Core Concepts of Mobile and Wireless Networks

What Is Mobile Computing?

Mobile computing is a technology that allows the transmission of data, voice, and video through a computer or any other wireless-enabled device without being connected to a fixed physical link. It enables users to perform computing tasks anytime and anywhere using mobile devices such as smartphones, laptops, tablets, and PDAs connected through wireless networks.

Key Features of Mobile Computing

• Ubiquitous Access: Users can access services and information from any location.
• Wireless Communication: Relies on wireless networks like Wi-Fi, cellular (4G, 5G), and Bluetooth.
• Real-time Connectivity: Provides continuous connection to networks and services.
• Portability: Devices are designed to be lightweight and easily carried.
• Location Independence: The user's location does not restrict access to services.

User Mobility vs. Device Portability

• User Mobility: This refers to the ability of a user to move from one location to another while maintaining access to computing services without interruption. The network automatically handles location changes using techniques such as handoff and roaming, ensuring that services, applications, and session continuity remain unaffected. For example, making a voice call while traveling in a car without the call dropping. Its importance lies in enabling seamless communication, location tracking, and a consistent Quality of Service (QoS).
• Device Portability: This refers to the capability of a computing device to be easily carried and used in different locations. Portable devices are lightweight, battery-powered, and have wireless connectivity, allowing them to operate in various environments without a fixed network infrastructure. Examples include laptops, tablets, and smartphones used in offices, homes, or public spaces. Its importance lies in providing convenience for a mobile workforce, on-demand access to information, and flexible work environments.

Key Applications of Mobile Computing

• Business and Office Work

  Mobile computing enables employees to work remotely using laptops, smartphones, and tablets connected via wireless networks.

  • Examples: Accessing emails and business documents on the go; video conferencing with tools like Zoom and Microsoft Teams.
  • Benefits: Increased productivity, flexibility, and a reduced need for physical office spaces.

• Mobile Banking and Financial Services

  Banking services are accessible through mobile apps, allowing customers to conduct transactions anytime.

  • Examples: UPI payments (Google Pay, PhonePe), checking account balances, and transferring funds.
  • Benefits: Saves time, provides 24/7 access, and supports cashless transactions.

• Healthcare and Telemedicine

  Mobile computing enables doctors and patients to connect remotely for consultations, prescriptions, and health monitoring.

  • Examples: Remote patient monitoring devices connected via mobile apps; telemedicine platforms like Practo and Apollo 24/7.
  • Benefits: Provides timely medical assistance, especially in rural and underserved areas.

• Education and E-Learning

  Students and teachers can access study materials, attend virtual classes, and submit assignments through mobile devices.

  • Examples: E-learning platforms like Google Classroom, BYJU’S, and Coursera.
  • Benefits: Offers flexibility in learning and provides access to global educational resources.

• Location-Based Services and Navigation

  Mobile devices use GPS to provide navigation, real-time traffic updates, and location-based recommendations.

  • Examples: Google Maps, ride-sharing apps like Ola and Uber, and food delivery tracking.
  • Benefits: Saves time and offers convenience in travel and deliveries.

• Entertainment and Media

  Mobile computing provides on-the-go access to music, movies, games, and social media.

  • Examples: Streaming services like Netflix and YouTube, music apps like Spotify, and mobile games like PUBG Mobile.
  • Benefits: Delivers on-demand entertainment and enhances user engagement.

Common Challenges in Mobile Computing

1. Limited Bandwidth

   Wireless networks typically have lower bandwidth compared to wired networks. This can lead to slower data transfer rates, affecting applications like high-definition video streaming and large file downloads.

2. Limited Battery Life

   Mobile devices rely on battery power, which restricts continuous usage. High-performance applications, GPS, and 5G connectivity drain the battery more quickly.

3. Smaller Screen and Input Constraints

   Mobile devices have small display sizes and limited input methods (e.g., touchscreens, small keyboards), which can impact user experience for complex tasks.

4. Security Issues

   Wireless communication is more vulnerable to security threats like eavesdropping, data interception, unauthorized access, and malware attacks.

5. Network Reliability and Handoff Issues

   As mobile users move between different coverage areas, the handoff process between base stations can sometimes cause delays or disconnections.

6. Limited Processing Power and Storage

   Portable devices generally have lower CPU power and less storage capacity compared to desktop computers or servers, which can limit their performance for resource-intensive applications.

Fundamentals of Cellular Networks

The Cellular Concept: Cells

A cell in mobile communication is the basic geographic unit of a cellular system. The entire service area is divided into these small regions, and each cell is served by its own Base Station (BS).

Frequency Reuse Explained

Frequency reuse is a core technique for using the same frequency channels in different cells that are sufficiently far apart to minimize interference. The goal is to maximize spectrum efficiency and support a large number of users within a limited frequency band.

The total available spectrum is divided into smaller frequency sets. Each cell in a cluster is assigned a unique set of frequencies to avoid adjacent cell interference. This cluster pattern (e.g., 3-cell or 7-cell) is then repeated across the entire coverage area. A frequency reuse factor is defined as 1/N, where N is the number of cells in a cluster.

3-Cell vs. 7-Cell Cluster Structures

• 3-Cell Structure: In a 3-cell cluster, the total frequency spectrum is divided into three groups. Each cell in the cluster uses a unique frequency band, and the pattern repeats.
  • Features: Frequency Reuse Factor = 1/3. It has a simple architecture but offers less capacity compared to larger clusters.
• 7-Cell Structure: In a 7-cell cluster, the spectrum is divided into seven groups. Each of the seven cells uses a different frequency band.
  • Features: Frequency Reuse Factor = 1/7. This structure is more efficient, widely used in real networks, and reduces co-channel interference compared to a 3-cell structure.

Handoff (Handover) in Mobile Networks

Handoff (or Handover) is the process of transferring an ongoing call or data session from one Base Station (BS) to another without interruption. It ensures continuous service as a user moves from one cell's coverage area into another. When a mobile user moves, the signal strength from the serving BS decreases while the signal from a neighboring BS increases. At a certain threshold, control of the call is handed over to the new BS.

Types of Handoffs

• Hard Handoff: Also called "break-before-make." The current connection is broken before a new connection is established with the next BS. This is used in 2G GSM (TDMA/FDMA) systems and can sometimes cause slight interruptions.
• Soft Handoff: Also called "make-before-break." The mobile terminal connects to two or more BSs simultaneously during the transition. This is used in CDMA and 3G systems, providing a smoother transition with less probability of a dropped call.
• Intra-cell Handoff: Occurs within the same cell, typically to change a frequency or channel due to interference.
• Inter-cell Handoff: The most common type, occurring between two different cells as a user moves across cell boundaries.
• Intra-system Handoff: Occurs within the same cellular system (e.g., between two BSs of a GSM network).
• Inter-system Handoff: Occurs between different systems or networks (e.g., from 4G to Wi-Fi). This is more complex as it requires network authentication and compatibility checks.

Key Characteristics of Handoffs

• Seamlessness: The handoff should be smooth, without call drops or data loss.
• Low Latency: The process must be fast to maintain service continuity.
• Reliability: It must provide a stable connection during user mobility.
• Efficiency: It should minimize signaling overhead and use network resources effectively.
• Transparency: The process should be invisible to the user.
• Adaptability: It should work across different networks and mobility patterns.

Wireless Network Generations and Technologies

Evolution of Wireless Communication

1G (First Generation)

• Advantages: Enabled mobile voice communication for the first time, provided wider coverage than earlier radio telephony, and increased capacity by using the cellular concept (frequency reuse).
• Disadvantages: As an analog system, it suffered from poor voice quality, noise, and distortion. It had no security, low capacity, poor handoff performance, and offered no data services. Handsets were bulky with high power consumption.

3G (Third Generation)

The Third Generation (3G) of wireless communication was developed to provide high-speed data transmission and better voice quality compared to 2G. It is based on the International Mobile Telecommunications-2000 (IMT-2000) standard set by the ITU (International Telecommunication Union). 3G networks enabled multimedia communication such as video calling, mobile internet, and online gaming.

Key Objectives of 3G

• Provide higher data rates for mobile internet access.
• Enable global roaming with a single device.
• Support multimedia services (voice, video, data) on the same network.

Features of 3G

• High Data Rates: Up to 2 Mbps for stationary or slow-moving users.
• Enhanced Voice Quality: Clearer audio with reduced noise and interference.
• Multimedia Support: Enabled video calling, mobile TV, video conferencing, and online gaming.
• Global Roaming: A single SIM could be used in multiple countries where 3G was supported.
• Packet-Switched Data Transmission: Ensured efficient use of network resources for internet and multimedia.
• Always-On Connectivity: Allowed users to remain continuously connected to the internet.

Technologies Used in 3G

• WCDMA (Wideband Code Division Multiple Access)
• CDMA2000
• TD-SCDMA (Time Division-Synchronous Code Division Multiple Access)

Commonly Used Frequency Ranges

• 1G (AMPS): 824–894 MHz band with a 30 kHz channel bandwidth.
• 2G (GSM): 890–915 MHz (uplink) and 935–960 MHz (downlink) bands with a 200 kHz channel bandwidth.
• 3G (UMTS/WCDMA): 1920–1980 MHz (uplink) and 2110–2170 MHz (downlink) bands.
• 4G (LTE): Various bands from 700 MHz to 2.6 GHz, depending on the region.
• 5G: Sub-6 GHz bands (e.g., 3.3–3.6 GHz) and mmWave bands (e.g., 24–86 GHz).

Core Communication Networks and Services

Public Switched Telephone Network (PSTN)

The Public Switched Telephone Network (PSTN) is the traditional, circuit-switched telephone network used for voice communication across the world. It connects telephones through a global system of switching offices, transmission lines, and signaling systems. Initially designed for analog voice calls, the PSTN has evolved to support digital signals and integrate with mobile networks and the Internet.

Features of PSTN

• Circuit-Switched Network: A dedicated physical path is established between the caller and receiver for the duration of the call.
• Global Coverage: Interconnected telephone exchanges provide worldwide communication.
• Reliability: Offers stable and predictable call quality.
• Fixed-line Communication: Uses copper wires, fiber optics, and microwave links.

Components of PSTN

• Subscriber Loop (Local Loop): The physical connection between a customer’s telephone and the local exchange.
• Local Exchange (Central Office): A switching facility that connects subscribers in a local area and handles call routing.
• Trunk Lines: High-capacity lines (copper, fiber optic, or microwave) connecting exchanges.
• Tandem Exchange: A switch that connects multiple local exchanges within a region.
• International Gateway Exchange: Handles calls between countries using undersea cables or satellites.
• Signaling System (e.g., SS7): Used for call setup, routing, and disconnection.

Public Communication Service (PCS)

Public Communication Service (PCS) refers to a set of wireless communication capabilities that provide integrated voice, data, and multimedia services to the public. It is a mobile communication system designed to deliver personalized, location-independent communication. PCS is often considered an extension of cellular telephony with enhanced features and broader service capabilities.

Key Characteristics of PCS

• Personal Mobility: Users can be reached anywhere using a single personal telephone number, regardless of their device or location.
• Service Flexibility: Supports a wide range of services, including voice, text, video, and data.
• Location Independence: Service is accessible from different geographic areas without changing numbers.
• Integration of Networks: Combines wireless, wired, and internet-based networks.

Services Offered by PCS

• Voice Communication
• Short Message Service (SMS) and Multimedia Messaging Service (MMS)
• Data Services
• Video Communication
• Location-Based Services
• Paging and Fax Services

Components of PCS

• Mobile Stations (MS): User devices like mobile phones, tablets, and laptops.
• Base Stations (BS): Provide wireless coverage and connect devices to the network.
• Switching Centers: Route calls and data to their destinations.
• Databases: Store subscriber information for authentication and billing.
• Public Network Interfaces: Connect the PCS network to the PSTN, ISDN, or the Internet.

Protocols and Access Methods in Wireless Communication

Medium Access Control (MAC)

Medium Access Control (MAC) is a sublayer of the Data Link Layer in the OSI model. It is responsible for controlling how multiple devices share a common communication medium. In mobile computing, since the radio spectrum is shared, MAC ensures efficient, fair, and collision-free access.

Functions of MAC

• Channel Allocation: Determines how the radio channel is divided among users (e.g., FDMA, TDMA, CDMA, OFDMA).
• Collision Avoidance: Prevents multiple devices from transmitting simultaneously, using protocols like CSMA/CA (used in Wi-Fi).
• Addressing: Provides unique MAC addresses to devices for identification on the network.
• Scheduling: Decides the order in which users can access the channel to ensure fairness.
• Error Detection: Detects transmission errors using techniques like CRC (Cyclic Redundancy Check).

In wireless networks, MAC must address challenges like the hidden and exposed terminal problems, limited bandwidth, user mobility, and dynamic topology.

The Hidden and Exposed Terminal Problems

• Hidden Terminal Problem: This occurs when two nodes are out of each other’s range but can both transmit to a common receiver. Because they are "hidden" from each other, they cannot detect a potential collision at the receiver. This increases packet collisions and reduces network performance.
• Exposed Terminal Problem: This occurs when a node is unnecessarily prevented from sending data because it senses the medium is busy due to a nearby transmission, even though its own transmission would not cause interference at the intended receiver. This leads to the under-utilization of channel capacity.

Wireless Application Protocol (WAP)

Wireless Application Protocol (WAP) is a standard protocol that enables mobile devices like smartphones to access internet-based services over wireless networks with limited bandwidth. It follows a layered architecture similar to the OSI model.

WAP Layered Architecture

1. Application Layer (WAE – Wireless Application Environment): Provides applications and services to users. It supports WML (Wireless Markup Language), WMLScript, and micro-browsers.
2. Session Layer (WSP – Wireless Session Protocol): Handles communication sessions between the client and server, offering both connection-oriented and connectionless modes.
3. Transaction Layer (WTP – Wireless Transaction Protocol): Provides lightweight request/response transaction services, reducing overhead compared to TCP.
4. Security Layer (WTLS – Wireless Transport Layer Security): Ensures data security through authentication, data integrity, and encryption, similar to SSL/TLS.
5. Transport Layer (WDP – Wireless Datagram Protocol): Provides a common interface to the upper layers, ensuring data transport over different bearer services (e.g., GSM, CDMA, SMS).

Comparing TDMA and CDMA

Understanding Ad-hoc Wireless Networks

An ad-hoc network is a decentralized type of wireless network where nodes communicate directly with each other without relying on a central access point or router.

Key Design Issues in Ad-hoc Networks

• Routing: A major challenge because the network topology changes frequently due to node mobility.
• Limited Bandwidth: The wireless medium has low bandwidth, and performance can be degraded by channel contention and interference.
• Energy Constraints: Mobile nodes are powered by batteries with limited capacity, making energy efficiency critical.
• Security: Ad-hoc networks are vulnerable to attacks like eavesdropping, spoofing, and denial-of-service since there is no central authority.
• Quality of Service (QoS): Maintaining guaranteed bandwidth and low delay for applications like video streaming is difficult due to the dynamic topology and limited resources.
• Scalability: As the number of nodes increases, routing and resource management become more complex.
• Mobility Management: Frequent link breakages due to random node movement require efficient mobility management techniques.
• Interference and Collision: Overlapping wireless signals cause interference and packet collisions, requiring effective MAC protocols.

Ad-hoc Routing Protocols

DSDV (Destination Sequenced Distance Vector)

DSDV is a proactive (table-driven) routing protocol. Each node maintains a routing table with the shortest path to every other node. Sequence numbers are used to avoid routing loops and ensure the freshness of routes.

Benefits of DSDV

• Provides loop-free routing due to sequence numbers.
• Ensures immediate route availability since tables are always maintained.
• Suitable for small to medium-sized networks with less mobility.
• Offers low latency in route discovery.
• Simple and easy to implement.

AODV (Ad-hoc On-demand Distance Vector)

AODV is a reactive (on-demand) protocol. Routes are established only when required using Route Request (RREQ) and Route Reply (RREP) messages. Sequence numbers are also used to ensure route freshness.

Benefits of AODV

• Reduces routing overhead since routes are created only when needed.
• Adapts well to high mobility and dynamic networks.
• Provides loop-free routes using sequence numbers.
• Uses bandwidth efficiently by not maintaining all routes at all times.
• Suitable for large ad-hoc networks with frequent topology changes.