UNIT 4: WIRELESS NETWORKS - Short Notes

(Based on RGPV Past Papers 2022–2025)

I. FOUNDATIONS OF WIRELESS COMMUNICATION

Characteristics of Wireless Medium:

• Shared broadcast channel (multiple users compete).

• Mobility support (nodes can move).

• Limited bandwidth and high path loss.

• Susceptible to interference, noise, and multipath.

• Power-constrained devices (battery life critical).

Channel Impairments:

1. Multipath Propagation:

   • Signals arrive via reflection, diffraction, scattering → multiple copies with different delays.

   • Causes delay spread and inter-symbol interference (ISI).

   • Mitigation: Equalization, OFDM.

2. Doppler Shift:

   • Frequency shift due to relative motion: $$\displaystyle f_d = \frac{v}{\lambda} $$, where $v$ = relative velocity, $\lambda$ = wavelength.

   • Causes fast fading (channel varies within symbol duration).

3. Fading:

   • Small-scale: Fast/slow fading (relative to symbol rate), flat/frequency-selective (relative to bandwidth).

   • Large-scale: Path loss (log-distance), shadowing (obstructions).

Medium Access Challenges:

• Hidden Terminal Problem: Two nodes cannot sense each other but interfere at a common receiver (e.g., A and C both send to B).

  • Solution: RTS/CTS handshake (virtual carrier sensing).

• Exposed Terminal Problem: A node defers transmission because it senses a transmission that does not interfere with its intended receiver (e.g., B transmits to A, C senses B but could transmit to D).

  • Mitigation: RTS/CTS, directional antennas, adjusted carrier sense thresholds.

[!TIP] Hidden terminals cause collisions; exposed terminals reduce spatial reuse. 802.11 uses RTS/CTS to mitigate both but adds overhead.

II. WIRELESS STANDARDS AND NETWORK ARCHITECTURES

A. IEEE 802.11 WLAN Family

Protocol Architecture (Layers):

• LLC (Logical Link Control): Common interface to upper layers, error control.

• MAC (Medium Access Control): Access to shared medium, framing, security.

• PHY (Physical): Modulation, coding, transmission.

Physical Layer Technologies:

• FHSS (Frequency-Hopping Spread Spectrum): 802.11 legacy (1–2 Mbps).

• DSSS (Direct-Sequence Spread Spectrum): 802.11b (up to 11 Mbps).

• OFDM (Orthogonal Frequency Division Multiplexing): 802.11a/g/n/ac (54 Mbps to >1 Gbps).

• HR-DSSS (High-Rate DSSS): 802.11b+ (up to 22 Mbps).

• MIMO-OFDM: 802.11n/ac (multiple spatial streams).

MAC Layer Functions:

• DCF (Distributed Coordination Function): Primary access method, CSMA/CA with binary backoff.

• PCF (Point Coordination Function): Optional, centralized polling (infrastructure mode).

• MAC Management: Scanning, authentication, association, power management.

MAC Frame Structure:

• Control Frames: RTS, CTS, ACK.

• Data Frames: Carry payload.

• Management Frames: Beacon, probe, authentication.

Comparison with HIPERLAN:

B. IEEE 802.16 (WiMAX)

• Fixed WiMAX (802.16-2004): Stationary terminals, OFDM/OFDMA, up to 75 Mbps.

• Mobile WiMAX (802.16e): Supports mobility (up to 120 km/h), handover, sleep modes.

• Key Mobility Enhancements:

  • Fast Handover: Predictive or reactive, with context transfer.

  • Power Management: Sleep and idle modes to conserve battery.

  • Mobility Classes: Different QoS for real-time/non-real-time traffic.

C. Cellular Systems: 2G to 4G Evolution

GSM to UMTS Evolution:

UMTS Architecture:

• Core Network (CN):

  • MSC (Mobile Switching Center): Circuit-switched call control.

  • VLR (Visitor Location Register): Temporary data for roaming subscribers.

  • HLR (Home Location Register): Permanent subscriber database.

  • SGSN (Serving GPRS Support Node): Packet-switched core, mobility management.

  • GGSN (Gateway GPRS Support Node): Connects to external IP networks.

• UTRAN (UMTS Terrestrial Radio Access Network):

  • Node B: Base station (radio transmission).

  • RNC (Radio Network Controller): Controls Node Bs, radio resource management, handovers.

• Interactions: Mobile station → Node B (Uu interface) → RNC (Iub) → CN (Iu-CS/Iu-PS). SGSN/GGSN handle packet data sessions.

E-UTRAN (LTE) Architecture:

• eNodeB (evolved Node B): Integrates RNC and Node B functions. Handles scheduling, radio resource control, handover decisions.

• Flat Architecture: No RNC; eNodeBs connect directly to EPC.

• EPC (Evolved Packet Core):

  • MME (Mobility Management Entity): Control plane, authentication, bearer management.

  • S-GW (Serving Gateway): User plane, data routing, handover anchor.

  • P-GW (Packet Data Network Gateway): Connects to external networks, IP allocation, policy enforcement.

• Interfaces: S1 (eNodeB–EPC), X2 (eNodeB–eNodeB for handover).

3GPP Role and Objectives:

• Role: Standardization body for GSM, UMTS, LTE, LTE-A (and 5G NR).

• LTE/LTE-A Design Principles:

  • All-IP network (no circuit switching).

  • Flat architecture (reduce latency).

  • High spectral efficiency (OFDMA, MIMO).

  • Flexible bandwidth (1.4–20 MHz).

D. HIPERLAN

• Key Features:

  • High data rate (up to 54 Mbps).

  • Uses OFDM, supports ad-hoc and infrastructure modes.

  • Quality of Service (QoS) via priority channels.

• Advantages over IEEE 802.11:

  • Better QoS for real-time traffic.

  • Higher data rates in early versions.

  • Different MAC (EY-NPMA) reduces collisions.

E. Wireless ATM

• Architecture: Mobile terminal ↔ base station ↔ ATM switch (wired).

• Components:

  • Mobile terminal (MT).

  • Base station (BS): radio access.

  • ATM switch: core network, cell relay.

• Enabling High-Speed Data:

  • ATM provides QoS (CBR, VBR, ABR).

  • Fixed-size cells (53 bytes) reduce processing delay.

• Major Research Challenges:

  • Mobility management (handover with QoS).

  • Wireless link errors (high BER).

  • Integration with existing ATM infrastructure.

III. ADVANCED RADIO TECHNOLOGIES

A. Antenna Systems

• SISO (Single-Input Single-Output): One antenna at transmitter and receiver.

• MIMO (Multi-Input Multi-Output): Multiple antennas at both ends.

• Advantages of MIMO:

  1. Spatial Multiplexing: Parallel data streams → capacity increase (up to $$\displaystyle \min(N_t, N_r) $$ times).

  2. Diversity: Multiple copies improve reliability (e.g., Alamouti code).

  3. Array Gain: Beamforming increases SNR.

• MIMO in LTE:

  • Downlink: up to 4x4 MIMO, MU-MIMO (multi-user).

  • Uplink: up to 2x2 MIMO.

B. Orthogonal Frequency Division Multiplexing (OFDM)

Transmitter Block Diagram:

1. Serial-to-parallel conversion.

2. QAM/PSK modulation per subcarrier.

3. IFFT (inverse FFT) → time-domain OFDM symbols.

4. Cyclic Prefix (CP) insertion → mitigate ISI.

5. Parallel-to-serial, RF upconversion.

Receiver Block Diagram:

1. RF downconversion, serial-to-parallel.

2. CP removal.

3. FFT → frequency-domain subcarriers.

4. QAM/PSK demodulation.

5. Parallel-to-serial.

Principle of Orthogonal Subcarriers:

• Subcarrier spacing $$\displaystyle \Delta f = \frac{1}{T} $$, where $T$ = symbol duration.

• Orthogonality: $$\displaystyle \int_0^T e^{j2\pi n\Delta f t} e^{-j2\pi m\Delta f t} dt = 0 $$ for $n \neq m$.

• No inter-carrier interference (ICI) under ideal conditions.

Advantages against Frequency-Selective Fading:

• Converts wideband frequency-selective channel into parallel flat-fading subchannels.

• Simple equalization (one-tap per subcarrier).

• Robust to multipath delay spread (CP absorbs ISI).

C. OFDM-MIMO Hybrid Systems

• Addressing Channel Variability: MIMO provides spatial diversity; OFDM handles frequency selectivity.

• Mitigating ISI: CP in OFDM eliminates inter-symbol interference; MIMO spatial streams are independently modulated per subcarrier.

• Combined Benefits:

  • High data rates via spatial multiplexing over many subcarriers.

  • Improved link reliability via diversity.

  • Efficient use of bandwidth (OFDM) and spatial resources (MIMO).

IV. MOBILITY MANAGEMENT

A. Mobile IP (IPv4 and IPv6)

Core Components:

• Home Agent (HA): Router in home network, tunnels packets to mobile node’s care-of address.

• Foreign Agent (FA): Optional, in visited network, provides care-of address and routing.

• Care-of Address (CoA): Temporary IP address in visited network (via FA or DHCP).

• Mobile Node (MN): Moves between networks.

• Correspondent Node (CN): Communicating peer.

Data Forwarding Process (Away from Home):

1. MN obtains CoA in foreign network.

2. MN registers CoA with HA (via FA if present).

3. CN sends packets to MN’s home address → HA intercepts, tunnels to CoA.

4. MN sends packets directly to CN (if route optimization) or via HA (triangular routing).

Optimizations:

• Triangular Routing: MN sends directly to CN using CoA as source; CN replies to home address → packets tunneled from HA.

• Route Optimization: MN sends binding update to CN; CN caches CoA and sends directly to MN, avoiding HA detour.

B. General Mobility Management Concepts

• Handover Types:

  • Horizontal: Same access technology (e.g., LTE to LTE).

  • Vertical: Different technologies (e.g., Wi-Fi to LTE).

  • Intra-RAT / Inter-RAT: Within/between radio access technologies.

• Handover Signaling:

  1. Measurement (MN reports signal strength).

  2. Decision (network triggers handover).

  3. Execution (resource allocation, data forwarding).

• Location Management:

  • Registration: MN updates location (VLR/HLR in cellular, HA in Mobile IP).

  • Paging: Network broadcasts paging message to locate MN.

V. TRANSPORT LAYER PROTOCOLS FOR WIRELESS

A. Traditional TCP

• Congestion Control Mechanisms:

  1. Slow Start: cwnd doubles per RTT until ssthresh.

  2. Congestion Avoidance: cwnd increases by 1 per RTT (additive increase).

  3. Fast Retransmit: 3 duplicate ACKs trigger retransmission without wait.

  4. Fast Recovery: After fast retransmit, halve cwnd, continue sending new data.

• Significance: Prevents network collapse due to congestion.

• Limitations in Wireless:

  • Packet loss due to channel errors misinterpreted as congestion → unnecessary cwnd reduction.

  • High RTT variations → poor RTT estimation.

  • Handover disruptions → timeouts.

B. TCP Variants for Wireless/Mobile Networks

C. Mobile TCP Adaptations

• Selective Retransmission (SACK): Retransmit only lost packets.

• Connection Freezing: Suspend TCP during handover, resume after.

• Delayed ACKs: Reduce ACK overhead.

• Explicit Congestion Notification (ECN): Mark packets instead of drop.

• TCP-Friendly Rate Control (TFRC): Rate-based for multimedia.

D. Congestion Window Management

• Principle: cwnd limits unacknowledged data. Throughput ≈ cwnd / RTT.

• Wireless Impact:

  • Packet losses (non-congestion) cause cwnd reduction → throughput drop.

  • Variable RTT → inaccurate cwnd adjustment.

• Consequences:

  • Underutilization of bandwidth.

  • Increased latency.

VI. WIRELESS SENSOR NETWORKS (WSNs)

A. WSN Architecture

• Sensor Node Components:

  • Sensing (transducer).

  • Processing (microcontroller).

  • Communication (radio transceiver).

  • Power (battery/energy harvesting).

• Network Topology:

  • Star: Single-hop to sink.

  • Multi-hop: Mesh, tree, cluster-based.

• Differences from Wired Networks:

  • Resource-constrained (energy, memory, CPU).

  • Dynamic topology (node failure, mobility).

  • Data-centric (query-based, not address-based).

  • Large-scale deployment (thousands of nodes).

B. WSN Applications and Use Cases

• Environmental Monitoring: Forest fires, pollution, agriculture.

• Military: Surveillance, target tracking, battlefield monitoring.

• Health: Patient monitoring, drug administration.

• Smart Homes: Automation, security.

• Industrial: Process control, inventory tracking.

C. Topology Management

• Importance:

  • Energy efficiency (network lifetime).

  • Coverage and connectivity.

  • Robustness to node failures.

• Techniques:

  • Clustering: Group nodes into clusters (e.g., LEACH), cluster heads aggregate data.

  • Sleep Scheduling: Nodes alternate between active/sleep to save energy (duty cycling).

D. Routing Protocols

Proactive (Table-Driven):

• DSDV (Destination-Sequenced Distance-Vector): Distance-vector with sequence numbers to avoid loops.

• OLSR (Optimized Link State Routing): Link-state with MPR (multi-point relays) to reduce overhead.

• Advantages: Immediate route availability.

• Limitations: High overhead (periodic updates), poor scalability.

Reactive (On-Demand):

• AODV (Ad-hoc On-Demand Distance Vector): RREQ/RREP flood, maintains routes only when needed.

• DSR (Dynamic Source Routing): Source routing, route caching.

• Advantages: Low overhead in stable networks.

• Limitations: Route discovery delay, flooding overhead.

Routing Metrics: Hop count, energy, latency, link quality.

E. Underwater Wireless Sensor Networks (UWSN)

• Architecture:

  • Sensor nodes (underwater, acoustic communication).

  • Surface gateway (relays to onshore).

  • Offshore control station (data processing).

• Main Applications:

  • Oceanographic data collection (temperature, salinity).

  • Defense (submarine detection, port security).

  • Disaster prevention (tsunami early warning).

• Specific Challenges:

  • High Propagation Delay: Acoustic speed ~1500 m/s → RTT seconds.

  • Limited Bandwidth: Tens of kHz due to absorption.

  • Node Mobility: Water currents cause drift.

  • Harsh Environment: Corrosion, pressure, biofouling.

  • Energy: Battery replacement difficult; energy harvesting limited.

F. Security in WSN

• Security Challenges:

  • Resource constraints (limited computation, energy).

  • Unattended deployment (physical attacks).

  • Adversarial environment (eavesdropping, jamming).

• Techniques:

  • Data Confidentiality: Encryption (AES-128, lightweight ciphers like SPECK).

  • Data Integrity: MACs (HMAC-SHA256), checksums, hash chains.

  • Data Authenticity: Symmetric/asymmetric cryptography, key management (e.g., LEAP+, TinySec).

G. Sensor Node Technologies

• Hardware Platforms:

  • Microcontrollers: MSP430, ARM Cortex-M.

  • Radios: Zigbee (CC2420), Bluetooth Low Energy, custom ASICs.

  • Sensors: Temperature, humidity, accelerometer.

• Energy Sources: Batteries (Li-ion), solar, vibration harvesters.

• Examples: TelosB, MicaZ, Imote2.

H. Coverage and Placement

• Practical Needs:

  • Full area coverage (no blind spots).

  • Network connectivity (to sink).

  • Fault tolerance (redundancy).

• Node Placement Strategies:

  • Grid: Deterministic, uniform coverage.

  • Random: Poisson point process, simple but may have holes.

  • Deterministic: Optimized for coverage/connectivity (e.g., force-based algorithms).

VII. INTERNET OF THINGS (IoT)

A. IoT Architecture

• Three-Layer Model:

  1. Perception Layer: Sensors/actuators, data acquisition.

  2. Network Layer: Gateways, communication networks (Wi-Fi, cellular, LPWAN).

  3. Application Layer: Services, analytics, user interfaces.

• Key Components:

  • Sensors/actuators.

  • Gateways (edge computing).

  • Cloud platforms (data storage, processing).

  • Applications (smart city, industrial IoT).

B. Design Principles and Capabilities

• Scalability: Support billions of devices.

• Interoperability: Heterogeneous devices/ protocols.

• Security: End-to-end encryption, authentication.

• Energy Efficiency: Low-power operation, duty cycling.

C. Emerging IoT Standards

D. Case Study: Sensor Body Area Network (BAN)

• Architecture:

  • Wearable sensors (ECG, glucose, motion).

  • Personal server (smartphone/ gateway).

  • Healthcare provider network (cloud).

• Applications:

  • Continuous health monitoring.

  • Fall detection for elderly.

  • Sports performance tracking.

• Implementation Challenges:

  • Interference (with other wireless devices).

  • Security (sensitive health data).

  • Battery life (continuous operation).

  • Comfort and wearability.

VIII. SHORT-RANGE WIRELESS TECHNOLOGIES

A. Bluetooth

• Piconet:

  • Star topology: 1 master, up to 7 active slaves.

  • Coverage: ~10 m (Class 2).

  • Device interaction: Master-slave, time-division duplex.

• Scatternet:

  • Interconnected piconets (devices participate in multiple piconets).

  • Scalability: More than 8 devices via bridges.

  • Complex scheduling (time-sharing between piconets).

• Frequency Hopping: 79 channels (1 MHz spacing), 1600 hops/sec.

• Data Rates:

  • BR/EDR: Up to 3 Mbps.

  • BLE (Bluetooth Low Energy): Up to 2 Mbps, lower power.

B. IEEE 802.15 WPAN

• Standards Overview:

  • 802.15.1: Bluetooth.

  • 802.15.4: Low-rate WPAN (Zigbee, 6LoWPAN).

  • 802.15.3: High-rate WPAN (Wireless USB).

  • 802.15.6: Body Area Networks.

• Use Cases:

  • 802.15.4: Sensor networks, home automation (250 kbps).

  • 802.15.3: Multimedia streaming (up to 100 Mbps).

C. Zigbee

• Architecture:

  • Coordinator: Forms network, stores network info.

  • Router: Extends network, forwards data.

  • End Device: Sleepy, communicates only with parent.

• Low-Power, Low-Data-Rate:

  • Data rate: 250 kbps (2.4 GHz).

  • Range: 10–100 m.

  • Applications: Home automation, industrial control.

D. Interoperability (802.11 and Bluetooth)

• Coexistence Challenges: Both operate in 2.4 GHz ISM band → interference.

• Techniques:

  • Adaptive Frequency Hopping (AFH): Bluetooth avoids Wi-Fi channels.

  • Time Division: Coordinate transmission times.

  • Power Control: Reduce transmit power to limit interference.

  • Packet Scheduling: Prioritize latency-sensitive traffic.

IX. NETWORK LAYER AND ADDRESSING

A. IPv4 vs. IPv6

B. TCP vs. UDP

X. SPECIALIZED TOPICS

A. GPS and GAGAN

• GPS-Aided GEO Augmented Navigation (GAGAN):

  • Satellite-based augmentation system (SBAS) using geostationary satellites.

  • Purpose: Improve accuracy, integrity, and availability for aviation/navigation in India (and other regions).

  • Provides correction signals for ionospheric errors, satellite orbit/clock errors.

  • Enables Category I/II precision approaches.

B. CSMA/CD

• Principles:

  • Carrier Sense: Listen before transmit.

  • Collision Detection: Monitor channel during transmission; if collision, abort.

  • Binary Backoff: Random wait time after collision.

• Relevance: Used in wired Ethernet (10BASE-T, 100BASE-TX).

• Contrast with CSMA/CA in Wireless:

  • Wireless cannot detect collisions (hidden terminal, capture effect).

  • Uses RTS/CTS and ACKs to avoid collisions.

C. WiMAX (Recap)

• Broadband Wireless Access: IEEE 802.16 standard.

• Key Features:

  • OFDMA (downlink/ uplink).

  • Fixed and mobile versions.

  • Up to 1 Gbps (theoretical), range up to 50 km.

  • QoS classes (UGS, rtPS, nrtPS, BE).

• Applications: Last-mile broadband, cellular backhaul, IoT.

Exam Tips:

• Focus on comparisons (e.g., proactive vs reactive routing, IPv4 vs IPv6, TCP variants).

• Diagrams are crucial for UMTS, E-UTRAN, OFDM, MIMO, WSN, IoT, Mobile IP.

• Formulas: Doppler shift $$\displaystyle f_d = v/\lambda $$, OFDM orthogonality $$\displaystyle \Delta f = 1/T $$, MIMO capacity $$\displaystyle C = \min(N_t, N_r) \log_2(1+\text{SNR}) $$.

• Short notes (3–4 marks) require concise definitions and key points; long answers (7 marks) need diagrams and detailed explanations.

• High-frequency topics: WSN routing, UMTS/LTE architecture, MIMO/OFDM, TCP variants, IoT standards, Mobile IP.