Wireless and Mobile Computing (IT-602) - Unit 5 Short Notes

UNIT 5: Wireless and Mobile Computing

I. Wireless Network Reference Model and Fundamentals

Reference Model for Wireless and Mobile Networks

A layered architecture that adapts the traditional OSI/TCP-IP model to address challenges of mobility, wireless transmission, and resource constraints.

• Key Interfaces: Between mobile host and base station (physical/link layer), and between base station and fixed network (network layer).

• Mobility Challenges: Handoff management, location tracking, variable bandwidth, high bit error rates, power constraints.

[!TIP] Exam Focus: Be prepared to draw and label the reference model, showing the mobile host, base station, and fixed network with key protocol layers.

Wave Propagation Models

Radio waves propagate via three primary mechanisms, each with distinct characteristics and frequency dependencies.

Antenna Fundamentals: Helical Antenna

A helical antenna is a spring-like (helix) conductor that operates in two primary modes:

1. Normal Mode (Broadside): Helix diameter and spacing are small (~λ/10). Radiates perpendicular to axis. Low gain, used for mobile communications.

2. Axial Mode (End-Fire): Helix circumference ≈ λ, spacing ≈ 0.2λ. Radiates along the axis. Circular polarization, high gain, wide bandwidth. Used for satellite communication, telemetry.

Key Parameters: Pitch angle (α), number of turns (N), circumference (C). Gain ≈ 11n + 8.5 dB (for n>3).

II. Multiple Access and Channel Allocation Techniques

Fundamentals of Multiple Access

Allows multiple users to share a common communication medium (spectrum). Main categories:

• FDMA: Frequency Division; each user gets a dedicated frequency band.

• TDMA: Time Division; users share frequency but transmit in assigned time slots.

• CDMA: Code Division; all users transmit simultaneously on same frequency, separated by unique codes.

• SDMA: Space Division; uses directional antennas to create spatial channels.

Demand Assigned Multiple Access (DAMA)

A reservation-based protocol. A central controller (e.g., satellite) assigns channels on-demand.

• Working: Users send a request packet on a common request channel. Controller queues requests and assigns a free traffic channel via a grant channel.

• Use Case: Efficient for bursty traffic with low utilization (e.g., VSAT networks).

Packet Reservation Multiple Access (PRMA)

Combines TDMA with slotted ALOHA reservation. Used in satellite and cellular systems (e.g., GPRS).

• Slotted Structure: Time is divided into slots/frames.

• Reservation: A user with a packet to send contends for a slot in the next frame. If successful, that slot is reserved for that user in subsequent frames until the packet burst is complete.

• Priority: Can incorporate priority levels for different traffic types (voice vs. data).

Priority-Based Multiple Access Schemes

Implementation involves modifying contention resolution to favor high-priority users.

• Mechanism: Assign different contention window sizes or backoff algorithms. High-priority users use smaller windows or lower backoff limits, increasing their chance of winning contention.

• Example: In IEEE 802.11e (WMM), voice (AC_VO) has shorter AIFS and smaller CWmin than best-effort data.

Hidden Terminal Problem

Occurs when two stations (A and C) are out of range of each other but within range of a common receiver (B).

• Result: A and C cannot sense each other's transmission. They may transmit to B simultaneously, causing a collision at B.

• Solution: RTS/CTS (Request-to-Send/Clear-to-Send) handshake. A sends RTS to B, B replies with CTS (which A and C can hear). C, hearing CTS, defers transmission.

Exposed Terminal Problem

Occurs when a station (B) is within range of a transmitting station (A) but the intended receiver (C) is out of A's range.

• Result: B, sensing A's transmission, defers even though its transmission to C would NOT interfere with A's transmission to its own receiver.

• Impact: Reduces spatial reuse and network capacity.

• Solution: More complex protocols like MACAW or directional antennas.

[!TIP] Common Pitfall: Hidden terminal causes collision at receiver; exposed terminal causes unnecessary deferral. RTS/CTS solves hidden but can worsen exposed.

III. MAC Layer: Principles and Protocols

General MAC Functions & Challenges

• Functions: Frame delimiting, addressing, error control (often limited), access control, synchronization.

• Wireless Challenges: Hidden/exposed terminals, asymmetric channels (downlink vs. uplink), power saving, mobility support.

IEEE 802.11 Family (WLAN)

• Architecture: Two modes:

  1. Infrastructure Mode: Stations (STAs) communicate via an Access Point (AP). AP connects to distribution system (wired LAN).

  2. Ad Hoc Mode (IBSS): Direct STA-to-STA communication without AP.

• IEEE 802.11a:

  • Band: 5 GHz UNII bands (5.15-5.35 GHz, 5.725-5.825 GHz).

  • PHY: OFDM (Orthogonal Frequency Division Multiplexing). 52 subcarriers (48 data, 4 pilot). Data rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps.

  • MAC: Same CSMA/CA as 802.11b/g but with shorter slot time (9 µs). Supports QoS via 802.11e.

HIPERLAN (High Performance Local Area Network)

European standard (ETSI). Key versions:

• HIPERLAN-1 Channel Access (EY-NPMA - Elimination Yield - Non-Preemptive Priority Multiple Access):

  1. Prioritization Phase: Contend for priority based on traffic class.

  2. Elimination Phase: Contending stations randomly wait; winner eliminates others.

  3. Yield Phase: Winner transmits; others yield for a random time.

• Physical Layer (HIPERLAN/1): Uses GMSK modulation in 23.5 MHz channels at 5 GHz.

• MAC Layer: Connection-oriented, supports QoS, power saving, and mobility.

Bluetooth

• Network Architecture: Piconet (1 master, up to 7 active slaves). Scatternet: multiple interconnected piconets.

• Physical Layer: FHSS (Frequency Hopping Spread Spectrum) in 2.4 GHz ISM band. 79 hopping channels (1 MHz spacing), 1600 hops/sec. GFSK modulation.

• MAC Layer: TDMA + TDD with master-driven polling. Master controls 625 µs slots. Slaves only transmit in slots allocated by master. Uses parked state for inactive slaves.

DECT (Digital Enhanced Cordless Telecommunications)

• Standard: ETSI, primarily for cordless phones.

• PHY: TDMA/TDD in 1.728 MHz channels in 1.88-1.90 GHz band. 10 slots/frame (12.5 ms). GFSK modulation.

• MAC: Dynamic channel allocation (DCA). Supports fast handoff between base stations (radio fixed part). Multi-carrier option (DECT/UMTS) for higher data.

TETRA (Terrestrial Trunked Radio)

• Standard: ETSI, for professional mobile radio (PMR) - police, emergency, transport.

• PHY: TDMA/FDD in 400 MHz band. 4 time slots per carrier (14.4 kbps/slot). π/4-DQPSK modulation.

• MAC: Supports group calls (one-to-many), individual calls, broadcast. Fast call set-up (<300 ms). Switched mode (circuit) and packet mode (IP).

IV. Cellular Communication Systems

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 frequency can be reused in non-adjacent cells, enabling frequency reuse and high capacity.

• Core Components:

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

  • Base Station (BS) / Node B: Radio equipment in cell.

  • Base Station Controller (BSC) / RNC: Manages multiple BSs, handles handoff, channel allocation.

  • Mobile Switching Center (MSC): Core network switch. Connects calls, manages mobility (location registers), interfaces to PSTN.

  • Home Location Register (HLR) / Visitor Location Register (VLR): Databases for subscriber location and profile.

Key Cellular Concepts

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

   • Types: Hard Handoff (break-before-make, GSM):旧连接释放，新连接建立。Soft Handoff (make-before-break, CDMA): MS temporarily connected to multiple BSs.

   • Procedure: MS measures signal strength from neighboring BSs (via MAHO - Mobile Assisted Handoff). Reports to current BS/BSC. Decision by network (MSC/BSC). New traffic channel assignment.

2. Sectorization: Using directional antennas (typically 3 sectors of 120° or 6 of 60°) at a BS site.

   • Benefits: Reduces co-channel interference, increases capacity (each sector can reuse frequencies independently), improves signal quality.

GSM (Global System for Mobile Communications)

• Architecture: Circuit-switched core network. Key interfaces: Um (MS-BS), A-bis (BS-BSC), A (BSC-MSC), Iu (UMTS).

• Services: Teleservices (voice, SMS, fax), Bearer Services (data transmission), Supplementary Services (call forwarding, hold).

• Handover in GSM: Primarily hard handoff. Controlled by BSC (intra-BSC) or MSC (inter-BSC/MSC). Uses MAHO. Requires new time slot assignment on new BS frequency.

GPRS (General Packet Radio Service)

• Architecture: Packet-switched overlay on GSM. Introduces new network elements:

  • SGSN (Serving GPRS Support Node): Similar to MSC for packet domain. Tracks MS location, performs authentication, IP address assignment.

  • GGSN (Gateway GPRS Support Node): Gateway to external PDNs (Internet). Acts as router, performs IP address translation, charging.

  • PCU (Packet Control Unit): Often integrated into BSC, handles packet traffic on radio interface.

• Comparison: GSM vs. GPRS

  | Feature | GSM | GPRS | | :--- | :--- | :--- | | Switching | Circuit-Switched (CS) | Packet-Switched (PS) | | Channel Allocation | Dedicated channel per call | Shared channels, on-demand | | Data Rate | ~9.6 kbps (full rate) | Up to ~171 kbps (theoretical, 4 timeslots) | | Architecture | MSC, HLR, VLR | SGSN, GGSN added | | Billing | Per-second/time | Per-data-volume (KB/MB) | | Always-On | No (call setup required) | Yes (virtual session) |

UMTS (Universal Mobile Telecommunications System)

• 3G Standard (IMT-2000). Based on W-CDMA (Wideband CDMA).

• Architecture (UMTS Terrestrial Radio Access Network - UTRAN):

  • UE (User Equipment): 3G phone.

  • Node B: Equivalent to BS.

  • RNC (Radio Network Controller): Equivalent to BSC, but handles W-CDMA radio resource management.

  • Core Network: Can be GSM-MAP (for interworking) or all-IP (release 5+ with MSC server, media gateway).

• Key Features: Higher data rates (2 Mbps indoor, 384 kbps outdoor), soft handoff (CDMA property), support for multimedia services, packet-switched core (with GGSN).

V. Wireless LANs (WLAN)

IEEE 802.11 WLAN Architecture (Detailed)

• Modes: Infrastructure (with AP) and Ad Hoc (IBSS).

• Components:

  • Station (STA): Any device with 802.11 NIC.

  • Access Point (AP): Bridge between wireless and wired LAN. Connects to Distribution System (DS) (typically Ethernet).

  • Distribution System (DS): The wired backbone (e.g., LAN switch).

  • Portal: Bridge to non-802.11 networks.

• Services:

  • Station Services: Authentication, deauthentication, privacy (WEP/WPA).

  • Distribution System Services: Association, disassociation, distribution, integration, reassociation.

Frequency Bands in WLAN

Comparison: 802.11 vs. HIPERLAN

VI. Ad Hoc and Sensor Networks

Ad Hoc Networks

• Characteristics:

  • Infrastructure-less, autonomous.

  • Dynamic topology (nodes move, join, leave).

  • Multi-hop routing.

  • Limited resources (battery, bandwidth).

  • Distributed operation, peer-to-peer.

• Performance Issues:

  • Routing: Frequent topology changes, route discovery overhead.

  • Security: Vulnerable to insider attacks (no central authority), difficult key management.

  • Scalability: Routing protocols may not scale well with network size.

  • QoS: Hard to guarantee due to dynamic links.

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

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

• On-demand (reactive) protocol.

• Two Main Mechanisms:

  1. Route Discovery: Source floods Route Request (RREQ) packets. Each intermediate node appends its address to the packet. When RREQ reaches destination, it replies with Route Reply (RREP) containing the accumulated route path. Source caches this route.

  2. Route Maintenance: If a link breaks during transmission, source is notified via Route Error (RERR). Source initiates new route discovery.

• Advantages: Simple, no periodic routing updates, source routing allows easy loop detection.

• Disadvantages: Route discovery overhead can be high; stale route cache can cause errors.

Wireless Sensor Networks (WSN)

• Architecture:

  • Sensor Nodes: Hundreds/thousands of tiny, low-cost, battery-powered nodes with sensing, processing, radio. Dense deployment.

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

  • Gateway: Connects WSN to Internet (often same as sink).

  • Management Node: User interface to monitor/control network.

• Applications:

  • Environmental monitoring (forest, ocean, agriculture).

  • Military (intrusion detection, target tracking).

  • Health monitoring (patient, body area).

  • Industrial automation (smart buildings, inventory).

VII. Security in Wireless Networks

Firewall in Wireless Context

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

• Issues in Firewall Design for Wireless:

  1. Mobile Hosts as Firewall Ends: A mobile host's IP address changes frequently (roaming), making static access control lists (ACLs) ineffective.

  2. Wireless Link Vulnerability: The "last hop" (wireless link between mobile and AP/BS) is inherently insecure; firewalls at the wired network edge cannot inspect or protect this segment.

  3. VPN Overhead: Often requires IPsec VPN from mobile host, adding latency and processing burden.

  4. Stateful Inspection Challenges: Tracking connection state for mobile hosts that change IP addresses is complex.

  5. Placement Dilemma: Should firewall be at the mobile host (resource-constrained) or at the network edge (cannot protect wireless link)?

Intrusion Detection Systems (IDS) for Wireless

• Purpose: Monitor network traffic for malicious activity or policy violations.

• Types for Wireless:

  • Network-based IDS (NIDS): Sensors placed at strategic points (e.g., near APs) to monitor wireless traffic. Detects rogue APs, MAC spoofing, DoS attacks.

  • Host-based IDS (HIDS): Software on individual mobile devices. Monitors system logs, file integrity.

  • Wireless-specific Detection: Focus on 802.11-specific attacks: deauthentication/disassociation floods, fake authentication, evil twin (rogue AP), wireless scanning.

Password Management

• Challenges in Wireless/Mobile:

  • Device Theft/Loss: Physical compromise of device with stored credentials.

  • Small Form Factor: Difficult to enter complex passwords on small keypads/touchscreens.

  • Frequent Authentication: Re-authentication during handoffs or after idle periods.

  • Memory Constraints: Storing multiple strong passwords for different services.

  • Shoulder Surfing: Easy to observe PIN/password entry in public places.

• Methods of Password Management:

  • One-Time Passwords (OTP): Token-based or SMS-based. Mitigates replay attacks.

  • Biometrics: Fingerprint, face recognition. Convenient but has false acceptance/rejection rates.

  • Graphical Passwords: Select images/points on screen. More memorable but vulnerable to shoulder surfing/smudge attacks.

  • Password Managers: Encrypted storage of multiple passwords behind a single strong master password.

  • Multi-Factor Authentication (MFA): Combine password with something you have (token) or are (biometric).

VIII. Mobility Support in TCP/IP

Challenges of TCP in Mobile Environments

• High Bit Error Rate (BER): Wireless links have higher BER than wired, causing packet loss interpreted by TCP as congestion.

• Handoff Delays: During handoff, packets may be lost or delayed for seconds, triggering TCP's retransmission timeout (RTO) and congestion control (slow start), unnecessarily reducing throughput.

• Route Changes: Path changes can cause packet reordering, which TCP may interpret as loss.

• Asymmetric Links: Downlink often has higher bandwidth than uplink (ACKs).

Classical Approaches to Make TCP Mobile

Comparison Summary: I-TCP breaks end-to-end semantics. Snooping is lightweight but fails on long breaks. M-TCP best for long disconnections but most complex.

IP Encapsulation Techniques

Used to tunnel IP packets (mobile node's home address) through foreign network to Home Agent (HA).

Dynamic Host Configuration Protocol (DHCP) for Wireless

• Standard DHCP: Provides IP address, subnet mask, gateway, DNS server to clients dynamically.

• In Wireless/Mobile Context:

  • DHCP Relay: Foreign Agent (FA) or Home Agent (HA) relays DHCP requests from mobile host to central DHCP server.

  • Fast Handoff: DHCP for Mobile IP can be optimized. When MH moves to new subnet, it can obtain a new Care-of Address (CoA) quickly via DHCP from the new FA, reducing latency.

  • Challenge: Lease times must be short due to mobility, but frequent renewal causes signaling overhead.

Snooping TCP (Detailed Operation)

1. Setup: Foreign Agent (FA) is on the path between CH and MH. FA maintains a connection cache entry for each active TCP connection involving an MH.

2. Data Transfer (CH -> MH):

   • CH sends data packet to FA (dest=MH's CoA).

   • FA forwards to MH. FA snoops (copies) the packet and buffers it.

   • MH sends ACK back to FA (and then to CH).

   • If FA detects MH's ACK is lost (no ACK received from MH within timeout), FA locally retransmits the buffered packet to MH.

   • FA sends ACK to CH immediately after receiving MH's ACK (or after local retransmit success). This keeps CH's congestion window high.

3. Data Transfer (MH -> CH): Straightforward; FA just forwards packets.

4. Handoff: When MH moves to new FA, old FA may forward buffered packets to new FA (if buffered). New FA starts snooping new connection.

5. Failure: If MH is disconnected for longer than CH's RTO, CH will timeout, enter slow start, and break the connection. Snooping TCP cannot prevent this.

Key Point: Snooping TCP hides wireless losses from CH but cannot hide long disconnections (handoff delays > RTO). It's a link-level solution.