Wireless and Mobile Computing (IT-602) - Unit 2 Short Notes

1.0 Radio Wave Propagation & Antennas

1.1 Propagation Models

Radio wave propagation describes how electromagnetic waves travel from transmitter to receiver. The dominant mechanism depends on frequency and distance.

[!TIP] Exam Focus: Be prepared to sketch the Earth's ionospheric layers for sky wave and the radio horizon for LOS. Ground wave is often confused with sky wave—remember ground wave stays close to Earth.

1.2 Antenna Types: Helical Antenna

A helical antenna is a wire wound in a helix shape, often mounted over a ground plane.

• Operation: Operates in two primary modes:

  1. Normal Mode: Helix diameter small compared to wavelength. Radiates like a dipole, perpendicular to axis. Low gain, used in compact devices.

  2. Axial Mode (Most Common): Helix circumference ≈ wavelength. Radiates circularly polarized waves along the antenna axis. High gain, wide bandwidth.

• Characteristics:

  • Circular Polarization: Reduces signal degradation due to polarization mismatch.

  • Wide Bandwidth: Typically 10-15%.

  • Gain: Proportional to number of turns; can be 10-15 dBi.

  • Input Impedance: ~140 Ω in axial mode.

• Applications: Satellite communication, space telemetry, RFID, GPS (where circular polarization is beneficial).

[!TIP] Exam Focus: Always specify axial mode when discussing helical antennas for high-gain applications. Sketch should show helix, ground plane, and radiation pattern (donut-shaped along axis).

2.0 Cellular Communication Fundamentals

2.1 Cellular Network Architecture & Concept

• Why "Cellular"? The service area is divided into small geographic regions called cells. Each cell has a base station (BS). The same frequencies can be reused in non-adjacent cells, enabling frequency reuse and efficient spectrum utilization. This is the core cell concept.

• Basic Architecture:

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

  • Base Station Subsystem (BSS): Contains Base Transceiver Station (BTS) (radio equipment) and Base Station Controller (BSC) (manages multiple BTSs, handoffs).

  • Network Switching Subsystem (NSS): Contains Mobile Switching Center (MSC) (core switch, routing, mobility management), Home Location Register (HLR) (permanent subscriber data), Visitor Location Register (VLR) (temporary data for visiting mobiles), Authentication Center (AuC), Equipment Identity Register (EIR).

  • Public Switched Telephone Network (PSTN): Connects cellular network to landlines.

  • Gateway MSC (GMSC): Connects MSC to PSTN.

[!DIAGRAM: CANVAS: Draw hexagonal cells, each with a BS. Show BTS connected to BSC, multiple BSCs connected to MSC, MSC connected to PSTN/GMSC. Label HLR/VLR attached to MSC.]

2.2 Key Cellular Concepts

• Handoff / Handover:

  • Definition: The process of transferring an ongoing call from one cell (or channel) to another as the mobile user moves, without dropping the call.

  • Types:

    1. Hard Handoff: Break-before-make. Connection to old BS is terminated before new one is established (used in GSM, TDMA).

    2. Soft Handoff: Make-before-break. Mobile connects to new BS before releasing old one (used in CDMA). Provides diversity gain.

    3. Intra-cell Handoff: Changing channels within the same cell (due to interference).

    4. Inter-cell Handoff: Moving to a different cell.

• Sectorization:

  • Definition: Dividing a cell into smaller sectors (typically 2, 3, or 6) using directional antennas at the BS.

  • Purpose:

    • Reduces co-channel interference.

    • Increases system capacity (more frequency reuse).

    • Improves signal quality by focusing energy.

3.0 Multiple Access Techniques

3.1 Overview

Allows multiple users to share a finite spectrum simultaneously.

3.2 Specific Multiple Access Schemes

• Demand Assigned Multiple Access (DAMA):

  • Channels are assigned on-demand based on traffic request.

  • A pool of channels is managed by a central controller (e.g., satellite hub).

  • Efficient for bursty traffic (e.g., VSAT networks).

• Packet Reservation Multiple Access (PRMA):

  • Combines TDMA with slotted ALOHA.

  • Time slots are grouped into frames. A user reserves a slot by sending a packet in a contention slot.

  • Once reserved, slot is used in subsequent frames until release. Used in satellite and wireless LANs.

• Priority-Based Multiple Access:

  • Users are assigned priority levels.

  • Access protocols (e.g., in HIPERLAN) give preference to high-priority traffic.

  • Implemented via contention window adjustments or reservation mechanisms.

3.3 HIPERLAN-1 Channel Access Methods

HIPERLAN-1 uses three complementary channel access mechanisms:

1. EY-NPMA (Elimination-Yield Non-Preemptive Priority Multiple Access):

   • A contention-based protocol with priority support.

   • Phases: Prioritization (high-priority nodes win), Elimination (contending nodes back off based on random number), Yield (lowest priority yields).

2. R-ALOHA (Reservation ALOHA):

   • Slotted ALOHA with reservation.

   • Successful transmission in a slot reserves that slot for the same user in subsequent frames.

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

   • Used for best-effort traffic.

   • Nodes sense channel; if idle, transmit after a random backoff. Includes RTS/CTS to mitigate hidden terminals.

[!TIP] Exam Focus: HIPERLAN-1's three access methods serve different traffic types: EY-NPMA for prioritized, R-ALOHA for reserved, CSMA/CA for best-effort. Be ready to contrast them.

4.0 Cellular System Standards: GSM & GPRS

4.1 Global System for Mobile Communications (GSM)

• Architecture & Components (Detailed):

  • Mobile Station (MS): ME (Mobile Equipment) + SIM (Subscriber Identity Module).

  • Base Station Subsystem (BSS): BTS + BSC.

  • Network Switching Subsystem (NSS): MSC + HLR + VLR + AuC + EIR.

  • Operation Support Subsystem (OSS): For network management.

  • ** Interfaces:** A (BSC-MSC), Abis (BTS-BSC), Um (MS-BTS, radio).

• Services Available:

  • Voice: Circuit-switched telephony.

  • Data: Circuit-switched data (CSD) up to 9.6 kbps.

  • SMS: Short Message Service (store-and-forward).

  • Supplementary: Call forwarding, barring, conferencing.

4.2 General Packet Radio Service (GPRS)

• Architecture & Components (Detailed):

  • New Network Elements:

    • Serving GPRS Support Node (SGSN): Tracks MS location, handles session management, authentication (similar to VLR for packet domain).

    • Gateway GPRS Support Node (GGSN): Gateway to external PDNs (Internet, X.25). IP address assignment, tunneling.

    • PCU (Packet Control Unit): Often added to BSC to handle packet switching.

  • Existing Elements: BSS, HLR (now stores GPRS profile), MSC (for voice).

  • Interfaces: Gb (BSS-SGSN), Gn (SGSN-GGSN), Gi (GGSN-PDN), Gs (SGSN-MSC for coordination).

• Working Principle:

  • Mobile requests PDP context (Packet Data Protocol address, e.g., IP).

  • SGSN authenticates, checks HLR, assigns IP (from GGSN pool).

  • Data packets routed via SGSN to GGSN, then to Internet. Always-on, packet-switched.

4.3 Comparison: GSM vs. GPRS

[!TIP] Exam Focus: The fundamental difference is circuit vs. packet switching. GPRS is an overlay on GSM, sharing radio infrastructure but adding core packet nodes (SGSN/GGSN).

5.0 Wireless Protocols & Standards

5.1 Bluetooth

• Network Architecture:

  • Piconet: Basic unit. 1 master (controls clock, hopping sequence) + up to 7 active slaves. 1 master can have 255 parked slaves.

  • Scatternet: Multiple piconets interconnected via a slave that participates in more than one piconet (time-division multiplexing).

• Physical Layer:

  • Frequency: 2.4 GHz ISM band (2402-2480 MHz).

  • Modulation: FHSS (Frequency Hopping Spread Spectrum). 79 channels (1 MHz spacing), hopping rate 1600 hops/sec.

  • Data Rate: Basic Rate: 1 Mbps (BR), Enhanced Data Rate: 2-3 Mbps (EDR).

• MAC Layer:

  • Access Method: TDMA + FHSS. Master allocates time slots (625 µs) in a polling fashion.

  • Packet Structure: Access code (synchronization), header (ARQN, SEQN), payload.

  • Error Control: ARQ (Automatic Repeat Request) with 1/3 rate FEC for headers.

5.2 Digital Enhanced Cordless Telecommunications (DECT)

• Architecture & Features:

  • Frequency: 1.9 GHz band (1880-1990 MHz in Europe).

  • Access: TDMA/TDD. 10 frames/sec, each frame has 24 time slots (12 downlink, 12 uplink).

  • Cells: 10/100 cells (microcellular). Handoff between cells seamless.

  • Focus: High-quality voice, but supports data (up to 552 kbps).

  • Architecture: Portable part (PP) + fixed part (FP). Interworking with PSTN/ISDN via DECT base station.

5.3 Terrestrial Trunked Radio (TETRA)

• Architecture & Features (Public Safety Focus):

  • Frequency: 380-470 MHz (UHF).

  • Access: 4-slot TDMA per carrier (25 kHz channel).

  • Mode: Trunked mode (like cellular) + direct mode (device-to-device, no infrastructure).

  • Key Features: Group call (push-to-talk), priority call, emergency call, wide area coverage, robust security (encryption, authentication).

  • Users: Police, fire, ambulance, military, transport.

5.4 Universal Mobile Telecommunications System (UMTS)

• Overview & Key Features (3G):

  • Goal: Provide mobile broadband (voice + high-speed data).

  • Air Interface: W-CDMA (Wideband CDMA). 5 MHz carrier, chip rate 3.84 Mcps.

  • Architecture: UTRAN (UMTS Terrestrial Radio Access Network) with Node B (BS) and RNC (Radio Network Controller). Core network evolves from GSM (MSC, SGSN, GGSN).

  • Data Rates: Up to 2 Mbps (indoor), 384 kbps (outdoor).

  • Services: Video calling, mobile internet, multimedia messaging.

5.5 HIPERLAN (High Performance Radio LAN)

• HIPERLAN Versions (Overview & Sketches):

  • HIPERLAN/1 (1996): 5 GHz band, up to 23 Mbps, EY-NPMA access. For data.

  • HIPERLAN/2 (1999): OFDM physical layer, QoS support via DFP (Dynamic Frequency Selection) and DCA (Dynamic Channel Allocation). Converges with 3G (UMTS). Focus on multimedia.

  • HIPERLAN/3: Designed for fast-moving vehicles (high mobility).

  • HIPERLAN/4: Also known as HIPERACCESS, for broadband wireless access (point-to-multipoint).

  [!DIAGRAM: SEARCH: "HIPERLAN versions comparison diagram"]

• Physical Layer (HIPERLAN-1):

  • Frequency: 5 GHz band (5.15-5.30 GHz & 5.47-5.725 GHz).

  • Modulation: GFSK (Gaussian Frequency Shift Keying).

  • Data Rates: 1.5, 2, and 4 Mbps (optional 8, 16, 23 Mbps with coding).

• Access Control Sublayer (HIPERLAN-1):

  • Implements the three channel access methods (EY-NPMA, R-ALOHA, CSMA/CA) as described in Section 3.3.

  • Provides prioritization and reservation for different traffic types.

6.0 Wireless Local Area Networks (WLANs)

6.1 IEEE 802.11 Family

• Architecture of 802.11 LAN:

  • Infrastructure Mode: Devices communicate via an Access Point (AP). AP connects to wired DS (Distribution System, usually Ethernet). Basic Service Set (BSS) = one AP + associated stations.

  • Ad-hoc (IBSS) Mode: Peer-to-peer network without AP. Stations communicate directly. Independent BSS.

  • Components: Station (STA), AP, DS (wired backbone).

  [!DIAGRAM: SEARCH: "802.11 infrastructure vs ad-hoc mode diagram"]

• Physical Layer Technologies & Frequencies:

  • 802.11b/g/n: 2.4 GHz ISM band. Uses DSSS (b) and OFDM (g/n). 2.4 GHz propagates well through walls but crowded (Bluetooth, microwave).

  • 802.11a/n/ac/ax: 5 GHz band. Uses OFDM (a) and OFDMA (ax). Less interference, more non-overlapping channels, but shorter range (higher frequency).

  • Why specific waves/frequencies? 2.4 GHz offers better range/penetration; 5 GHz offers more bandwidth, less congestion. Regulatory domains (FCC, ETSI) allocate these ISM bands license-free.

• Specific Standard: IEEE 802.11a:

  • Frequency: 5 GHz band (5.15-5.35 GHz, 5.725-5.825 GHz).

  • Modulation: OFDM (52 subcarriers, 48 data + 4 pilots).

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

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

  • Channels: 12 non-overlapping 20 MHz channels (in US).

6.2 Wireless LAN Communication Waves

• Types of Waves Used: Primarily radio waves in microwave frequency bands (2.4 GHz, 5 GHz). Some systems (IR) exist but are niche.

• Reasoning for Wave Selection:

  1. Propagation: Radio waves penetrate non-metallic obstacles (walls), enabling indoor coverage.

  2. Bandwidth: Microwave bands offer sufficient bandwidth for high data rates (tens to hundreds of Mbps).

  3. Regulation: ISM bands are license-free, reducing deployment cost.

  4. Antenna Size: At GHz frequencies, antennas are compact.

  5. Attenuation: Higher frequencies (5 GHz) suffer higher free-space path loss, limiting range but enabling frequency reuse.

7.0 Ad Hoc & Sensor Networks

7.1 Mobile Ad Hoc Networks (MANETs)

• Definition & Characteristics:

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

  • Characteristics:

    • Dynamic topology (nodes move freely).

    • Multi-hop routing (no fixed infrastructure).

    • Limited bandwidth, battery power, security vulnerable.

    • Distributed operation.

• Performance Issues:

  • Routing: Frequent topology changes require efficient, adaptive routing protocols (e.g., DSR, AODV).

  • Security: Vulnerable to eavesdropping, spoofing, wormholes (no central authority).

  • Power: Battery-powered nodes; energy-efficient protocols critical.

  • QoS: Difficult to guarantee due to mobility and contention.

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

7.2 Wireless Sensor Networks (WSNs)

• Architecture:

  • Sensor Nodes: Small, cheap, battery-powered. Sense (temperature, light, motion), process, transmit. Have limited processing, memory, energy.

  • Sink/Base Station: Collects data from sensor nodes. May connect to external network (Internet).

  • Typical Topology: Many sensor nodes → multi-hop → sink → base station → user.

  [!DIAGRAM: SEARCH: "wireless sensor network architecture diagram"]

• Applications:

  • Environmental Monitoring: Forest fires, pollution, habitat.

  • Health Monitoring: Patient vital signs.

  • Industrial: Machine monitoring, inventory.

  • Military: Intrusion detection, battlefield surveillance.

  • Smart Home: Automation, security.

8.0 Mobility Support in Network Layer & Transport Layer

8.1 Mobility Management & Tunneling

• Reference Model for Wireless/Mobile Networks:

  • Layered Approach: Application, Transport, Network, Link, Physical.

  • Mobility handled at Network Layer (IP) and above.

  • Key Entities: Mobile Node (MN), Home Agent (HA), Foreign Agent (FA), Correspondent Node (CN).

  • Tunneling: Encapsulating IP packet from HA to FA with new IP header.

  [!DIAGRAM: SEARCH: "mobile IP reference model diagram"]

• Tunneling & Encapsulation Protocols:

  1. IP-in-IP Encapsulation (RFC 2003):

     • Original IP packet is encapsulated inside a new IP packet.

     • Outer header: HA→FA. Inner header: CN→MN.

     • Header Fields: Outer IP header (src=HA, dst=FA), then original IP packet.

  2. Minimal Encapsulation (RFC 2004):

     • Reduces overhead by not duplicating IP header fields.

     • Outer header: HA→FA. Inner: modified original header (MN's home address moved to option field).

     • More efficient than IP-in-IP.

  3. Generic Routing Encapsulation (GRE - RFC 2784):

     • Generic tunneling protocol for any network layer protocol.

     • Adds GRE header (flags, protocol type) between outer and inner headers.

     • Used in VPNs, mobile IP.

8.2 Routing in Ad Hoc Networks: Dynamic Source Routing (DSR)

• Operation:

  • Source Routing: Entire route is stored in packet header by source.

  • Two Main Mechanisms:

    1. Route Discovery: Source broadcasts Route Request (RREQ) with destination address. Each node appends its address to RREQ. When RREQ reaches destination, it sends back Route Reply (RREP) with full route path (either via reverse path or new RREQ).

    2. Route Maintenance: If link breaks during transmission, source notified via Route Error (RERR). Source can try alternate route from cache or initiate new discovery.

  • Route Caching: Every node caches routes it learns, reducing discovery overhead.

8.3 Configuration & Addressing: DHCP for Mobile Environments

• Dynamic Host Configuration Protocol (DHCP):

  • Automatically assigns IP addresses, subnet masks, default gateways, DNS servers to hosts.

  • Process: DORA - Discover, Offer, Request, Acknowledgement.

  • For Mobile Hosts: When MN moves to a new foreign network, it needs a Care-of Address (CoA). DHCP can be used by FA to assign this temporary IP address to the visiting MN.

8.4 Transport Layer Challenges & Classical Approaches

• Problems with Standard TCP:

  • Fading & Bit Errors: TCP interprets as congestion, reduces window unnecessarily.

  • Handoff: Long disconnection during handoff causes TCP timeout, window collapse.

  • High RTT & Variable RTT: Poor congestion window adjustment.

  • Asymmetric Links: Uplink/downlink bandwidth mismatch.

• Classical Approaches to Improve TCP:

  | Approach | Principle | Advantages | Disadvantages | |--------------|---------------|----------------|-------------------| | Indirect TCP (I-TCP) | Break TCP connection at FA. FA has separate TCP with MN (wireless) and with CN (wired). | Isolates wireless losses; no CN modifications. | Inefficient (two TCP connections); FA bottleneck. | | Snooping TCP | FA snoops packets between MN and CN. Buffers data, does local retransmission for wireless losses. | No CN changes; maintains end-to-end semantics. | FA must parse TCP; complex buffering. | | Mobile TCP (M-TCP) | Split connection at FA, but FA shuts down TCP connection to CN during disconnection. Uses window reduction to avoid CN congestion. | Prevents CN window collapse; efficient during disconnection. | Requires FA modifications; not pure end-to-end. | | Fast Retransmit/Recovery Modifications | Modify TCP's fast retransmit to distinguish wireless vs. congestion loss (e.g., using ECN or explicit loss notification). | Maintains end-to-end; efficient. | Requires router/FA support; complex. |

[!TIP] Exam Focus: Know the core idea of each TCP variant: I-TCP (break connection), Snooping (local retransmit), M-TCP (window shutdown during disconnection). Compare based on end-to-end semantics and efficiency.

9.0 Medium Access Control (MAC) Sublayer Issues

9.1 Wireless Channel Access Problems

• Hidden Terminal Problem:

  • Cause: Two stations (A, C) are out of range of each other but both within range of a common receiver (B). A and C cannot sense each other's transmission.

  • Effect: A and C may transmit to B simultaneously, causing collision at B.

  • Solution: RTS/CTS handshake (as in 802.11).

  [!DIAGRAM: SEARCH: "hidden terminal problem diagram"]

• Exposed Terminal Problem:

  • Cause: Station B is transmitting to A. Station C is within range of B but not of A. C wants to transmit to D (out of range of B).

  • Effect: C senses B's transmission and defers, even though C's transmission to D would not interfere with B's transmission to A. Reduces spatial reuse.

  • Solution: Careful RTS/CTS design; directional antennas.

9.2 MAC Layer Protocols

• General Functions & Challenges: Channel access control, addressing, error control (often via ARQ), fragmentation/reassembly. Challenges: hidden/exposed terminals, capture effect, fairness.

• MAC Layer of Bluetooth: See Section 5.1.3 (TDMA + FHSS, master-slave polling).

• MAC Layer of HIPERLAN: See Section 3.3 & 5.5.3 (EY-NPMA, R-ALOHA, CSMA/CA with prioritization).

10.0 Security in Wireless & Mobile Environments

10.1 Firewalls

• Definition & Purpose: A firewall is a network 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 Firewall Design for Wireless/Mobile Networks:

  1. Dynamic IP Addresses: Mobile nodes often get temporary IPs (via DHCP), making static rule-based filtering difficult.

  2. Roaming: When MN moves between networks, firewall policies must follow or be consistent across networks.

  3. Wireless Link Exposure: The "last hop" is wireless, vulnerable to eavesdropping; firewall alone cannot secure this segment.

  4. VPN Integration: Need to allow VPN traffic (IPsec, SSL) through while maintaining security.

  5. Application-Layer Threats: Modern firewalls (NGFW) must inspect application traffic, which is complex in mobile environments.

10.2 Intrusion Detection Systems (IDS)

• Definition & Types:

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

  • Types:

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

    • Host-based IDS (HIDS): Monitors activities on a single host (e.g., file integrity, log analysis).

• Role in Wireless Security:

  • Detect wireless-specific attacks: Rogue APs, MAC spoofing, DoS (deauthentication attacks), abnormal traffic patterns.

  • Monitor 802.11 management frames (beacons, probes).

  • Often integrated with Wireless Intrusion Detection/Prevention Systems (WIDS/WIPS).

10.3 Password Management

• Challenges in Wireless/Mobile Context:

  1. Theft/Loss: Mobile devices easily lost/stolen; passwords must protect data.

  2. Shoulder Surfing: Users enter PINs/passwords in public places.

  3. Small Keypads: Complex passwords hard to enter on mobile keyboards.

  4. Frequent Authentication: Users may need to re-authenticate often (e.g., after handoff, screen lock).

  5. Synchronization: Passwords must sync across devices/services.

• Various Methods & Mechanisms:

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

  2. Hardware Tokens: RSA SecurID, smart cards (two-factor).

  3. One-Time Passwords (OTP): Time-based (TOTP) or counter-based.

  4. Graphical Passwords: Select images/points on screen (easier on touchscreens).

  5. Password Managers: Securely store and auto-fill complex passwords.

  6. Multi-Factor Authentication (MFA): Combine password with SMS OTP, biometrics.

[!TIP] Exam Focus: For password management, link challenges to mobile-specific issues (loss, small keypad). Methods should address these (biometrics for convenience, MFA for security).