Unit 3: Basics of Wireless, Sensors and Applications

Syllabus:

Basics of Wireless, Sensors and Applications: Applications, Classification of sensor networks, Architecture of sensor network, Physical layer, MAC layer, Link layer.

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Section 1: Introduction to Wireless Sensor Networks (WSN)

1.1 What is a WSN?

A Wireless Sensor Network (WSN) is a distributed network of small, low-power devices called sensor nodes (or motes) that collaboratively sense, process, and transmit data about physical or environmental conditions to a central location called the base station or sink.

Each sensor node is capable of:

• Sensing physical phenomena (temperature, humidity, pressure, motion, etc.)
• Local data processing
• Wireless communication with neighboring nodes or the base station

WSNs operate without fixed infrastructure and are often deployed in harsh, inaccessible, or remote environments.

Key characteristics of WSN:

• Large scale — hundreds to thousands of nodes
• Resource constrained — limited battery, memory, and processing power
• Self-organizing — nodes form network autonomously
• Data-centric — users query data, not specific nodes
• Application-specific — design varies by application

1.2 WSN vs MANET — Key Differences

1.3 Key Constraints of Sensor Nodes

Energy

• Sensor nodes are battery-powered with no or limited recharging capability.
• Energy is the most critical constraint — determines network lifetime.
• Energy consumption breakdown:
  • Communication (radio) — dominant (~70–80%)
  • Sensing — moderate
  • Processing — relatively low
• Nodes must minimize idle listening, overhearing, and collisions.

Memory

• Typical sensor nodes have very limited RAM (4–256 KB) and Flash memory (32 KB–1 MB).
• Limits the complexity of algorithms that can run on the node.
• Protocols must be lightweight and compact.

Processing Power

• Microcontrollers like ATmega128, MSP430, or ARM Cortex-M are used.
• Clock speeds in the range of 4–16 MHz — far less than general-purpose CPUs.
• Complex computations (encryption, compression) must be carefully designed.

Communication Range

• Typical range: 10 m to 100 m (depends on power and environment).
• Long-range transmission is costly in energy — multi-hop routing is used instead.
• Radio transceivers operate in the ISM (Industrial, Scientific, Medical) bands.

Section 2: Applications of Sensor Networks

WSNs are deployed across a wide range of domains. The key driving factor is the ability to place sensors in locations that are inaccessible, hazardous, or impractical for wired sensing.

2.1 Military Applications

Military applications were among the first motivators for WSN research.

• Battlefield surveillance — monitor troop movements, detect enemy activity
• Enemy tracking — track vehicles, equipment, and personnel positions
• Nuclear, biological, chemical (NBC) detection — sense hazardous agents
• Damage assessment — evaluate post-attack damage in real time
• Smart weapons guidance — sensors embedded in munitions

Example: DARPA-funded programs like SensIT and Smart Dust were early WSN research initiatives for military use.

2.2 Environmental Monitoring

• Forest fire detection — deploy nodes across forests; detect temperature/smoke anomalies before fires spread
• Flood monitoring — measure water levels in rivers and trigger alerts
• Weather monitoring — distributed sensing of temperature, humidity, wind, precipitation
• Landslide detection — detect ground movement patterns
• Volcanic activity monitoring — deploy sensors near volcanoes to detect seismic activity
• Ecosystem monitoring — track wildlife behavior, habitat conditions (e.g., Great Duck Island project)

2.3 Healthcare Applications

• Patient monitoring — continuous monitoring of heart rate, blood pressure, SpO2, temperature
• Hospital asset tracking — track wheelchairs, IV pumps, defibrillators using RFID + sensors
• Elderly care — detect falls, monitor vital signs, alert caregivers
• Drug administration — ensure correct dosage and timing
• Post-operative monitoring — track recovery outside ICU
• Epidemic detection — monitor spread of infectious diseases

2.4 Industrial Applications

• Machine health monitoring — detect vibration, temperature anomalies in motors/turbines
• Process control — monitor chemical, pressure, flow in industrial pipelines
• Structural health monitoring — monitor bridges, dams, buildings for cracks, stress
• Supply chain management — track goods in warehouses and during transit
• Worker safety — detect toxic gas levels in mines or chemical plants

2.5 Smart Home Applications

• Automation — automatic control of lighting, HVAC, appliances based on occupancy
• Security — motion detectors, door/window sensors, surveillance
• Energy management — monitor and optimize power usage
• Smart meters — real-time electricity/water/gas consumption tracking
• Elderly/children monitoring — detect unusual activity patterns

2.6 Agriculture (Precision Farming)

• Soil monitoring — soil moisture, pH, nitrogen levels for optimal irrigation and fertilization
• Crop monitoring — detect crop diseases, pest activity
• Microclimate sensing — temperature and humidity inside greenhouses
• Livestock tracking — GPS + sensors to track animal health and location
• Irrigation automation — activate/deactivate irrigation based on soil moisture thresholds

2.7 Disaster Management

• Earthquake early warning — detect seismic activity, estimate magnitude
• Tsunami detection — underwater pressure sensors detect wave patterns
• Flood early warning — river/reservoir level monitoring
• Post-disaster search and rescue — deploy sensors in rubble to detect survivors
• Infrastructure assessment — monitor buildings after disasters

2.8 Transportation

• Traffic monitoring — measure vehicle flow, detect congestion, control traffic lights
• Vehicle tracking — fleet management, cargo tracking
• Parking management — detect occupancy of parking spots
• Road condition monitoring — ice detection, pothole detection
• Railway safety — track status of rail lines, bridge loads

Section 3: Classification of Sensor Networks

Sensor networks can be classified along multiple dimensions based on deployment, mobility, communication model, and other factors.

3.1 By Deployment Strategy

3.2 By Mobility

3.3 By Communication Model

Multi-hop is preferred when nodes are far from the base station, as it saves transmit energy.

Single-hop:                     Multi-hop:
  [S1] ----\                    [S1] -> [S2] -> [S3] -> [BS]
  [S2] ----- [BS]              [S4] -> [S5] ---------> [BS]
  [S3] ----/

3.4 By Data Aggregation

3.5 By Power Source

3.6 By Sensing Type

Multimedia WSNs (MWSNs) have additional challenges: higher data rates, more processing, and much more energy consumption.

Section 4: Architecture of Sensor Network

4.1 Sensor Node — Block Diagram

A sensor node (mote) typically consists of four main subsystems:

+-------------------------------------------------------+
|                   SENSOR NODE                         |
|                                                       |
|  +------------------+    +------------------------+  |
|  |  SENSING UNIT    |    |   PROCESSING UNIT      |  |
|  |                  |    |                        |  |
|  | [Sensors]        |    | [Microcontroller]      |  |
|  |    |             |    |    |                   |  |
|  | [ADC]            |    | [Memory]               |  |
|  |  (Analog to      |    |  (Flash + SRAM)        |  |
|  |   Digital)       |    |                        |  |
|  +--------+---------+    +-----------+------------+  |
|           |                          |               |
|           +---------+  +-------------+               |
|                     |  |                             |
|              +------v--v-------+                     |
|              | COMMUNICATION   |                     |
|              |     UNIT        |                     |
|              | [Radio          |                     |
|              |  Transceiver]   |                     |
|              |  (TX + RX)      |                     |
|              +-----------------+                     |
|                                                       |
|  +---------------------------------------------------+|
|  |              POWER UNIT                           ||
|  |  [Battery] + [Energy Harvesting (optional)]       ||
|  |  [Power Management Circuit]                       ||
|  +---------------------------------------------------+|
+-------------------------------------------------------+

Sensing Unit:

• Sensors — transduce physical quantity into analog electrical signal
• ADC (Analog-to-Digital Converter) — converts analog signal to digital for processing

Processing Unit:

• Microcontroller — executes sensing, processing, MAC, routing protocols
• Memory — Flash (program storage) and SRAM (data/stack)

Communication Unit:

• Radio Transceiver — handles wireless transmission and reception
• Typically a short-range low-power radio (e.g., CC2420, nRF24L01)

Power Unit:

• Battery — primary energy source (AA, coin cell, or custom)
• Energy harvesting — solar panel, piezoelectric, thermoelectric (optional)
• Power regulation circuit — converts and regulates voltage

4.2 Network Architecture

A WSN follows a layered network architecture:

+------------------+
|   User / Cloud   |   <-- Applications, dashboards, data analytics
+--------+---------+
         |
     (Internet)
         |
+--------+---------+
|   Base Station   |   <-- Sink node; collects all data, high-power device
|      (Sink)      |       connected to the internet
+--------+---------+
         |
    (Multi-hop
     wireless)
         |
+--------+---------+
|  Cluster Heads   |   <-- Aggregator nodes; collect data from member nodes
+--------+---------+
         |
+--------+---------+
|  Sensor Nodes    |   <-- Leaf nodes; sense and transmit data
+------------------+

Sensor Nodes: Smallest, most numerous, most resource-constrained. Sense the environment and send data to cluster heads or directly to the base station.

Cluster Heads (CHs): Elected periodically; aggregate data from cluster members. Have higher energy or are rotated among nodes. Reduce total traffic.

Base Station (Sink): Powerful node (often connected to mains power) that receives data from the entire network. Acts as a gateway to the internet.

Internet / User: End user accesses data via web/mobile applications.

4.3 Data Flow Diagram

Physical World
     |
     | (Physical phenomenon)
     v
[Sensor Node]
     |  Sense -> ADC -> Process -> Compress/Encrypt
     |
     v (Radio transmission)
[Neighboring Node / Cluster Head]
     |  Aggregate / Forward
     v
[Base Station / Sink]
     |  Protocol conversion, store data
     v
[Internet]
     |
     v
[User Application]
     (Query, visualization, alerts)

4.4 Network Topology Types

Flat Architecture

All sensor nodes are peers. Each node communicates directly or via multi-hop to the base station. No special roles.

[S1] - [S2] - [S3]
  \             |
   [S4] - [S5]-[BS]

✅ Simple, robust, decentralized
❌ Poor scalability, high energy cost for nodes far from BS

Hierarchical / Cluster-based Architecture

Nodes organized into clusters. Each cluster has a Cluster Head (CH) that aggregates data.

Cluster 1:          Cluster 2:
[S1][S2][S3]        [S6][S7][S8]
      |                   |
    [CH1]               [CH2]
       \                 /
        ----> [BS] <----

✅ Scalable, energy-efficient (data aggregation), extends lifetime
❌ CH may become energy bottleneck; requires CH election protocol (e.g., LEACH)

Tree-based Architecture

Hierarchical tree rooted at the base station.

              [BS]
             /    \
          [CH1]  [CH2]
          / \      / \
        [S1][S2] [S3][S4]

✅ Structured, easy routing
❌ Vulnerable to node failure (single point)

Section 5: Physical Layer

The Physical (PHY) layer is responsible for the actual transmission and reception of raw bits over the wireless channel.

5.1 Wireless Transmission Basics

• Data is transmitted as electromagnetic waves through the air.
• Key parameters: frequency, bandwidth, modulation, transmit power, antenna gain
• Wireless channel introduces path loss, shadowing, multipath fading, and interference

5.2 Frequency Bands Used in WSN (ISM Bands)

WSNs primarily use ISM (Industrial, Scientific, Medical) bands — unlicensed spectrum:

Trade-offs:

• Lower frequency → better range, less path loss, but narrower bandwidth
• Higher frequency → more channels available, but more path loss

5.3 Modulation Techniques

Modulation converts digital data into analog signals suitable for wireless transmission.

BPSK — Binary Phase Shift Keying

• Two phase states: 0° and 180°
• 1 bit per symbol
• Simple, robust, used in low-data-rate WSN
• Used in 868/915 MHz band of IEEE 802.15.4

Bit:  0     1     0     1
      |     |     |     |
Phase: 0°   180°  0°   180°

QPSK — Quadrature Phase Shift Keying

• Four phase states: 0°, 90°, 180°, 270°
• 2 bits per symbol → higher spectral efficiency
• Used in 2.4 GHz band of IEEE 802.15.4 (O-QPSK variant)

Dibits: 00=0°, 01=90°, 10=180°, 11=270°

FSK — Frequency Shift Keying

• Different frequencies represent different bits
• More immune to amplitude noise
• Used in Bluetooth and some proprietary WSN radios

5.4 IEEE 802.15.4 Standard (Zigbee Physical Layer)

IEEE 802.15.4 is the foundational standard for low-rate wireless personal area networks (LR-WPAN), used by Zigbee, 6LoWPAN, and WirelessHART.

PHY Frame Structure (IEEE 802.15.4):

+----------+----------+----------+----------+----------+
| Preamble | SFD      | PHR      |  PHY     | Payload  |
| (4 bytes)|(1 byte)  |(1 byte)  | Payload  | (PSDU)   |
|          |          |          | Length   | 0-127 B  |
+----------+----------+----------+----------+----------+

• Preamble: synchronization
• SFD (Start Frame Delimiter): marks start of data
• PHR (PHY Header): contains payload length
• PSDU: actual data payload

5.5 Propagation Models

Propagation models describe how signal strength drops as distance increases.

Free Space Model

Analogy: Like a light bulb in an empty room — the further you go, the dimmer it gets, and it drops off with the square of the distance.

• Signal power decreases with d² (double the distance → 4× weaker signal)
• Assumes clear line-of-sight, no obstacles or reflections
• Optimistic model — real environments are messier

Two-Ray Ground Reflection Model

Analogy: Like shouting across an empty field — your voice travels directly to the listener, but also bounces off the ground and arrives slightly later, partially cancelling your direct signal.

• More realistic — signal travels via two paths: direct path + ground-reflected path
• Signal power decreases with d⁴ — drops off much faster than free space
• Used for longer distances over flat terrain (outdoor fields, farmland)

5.6 Energy Consumption at Physical Layer

The PHY layer is the dominant energy consumer in a sensor node.

Key insight: Idle listening wastes almost as much energy as active receiving. MAC protocols exploit sleep scheduling to minimize idle listening.

Section 6: MAC Layer

The Medium Access Control (MAC) layer controls how sensor nodes share the wireless channel. In WSN, MAC design is energy-centric, unlike traditional MACs that focus on throughput and fairness.

6.1 Role of MAC in WSN

• Coordinate channel access — prevent collisions among nodes
• Energy efficiency — minimize idle listening, overhearing, collisions
• Latency — acceptable delay for data delivery
• Scalability — support varying network sizes
• Adaptability — handle changes in network topology

6.2 Sources of Energy Waste at MAC Layer

These are the four primary causes of energy wastage:

Energy Waste Sources:
┌─────────────────────┐
│  Idle Listening     │  ← Most common waste
│  Overhearing        │  ← Dense networks
│  Collision          │  ← Contention-based MACs
│  Control Overhead   │  ← Excessive handshaking
└─────────────────────┘

6.3 Types of MAC Protocols

WSN MAC protocols fall into three categories:

Schedule-Based (TDMA)

• Time is divided into slots; each node is assigned specific slots to transmit.
• No collisions, no idle listening during off-slots.
• Examples: TDMA, TRAMA, S-MAC (partial)

✅ No collisions
✅ Predictable latency
✅ Energy-efficient
❌ Requires time synchronization
❌ Less adaptive to dynamic traffic

Contention-Based (CSMA)

• Nodes compete for the channel; listen before transmitting.
• Examples: CSMA/CA (IEEE 802.15.4), ALOHA

✅ No synchronization needed
✅ Adapts to traffic load
❌ Collisions possible
❌ Idle listening waste

Hybrid

• Combines TDMA and CSMA features.
• Examples: Z-MAC, TDMA+CSMA hybrid

✅ Flexibility of CSMA + efficiency of TDMA
❌ More complex design

6.4 S-MAC Protocol (Sensor MAC)

S-MAC is one of the most important energy-efficient MAC protocols for WSN.

Key Idea: Nodes follow a periodic sleep/listen cycle. During the listen period, they can send/receive. During sleep, the radio is OFF.

Operation:

1. Each node follows a duty cycle: Listen for a short period, then sleep.
2. Nodes exchange SYNC packets to synchronize their schedules.
3. Neighboring nodes form virtual clusters — nodes with the same schedule.
4. Nodes on different schedules have border nodes that follow both schedules.

Duty Cycle:

|<--------- Period T -------->|
|  Listen  |     Sleep        |  Listen  |     Sleep        |
|<--Tl--->|<-------Ts------->|<--Tl--->|<-------Ts------->|

Duty cycle = Tl / T  (typically 10%)

Frame Structure during Listen:

|  SYNC  |  DATA   |
|  slot  |  slot   |

• SYNC slot: Nodes exchange synchronization information
• DATA slot: Actual data transmission using RTS/CTS/DATA/ACK

Virtual Clusters:

+--Group A (same schedule)--+    +--Group B--+
| [S1] [S2] [S3]            |    | [S4] [S5] |
|        |                  |    |    |       |
|      [S3]=border node=====+====+=[S4]       |
+---------------------------+    +-----------+

Border nodes follow both Group A and Group B schedules.

Message Passing (RTS/CTS):

Sender:   |--RTS-->|         |--DATA-->|
Receiver: |        |<--CTS---|         |--ACK-->|

Nodes that overhear RTS or CTS go to sleep for the duration of the transmission (reduces overhearing).

✅ Reduces idle listening dramatically
✅ Good for low-traffic WSN
❌ Latency increases with more hops (adaptive listening reduces this)
❌ Border nodes wake up more, consuming extra energy

6.5 IEEE 802.15.4 MAC

The IEEE 802.15.4 MAC supports two modes:

Beacon-Enabled Mode (Superframe Structure)

A coordinator (e.g., cluster head or PAN coordinator) sends periodic beacons to synchronize the network.

|<---------------------- Superframe ---------------------------->|
|  Beacon | CAP (Contention Access Period) | CFP (TDMA slots)  |
|         | CSMA-CA based                  | GTS (Guaranteed   |
|         |                                | Time Slots)       |

Active: |<-------Active portion-------->|<----Inactive------->|
                                         (nodes sleep here)

• CAP: Contention-based (CSMA-CA) for general data
• CFP: TDMA-like guaranteed time slots (GTS) for time-critical data

Non-Beacon Mode

No synchronization. Nodes use unslotted CSMA-CA:

1. Node wants to transmit → Backs off random number of slots
2. Perform CCA (Clear Channel Assessment) — check if channel is idle
3. If idle → transmit; If busy → wait and retry
4. After fixed retries, declare failure

Slotted CSMA-CA (Beacon mode):

CW=2 (initial contention window)
BE=macMinBE (initial backoff exponent)

1. Set NB=0, CW=2, BE=macMinBE
2. Delay random(0, 2^BE - 1) backoff periods
3. Perform CCA
4. If idle: decrement CW; if CW=0, transmit
5. If busy: NB++, BE=min(BE+1, macMaxBE), CW=2
6. If NB > macMaxCSMABackoffs: declare failure

6.6 Z-MAC (Zebra MAC)

Z-MAC is a hybrid MAC protocol that uses:

• CSMA under low traffic (flexible, low latency)
• TDMA under high traffic (efficient, collision-free)

Each node is assigned a TDMA slot but can also use others' slots when idle. Switches between modes based on detected contention.

6.7 Comparison: TDMA vs CSMA/CA for WSN

Section 7: Link Layer

The Link Layer (Data Link Layer) ensures reliable frame delivery between directly connected nodes. In WSN, links are often lossy due to interference, fading, and node mobility, making link-layer reliability critical.

7.1 Role of Link Layer in WSN

• Framing: Package data into frames with headers and trailers
• Error detection: Detect bit errors introduced by the channel
• Error correction: Recover from errors without retransmission (FEC)
• Retransmission: Request retransmission of lost/corrupted frames (ARQ)
• Link quality estimation: Measure channel quality for routing decisions

7.2 Error Detection: CRC

CRC (Cyclic Redundancy Check) is the standard error detection method in WSN.

How it works:

1. Sender divides the message polynomial by a generator polynomial G(x)
2. The remainder is appended to the frame as the CRC field
3. Receiver divides the received frame by G(x)
4. If remainder is 0 → no error; else → error detected

Sender:
  Message M:   1101011011
  Generator G: 10011 (CRC-4)
  
  Append zeros: 11010110110000
  Divide by G:  compute remainder R
  Transmitted:  Message + R

Receiver:
  Divide (Message + R) by G
  Remainder = 0 → no error
  Remainder ≠ 0 → error detected → discard frame

Common CRC variants:

• CRC-8: 8-bit check (simple, lightweight for WSN)
• CRC-16: 16-bit check (used in many protocols)
• CRC-32: 32-bit check (stronger, used in Ethernet)

IEEE 802.15.4 uses CRC-16 (ITU-T polynomial: 0x1021)

✅ Can detect all single-bit errors, burst errors
❌ Cannot correct errors — only detection

7.3 Error Correction: FEC

FEC (Forward Error Correction) allows the receiver to correct errors without requesting retransmission.

How it works:

• Sender adds redundant bits (parity/coding bits) to the data
• Receiver uses the redundancy to detect AND correct errors

Common FEC codes:

In WSN: Simple FEC like Hamming code is preferred due to low computational overhead.

Trade-off:

• FEC adds overhead (larger frames → more energy to transmit)
• FEC saves retransmission energy in high-error environments
• FEC is preferred when link error rate is high and retransmission cost is high

7.4 ARQ — Automatic Repeat Request

ARQ relies on the receiver to detect errors (using CRC) and request retransmission.

Three main ARQ protocols:

Stop-and-Wait ARQ

Sender transmits one frame and waits for ACK before sending next.

Sender:   [Frame 0] ---------> 
Receiver:           <--------- [ACK 0]
Sender:   [Frame 1] ---------> 
Receiver:           <--------- [ACK 1]
          ...

If no ACK within timeout → retransmit same frame.

✅ Simple to implement
❌ Low efficiency — channel idle while waiting for ACK
❌ High latency

Go-Back-N ARQ

Sender can transmit N frames (window size N) without waiting for ACK.

Window size N=4:
Sender:   [0][1][2][3][4] ...
                  ^-- Error at frame 2
Receiver:          NACK-2
Sender:   [2][3][4][5] ...   (go back to frame 2, resend all)

If frame i is corrupted → receiver rejects all subsequent frames → sender retransmits from frame i.

✅ Better channel utilization than stop-and-wait
❌ Retransmits correctly received frames — wasteful

Selective Repeat ARQ

Sender transmits up to N frames; receiver buffers out-of-order frames.

Window size N=4:
Sender:   [0][1][2][3][4] ...
                  ^-- Error at frame 2
Receiver: Accepts 0,1,3,4 (buffers out-of-order)
          NACK-2
Sender:   [2] only (selective retransmit)

✅ Most efficient — only lost frames are retransmitted
❌ More complex — requires receiver buffering

ARQ in WSN Context

In WSN, Stop-and-Wait or simple link-layer retransmission is often used due to memory constraints. Higher-layer protocols may handle end-to-end reliability.

7.5 Link Quality Indicators

RSSI — Received Signal Strength Indicator

• Measures the power level of the received signal in dBm
• Higher RSSI → stronger signal → better link quality
• Affected by distance, obstacles, interference

RSSI Range (typical):
  -40 dBm : Excellent
  -70 dBm : Good
  -85 dBm : Fair
  -100 dBm: Very poor / packet loss likely

LQI — Link Quality Indicator

• A composite metric measuring the quality of the received signal at the PHY layer
• In IEEE 802.15.4: LQI is computed from the energy of received symbols and correlation with ideal symbols
• Range: 0–255 (higher = better)
• Used by routing protocols to select reliable paths

PRR — Packet Reception Rate

• Fraction of successfully received packets over a period
• More accurate than RSSI/LQI as it measures actual link performance

PRR = (Packets received) / (Packets sent)

7.6 Why Link-Layer Reliability Matters in WSN

WSN links are inherently lossy:

• Multipath fading — reflections cause constructive/destructive interference
• Interference — other devices using the same ISM band (WiFi, Bluetooth, microwaves)
• Node mobility — changing topology in mobile WSNs
• Environmental dynamics — moving objects, weather changes

Consequences of lossy links:

• Increased retransmissions → more energy consumption
• Reduced throughput
• Higher end-to-end latency

Link-layer mechanisms to address lossy links:

• CRC for error detection
• FEC for error correction without retransmission
• ARQ for reliable delivery
• Link quality estimation (RSSI, LQI) to select better routes
• Adaptive modulation — switch to lower-rate, more robust modulation on bad links

Quick Revision Points

WSN Fundamentals

• WSN = distributed network of sensor nodes sensing physical phenomena and reporting to a base station
• Key constraints: energy (dominant), memory, processing, communication range
• WSN differs from MANET in: data-centric, mostly static, many-to-one traffic, thousands of nodes

Applications

• Military, environmental monitoring, healthcare, industrial, smart home, agriculture, disaster management, transportation
• Motivating factor: sensing in inaccessible, hazardous, or large-scale environments

Classification

• Deployment: random vs deterministic
• Mobility: static vs mobile vs hybrid
• Communication: single-hop vs multi-hop
• Data: with/without aggregation
• Power: battery vs energy harvesting
• Sensing: scalar vs multimedia

Architecture

• Sensor node = Sensing unit + Processing unit + Communication unit + Power unit
• Network layers: Sensor nodes → Cluster Heads → Base Station → Internet → User
• Topologies: Flat, Hierarchical/Cluster-based, Tree-based
• LEACH is a classic cluster-based protocol with random CH rotation

Physical Layer

• ISM bands: 868 MHz (Europe), 915 MHz (Americas), 2.4 GHz (Global)
• Modulation: BPSK (868/915 MHz), O-QPSK (2.4 GHz) in IEEE 802.15.4
• IEEE 802.15.4: data rate 250 kbps at 2.4 GHz, uses DSSS
• Propagation: free space (d²), two-ray (d⁴)
• Energy: TX dominant; idle listening nearly as costly as RX

MAC Layer

• Energy waste: idle listening, overhearing, collision, control overhead
• TDMA: no collision, needs sync, energy-efficient
• CSMA/CA: no sync needed, flexible, collisions possible
• S-MAC: periodic sleep/listen, virtual clusters, RTS/CTS during listen
• IEEE 802.15.4 MAC: beacon-enabled (superframe, CAP+CFP) and non-beacon (unslotted CSMA-CA)
• Z-MAC: hybrid CSMA+TDMA

Link Layer

• CRC: error detection (IEEE 802.15.4 uses CRC-16)
• FEC: error correction (Hamming, Reed-Solomon) — no retransmission needed
• ARQ: Stop-and-Wait (simple, low memory), Go-Back-N, Selective Repeat (most efficient)
• RSSI: received power level; LQI: link quality (0–255); PRR: packet reception rate
• WSN links are lossy → link-layer reliability is critical

Expected Exam Questions

PYQs will be added after analysis — check back soon.

These notes were compiled by Deepak Modi
Last updated: May 2026