UNIT 3: Wireless and Mobile Computing - Short Notes

1. Wireless Propagation Fundamentals & Antennas

Propagation Models

Wireless signals travel from transmitter to receiver via different paths. The three fundamental models are:

Diagram:

DiagramSEARCH: "ground wave sky wave line of sight propagation comparison diagram"

Helical Antenna

A circularly polarized antenna with a helical (spring-like) shape.

• Construction: A conducting wire wound in a helix around a cylindrical former, fed from one end.

• Operation: The circumference (C) of one turn ≈ wavelength (λ). Operates in two modes:

  1. Normal Mode (Axial Mode): C < 0.75λ. Radiates broadside to the axis (like a dipole). Low gain, used in mobile communications.

  2. Axial Mode: C ≈ 1λ. Radiates along the helix axis (end-fire). Circular polarization is a key feature. High gain, used in satellite communications.

• Polarization: The sense of rotation (right-hand or left-hand) determines polarization.

Exam Tip: Be prepared to sketch the helical antenna and label circumference, spacing, and feed point. Distinguish between normal and axial modes clearly.

2. Multiple Access Techniques

Fundamentals

Multiple Access allows multiple users to share a common communication medium (e.g., radio spectrum). The core need is to maximize spectral efficiency while ensuring fair access and minimizing collisions.

Contention-Based Access (Random Access)

Users compete for channel access. No prior reservation.

• Implementation: ALOHA and its variants (Slotted ALOHA, CSMA, CSMA/CD, CSMA/CA).

• Mechanism: A user transmits whenever it has data. Collisions occur if multiple users transmit simultaneously. Requires collision resolution protocols (binary backoff).

• Advantage: Simple, no scheduling overhead.

• Disadvantage: Performance degrades rapidly with high load (throughput < 50% for pure ALOHA).

Contention-Free / Reservation-Based Access

Channel access is scheduled to avoid collisions.

• Demand Assigned Multiple Access (DAMA):

  • Principle: A central controller (e.g., satellite) assigns time slots or frequency channels on-demand.

  • Operation: 1. Request Phase: Users send brief requests on a common channel. 2. Assignment Phase: Controller broadcasts a schedule. 3. Transmission Phase: Users transmit in assigned slots.

  • Use Case: Satellite networks with bursty traffic.

• Packet Reservation Multiple Access (PRMA):

  • Principle: Combines contention for reservations with slotted transmission. Used in satellite and cellular (e.g., GPRS).

  • Operation: Time is divided into slots grouped into frames. Users contend for slots in a frame. If a user successfully transmits a packet in a slot, it reserves that slot position in subsequent frames for a fixed number of frames (until the packet burst ends).

  • Key: Uses permission probabilities to control contention intensity.

Comparison of Multiple Access Schemes

Exam Tip: Be ready to contrast DAMA (centralized, request/assign) with PRMA (distributed contention with reservation). Know that PRMA is the basis for GPRS's MAC layer.

3. Cellular Communication Systems

System Architecture & Fundamentals

A cellular system divides a service area into cells, each served by a Base Station (BS).

Diagram:

DiagramCANVAS: "Cellular System Architecture showing Mobile Station (MS), Base Transceiver Station (BTS), Base Station Controller (BSC), Mobile Switching Center (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AUC), Equipment Identity Register (EIR). Show interfaces like A, Abis, Ater."

• Core Components:

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

  • Base Transceiver Station (BTS): Radio transceiver per cell.

  • Base Station Controller (BSC): Manages multiple BTSs (handoff, frequency allocation).

  • Mobile Switching Center (MSC): Core switch. Connects calls, handles mobility (roaming), interfaces with PSTN.

  • Network Subsystem: HLR, VLR, AUC, EIR (databases for subscriber info, location, authentication).

• Why "Cellular"?

  1. Cell: Hexagonal (ideal) coverage area from a BS.

  2. Frequency Reuse: Same frequency channels can be reused in non-adjacent cells to increase capacity.

  3. Cluster: A group of N cells (N = i² + ij + j²) that use all available channels exactly once before reuse. Capacity ∝ 1/N.

Key Concepts & Procedures

• Handoff / Handover:

  • Definition: Transferring an ongoing call from one cell to another as the user moves.

  • Types:

    • Intra-cell: Channel change within same cell (due to interference).

    • Inter-cell: Cell-to-cell transfer (most common).

    • Inter-BSC / Inter-MSC: Handoff involving different controllers/switches.

  • Decision Parameters: Signal strength (threshold, hysteresis), signal quality (BER), distance to BS, cell load, MS velocity.

• Sectorization:

  • Definition: Using directional antennas (e.g., 120° or 60°) at a BS to divide a cell into multiple sectors.

  • Purpose: Reduces co-channel interference, increases capacity (more channels per cell site).

  • Implementation: Each sector has its own set of frequencies and often its own transceiver.

2G: GSM (Global System for Mobile)

Diagram:

DiagramCANVAS: "GSM Architecture: 1. Mobile Station (MS) with SIM. 2. Radio Subsystem: BTS, BSC. 3. Network Subsystem: MSC, HLR, VLR, AUC, EIR. 4. Operation & Support Subsystem (OSS). Show interfaces: Um (MS-BTS), Abis (BTS-BSC), A (BSC-MSC), E (MSC-HLR), etc."

• Architecture Subsystems:

  1. Radio Subsystem (RSS): BTS & BSC. Manages radio resources.

  2. Network Switching Subsystem (NSS): MSC, HLR, VLR, AUC, EIR. Call control & mobility.

  3. Operation & Support Subsystem (OSS): Network management.

• Services:

  • Teleservices: Bearer of information (voice, SMS, fax).

  • Bearer Services: Transmission capability (circuit-switched data, packet-switched via GPRS).

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

• Handover: Primarily Mobile-Assisted Handover (MAHO). MS measures neighbor BS signal strength and reports to BSC/MSC, which makes the decision.

2.5G: GPRS (General Packet Radio Service)

Diagram:

DiagramCANVAS: "GPRS Architecture overlay on GSM. Show new elements: SGSN (Serving GPRS Support Node), GGSN (Gateway GPRS Support Node), PCU (Packet Control Unit). Interfaces: Gb (BSC-SGSN), Gn (SGSN-GGSN), Gi (GGSN-External PDN), Gs (SGSN-MSC). Highlight that MS now connects to both circuit-switched (MSC) and packet-switched (SGSN) domains."

• Key New Nodes:

  • SGSN: Similar to MSC for packet domain. Handles mobility, authentication, packet routing to/from MS.

  • GGSN: Gateway to external packet networks (Internet, X.25). IP address assignment, tunneling.

  • PCU: Added to BSC to manage packet traffic on radio channels.

• Comparison with GSM:

  | Feature | GSM (2G) | GPRS (2.5G) | | :--- | :--- | :--- | | Switching | Pure Circuit-Switched (CS) | Packet-Switched (PS) overlay on CS network | | Data | Slow (9.6 kbps), connection-oriented | "Always-on", packet-by-packet, shared channels | | Resources | Dedicated channel per call | Dynamic channel allocation (1-8 TDMA slots) | | Architecture | MSC-centric | SGSN/GGSN for data path | | Speed | ~9.6 kbps | ~20-40 kbps (theoretical 171 kbps) | | Billing | Per-second/minutes | Per-kilobyte of data transferred |

3G: UMTS (Universal Mobile Telecommunications System)

• Architecture:

  • UTRAN (UMTS Terrestrial Radio Access Network): Replaces GSM's BSS. Consists of Node B (BS) and RNC (Radio Network Controller, like advanced BSC).

  • Core Network (CN): Evolved from GSM NSS. Includes MSC/VLR (for CS), SGSN/GGSN (for PS), and HSS (Home Subscriber Server, merged HLR/AUC).

• Key Features: Wideband CDMA (WCDMA), higher data rates (144 kbps - 2 Mbps), support for multimedia services, soft handover (mobile can connect to multiple Node Bs simultaneously).

Exam Tip: For GSM vs. GPRS, focus on the new nodes (SGSN, GGSN) and the shift from circuit to packet switching. For UMTS, know UTRAN components (Node B, RNC) and the use of WCDMA.

4. Wireless Local Area Network (WLAN) Technologies

IEEE 802.11 Family

Diagram:

DiagramSEARCH: "802.11 architecture BSS ESS IBSS diagram"

• General Architecture:

  • Station (STA): Any wireless device.

  • Access Point (AP): Connects wireless STAs to wired network.

  • Basic Service Set (BSS): One AP + associated STAs. Infrastructure mode.

  • Extended Service Set (ESS): Multiple BSSs connected by a distribution system (wired LAN). Enables roaming.

  • Independent BSS (IBSS): Ad-hoc mode. STA-to-STA without AP.

• 802.11a:

  • PHY Layer: Uses OFDM (Orthogonal Frequency Division Multiplexing) in the 5 GHz band.

  • Why 5 GHz? Less crowded than 2.4 GHz, more non-overlapping channels (12 vs. 3), but shorter range and worse penetration through walls.

  • MAC Layer: Same as 802.11 (CSMA/CA), but supports higher rates (6-54 Mbps).

• Physical Layer Frequency Bands (General):

  • 2.4 GHz ISM Band: Used by 802.11b/g/n. 3 non-overlapping 22 MHz channels (1,6,11). Prone to interference (microwaves, Bluetooth).

  • 5 GHz UNII Band: Used by 802.11a/n/ac/ax. More channels (up to 25 non-overlapping 20 MHz), less interference, shorter range.

HIPERLAN (High Performance Radio LAN)

• HIPERLAN/1:

  • Architecture: Ad-hoc (peer-to-peer) and infrastructure modes. Uses EY-NPMA (Elimination-Yield Non-Preemptive Multiple Access) as primary channel access.

  • Channel Access Methods:

    1. EY-NPMA: Prioritized access. Has elimination, yield, and non-preemptive phases.

    2. CA (Contention Access): For low-priority traffic (slotted ALOHA-like).

    3. Priority Access: For high-priority traffic (short pre-allocated slots).

• HIPERLAN/2:

  • Architecture: Connection-oriented, centrally controlled by an AP (Access Point). Similar to 802.11 infrastructure mode.

  • Key Features: QoS support (via time-division duplexing and traffic contracts), high data rate (up to 54 Mbps), supports both packet and ATM cells.

• Comparison:

  | Feature | HIPERLAN/1 | HIPERLAN/2 | 802.11 (b/g) | | :--- | :--- | :--- | :--- | | Mode | Ad-hoc & Infra | Infrastructure only | Both | | MAC | EY-NPMA (priority) | Connection-oriented, TDMA/TDD | CSMA/CA (contention) | | QoS | Limited | Strong, guaranteed | Best-effort (WMM optional) | | Data Rate | ~23 Mbps | Up to 54 Mbps | 11-54 Mbps |

Common WLAN Issues: Hidden & Exposed Terminal

• Hidden Terminal Problem:

  • Cause: Two STAs (A & C) cannot hear each other but both can communicate with AP (B). Due to distance/obstruction.

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

  • Solution: RTS/CTS (Request-to-Send/Clear-to-Send) handshake. A sends RTS to B, B replies CTS (heard by C), C defers transmission.

• Exposed Terminal Problem:

  • Cause: STA (B) is within range of a transmitting STA (A) but not of the intended receiver (C). A is transmitting to some other node (D).

  • Effect: B unnecessarily defers its transmission to C (which would have been successful) because it senses carrier from A.

  • Effect: Reduces spatial reuse and overall capacity.

  • Solution: RTS/CTS also helps here (CTS from C to B is not heard by A).

Exam Tip: Draw a 4-node diagram (A, B, C, D) to clearly show the hidden and exposed terminal scenarios. Explicitly state that RTS/CTS solves both by creating a "virtual carrier sense" around the transmitter-receiver pair.

5. Wireless Personal Area Network (WPAN) & Specialized Protocols

Bluetooth

• Network Architecture:

  • Piconet: Fundamental unit. 1 Master (controls clock, hopping sequence) + up to 7 active Slaves. Master's clock & address define the network.

  • Scatternet: Interconnection of multiple piconets. A device can be Master in one piconet and Slave in another (or Slave in multiple). Allows extended coverage.

• Physical Layer:

  • Frequency Hopping Spread Spectrum (FHSS): 79 channels (1 MHz each) in 2.4 GHz ISM band. Hopping rate: 1600 hops/sec.

  • Modulation: Gaussian FSK (GFSK).

  • Power Classes: Class 1 (100mW, 100m), Class 2 (2.5mW, 10m), Class 3 (1mW, 1m).

• MAC Layer:

  • Polling-Based Access: Master controls medium. Uses Time Division Duplex (TDD) with slots (625 µs).

  • Slot Timing: Master-to-slave transmission starts at even slot time, slave-to-master at odd slot time.

  • Addressing: 48-bit IEEE device address (BD_ADDR) + 3-bit active member address (AM_ADDR) in piconet.

  • Parked State: Up to 255 devices can be "parked" (inactive) in a piconet.

DECT (Digital Enhanced Cordless Telecommunications)

• Architecture: Cordless standard, not cellular. Fixed part (Base Station, connected to PSTN) and Portable Part (handset).

• Application Domain: Primarily residential/office cordless telephony. Also supports low-rate data (DECT Data Services - DDS).

• Key Feature: Uses TDMA/TDD in 1.88-1.90 GHz band. 10 carriers, each with 12 time slots (6 for downlink, 6 for uplink). Supports fast handover between base stations.

TETRA (Terrestrial Trunked Radio)

• Architecture: Professional Mobile Radio (PMR) standard for public safety (police, fire, ambulance), transport, utilities.

• Application Domain: Mission-critical, group communication (push-to-talk), with strong emphasis on reliability, security, and group call.

• Key Feature: Uses TDMA (4 slots per carrier) in 380-470 MHz band. Supports direct mode operation (DMO) where radios communicate peer-to-peer without infrastructure.

Exam Tip: Contrast Bluetooth (short-range, ad-hoc piconet, consumer) with DECT (cordless phone, fixed infrastructure) and TETRA (long-range PMR, group comms, mission-critical). Remember Bluetooth's scatternet and TETRA's DMO.

6. Ad Hoc & Mobile Networks

Ad Hoc Networks

• Definition: A self-configuring, infrastructure-less network of mobile nodes connected by wireless links.

• Key Characteristics:

  • No central administration or fixed infrastructure.

  • Dynamic topology (nodes move freely).

  • Multi-hop routing (nodes act as routers).

  • Limited resources (battery, bandwidth).

• Performance Issues:

  1. Routing: Finding and maintaining routes in a dynamic topology is the primary challenge.

  2. Security: Vulnerable to eavesdropping, spoofing, insider attacks (no central authority).

  3. QoS: Difficult to guarantee due to dynamic links and contention.

  4. Power Consumption: Battery-powered nodes; routing must be energy-efficient.

• Applications: Military battlefield, emergency/disaster relief, sensor networks, conferencing, vehicular networks (VANET).

MANET Routing: Dynamic Source Routing (DSR)

• Operation: Source routing protocol. The source node specifies the complete hop sequence in the packet header.

• Two Main Mechanisms:

  1. Route Discovery: Source broadcasts Route Request (RREQ) packets. Each node appends its address to the packet. When RREQ reaches destination, it unicasts Route Reply (RREP) back along the recorded path. Source caches the route.

  2. Route Maintenance: If a link breaks during transmission, a Route Error (RERR) is sent back to source. Source initiates new discovery. Nodes also use passive acknowledgments (hearing next-hop transmit) and link-layer acknowledgments.

• Advantages: No periodic routing traffic (on-demand), simple, caches multiple routes.

• Disadvantages: Route caching can lead to stale routes in high mobility. RREQ flood can cause control overhead.

Reference Model for Wireless & Mobile Networks

Diagram:

DiagramSEARCH: "wireless and mobile networks reference model cross layer design diagram"

• Layered Architecture: Based on OSI/TCP-IP but with adaptations:

  • Physical & MAC: Handle wireless medium, errors, access.

  • Network: IP with mobility support (Mobile IP).

  • Transport: TCP/UDP with adaptations for wireless losses.

  • Application: May be aware of mobility (e.g., VoIP).

• Cross-Layer Design: Crucial in wireless. Information is shared across layers to optimize performance (e.g., MAC informs network layer about link quality for routing decisions; physical layer informs transport about error rates).

7. Mobility & Transport Layer Support

Mobile IP

Enables a mobile node to maintain permanent IP address while moving.

• Key Entities:

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

  • Foreign Agent (FA): Router in visited network. Provides Care-of Address and forwards tunneled packets.

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

  • Mobile Node (MN): The moving host.

• Tunneling & Encapsulation: HA encapsulates original IP packet (with MN's home address) inside a new IP packet addressed to CoA. FA decapsulates and delivers to MN.

• Encapsulation Types:

  1. IP-in-IP Encapsulation (RFC 2003): Original IP packet is payload of new IP packet. Simple, but adds 20-byte header.

  2. Minimal Encapsulation (RFC 2004): Reduces overhead by removing redundant fields from original header. Saves ~8-12 bytes.

  3. Generic Routing Encapsulation (GRE - RFC 1701): More general, supports multiple protocols. Adds GRE header (4 bytes) + new IP header.

Process: 1. Agent Discovery (MN finds HA/FA). 2. Registration (MN registers CoA with HA). 3. Tunneling (HA tunnels packets to CoA). 4. Decapsulation (FA or MN removes outer header).

DHCP in Mobile Environments

• Role: Dynamic Host Configuration Protocol automatically assigns IP addresses, subnet masks, default gateways, DNS servers to devices joining a network.

• In Mobile Context: When a mobile node enters a new foreign network (with FA), it can use DHCP to obtain a Co-located CoA dynamically. This avoids dependency on FA's address and allows direct routing (triangle routing optimization).

TCP for Mobile Environments

• Challenges:

  1. High Bit Error Rate (BER): Wireless links have higher errors than wired → TCP interprets as congestion → unnecessary slow start.

  2. Long Latency & Variable Delay: Satellite links, handoffs cause delays → spurious timeouts.

  3. Frequent Disconnections: Handoffs, power save mode → TCP connection breaks.

• Classical Approaches & Trade-offs:

  | Approach | Principle | Advantages | Disadvantages | | :--- | :--- | :--- | :--- | | Indirect TCP (I-TCP) | Split TCP connection at FA. FA acts as proxy to HA. | Hides wireless errors from wired TCP; no changes to fixed hosts. | Triangle routing (inefficient); FA stateful; breaks end-to-end semantics. | | Snooping TCP | FA snoops packets (ACKs, data) between MN & CN. Retransmits lost packets locally. | Maintains end-to-end connection; hides wireless losses. | FA must buffer packets; works only for downlink; complex. | | Mobile TCP (M-TCP) | FA breaks connection into two: MN-FA (wireless, reliable) and FA-CN (wired, standard TCP). FA uses window scaling to avoid congestion on wireless part. | Separates wired/wireless problems; efficient. | Requires FA modification; asymmetric connection. | | Fast Retransmit/Recovery Adaptations | Modify TCP's congestion control (e.g., Eifel algorithm) to distinguish wireless vs. congestion loss using timestamps or SACK. | End-to-end; no proxies. | Requires changes to TCP stack; not universally deployed. |

Exam Tip: For TCP approaches, focus on where the connection is split (I-TCP, M-TCP) or how losses are hidden (Snooping). Contrast their impact on end-to-end semantics and triangle routing.

8. Security in Wireless & Mobile Networks

Firewalls

• Definition: A security device (hardware/software) that monitors and controls incoming/outgoing network traffic based on predetermined security rules.

• Purpose: Establish a barrier between trusted internal network and untrusted external network (e.g., Internet).

• Issues in Wireless/Mobile Design:

  1. Dynamic IP Addresses: Mobile nodes change IP addresses (roaming). Traditional firewalls rely on static IPs for rules.

  2. Roaming & Handoffs: Security policy must follow the user across networks. Requires distributed or mobile-aware firewalls.

  3. Application-Layer Gateways (ALG): Needed for protocols like SIP, FTP that embed IP addresses in payload. Complex to implement for all mobile apps.

  4. Wireless Link Vulnerability: Firewall protects network edge but cannot secure the airlink between mobile node and AP/BS. Requires link-layer encryption (WPA2/WPA3).

  5. Device Theft/Loss: Mobile devices are prone to loss. Firewall rules on device itself (personal firewall) become critical.

Intrusion Detection Systems (IDS)

• Definition: Monitors network or system activities for malicious actions or policy violations.

• Types:

  • NIDS (Network-based IDS): Monitors network traffic (e.g., on wired backbone). Can detect attacks on wireless infrastructure but cannot see airlink directly unless placed at AP.

  • HIDS (Host-based IDS): Runs on individual mobile device. Monitors system calls, file integrity. Crucial for detecting malware on lost/stolen devices.

• Relevance to Wireless: Wireless networks are inherently more accessible. NIDS must be placed to monitor wireless segments. Wireless IDS (WIDS) specifically monitors 802.11 frames for attacks (e.g., deauthentication floods, evil twin APs).

Password Management

• Methods:

  1. One-Time Passwords (OTP): Password valid for single session/transaction. Generated by token or algorithm (e.g., RSA SecurID, TANs). Resists replay attacks.

  2. Biometrics: Fingerprint, face, iris. "Something you are". High security but privacy concerns, false rejects.

  3. Challenge-Response: Server sends random challenge; client responds using a shared secret (e.g., password + hash). Preents replay.

  4. Graphical Passwords: Select images/points on screen. Easier to remember, vulnerable to shoulder surfing.

• Challenges:

  • Usability vs. Security: Complex passwords are secure but hard to remember on small devices.

  • Device Theft: Stolen device with saved passwords is a major risk. Requires remote wipe capabilities.

  • Small Input Devices: Typing long alphanumeric passwords on mobile keypads is cumbersome.

  • Synchronization: OTP tokens or time-based algorithms must be synced with server.

9. Wireless Sensor Networks (WSN)

Architecture

Diagram:

DiagramCANVAS: "WSN Architecture: 1. Sensor Nodes (with sensor, processor, memory, transceiver, battery). 2. Base Station/Gateway (more powerful, connects to Internet). 3. Hierarchical/Flat topology. Show data flow: Sensors -> multi-hop -> BS -> User."

• Typical Layered Architecture (similar to TCP/IP but constrained):

  • Physical Layer: Handles signal modulation, frequency selection.

  • Data Link Layer (MAC): Controls access to shared medium (e.g., S-MAC, B-MAC). Crucial for energy efficiency.

  • Network Layer: Handles routing (e.g., directed diffusion, LEACH, GPSR).

  • Transport Layer: Often minimal or absent. Provides reliability if needed (e.g., for critical data).

  • Application Layer: Defines specific monitoring tasks (temperature, motion).

• Components:

  • Sensor Node (Mote): Low-cost, low-power, limited processing/memory. Battery-powered (often irreplaceable).

  • Base Station (Sink/Gateway): More powerful node that aggregates data and connects WSN to external networks (Internet).

  • Task Manager/User: The entity that queries and receives data.

Applications

• Environmental Monitoring: Forest fire detection, habitat monitoring, precision agriculture (soil moisture).

• Military: Enemy tracking, battlefield surveillance, nuclear/chemical attack detection.

• Health: Patient monitoring, drug administration, wearable sensors.

• Smart Homes/Buildings: HVAC control, lighting, security.

• Industrial: Machine monitoring, supply chain management.

Key Distinction from MANET: WSN nodes are densely deployed, resource-constrained (energy!), and data-centric (report events, not address-specific). MANET nodes are more powerful, address-centric, and focus on connectivity.

Final Exam Checklist:

• [ ] Draw & Explain: Propagation models, Cellular architecture, GSM/GPRS/UMTS architecture, Bluetooth piconet/scatternet, WSN architecture.

• [ ] Compare & Contrast: GSM vs GPRS, HIPERLAN/1 vs /2, TCP approaches (I-TCP, Snooping, M-TCP).

• [ ] Define & Describe: Handoff, Sectorization, Hidden/Exposed Terminal, Ad Hoc Network, Mobile IP entities.

• [ ] Explain Principles: DAMA, PRMA, FHSS (Bluetooth), RTS/CTS, Cross-layer design.

• [ ] List Challenges: WSN (energy), Password management, Firewall design for mobile, Ad Hoc routing.