Mobile Computing Unit 1 Introduction.ppt

20CS501 – Mobile Computing
III Year / VI Semester
Dr.R.Sathya
AP / IT

Objectives
The Student should be made to:
Understand the basic concepts of mobile computing
Be familiar with the network protocol stack
Learn the basics of mobile telecommunication system
Be exposed about the Ad-Hoc networks
Gain knowledge about different mobile platforms and
application development

Unit I - Introduction
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

Mobile Computing
The ability to compute remotely while on the
move.
 Concepts:
 Mobile Communication
 Mobile Computing

Mobile Computing
 Mobile Computing denotes the capability to
automatically carry out certain processing related to
service invocations on a remote computer.
 Mobile Communication provides the capability to
change location while communicating to invoke
computing services to some remote computers.

Mobile Computing vs. Wireless
Networking
 Mobile computing essentially denotes
accessing information and remote
computational services while on the move,
wireless networking provides the basic
communication infrastructure necessary to
make this possible.

Networking
 Mobile computing also requires the applications themselves
– their design and development, and the hardware at the
client and server sides.
 Mobile computing includes the area of wireless networking.
 Wireless networking is increasingly replacing traditional
networks because of the low setup time and low initial
investment required to set up the wireless network.

Networking
 Wireless networks can be classified into two basic
types.
 Extension of Wired Networks: It uses fixed infrastructures
such as base stations to provide essentially single hop
wireless communication with a wired network.
 Ad Hoc Network: It does not use any fixed infrastructure
and is based on multi-hop wireless communication.

Networking
 One popular example of a fixed infrastructure wireless
network is a Wireless LAN (WLAN) that implements
the IEEE 802.11 protocol.
 Only the last hop is through the wireless medium.
 An access point (AP) provides the last hop
connectivity of the mobile nodes to a wired network.

Networking
 All communication goes through APs which perform
bridging between the wireless and the wired mediums.
 A station must be recognized by an AP to be able to
connect to the network.
 The AP may require authentication and this in turn is
used as the basic means to keep out the unauthorized
users.

Networking
 In an infrastructureless network, the communication
between hosts occurs directly or via a few intermediate
nodes that form the hops.
 The Bluetooth technology can also be used to establish
direct wireless connection of a cell phone with devices
such as printers, cameras, scanners, laptop and desk
computers.

Networking
 One of the objectives of the Bluetooth
technology is to enable users to easily connect
to a wide range of personal computing and
telecommunication devices, without the need
to buy, carry or lay out cables.

Networking
 Bluetooth promises to eliminate the need to
purchase additional or proprietary cabling and
configuration exercises needed to connect
individual devices.

Networking
 Ad hoc network – Mobile Ad hoc Network
(MANET).
 It is a collection of mobile nodes that form a
network on the fly without requiring the
support of any fixed infrastructure.

Mobile Computing Applications
 Vehicles:
 Transmission of news, road condition, weather, music via
Digital Audio Broadcasting.
 Personal communication using GSM
 Position via GPS
 Local ad-hoc network with vehicles close-by to prevent
accidents.
 Vehicle data can be transmitted in advance for maintenance

 Medical
Doctors in hospital are now using Wireless Tablet PC /
WLAN to collect and share patient information.
 Sales:
 Sales representation are using Table PCs with Smart
phones for presentation.
 Sharing and Accessing information among offices

 Credit Card Verification
Speed up the transaction process
 Taxi/Truck Dispatch
A central computer to be able to track and receive
status information from all of its mobile secure
delivery vans.

Electronic Mail/Paging
 It allows him/her to keep in touch with any
colleagues.
 Access to the Internet, using mobile computing
technology, allows the individual to have vast
arrays of knowledge at his/her fingertips.

Mobile Computing Characteristics
 Ubiquity – present everywhere
 The ability of a user to perform computations from
anywhere and at anytime.
 Location awareness - GPS
 It provides information about the current location of a
user to a tracking station.
 Example: Traffic control and emergency services

 Adaption:
 The ability of a system to adjust to bandwidth
fluctuation without inconveniencing the user.
 Broadcast:
 Efficient delivery of data can be made simultaneously
to hundreds of mobile users.
 Advertising information about nearby location.

 Personalization:
 A mobile environment can be easily personalized
according to a user’s profile.
 The user avail information with their hand-held
devices.

Structure of Mobile Computing
Application
Presentation (Tier - 1)
Application (Tier – 2)
Data (Tier – 3)

Application
Presentation
Tier
Application
Tier
Data
Tier

Application
 Presentation Tier
 It concerns the user interface.
 A good user interface facilitates the user to issue requests and to
present the results to them meaningfully.
 The programs at this layer run on the client’s computer.
 This layer usually includes web browsers and customized
programs for dissemination of information and for collection of
data from the user.

Application
 Application Tier
 Responsibility of making logical decisions and
performing calculations.
 It also moves and processes data between the
presentation and data layers.
 It is like an “engine” of an automobile.

Application
 Application Tier
 It performs the processing of user input, obtaining
information and them making decisions.
 This layer is implemented using technology like
Java, .NET services, etc.
 It is database independent
 It is implemented on a fixed server.

Application
 Data Tier
 It is responsible for providing the basic facilities
of data storage, access and manipulation
 It contains a database.
 The information is stored and retrieved for this
database.

MAC Protocols
 In wireless network, multiple nodes may contend
to transmit on the same shared channel at the
same time.
 In this situation, the transmitted data would get
garbled unless a suitable medium access
arbitration scheme is deployed.

MAC Protocols
 It is the responsibility of the medium access
control (MAC) protocol to perform the task.
 To enforce discipline in the access of a shared
channel when multiple node contend to access
that channel.

MAC Protocols
 Maximization of the utilization of the channel
 Minimization of average latency of transmission.
 It ensures that no node has to wait for an unduly
long time, before it is allowed to transmit.

MAC Protocols
 Properties:
 It should implement some rules that help to enforce
discipline when multiple nodes contend for a shared
channel.
 It should help maximize the utilization of the channel.
 Channel allocation need to be fair.

MAC Protocols
 Properties:
 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.

MAC Protocols
 Issues:
 It is difficult to implement a collision detection scheme in
a wireless environment, since collisions are hard to be
detected by the transmitting nodes.
 Also, in infrastructure-less networks, the issue of hidden
and exposed terminals make a MAC protocol extremely
inefficient, unless special care is taken to overcome these
problems.

MAC Protocols
 The Hidden Terminal Problems:

MAC Protocols
 The transmission range of A reaches B, but not C.
 The transmission range of C reaches B, but not A.
 Finally, the transmission range of B reaches A and
C, i.e., A cannot detect C and vice versa

MAC Protocols
 A starts sending to B, C does not receive this
transmission. C also wants to send something to B
and senses the medium.
 The medium appears to be free, the carrier sense
fails. C also starts sending causing a collision at B.

MAC Protocols
 But A cannot detect this collision at B and
continues with its transmission. A is hidden for C
and vice versa.
 While hidden terminals may cause collisions, the
next effect only causes unnecessary delay.

MAC Protocols
 The Exposed Terminal Problems:

MAC Protocols
 Node D is within node A’s transmission range.
 Node B is within the transmission range of both the
nodes A and C, but the nodes A and C are not within
each other’s transmission range.
 MAC protocols usually inhibit transmission when
transmission from another terminal is detected.

MAC Protocols
 As a result, node A will not be able to transmit to any
node when B is transmitting to C.
 On the other hand, had A transmitted to D, it would
have been received correctly by D and B’s
transmission would have also been correctly received
at C.

MAC Protocols
 The problem arose only because A and B are within
each other’s transmission range, though the destination
nodes are in the transmission range of only one of the
nodes.
 In other words, the problem occurs because A is
exposed to B’s transmission.

MAC Protocols
 The overall effect of this problem is that it leads to
inefficient spectrum usage as well as unnecessary
transmission delays unless these are carefully
addressed by a wireless MAC protocol.

MAC Protocols Categories
 Fixed assignment schemes
 Random assignment schemes
 Reservation-based schemes

Fixed assignment schemes
 Categories:
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)

 Categories - Frequency Division Multiple
Access (FDMA):
 In FDMA, the available bandwidth (frequency
range) is divided into many narrower frequency
bands called channels.

 Categories - Frequency Division Multiple
Access (FDMA):
 A division of the existing bandwidth into many
channels (showns as Ch 1, Ch 2, etc.).

 Categories - Frequency Division Multiple Access
(FDMA):
 For full duplex communication to take place, each
user is allocated a forward link (channel) for
communicating from it (mobile handset) to the base
station (BS), and a reverse channel for communicating
from the BS to it.

(FDMA):
 Thus, each user making a call is allocated two unique
frequency bands (channels), one for transmitting and the
other for receiving signals during the call.
 Obviously, when a call is underway, no other user would
be allocated the same frequency band to make a call.

(FDMA):
 Unused transmission time in a frequency band that occurs
when the allocated caller pauses between transmissions, or
when no user is allocated a band, goes idle and is wasted.
 FDMA, therefore, does not achieve a high channel
utilization.

 Categories - Time Division Multiple Access (TDMA):
 TDMA is an access method in which multiple nodes are
allotted different time slots to access the same physical
channel.
 That is, the timeline is divided into fixed-sized time slots
and these are divided among multiple nodes who can
transmit.

 Categories - Time Division Multiple Access
(TDMA):
 Note that in this case, all sources use the same
channel, but take turns in transmitting.

 Categories - Time Division Multiple Access
(TDMA):
 The Figure shows the situation where time slots are
allocated to users in a round robin manner, with each
user being assigned one time slot per frame.
 Unused time slots go idle, leading to low channel
utilization.

 Categories - Code Division Multiple Access (CDMA):
 In CDMA, 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.

 Categories - Code Division Multiple Access
(CDMA):

(CDMA):
 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.
 The bandwidth of this medium is much larger than the
space that would be allocated to each packet transmission
during FDMA
 The signals can be distinguished from each other by means
of a special coding scheme that is used

(CDMA):
 This is done with the help of a frequency spreading
code known as the m-bit pseudo-noise (PN) code
sequence.
Using m bits, 2m
– 1 different codes can be obtained.
From these codes, each user will use only one code.

 It is possible to distinguish transmissions from different nodes by
ensuring some properties on the codes.
 A code for a user should be orthogonal (that is, non-interfering) to the
codes assigned to other nodes.
 The term “orthogonal” means that the vector inner product is zero, and
good autocorrelation uses the bipolar notation where a code sequence
of binary 0 is represented as –1 and binary 1 is represented as +1.

(CDMA):
 On the receiving end, only the same PN sequence
is able to demodulate the signal to successfully
convert the input data.

(CDMA):
 Each sender has a unique random number key, and
the sender XORs the signal with this random
number key. The receiver can “tune” into this
signal if it knows the pseudorandom number.

 Consider an example, where X, Y are the transmitters and
Z is a receiver.
 Sender X_data = 1 and X_Key = (010011). Its
autocorrelation representation is (–1, +1, –1, –1, +1, +1).
The signal to be calculated at sender X is Xs = X_data *
X_key = +1*X_key = (–1, +1, –1, –1, +1, +1).

(CDMA):
 Sender Y_data = 0 and Y_key = (110101) and the
signal to be sent at Y is Ys = –1*Y_key = –1*(+1,
+1, –1, +1, –1, +1) = (–1, –1, +1, –1, +1, –1).

(CDMA):
 The signal received by receiver Z is Xs + Ys = (–1,
+1, –1, –1, +1, +1) + (–1, –1, +1, –1, +1, –1) = (–2, 0,
0, –2, +2, 0).
 At the receiver, in order to receive the data sent by
sender X, the signal Z is dispread.

(CDMA):
 So now if Z wants to get information of sender 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.

(CDMA):
 Similarly, the information of sender Y data may be
obtained as 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.

Random assignment schemes
 Schemes:
ALOHA
Slotted ALOHA
CSMA
CSMA/CD
CSMA/CA

 Schemes – ALOHA:
 The basic (pure) ALOHA scheme , is a simple
protocol.
 If a node has data to send, it begins to transmit.
 The first step implies that Pure ALOHA does not
check whether the channel is busy before transmitting.

 If the frame successfully reaches the destination
(receiver), the next frame is sent. If the frame fails to
be received at the destination, it is sent again.
 The simple ALOHA scheme works acceptably, when
the chances of contention are small (i.e., when a small
number of senders send data infrequently).

 The collisions can become unacceptably high if the number of
contenders for transmission is high.
 An improvement over the pure ALOHA scheme is the slotted
ALOHA.
 The time is divided into equal-sized slots in which a packet can be
sent. Thus, the size of the packet is restricted.
 A node wanting to send a packet, can start to do so only at the
beginning of a slot.

 The slotted ALOHA system employs beacon signals
that are sent at precise intervals that mark the
beginning of a slot, at which point the nodes having
data to send can start to transmit.
 Again, this protocol does not work very well if the
number of stations contending to send data is high.

 Schemes – CSMA:
 CSMA - Carrier Sense Multiple Access
 A node senses the medium before starting to transmit.
 If it senses that some transmission is already
underway, it defers its transmission.
 Techniques: collision detection (CSMA/CD) and the
collision avoidance (CSMA/ CA)

 In a wireless network the CSMA/CD technique does
not work very well.
 In the CSMA/CD technique, the sender starts to
transmit if it senses the channel to be free. But, even if
it senses the channel to be free, there can be a collision
(why?) during transmission.

 In a 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.
 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 retransmissions and severe wastage
of channel utilization.
In contrast, in a wired network when a node
detects a collision, it immediately stops
transmitting, thereby minimizing channel wastage.

 In a wireless network, a collision avoidance scheme works
much better compared to a collision detection-based
scheme.
 A collision avoidance scheme is based on the idea that it is
necessary to prevent collisions at the moment they are most
likely to occur, that is, when the bus is released after a
packet transmission.

 During the time a node is transmitting 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.

 In the collision avoidance scheme, all nodes are forced to wait
for a random time and then sense the medium again, before
starting their transmission.
 If the medium is sensed to be busy, a node waiting to transmit
waits for a further random amount of time and so on.
 The chance of two nodes starting to transmit at the same time
would be greatly reduced.

Reservation-based schemes
 A basic form of the reservation scheme is the RTS/CTS
scheme.
 In an RTS/CTS scheme, a sender transmits an RTS (Ready
to Send) packet to the receiver before the actual data
transmission.
 On receiving this, the receiver sends a CTS (Clear to Send)
packet, and the actual data transfer commences only after
that.

 When the other nodes sharing the medium 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 protocols.

 MACA - Multiple Access Collision Avoidance:
 MACA solves the hidden/exposed terminal problems by regulating the
transmitter power.
 A node running MACA requests to use the medium by sending an RTS
to the receiver. Since radio signals propagate omni-directionally, every
terminal within the sender’s radio range will hear this and then refrain
from transmitting.
 As soon as the receiver is ready to receive data, it responds with a
CTS.

 MACA - Multiple Access Collision
Avoidance:

 Before the start of its transmission, it sends a Request To
Send.
 B receives the RTS that contains the sender’s name and the
receiver’s name, as well as the length of the future
transmission.
 In response to the RTS, an acknowledgment from B is
triggered indicating Clear To Send (CTS).

 The CTS contains the names of the sender and receiver,
and the length of the planned transmission.
 This CTS is heard by C and the medium is reserved for use
by A for the duration of the transmission.
 On receipt of a CTS from B, C refrains from transmitting
anything for the time indicated in the CTS.

Avoidance:
 Thus a collision cannot occur at B during data
transmission, and the hidden terminal problem is
solved.

 Though this is a collision avoidance protocol, a
collision can occur during the sending of an RTS.
 Both A and C could send an RTS at same time. But an
RTS occurs over a very small duration compared to the
duration of data transmission.
 Thus the probability of collision remains much less.

Avoidance:
 B resolves this contention problem by
acknowledging only one station in the CTS.
 No transmission occurs without an appropriate
CTS.

 Assume that B needs to transmit to A. B has to
transmit an RTS first.
 The RTS would contain the names of the receiver (A)
and the sender (B).
 C does not act in response to this message as it is not
the receiver, but A responds with a CTS.

Avoidance:
 C does not receive this CTS and concludes that A
is outside the detection range.
 Thus C can start its transmission assuming that no
collision would occur at A.

Mobile Computing Unit 1 Introduction.ppt

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Mobile Computing Unit 1 Introduction.ppt