Mobile Computing - Unit 1 material

1. Prepared By,
Mrs. V. Deepa Priya AP/IT
Kamaraj College of Engineering and Technology, VIrudhunagar

2. Mobile Computing – Mobile Computing Vs
wireless Networking – Mobile Computing
Applications – Characteristics of Mobile
computing – Structure of Mobile Computing
Application. MAC Protocols – Wireless MAC
Issues – Fixed Assignment Schemes – Random
Assignment Schemes – Reservation Based
Schemes.

3.  Mobile Computing refers a technology that
allows transmission of data, voice and video
via a computer or any other wireless enabled
device.
 It is free from having a connection with a
fixed physical link.
 It facilitates the users to move from one
physical location to another during
communication.
 People to access information from anywhere
and at anytime.

4.  The concept of Mobile Computing can be
divided into three parts:
◦ Mobile Communication
◦ Mobile Hardware
◦ Mobile Software

5.  Mobile Communication specifies a framework that is
responsible for the working of mobile computing
technology.
 In this case, mobile communication refers to an
infrastructure that ensures seamless and reliable
communication among wireless devices.
 This framework ensures the consistency and
reliability of communication between wireless
devices.
 The mobile communication framework consists of
communication devices such as protocols, services,
bandwidth, and portals necessary to facilitate and
support the stated services.
 These devices are responsible for delivering a smooth
communication process.

7.  Fixed and Wired: In Fixed and Wired
configuration, the devices are fixed at a position,
and they are connected through a physical link to
communicate with other devices.
◦ For Example, Desktop Computer.
 Fixed and Wireless: In Fixed and Wireless
configuration, the devices are fixed at a position,
and they are connected through a wireless link to
make communication with other devices.
◦ For Example, Communication Towers, WiFi router

8.  Mobile and Wired: In Mobile and Wired
configuration, some devices are wired, and
some are mobile. They altogether make
communication with other devices.
◦ For Example, Laptops.
 Mobile and Wireless: In Mobile and Wireless
configuration, the devices can communicate
with each other irrespective of their position.
They can also connect to any network without
the use of any wired device.
◦ For Example, WiFi Dongle.

9.  Mobile hardware consists of mobile devices
or device components that can be used to
receive or access the service of mobility.
Examples of mobile hardware can be
smartphones, laptops, portable PCs, tablet
PCs, Personal Digital Assistants, etc.

10.  Mobile software is a program that runs on
mobile hardware.
 This is designed to deal capably with the
characteristics and requirements of mobile
applications.
 This is the operating system for the appliance
of mobile devices.
 In other words, you can say it the heart of the
mobile systems.
 This is an essential component that operates
the mobile device.

11.  Web or Internet access.
 Global Position System (GPS).
 Emergency services.
 Entertainment services.
 Educational services.

12.  Mobile computing denotes accessing
information and remote computational
services while on the move
 Wireless networking provides the basic
communication infrastructure necessary to
make this possible.
 So, Mobile computing is based on wireless
networking and helps one to invoke
computing services on remote servers while
on the move: be it be office, home,
conference, hotel and so on.

13.  Wireless networking is important ingredient
of mobile computing
 Mobile computing also requires the
application themselves-their design,
development and the hardware at the client
and server sides.
 Wireless networking is increasingly replacing
traditional networks because of the low setup
time and low initial setup time and low initial
investment required to set up the wireless
network.

14.  Wireless networking is divided into two basic
types : 1) Extension of wired network 2)Adhoc
Network
◦ Extension of wired network – uses fixed
infrastructure such as base stations to provide
single hop wireless communication with a wired
network

15. ◦ Ad-hoc Network
 It does not uses any fixed infrastructure and it is based
on multi-hop wireless communication

16.  One popular example of a fixed infrastructure is
wireless LAN(WLAN) that implements the
IEEE802.11 protocol.
 In this diagram, the access point (AP) provides
the last hop connectivity of the mobile nodes to a
wired network
 AP’s – Bridging between wireless and wired
mediums.

17.  A recent development in wireless networking of
various types of devices using Bluetooth technology
 It is the direct wireless connection of a cell phone
with devices such as printers, cameras, scanners,
laptop and desktop computers.
 Bluetooth replaces the cables and infrared for
exchanging information between devices.
 Bluetooth enables the users to easily connect to a
wide range of personal computing and
telecommunication devices without the need of buy,
carry or lay out cables.
 Bluetooth technology setting up Personal Area
Networks(PAN) known as piconets and ad hoc nework
known as scatternet.

18.  Bluetooth Example:

19.  Ad-hoc network is also known as Mobile Ad-
hoc Network (MANET).
 It is the collection of mobile nodes that form
a network on the fly without requiring the
support of any fixed infrastructure.
 Wireless sensor network are a special type of
wireless ad hoc networks.

20.  Business:
◦ Managers can use mobile computers say, critical
presentations to major customers. They can access the
latest market share information. To enable the company
to keep track of all activities of their travelling
employees, to keep databases consistent etc. With
wireless access, the laptop can be turned into a true
mobile office, but efficient and powerful synchronization
mechanisms are needed to ensure data consistency.
 Infotainment:
◦ Now a days there is a huge market of Entertainment for
humans while they are on move. Watching movies,
listening music and playing a game is become a part of
life for entertainment. In this case mobile computing
performs a major role to provide uninterrupted internet
connection to digital devices.

21.  Replacement of Wired Networks:
◦ wireless networks can also be used to replace wired
networks. e.g., remote sensors, for tradeshows, or in
historic buildings. Due to economic reasons, it is often
impossible to wire remote sensors for weather forecasts,
earthquake detection, or to provide environmental
information.
 Credit Card Verification
◦ At Point of Sale (POS) terminals in shops and supermarkets,
when customers use credit cards for transactions, the
intercommunication required between the bank central
computer and the POS terminal, in order to effect
verification of the card usage, can take place quickly and
securely over cellular channels using a mobile computer
unit. This can speed up the transaction process and relieve
congestion at the POS terminals.

22.  Emergencies:
◦ An ambulance with a high-quality wireless connection to a
hospital can carry vital information about injured persons
to the hospital from the scene of the accident. All the
necessary steps for this particular type of accident can be
prepared and specialists can be consulted for an early
diagnosis.
 Tourism:
◦ Tourism is the largest industry for all the countries. Mostly
tourism places are at remote location from the developed
cities. In this case wireless communication performs a vital
role in connectivity for people who are enjoying their tour.
They are continuously in contact with the family and friends
and searching for travel services, hotel services food
services etc.

23.  E-Governance:
◦ Governments are using many communication services to
update their rural areas. To provide health, education,
safety , farming , weather forecast and many other
related information to governance, governments are
connecting rural areas with head quarter offices for
monitoring.
 Education:
◦ 2020 COVID Pandemic teach us a great lesson to provide
distance education to all who can not reach to the
college or schools. Wireless communication and digital
devices are the backbone for such distance learning
concept. Now all private and public industries are
moving towards the online education with help of good
wireless connectivity.

24.  Manage Personal Records :
◦ Some mobile applications allows user to manage
their personal records such as day to day activities,
some useful notes, etc…
 Social Media and Group Message :
◦ Some mobile applications allows user to keep in
touch with their friends and relatives by sending
messages, images, audio and video clips.
 Transaction :
◦ Some mobile applications allow the facility of
transaction such as recharge mobile, pay bills etc…

25.  1. Portability - The Ability to move a device within a learning environment or to
different environments with ease.
 2. Social Interactivity - The ability to share data and collaboration between users.
 3. Context Sensitivity - The ability to gather and respond to real or simulated data unique to
a current location, environment, or time.
 4. Connectivity - The ability to be digitally connected for the purpose of communication of
data in any environment.
 5. Individual - The ability to use the technology to provide scaffolding on difficult activities
and lesson customization for individual learners.
 6. Small Size - Mobile devices are also known as handhelds, palmtops and smart phones due
to their roughly phone-like dimensions. A typical mobile device will fit in the average adult's
hand or pocket. Some mobile devices may fold or slide from a compact, portable mode to a
slightly larger size, revealing built-in keyboards or larger screens. Mobile devices make use of
touch screens and small keypads to receive input, maintaining their small size and
independence from external interface devices. The standard form of a mobile device allows the
user to operate it with one hand, holding the device in the palm or fingers while executing its
functions with the thumb.
 7. Wireless Communication - Mobile devices are typically capable of communication
with other similar devices, with stationary computers and systems, with networks and portable
phones. Base mobile devices are capable of accessing the Internet through Bluetooth or Wi-Fi
networks, and many models are equipped to access cell phone and wireless data networks as
well. Email and texting are standard ways of communicating with mobile devices, although
many are also capable of telephony, and some specialized mobile devices, such as RFID and
barcode.

26.  Ubiquity:
◦ Ubiquity is present everywhere.
◦ The ability of a user to perform computations from anywhere and
at anytime.
◦ Example : A business executive can receive business notifications
and issue business transactions as long he is in the wireless
coverage area
 Location Awareness
◦ A hand held device equipment with GPS can transparently provide
information about the current location of a user to a tracking
station
◦ Example: a person travelling by road in a car, may need to find
out a car maintenance service that may be available nearby.
◦ Through mobile communication he easily track the near by car
maintenance service.
◦ Other examples: traffic control, fleet management and emergency
services.

27.  Adaptation
◦ Ability of a system to adjust to bandwidth fluctuation without
inconveniencing the user.
◦ It is crucial, because of intermittent disconnections and bandwidth
fluctuations that can arise due to a number of factors such as
handoff, obstacles, environmental noise etc.,
 Broadcast
◦ Efficient delivery of data can be made simultaneously to hundreds
of mobile users.
◦ Example: all users at a specific location such as those near a
railway station, may be sent advertising information by a taxi
service operator
 Personalization
◦ It is easily personalized according to a user’s profile.
◦ Example – Mobile user may need only a certain types of
information from specific sources.

30.  Presentation Tier
◦ It is the user interface
◦ The user has to issue requests and to present the
results to them meaningfully
◦ Programs at this layer run on the client’s machine
◦ It includes web browser and customized client
programs for dissemination of information and for
collection of data from the user.

31.  Application Tier
◦ This layer makes logical decisions and performing
calculation
◦ It moves and process data between the presentation
and data layers.
◦ It performs the processing of user input, obtaining
information and then making decisions.
◦ It can be implemented by using Java, .NET services,
cold fusion etc.,
◦ It is independent of the database design
◦ This layer of functionalities is usually implemented
on a fixed server.

32.  Data Tier
◦ It provides the basic facilities of data storage,
access, and manipulation.
◦ This layer contains a database.
◦ The information is stored and retrieved from this
database.
◦ This layer is also implemented on a fixed server.

33.  Media access control (MAC) protocols enforce a
methodology to allow multiple devices access to a
shared media network.
 Before LANs, communication between computing
devices had been point-to-point.
 That is, two devices were connected by a dedicated
channel.
 LANs are shared media networks, in which all devices
attached to the network receive each transmission
and must recognize which frames they should accept.
 Media sharing reduced the cost of the network
 Objective of MAC protocol are maximization of the
utilization of the channel and minimization of
average latency of transmission.

34.  MAC protocol should implement some rules – it
helps to enforce discipline when multiple nodes
connect for a shared channel.
 It should help maximize the utilization of the
channel.
 Channel allocation needs to be fair. No node
should be discriminated against at any time and
made to wait for a long time for transmission
 It should be capable of supporting several types
of traffic having different maximum and average
bit rates.
 It should be robust in the face of equipment
failures and changing network conditions

35.  Bandwidth Efficiency –
The shortage of data transfer capacity assets in these
networks requires its proficient use. To evaluate this,
we could state that
bandwidth capacity is the proportion of the
bandwidth used for data transmission to the
complete accessible bandwidth capacity.
 Quality of Service Support –
Quality of service support is difficult due to the
mobility of the nodes. Once a node moves out of
reach, the reservation in it is lost. In these networks,
QoS is extremely important because if it is being
used in military environments, the service support
needed time to time.

36.  Synchronization –
Some instruments must be found so as to
give synchronization among the nodes.
Synchronization is significant for directing
the bandwidth reservation.
 Hidden Terminal Problem –
When there are two nodes, both are outside
of each other’s range and try to communicate
with same node within their range at the
same time, then there must be packet
collision.

37.  Hidden Terminal Problem (Contd)
◦ Node A is in the radio range of the node B, and B is
also within the radio range of C.
◦ Node A and Node C are not in the radio range of
each other.
◦ If both A and C start to transmit to B at the same
time, the data received at node B would get
garbled.
◦ This situation can arise because A and C are
“hidden” from each other because they are outside
each other’s network range.

38.  Exposed Terminal Problem:
◦ In wireless LANs (wireless local area networks), the
exposed terminal problem is a transmission
problem that arises when a transmitting station is
prevented from sending frames due to interference
with another transmitting station

39.  Exposed Terminal Problem: (Contd..)
◦ Node B is within the transmission range of A and C
◦ Node D is within the transmission range of C
◦ Node A is within the transmission range of B
◦ Node A will not be able to transmit to any node
when B is transmitting to C.
◦ The problem occurs because A is exposed to B’s
transmission.

40.  The MAC protocols can be broadly divided
into the following three categories.
◦ Fixed assignment schemes
◦ Random assignment schemes
◦ Demand based Scheme.

41.  It is usually called circuit-switched schemes.
 In fixed assignment schemes, the resources required for a
call are assigned for the entire duration of the call.
 On the other hand, the random assignment schemes and
the reservation schemes are called packet switched
schemes
 The random assignment schemes are connection less and
packet switching schemes.
 In this no resource reservations are made, the nodes
simply start to transmit as soon as they have packet to
send
 In reservation schemes, a node makes explicit reservation
to a connection based packet switching scheme.
 The reservation based MAC schemes are suitable to handle
calls with widely varying traffic characteristics.

42.  It is categorized into three parts
◦ Frequency Division Multiple Access (FDMA)
◦ Time Division Multiple Access (TDMA)
◦ Code Division Multiple Access (CDMA)
 Example:
◦ Consider a student’s common room (Channel) in which many
students want to communicate with each other.
◦ If the student want to avoid cross-talk in the on going process,
then either the students could take turns in speaking(time
division) or they could speak in different pitches(frequency
division) or they could speak in different language (Code Division)
◦ In CDMA, when the students who are speaking the same language
understand each other, but the rest of the students cannot.
◦ In this case many codes occupy the same channel, but only the
users who share a specific code will be able to understand each
other

43.  In FDMA, the available bandwidth is divided
into many narrower frequency bands called
channels.

44.  In the previous example shows a division of the existing
bandwidth into many channels (channel1, channel2, etc.,)
 For full duplex communication to take place, each user is
allocated a forward link for communicating from it (Mobile
handset) to base station (BS)
 Reverse channel for communicating from the BS to it.
 Each user making a call is allocated two unique frequency
bands (Channels) one for transmitting and other for
receiving signals during the call.
 When call is underway, no other user would be allocated
the same frequency band to make a call.
 Unused transmission happens when the allocated caller
pauses between transmission or when no user is allocated
a band, goes idle and wasted.
 FDMA does not achieve a high channel utilization.

45.  Forward link and reverse link

46.  TDMA is an access method in which multiple
nodes are allotted different time slots to
access the same physical channel.
 The time line is divided into fixed sized time
slots.
 These are divided among multiple nodes who
can transmit.
 In this case, all sources use the same channel,
but take turns in transmitting

47.  Time slots are allocated to the user in the
round robin manner
 In TDMA, each user of the channel owns the
channel for exclusive use for one time slot at
a time in a round robin fashion.
Ch1 Ch2 … ChN Ch1 Ch2 … ChN

48.  Multiple users are allotted different codes that consist
of sequences of 0 and 1 to access the same channel.
 A special coding scheme is used that allows signals
from multiple users to be multiplexed over the same
physical channel.
 Three different users who have been assigned
separate codes are multiplexed on the same physical
channel.
 In CDMA, multiple users use the same frequency at
the same time and no time scheduling is applied.
 All the senders send signals simultaneously through a
common medium

50.  How to distinguish transmission from
different nodes
◦ Two vectors are said to be orthogonal if their inner
product = 0 Let p and q be two vectors and
suppose p = (2,5,0) and q = (0,0,17) then the inner
product of p*q = (2*0+5*0+0*17) = 0
◦ For good autocorrelation, binary 0 is represented as
-1 and binary 1 is represented as +1.
◦ Let the binary sequence be 1001, then the
representation sequence is +1-1-1+1.

51.  All nodes transmit on the same frequency at
the same time using the entire bandwidth of
the transmission channel.
 Each sender has unique random number key,
and the send XORs the signal with this
random number key.
 The receiver can “tune” into his signal if it
knows the pseudorandom number.
 Consider an example, where X,Y are the
transmitters and Z is a receiver.

52.  Sender X_data = 1 and X_Key = (010011).
 Its autocorrelation representation is (-1, +1, -1,
-1, +1, +1)
 The signal is calculated at sender X is
Xs=X_data*X_key = +1*X_key = (-1, +1, -1, -
1,+1,+1)
 Similary Sender Y_data = 0 and Y_key = (110101)
 Its autocorrelation representation is (+1, +1, -1,
+1, -1,+1)
 Ys = Y_data*Y_Key = -1*X_key = (-1, -1, +1, -1,
+1, -1)
 Xs+Ys = (-2,0,0,-2,+2,0)

53.  At the receiver, in order to receive the data
sent by send X, the signal Z is dispread.
 If Z wants to get information of send X data,
then Z*X_key = (-2,0,0,-2,+2,0)*(-1, +1,-1,-
1,+1,+1) = 2+0+0+2+2+0 = 6>0 (positive)
that is the original bit was a 1.
 Similarly the information of send Y data may
be obtained Z*Y_key = (-2,0,0,-2,+2,0) *
(+1, +1, -1, +1, -1, +1) = -2+0+0-2-2+0
= -6 < 0 (negative) so the Y data original bit
was a 0.

54.  There are number of random assignment
schemes are used in MAC protocol. They are
◦ ALOHA
◦ Slotted ALOHA
◦ CSMA
CSMA/CD
◦ CSMA/CA

55.  ALOHA
◦ ALOHA is a simple protocol.
◦ If a node has data to send, it begins to transmit.
◦ PURE ALOHA does not check whether the channel is
busy before transmitting.
◦ If the frame successfully reaches the destination the
next frame is sent.
◦ If the frame fails to be received at the destination, it
is sent again.
◦ If the transmission is high, the collision also high in
simple ALOHA

56.  ALOHA

57.  Slotted ALOHA
◦ An improvement in pure ALOHA scheme is the slotted
ALOHA scheme.
◦ In slotted aloha scheme the chances of collision are
attempted to be reduced by enforcing the following
restrictions.
◦ The time is divided into equal sized slots in which a packet
can be sent.
◦ The size of the packet is restricted.
◦ A node wants to send a packet, can start to do so only at
the beginning of the slot.
◦ The slotted ALOHA system employs beacon signals that are
sent at precise intervals that mark the beginning of a slot.
◦ The protocol does not work well if the number of stations
contending to send data is high.
◦ CSMA schemes works better than ALOHA scheme.

59.  CSMA
◦ Popular MAC arbitration technique is the Carrier
Sense Multiple Access (CSMA)
◦ A node sense the medium before starting to
transmit.
◦ If it senses that some transmission is already
underway, it defers its transmission.
◦ Two popular extensions of the basic CSMA
technique.
 CSMA/CD – Collision Detection
 CSMA/CA – Collision Avoidance

60.  CSMA/CD
◦ In wireless network the CSMA/CD technique does not
work very well.
◦ In CSMA/CD technique, the sender starts to transmit if it
senses the channel to be free.
◦ If it senses the channel to be free, there can be collision
(Why?) during transmission.
◦ In wired network, collision detection technique is simple.
◦ In wireless network, it is very difficult for a transmitting
node to detect a collision, since any received signal from
other nodes would be too feeble compared to its own
signal and can easily be masked by noise.

61.  CSMA/CD
◦ As a result, a transmitting node would continue to
transmit the frame and only the destination node
would notice the corrupted frame after it computes
the checksum
◦ This leads to retransmission and severe wastage of
channel utilization.
◦ In wired network when a node detects a collision, it
immediately stops transmitting, thereby minimizing
channel wastage.

62.  CSMA/CA
◦ In wireless network, a collision avoidance scheme works much better
compared to a collision detection based scheme.
◦ Collision Avoidance scheme is based on the idea that it is necessary to
prevent collision at the moment they are most likely to occur, that is when
the bus is released after a packet transmission.
◦ During the time of transmission on the channel, several nodes might be
wanting to transmit.
◦ These nodes would be monitoring the channel and waiting for it to
become free.
◦ The moment the transmitting node completes its transmission, these
waiting nodes would sense the channel to be free and would all start
transmitting at the same time.
◦ To overcome such collisions, all nodes are forced to wait for a random
time and then sense the medium again, before starting their transmission.
◦ If the medium is still busy, the node has to wait for another random
amount of time and sense the medium again.

63. CSMA/CD

64.  The basic form of reservation scheme is RTS/CTS
scheme
 A sender transmits the RTS(Ready to Send) packet
to the receiver before the actual data
transmission.
 In receiving end, the receiver sends a CTS (Clear
to Send) packet, and the actual data transfer
commences only after that.
 When other nodes sharing the medium sense the
CTS packet, they refrain from transmitting until
the transmission from sending node is complete.

65.  In contention based MAC protocol, the node wants to
reserve the medium by using an appropriate control
message.
 A reservation of the medium can be achieved by RTS/CTS
scheme.
 A sender transmits RTS (Ready to Send) packet to the
receiver before the actual data transmission.
 On receiving side, the receiver sends CTS(Clear to Send)
packet, and the actual data transfer commences only after
that.
 If the other nodes sharing the medium, it sense the CTS
packet they refrain from transmitting until the
transmission from the sending node is complete.
 A few examples of RTS-CTS based MAC protocols are
MACA, MACAW, MACA-BI, PAMAS, DBTMA, MARCH, S-
MAC.

66.  It solves the hidden/exposed terminal
problems by regulating the transmitter
power.
 A node sends the RTS to the receiver.
 Radio signal is omini-directional, so every
node within the sender’s radio range will hear
this and then refrain from transmitting
 The receiver is ready to receive data, it
responds with a CTS.

68.  MACA is a non-persistent slotted protocol.
This implies that if the medium is detected as
busy, a station waits for a random time
period after the beginning of a time slot and
then it sends an RTS. This assures fair access
to the medium.

70.  The whole process will work as follows:
 A will send RTS frame to the B
 Then b will send CTS frame to A
 When CTS frame is received by A then it will
start sending data frame to B
 Upon receiving data successfully it will send
acknowledgment frame(ACK)

71.  Solution to Hidden/Exposed Terminal
Problem:
◦ MACA protocol uses RTS and CTS to avoid hidden
and exposed terminal problem.
◦ In hidden terminal problem two nodes try to
contact same node at a same time which can create
collision to combat this if two nodes send RTS to
same node then the node which receives CTS will
send the data not the other one which will avoid the
collision.