UNIT 4: Wireless and Mobile Computing - Exam-Focused Notes

I. FUNDAMENTALS OF WIRELESS TRANSMISSION & PROPAGATION

A. Radio Wave Propagation Models

Wireless signals travel via three primary mechanisms:

[!TIP] Exam Focus: Be prepared to sketch the Earth's layers for sky wave (D, E, F layers) and explain why higher frequencies (microwave) rely on LOS.

B. Helical Antenna

• Construction: A conducting wire wound in a helix shape, mounted over a ground plane.

• Operation: Radiates circularly polarized waves. The pitch angle (α) determines mode:

  • Normal Mode (α ≈ 0°): Broadside radiation, narrow bandwidth, used in HF/VHF.

  • Axial Mode (α ≈ 12°-14°): End-fire radiation along the helix axis, wide bandwidth, used in satellite communication.

• Key Parameters:

  • Circumference C ≈ λ for axial mode.

  • Gain increases with number of turns N.

  • Input Impedance: ~140 Ω for axial mode.

[!TIP] Common Mistake: Confusing normal and axial mode radiation patterns. Axial mode radiates along the helix axis (like a directional arrow).

II. MULTIPLE ACCESS TECHNIQUES

A. Core Concepts

• Definition: Methods to allow multiple users to share a finite radio spectrum efficiently.

• Need: Maximize channel utilization, ensure fairness, support Quality of Service (QoS).

• Priority-Based Schemes: Users are assigned priorities (static/dynamic). Higher priority users get immediate access; lower priority users contend or wait.

B. Specific Techniques

[!TIP] Exam Focus: Contrast DAMA (centralized, reservation) with PRMA (slotted, contention-based reservation). PRMA is more dynamic.

III. CELLULAR COMMUNICATION SYSTEMS

A. Architecture & Fundamentals

• Overall Architecture: Divides service area into cells (hexagonal zones). Each cell has a Base Station (BS). BSs connect to a Mobile Switching Center (MSC) which links to PSTN and databases (HLR, VLR).

• Why "Cellular"? The term comes from the cellular structure—the coverage area is partitioned into small, contiguous cells, allowing frequency reuse.

• Sectorization: Using directional antennas (typically 3 sectors of 120° each) at a BS to:

  1. Reduce co-channel interference.

  2. Increase capacity (same frequency can be reused in different sectors).

  3. Allow frequency planning per sector.

B. Key Procedures & Services

• Handoff / Handover: Process of transferring an ongoing call from one BS to another as the Mobile Station (MS) moves.

  • Types: Intra-cell (same BS, different channel), Inter-cell (different BS), Inter-MSC.

  • Process: MS measures signal strength → reports to current BS → MSC evaluates → if threshold crossed, MSC allocates new channel & BS → command sent to MS → connection switched.

• GSM Services:

  • Bearer Services: Data transmission (circuit-switched, packet-switched via GPRS).

  • TeleServices: Voice, emergency calls, SMS.

  • Supplementary Services: Call forwarding, call waiting, caller ID.

[!TIP] Exam Focus: Handoff is network-controlled (NCHO) in GSM. MSC makes the decision based on measurements from MS and BS.

IV. EVOLUTION OF CELLULAR TECHNOLOGIES & ARCHITECTURES

A. Global System for Mobile Communications (GSM)

• Complete Architecture:

  
  [MS] <--Um--> [BTS] <--Abis--> [BSC] <--A--> [MSC]
  
                             |                  |
  
                          [TC]               [VLR]
  
                             |                  |
  
                          [HLR] <------------|
  
  

  • MS (Mobile Station): Mobile phone + SIM.

  • BTS (Base Transceiver Station): Radio equipment in cell.

  • BSC (Base Station Controller): Manages multiple BTSs, handles radio resource management.

  • MSC (Mobile Switching Center): Core switch, call routing, handoff control.

  • HLR (Home Location Register): Permanent database (subscriber data, current VLR).

  • VLR (Visitor Location Register): Temporary database for visiting subscribers.

  • EIR (Equipment Identity Register): Tracks stolen/defective IMEIs.

  • AuC (Authentication Center): Generates authentication vectors.

  • TC (Transcoder): Rate conversion (PCM 64 kbps ↔ GSM 13 kbps).

[!TIP] Diagram Must-Know: Draw the full GSM architecture with all components and interfaces (Um, Abis, A, etc.). Label data flow for a call setup.

B. General Packet Radio Service (GPRS)

• Complete Architecture (Packet-Switched Core):

  
  [MS] <--Um--> [BTS] <--Abis--> [BSC/SGSN] <--Gn--> [GGSN] --> PDN (Internet/ISP)
  
                             |                     |
  
                          [HLR] <--Gc--> [GGSN]
  
  

  • SGSN (Serving GPRS Support Node): Equivalent of MSC for packet data. Tracks MS location, authenticates, manages sessions.

  • GGSN (Gateway GPRS Support Node): Gateway to external PDNs (Packet Data Networks). Assigns IP addresses, performs tunneling.

  • Key Change from GSM: Introduces packet-switched domain alongside circuit-switched. BSC is upgraded to BSC with PCU (Packet Control Unit) or replaced by SGSN in some architectures.

C. Universal Mobile Telecommunications System (UMTS)

• Architecture (3G): Based on W-CDMA (Wideband CDMA).

  • UE (User Equipment): 3G phone.

  • UTRAN (UMTS Terrestrial Radio Access Network): Consists of Node B (BS) and RNC (Radio Network Controller). RNC is like BSC but with more intelligence (macro diversity, soft handoff).

  • Core Network (CN): MSC (for circuit-switched) + SGSN/GGSN (for packet-switched) – often called GPRS Core Network.

• Key Feature: Soft Handoff (mobile can be connected to multiple Node Bs simultaneously during handoff) due to CDMA's wideband nature.

D. Comparative Analysis: GSM vs. GPRS

[!TIP] Exam Focus: GPRS is an overlay on GSM. It reuses the radio infrastructure (BTS/BSC) but adds new core nodes (SGSN/GGSN). Be ready to draw both architectures side-by-side.

V. WIRELESS LOCAL AREA NETWORK (WLAN) PROTOCOLS & STANDARDS

A. IEEE 802.11 Family

• IEEE 802.11a:

  • Band: 5 GHz UNII band.

  • Modulation: OFDM (Orthogonal Frequency Division Multiplexing).

  • Data Rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps.

  • Range: Shorter than 802.11b/g due to higher frequency.

  • Channels: 12 non-overlapping 20 MHz channels.

• Hidden Terminal Problem: Node A and C cannot hear each other (out of range) but both can transmit to B, causing collision at B.

• Exposed Terminal Problem: Node B is transmitting to A. Node C (in range of B but not A) defers even though its transmission to D would not interfere at A.

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

  1. Sender sends RTS to receiver.

  2. Receiver replies with CTS (includes duration field).

  3. All nodes hearing RTS/CTS set NAV (Network Allocation Vector) to defer.

  • Trade-off: RTS/CTS adds overhead; used only for large frames.

[!TIP] Diagram Must-Know: Draw the hidden terminal scenario (A-B-C in a line, A and C out of range). Show how RTS/CTS reserves the channel.

B. HIPERLAN (High Performance Radio LAN)

• Architecture: Connection-oriented, supports both infrastructure and ad-hoc modes.

• HIPERLAN Versions:

  • HIPERLAN/1: 23.5 Mbps @ 5 GHz. Uses Channel Access Method (CAM).

  • HIPERLAN/2: ~54 Mbps @ 5 GHz. OFDM physical layer. Supports QoS via Central Controller (CC) in infrastructure mode. Eliminates hidden terminal via centralized scheduling.

• HIPERLAN/1 Channel Access (CAM):

  1. Prioritized Carrier Sense (P-CS): 8 priority levels.

  2. Prioritized Random Access (P-RA): Contention with backoff.

  3. Prioritized Elimination by Random Backoff (PERB): For collision resolution.

• Physical Layer & MAC Sublayer:

  • Physical Layer: GFSK (HIPERLAN/1), OFDM (HIPERLAN/2).

  • MAC Sublayer: Connection-Oriented. Uses MAC connections (like virtual circuits) identified by Connection Identifier (CID). Supports broadcast/multicast via special CIDs.

C. Other Standards

[!TIP] Exam Focus: DECT is not a WLAN standard; it's for cordless telephony. TETRA is for professional mobile radio (PMR). HIPERLAN/2 is QoS-focused.

VI. PERSONAL AREA NETWORKS (PAN)

A. Bluetooth

• Network Topology:

  • Piconet: Basic unit. 1 Master (determines hopping sequence) + up to 7 Slaves (active). Total 8 devices. Uses FHSS (Frequency Hopping Spread Spectrum) in 2.4 GHz ISM band (79 channels, 1 MHz spacing, 1600 hops/sec).

  • Scatternet: Multiple piconets interconnected. A device can be Master in one piconet and Slave in another (or Slave in multiple). Uses different hopping sequences per piconet.

• Physical Layer: GFSK (Basic Rate), optionally π/4-DQPSK or 8-DPSK (Enhanced Data Rate). Time Division Duplex (TDD).

• MAC Layer Protocol:

  • Slotted Time Division Duplex: Time divided into 625 µs slots.

  • Packet Structure: Access code (72 bits) + header (54 bits) + payload (0-2745 bits).

  • Hopping: Synchronized to slot boundaries. Master's clock and BD_ADDR determine hop sequence.

  • Polling: Master polls slaves in round-robin. Slave can transmit only in slot assigned by master or after polling.

[!TIP] Diagram Must-Know: Draw a piconet with 1 master and 3 slaves. Show time slots with master-to-slave and slave-to-master transmissions. Show a scatternet with two overlapping piconets.

VII. MOBILE NETWORKING & IP SUPPORT

A. Reference Model

• Layered Architecture: Extends standard Internet model.

  • Physical/Link: Wireless link (802.11, Bluetooth, etc.).

  • Network: Mobile IP handles mobility at IP layer.

  • Transport: TCP variants (I-TCP, Snooping) handle mobility.

  • Application: Unchanged.

B. Mobile IP

• Key Concepts:

  • Home Agent (HA): Router in home network. Stores Care-of Address (CoA). Tunnels packets to CoA.

  • Foreign Agent (FA): Router in foreign network. Provides CoA (often its own IP) and forwards packets to MS.

  • Mobile Node (MN): The mobile device.

  • Care-of Address (CoA): Temporary IP address in foreign network. Can be FA CoA (FA's IP) or Co-located CoA (MN's own temporary IP).

• Tunneling Mechanisms: Encapsulate original IP packet inside a new IP header for delivery to CoA.

  | Mechanism | Full Form | Header Fields | Use Case | | ------------------- | -------------------------------------- | --------------------------------------------------------------------------------- | -------------------------------- | | IP-in-IP | IP Encapsulation | New IP header (src=HA, dst=CoA) + original IP packet. | Simple tunneling. | | Minimal Encaps. | Minimal Encapsulation | New IP header (src=HA, dst=CoA) + minimal header (type, S/D port, etc.) + original. | Avoids double header overhead. | | GRE | Generic Routing Encapsulation | GRE header (flags, type, key, seq#) + original packet. | Supports multiple protocols. |

[!TIP] Exam Focus: Know the three agents (MN, HA, FA) and their roles. Tunneling is triangular routing: CN → HA → FA → MN. Draw the packet format for each encapsulation type.

C. DHCP for Mobile Environments

• Role: Automatically assigns IP address, subnet mask, default gateway, DNS servers to MN when it enters a new foreign network.

• Operation:

  1. MN broadcasts DHCP Discover.

  2. FA (or DHCP server) responds with DHCP Offer.

  3. MN sends DHCP Request.

  4. FA/server sends DHCP ACK with configuration.

• Mobile Context: DHCP lease time is often short due to mobility. Can be combined with Mobile IP (CoA assigned via DHCP).

VIII. AD HOC & SENSOR NETWORKS

A. Mobile Ad Hoc Networks (MANETs)

• Definition: Infrastructure-less, self-configuring network of mobile nodes connected wirelessly.

• Characteristics: Dynamic topology, limited bandwidth/battery, multi-hop routing, distributed operation.

• Performance Issues:

  • Routing: Frequent route breaks, control overhead.

  • Security: Vulnerable to eavesdropping, blackhole attacks.

  • Power: Energy-aware routing critical.

• Applications: Military, emergency response, sensor networks, vehicular networks.

• Dynamic Source Routing (DSR):

  • Operation: Source routing. MN discovers route via Route Discovery (RREQ/RREP flooding). Entire path stored in packet header.

  • Route Maintenance: If link breaks, Route Error (RERR) sent back to source. Source initiates new discovery.

  • Advantage: No periodic routing table updates (reactive). Disadvantage: Large header overhead, poor scalability.

B. Wireless Sensor Networks (WSNs)

• Architecture:

  • Sensor Nodes: Sense, process, transmit data. Constrained in power, computation, memory.

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

  • Typical Topology: Many sensors → multi-hop → sink → BS → user.

  
  [Sensors] --> [Relay Nodes] --> [Sink] --> [Base Station] --> [User]
  
  

• Applications:

  • Environmental: habitat monitoring, weather.

  • Health: patient monitoring.

  • Military: intrusion detection, surveillance.

  • Smart homes/buildings.

[!TIP] Diagram Must-Know: Draw WSN architecture showing densely deployed sensors, multi-hop routing to a sink, and connection to a remote base station. Label data flow direction.

IX. TRANSPORT LAYER PROTOCOLS FOR MOBILE ENVIRONMENTS

Classical Approaches Comparison

[!TIP] Exam Focus: Compare I-TCP (break end-to-end, use proxy) vs Snooping (maintain end-to-end, FA buffers/snoops). I-TCP hides mobility from CN; Snooping tries to preserve end-to-end.

X. SECURITY IN WIRELESS & MOBILE NETWORKS

A. Firewalls

• Definition: Network security system that monitors and controls incoming/outgoing traffic based on predetermined security rules.

• Design Issues:

  • Performance: Deep packet inspection is slow; need hardware acceleration.

  • Encrypted Traffic: Cannot inspect payload of SSL/TLS/VPN traffic (creates blind spots).

  • Application-Level Gateways: Understand application protocols (e.g., FTP) but are complex and protocol-specific.

  • Wireless Context: Must also secure the air interface (e.g., 802.11i/WPA2). Firewalls at the edge of WLAN protect the wired network.

B. Intrusion Detection Systems (IDS)

• Types:

  • Network-based IDS (NIDS): Monitors network traffic (e.g., Snort).

  • Host-based IDS (HIDS): Monitors single host (e.g., file integrity).

  • Wireless IDS (WIDS): Specifically monitors 802.11 traffic for attacks (e.g., rogue APs, deauthentication floods).

• Role in Wireless: Detect passive eavesdropping, active attacks (jamming, spoofing), misconfigured/malicious APs.

C. Password Management

• Challenges in Wireless/Mobile:

  • Small keypads/screens → difficult to enter long passwords.

  • Public spaces → shoulder surfing risk.

  • Frequent network changes → multiple credentials.

  • Device loss/theft → stored passwords compromised.

• Methods:

  • One-Time Passwords (OTP): Token-based (e.g., RSA SecurID) or SMS-based.

  • Biometrics: Fingerprint, face recognition (convenient but privacy concerns).

  • Certificates: Public key infrastructure (PKI) for mutual authentication.

  • Single Sign-On (SSO): One credential for multiple services.

XI. MAC LAYER (GENERAL)

• Functions in Wireless:

  1. Channel Access Control: Multiple access (CSMA/CA, TDMA, etc.).

  2. Frame Format: Define structure (preamble, header, payload, CRC).

  3. Addressing: MAC addresses (48-bit).

  4. Error Control: ARQ (Automatic Repeat reQuest).

  5. Synchronization: Frame timing, clock recovery.

• Challenges in Wireless:

  • Hidden/Exposed Terminal: As discussed in 802.11.

  • Signal Fading/Interference: Causes burst errors.

  • Limited Battery Power: Need energy-efficient protocols.

  • Mobility: Topology changes frequently.

  • Noise & Capture: Collisions may not always cause complete packet loss (capture effect).

[!TIP] Exam Focus: Contrast with wired MAC (e.g., Ethernet uses CSMA/CD, no hidden terminals). Wireless MAC must use RTS/CTS or scheduling to mitigate hidden terminals.

Final Exam Strategy:

1. Diagrams are mandatory for GSM, GPRS, WSN, Bluetooth piconet, propagation models, hidden terminal.

2. Comparison tables (GSM vs GPRS, TCP variants) score high.

3. Define terms precisely (Handoff, Sectorization, CoA).

4. Link concepts: e.g., Sectorization → reduces co-channel interference → increases capacity.

5. Prioritize: GSM, GPRS, Bluetooth, Mobile IP, Ad Hoc, Propagation are high-frequency.

\boxed{\text{Revise past papers 2025 & 2023 using this blueprint.}}