Digital Image Processing (EC-803 (B)) - Unit 4 Short Notes

UNIT 4: Wireless Communication Technologies

I. Wireless Local Area Networks (WLAN)

IEEE 802.11 Protocol Architecture

• Protocol Stack:

  • Physical Layer (PHY): Defines modulation, coding, and transmission characteristics for specific frequency bands (2.4 GHz, 5 GHz).

  • Medium Access Control (MAC) Layer: Controls access to the shared wireless medium. Implements CSMA/CA.

  • Logical Link Control (LLC) Layer: Provides interface to higher network layers (e.g., IP), handles error control and flow control.

• Physical Layer Technologies:

  • FHSS (Frequency-Hopping Spread Spectrum): Rapidly switches carrier frequency among many channels.

  • DSSS (Direct-Sequence Spread Spectrum): Spreads signal over a wider bandwidth using a pseudo-random code.

  • OFDM (Orthogonal Frequency Division Multiplexing): Splits data stream into multiple parallel subcarriers. Key for 802.11a/g/n/ac/ax. Provides high data rates and robustness against ISI.

• MAC Layer Functions: Frame delimiting, addressing, error checking (FCS), and coordination of channel access. The MAC sublayer handles association, authentication, and power management.

Medium Access Control in 802.11

• CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance):

  1. Sense Channel: If idle for DIFS (DCF Interframe Space), proceed.

  2. Random Backoff: If busy, wait random time (slots) to avoid collisions.

  3. Transmit: Send frame.

  4. ACK: Receiver sends ACK after SIFS (Short Interframe Space). No ACK implies collision/loss.

• Frame Types: Management (association, authentication), Control (RTS, CTS, ACK), Data.

• Hidden Terminal Problem: Node A & C cannot hear each other but both transmit to B, causing collision at B.

  • Mitigation: RTS/CTS (Request to Send / Clear to Send) handshake. Virtual carrier sensing via NAV (Network Allocation Vector).

• Exposed Terminal Problem: Node B transmits to A. Node C (near B, far from A) defers even though it could transmit to D without interfering with A.

  • Mitigation: More aggressive use of RTS/CTS, directional antennas.

CSMA/CD Protocol (Ethernet)

• Principle: Collision Detection (CD). Stations sense medium while transmitting.

• Operation:

  1. Listen before talk (Carrier Sense).

  2. Transmit while sensing. On collision, transmit jam signal.

  3. Binary Exponential Backoff: Wait random time k * slot time, where k doubles after each collision (up to limit).

• Key Difference from CSMA/CA: CD is possible in wired media (signal strength at sender). CA is mandatory in wireless (collision detection is difficult).

Comparison: IEEE 802.11 vs. HIPERLAN

Interface between 802.11 and Bluetooth

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

• Solutions:

  • Frequency Hopping: Bluetooth hops rapidly; 802.11 uses channels.

  • Packet Scheduling: Coordinate transmission times.

  • Physical Separation: Use different channels (e.g., 802.11 on channel 1, Bluetooth avoids).

  • Adaptive Frequency Hopping (AFH): Bluetooth detects and avoids 802.11 channels in use.

[!TIP] Exam Focus: CSMA/CA steps, RTS/CTS for hidden terminal, and the fundamental difference between CSMA/CA and CSMA/CD (collision avoidance vs. detection) are very frequent.

II. Cellular Mobile Communication Systems

Evolution from GSM to UMTS

UMTS Network Architecture

• UE (User Equipment): Mobile device (phone, modem).

• UTRAN (UMTS Terrestrial Radio Access Network):

  • Node B: Base Station (equivalent to BTS in GSM). Handles radio transmission/reception.

  • RNC (Radio Network Controller): Controls multiple Node Bs. Handles radio resource management, handover decisions, and mobility anchoring.

• Core Network (CN):

  • MSC (Mobile Switching Center): Circuit-switched domain (voice).

  • SGSN (Serving GPRS Support Node): Packet-switched domain (data). Mobility management, session management.

  • GGSN (Gateway GPRS Support Node): Gateway to external PDNs (Internet). IP address assignment, charging.

• Key Interfaces:

  • Iub: Between RNC and Node B (control & user plane).

  • Iu: Between RNC and Core Network (Iu-CS to MSC, Iu-PS to SGSN).

LTE and LTE-Advanced (3GPP Standards)

• 3GPP Role: Standards organization (partners: ARIB, ATIS, CCSA, ETSI, TSDSI, TTA, TTC). Defines specifications for GSM, UMTS, LTE, 5G.

• E-UTRAN (Evolved UTRAN) Architecture (All-IP, Flat):

  • eNodeB (eNB): Single node replaces RNC & Node B. Handles radio resource control, scheduling, and handover (no centralized RNC). Directly connects to EPC (Evolved Packet Core).

  • EPC Components:

    • MME (Mobility Management Entity): Control plane. Handles authentication, bearer management, UE mobility (idle mode tracking).

    • S-GW (Serving Gateway): User plane. Local mobility anchor, packet routing/forwarding, lawful intercept.

    • P-GW (Packet Data Network Gateway): User plane. External PDN connection point. IP address allocation, policy enforcement, charging.

• Mobility & Resource Allocation:

  • Hierarchy: eNB (scheduler) → MME (mobility control) → P-GW (PDN connection).

  • Handover: eNB-to-eNB (X2 interface) or via MME (S1 interface).

  • Resource Allocation: Dynamic scheduling by eNB in 1 ms TTI (Transmission Time Interval) subframes.

Mobility Management in Cellular Networks

• Handover (HO) Types:

  • Hard HO: Connection to target cell before breaking from source (UMTS, GSM).

  • Soft HO: Connection to target before breaking, macrodiversity (multiple cells serve UE) – CDMA only.

  • Softer HO: Soft HO within same Node B (different sectors).

• Location Management:

  • Location Update: UE informs network of new Location Area (LA) or Tracking Area (TA) when moving.

  • Paging: Network broadcasts paging message in LA/TA to locate idle UE for incoming call/data.

[!TIP] Key Distinction: UMTS has RNC (centralized control), LTE has flat architecture with eNB handling radio control directly. MME is control plane only; S-GW/P-GW are user plane.

III. MIMO and OFDM Technologies

SISO vs. MIMO Systems

Orthogonal Frequency Division Multiplexing (OFDM)

• Principle: High-rate data stream is split into N parallel low-rate streams, each modulating a subcarrier.

• Orthogonality: Subcarrier spacing Δf = 1/T (where T is symbol duration). No Inter-Carrier Interference (ICI).

$$ \text{Subcarrier Spacing: } \Delta f = \frac{1}{T_{sym}} $$

• Transmitter Block Diagram:

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

• Receiver Block Diagram:

  Antenna → ADC → Remove CP → Serial-to-Parallel → FFT → Channel Estimation/Equalization → Demapper → Parallel-to-Serial → Output Data

• Cyclic Prefix (CP): Copy of end of OFDM symbol prepended. Converts linear convolution to circular, eliminating ISI from multipath delays < CP length.

• Advantages:

  • Robust against multipath delay spread (ISI) via CP.

  • Narrowband fading affects only few subcarriers (easier equalization).

  • Efficient FFT/IFFT implementation.

  • Flexible spectrum shaping.

OFDM-MIMO Integration (MIMO-OFDM)

• How it Addresses Challenges:

  1. Channel Variability (Fading): Spatial diversity (e.g., Alamouti code) across MIMO antennas provides independent fading paths → reliable reception.

  2. ISI: OFDM's CP handles multipath delay spread within each MIMO stream.

• Application: LTE (DL: OFDMA, UL: SC-FDMA), WiMAX (802.16e/m), Wi-Fi (802.11ac/ax). Enables high data rates in frequency-selective fading channels.

[!TIP] Core Concept: OFDM handles frequency-selective fading (time domain ISI → frequency domain flat fading per subcarrier). MIMO adds spatial dimension for multiplexing/diversity. Together, they maximize capacity and reliability in mobile channels.

IV. Wireless Sensor Networks (WSN)

WSN Architecture and Node Components

• Sensor Node (Mote):

  • Sensing Unit: Transducer (temperature, humidity, etc.).

  • Processing Unit: Microcontroller/CPU.

  • Communication Unit: Radio transceiver (e.g., IEEE 802.15.4).

  • Power Unit: Battery (often irreplaceable), energy harvesting.

  • Memory: Small RAM/Flash.

• Network Deployment:

  • Star: All nodes to single sink/base station. Simple, single point of failure.

  • Mesh: Multi-hop, peer-to-peer. Robust, scalable.

  • Tree: Hierarchical, data converges to root (sink).

Routing Protocols in WSN

Topology Management

• Definition: Techniques to organize network structure (clusters, sleep schedules) to meet application goals.

• Importance: Conserves energy (primary constraint), reduces collisions, extends network lifetime, ensures coverage.

• Techniques:

  • Clustering: Elect Cluster Heads (CH). CHs aggregate data, reduce long-range transmissions. (e.g., LEACH).

  • Sleep Scheduling: Nodes alternate between active/sleep states to save energy while maintaining connectivity/coverage.

Security in WSN

• Challenges: Resource constraints (energy, computation, memory), unattended deployment, adversarial environment.

• Techniques:

  • Confidentiality: Symmetric encryption (AES, lightweight ciphers like SPECK, SIMON).

  • Integrity: Message Authentication Codes (MACs) like HMAC.

  • Authenticity: Symmetric key pre-distribution, public-key (elliptic curve for efficiency).

  • Key Management: Key predistribution schemes (e.g., random key pool), key establishment protocols.

Underwater Wireless Sensor Networks (UWSN)

• Architecture: Sensor nodes (anchored/mobile) with acoustic modems. Sink nodes (surface buoys) relay to onshore via RF/satellite.

• Applications: Oceanography (monitoring), disaster prevention (tsunami detection), oil/gas exploration, military surveillance.

• Unique Challenges:

  • High Propagation Delay: ~1.5 sec/km (vs. ~3 µs/km in RF). Very long latency.

  • Limited Bandwidth: kHz range (vs. MHz/GHz).

  • High Path Loss & Multipath.

  • Node Mobility: Water currents cause drift.

  • 3D Deployment: Vertical dimension.

[!TIP] WSN Priority: Energy efficiency is paramount. Routing protocols must minimize hop count and transmission power. LEACH (clustering) and AODV (reactive) are classic exam answers.

V. Mobility and Transport Layer Protocols

Mobile IP

• Components:

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

  • Home Agent (HA): Router in MN's home network. Stores Care-of Address (CoA), tunnels packets to MN.

  • Foreign Agent (FA): Router in visited network. Provides CoA (often its own address), offers default routing.

  • Correspondent Node (CN): Communication peer.

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

• Data Forwarding (Triangle Routing):

  1. CN sends packet to MN's Home Address.

  2. HA intercepts (proxy ARP), tunnels packet (encapsulation) to MN's CoA.

  3. FA (or MN) decapsulates, delivers to MN.

  4. MN's replies go directly to CN (using source IP = Home Address).

• Optimizations:

  • Route Optimization (Bidirectional Tunneling): MN sends Binding Update to CN with its current CoA. CN can then send directly to MN's CoA, avoiding HA detour (triangle routing).

  • Foreign Agent Care-of Address: FA provides CoA, reducing MN's need for co-located CoA.

TCP for Wireless and Mobile Networks

• Limitations of Traditional TCP:

  • Congestion vs. Loss Confusion: TCP interprets all packet loss as congestion, reduces window unnecessarily in wireless (loss due to fading, handover).

  • Long RTTs: Inaccurate RTT estimation, poor timeout.

  • Handover Disruptions: Temporary loss during HO triggers slow start.

• TCP Variants & Enhancements:

  | Variant | Key Enhancement | Mechanism | | :--- | :--- | :--- | | TCP Tahoe | Fast Retransmit, Slow Start | On triple duplicate ACK or timeout → ssthresh = cwnd/2, cwnd=1 (slow start). | | TCP Reno | Fast Recovery | On triple dup ACK → ssthresh=cwnd/2, cwnd=ssthresh+3, then additive increase. | | TCP New-Reno | Partial ACK Handling | Fast Recovery until all data in flight ACKed; handles multiple losses in one window better. | | TCP Vegas | Delay-Based Congestion Control | Monitors RTT increase (vs. min RTT) to detect congestion before loss. Adjusts cwnd proactively. | | Mobile TCP (M-TCP) | Split Connection | Shim layer at FA. Maintains TCP connection with HA over stable wired link. MN uses short-lived local TCP to FA. FA masks wireless loss from HA. |

Congestion Window Management

• Principle: cwnd limits amount of unacknowledged data.

• Phases:

  1. Slow Start: cwnd doubles per RTT (cwnd += 1 per ACK) until ssthresh.

  2. Congestion Avoidance: cwnd increases by 1/cwnd per ACK (≈ +1 per RTT).

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

• Consequence in Wireless: Spurious cwnd reduction on non-congestion loss → underutilization of bandwidth.

[!TIP] TCP Focus: Know the difference between Reno and New-Reno (partial ACK handling). M-TCP uses split connection to isolate wireless loss. Vegas uses RTT, not loss, for congestion detection.

VI. Personal Area Networks (PAN) and Short-Range Wireless

Bluetooth Technology

• Piconet Topology:

  • 1 Master, up to 7 Active Slaves. Master controls frequency-hopping sequence and timing.

  • Parked/Sniff/Hold modes for more than 7 devices (low power).

  • Coverage: ~10 m (Class 2).

• Scatternet Topology:

  • Multiple interconnected piconets. A device can be Master in one piconet, Slave in another.

  • Bridge device relays traffic between piconets.

  • Complex synchronization, reduced throughput.

• Comparison:

  | Feature | Piconet | Scatternet | | :--- | :--- | :--- | | Scale | Max 8 active devices | More devices via inter-piconet links | | Complexity | Low (single master) | High (synchronization, role switching) | | Throughput | Higher (no relaying) | Lower (relay overhead) | | Use Case | Simple device pairing (headset, mouse) | Larger ad-hoc groups (file sharing) |

IEEE 802.15 WPAN Standards

• 802.15.1: Bluetooth (legacy).

• 802.15.4: Basis for Zigbee, 6LoWPAN. Low-rate, low-power. Defines PHY and MAC.

• 802.15.3: High-rate WPAN (e.g., for multimedia).

• 802.15.6: Body Area Networks (BAN) for medical/healthcare.

Zigbee Protocol

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

• Features:

  • Low-Power, Low-Data-Rate (250 kbps @ 2.4 GHz).

  • Large Network Size: ~65,000 nodes.

  • Topologies: Star, tree, mesh (with routing).

  • Beacon-enabled (for synchronization) or non-beacon.

• Applications: Home automation (lighting, security), industrial monitoring, WSNs (due to mesh support).

[!TIP] Bluetooth vs. Zigbee: Bluetooth is for cable replacement (audio, files), higher data rate. Zigbee is for low-power, large-scale monitoring/control with mesh networking.

VII. Internet of Things (IoT)

IoT Architecture

• Three-Layer Model:

  1. Perception Layer: Sensors/Actuators + data acquisition (RFID, NFC).

  2. Network Layer: Gateways, networks (WLAN, cellular, LPWAN) for data transport.

  3. Application Layer: User applications, analytics, cloud services.

• Five-Layer Model (More Detailed):

  1. Perception

  2. Transport/Network (specific network techs)

  3. Processing (edge/cloud computing, data storage)

  4. Application

  5. Business (business models, management).

• Key Components: Sensors, Actuators, Gateways (protocol translation), Cloud Platform, End-user applications.

Design Principles and Capabilities

• Scalability: Support billions of devices.

• Interoperability: Standard protocols (MQTT, CoAP, HTTP/2) for device-cloud communication.

• Security-by-Design: Authentication, encryption from ground up (not add-on).

• Capabilities: Connectivity, Sensing, Data Analytics, Automation.

Emerging IoT Standards (Networking Engineer Focus)

• LoRaWAN (Long Range WAN): LPWAN technology. Uses Chirp Spread Spectrum. Long range (~15 km), low power, low data rate. Star-of-stars topology (gateways to network server).

• NB-IoT (Narrowband IoT): Cellular-based LPWAN (3GPP). Uses licensed spectrum, deep indoor penetration, high reliability.

• MQTT (Message Queuing Telemetry Transport): Lightweight publish/subscribe messaging protocol over TCP/IP. Ideal for constrained devices.

Case Study: Sensor Body Area Network (BAN)

• Implementation: Wearable/implantable sensors (ECG, glucose, motion) on body. IEEE 802.15.6 standard for short-range, low-power, reliable communication.

• Gateway: Smartphone or dedicated hub aggregates data.

• Applications: Healthcare monitoring (remote patient, elderly care), fitness tracking, military performance.

• Challenges: Battery life, security of medical data, interference, comfort.

VIII. Other Wireless Technologies and Systems

WiMAX (IEEE 802.16)

• Fixed WiMAX (802.16-2004): Point-to-multipoint, fixed terminals. Last-mile broadband.

• Mobile WiMAX (802.16e): Adds mobility support. Features:

  • OFDMA (downlink), SC-FDMA (uplink).

  • Fast HO (< 50 ms), MIMO support.

  • QoS classes: UGS, rtPS, nrtPS, BE.

• Key Enhancements for Mobility: Sleep mode for power saving, fast HO with predictive algorithms, MIMO for robust links.

Wireless ATM

• Architecture: Extends ATM cell-switching to wireless. WATM Forum standards.

  • Mobile Terminal ↔ Base Station (wireless link) ↔ WATM Switch (mobility management) ↔ Fixed ATM Network.

• Role: Aimed for high-speed data, QoS guarantees (CBR, VBR) in mobile environments (e.g., vehicular).

• Biggest Research Challenges:

  1. Handover with QoS: Maintaining VPI/VCI and QoS during HO across WATM switches.

  2. Scalability: Managing many mobile terminals with per-flow state.

  3. Wireless Link Errors: High BER requires robust error control without violating ATM cell timing.

GPS and GAGAN

• GPS (Global Positioning System): US-owned satellite-based navigation. Provides position, velocity, time.

• GAGAN (GPS-Aided GEO Augmented Navigation):

  • Purpose: Satellite-based augmentation system (SBAS) for India/Indian region.

  • Functionality: Uses geostationary satellites to broadcast correction signals (for GPS satellite orbits, clock errors) and integrity messages. Improves accuracy, availability, and integrity for aviation (Cat I/II/III approaches) and other users.

IPv4 vs. IPv6 Addressing

TCP vs. UDP

IX. Wireless Network Challenges and Issues

Medium Access Problems (Recap & Mitigation)

• Hidden Terminal: Mitigated by RTS/CTS and physical carrier sensing.

• Exposed Terminal:

  • Problem: Node defers unnecessarily, reducing spatial reuse.

  • Mitigation Techniques:

    1. Use RTS/CTS selectively (only for long packets).

    2. Directional Antennas: Transmit/receive in specific direction, reducing interference area.

    3. Protocol Modifications: Allow transmission if sensed channel is busy but received signal strength is below threshold (i.e., not from intended receiver).

Effects of Multipath and Doppler Spread

• Multipath:

  • Cause: Reflections from objects.

  • Effect: Constructive/destructive interference → fading. Causes ISI if delay spread > symbol period.

  • Mitigation: OFDM (with CP), equalization, diversity.

• Doppler Spread:

  • Cause: Relative motion between Tx/Rx (or scatters).

  • Effect: Frequency shift → time-varying channel (fast fading). Reduces coherence time.

  • Mitigation: Fast channel tracking, robust modulation/coding, MIMO (spatial diversity helps).

General Security Considerations

• Threats:

  • Eavesdropping: Passive listening on wireless medium.

  • Jamming: Deliberate interference at PHY layer.

  • Masquerading/Spoofing: Fake access points, MAC spoofing.

  • Replay Attacks: Capturing and retransmitting valid packets.

• Mechanisms:

  • Encryption: WPA2/WPA3 (Wi-Fi), AES-CCM (802.15.4), lightweight ciphers (WSN).

  • Authentication: 802.1X (EAP), pre-shared keys, certificate-based.

  • Integrity: MIC (Message Integrity Code), HMAC.

  • Anti-Jamming: Spread spectrum (FHSS/DSSS), low-probability-of-intercept (LPI) techniques.

[!TIP] Final Exam Checklist: Be able to draw/explain OFDM transmitter/receiver, compare SISO/MIMO, list WSN routing types with examples, explain Mobile IP tunneling, differentiate TCP variants, and describe Bluetooth topologies. Diagrams for UMTS, E-UTRAN, WSN architecture are high-yield.