives
Minimize energy consumption
Maximize network lifetime
Reduce communication overhead
Provide reliable delivery
Meet delay and QoS requirements
Support scalability and failures
15. Fundamentals and Challenges of WSN Routing 14 Marks
Energy constraint: Frequent forwarding depletes batteries.
Scalability: Protocols must support thousands of nodes.
Dynamic topology: Nodes may fail or move.
Data redundancy: Nearby nodes generate similar data.
Unreliable links: Wireless channels suffer interference and loss.
Limited resources: Routing tables and algorithms must remain small.
Coverage requirements: Routing should not rapidly exhaust critical nodes.
Load balancing: Traffic should not overload a few nodes.
Heterogeneity: Nodes may have different capabilities.
Mobility: Sink or sensor nodes may move.
QoS: Some data requires low delay or high reliability.
Security: Malicious nodes may alter routes or packets.
16. Taxonomy of Routing and Data Dissemination Protocols 14 Marks
Data-centric routing identifies communication by data attributes or interests instead of unique node addresses.
Basic Operation
The sink sends an interest or query describing required data.
Nodes matching the query prepare or forward data.
Routes may be reinforced based on received data quality.
Duplicate information is aggregated or suppressed.
Example query: “Report temperature above 50°C from the northern region every 10 seconds.”
Advantages
Reduces redundant transmissions
Supports application-specific queries
Enables aggregation
Does not require global node addressing
Limitations
Query and interest dissemination creates overhead.
Naming sensed data can be complex.
May be unsuitable for continuous high-rate traffic.
21. Multipath-Based Routing Protocols 14 Marks
Multipath routing maintains two or more paths between source and sink for reliability, fault tolerance or load balancing.
Path 1: A ---- B
/ \
Source S Sink D
\ /
Path 2: C ---- E
Uses
Backup path after failure
Parallel packet transmission
Load balancing
Improved reliability
Fault tolerance
Advantages
Continues after link or node failure
Balances energy consumption
May improve throughput
Limitations
More routing-state overhead
Route maintenance consumes energy
Duplicate packets may increase traffic
22. Mobility-Based Routing Protocols 14 Marks
Mobility-based protocols support movement of sensor nodes, sinks or data collectors.
Types of Mobility
Mobile sensor nodes
Mobile sink
Mobile relay or data mule
Mobile event source
Benefits of Mobile Sink
Reduces energy burden near a fixed sink.
Improves data collection from isolated regions.
May balance network energy consumption.
Challenges
Frequent route updates
Changing connectivity
Delivery delay
Movement prediction
Scheduling communication with mobile collector
23. QoS-Based Routing Protocols 14 Marks
Quality-of-Service-based routing selects routes that satisfy application requirements such as delay, reliability, bandwidth, packet loss and energy consumption.
QoS Parameters
End-to-end delay
Packet delivery ratio
Reliability
Available bandwidth
Jitter
Energy consumption
Applications
Emergency detection
Industrial control
Multimedia sensor networks
Healthcare monitoring
Challenges
QoS and energy requirements may conflict.
Wireless link quality changes.
Resource constraints limit complex algorithms.
Different traffic classes require prioritization.
24. Heterogeneity-Based Routing Protocols 14 Marks
Heterogeneity-based routing uses differences in node energy, processing power, communication range or sensing capability to assign suitable routing roles.
Heterogeneous Node Types
Normal sensor nodes
High-energy nodes
Powerful cluster heads
Gateway nodes
Specialized sensing nodes
Advantages
Powerful nodes handle expensive tasks.
Can improve network lifetime.
Supports advanced processing and long-range communication.
Improves scalability.
Limitations
Network design becomes more complex.
Powerful nodes may become critical failure points.
Cost and deployment planning increase.
25. Data Dissemination in WSN 14 Marks
Data dissemination is the process of distributing queries, interests, sensed information or control messages among sensor nodes and sinks.
Dissemination Models
Query-driven: Sink requests specific information.
Event-driven: Nodes report when an event occurs.
Time-driven: Nodes report periodically.
Continuous: Data is streamed continuously.
Flooding
Every node forwards a received packet to its neighbors.
Simple and robust
Causes implosion, overlap and resource blindness
Gossiping
Each node forwards the packet to one randomly selected neighbor.
Reduces duplicate transmissions
May create high delay
Key Design Goals
Energy efficiency
Low delay
Scalability
Reliable delivery
Redundancy reduction
Adaptability
Unit 2 Quick Revision
MAC controls access to the shared wireless channel.
Main MAC categories are contention-based, contention-free and hybrid.
Energy waste occurs due to collision, idle listening, overhearing and overhead.
S-MAC uses periodic sleep and listen schedules.
T-MAC uses an adaptive active period.
B-MAC uses low-power listening and long preambles.
TDMA assigns separate transmission slots.
Hybrid MAC combines contention and scheduled access.
WSN routing must minimize energy and support many constrained nodes.
Location-aided protocols use node position.
Layered routing forms clusters.
In-network processing reduces transmitted data.
Data-centric routing uses named data and interests.
Multipath routing improves reliability and load balancing.
Mobility-based routing supports moving nodes or sinks.
QoS routing considers delay, reliability and bandwidth.
Heterogeneity-based routing uses different node capabilities.
Important RGPV Exam Questions
Long Answer Questions
Define MAC protocol and explain its need and functions in WSN.
Explain the major sources of energy waste at the MAC layer.
Discuss the objectives and challenges of MAC design for WSN.
Explain contention-based MAC protocols with advantages and limitations.
Explain S-MAC with a neat diagram.
Explain T-MAC and compare it with S-MAC.
Write a detailed note on B-MAC.
Explain contention-free and TDMA-based MAC protocols.
What are hybrid MAC protocols? Explain their operation.
Compare contention-based, contention-free and hybrid MAC protocols.
Define routing in WSN and explain its objectives.
Discuss major routing challenges in sensor networks.
Explain the taxonomy of routing and data-dissemination protocols.
Explain location-aided routing protocols.
Explain layered and in-network processing-based protocols.
Explain data-centric routing with suitable example.
Explain multipath-based routing protocols.
Discuss mobility-based routing protocols.
Explain QoS-based routing protocols.
Explain heterogeneity-based routing protocols.
Define data dissemination and explain flooding and gossiping.
Short Answer Questions
Define MAC protocol.
What is idle listening?
Define overhearing.
What is the hidden-terminal problem?
What is S-MAC?
Define T-MAC.
What is low-power listening?
Define TDMA.
What is hybrid MAC?
Define routing in WSN.
What is data-centric routing?
Define in-network processing.
What is multipath routing?
What is QoS routing?
Define flooding and gossiping.
Exam Tip: S-MAC, T-MAC, TDMA comparison, routing taxonomy, data-centric routing and in-network processing are high-priority Unit 2 topics.
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MAC schedules, routing taxonomy and architecture diagrams.
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Frequently Asked Questions
MAC protocol shared wireless channel par nodes ke transmission ko coordinate karta hai aur collision, idle listening aur energy waste ko reduce karta hai.
Contention-based MAC mein nodes channel access ke liye compete karte hain aur carrier sensing ya random backoff use karte hain.
S-MAC periodic listen aur sleep schedule use karta hai. Sleep period mein node radio off kar deta hai.
S-MAC fixed active period use karta hai, jabki T-MAC traffic ke according adaptive active period use karta hai.
Data-centric routing node address ke badle data attributes, interests aur queries ke basis par communication karti hai.
In-network processing aggregation, filtering aur duplicate removal se transmitted data kam karta hai, jisse energy aur bandwidth save hoti hai.
Multipath routing reliability, fault tolerance aur load balancing improve karne ke liye multiple routes maintain karti hai.