UNIT 2: WIRELESS NETWORKS – SHORT NOTES

1. INTRODUCTION & WIRELESS CHANNEL CHARACTERISTICS

Wireless Medium Characteristics:

• Bandwidth Constraints: Limited and shared spectrum; regulated by bodies (e.g., FCC, ITU). Requires efficient modulation and multiple access techniques.

• Interference: From other users (co-channel, adjacent channel) and natural sources (noise). Degrades Signal-to-Interference-plus-Noise Ratio (SINR).

• Signal Propagation Effects:

  • Path Loss: Signal strength decreases with distance (∝ $$\displaystyle d^n $$, where $n$ is path loss exponent).

  • Shadowing: Large-scale signal fluctuations due to obstacles (buildings, hills). Modeled by log-normal distribution.

  • Multipath Fading: Constructive/destructive interference from multiple signal paths (delays, phases). Causes flat fading (if symbol duration >> delay spread) or frequency-selective fading (if symbol duration < delay spread). Leads to Inter-Symbol Interference (ISI).

  • Doppler Shift: Frequency shift due to relative motion between transmitter/receiver or scatterers. $$\displaystyle f_d = \frac{v}{\lambda} \cos \theta $$, where $v$ is velocity, $\lambda$ wavelength, $\theta$ angle. Causes time-selective fading (channel varies over time).

[!TIP] Exam Focus: Be ready to explain how multipath causes ISI and how OFDM combats it. Doppler shift's impact on high-mobility scenarios (e.g., trains, vehicles) is a common question.

2. WIRELESS LOCAL AREA NETWORKS (WLANs) – IEEE 802.11

Protocol Architecture (Layered Structure)

Similar to OSI/ TCP-IP model. Key layers:

• Physical Layer (PHY): Defines modulation, coding, frequency bands (2.4 GHz, 5 GHz).

• MAC Layer: Controls access to shared medium. Includes MAC Management sub-layer.

PHY Specifications & Technologies

• DSSS (Direct Sequence Spread Spectrum): Spreads signal over wider band using pseudo-noise code. Used in 802.11b (1-11 Mbps).

• OFDM (Orthogonal Frequency Division Multiplexing): Splits high-rate data into parallel low-rate streams on orthogonal subcarriers. Robust against frequency-selective fading. Core of 802.11a/g/n/ac/ax (54 Mbps to multi-Gbps).

MAC Layer Functions

• CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance): "Listen before talk." If medium idle for DIFS, transmit. If busy, backoff (random slot time). Uses binary exponential backoff.

• MAC Management:

  • Association/Reassociation: Station joins/roams within an ESS (Extended Service Set).

  • Authentication: verifies station identity (Open System or Shared Key).

  • Synchronization: Beacon frames from AP.

• Frame Formats: Include MAC headers (addresses: DA, SA, BSSID), frame control, duration/ID, payload, FCS.

Hidden & Exposed Terminal Problems

[!TIP] Exam Focus: Diagrammatically explain hidden/exposed terminal problems with 4 nodes (A-B-C-D). RTS/CTS handshake sequence is frequently asked.

Comparison: IEEE 802.11 vs. HIPERLAN

802.11 & Bluetooth Interface/Coexistence

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

• Solutions:

  1. Frequency Hopping Spread Spectrum (FHSS): Bluetooth hops 1600 times/sec across 79 channels. 802.11 uses fixed channels.

  2. Adaptive Frequency Hopping (AFH): Bluetooth identifies and avoids 802.11 channels in use.

  3. Time Division: Coordinate activity (e.g., Bluetooth uses slots; 802.11 can be silent during Bluetooth transmission).

  4. Physical Separation: Use 5 GHz band for 802.11 (802.11a/n/ac/ax) to avoid overlap.

3. WIRELESS METROPOLITAN AREA NETWORKS (WMANs) – WiMAX (IEEE 802.16)

Fixed vs. Mobile WiMAX (Key Differences)

Key Mobile WiMAX Enhancements for Mobility

• Handover Mechanisms: Macro-diversity Handover (MDHO): Multiple BSs transmit same data during handover. Fast Base Station Switching (FBSS): Active set of BSs, fast switch.

• Power Management: Sleep Mode: Periodic awake/power save. Idle Mode: Reduced network registration, paging.

• Link Adaptation: Adaptive Modulation and Coding (AMC) per subchannel based on channel conditions.

OFDM-MIMO in WiMAX

• OFDM: Combats ISI from multipath by turning frequency-selective channel into flat fading subchannels. Provides diversity.

• MIMO (Multiple-Input Multiple-Output): Uses multiple antennas at TX/RX.

  • Spatial Multiplexing: Increases data rate (capacity ∝ min($$\displaystyle N_t $$, $$\displaystyle N_r $$)).

  • Diversity Gain: Improves reliability (e.g., Alamouti coding).

• OFDM-MIMO Integration: MIMO operates on each OFDM subcarrier. Simplifies equalization. WiMAX uses STC (Space-Time Coding) and beamforming. Addresses channel variability by adapting per subcarrier and spatial streams.

4. CELLULAR MOBILE NETWORKS (3G/4G)

4.1 Universal Mobile Telecommunications System (UMTS)

• Network Architecture:

  • UE (User Equipment): Mobile phone/device.

  • Access Network (UTRAN): Node B (base station) + RNC (Radio Network Controller). RNC manages radio resources, handovers, connects to Core Network.

  • Core Network (CN): Circuit-Switched (CS): MSC, VLR, HLR (voice, SMS). Packet-Switched (PS): SGSN, GGSN (data).

• Component Interaction (Data Session):

  1. UE attaches via Node B → RNC.

  2. RNC routes to SGSN (PS) or MSC (CS).

  3. SGSN connects to external packet network via GGSN.

  4. GGSN assigns IP address, tunnels packets to UE.

• Evolution from GSM:

  | Feature | GSM (2G) | UMTS (3G) | | :--- | :--- | :--- | | Air Interface | TDMA/FDMA (GSM) | WCDMA (CDMA) | | Peak Data Rate | ~9.6 kbps (GPRS: ~114 kbps) | 2 Mbps ( indoor), 384 kbps (mobile) | | Access | Circuit-switched (voice), packet-switched (GPRS) | Packet-switched (PS) core for all services | | Services | Voice, SMS, low-rate data | Mobile broadband, video calling, multimedia |

4.2 Long-Term Evolution (LTE) / E-UTRAN

• E-UTRAN Architecture (Simplified "Flat"):

  • eNodeB (eNB): Single node replaces Node B + RNC. Controls radio resources, scheduling, handovers, connects directly to Core Network (EPC). No RNC.

  • EPC (Evolved Packet Core):

    • MME (Mobility Management Entity): Signaling, authentication, mobility (idle mode paging, handover decisions).

    • S-GW (Serving Gateway): Data anchor, local mobility anchor, packet routing/forwarding.

    • P-GW (PDN Gateway): External network interface, IP address allocation, policy enforcement, charging.

• Mobility Management & Resource Allocation:

  • Mobility: Handovers controlled by eNBs (X2 interface) or MME (S1 interface). Idle mode tracking via TAU (Tracking Area Update).

  • Resource Allocation: Dynamic, scheduler-based in eNB. Uses OFDMA (downlink) / SC-FDMA (uplink). Allocates Resource Blocks (RBs) in time-frequency grid based on channel conditions (AMC) and QoS.

4.3 3GPP (3rd Generation Partnership Project)

• Role: Global standards organization developing specifications for mobile telecommunications (GSM, UMTS, LTE, 5G NR). Collaboration between ARIB (Japan), ATIS (USA), CCSA (China), ETSI (Europe), TSDSI (India), TTA (Korea).

• Key Objectives for LTE/LTE-A:

  1. High Performance: Peak rates (100 Mbps DL / 50 Mbps UL), low latency (<10 ms).

  2. Simplified Architecture: All-IP, flat network (E-UTRAN/EPC), reduced cost.

  3. Flexible Spectrum: Operates in 1.4 MHz to 20 MHz blocks, paired/unpaired.

  4. Seamless Mobility: Optimized handovers, support for high speeds.

  5. Backward Compatibility: With 2G/3G (CS fallback).

5. WIRELESS PERSONAL AREA NETWORKS (WPANs)

Bluetooth Technology

• Piconet Topology:

  • Master-Slave structure. 1 master, up to 7 active slaves.

  • Master controls clock, hopping sequence.

  • Time-division duplex (TDD): Master transmits in even slots, slaves in odd slots.

  • Coverage: ~10 m (Class 2).

• Scatternet Topology:

  • Multiple interconnected piconets.

  • A device can be master in one piconet, slave in another (bridge).

  • Increases total nodes, coverage, but adds complexity (synchronization, scheduling).

• Comparison: Piconet vs. Scatternet

  | Feature | Piconet | Scatternet | | :--- | :--- | :--- | | Topology | Single star | Multiple interconnected stars | | Scalability | Limited (≤8 active devices) | Higher (many devices) | | Coverage | Limited by master | Extended via bridges | | Device Interaction | Simple, master-controlled | Complex, time-sharing between piconets | | Complexity | Low | High (synchronization, interference) |

IEEE 802.15 WPAN Standards & Applications

• 802.15.1: Bluetooth (legacy).

• 802.15.4: Low-Rate WPAN (LR-WPAN). Basis for Zigbee, WirelessHART, MiWi. Low power, low data rate (250 kbps), long battery life. Used in IoT, home automation, sensor networks.

• 802.15.3: High-Rate WPAN (HR-WPAN). For multimedia (video, audio). Uses TDMA, QoS.

• 802.15.6: Body Area Networks (BAN). For medical/implant devices. Ultra-low power, short range.

Zigbee Technology: Architecture & Use Cases

• Architecture: Based on 802.15.4 PHY/MAC. Adds Network (NWK) and Application (APL) layers.

  • Topologies: Star, Tree, Mesh (most robust, self-healing).

  • Device Types: Coordinator (forms network), Router (extends network), End Device (low power, sleeps).

• Use Cases: Home automation (lights, thermostats), industrial monitoring, smart metering, healthcare monitoring. Low power, low data rate, large network size (65k+ nodes).

6. WIRELESS SENSOR NETWORKS (WSNs)

WSN Architecture & Components

[Sensor Nodes] (Sense, Process, Transmit)

        |

        | (Multi-hop, wireless)

        |

[Sink/Base Station] (Aggregates data)

        |

        | (Wired/Long-range wireless)

        |

[Gateway] (Connects to Internet/Backbone)

        |

[User/Control Center]

• Sensor Node: Microcontroller, sensors, transceiver, power source (battery). Resource-constrained (CPU, memory, energy, bandwidth).

• Sink/Base Station: More powerful; collects data from network, may perform data fusion.

• Gateway: Interface to external networks (Internet, satellite).

WSN vs. Traditional Wired Networks

Underwater Wireless Sensor Networks (UWSNs)

• Architecture: Sensor nodes deployed underwater (anchored or drifting). Sink may be surface buoy or autonomous underwater vehicle (AUV). Communication via acoustic waves (primary), RF/optical for short ranges.

• Main Applications:

  • Oceanographic data collection (temperature, salinity).

  • Environmental monitoring (pollution, oil spills).

  • Disaster prevention (tsunami detection).

  • Military (surveillance, mine detection).

  • Navigation (AUV positioning).

• Specific Challenges:

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

  • Limited Bandwidth: Narrow bandwidth (tens of kbps).

  • High Bit Error Rate: Multipath, Doppler, noise.

  • Node Mobility: Drifting nodes, 3D topology.

  • Energy Constraints: Battery replacement difficult; energy harvesting limited.

  • Limited Storage & Processing.

Routing Protocols in WSNs

Topology Management in WSNs

• Definition: Controlling network connectivity by activating/deactivating nodes or adjusting transmission power to maintain a connected topology while conserving energy.

• Importance:

  • Energy Efficiency: Turns off redundant nodes (sleep scheduling).

  • Network Longevity: Prolongs system lifetime.

  • Robustness: Maintains connectivity despite node failures.

  • Coverage: Ensures monitored area remains covered.

  • Techniques: Topology Control (power adjustment), Connected Dominating Set (CDS) for backbone, sleep/wakeup scheduling.

Security Challenges in WSNs

• Threats/Vulnerabilities:

  • Node Capture: Physical access → key extraction.

  • Eavesdropping: Passive listening on wireless link.

  • Message Tampering/Replay: Altering/injecting packets.

  • Sybil Attack: Node pretends to have multiple identities.

  • Wormhole/Sinkhole: Attracts/redirects traffic.

  • Denial-of-Service (DoS): Jamming, resource exhaustion.

• Techniques for CIA:

  • Confidentiality: Symmetric-key cryptography (e.g., AES) due to resource constraints. Key management is critical.

  • Integrity: Message Authentication Codes (MACs) (e.g., CBC-MAC).

  • Authenticity: Pairwise keys, public-key (used sparingly, e.g., for key establishment), broadcast authentication (e.g., μTESLA).

  • Other: Secure routing protocols, intrusion detection, key predistribution schemes.

Sensor Node Technologies & Network Classification

• Node Tech: MEMS sensors, low-power MCUs (e.g., ARM Cortex-M), RF transceivers (e.g., CC2420 for 802.15.4), energy sources (batteries, solar).

• Network Classification:

  • By Structure: Flat (homogeneous), Hierarchical (clustered) (e.g., LEACH), Location-based.

  • By Communication: Single-hop (star), Multi-hop.

  • By Application: Event-driven, Time-driven, Query-based, Hybrid.

Coverage & Placement Strategies

• Goal: Ensure complete coverage of monitored area with minimum overlap (to save energy) and connectivity.

• Placement: Deterministic (planned, for static fields), Random (aerial drop, disaster).

• Strategies: Grid placement, probabilistic models. Use coverage algorithms to schedule sleep/wake cycles while maintaining coverage/connectivity.

Applications of WSNs

• Environmental: Forest fire detection, precision agriculture, habitat monitoring.

• Healthcare: Patient monitoring, drug delivery.

• Industrial: Structural health monitoring, inventory control, smart buildings.

• Military: Surveillance, target tracking, battlefield monitoring.

• Smart Cities: Traffic monitoring, waste management.

7. INTERNET OF THINGS (IoT)

IoT Architecture (Layered View)

1. Perception Layer (Physical)

   - Sensors, Actuators, RFID, QR Codes

   - Data acquisition from physical world.

2. Network Layer (Transport)

   - Gateways, networks (Wi-Fi, Bluetooth, Zigbee, LPWAN, Cellular).

   - Data transmission to processing centers.

3. Middleware/Processing Layer (Service)

   - Cloud/Edge computing, data storage, analytics.

   - Device management, service discovery.

4. Application Layer

   - End-user applications (smart home, smart city, industrial IoT).

   - APIs, dashboards.

5. Business Layer

   - Business models, analytics, decision-making.

Key Components of IoT Systems

1. Things/Devices: Sensors, actuators, embedded systems.

2. Connectivity: Communication protocols & networks (short/long-range).

3. Data Processing: Cloud platforms (AWS IoT, Azure IoT), edge computing.

4. User Interface: Mobile apps, web portals.

5. Security: Device security, data encryption, authentication.

Main Design Principles & Capabilities

• Principles: Interoperability, Scalability, Security/Privacy by Design, Energy Efficiency, Modularity.

• Required Capabilities: Unique identification (IP, EPC), Sensing/Actuation, Connectivity, Data analytics, Autonomy (self-* properties), Manageability.

Emerging IoT Standards for Networking Engineers

• LPWAN: LoRaWAN (long-range, low-power), NB-IoT (narrowband cellular), Sigfox.

• IP-based: 6LoWPAN (IPv6 over Low-Power WPAN), CoAP (Constrained Application Protocol - RESTful for devices).

• Messaging: MQTT (lightweight publish-subscribe).

• Frameworks: AllJoyn, IoTivity (device interoperability).

• 5G: mMTC (massive Machine-Type Communications) for massive IoT.

Sensor Body Area Network (BAN) Case Study

• Implementation Example: Chronic Disease Monitoring.

• Sensors: ECG patch, glucose monitor, blood pressure cuff, motion sensor (accelerometer).

• Architecture:

  • On-body sensors (wearable/implantable) → Body Hub (smartphone/device) via Bluetooth Low Energy (BLE) / Zigbee.

  • Body Hub aggregates, pre-processes data → transmits via Wi-Fi/4G/5G to Cloud/Medical Server.

  • Application: Doctor's dashboard, patient alerts, emergency response.

• Challenges: Ultra-low power, reliability, security/privacy (HIPAA compliance), interoperability of medical devices, regulatory approval.

8. PHYSICAL LAYER ADVANCED TECHNIQUES

SISO vs. MIMO Systems

Advantages of MIMO over SISO

1. Higher Data Rates (Spatial Multiplexing): Parallel transmission of independent data streams → capacity increase linearly with $$\displaystyle \min(N_t, N_r) $$.

2. Improved Signal Reliability (Diversity): Multiple copies of signal via different paths → reduces fading impact (e.g., Alamouti STC).

3. Extended Coverage (Beamforming): Focuses energy in specific direction → increases SNR at receiver.

4. Interference Suppression: Can null out interference from specific directions.

OFDM (Orthogonal Frequency Division Multiplexing)

• Principle: High-rate serial data stream converted to parallel low-rate streams modulated onto orthogonal subcarriers.

  • Orthogonality: Subcarrier spacing $$\displaystyle \Delta f = 1/T_s $$, where $$\displaystyle T_s $$ is symbol duration. Prevents ICI (Inter-Carrier Interference) even with overlapping spectra.

• Transmitter Block Diagram:

  
  Serial Data → Serial-to-Parallel → QAM/PSK Mapper → IFFT (N-point) → Parallel-to-Serial → Add CP → DAC → RF → Antenna
  
  

• Receiver Block Diagram:

  
  Antenna → ADC → Remove CP → Serial-to-Parallel → FFT (N-point) → Channel Equalization → Demapper → Parallel-to-Serial → Data
  
  

• Key Feature: Cyclic Prefix (CP): Copy of end of symbol prepended. Turns linear convolution (multipath) into circular convolution, allowing simple single-tap equalization per subcarrier in frequency domain. Combats ISI.

OFDM-MIMO Integration

• How it Addresses Challenges:

  1. Channel Variability (Frequency-Selective Fading): OFDM converts wideband channel into many flat-fading subcarriers. MIMO processing (e.g., eigen-beamforming) can be applied per subcarrier, adapting to its specific channel state.

  2. Inter-Symbol Interference (ISI): OFDM's CP eliminates ISI between OFDM symbols. MIMO's spatial processing operates on ISI-free OFDM symbols.

• Benefits in Wireless (LTE, WiMAX):

  • High Spectral Efficiency: Combines MIMO's spatial multiplexing with OFDM's dense packing.

  • Robustness: Handles multipath and mobility better than single-carrier MIMO.

  • Flexible Resource Allocation: Can assign different MIMO modes (spatial multiplexing, diversity, beamforming) to different users/subcarriers based on channel conditions.

9. MEDIUM ACCESS CONTROL (MAC) LAYER ISSUES

General Challenges in Wireless MAC

• Hidden Terminal Problem: (See Section 2).

• Exposed Terminal Problem: (See Section 2).

• Collision vs. Capture: In wireless, collisions may not be detectable (can't hear while transmitting). Capture effect (stronger signal wins) is possible but not reliable.

• Limited Bandwidth & Energy: MAC must be efficient to conserve power and bandwidth.

• Asymmetric Links: Different link qualities in opposite directions.

CSMA/CD Limitations in Wireless

• CSMA/CD (Wired Ethernet): Listen while transmitting → collision detection → abort transmission.

• Why it Fails in Wireless:

  1. Inability to Detect Collisions: Signal strength from distant nodes is below noise floor (capture effect). Transmitter cannot hear collision while transmitting.

  2. Hidden Terminal Problem: Collision may occur at receiver, not at transmitter.

  3. Signal Attenuation: Transmission power >> reception sensitivity.

• Result: Wireless uses CSMA/CA (Collision Avoidance) instead.

Comparison of MAC Protocols

10. NETWORK LAYER: MOBILITY, ROUTING & ADDRESSING

10.1 Mobile IP

• Components:

  • Mobile Node (MN): Device changing point of attachment.

  • Home Agent (HA): Router in MN's home network. Tunnels packets to MN's care-of address.

  • Foreign Agent (FA): Router in visited network. Provides care-of address (CoA) to MN, may deliver tunneled packets.

  • Correspondent Node (CN): Communication partner.

• Data Forwarding Process (Triangle Routing):

  1. CN sends packet to MN's permanent home address.

  2. HA intercepts packet, encapsulates (tunnels) it to MN's CoA (registered with HA).

  3. FA (or MN itself if CoA is co-located) decapsulates and delivers to MN.

  4. MN sends packets directly to CN (source routing or reverse tunneling if CN doesn't support MIP).

• Optimizations:

  • Route Optimization: MN sends binding updates to CN. CN can then directly tunnel packets to MN's CoA, avoiding triangle routing (shorter path, less load on HA).

  • Foreign Agent Care-of Address (FACoA): FA assigns CoA → simplifies MN's processing.

  • Co-located Care-of Address: MN obtains temporary IP from visited network (e.g., via DHCP) → no FA needed.

10.2 Mobility Management

• Concepts:

  • Handover/Handoff: Process of transferring an ongoing session from one access point/base station to another as MN moves.

  • Location Update: MN informs network of its new location (e.g., new cell, new tracking area). Signaling overhead.

  • Paging: Network broadcasts paging messages in a group of cells (paging area) to locate an idle MN. Reduces location update frequency.

• Importance: Enables seamless communication, session continuity, efficient resource utilization (paging), and network scalability.

10.3 Addressing: IPv4 vs. IPv6

11. TRANSPORT LAYER PROTOCOLS FOR WIRELESS

Limitations of Traditional TCP in Wireless

• High Packet Loss: Due to bit errors (not congestion) → TCP interprets as congestion → unnecessary cwnd reduction.

• Variable Latency & Handovers: Causes spurious timeouts or duplicate ACKs → degrades throughput.

• Asymmetric Links: Uplink/downlink capacity mismatch.

• Frequent Disconnections: Breaks connections.

TCP Variants & Enhancements

Congestion Window Management

• Principle: TCP sender maintains cwnd (congestion window) limiting unacknowledged data. Adjusted based on congestion signals (loss, ECN, RTT increase).

• Phases:

  • Slow Start: cwnd doubles per RTT (exponential growth) until ssthresh.

  • Congestion Avoidance: cwnd increases by ~1 per RTT (linear).

  • Fast Retransmit/Recovery: On 3 dupACKs, set ssthresh = cwnd/2, cwnd = ssthresh + 3 (Reno) or ssthresh (Tahoe).

• Practical Consequences in Wireless:

  • Throughput Collapse: Wireless loss triggers cwnd reduction → throughput drops significantly.

  • Starvation: After spurious timeout, cwnd drops to 1 → slow recovery.

  • Solutions: Split connections (I-TCP), explicit loss notification (M-TCP), delay-based (Vegas), wireless-aware TCP (W-TCP, TCP-Westwood+).

12. SECURITY IN WIRELESS NETWORKS

Security Issues Specific to WSNs

• Node Capture Attack: Physical access → extract keys.

• False Data Injection: Malicious node sends fake sensor readings.

• Sinkhole/HELLO Flood: Attractor node attracts/redirects traffic.

• Wormhole: Tunnels packets between distant points → disrupts routing.

• Selective Forwarding: Drops specific packets.

• Resource Exhaustion: Sends requests to drain battery.

• Lack of Physical Protection: Nodes deployed in open/unattended areas.

• Scalability: Security mechanisms must be lightweight for thousands of nodes.

Security Mechanisms in WSNs

• Encryption: Symmetric-key (AES, RC5) due to resource limits. Key management is critical (pre-distribution, key predistribution schemes like q-composite, polynomial-based).

• Authentication: Message Authentication Codes (MACs) (e.g., CBC-MAC) for data integrity & source authentication. Broadcast authentication (e.g., μTESLA using delayed key disclosure).

• Key Management: Key predistribution (keys loaded before deployment), key establishment (Ethernet, public-key for initial setup), key refreshment.

• Secure Routing: Secure AODV, INSENS (intrusion-tolerant routing).

• Intrusion Detection: Local monitoring, anomaly detection.

General Wireless Security Considerations

• Eavesdropping: Wireless medium is open → encryption essential (WPA2/WPA3 for Wi-Fi, LTE encryption).

• Jamming: Deliberate interference at PHY layer → frequency hopping, spread spectrum, detection.

• Man-in-the-Middle: Attacker relays/alters communication → mutual authentication (certificates, pre-shared keys).

• Unauthorized Access: Authentication (802.1X, EAP), MAC filtering (weak).

• Denial-of-Service: Flooding, resource exhaustion attacks → rate limiting, filtering, secure bootstrapping.

13. SPECIALIZED SYSTEMS & SHORT NOTE TOPICS

Wireless ATM (Asynchronous Transfer Mode)

• Architecture: ATM cells (53 bytes) transported over wireless links. Requires Wireless Access Point (WAP) that interfaces between wireless mobile terminals and fixed ATM network. Handles radio resource management, handover (ATM VPC/VCI switch), error correction (due to higher BER than fiber).

• High-Speed Data Transmission: ATM provides QoS classes (CBR, VBR, ABR, UBR) for different traffic types. Fixed-length cells simplify switching. Wireless ATM aimed to extend these QoS guarantees to mobile users.

• Major Research Challenges:

  • High Bit Error Rate (BER): Wireless channels need robust FEC, ARQ → conflicts with ATM's low latency.

  • Handover with QoS Guarantee: Maintaining cell loss rate, delay during handover.

  • Radio Resource Allocation: Efficiently sharing limited bandwidth among users with different QoS.

  • Power Control & Mobility Management.

  • Complexity & Cost: WAPs and mobile terminals became too expensive; overtaken by IP-based solutions (4G/5G).

GPS-Aided GEO Augmented Navigation (GAGAN)

• Purpose: Satellite-Based Augmentation System (SBAS) for Indian Regional Navigation Satellite System (IRNSS) and GPS. Improves accuracy, integrity, availability of GNSS signals over a defined service volume (India & surrounding region).

• Augmentation of GPS:

  • Accuracy: Corrects satellite orbit/clock errors, ionospheric delays via reference stations on ground. Provides correction data (via geostationary satellite).

  • Integrity: Monitors satellite health; alerts user within 6 seconds if satellite is faulty (critical for aviation).

  • Availability: Increases number of usable satellites in view.

• Implementation: ISRO + AAI. Uses GEO satellites (GSAT series) to broadcast correction signals. Supports Category I precision approach for aviation.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

• Principle: Wired Ethernet MAC. Stations sense carrier; if idle, transmit. Listen while transmitting; if collision detected (signal > threshold), abort transmission immediately and send jam signal. Stations wait random backoff time before retrying.

• Limitation in Wireless: Cannot detect collisions while transmitting due to signal attenuation and hidden terminals. Hence, CSMA/CA is used.

OFDM-MIMO as Combined Technique

• Integration: MIMO spatial processing applied to each OFDM subcarrier in frequency domain.

• Benefits:

  • Simplifies MIMO Equalization: Frequency-selective channel becomes flat per subcarrier → single-tap equalizers.

  • Combats ISI & Fading: OFDM handles delay spread; MIMO provides diversity/multiplexing.

  • Flexible Adaptation: Can assign different MIMO modes (spatial streams, coding rate) per subcarrier/user based on Channel State Information (CSI).

• Used in: WiMAX (802.16e/m), LTE/LTE-Advanced, Wi-Fi (802.11n/ac/ax).

Underwater WSN (UWSN)

• See Section 6.3 (UWSN) for full details.

• Key Distinction from Terrestrial WSN: Acoustic communication (vs. RF) → high delay, low bandwidth, high error, mobility. Requires specialized routing (e.g., depth-based, vector-based), localization (range-free), and MAC protocols (handshake-based due to long propagation).

IEEE 802.15 WPAN Standards Overview

• 802.15.1: Bluetooth (legacy).

• 802.15.2: Coexistence with 802.11 (guidelines).

• 802.15.3: High-Rate WPAN (HR-WPAN) for multimedia.

• 802.15.4: Low-Rate WPAN (LR-WPAN). Basis for Zigbee, WirelessHART. Defines PHY (O-QPSK, BPSK) and MAC (CSMA/CA, beacon-enabled).

• 802.15.5: Mesh networking for WPANs.

• 802.15.6: Body Area Networks (BAN) for medical/implant. Uses narrowband, ultra-wideband (UWB).

• 802.15.7: Visible Light Communication (VLC) WPAN.

END OF UNIT 2 NOTES