21IT305 – MOBILE COMPUTING

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 Unit I
INTRODUCTION
 Table of Contents
1. Introduction to Mobile Computing
2. Generations of Mobile Communication Technologies
3. Multiplexing
4. Spread spectrum
5. MAC Protocols
6. Cellular Systems
7. GSM – Services & Architecture
8. Handover
9. Security.

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 Introduction to Mobile Computing
Definition:
Mobile Computing is a technology that allows transmission of data,
voice and video via a computer or any other wireless enabled device
without having to be connected to a fixed physical link.
In this technology, data transmission is done wirelessly with the
help of wireless devices such as mobiles, laptops etc.
The concept of Mobile Computing can be divided into three parts:
1. Mobile Communication
2. Mobile Hardware
3. Mobile Software

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Mobile Communication
 Mobile Communication specifies a framework that is
responsible for the working of mobile computing technology
 It refers to an infrastructure that ensures seamless and reliable
communication among wireless devices and ensures the
consistency and reliability of communication between wireless
devices.
 Examples: Desktops, Wi-Fi, Laptops, Wi-Fi Dongles

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Mobile Hardware
 Mobile hardware consists of mobile devices or device
components that can be used to receive or access the service of
mobility.
 Examples: smart phones, laptops, portable PCs, tablet PCs,
Personal Digital Assistants, etc.
 These devices are inbuilt with a receptor medium that can send
and receive signals.
 These devices are capable of operating in full-duplex.

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Mobile Software
 Mobile software is a program that runs on mobile hardware.
 This is the operating system for the appliance of mobile devices.
 It is the heart of the mobile systems.
 Examples: Social Media Apps, E-commerce Apps etc…

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Characteristics of Mobile Computing
 Portability
 Social Interactivity
 Context Sensitivity
 Connectivity
 Individual
 Small Size
 Wireless Communication

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Evolution of Mobile Computing
 The main idea of Mobile computing was evolving since the
1990s.
 It has evolved from two-way radios to modern day
communication devices.
 The list of most common forms of devices used in mobile
computing are:
1.Portable Computers
2. Personal Digital Assistant/Enterprise Digital Assistant (PDA or EDA)
3. Ultra-Mobile PC
4. Laptop
5. Smartphone
6. Tablet Computers
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7. Wearable computers
8. E-reader
Functions of Mobile Computing
A computing environment is defined as mobile if it supports one or
more of these characteristics:
1. User mobility
2. Network mobility
3. Device mobility
4. Session mobility
5. Service mobility
6. Host mobility

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Mobile Computing Vs Wireless Networking

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Advantages and Disadvantages of Mobile
Computing
Advantages:
 Enhanced Productivity
 Location Flexibility
 Saves Time
 Support Cloud Computing
 Entertainment
Disadvantages:
 Poor Quality of Connectivity
 Security Issues
 High on Power Consumption 22/01/2025 12

 low data transmission rates, High data losses, Frequent network issues etc
Applications of Mobile Computing
 Vehicles
 Emergencies
 Business
 Credit Card Verification
 Replacement of Wired Networks
 Infotainment

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Generations of Mobile Computing
 First Generation (1G)
 Second Generation(2G)
 Third Generation(3G)
 Fourth Generation(4G)
 Fifth Generation(5G)

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Contd….

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Multiplexing
 Multiplexing is a technique in which, multiple simultaneous
analog or digital signals are transmitted across a single data link.
 The concept behind it is very simple: Proper Resource Sharing
and its Utilization.
 It can be classified into four types.

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Frequency Division Multiplexing (FDM)
 In Frequency Division , the frequency dimension spectrum is split into bands of
smaller frequency. It is used because of the fact that, a number of frequency band
can work simultaneously without any time constraint. Ex: Radio station in a
particular region

Advantages of FDM
 This concept is applicable on both analog signals as well as digital signals.
 Simultaneous signal transmission feature.
Disadvantages of FDM
 Less Flexibility. 22/01/2025 17

 Bandwidth wastage is high and can be an issue.

Time Division Multiplexing (TDM)
 Time Division is used for a particular amount of time in which the whole
spectrum is used. Time frames of same intervals are made such that the entire
frequency spectrum can be accessed at that time frame.
Example : ISDN(Integrated Service for Digital Network) telephonic service

Advantages of TDM
 Single user at a time.
 Less complex and more flexible architecture.
Disadvantages of TDM 22/01/2025 18
 Difficult to implement.
Code Division Multiplexing (CDM)
 In Code Division Multiplexing, every channel is allotted with a unique code so
that each of these channels can use the same spectrum simultaneously at same
time. Example : Cell Phone Spectrum Technology(2G, 3G etc.)

Advantages of CDM
 Highly Efficient.
 Less Inference.
Disadvantages of CDM 22/01/2025 19
 Less data transmission rates.

Space Division Multiplexing (SDM):

 Space Division can be called as the combination of concepts of Frequency

Division Multiplexing and Time Division Multiplexing.
 In SDM, the goal is to pass messages or data parallelly with the use of specific
frequency at certain interval of time.
 It means, a particular channel for some amount of time will be used against a
certain frequency band. Example : GSM(Global Service For Mobile) Technology.
Advantages of SDM
 High Data transmission rate.
 Optimal Use of Time and Frequency bands.
Disadvantages of SDM
 Inference Problems.
 High inference losses. 22/01/2025 20
Spread Spectrum
 When transmitted signals of certain frequencies are varied slightly in order to obtain greater
bandwidth as compared to initial bandwidth is known as Spread Spectrum.
 It helps in transmission of radio signals because they can easily reduce the noise and other issues
that are data resistant.
 It can be broadly categorized into two:

 The major reason of spectrum technology being used is because of its proper bandwidth utilization
ability.
Applications:
 LAN technology. 22/01/2025 21

 Satellite communication technology

Frequency Hopping Spread Spectrum (FHSS)
FHSS divides the available bandwidth into multiple channels and randomly selects
frequency slots based on channel occupancy for signal transmission. Transmitters and
receivers hop between channels at set time intervals, combining Frequency Division
Multiplexing (FDM) and Time Division Multiplexing (TDM).

Slow Hopping: Multiple bits transmitted on the same frequency.

Fast Hopping: Individual bits transmitted on different frequencies.
Advantages:
 High security and High Efficiency
 Simple implementation compared to Direct Sequence Spread Spectrum (DSSS)
Disadvantages: 22/01/2025 22

 Less robust performance in challenging environments.

Direct Sequence Spread Spectrum (DSSS)
DSSS splits data into smaller blocks, attaches each with a high data rate bit
sequence, and transmits them from sender to receiver. The receiver uses the bit
sequence to recombine and recover lost data, reducing noise and interference.
 Advantages:
 Hard to detect signals
 Lower chances of jamming
 Resistant to noise
 Disadvantages:
 Slower process
 Requires wide-band channels
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MAC Protocols

 In a wireless network, multiple nodes may contend to transmit on the same

shared channel at the same time. It is the responsibility of the medium access
control (MAC) protocol to perform this task.
 Its key responsibilities include:
1. Channel Access Control: Ensures orderly and fair channel access.
2. Maximizing Utilization: Optimizes channel use.
3. Minimizing Latency: Reduces transmission delays while ensuring fairness among nodes .

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Wireless MAC Issues
MAC protocols in wireless networks are more complex compared to wired networks due to
several inherent challenges:

1.Collision Detection Difficulty:

Unlike wired networks, detecting collisions is hard in wireless environments because
transmitting nodes cannot easily sense if other nodes are transmitting simultaneously.
2.Hidden Terminal Problem:
Occurs when two nodes are out of each other's transmission range but attempt to send
data to a common receiver, causing collisions at the receiver.
3.Exposed Terminal Problem:
Happens when a node mistakenly assumes the channel is busy because it detects
another transmission, even though its intended receiver is out of range of that transmission,
leading to unnecessary transmission delays.

These challenges require specialized techniques like handshaking protocols, collision avoidance,
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and signal scheduling to ensure efficient and reliable communication in wireless networks.
Taxonomy of MAC Protocols
A large number of MAC protocols have been proposed. These MAC
protocols can be broadly divided into the following three categories:
1. Fixed assignment schemes or circuit-switched schemes
2. Random assignment schemes
3. Reservation based schemes or packet-switched schemes

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Fixed Assignment Schemes
In fixed assignment schemes, the resources required for a call are assigned
for the entire duration of the call.
A few important categories of fixed assignment MAC protocols are the
following:
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)

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Frequency Division Multiple Access (FDMA)
Divides the available frequency spectrum into several
smaller frequency bands called channels. Each user is assigned a
unique pair of channels (one for transmitting and one for receiving).
 Full Duplex Communication: Separate channels are used for
sending and receiving signals.
 Limitations:
Unused bandwidth is wasted when users are idle or pause
between transmissions.
Inefficient when user activity is intermittent.

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Time Division Multiple Access (TDMA)

Divides the communication timeline into fixed time slots. Each user is
assigned a specific time slot to transmit data on the same frequency channel.
Transmission Process:
Users transmit in turns, ensuring no overlap.
Time slots are allocated in a round-robin fashion.
Advantages:
Supports both voice and data communication.
Enables services like SMS, fax, and video conferencing.
High data rates (64 kbps to 120 Mbps).
Disadvantages:
Users may face disconnections when moving to a new cell if all time slots
are occupied.
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Susceptible to multipath distortion, causing data errors if signals are
Code Division Multiple Access (CDMA)

Uses unique binary codes assigned to each user, enabling multiple users to
transmit simultaneously over the same frequency channel.
Each user’s signal is encoded with a unique code sequence.
The receiver decodes signals using the corresponding code, distinguishing
between overlapping transmissions.
Advantages:
High spectral efficiency and capacity.
Better coverage in rural and low-signal areas.
Fewer dropped calls due to extended range from base stations.
Disadvantages:
Channel Pollution: Occurs when signals from multiple base stations
interfere, reducing call quality.
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Handset Limitations: Phones are tied to carriers since network
Cellular Systems
Cellular systems use Space Division Multiplexing (SDM), where each base
station (transmitter) covers a specific geographic area called a cell. Cell sizes vary
based on the environment:
 Small Cells: Hundreds of meters in cities.
 Medium Cells: Up to tens of kilometers in rural areas.
 Large Cells: Tens of meters in buildings.

Cell shapes are irregular due to environmental factors like buildings, terrain,
weather, and system load. Mobile stations within a cell communicate with the base
station for seamless mobile telecommunication services.

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Cellular System Infrastructure

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GSM SERVICES AND ARCHITECTURE

 GSM (Global System for Mobile Communications) is a widely used digital

mobile system with over 800 million users in 190+ countries.
 It was developed in the 1980s to replace incompatible analog systems in Europe.
 Established by the Group Special Mobile (GSM) under ETSI, it became a 2G
cellular standard, supporting voice and data services using digital technology.

GSM Services
1. Tele Services
2. Bearer or Data Services
3. Supplementary Services

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Contd….
1.Tele Services:
• Voice Calls: Supports standard telephony at 13 kbps and emergency calls.
• Videotext and Facsimile: Enables text and fax transmissions.
• Short Messaging Service (SMS): Allows sending and receiving text messages, including news,
financial, and location-based data.

2. Bearer or Data Services:

• Enables data transmission with rates up to 9600 bit/s.
• Transparent Services: Use only physical layer functions with constant delay but no error
correction.
• Non-Transparent Services: Add protocols for error correction and flow control using radio link
protocols (RLP).
• Data can be transmitted synchronously or asynchronously with speeds from 300 bit/s to 9.6
kbit/s.

3.Supplementary Services:
• Offers additional features like call forwarding, user identification, and call34redirection.
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• Supports ISDN-like functions such as closed user groups and multiparty communication,
benefiting businesses with private GSM networks.
GSM Architecture
 GSM comes with a hierarchical, complex system architecture comprising many
entities, interfaces, and acronyms.
 A GSM system consists of three subsystems,
 The radio sub system (RSS)
 The network and switching subsystem (NSS)
 The operation subsystem (OSS).

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Mobile Station
The Mobile Station is made up of two entities:
1.Mobile Equipment (ME)
2.Subscriber Identity Module (SIM)

Mobile Equipment (ME):

 Portable, vehicle mounted, hand held device
 Uniquely identified by an IMEI (International Mobile Equipment Identity)
 Voice and data transmission
 Monitoring power and signal quality of surrounding cells for optimum handover
 Power level : 0.8W – 20 W
 160 character long SMS.
Subscriber Identity Module(SIM):
 Smart card contains the International Mobile Subscriber Identity (IMSI)
 Allows user to send and receive calls and receive other subscribed services
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 Protected by a password or PIN

 Base Station Subsystem(BSS)
Base station subsystem is composed of two parts that communicate across the
standardized Abis interface allowing operation between components made by different suppliers.

Base Transceiver Station (BTS)

Base Station Controller (BSC)

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Base Transceiver Station(BTS)
 Encodes, encrypts, multiplexes, modulates and feeds the RF signals to the antenna.
 Communicates with Mobile station and BSC
 Consists of Transceivers (TRX) units

Base Station Controller (BSC)

 Manages Radio resources for BTS
 Assigns Frequency and time slots for all MS’s in its area
 Handles call set up
 Handover for each MS
 It communicates with MSC and BTS
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Network Switching Subsystem(NSS)
The system contains the following functional units
 Mobile Switching Center (MSC)
 Home Location Register (HLR)
 Visitor Location Register (VLR)
 Authentication Center (AUC)
 Equipment Identity Register (EIR)

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Mobile Switching Center (MSC)
 Heart of the network
 Manages communication between GSM and other networks
 Billing information and collection
 Mobility management
- Registration
- Location Updating
- Inter BSS and inter MSC call handoff

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Home Location Registers(HLR)
 Stores information about each subscriber that belongs to it MSC in permanent
and temporary fashion.
 As soon as mobile subscriber leaves its current local area, the information in the
HLR is updated.
 Database contains IMSI, MSISDN, prepaid/ postpaid, roaming restrictions,
supplementary services.

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Visitor Location Registers (VLR)
 Temporary database which updates whenever new MS enters its area, by HLR
database
 Assigns a TMSI to each MS entering the VLR area which keeps on changing.
 Controls those mobiles roaming in its area
 Database contains IMSI, MSISDN, Location Area, authentication key

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Authentication Center (AUC)
 Contains the algorithms for authentication as well as the keys for encryption.
 Protects network operators from fraud.
 Situated in special protected part of the HLR.

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Equipment Identity Register (EIR)
 Stores all devices identifications registered for this network.
 Database that is used to track handsets using the IMEI (International Mobile
Equipment Identity)
 Prevents calls from stolen, unauthorised or defective mobile devices

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HANDOVER

Handover is the process of transferring an ongoing call or data session from one base
station (or cell) to another without disconnecting the service. It ensures continuous connectivity as a
user moves through different coverage areas.
 Two Types:
1.Hard Handover
2.Soft Handover

Hard Handover
 In UMTS, hard handovers switch connections between antennas, systems, or frequencies.
UTRA TDD switches between time slots, while inter-frequency handovers require more
advanced receivers.
 Switching to GSM, other IMT-2000 systems, or satellites is also a hard handover because
different frequencies are used.
 To allow the device (UE) to connect to GSM or other bands,22/01/2025
compressed mode 45
pauses
transmission temporarily. Data loss is avoided by sending data faster or reducing the amount sent.
Soft Handover

 In soft handover, a mobile device (UE) connects to multiple base stations

simultaneously during a handover process. This is possible in UMTS UTRA
FDD because the same frequency is used across neighboring cells.
Advantage:
 Smooth transition without dropping the connection, as data is received from
multiple sources.
 The mobile device gradually disconnects from one base station while
maintaining connections with others, improving call quality and reliability.
 Soft handover is ideal for CDMA-based systems like UMTS, where overlapping
cell coverage enables this seamless connection.

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SECURITY
 GSM is one of the most secure mobile telecommunication systems, ensuring end-
to-end security through standardized methods, encryption, and subscriber
anonymity.
 GSM ensures user privacy by assigning Temporary Identification Numbers
(TMSI) to subscribers, concealing their actual identities. Communication privacy
is further secured using encryption algorithms and frequency hopping, making
transmissions difficult to intercept or trace.

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Mobile Station Authentication
 Uses a challenge-response mechanism involving a 128-bit Random Number
(RAND) and a 32-bit Signed Response (SRES).
 The Subscriber Authentication Key (Ki) is stored securely in the SIM and
authentication databases (AUC, HLR, VLR) and never transmitted over the
network.
 If the calculated SRES matches the one from the subscriber, authentication is
successful; otherwise, the connection is terminated.

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Signalling and Data Confidentiality

• The SIM generates a 64-bit Ciphering Key (Kc) using the A8 algorithm and the
RAND from the authentication process.
• The A5 algorithm ensures encrypted voice and data communication using Kc.
• Ciphering keys are updated periodically for enhanced security and to resist
eavesdropping.

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Subscriber Identity Confidentiality

 The system assigns a Temporary Mobile Subscriber Identity (TMSI) after

authentication to hide the actual International Mobile Subscriber Identity (IMSI).
 The TMSI is valid within a specific Location Area (LA) and is supplemented by
the Location Area Identification (LAI) for secure communication outside the
area.
These layered security mechanisms protect GSM communications from
unauthorized access, ensuring data privacy and subscriber anonymity.

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