Mc Answer Key 1

MOBILE COMPUTINGAnswer Key

Question & Answers

Introduction

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

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Spread Spectrum Overview 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 Cellular telephones 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).

Questions & Answers

Medium Access Control

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

Questions & Answers

GSM

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.

Questions & Answers

Wireless LAN

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.

Questions & Answers

Mobile Network Layer

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.

Questions & Answers

Mobile Transport Layer

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.

Questions & Answers

Data Broadcasting

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”

Questions & Answers

Mobile Ad hoc Networks (MANETs)

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.

Questions & Answers

Support for mobility

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

Questions & Answers

Miscellaneous

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.

Mc Answer Key 1

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