Speech Processing (EC-803 (C)) - Unit 2 Short Notes

UNIT 2: WIRELESS NETWORKS TECHNOLOGIES & ARCHITECTURES

I. WIRELESS NETWORK ARCHITECTURES & STANDARDS

A. Cellular Network Generations & Standards

Evolution: GSM → UMTS (3G)

[!TIP] Exam often asks for comparative evolution. Focus on shift from circuit-switched (GSM) to packet-switched (UMTS) core.

UMTS Network Architecture

• UE (User Equipment): Mobile device.

• Node B: Base station (analogous to BTS in GSM).

• RNC (Radio Network Controller): Manages Node Bs, radio resources, handovers.

• Core Network:

  • MSC/VLR: Circuit-switched calls, mobility management.

  • SGSN (Serving GPRS Support Node): Packet-switched mobility, data routing.

  • GGSN (Gateway GPRS Support Node): Gateway to external PDNs (Internet).

Interaction Flow (Data): UE ↔ Node B ↔ RNC ↔ SGSN ↔ GGSN ↔ Internet.

3GPP (3rd Generation Partnership Project)

• Role: Global standards body for GSM, UMTS, LTE, 5G NR.

• Objectives: Develop evolved standards (LTE/LTE-A) focusing on:

  • High data rates (100 Mbps DL, 50 Mbps UL for LTE).

  • Low latency.

  • Improved spectral efficiency.

  • All-IP flat architecture.

E-UTRAN (LTE) Architecture

• Key Philosophy: Simplified, flat, all-IP. No RNC.

• Components:

  • eNodeB (evolved Node B): Single entity combining Node B + RNC functions. Handles MAC scheduling, radio resource management, handover decisions.

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

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

  • P-GW (PDN Gateway): User plane. Interface to external PDNs, IP address allocation, policy enforcement.

• Interface: eNodeB connects directly to MME/S-GW via S1 interface. eNodeBs interconnect via X2 interface.

[!TIP] Remember "e" for evolved and flat. Key contrast with UMTS: No RNC, eNodeB is smarter.

B. Broadband Wireless Access (BWA) Standards

IEEE 802.16 (WiMAX)

• Fixed WiMAX (802.16-2004): Static, point-to-multipoint. No mobility support.

• Mobile WiMAX (802.16e-2005): Key enhancements for mobility:

  • Hard Handover: Break-before-make (like GSM).

  • Fast Fourier Transform (FFT) scalability: Supports different channel bandwidths (1.25–20 MHz).

  • MIMO support (optional).

  • Improved power management for mobile devices.

  • Security: PKMv2 (Privacy Key Management) for authentication/encryption.

Wireless ATM

• Concept: Extend ATM cell-switching to wireless domains. Aims for QoS-guaranteed high-speed data.

• Architecture:

  • Base Station (BS): Connects wireless terminals to ATM switch.

  • Mobile Terminal (MT): Runs ATM adaptation layer (AAL).

  • Mobile Switching Center (MSC): Handles mobility (handovers, location updates).

• Role: Enables seamless integration of wireless and wired ATM networks, supporting constant-bit-rate (CBR) and variable-bit-rate (VBR) services.

C. Wireless Personal Area Networks (WPANs)

Bluetooth

[!TIP] Scatternet is theoretical/rarely implemented; most Bluetooth networks are single piconets.

IEEE 802.15 WPAN

• Scope: Standards for low-rate, low-power, short-range networks.

• Key Standards:

  • 802.15.1: Bluetooth (legacy).

  • 802.15.4: Basis for Zigbee and 6LoWPAN. Defines PHY/MAC for low-data-rate (<250 kbps) networks.

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

  • 802.15.6: Body Area Networks (BAN).

Zigbee

• Based on: IEEE 802.15.4 (PHY/MAC).

• Architecture:

  • Zigbee Coordinator (ZC): Forms network, stores info.

  • Zigbee Router (ZR): Extends network, relays data.

  • Zigbee End Device (ZED): Low-power, talks only to parent (ZR/ZC).

• Topologies: Star, Tree, Mesh.

• Key Features: Ultra-low power, large node count (>65,000), self-healing mesh, 128-bit AES encryption.

D. Wireless Local Area Networks (WLANs)

IEEE 802.11 Protocol Architecture

+-----------------------+

|  Application          |

+-----------------------+

|  802.11 MAC Sublayer  | <-- DCF, PCF, MAC Management

+-----------------------+

|  PHY Sublayer         | <-- FHSS, DSSS, OFDM, etc.

+-----------------------+

• Physical Layer: Defines modulation (DSSS, OFDM), frequency bands (2.4 GHz, 5 GHz), data rates.

• MAC Management Sublayer: Handles scanning, authentication, association, power management.

HIPERLAN (High-Performance Radio LAN)

• Key Features vs. 802.11:

  • Connection-Oriented: Supports QoS via prioritized channel access.

  • Multihop Support: Can form ad-hoc mesh networks natively.

  • Channel Access: Elimination-by-Choosing (similar to TDMA) + Prioritized contention.

  • Higher Data Rates: Up to 54 Mbps (HIPERLAN/2).

• Advantage: Better QoS and ad-hoc support than original 802.11 (which was infrastructure-focused).

MAC Layer Functions in 802.11 (DCF Mode)

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

  1. Sense channel.

  2. If idle, wait DIFS, then transmit.

  3. If busy, defer, backoff (random slot time).

• RTS/CTS (Optional): For hidden terminal problem.

• Frame Structure: Contains address fields (up to 4 addresses for distribution system), sequence control, frame check sequence (FCS).

II. ADVANCED TRANSMISSION & ANTENNA TECHNOLOGIES

A. Fundamental Concepts

SISO vs. MIMO

Challenges in Wireless Channels

• Multipath Propagation: Signals take multiple paths → constructive/destructive interference → fading.

• Doppler Effect: Relative motion → frequency shift → time-varying channel.

• Inter-Symbol Interference (ISI): Delayed multipath copies overlap with subsequent symbols → symbol detection errors.

B. Orthogonal Frequency Division Multiplexing (OFDM)

Principle:

• High-rate serial data stream is split into N parallel low-rate streams.

• Each stream modulates a subcarrier.

• Subcarriers are orthogonal (spacing = 1/T_sym), so spectra overlap but no ICI.

• Mathematical Basis: Use IFFT at transmitter, FFT at receiver.

$$s(t) = \sum_{k=0}^{N-1} X_k e^{j2\pi k t / T_{sym}}, \quad 0 \le t < T_{sym}$$

where $$\displaystyle X_k $$ are modulated symbols, $$\displaystyle T_{sym} $$ is symbol duration.

OFDM Transmitter Block Diagram:

Serial Data → S/P → QAM/Mapper → IFFT → Parallel-to-Serial → **Cyclic Prefix Insertion** → D/A → RF → Antenna

OFDM Receiver Block Diagram:

Antenna → RF → A/D → **Cyclic Prefix Removal** → Serial-to-Parallel → FFT → Demapper → P/S → Output Data

Cyclic Prefix (CP):

• Copy last N_cp samples of OFDM symbol to front.

• Purpose: Convert linear convolution (multipath) to circular convolution, allowing simple frequency-domain equalization (single tap per subcarrier).

• Trade-off: CP reduces spectral efficiency but provides ISI protection.

[!TIP] OFDM turns a frequency-selective fading channel into N flat-fading subchannels.

C. MIMO & OFDM-MIMO

MIMO System Explanation

1. Spatial Multiplexing: Independent data streams sent from each Tx antenna. Capacity increases linearly with min(N_t, N_r).

$$C = \min(N_t, N_r) \cdot B \cdot \log_2(1 + \text{SNR}) \quad \text{(ideal, uncoded)}$$

2. Diversity: Same data sent from multiple antennas (Alamouti code) → reliability gain.

3. Beamforming: Weighted transmission from multiple antennas → steer beam toward receiver → ↑ SNR.

OFDM-MIMO Integration (e.g., in LTE, WiMAX)

• Why combine? MIMO needs flat-fading per stream. OFDM provides flat subchannels.

• Implementation: Apply MIMO encoding per subcarrier.

  • For each subcarrier k, MIMO encoder (e.g., spatial multiplexing matrix) acts on the symbol vector.

  • Resulting MIMO streams are mapped to transmit antennas.

• Addresses:

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

  • ISI: OFDM's CP eliminates inter-symbol interference from multipath.

[!TIP] OFDM-MIMO = OFDM (handles frequency domain) + MIMO (handles spatial domain). Foundation of 4G/5G.

III. MEDIUM ACCESS CONTROL (MAC) PROTOCOLS & CHALLENGES

A. MAC in Infrastructure & Ad-hoc Networks

802.11 MAC Operation (DCF - Distributed Coordination Function)

1. Carrier Sense: Listen to channel.

2. If idle > DIFS: Transmit immediately.

3. If busy: Wait until idle + DIFS, then start random backoff (choose slot in [0, CW-1]).

4. After backoff: Sense again. If idle > DIFS, transmit.

5. ACK: Receiver sends ACK after SIFS. No ACK → retransmission (up to 7 times).

PCF (Point Coordination Function): Optional, centralized polling by AP (infrastructure only).

Hidden Terminal Problem

• Scenario: A, B, C. A & C can't hear each other, both can talk to B.

• Problem: A & C transmit to B simultaneously → collision at B.

• Mitigation: RTS/CTS (Request-to-Send / Clear-to-Send).

  1. A → B: RTS (includes duration).

  2. B → A & C (heard by all): CTS (includes duration).

  3. C defers upon hearing CTS (even if can't hear A).

  4. A transmits data → B sends ACK.

Exposed Terminal Problem

• Scenario: A, B, C, D. A talks to B, C talks to D. B & C are in range.

• Problem: C hears RTS from A→B (or CTS from B→A) → unnecessarily defers its transmission to D, even though C-D transmission wouldn't interfere with A-B.

• Impact: Reduced spatial reuse, lower capacity.

• Mitigation:

  • RTS/CTS with proper threshold: Only use RTS/CTS if packet size > threshold.

  • CTS-to-self: In 802.11e/g, CTS uses same rate as data, heard only by nearby nodes.

  • Power control: Transmit at minimum required power.

[!TIP] Hidden Terminal → Collision. Exposed Terminal → Unnecessary deferral. Both reduce throughput.

B. MAC for Sensor Networks (WSNs)

Topology Management

• Definition: Organizing nodes into efficient network structure (clusters, trees, grids).

• Why Essential?

  • Energy Efficiency: Reduces radio on-time (e.g., cluster heads aggregate data).

  • Scalability: Manages large number of nodes.

  • Robustness: Provides redundancy, fault tolerance.

  • Load Balancing: Prevents early death of certain nodes.

MAC Design Constraints for WSNs

1. Energy Efficiency (Primary): Minimize radio usage (duty cycling, low-power listening).

2. Scalability: Protocol overhead must not grow with network size.

3. Latency vs. Energy Trade-off: Deep sleep saves power but increases delay.

4. Collision Avoidance: Low-power listen (LPL) or scheduled access (TDMA) to avoid costly collisions.

5. Overhead Minimization: Small packet headers, simple algorithms.

IV. WIRELESS SENSOR NETWORKS (WSNs)

A. WSN Fundamentals

Definition & Differentiation from Wired Networks

• WSN: Dense deployment of autonomous, resource-constrained nodes collaborating to monitor physical/environmental conditions.

• Key Differences:

  • Node Density: Often hundreds/thousands vs. few in wired.

  • Power: Battery-powered, often irreplaceable vs. mains-powered.

  • Failure Rate: High (unattended, harsh environments).

  • Topology: Often ad-hoc, multi-hop vs. fixed.

  • Traffic: Many-to-one (convergecast) vs. peer-to-peer.

  • Addressing: Data-centric ("event in region X") vs. node-centric ("send to node Y").

WSN Architecture

[Sensor Nodes] → (Multi-hop) → [Sink/Base Station] → [Gateway] → [Management Station/Internet]

• Sensor Node: Sensing, processing, communication, power units.

• Sink/Base Station: Collects data from network, connects to external world.

• Gateway: Protocol translation (e.g., 802.15.4 to Ethernet/IP).

• Management Station: Configuration, data analysis.

Node Components

1. Sensing Unit: Transducer + ADC.

2. Processing Unit: Microcontroller/CPU, memory.

3. Communication Unit: Radio transceiver (e.g., CC2420 - 802.15.4).

4. Power Unit: Battery + power management (DC-DC converters).

Application Areas

• Environmental (forest fire, habitat monitoring).

• Health (patient monitoring, BAN).

• Industrial (machine monitoring, process control).

• Military (surveillance, target tracking).

• Smart Home/Building.

B. Underwater WSNs (UWSNs)

Architecture Specifics

• Nodes: Underwater modems (acoustic), sensors (pressure, temperature, video).

• Deployment: Anchored to seabed or floating.

• Communication: Acoustic waves (not RF). Speed ~1500 m/s.

• Topology: 3D deployment (vertical & horizontal).

Main Applications & Use Cases

• Oceanographic data collection (temperature, salinity).

• Pipeline/offshore structure monitoring.

• Disaster prevention (tsunami detection).

• Marine archaeology.

• Assisted navigation.

Key Challenges

C. Routing in WSNs

Classification

Routing Metrics (Beyond Hop Count)

• Energy Efficiency: Minimize total energy, balance load (avoid dying nodes).

• Latency: Critical for event-driven applications.

• Robustness/Fault Tolerance: Multi-path, redundancy.

• Data-Centric: Attribute-based addressing (e.g., Directed Diffusion).

D. Security in WSNs

Security Challenges

1. Resource Constraints: Limited CPU, memory, energy → heavy crypto (RSA) often infeasible.

2. Unattended Operation: Nodes deployed in hostile/unmonitored areas → physical capture.

3. Ad-hoc Nature: No fixed infrastructure → vulnerable to insider attacks.

4. Scalability: Security mechanism must scale to thousands of nodes.

5. Wireless Medium: Eavesdropping, jamming, spoofing easy.

Techniques for CIA Triad

• Confidentiality:

  • Symmetric-key crypto: AES (preferred over DES/3DES).

  • Key Management: Pre-distribution (e.g., Eschenauer-Gligor), pairwise keys, cluster-based keys.

• Integrity:

  • Message Authentication Codes (MACs): HMAC with shared key.

  • Digital Signatures: Often too heavy; use micro-MACs.

• Authenticity:

  • Mutual Authentication: Node-to-node, node-to-sink.

  • Techniques: Challenge-response, public-key (ECC for efficiency).

• Additional:

  • Intrusion Detection: Local monitoring (watchdog), global detection (sink-based).

  • Secure Routing: Secure AODV (SAODV) with digital signatures for route control packets.

  • Resilience: Node revocation, key refreshment.

[!TIP] WSN security focuses on lightweight symmetric crypto and robust key management.

V. MOBILITY MANAGEMENT

A. Mobile IP

Core Concepts

• Home Agent (HA): Router in MN's home network. Tunnels packets to CoA.

• Foreign Agent (FA): Router in visited network. Provides CoA, forwards packets.

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

  • FA CoA: Address of FA (shared by many MNs).

  • Co-located CoA: MN gets its own address (e.g., via DHCP).

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

Data Forwarding Process (Away from Home)

1. Registration: MN → FA (via Agent Advertisement/Solicitation) → FA → HA (registers CoA).

2. Correspondent Node (CN) → MN:

   • CN sends packet to MN's Home Address.

   • HA intercepts (proxy ARP), tunnels packet to MN's CoA (encapsulation: IP-in-IP).

   • FA decapsulates, delivers to MN.

3. MN → CN: Direct (no tunnel). Source IP = Home Address.

4. Triangular Routing: CN→HA→MN path is often suboptimal.

Optimizations

• Route Optimization (RFC 4725):

  • MN sends Binding Update to CN with current CoA.

  • CN caches binding, sends packets directly to CoA (bypassing HA).

  • Uses Return Routability procedure to secure binding updates.

• Foreign Agent Care-of Address: FA CoA (shared) reduces CoA allocation overhead.

B. Cellular Mobility Management

Handover (Handoff) Process

1. Measurement: MN measures neighbor cell signals.

2. Decision: Network (RNC/eNodeB) or MN decides to handover.

3. Execution:

   • Hard Handover (GSM, LTE): Break-before-make. Connection to old BS released before new BS connection. Simple, but possible drop.

   • Soft Handover (CDMA): Make-before-break. MN maintains connections to multiple BSs simultaneously (diversity). Complex, no drop.

4. Resource Allocation: New BS allocates channels.

5. Data Forwarding: Old BS forwards buffered data to new BS (via core network).

Location Management

• Location Area (LA): Group of cells. Paging is done per LA.

• Updating Strategies:

  • Always Update: MN updates on every cell change → high signaling, low paging cost.

  • Never Update: MN never updates → low signaling, high paging cost (must page entire network).

  • Time-Based: Update if time since last update > T.

  • Distance-Based: Update if distance from last update > D.

  • Hybrid: Combine above.

GTP (GPRS Tunneling Protocol)

• Role in LTE/EPC: Tunnels user data and control messages between S-GW and P-GW (and between S-GWs during handover).

• User Plane (GTP-U): Tunnels user data packets (IP payload).

• Control Plane (GTP-C): Manages tunnel endpoints, carries session/path management messages.

• Key Function: Maintains IP address continuity for UE during mobility (S-GW anchors).

VI. TRANSPORT LAYER FOR WIRELESS & MOBILE NETWORKS

A. Limitations of Standard TCP

• Packet Loss Interpretation: TCP assumes all loss = congestion. Wireless losses (fading, interference) trigger unnecessary congestion control (cwnd reduction).

• Variable RTT: Handovers, queuing → RTT fluctuations → spurious timeouts, poor RTT estimation.

• Frequent Disconnections: Handovers can cause silent periods → TCP timeout → cwnd → 0, slow recovery.

• Asymmetric Links: Uplink often weaker → ACK loss → performance degradation.

B. TCP Variants for Wireless/Mobile

[!TIP] I-TCP & M-TCP use split-connection approach. Vegas uses delay-based congestion detection (vs. loss-based).

VII. INTERNET OF THINGS (IoT)

A. IoT Architecture

Layered Architecture

[Application Layer]      : Smart apps, analytics, cloud services.

[Network/Transport Layer]: IP-based routing, MQTT/CoAP, LPWAN.

[Middleware]             : Data management, device management, security.

[Perception Layer]       : Sensors/Actuators, data acquisition, short-range comms (BLE, Zigbee).

• Key Components:

  • Things/Devices: "Smart" objects with sensing/actuation + connectivity.

  • Gateways: Protocol translation (e.g., 802.15.4 ↔ Ethernet), edge processing.

  • Cloud/Edge Computing: Data storage, analytics, application hosting.

  • Applications: Domain-specific (smart city, health, industry).

Design Principles & Capabilities

• Addressing: Unique IDs (IPv6, EPC, uCode).

• Identification: Auto-ID (RFID, NFC, barcode).

• Sensing: Environmental, location, biometric.

• Communication: Short-range (WPAN), long-range (LPWAN), cellular (NB-IoT).

• Architectural: Scalability, interoperability, security-by-design.

• Capabilities: Ubiquitous sensing, data-driven automation, remote control.

B. IoT Standards & Enablers

Emerging IoT Networking Standards

• LPWAN (Low-Power Wide-Area Network):

  • LoRaWAN: Star-of-stars topology, unlicensed bands, long-range (~10 km), very low power.

  • NB-IoT (Narrowband IoT): Cellular (LTE-based), licensed bands, deep indoor coverage, high reliability.

• M2M Standards: 3GPP LTE-M, EC-GSM-IoT (evolution of GSM for IoT).

Role of IPv6 & 6LoWPAN

• IPv6: Provides ~3.4×10³⁸ addresses → essential for massive IoT.

• 6LoWPAN (IPv6 over Low-Power WPAN):

  • Adaptation layer between IPv6 and IEEE 802.15.4.

  • Header compression: Compresses 40-byte IPv6 header to ~10-20 bytes.

  • Fragmentation/Reassembly: Handles small MTU (127 bytes) of 802.15.4.

  • Enables IP-addressable sensor nodes.

VIII. SPECIALIZED TECHNOLOGIES & SHORT NOTE TOPICS

A. Navigation & Augmentation

GAGAN (GPS-Aided GEO Augmented Navigation)

• Purpose: Satellite-Based Augmentation System (SBAS) for India. Improves GPS accuracy, integrity, availability for aviation (CAT I/II/III approaches).

• System: Geostationary satellites (GSAT-8, GSAT-10, GSAT-15) broadcast correction signals (ionospheric, satellite orbit/clock).

• Benefit: Provides < 3 m horizontal accuracy and high integrity (alerts within 6 sec if GPS unusable).

B. Comparative Analyses

IPv4 vs. IPv6

TCP vs. UDP

C. Other Specific Technologies

WiMAX (IEEE 802.16) Overview

• Goal: "Last-mile" broadband wireless access (alternative to DSL/Cable).

• Key Features:

  • OFDM-based PHY (256-2048 FFT points).

  • TDD/FDD support.

  • Mesh and point-to-multipoint topologies.

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

  • Mobility: Up to 120 km/h (802.16e).

• Status: Largely superseded by LTE, but used in some rural/backhaul scenarios.

IEEE 802.15 WPAN Scope

• Focus: Personal operating space (~10 m).

• Standards:

  • 802.15.1: Bluetooth (legacy).

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

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

  • 802.15.6: Body Area Networks (BAN).

  • 802.15.7: Visible Light Communications (VLC).

Interface between 802.11 and Bluetooth

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

• Co-existence Mechanisms:

  1. Frequency Hopping (FHSS) vs. DSSS/OFDM: Bluetooth hops 1600 times/sec; 802.11 uses fixed channels. Time-domain coordination possible.

  2. Packet Scheduling: Coordinate transmission times to avoid overlap.

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

  4. Power Control: Reduce Bluetooth power when 802.11 active.

  5. Time Division: Use time slots (e.g., Bluetooth in one slot, 802.11 in next).

CSMA/CD (Contrast with CSMA/CA)

• CSMA/CD (Ethernet - Wired):

  • Collision Detection: Listen while transmitting. If collision → abort immediately (jam signal).

  • Efficiency: High (short distances, no hidden terminals).

  • Not used in wireless: Can't detect collision (transmit power >> receive power; hidden terminals).

• CSMA/CA (Wireless):

  • Collision Avoidance: RTS/CTS or inter-frame spaces + backoff.

  • No collision detection.

  • Must handle hidden/exposed terminals.

[!TIP] CSMA/CD = "Detect" (wired). CSMA/CA = "Avoid" (wireless). Fundamental difference due to channel characteristics.