PES INSTITUTE OF TECHNOLOGY, BANGALORE-560085

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24 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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PES INSTITUTE OF TECHNOLOGY, BANGALORE
-
560085

Subject:
Wireless and Mobile Networks





C
ode:

10EC0131

Class
: II SEM MTECH








Staff
: Kavitha Y.C

INTERNAL TEST
-

1 SOLUTIONS

Part
-
1


1.

Explain


a) Duplexing methods







(
10 Marks
)



b)
Multiple a
c
cess methods


c) Wireless switching technology

sol:

Duplexing methods:

Types:


1.
Frequency division duplexing




2.
Time division duplexing

Frequency division duplexing




FDD is employed in radio systems to provide an uplink and a

downlink radio channel between the sender and
the receiver.



The frequencies are separated by a duplex spacing.




User tunes
between

an uplink and a downlink frequencies to transmit and receive respectively.


Time division duplexing:




In TDD the two
directions of the transmission are created by alternating the transmission in time the maximum
bit rate B
max
supportable for a given bandwidth W is allocated between the two directions.



If this bit rate is equally divided between the two directions each di
rection of transmission can support at most
B
max
/2.










Multiple access methods
:

Method of creating multiple channels for each transmission direction.

Types:

1.

Frequency division

multiple access

2. Time division multiple access

3. Code division mult
iple access


Frequency division

multiple access:



FDMA is a channel access method in

multiple access protocols as a channelization protocol.



It gives user an individual allocation of one or several frequency bands so that receivers can discriminate among
th
em by tuning to the desired channel



FDMA is a basic technology in analog advanced mobile phone
service (
AMPS) in North America.


Time division multiple access:



TDMA is a channel access method

for shared medium networks



It allows several users to share the

same frequency channel by dividing the signal into different time slots.



Users transmit in rapid
succession

one after the other
each using his

own time slot



TDMA is used in digital 2G cellular
systems such

as GSM, IS
-
136
, PDC, DECT

for portable phones.

Code division multiple access:



CDM
A

is a form of multiplexing where the transmitter encodes the signal using a pseudorandom sequence that
the receiver also knows and can use to decode the received signal.



Each
different random

sequence corresponds to a dif
ferent communication channel.




CDMA is used in Mobile phone standard like IS
-
95
.

Wireless switching technology


Types:

Packet switching



Virtual circuits
:

Types:


i. Switched virtual
circuits (
SVC)



ii. Permanent virtual
circuits (
PVC)




Packet switchi
ng:



Packet switched communication uses short bursts of information that use channels only for short periods of
time.



Packet switching is a more efficient system for transmitting data because it shares spectrum and bandwidth.



In packet switching the packets from all
the computers moves in
pipelined

fashion over the
network.
This

network is connected to the switch of the WAN with data circuit terminating equipment.




Virtual circuits:


It is a logical circuit created within a s
hared network between two network devices.

Switched virtual
circuits (
SVC):



SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is
complete.



Communication over SVC consist of three phases :

Circuit

establi
shment

: involves creating VC b/
w source

& destination


Data

transfer


: involves transmitting

data b/w the devices over VC



Circuit

termination


: involves

tearing down the VC b/w source & destination.


Permanent virtual
circuits (
PVC):



It’s a permanentl
y established VC that
consists

of one mode data transfer.



Its used in situations in which data transfer b/w devices is constant.



It decreases the bandwidth use associated with establishment and termination of VC

but increases the cost



2).

Discuss the
wireless communication problems
.





(
10 Marks
)

The wireless communication problems are

i.

Shared media
: wireless media offers lower
bandwidth;

hence effective utilization of the bandwidth is
necessary for wireless networks.

ii.

Increased bit error rate
: wireless

N/w s media is more prone to
errors due

to obstacles coming in b/w the Tx
& Rx and interference caused by
neighboring

Tx

iii.

Lower radio transmission
power
: mobile

units are compact in siz
e and works on battery with scarce energy
resources. The mobile nodes l
imit transmission power to avoid interference.

iv.

Scattering
:

scattering occurs when the material through which the wave travels has objects with dimension
that are small compared to the wavelength and where number of obstacles per unit volume is large.

v.

Refle
ction
: Reflection occurs when a propagating electromagnetic waves strikes an object which has very
large dimensions compared to the wavelength of the propagating wave.

vi.

Diffraction
: Diffraction occurs when the radio path b/w the Tx and Rx is obstructed by a

surface that has
sharp irregularities
.

vii.

Multipath propagation

: the multipath propagation in mobile radio is caused by reflection and scattering
from buildings, trees and other obstacles along the radio path

viii.

Fading
: signal fading can drastically affect the

performance of a wireless communication
system. Fading

can
degrade the BER performance of digital communication sys, resulting in data loss.

ix.

Path loss
: path loss b/w the Tx& Rx is a key consideration while designing a wireless network. Expected
levels of
path loss provide valuable information when determining requirements for transmit power levels,
receiver sensitivity and SNR.

x.

RF signal interference
: the process of transmitting & receiving radio and laser signals through the air makes
wireless systems
vul
nerable

to atmospheric noise and transmission
from other systems
.










PART
-

2



3)
.


Discuss the design issues in wireless networks.





(
10 Marks
)



The various design issues in wireless networks are


Traffic and

resource allocation
: Each accepted has a certain traffic contract that describes the traffic
type and

resource requirements. A slot scheduler is responsible to assign slots in a transmission frame according to the
various traffic contracts.

Flow control
:

a connection involve
s buffering at several places on the path b/w the sender and the receiver.
Traffic type requirements concerning delay and implementation
restrictions on the buffer capacity generally
limit the amount of buffer space available to a connection.

Error control
: The high BER is typical for a wireless link, many packets can be corrupted during transmission. If
this rate exceeds the allowable packet loss rate of a connection, an effective and efficient error control scheme
must be implemented to handle such situat
ions.

Security and privacy
:

network security refers
to the protection of info and resources from loss, corruption and
improper use. Eavesdropper may listen to the traffic in real time or record it for future cryptanalysis. To
overcome the spoofing proper authentication and
access

control mechanism need to b
e put up in wireless
network.

Mobility
:

the

mobility of the wireless node
enforces handover procedures when the node moves from one area
to another. As the radius of an area decreases handover situation will be encountered
frequently. The

task of
link laye
r is to provide the higher layers of the mobile with info about which areas are in range and provide
services to actually handle the handover.

Routing
: multihop wireless networks without any infrastructure pose bigger challenges in computing proper and
ef
ficient routes for source destination
pairs. This

is mainly due to node mobility, ie network topology frequently
changes leading to unstable and improper routes.

Reachability

and
path minimization

are some important
features of routing.

Qos Management
: Qua
lity of service management is essential when a connection is initiated or when the
operating conditions have changed.


4)
.

Explain the properties of WBAN.






(
10 Marks
)





5)
.


Discuss the design issues of WBAN.






(
10 Marks
)




PART
-
3

6)

Explain the physical layer techniques of WBAN




(
10 Marks
)


The physical layer techniques are



Inductive

coupling,



Radio

frequency (RF) communication,


Antennas

and


Modulation

techniques.



7)

What is Scheduled protocol of a WBAN? Explain
with an example
.




(
10 Marks
)


In scheduled protocol for WBAN adopts common sleep/wakeup schedules. The nodes remain
awake only for a brief contention period to coordinate. Scheduling allows the nodes to operate in low
duty cycles

.it only wakes up and sends when the receiver is listening. The premise
of scheduled

protocols is the cost of coordination is minimal compared to reductions in time spent in listening for
potential transmissions.

Examples: sensor MAC(S
-
MAC
) and

timeout MA
C( T
-
MAC)







8).

W
hat

is
Low power

listening protocol of
a WBAN
? Explain with an example.


(
10 Marks
)



Low power
listening is

present in Wireless sensor
MAC (Wise MAC
) and
Berkeley

MAC (
B
-
MAC).

It allows a sleeping node to
check channel

activity with a brief, low power “channel active” probes
.these protocols replaces the relatively long wake
up interval

in S
-
MAC and T MAC with a very short
channel polling
time. The

nodes randomly poll the channel with
predefined polling

period. To

wake
up
the receiver the sender uses a long preamble before each packet, which is
at least

the length of the
polling period
.

Example
B
-
MAC