Android is a mobile operating system developed by Google, based on Linux, designed for touchscreen devices like smartphones and tablets. It uses touch gestures and virtual keyboards for input. Variants are used on other devices like TVs, cars, watches. As of 2015 it had the largest market share of any OS. It was developed initially by Android Inc, which Google acquired in 2005. The Google Play store has over a million apps available.

1. Android is a mobile operating system (OS) currently developed by Google, based on
the Linux kernel and designed primarily for touchscreen mobile devices such as smartphones and tablets. Android's user
interface is based on direct manipulation, using touch gestures that loosely correspond to real-world actions, such as
swiping, tapping and pinching, to manipulate on-screen objects, along with a virtual keyboard for text input. In addition to
touchscreen devices, Google has further developed Android TV for televisions, Android Auto for cars, and Android
Wear for wrist watches, each with a specialized user interface. Variants of Android are also used on notebooks, game
consoles, digital cameras, and other electronics. As of 2015, Android has the largest installed base of all operating
systems.[11]
It is the second most commonly used moblie operating system in the United States, while iOS is the first.[12]
Initially developed by Android, Inc., which Google bought in 2005,[13]
Android was unveiled in 2007, along with the
founding of the Open Handset Alliance – a consortium of hardware, software, and telecommunication companies
devoted to advancing open standards for mobile devices.[14]
As of July 2013, the Google Play store has had over one
million Android applications ("apps") published, and over 50 billion applications downloaded.[15]
An April–May 2013 survey
of mobile application developers found that 71% of developers create applications for Android,[16]
and a 2015 survey
found that 40% of full-time professional developers see Android as their priority target platform, which is comparable
to Apple's iOS on 37% with both platforms far above others.[17]
At Google I/O 2014, the company revealed that there were
over one billion active monthly Android users, up from 538 million in June 2013.[18]
Compact disc (CD) is a digital optical disc data storage format. The format was
originally developed to store and play only sound recordings but was later adapted for storage of data ( CD-ROM).
Several other formats were further derived from these, including write-once audio and data storage (CD-R), rewritable
media (CD-RW), Video Compact Disc (VCD), Super Video Compact Disc (SVCD), Photo CD, PictureCD, CD-i,
and Enhanced Music CD. Audio CDs and audio CD players have been commercially available since October 1982.
Standard CDs have a diameter of 120 millimetres (4.7 in) and can hold up to about 80 minutes of uncompressed audio or
about 700 MiB of data. The Mini CD has various diameters ranging from 60 to 80 millimetres (2.4 to 3.1 in); they are
sometimes used for CD singles, storing up to 24 minutes of audio, or delivering device drivers.

2. At the time of the technology's introduction in 1982, a CD had greater capacity than a personal computer hard drive. By
2010 hard drives commonly had capacities exceeding those of CDs by a factor of several thousand.
In 2004, worldwide sales of audio CDs, CD-ROMs and CD-Rs reached about 30 billion discs. By 2007, 200 billion CDs
had been sold worldwide.[1]
CDs are increasingly being replaced by other forms of digital storage and distribution, with the
result that audio CD sales rates in the U.S. have dropped about 50% from their peak; however, they remain one of the
primary distribution methods for the music industry
VD ( "digital versatile disc"[4][5]
or "digital video disc"[6]
) is a digital optical
disc storage format invented and developed byPhilips, Sony, Toshiba, and Panasonic in 1995. The medium can store
any kind of digital data and is widely used for software and other computer files as well as video programs watched
using DVD players. DVDs offer higher storage capacity than compact discswhile having the same dimensions.
Pre-recorded DVDs are mass-produced using molding machines that physically stamp data onto the DVD. Such discs
are known as DVD-ROM, because data can only be read and not written or erased. Blank recordable DVD discs (DVD-
R and DVD+R) can be recorded once using a DVD recorder and then function as a DVD-ROM. Rewritable DVDs (DVD-
RW, DVD+RW, and DVD-RAM) can be recorded and erased many times.
DVDs are used in DVD-Video consumer digital video format and in DVD-Audio consumer digital audio format as well as
for authoring DVD discs written in a special AVCHD format to hold high definition material (often in conjunction with
AVCHD format camcorders). DVDs containing other types of information may be referred to as DVD data discs.
1G (or 1-G) refers to the first generation of wireless telephone technology (mobile telecommunications). These are
the analog telecommunications standards that were introduced in the 1980s and continued until being replaced
by 2G digital telecommunications. The main difference between the two mobile telephone systems (1G and 2G), is that
the radio signals used by 1G networks are analog, while 2G networks are digital.
Although both systems use digital signaling to connect the radio towers (which listen to the handsets) to the rest of the
telephone system, the voice itself during a call is encoded to digital signals in 2G whereas 1G is only modulated to higher

3. frequency, typically 150 MHz and up. The inherent advantages of digital technology over that of analog meant that 2G
networks eventually replaced them almost everywhere.
One such standard is NMT (Nordic Mobile Telephone), used in Nordic countries, Switzerland, the Netherlands, Eastern
Europe and Russia. Others include AMPS (Advanced Mobile Phone System) used in North
America and Australia,[1]
TACS (Total Access Communications System) in the United Kingdom, C-450 in West
Germany, Portugal and South Africa, Radiocom 2000[2]
in France, TMA in Spain, and RTMI in Italy. In Japan there were
multiple systems. Three standards, TZ-801, TZ-802, and TZ-803 were developed by NTT (Nippon Telegraph and
Telephone Corporation[3]
), while a competing system operated by DDI (Daini Denden Planning, Inc.[3]
) used the JTACS
(Japan Total Access Communications System) standard.
2G (or 2-G) is short for second-generation wireless telephone technology. Second generation 2G cellular telecom
networks were commercially launched on the GSM standard inFinland by Radiolinja (now part of Elisa Oyj) in
1991.[1]
Three primary benefits of 2G networks over their predecessors were that phone conversations were digitally
encrypted; 2G systems were significantly more efficient on the spectrum allowing for far greater mobile phone
penetration levels; and 2G introduced data services for mobile, starting with SMStext messages. 2G technologies
enabled the various mobile phone networks to provide the services such as text messages, picture messages and MMS
(multi media messages). All text messages sent over 2G are digitally encrypted, allowing for the transfer of data in such
a way that only the intended receiver can receive and read it.
After 2G was launched, the previous mobile telephone systems were retroactively dubbed 1G. While radio signals on 1G
networks are analog, radio signals on 2G networks aredigital. Both systems use digital signaling to connect the radio
towers (which listen to the handsets) to the rest of the telephone system.
2G has been superseded by newer technologies such as 2.5G, 2.75G, 3G, and 4G; however, 2G networks are still used
in many parts of the world.
3G, short form of third generation, is the third generation of mobile telecommunications technology.[1]
This is based
on a set of standards used for mobile devices and mobile telecommunications use services and networks that comply
with the International Mobile Telecommunications-2000 (IMT-2000) specifications by the International
Telecommunication Union.[2]
3G finds application in wireless voice telephony, mobile Internet access, fixed
wireless Internet access, video calls and mobile TV.
3G telecommunication networks support services that provide an information transfer rate of at least 200 kbit/s. Later 3G
releases, often denoted 3.5G and 3.75G, also providemobile broadband access of
several Mbit/s to smartphones and mobile modems in laptop computers. This ensures it can be applied to wireless voice
telephony, mobile Internet access, fixed wireless Internet access, video calls and mobile TV technologies.
A new generation of cellular standards has appeared approximately every tenth year since 1G systems were introduced
in 1981/1982. Each generation is characterized by new frequency bands, higher data rates and non–backward-
compatible transmission technology. The first 3G networks were introduced in 1998 and fourth generation " 4G" networks
in 2008.
4G, short for fourth generation, is the fourth generation of mobile telecommunications technology, succeeding 3G and
preceding 5G. A true 4G system must provide capabilities defined by ITU in IMT Advanced. Potential and current
applications include amended mobile web access, IP telephony, gaming services, high-definition mobile TV, video
conferencing, 3D television, and cloud computing.

4. Two 4G candidate systems are commercially deployed: the Mobile WiMAX standard (first used in South Korea in 2007),
and the first-release Long Term Evolution (LTE) standard (in Oslo, Norway and Stockholm, Sweden since 2009). It has
however been debated if these first-release versions should be considered to be 4G or not, as discussed in thetechnical
definition section below.
In the United States, Sprint (previously Clearwire) has deployed Mobile WiMAX networks since 2008,
while MetroPCS became the first operator to offer LTE service in 2010. USB wireless modems were among the first
devices able to access these networks, with WiMAX smartphones becoming available during 2010, and LTE
smartphones arriving in 2011. 3G and 4G equipment made for other continents are not always compatible because of
different frequency bands. Mobile WiMAX is not available for the European market as of April 2012.