Mobile Computing Interview Questions

This section covers a range of mobile computing interview questions, focusing on its fundamental concepts, challenges, technologies, and applications.

What is Mobile Computing?

Mobile computing allows data, voice, and video communication using wireless devices (laptops, smartphones, tablets, etc.) from various locations and networks. It involves distributed systems and mobile communication protocols.

Building Blocks of Mobile Computing

Mobile computing relies on:

• Portable computing devices (laptops, smartphones, etc.).
• Wireless communication technologies (Wi-Fi, cellular networks).

Mobility in Mobile Computing

Mobility refers to the freedom of movement for both users and devices across different geographical locations and networks. It's a defining characteristic of mobile computing.

Types of Mobility

• User Mobility: Users accessing services from various locations and networks.
• Device Mobility: Devices maintaining connectivity while moving.

Applications of Mobile Computing

• Mobile device applications.
• Location-based services.
• Vehicle telematics.
• Emergency medical services.
• Wireless networking.
• Infotainment.
• Business applications.

Challenges in Mobile Communication

• Signal interference.
• Limited bandwidth.
• Frequency regulation and spectrum allocation.
• Transmission delays and variability.
• Security vulnerabilities.

SIM Card (Subscriber Identity Module)

A SIM card is a small chip containing user information (IMSI - International Mobile Subscriber Identity, which identifies the subscriber on the network) and security credentials for accessing cellular networks. It is also used to store other data like contact details, and SMS messages.

Information Stored on a SIM Card

• IMSI (International Mobile Subscriber Identity)
• PIN (Personal Identification Number)
• PUK (PIN Unblocking Key)
• Authentication Key (KI)
• Subscriber information (phone number, service details, etc.)

How a SIM Card Works

The SIM card provides the identity and authentication details to connect to the cellular network, enabling mobile communication. It helps your device communicate with the network provider, which in turn facilitates various services (calls, texts, data).

Advantages of Mobile Computing

• Increased productivity.
• Improved portability and accessibility.
• Enhanced entertainment options.
• Cloud computing capabilities.

Disadvantages of Mobile Computing

• Unreliable connectivity.
• Security vulnerabilities.
• Limited battery life.

Guard Space in Mobile Computing

Guard space is the frequency band between channels to prevent interference.

Guard Space in CDMA Systems

In CDMA (Code-Division Multiple Access) systems, guard space is maintained by using orthogonal spreading codes. Lower correlation between codes means better separation of users.

Multiplexing in Mobile Computing

• Frequency-Division Multiplexing (FDM): Divides the frequency spectrum into channels.
• Time-Division Multiplexing (TDM): Divides time into slots for different users.

Fundamental Propagation Behaviors

• Ground wave: Low frequencies ( < 2 MHz).
• Sky wave: Frequencies between 2 MHz and 30 MHz.
• Line-of-sight: High frequencies ( > 30 MHz).

Multipath Propagation

Multipath propagation occurs when radio waves travel multiple paths to reach the receiver. This causes signal distortion and fading.

Analog Modulation Schemes

• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)

Phase-Locked Loops (PLLs)

PLLs are control systems that synchronize the phase and frequency of signals. They're crucial for maintaining synchronization in mobile communication systems.

Hopping Sequence (FHSS)

A hopping sequence defines the pattern of frequencies used in FHSS (Frequency Hopping Spread Spectrum), enhancing security and resistance to interference.

Dwell Time

Dwell time is the duration a frequency-hopping system stays on a particular frequency before hopping to another.

Advantages of Cellular Systems

• Increased capacity.
• Reduced transmission power.
• Localized interference.
• Improved robustness.

Fixed Channel Allocation (FCA) vs. Borrowing Channel Allocation (BCA)

FCA: Frequencies are permanently assigned to cells. BCA: Cells can borrow frequencies from neighbors to handle varying traffic loads.

Authentication Center (AuC)

The AuC is responsible for verifying user identities and securing communication on the network. It manages authentication and encryption keys.

Bursts and Normal Bursts

In mobile communication, data is transmitted in bursts. Normal bursts carry user data within a time slot.

Traffic and Control Multiframes

In GSM, data and control information are organized into multiframes:

• Traffic multiframe: A collection of 26 time slots (bursts), of which 24 are typically for user data transmission. A traffic multiframe has a duration of 120 milliseconds.
• Control multiframe: Consists of 51 TDMA frames for signaling and control channels.

Mobile IP (MIP)

Mobile IP allows mobile devices to maintain their IP address even when they move between different networks. This ensures continuous connectivity. It manages this through a home agent (in the home network) and a foreign agent (in the visited network).

Importance of Mobile IP

Mobile IP is crucial because it enables seamless network roaming for mobile devices without requiring users to change their IP addresses.

Components of Mobile IP

• Mobile Node (MN): The mobile device.
• Home Agent (HA): Manages the MN's home network.
• Foreign Agent (FA): Manages the network the MN is currently visiting.
• Care-of Address (CoA): The MN's temporary address in the foreign network.
• Correspondent Node (CN): The device communicating with the MN.
• Home Network: The MN's original network.
• Foreign Network: The network the MN is currently visiting.

GSM (Global System for Mobile Communications)

GSM is a standard for second-generation (2G) digital cellular networks. It's a widely used technology providing mobile voice and data services.

GSM's First Deployment

GSM was first deployed in Finland in December 1991.

GSM System Protocol Architecture

GSM uses a layered protocol architecture where lower layers provide services to higher layers. Key layers are:

• Layer 1 (Physical): Radio signals.
• Layer 2 (Data Link): LAPDm (Link Access Procedure on the D channel).
• Layer 3 (Network): Radio Resource Management (RR), Mobility Management (MM), Connection Management (CM).

TCP (Transmission Control Protocol)

TCP is a connection-oriented protocol that provides reliable data transmission. It divides messages into packets, numbers them for reassembly, and ensures that all packets are received correctly.

How TCP Works

1. Messages are broken into packets.
2. Packets are sent through the network.
3. Packets are reassembled at the destination.
4. A connection is established before data transmission and maintained until it's complete.

OVSF (Orthogonal Variable Spreading Factor)

OVSF is a technique used in UMTS (Universal Mobile Telecommunications System) to separate different users' data streams using orthogonal codes.

Polling in Mobile Communication

Polling is a method where one device repeatedly checks the status of other devices. It's a simple but less efficient communication technique compared to event-driven methods.

BSSGP (Base Station System GPRS Protocol)

BSSGP is used in GPRS (General Packet Radio Service) networks for communication between the Base Station Subsystem (BSS) and the Serving GPRS Support Node (SGSN).

WLAN (Wireless Local Area Network)

WLANs use radio waves for wireless communication within a limited area (like a home or office). They usually require a wireless access point.

Advantages and Disadvantages of WLANs

Infrastructure vs. Ad-hoc Networks

Infrared (IR) Transmission

IR uses infrared light waves for short-range wireless communication. It's used in many devices.

Applications of Infrared in Mobile Communication

• Remote controls
• Wireless peripherals
• Short-range wireless communication