Mobile computing

Chanderprabhu Jain College of Higher Studies & School of Law
Plot No. OCF, Sector A-8, Narela, New Delhi – 110040
(Affiliated to Guru Gobind Singh Indraprastha University and Approved by Govt of NCT of Delhi & Bar Council of India)
Semester: THIRD Semester
PAPER CODE: 304
Name of the Subject:
MOBILE COMPUTING

UNIT 1
Wireless communication
Wireless communication involves the transmission of information over a distance
without the help of wires, cables or any other forms of electrical conductors.
Wireless communication is a broad term that incorporates all procedures and
forms of connecting and communicating between two or more devices using a
wireless signal through wireless communication technologies and devices.

Features of Wireless Communication
The evolution of wireless technology has brought many advancements
with its effective features.
• The transmitted distance can be anywhere between a few meters (for
example, a television's remote control) and thousands of kilometers
(for example, radio communication).
• Wireless communication can be used for cellular telephony, wireless
access to the internet, wireless home networking, and so on.
• Other examples of applications of radio wireless technology include
GPS units, garage door openers, wireless computer mice, keyboards
and headsets, headphones, radio receivers, satellite television,
broadcast television and cordless telephones.

Wireless - Advantages
Wireless communication involves transfer of information without any physical
connection between two or more points. Because of this absence of any 'physical
infrastructure', wireless communication has certain advantages. This would often
include collapsing distance or space

Early wireless
A number of wireless electrical signaling schemes including sending electric
currents through water and the ground using electrostatic and electromagnetic
induction were investigated for telegraphy in the late 19th century before practical
radio systems became available. These included a patented induction system by
Thomas Edison allowing a telegraph on a running train to connect with telegraph
wires running parallel to the tracks, a William Preece induction telegraph system
for sending messages across bodies of water, and several operational and proposed
telegraphy and voice earth conduction systems.

Radio frequency (RF)
Radio frequency (RF) is a measurement representing the oscillation rate of
electromagnetic radiation spectrum, or electromagnetic radio waves, from
frequencies ranging from 300 GHz to as low as 9 kHz. With the use of
antennas and transmitters, an RF field can be used for various types of wireless
broadcasting and communications.

RF technology
Many types of wireless devices make use of RF fields. Cordless and cellphones,
radio and television broadcast stations, Wi-Fi and Bluetooth, satellite
communications systems, and two-way radios all operate in the RF spectrum. In
addition, other appliances outside of communications, including microwave
ovens and garage-door openers, operate at radio frequencies. Some wireless
devices, like TV remote controls, some cordless computer keyboards and
computer mice, operate at IR frequencies, which have shorter electromagnetic
wavelengths.

Spread-spectrum
A spread-spectrum transmission offers three main advantages over a
fixed-frequency transmission:
• Spread-spectrum signals are highly resistant to narrowband
interference. The process of re-collecting a spread signal spreads out
the interfering signal, causing it to recede into the background.
• Spread-spectrum signals are difficult to intercept. A spread-spectrum
signal may simply appear as an increase in the background noise to a
narrowband receiver. An eavesdropper may have difficulty intercepting
a transmission in real time if the pseudorandom sequence is not
known.
• Spread-spectrum transmissions can share a frequency band with
many types of conventional transmissions with minimal interference.
The spread-spectrum signals add minimal noise to the narrow-
frequency communications, and vice versa. As a result, bandwidth can
be used more efficiently.

Cellular System
A cellular network or mobile network is a communication network
where the last link is wireless. The network is distributed over land areas
called cells, each served by at least one fixed-location transceiver, but
more normally three cell sites or base transceiver stations. These base
stations provide the cell with the network coverage which can be used for
transmission of voice, data, and other types of content. A cell typically
uses a different set of frequencies from neighboring cells, to avoid
interference and provide guaranteed service quality within each cell.

Direct-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum (DSSS) is a spread
spectrum modulation technique used to reduce overall signal interference. The
spreading of this signal makes the resulting wideband channel more noisy,
allowing for greater resistance to unintentional and intentional interference.
A method of achieving the spreading of a given signal is provided by the
modulation scheme. With DSSS, the message signal is used to modulate a bit
sequence known as a Pseudo Noise (PN) code; this PN code consists of a radio
pulse that is much shorter in duration (larger bandwidth) than the original
message signal.

Transmission method
Direct-sequence spread-spectrum transmissions multiply the data being
transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of
1 and −1 values; at a frequency much higher than that of the original signal.
The resulting signal resembles white noise, like an audio recording of "static".
However, this noise-like signal is used to exactly reconstruct the original data at
the receiving end, by multiplying it by the same pseudorandom sequence
(because 1 × 1 = 1, and −1 × −1 = 1). This process, known as "de-spreading",
mathematically constitutes a correlation of the transmitted PN sequence with the
PN sequence that the receiver already knows the transmitter is using.

Time-division multiplexing (TDM)
Time-division multiplexing (TDM) is a method of transmitting and receiving
independent signals over a common signal path by means of synchronized
switches at each end of the transmission line so that each signal appears on the
line only a fraction of time in an alternating pattern. It is used when the bit rate of
the transmission medium exceeds that of the signal to be transmitted. This form
of signal multiplexing was developed in telecommunications for telegraphy
systems in the late 19th century, but found its most common application in digital
telephony in the second half of the 20th century.

Code Division Multiplexing (CDM)
Code division multiplexing (CDM) is a networking technique in which
multiple data signals are combined for simultaneous transmission over a
common frequency band.
When CDM is used to allow multiple users to share a single
communications channel, the technology is called code division multiple
access (CDMA).

Amplitude-shift keying (ASK) is a form of amplitude modulation that
represents digital data as variations in the amplitude of a carrier wave. In an ASK
system, the binary symbol 1 is represented by transmitting a fixed-amplitude
carrier wave and fixed frequency for a bit duration of T seconds. If the signal
value is 1 then the carrier signal will be transmitted; otherwise, a signal value of
0 will be transmitted.
Any digital modulation scheme uses a finite number of distinct signals to
represent digital data. ASK uses a finite number of amplitudes, each assigned a
unique pattern of binary digits.

UNIT 2
Medium access control (MAC)
Medium access control (MAC) sublayer (also known as the media access
control sublayer) and the logical link control (LLC) sublayer together make up
the data link layer. Within that data link layer, the LLC provides flow control and
multiplexing for the logical link (i.e. EtherType, 802.1Q VLAN tag etc), while
the MAC provides flow control and multiplexing for the transmission medium.

Functions performed in the MAC sublayer
According to IEEE Std 802-2001 section 6.2.3 "MAC sublayer", the
primary functions performed by the MAC layer are:
• Frame delimiting and recognition
• Addressing of destination stations (both as individual stations and as
groups of stations)
• Conveyance of source-station addressing information
• Transparent data transfer of LLC PDUs, or of equivalent information in
the Ethernet sublayer
• Protection against errors, generally by means of generating and
checking frame check sequences
• Control of access to the physical transmission medium

Addressing mechanism
The local network addresses used in IEEE 802 networks and FDDI networks are
called media access control addresses; they are based on the addressing scheme
that was used in early Ethernet implementations. A MAC address is intended as a
unique serial number. MAC addresses are typically assigned to network interface
hardware at the time of manufacture. The most significant part of the address
identifies the manufacturer, who assigns the remainder of the address, thus
provide a potentially unique address. This makes it possible for frames to be
delivered on a network link that interconnects hosts by some combination of
repeaters, hubs, bridges and switches, but not by network layer routers.

Channel access control mechanism
The channel access control mechanisms provided by the MAC layer are
also known as a multiple access protocol. This makes it possible for
several stations connected to the same physical medium to share it.
Examples of shared physical media are bus networks, ring networks, hub
networks, wireless networks and half-duplex point-to-point links. The
multiple access protocol may detect or avoid data packet collisions if a
packet mode contention based channel access method is used, or
reserve resources to establish a logical channel if a circuit-switched or
channelization-based channel access method is used. The channel
access control mechanism relies on a physical layer multiplex scheme.

Space-division multiple access (SDMA) is a channel access method based on
creating parallel spatial pipes next to higher capacity pipes through spatial
multiplexing and/or diversity, by which it is able to offer superior performance in
radio multiple access communication systems. traditional mobile cellular
network systems, the base station has no information on the position of the
mobile units within the cell and radiates the signal in all directions within the cell
in order to provide radio coverage. This method results in wasting power on
transmissions when there are no mobile units to reach, in addition to causing
interference for adjacent cells using the same frequency, so called co-channel
cells.

Frequency division multiple access (FDMA) is a channel access
method used in multiple-access protocols as a channelization protocol.
FDMA gives users an individual allocation of one or several frequency
bands, or channels. It is particularly commonplace in satellite
communication. FDMA, like other multiple access systems, coordinates
access between multiple users.

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. The
users transmit in rapid succession, one after the other, each using its own
time slot. This allows multiple stations to share the same transmission
medium (e.g. radio frequency channel) while using only a part of its
channel capacity. TDMA is used in the digital 2G cellular systems such as
Global System for Mobile Communications (GSM), IS-136, Personal
Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless
Telecommunications (DECT) standard for portable phones.

Code-division multiple access (CDMA)
Code-division multiple access (CDMA) is a channel access method used
by various radio communication technologies.
CDMA is an example of multiple access, where several transmitters can
send information simultaneously over a single communication channel.
This allows several users to share a band of frequencies (see
bandwidth). To permit this without undue interference between the users,
CDMA employs spread spectrum technology and a special coding
scheme (where each transmitter is assigned a code).

Inhibit Sense Multiple Access (ISMA)
Compared to ALOHA or CSMA, the Inhibit Sense Multiple Access (ISMA)
radio system is supplemented by an outbound signalling of the status of
the channel: either "busy" or "idle". An example of such protocol is used
in the US CDPD system. When the base station receives an inbound
packet, a "busy" signal is broadcast to all mobiles to inhibit them from
transmission. In a practical system, this only occurs after a short
processing delay d1. The effect of this delay depends on its magnitude
relative to the duration of the data packet.. After termination of (all
contending) transmissions, the base station starts transmitting an "idle"
signal after a delay of duration d2.

Spread ALOHA Multiple access (SAMA)
A SPREAD ALOHA Multiple Access (SAMA) channel uses distinct chip
phases to label portions of a common SAMA channel and to reserve
specific phases by means of a random access packet in the same SAMA
channel. In the new SAMA reservation channel protocol, reservations
consisting of the specification of a given chip phase are transmitted in a
random access mode in a common SAMA channel. After a given user
has reserved a given chip phase, that chip phase can be used on a
reserved basis by that user to guarantee a certain quality of service. The
reservation can be made by a user who simply initiates transmissions
with a given chip phase and continues to use that chip phase if the user's
first packet (the packet making the reservation) is received correctly.

Telecommunications satellite links
Communications satellites are ideally placed to provide
telecommunications links between different places across the globe.
Traditional telecommunications links used direct "cables" linking different
areas. As a result of the cost of installation and maintenance of these
cables, satellites were seen as an ideal alternative. While still expensive
to put in place, they provided a high bandwidth and were able to operate
for many years.

Handover
In cellular telecommunications, the terms handover or handoff refer to
the process of transferring an ongoing call or data session from one
channel connected to the core network to another channel. In satellite
communications it is the process of transferring satellite control
responsibility from one earth station to another without loss or interruption
of service.

UNIT 3
Mobile internet
The mobile web, also known as mobile internet, refers to browser-based
Internet services accessed from handheld mobile devices, such as
smartphones or feature phones, through a mobile or other wireless
network. The shift to mobile web access has accelerated since 2007 with
the rise of larger multitouch smartphones, and since 2010 with the rise of
multitouch tablet computers.

Services For the mobile internet
Mobile Hotspot. Verizon offers Internet tethering (aka Mobile Hotspot) on
Palm WebOS phones (Pixi and Pre Plus). As of a few days ago, this was
a $40/month charge, over and above the minutes & data plans..
Overdrive 3G/4G. Sprint, at the least, offers a palm-sized device, the
Overdrive, that connects to the internet over 4G networks — degrading
supposedly gracefully to 3G where 4G is not available — and lets you
share that connection with other WiFi devices. The price of the Overdrive
varies between carriers, and there’s a data access charge of
approximately US$60/month.

• USB 3G cards. All new laptops (PC, Mac) most netbooks and
possibly other devices have USB ports. USB 3G cards also work over
a cellular network and have the flexibility of not being tied to a single
device. However, some carriers offer deals if you’re buying two or
more.
• Free or pay-per-use public WiFi. AT&T, Boingo and others offer a
variety of public WiFi options for their access points across the
country. (Boingo in particular is targeting Apple iPad users.)
• Other “personal hotspot” options. A few carriers offer devices that
look like WiFi routers (or possibly in vertical form factor) that can be
taken with you from city to city, plugged into a wall outlet, then used to
connect online over a cellular network, though not necessarily with 3G
let alone 4G.

3G netbook. Verizon and possibly other carriers are offering BOGO (buy one
get one) free deals where you can get 2 phones (plus monthly fee for
minutes + data).
PCMCIA laptop 3G cards. These cards plug into older MS Windows-based
laptops. Many newer laptops don’t have the PCMCIA slot anymore. Ask your
cellular provider for details. The cards are about the size of a business card,
but thicker.

WML Versions:
WAP Forum has released a latest version WAP 2.0. The markup
language defined in WAP 2.0 is XHTML Mobile Profile (MP). The WML
MP is a subset of the XHTML. A style sheet called WCSS (WAP CSS)
has been introduced alongwith XHTML MP. The WCSS is a subset of the
CSS2.
Most of the new mobile phone models released are WAP 2.0-enabled.
Because WAP 2.0 is backward compatible to WAP 1.x, WAP 2.0-enabled
mobile devices can display both XHTML MP and WML documents.

WML Decks and Cards:
A main difference between HTML and WML is that the basic unit of
navigation in HTML is a page, while that in WML is a card. A WML file can
contain multiple cards and they form a deck.
When a WML page is accessed from a mobile phone, all the cards in the
page are downloaded from the WAP server. So if the user goes to
another card of the same deck, the mobile browser does not have to send
any requests to the server since the file that contains the deck is already
stored in the wireless device.

WML - Environment
To develop WAP applications, you will need the following:
• A WAP enabled Web Server: You can enable your Apache or
Microsoft IIS to serve all the WAP client request.
• A WAP Gateway Simulator: This is required to interact to your WAP
server.
• A WAP Phone Simulator: This is required to test your WAP Pages
and to show all the WAP pages.

The WAP Model:
The figure below shows the WAP programming model. Note the
similarities with the Internet model. Without the WAP Gateway/Proxy the
two models would have been practically identical.

WML - Syntax
A WML program is typically divided into two parts: the document prolog and the
body. Consider the following code:
Following is the basic structure of a WML program:
<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.2//EN"
"http://www.wapforum.org/DTD/wml12.dtd">
<wml>
<card id="one" title="First Card">
<p>
This is the first card in the deck
</p>
</card>
<card id="two" title="Second Card">
<p>
Ths is the second card in the deck
</p>
</card>
</wml>

Testing Your Program:
Put above code in a file called test.wml file, and put this WML file locally
on your hard disk, then view it using an emulator.
This is by far the most efficient way of developing and testing WML files.
Since your aim is, however, to develop a service that is going to be
available to WAP phone users, you should upload your WML files onto a
server once you have developed them locally and test them over a real
Internet connection. As you start developing more complex WAP
services, this is how you will identify and rectify performance problems,
which could, if left alone, lose your site visitors.

UNIT 4
WAP is the de facto worldwide standard for providing Internet
communications and advanced telephony services on digital mobile phones,
pagers, personal digital assistants, and other wireless terminals − WAP
Forum.
WAP stands for Wireless Application Protocol. The dictionary definition of
these terms are as follows −
• Wireless − Lacking or not requiring a wire or wires pertaining to radio
transmission.
• Application − A computer program or piece of computer software that is
designed to do a specific task.
• Protocol − A set of technical rules about how information should be
transmitted and received using computers.

Who is behind WAP?
The Wireless Application Protocol (WAP) is a result of joint efforts taken
by companies teaming up in an industry group called WAP Forum
(www.wapforum.org).
On June 26, 1997, Ericsson, Motorola, Nokia, and Unwired Planet took
the initiative to start a rapid creation of a standard for making advanced
services within the wireless domain a reality. In December 1997, WAP
Forum was formally created and after the release of the WAP 1.0
specifications in April 1998, WAP Forum membership was opened to all.

Why is WAP Important?
Until the first WAP devices emerged, the Internet was a Internet and a
mobile phone was a mobile phone. You could surf the Net, do serious
research, or be entertained on the Internet using your computer, but this
was limited to your computer.
Now with the appearance of WAP, the scene is that we have the massive
information, communication, and data resources of the Internet becoming
more easily available to anyone with a mobile phone or communications
device.
WAP being open and secure, is well suited for many different applications
including, but not limited to stock market information, weather forecasts,
enterprise data, and games.

WAP Microbrowser
To browse a standard internet site you need a web browser. Similar way
to browse a WAP enables website, you would need a micro browser. A
Micro Browser is a small piece of software that makes minimal demands
on hardware, memory and CPU. It can display information written in a
restricted mark-up language called WML. Although, tiny in memory
footprint it supports many features and is even scriptable.
Today, all the WAP enabled mobile phones or PDAs are equipped with
these micro browsers so that you can take full advantage of WAP
technology.

WAP - WML Script
WMLScript (Wireless Markup Language Script) is the client-side scripting
language of WML (Wireless Markup Language). A scripting language is
similar to a programming language, but is of lighter weight. With
WMLScript, the wireless device can do some of the processing and
computation. This reduces the number of requests and responses to/from
the server.
This chapter will give brief description of all the important WML Script
components.

WML Script Functions
The user-defined functions are declared in a separate file having
the extension .wmls. Functions are declared as follows −
function name (parameters) { control statements; return var;}
The functions used are stored in a separate file with the extension
.wmls. The functions are called as the filename followed by a
hash, followed by the function name −
maths.wmls#squar()

WAP - WML Syntax
The topmost layer in the WAP architecture is made up of WAE (Wireless
Application Environment), which consists of WML and WML scripting
language.
WML scripting language is used to design applications that are sent over
wireless devices such as mobile phones. This language takes care of the
small screen and the low bandwidth of transmission. WML is an
application of XML, which is defined in a document-type definition.

URL
A Uniform Resource Locator (URL), colloquially termed a web
address, is a reference to a web resource that specifies its location on a
computer network and a mechanism for retrieving it. A URL is a specific
type of Uniform Resource Identifier (URI), although many people use the
two terms interchangeably. URLs occur most commonly to reference web
pages (http), but are also used for file transfer (ftp), email (mailto),
database access (JDBC), and many other applications.

THANK YOU

Mobile computing

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