MOBILE COMPUTING

Answer Key
Introduction
Question & Answers
Questions
 Define Mobile Computing, its applications and limitations
 Define - Mobile Computing, Mobile Communications, Passive
Computing, Ubiquitous networks, Mobile applications
 Define Spread Spectrum. Differentiate FHSS and DSSS
 Write short notes on DSSS and FHSS
 What is spread spectrum. Explain FHSS with a neat diagram.
 What is spread spectrum. Explain Direct Sequence Spread Spectrum
with a neat diagram.
 Define multiplexing. Explain in detail the various methods of
multiplexing available with neat diagram
 What is modulation? Explain the various methods available for digital
modulation with neat diagrams.
Definitions – 1/1
 Mobile Computing is an umbrella term used to describe
technologies that enable people to access network services at any
place, anytime, and anywhere.
 Mobile Communication -
 Pervasive Computing - Pervasive means ‘existing in all parts of a
place or thing’. The next generation of computing which takes into
account the environment in which information and communication
technology is used everywhere by everyone and at all times.
 Ubiquitous networks - A term that describes integration of
computers into practically all objects in our everyday environment,
endowing them with computing abilities
 Mobile applications -
Application of Mobile Computing
 Emergency services
 Flood
 Earthquake
 For Estate Agents
 In courts

 In companies

 Stock Information Collection/Control

 Credit Card Verification

 Taxi/Truck Dispatch

 Electronic Mail/Paging
Challenges and Limitations – 1/3
 Interference: Radio transmission cannot be protected
against interference using shielding as this is done in
coaxial cable or shielded twisted pair.
 Regulations and spectrum: Frequencies have to be
coordinated, and unfortunately, only a very limited amount
of frequencies are available
 Low bandwidth: Although they are continuously
increasing, transmission rates are still very low for wireless
devices compared to desktop systems. Local wireless
systems reach some Mbit/s while wide area systems only
offer some 10 kbit/s.
Challenges and Limitations – 2/3
 High delays, large delay variation: A serious problem for
communication protocols used in today’s Internet (TCP/IP) is the
big variation in link characteristics. In wireless systems, delays
of several seconds can occur, and links can be very asymmetrical
(i.e., the links offer different service quality depending on the
direction to and from the wireless device). Applications must be
tolerant and use robust protocols.
 Lower security, simpler to attack: Not only can portable
devices be stolen more easily, but the radio interface is also
prone to the dangers of eavesdropping. Wireless access must
always include encryption, authentication, and other security
mechanisms that must be efficient and simple to use.
Challenges and Limitations – 3/3
 Shared medium: Radio access is always realized via a
shared medium.
  Ad-hoc networking: Wireless and mobile computing
allows for spontaneous networking with prior set-up of an
infrastructure. However, this raises many new questions for
research: routing on the networking and application layer,
service discovery, network scalability, reliability, and
stability etc.
 Device
 Fit more functionality into single, smaller device.
 Need power and energy efficient
 Spread Spectrum Overview
9

 Spread Spectrum is a means of transmission in which the data

sequence occupies a bandwidth in excess of the minimum
bandwidth necessary to send it.
 Effectively the signal is mapped to a higher dimension signal space
 The signal component in each direction is sqrt(E/N)
 Signal spreading is done before transmission by using a
spreading sequence. The same sequence is used at the receiver
to retrieve the signal
 Spread Spectrum is most effective against interference
(intentional or non-intentional) with fixed energy.
 Main commercial applications in wireless and GPS.
Spread Spectrum Pros & Cons
 Pros
 Immunity from various noise and multipath distortion
 Including jamming
 Can hide/encrypt signals
 Only receiver who knows spreading code can retrieve signal
 Several users can share same higher bandwidth with little
interference
 Cellulartelephones
 Code division multiplexing (CDM)
 Code division multiple access (CDMA)

 Cons
 More complex implementation
Frequency Hopping Spread Spectrum (FHSS)

 Carrier changes frequency (HOPS) according to a

pseudorandom Sequence.
 Pseudorandom sequence is a list of frequencies. The carrier hops
through this lists of frequencies.
 The carrier then repeats this pattern.
 During Dwell Time the carrier remains at a certain frequency.
 During Hop Time the carrier hops to the next frequency.
 The data is spread over 83 MHz in the 2.4 GHz ISM band.
 This signal is resistant but not immune to narrow band
interference.
FHSS – Transmitter and Receiver
DSSS – 1/2
 Spread spectrum increases the bandwidth of the signal compared to
narrow band by spreading the signal.
 There are two major types of spread spectrum techniques: FHSS and
DSSS.
 FHSS spreads the signal by hopping from one frequency to another
across a bandwidth of 83 Mhz.
 DSSS spreads the signal by adding redundant bits to the signal prior to
transmission which spreads the signal across 22 Mhz.
 The process of adding redundant information to the signal is called
Processing Gain .
 The redundant information bits are called Pseudorandom Numbers
(PN).
DSSS – 2/2
 DSSS works by combining information bits (data signal) with higher
data rate bit sequence (pseudorandom number (PN)).
 The PN is also called a Chipping Code (eg., the Barker chipping code)
 The bits resulting from combining the information bits with the chipping
code are called chips - the result- which is then transmitted.
 The higher processing gain (more chips) increases the signal's resistance to
interference by spreading it across a greater number of frequencies.
 IEEE has set their minimum processing gain to 11. The number of chips in
the chipping code equates to the signal spreading ratio.
 Doubling the chipping speed doubles the signal spread and the required
bandwidth.
DSSS – Transmitter & Receiver
FHSS Vs DSSS – 1/2
 DSSS is more susceptible to narrow band noise.
 DSSS channel is 22 Mhz wide whereas
 FHSS is 79 Mhz wide.
 The FCC regulated that DSSS use a maximum of 1 watt of transmitter
power in Pt-to-Multipoint system.
 DSSS costs less then FHSS
 FHSS can have more systems co-located than DSSS.
 DSSS systems have the advantage in throughput
 The Wi-Fi alliance tests for DSSS compatibility
 No such testing alliance exists for FHSS
FHSS Vs DSSS – 2/2
 DSSS generally has a throughput of 5-6 Mbps while
FHSS is generally between 1-2 Mbps.
 Both FHSS and DHSS are equally insecure.
 DSSS has gained much wider acceptance due to its
low cost, high speed and interoperability.
 This market acceptance is expected to accelerate.
 FHSS advancement includes HomeRF and 802.15
(WPAN) (Bluetooth), however, it is expected to not
advance into the enterprise.
Multiplexing
 Multiplexing describes how several users can share
a medium with minimum or no interference. For
wireless communication, multiplexing can be
carried out in four dimensions: space, time,
frequency, and code. In this field, the task of
multiplexing is to assign space, time, frequency,
and code to each communication channel with a
minimum of interference and a maximum of
medium utilization.
Frequency Division Multiplexing
 (FDM) describes schemes to subdivide the
frequency dimension into several non-overlapping
frequency bands
 Separation of the whole spectrum into smaller
frequency bands
 A channel gets a certain band of the spectrum for the
whole time
Time Division Multiplexing
 A more flexible multiplexing scheme for typical
mobile communications is time division
multiplexing (TDM). Here a channel is given the
whole bandwidth for a certain amount of time, i.e.,
all senders use the same frequency but at different
points in time. Again, guard spaces, which now
represent time gaps, have to separate the different
periods when the senders use the medium.
Code Division Multiplexing
 All channels use the same frequency at the same time for transmission. Separation is now achieved
by assigning each channel its own ‘code’, guard spaces are realized by using codes with the
necessary ‘distance’ in code space, e.g., orthogonal codes. Implemented using spread spectrum
technology
 Advantages:
 The main advantage of CDM for wireless transmission is that it gives good protection against interference
and tapping. Different codes have to be assigned, but code space is huge compared to the frequency space.
Assigning individual codes to each sender does not usually cause problems.
 Bandwidth efficient
 No coordination and synchronization necessary 
 Good protection against interference and tapping
 Disadvantage
 The main disadvantage of this scheme is the relatively high complexity of the receiver. A receiver has to
know the code and must separate the channel with user data from the background noise composed of other
signals and environmental noise. 
 Additionally, a receiver must be precisely synchronized with the transmitter to apply the decoding correctly.
 To apply CDM, precise power control is required 
 Lower user data rates
Modulation
 digital modulation, - digital data (0 and 1) is
translated into an analog signal (baseband signal).
Digital modulation is required if digital data has to
be transmitted over a medium that only allows for
analog transmission.
 amplitude shift keying (ASK),
 frequency shift keying (FSK),
 and phase shift keying (PSK
Modulation – Amplitude Shift Keying

 Amplitude shift keying (ASK), is the most simple digital

modulation scheme. The two binary values, 1 and 0, are
represented by two different amplitudes. In the example, one of
the amplitudes is 0 (representing the binary 0).
 This simple scheme only requires low bandwidth, but is very
susceptible to interference. Effects like multi-path propagation,
noise, or path loss heavily influence the amplitude. In a wireless
environment, constant amplitude cannot be guaranteed, so ASK
is typically not used for wireless radio transmission.
 The wired transmission scheme with the highest performance,
namely optical transmission, uses ASK. Here, a light pulse may
represent a 1, while the absence of light represents a 0.
Modulation – Frequency Shift Keying

 A modulation scheme often used for wireless

transmission is frequency shift keying (FSK) (see
Figure 2.24). The simplest form of FSK, also called
binary FSK (BFSK), assigns one frequency f1 to
the binary 1 and another frequency f2 to the binary
0.
Modulation – Phase Shift Keying
 Finally, phase shift keying (PSK) uses shifts in the
phase of a signal to represent data. Figure 2.25
shows a phase shift of 180° or π as the 0 follows
the 1 (the same happens as the
 1 follows the 0). This simple scheme, shifting the
phase by 180° each time the value of data changes,
is also called binary PSK (BPSK).
Medium Access Control
Questions & Answers
Medium Access Control
 What is hidden terminal and exposed terminal problems? How are they handled in
wireless networks?
 Why MAC protocol does not implement CSMA/CD? How the hidden terminal and
fading problems are taken care?
 Explain the characteristics of Medium Access Control protocols.
 Explain ALOHA, PRMA, MACA, CSMA / CA, Polling
 Explain the following – CSMA, MACA, DAMA, PRMA
 Define Pure ALOHA, Slotted ALOHA, CSMA/CD, CSMA/CA and MACA
 What is MACA? Explain in detail with a neat diagram.
 Explain in detail Multiple Access with Collision Avoidance with a neat diagram.
 What is Channel Allocation? Explain with suitable diagram Fixed Channel Allocation.
 What is multiple accesses? Define the group access, random access, controlled access
and channelization
 Define Channelization, TDMA, FDMA and CDMA
GSM
Questions & Answers
GSM
 Explain the concept of handover mechanisms in GSM in detail. / Handovers in
GSM Systems – Discuss the various types available with neat diagram./What is
handover? Explain the various types of handover available with neat diagram.
 What is handoff? Explain with a suitable diagram “HIPERLAN-2 reference model”
 Explain with proper diagram Intra-EMAS-E/Intra-AP forward handover.
 Draw the GSM architecture and explain each block in detail. / In detail explain the
functional architecture of a GSM system with a neat diagram./Explain functional
architecture of a GSM system with a neat diagram.
 With a suitable diagram explain the role of GSM components BSC, MSC for
mobile communications.
 Explain the basic security services available in GSM systems using A3, A5, A8
algorithms with neat diagrams.
 Explain security of GSM in detail.
 Explain the cellular system and write about one cellular standard.
Wireless LAN
Questions & Answers
Questions
 What are the advantages and disadvantages of WLAN
 Explain Bluetooth protocol in detail.
 Write short note on security in Bluetooth.
 Write short note on Bluetooth protocol
 Compare and Contrast wired LANs with Wireless LANs
 Discuss at least two WLAN vulnerabilities
 Explain the location management requirements
transparency of mobility, Security, Efficiency and
Scalability and Identification.
Mobile Network Layer
Questions & Answers
Questions
 What is dynamic host configuration protocol? Explain its functions.
 Explain with suitable diagram how the IP packet delivery is done in a
wireless network.
 Explain the functions of agents in mobile IP in OSI network layer.
 What are mobile agents and their applications and uses in mobile computing.
 Briefly explain how mobile agents can be used for mobile clients (What
support can a mobile agent give for mobile clients.)
 Explain how agent discovery and registration is done during IP packet
delivery in mobile IPs with neat diagrams / Explain the following basic
functions in Mobile IP – Agent Discovery and Registration.
 Explain tunneling and reverse tunneling concept of Mobile IP.
 Explain the use of all the layers using the Simplified Reference Model of
Communication Systems with a neat diagram.
Mobile Transport Layer
Questions & Answers
Questions
 Explain two features namely flow control and congestion
control.
 Explain why traditional TCP features are failures in wireless
network.
 Explain Indirect TCP with Mobile TCP
 How traditional TCP function with reference to sliding
window concept.
 Describe snooping TCP. How it differs from indirect TCP?
What are the advantages and disadvantages of each one? /
Explain indirect TCP and Snooping TCP with neat diagrams.
Data Broadcasting
Questions & Answers
Questions
 What is communication asymmetry? Explain with suitable
example.
 Explain unidirectional broadcast systems with reference to audio
broadcast.
 Discuss the different broadcast models. List its advantages and
disadvantages.
 Explain in detail Digital Video broadcasting with a neat diagram.
 Explain in detail the Digital Audio broadcasting with a neat
diagram
 Discuss “Dissemination based Data Delivery Using Broadcast
Disks”
Mobile Ad hoc Networks (MANETs)
Questions & Answers
Questions
 What is mobile adhoc networks? Explain at least two
applications for which mobile adhoc networks are suitable.
 Explain the basic functioning of Adhoc networks with a
neat diagram
 Define mobile adhoc network. Explain the properties and
applications of MANET
 Write about adhoc networks and routing protocols used.
 Explain table driven routing algorithm of MANET
 Explain routing in adhoc networks using DSDV, DSR and
alternative metrics with neat diagrams.
Support for mobility
Questions & Answers
Questions
 Explain WAP architecture with diagram. / Explain
Wireless Application Protocol architecture.
 Give mobile computing architecture for a mobile
device.
 Explain at least five services provided in wireless
application protocol
Miscellaneous
Questions & Answers
Questions
 What are six contributions for saving energy in mobile
computing? Explain briefly at least four of them.
 How is energy conservation to be done in access
protocol for mobile computing and communication
(Transmission Scheduling Protocols & Reception
Scheduling Protocols)
 What is disconnected operation and how it is managed
in CODA FS?
 Explain the features and functions of Sync ML with
suitable diagram.
More Slides
Direct Sequence Spread Spectrum (DSSS) – 1/3

 The DSSS encoder spreads the data across a broad range of

frequencies using a mathematical key. The receiver uses the same
key to decode the data.
 Each bit represented by multiple bits using spreading code
 Spreading code spreads signal across wider frequency band
 In proportion to number of bits used
 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code
 One method:
 Combine input with spreading code using XOR
 Input bit 1 inverts spreading code bit
 Input zero bit doesn’t alter spreading code bit
 Data rate equal to original spreading code
 Performance similar to FHSS
Frequency Hopping Spread Spectrum (FHSS)

 For frequency hopping spread spectrum (FHSS) systems,

the total available bandwidth is split into many channels of
smaller bandwidth plus guard spaces between the channels.
 Transmitter and receiver stay on one of these channels for a
certain time and then hop to another channel.
 This system implements FDM and TDM.
 The pattern of channel usage is called the hopping
sequence, the time spend on a channel with a certain
frequency is called the dwell time.
 FHSS comes in two variants, slow and fast hopping