Wireless and Mobile Computing (IT-602) - Unit 1 Short Notes

UNIT 1: Wireless and Mobile Computing

I. Fundamentals of Wireless Communication

A. Wave Propagation Models

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

Key Point: Frequency dictates the dominant propagation mode. Lower frequencies bend with the Earth; higher frequencies travel straight.

B. Antenna Types and Characteristics

Helical Antenna: A broadband, circularly polarized antenna.

• Construction: A conducting wire wound in a helix (spring-like) shape, mounted over a ground plane.

• Operation: The helix dimensions (circumference, spacing) determine mode.

  • Normal Mode: Helix diameter << wavelength. Operates like an electric dipole. Low gain, omnidirectional.

  • Axial Mode: Helix circumference ≈ 1 wavelength, spacing ≈ 0.25λ. Radiates circularly polarized waves along the helix axis (end-fire). This is the primary mode for satellite communication.

• Advantages: Wide bandwidth, simple construction, high gain in axial mode, circular polarization reduces signal loss due to polarization mismatch.

C. Wireless Channel Characteristics

1. Hidden Terminal Problem:

• Scenario: Station A can communicate with Station B. Station C is within B's range but outside A's range. A and C cannot sense each other's transmissions.

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

• Consequence: Wasted bandwidth, retransmissions.

Classic Example: A---B---C (A and C hidden from each other).

2. Exposed Terminal Problem:

• Scenario: Station A is transmitting to Station B. Station C is within A's range (exposed to A's transmission) but cannot communicate with B (out of B's range).

• Issue: C unnecessarily defers its own transmission to another node D (which is within C's and D's range but far from A/B) because it senses the channel is busy (due to A's transmission).

• Consequence: Underutilization of network capacity; C could have transmitted to D without interfering with A→B.

Classic Example: A---B, C---D, with A and C within range, but B and D not. A→B blocks C→D unnecessarily.

II. Multiple Access and Medium Access Control

A. Multiple Access Techniques

Fundamental Schemes:

Demand Assigned Multiple Access (DAMA):

• A pool of channels (FDMA/TDMA) is shared.

• A user requests a channel from a central controller (e.g., satellite hub) when it has data.

• Controller assigns an available channel dynamically.

• Advantage: Efficient for bursty traffic; channels are allocated on-demand.

Packet Reservation Multiple Access (PRMA):

• Combines TDMA with slotted ALOHA contention.

• Time is divided into slots and frames.

• Users with packets contend for slots in the first frame.

• If contention is successful, the slot is reserved for that user in subsequent frames until the packet burst is complete.

• Used in: Satellite systems, some wireless LANs.

Priority-Based Multiple Access:

• Implements Quality of Service (QoS) by assigning different access priorities.

• Implementation Methods:

  1. Priority Queuing: Different queues with different backoff parameters.

  2. Different Inter-Frame Spacing (DIFS, SIFS in 802.11): High-priority traffic uses shorter waiting times.

  3. Slot Assignment: In TDMA, high-priority users get more or earlier slots.

B. MAC Layer Principles and Challenges

General MAC Functions:

• Frame Delimiting & Addressing: Identify start/end of frame, source/dest addresses.

• Channel Access Control: Govern when a station can transmit (contention-based or -free).

• Error Detection: CRC checks.

• Reliability: ACK/NACK mechanisms.

• Security: Basic encryption/authentication at link layer.

Contention-Based vs. Contention-Free:

III. Cellular Communication Systems

A. Cellular Architecture and Design Principles

• Why "Cellular"? The service area is divided into small geographic regions called cells. Each cell has a base station (BS). The same frequency bands can be reused in non-adjacent cells, enabling frequency reuse and spectral efficiency.

• Cell Structure: Typically modeled as hexagons (ideal coverage). Real coverage is irregular.

• Frequency Reuse: The set of frequencies used in a cell is called a channel set. The reuse factor (N) is the number of cells in a cluster where channel sets are unique. N = i² + j² + i*j (i, j are integers). Smaller N → higher capacity but more co-channel interference.

• Sectorization: A cell is divided into sectors (e.g., 120° or 60°) using directional antennas at the BS. Each sector uses a different channel set. Increases capacity without new cell sites, reduces interference.

B. Handoff/Handoever Mechanisms

Definition: The process of transferring an ongoing call or data session from one cell (source BS) to another (target BS) as the mobile user moves.

• Types:

  • Hard Handoff: Break-before-make. Connection with source BS is terminated before connection with target BS is established. Used in GSM, CDMA (often). Simple but possible drop if target fails.

  • Soft Handoff: Make-before-break. Mobile connects to multiple BSs simultaneously (in same frequency) during transition. Mobile combines signals (CDMA). No interruption, better reliability, but complex and uses more resources.

  • Softer Handoff: A special case of soft handoff where multiple BSs are in the same BSC (Base Station Controller).

C. GSM (Global System for Mobile Communications)

Architecture:

• Components:

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

  • BSS (Base Station Subsystem):

    • BTS (Base Transceiver Station): Radio transceiver, handles Um interface.

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

  • NSS (Network Switching Subsystem):

    • MSC (Mobile Switching Center): Core switch, call routing, mobility management.

    • HLR (Home Location Register): Permanent database (user profile, current VLR).

    • VLR (Visitor Location Register): Temporary database for visitors in an MSC area.

    • AUC (Authentication Center): Generates authentication parameters.

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

• Interfaces: Um (MS-BTS), Abis (BTS-BSC), A (BSC-MSC), I (MSC-HLR/VLR).

Services: Telephony, SMS, Data (Circuit-Switched, up to 9.6 kbps), Supplementary Services (call forwarding, barring).

Handover in GSM:

1. Measurement: MS measures signal strength of serving & neighboring cells (via BA list).

2. Report: MS sends measurement reports to serving BSC.

3. Decision: BSC/MSC decides if handoff needed.

4. Execution: MSC coordinates. Hard handoff: Old channel released, new channel allocated on target BS. Inter-BSC handoff involves MSC; intra-BSC handled by BSC.

D. GPRS (General Packet Radio Service)

Architecture (Overlay on GSM):

• New Network Elements:

  • SGSN: Mobility management & packet routing for users in its area. Analogous to MSC for packet data. Keeps track of user location (RAI - Routing Area), performs authentication.

  • GGSN: Gateway to external PDNs (Internet). Assigns IP addresses, performs packet filtering, charging data collection.

• Key Interfaces: Gb (BSS-SGSN), Gn (SGSN-GGSN within same PLMN), Gp (SGSN-GGSN between PLMNs), Gr (SGSN-HLR), Gs (SGSN-MSC for coordinated paging).

Comparison: GSM vs. GPRS

IV. Wireless Local Area Network (WLAN) Standards

A. IEEE 802.11 Family

Architecture:

• Basic Service Set (BSS): The fundamental building block.

  • Infrastructure BSS: Stations (STAs) communicate via an Access Point (AP). AP connects to distribution system (DS).

  • Independent BSS (IBSS): Ad-hoc mode; STAs communicate peer-to-peer without AP.

• Distribution System (DS): Typically a wired LAN (Ethernet) that interconnects APs. The portal entity connects DS to other LANs.

• Extended Service Set (ESS): Multiple BSSs connected via DS, providing coverage over larger area. Mobile stations can roam between APs.

Physical Layer Characteristics (General 802.11):

• Waves Used: Radio Waves (RF) primarily in 2.4 GHz ISM band (unlicensed) and 5 GHz UNII band.

• Why RF? Penetrates non-metallic walls, supports mobility, doesn't require line-of-sight (NLOS) for short ranges.

• Modulation/Spread Spectrum:

  • FHSS (Frequency-Hopping Spread Spectrum): 802.11 original (1 & 2 Mbps).

  • DSSS (Direct-Sequence Spread Spectrum): 802.11b (1, 2, 5.5, 11 Mbps).

  • OFDM (Orthogonal Frequency Division Multiplexing): 802.11a/g/n/ac/ax. High spectral efficiency.

IEEE 802.11a Standard Specifics:

• Frequency Band: 5 GHz (U-NII bands: 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz). Less congestion than 2.4 GHz.

• Physical Layer: OFDM with 52 subcarriers (48 data, 4 pilot). Supports data rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps.

• Modulation: BPSK, QPSK, 16-QAM, 64-QAM (higher rates use higher-order modulation).

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

• Key Advantage: Higher data rates, less interference from legacy 2.4 GHz devices (microwaves, Bluetooth).

B. Bluetooth

Network Architecture:

• Piconet: The basic unit. 1 master + up to 7 active slaves (plus 255 parked/sleeping). Master controls clock, hopping sequence, and polling (slaves respond only when polled).

• Scatternet: Multiple piconets interconnected. A device (master or slave) can participate in multiple piconets by time-multiplexing, but can be master in only one at a time.

Physical Layer Specifications:

• Frequency Band: 2.4 GHz ISM band (2400-2483.5 MHz).

• Modulation: GFSK (Gaussian Frequency Shift Keying) for basic rate (1 Mbps). Enhanced Data Rate (EDR) uses π/4-DQPSK (2 Mbps) and 8-DPSK (3 Mbps).

• Hopping: FHSS across 79 (or 40) 1-MHz channels. Hopping rate 1600 hops/sec. Provides resistance to interference and eavesdropping.

MAC Layer Protocols:

• Access Method: TDMA/TDD with polling (master-driven). Master controls medium access.

• Packet Structure: Slots of 625 µs. Packets can be 1, 3, or 5 slots long.

• Hopping Sequence: Determined by master's BD_ADDR and clock. All devices in piconet follow same sequence.

C. HIPERLAN (High Performance Radio LAN)

Different Versions:

• HIPERLAN/1: First standard (1996). Connection-oriented, supports QoS. Channel access: EY-NPMA (see below). Physical layer: 2.4 GHz, FHSS or DSSS (up to 23.5 Mbps).

• HIPERLAN/2: Connection-oriented, designed for QoS (like ATM). Channel access: Centralized, TDMA/TDD with dynamic slot assignment by a central controller (like a point coordinator). Physical layer: OFDM in 5 GHz band (up to 54 Mbps). More aligned with 802.11a.

• HIPERLAN/3 & /4: Focused on HIPERACCESS (broadband access) and HIPERLINK (short-range, high-speed). Not widely deployed.

Channel Access Methods in HIPERLAN-1: EY-NPMA

• EY-NPMA (Elimination-Yield Non-Preemptive Multiple Access): A contention-free protocol with priority support.

  1. Prioritization Phase: Contend based on priority (higher priority wins).

  2. Elimination Phase: Contend based on a random backoff within the winning priority group.

  3. Yield Phase: Lowest-numbered station in contention wins and transmits; others yield.

• Goal: Provide prioritized, collision-free access with low delay for high-priority traffic.

Physical Layer & MAC Sublayer (HIPERLAN-1):

• PLCP (Physical Layer Convergence Protocol): Preamble for synchronization, header for length/rate.

• Physical Layer: Supports both FHSS (79 channels, 1 MHz) and DSSS (CCK, 11 Mbps).

• MAC Sublayer: Implements EY-NPMA, connection-oriented, supports multihop (routing between HIPERLANs).

V. Mobile Ad Hoc Networks (MANETs)

A. Characteristics and Performance Issues

• Characteristics:

  • Infrastructure-less: No fixed base stations or routers.

  • Autonomous & Self-Configuring: Nodes act as both hosts and routers. Network formed spontaneously.

  • Dynamic Topology: Nodes move freely; links frequently break/reform.

  • Limited Resources: Battery power, bandwidth, processing capacity.

  • Multi-hop Routing: May require multiple hops to reach destination.

  • Security Vulnerable: Open medium, no central authority, mobile nodes.

• Performance Issues:

  • Routing: Finding and maintaining routes in a dynamic topology is the core challenge. Requires efficient, scalable, low-overhead protocols.

  • Security: Difficult to implement conventional security (no central CA). Vulnerable to spoofing, wormholes, DoS.

  • Resource Constraints: Power-aware routing and MAC crucial.

  • QoS: Supporting real-time traffic (voice/video) is hard due to route changes and contention.

  • Scalability: Protocols must work for small to large networks (100s of nodes).

B. Applications of Ad Hoc Networks

• Military Battlefield: Troop coordination, sensor networks.

• Emergency/Disaster Relief: Areas with no infrastructure (earthquakes, floods).

• Sensor Networks: Environmental monitoring, surveillance.

• Vehicular Ad Hoc Networks (VANETs): Car-to-car and car-to-infrastructure communication for safety/info.

• Conferencing/Meetings: Quick setup for sharing data.

• Home/Office Networks: Temporary networking without router.

• Robotics: Coordination among robot teams.

C. Routing Protocols for MANETs: Dynamic Source Routing (DSR)

• Type: On-demand (reactive) source-routed protocol.

• Key Idea: The source discovers the complete route to the destination and embeds the entire route (list of intermediate nodes) in the packet header.

• Two Main Mechanisms:

  1. Route Discovery (Flooding):

     • Source broadcasts Route Request (RREQ) packet with source, destination, and unique ID.

     • Each node appending its address to the RREQ's route record.

     • When RREQ reaches destination (or a node with fresh route to dest), it sends back Route Reply (RREP) along the reverse path (or via new RREQ if bidirectional).

     • Source caches the route.

  2. Route Maintenance:

     • If a link breaks during transmission, source is notified via Route Error (RERR).

     • Source removes broken link from its cache and may initiate new Route Discovery.

• Advantages: No periodic routing updates (low control overhead), source knows full path (can enforce policies), simple.

• Disadvantages: Route header overhead grows with path length (scales poorly). Stale route cache entries can cause problems. Flooding in discovery can cause "broadcast storm".

VI. Security in Wireless and Mobile Environments

A. Firewall Design

What is a Firewall? A security device (hardware/software) that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Acts as a barrier between trusted and untrusted networks.

Various Issues in Firewall Designing:

1. Placement: Where to place in the network (perimeter, between subnets, host-based). Affects what traffic is inspected.

2. Filtering Rules (Policy): Defining allow/deny rules based on IP, port, protocol, direction, state. Default policy (deny all vs allow all) is critical.

3. Stateful vs. Stateless: Stateful inspection tracks connection state (more secure, complex). Stateless filters packets independently (faster, less secure).

4. Performance: Firewall can become a bottleneck. Must balance security depth with throughput/latency.

5. Evasion Techniques: Attackers may use fragmentation, tunneling, or non-standard ports to bypass rules.

6. Management & Logging: Centralized management, rule updates, and comprehensive logging/auditing are essential.

7. Encrypted Traffic: Firewalls cannot inspect payload of encrypted traffic (SSL/TLS) without decryption (man-in-the-middle), creating a blind spot.

B. Intrusion Detection Systems (IDS)

Definition: A device or software that monitors network or system activities for malicious actions or policy violations and reports/responds.

Types in Wireless Contexts:

C. Password Management

Challenges in Mobile/Wireless Systems:

1. Small Keypad/Input: Difficult to enter long, complex passwords.

2. Shoulder Surfing: Passwords easily observed in public places.

3. Storage on Device: Password stored on mobile device (if cached) is vulnerable if device lost/stolen.

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

5. Weak User Choices: Tendency to choose simple, memorable passwords.

6. Synchronization: Passwords across multiple devices/services need management.

Different Methods of Password Management:

1. One-Time Passwords (OTP): Password valid for only one login session (e.g., via token, SMS). Resistant to replay.

2. Challenge-Handshake Authentication Protocol (CHAP): Server sends challenge; client responds using hash of password & challenge. Password not sent over network.

3. Public Key Infrastructure (PKI): Use digital certificates instead of passwords. More secure but complex management.

4. Biometrics: Fingerprint, face recognition. Avoids password entry but has privacy/false-match issues.

5. Single Sign-On (SSO): One authentication for multiple services. Reduces password fatigue but central point of failure.

6. Password Managers: Secure apps store encrypted passwords; user remembers one master password.

D. TCP Adaptations for Mobile Environments

Standard TCP interprets packet loss as congestion, leading to aggressive window reduction. In wireless networks, loss is often due to bit errors or handoffs, not congestion. This causes unnecessary throughput degradation.

Classical Approaches:

Snooping TCP (Detailed):

The Foreign Agent (FA) or Mobility Agent intercepts all TCP packets between CH and MH.

• Downlink (CH→MH): FA buffers data until MH ACKs. If MH ACK not received, FA retransmits locally.

• Uplink (MH→CH): FA forwards MH's ACKs to CH. If MH disconnects, FA can generate duplicate ACKs to keep CH's window open (prevent timeout), or buffer data from CH.

• Key: FA hides wireless losses from CH, making CH believe it's connected to a reliable fixed network.

VII. Wireless Sensor Networks (WSNs)

A. Architecture of Wireless Sensor Networks

• Sensor Node (Mote): The fundamental unit. Contains:

  • Sensor: Measures physical phenomenon (temp, light, sound, vibration).

  • Microcontroller/Processor: For local computation.

  • Memory: For code & data.

  • Transceiver: Typically low-power radio (e.g., 802.15.4/ZigBee, Bluetooth Low Energy).

  • Power Source: Small battery (often irreplaceable).

• Sink/Base Station: More powerful node. Collects data from sensors (via multi-hop), may perform aggregation, and connects to the outside world (WAN, internet).

• Connectivity: Typically multi-hop, mesh topology. Sensors relay each other's data to sink.

• Key Constraint: Severe energy limitation (battery-powered, often deployed in inaccessible areas). Drives all protocol design (duty cycling, low-power listening).

B. Applications and Use Cases of WSNs

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

• Industrial & Building Automation: Structural health monitoring (bridges, buildings), HVAC control, inventory tracking.

• Health Care: Patient monitoring, drug tracking, assisted living.

• Military: Surveillance, target tracking, battlefield monitoring, nuclear/chemical attack detection.

• Smart Cities: Traffic monitoring, waste management, smart lighting.

• Home Automation: Smart thermostats, security systems.

VIII. Other Wireless Protocols and Systems

A. DECT (Digital Enhanced Cordless Telecommunications)

• Protocol Overview: Standard for cordless telephones and wireless PBX. Operates in 1.9 GHz band (Europe, Asia) or 5.8 GHz (US). Uses TDMA/TDD.

• Architecture: Cellular but with very small cells (home/office). Handset ( Portable Part) communicates with a base station (Fixed Part). Multiple handsets per base. Base stations can be connected to a central switch (for multi-cell systems, e.g., offices).

• Key Features: Frequency hopping (fast, 100 hops/sec) across 10 carriers, each with 12 time slots (TDMA). Supports voice and low-rate data (up to 115 kbps in later versions). Excellent interference avoidance.

• Applications: Home/office cordless phones, wireless PBX systems, some wireless local loop (WLL).

B. TETRA (Terrestrial Trunked Radio)

• Protocol Overview: Standard for professional mobile radio (PMR) - public safety (police, fire, ambulance), transportation, utilities. Operates in 380-470 MHz band.

• Architecture: Trunked system (like cellular but for groups). Users belong to talkgroups. Central switching and management infrastructure (TETRA core network). Mobile Stations (MS) communicate with Base Stations (BS).

• Key Features: TDMA (4 time slots per carrier), FDMA (25 kHz channels). Supports group calls (push-to-talk), individual calls, data, short data service (SDS), telemetry. High reliability, fast call setup (<300ms), direct mode operation (DMO) for out-of-coverage peer-to-peer.

• Applications: Police, fire, ambulance dispatches, railway communications, airport ground staff.

C. UMTS (Universal Mobile Telecommunications System)

• Protocol Overview: The 3G standard for mobile broadband. Evolves from GSM, providing higher data rates (up to 2 Mbps indoor, 384 kbps outdoor).

• Role in 3G: UMTS is the radio access network (UTRAN) part of the 3GPP standard. Core network can be GSM-based (with GPRS) or all-IP.

• Architecture (Key New Element - UTRAN):

  • Node B: The UMTS base station (equivalent to BTS).

  • RNC (Radio Network Controller): Manages Node Bs, handles radio resource management, handovers, and connects to core network. New compared to GSM (BSC).

  • Core Network: Can be GSM-MAP (evolved from GSM) or IP-based (Release 5+).

• Air Interface (UTRA): Uses WCDMA (Wideband CDMA). 5 MHz carrier bandwidth, chip rate 3.84 Mcps. Supports both FDD (frequency division duplex, paired bands) and TDD (time division duplex, unpaired band).

• Applications: Mobile internet, video calling, mobile TV, broadband access on the go.

IX. Mobility Management and Network Layer Support

A. Reference Model for Wireless and Mobile Networks

The standard reference model is the Mobile IP architecture, which provides session continuity while the mobile node changes its point of attachment to the internet.

• Key Entities:

  • Mobile Node (MN): Device that changes its attachment point.

  • Home Agent (HA): Router in MN's home network. Maintains binding between MN's Home Address (permanent IP) and its current Care-of Address (CoA). Intercepts packets destined to Home Address and tunnels them to CoA.

  • Foreign Agent (FA): Router in the visited (foreign) network. Provides Care-of Address (often its own address on foreign link) to MN. Decapsulates tunneled packets and delivers to MN. May also act as default router for MN.

  • Correspondent Node (CH): The communication peer (could be fixed or mobile).

• Key Concept: Tunneling. HA encapsulates original IP packet (dest=HoA) in a new IP packet (dest=CoA) and sends it to FA/MN. FA/MN removes outer header.

B. Dynamic Host Configuration Protocol (DHCP)

Role in Mobile/Wireless Environments:

• Automatic IP Address Assignment: When a mobile node (laptop, phone) enters a new network (e.g., new Wi-Fi hotspot, cellular data network), it needs an IP address to communicate. DHCP provides this dynamically and automatically.

• Process:

  1. DHCP Discover: MN broadcasts request.

  2. DHCP Offer: DHCP server (often at the AP/gateway or network router) offers an IP address, lease time, subnet mask, default gateway, DNS servers.

  3. DHCP Request: MN selects an offer and requests it.

  4. DHCP ACK: Server confirms lease.

• Why Critical for Mobility? Eliminates manual configuration. Enables plug-and-play connectivity. Manages IP address pool efficiently as devices come and go. Provides other essential network parameters (DNS, gateway).

C. IP Tunneling and Encapsulation

Definition: Encapsulating one IP packet inside another IP packet to traverse a network that would not otherwise support the original packet (e.g., for Mobile IP, VPNs).

Common Methods:

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

   • Original IP packet (inner) is placed as payload of a new IP packet (outer).

   • Outer Header: New source (tunnel endpoint), new destination (other tunnel endpoint).

   • Inner Header: Unchanged (original source & destination addresses).

   • Use: Simple tunneling, Mobile IP (HA to FA/MN).

   • Header Fields: Outer IP header fields set by tunnel endpoints; inner header preserved.

2. Minimal Encapsulation (RFC 2004):

   • Designed for Mobile IP to reduce overhead.

   • Removes the inner header's original source address (since HA already knows it from binding) and protocol field (assumes IP-in-IP).

   • Only inner destination address (MN's CoA) and payload are kept from original packet.

   • Use: More efficient than full IP-in-IP for Mobile IP.

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

   • Generic tunneling mechanism. Can encapsulate a wide variety of protocols (IP, IPX, AppleTalk) inside IP.

   • Adds a GRE header between outer IP header and inner payload.

   • GRE header includes: Flags, Version, Protocol Type (type of inner packet), Key, Sequence Number (optional).

   • Use: VPNs (over IP), routing protocol tunneling (e.g., OSPF over MPLS), simple IP tunneling with more features (keys, sequencing).

Comparison of Header Overhead:

Exam Tip: Know the purpose and key difference of each. Minimal is for Mobile IP efficiency. GRE is the most flexible generic tunnel. IP-in-IP is the simplest.