Android
(operating system)
Android is a Linux-based
operating system for mobile devices such as smartphones
and tablet computers. It is developed by the Open Handset Alliance, led by Google, and other
companies. Google purchased the initial developer of the software, Android
Inc., in 2005. The unveiling of the Android distribution in 2007 was announced
with the founding of the Open Handset Alliance, a consortium of 86 hardware,
software, and telecommunication companies devoted to
advancing open standards for mobile devices. Google
releases the Android code as open-source, under the Apache License.
The Android Open Source Project (AOSP) is tasked
with the maintenance and further development of Android.
Android has a large community of developers writing applications ("apps")
that extend the functionality of the devices. Developers write primarily in a
customized version of Java. Apps can be downloaded from third-party
sites or through online stores such as Google Play
(formerly Android Market), the app store run by Google. In October 2011,
there were more than 500,000 apps available for Android, and the estimated number of applications
downloaded from the Android Market as of December 2011 exceeded 10 billion.
Android was listed as the best-selling smartphone platform worldwide in Q4
2010 by Canalys with over 300 million Android devices in use by February 2012. According to Google's Andy Rubin,
as of December 2011, there were over 700,000 Android devices activated every
day.
Android Open Source Project
The Android Open Source Project (AOSP)
is led by Google, and is tasked with the maintenance and development of
Android. According to the project "The goal of
the Android Open Source Project is to create a successful real-world product
that improves the mobile experience for end users." AOSP also maintains
the Android Compatibility Program, defining an "Android
compatible" device "as one that can run any application written by
third-party developers using the Android SDK and NDK",
to prevent incompatible Android implementations. The compatibility program is
also optional and free of charge, with the Compatibility Test Suite also
free and open-source.
Version history
Android has seen a number of updates since its original release, each fixing
bugs and
adding new features. Each version is named, in alphabetical order, after a dessert.
Recent releases
- 2.3
Gingerbread
refined the user interface, improved the soft keyboard and copy/paste features,
better native code support (which improves gaming performance), added SIP support (VoIP calls),
and added support for Near Field Communication.
- 3.0
Honeycomb
was a tablet-oriented release which
supports larger screen devices and introduces many new user interface
features, support for multi-core processors, hardware
acceleration for graphics and full system encryption. The first device
featuring this version, the Motorola Xoom tablet,
went on sale in February 2011.
- 3.1 Honeycomb, released in May 2011, added support
for extra input devices, USB host mode for transferring information directly
from cameras and other devices, and the Google Movies and Books apps.
- 3.2 Honeycomb, released in July 2011, added
optimization for a broader range of screen sizes, new "zoom-to-fill"
screen compatibility mode, loading media files directly from SD card, and an
extended screen support API. Huawei MediaPad is the first 7 inch tablet to
use this version.
- 4.0 Ice
Cream Sandwich,
announced on October 19, 2011, brought Honeycomb features to smartphones
and added new features including facial recognition unlock, network data
usage monitoring and control, unified social networking contacts, photography
enhancements, offline email searching, app folders, and information
sharing using NFC. Android 4.0.4 is the latest
Android version that is available to phones. The source code of Android
4.0.1 was released on November 14, 2011.
Android Architecture
The following diagram
shows the major components of the Android operating system. Each section is
described in more detail below
Linux
Android's kernel is based on the Linux kernel
and has further architecture changes by Google outside the typical Linux kernel
development cycle. Android does not have a native X Window System
nor does it support the full set of standard GNU libraries, and this
makes it difficult to port existing Linux applications or libraries to Android.
Certain features that Google contributed back to the Linux kernel, notably a
power management feature called wakelocks, were rejected by mainline kernel
developers, partly because kernel maintainers felt that Google did not show any
intent to maintain their own code. Even though Google announced in April 2010
that they would hire two employees to work with the Linux kernel community, Greg Kroah-Hartman, the current Linux kernel
maintainer for the -stable branch, said in December 2010 that he was concerned
that Google was no longer trying to get their code changes included in
mainstream Linux. Some Google Android developers hinted that "the Android
team was getting fed up with the process", because they were a small team
and had more urgent work to do on Android.
However, in September 2010, Linux kernel developer Rafael J. Wysocki added a
patch that improved the mainline Linux wakeup events framework. He said that
Android device drivers that use wakelocks can now be easily merged into
mainline Linux, but that Android's opportunistic suspend features should not be
included in the mainline kernel. In 2011 Linus Torvalds
said that "eventually Android and Linux would come back to a common
kernel, but it will probably not be for four to five years".
In December 2011, Greg Kroah-Hartman announced the start of the
Android Mainlining Project, which aims to put some Android drivers,
patches and features back into the Linux kernel, starting in Linux 3.3. further
integration being expected for Linux Kernel 3.4.
Features
The Android Emulator default home screen (v1.5)
Current features and specifications:
Handset layouts
The platform is
adaptable to larger, VGA, 2D graphics library, 3D graphics library based on OpenGL ES
2.0 specifications, and traditional smartphone layouts.
Storage
SQLite,
a lightweight relational database, is used for data
storage purposes.
Connectivity
Android supports
connectivity technologies including GSM/EDGE,
IDEN,
CDMA, EV-DO, UMTS,
Bluetooth,
Wi-Fi,
LTE,
NFC and WiMAX.
Messaging
SMS and MMS are available forms of
messaging, including threaded text messaging
and now Android Cloud To Device Messaging
(C2DM) is also a part of Android Push Messaging service.
Multiple language support
Android supports
multiple languages.
Web browser
The web browser
available in Android is based on the open-source WebKit
layout engine, coupled with Chrome's
V8 JavaScript engine. The browser
scores 100/100 on the Acid3
test on Android 4.0.
Java support
While most Android
applications are written in Java, there is no Java Virtual Machine in the platform
and Java byte code is not executed. Java classes are compiled into Dalvik
executables and run on Dalvik, a specialized virtual machine
designed specifically for Android and optimized for battery-powered mobile
devices with limited memory and CPU. J2ME
support can be provided via third-party applications.
Media support
Android supports the
following audio/video/still media formats: WebM,
H.263,
H.264
(in 3GP
or MP4
container), MPEG-4 SP,
AMR, AMR-WB
(in 3GP container), AAC, HE-AAC
(in MP4 or 3GP container), MP3,
MIDI,
Ogg Vorbis,
FLAC, WAV, JPEG,
PNG, GIF, BMP,
WebP.
Streaming media support
RTP/RTSP streaming (3GPP PSS,
ISMA),
HTML progressive download (HTML5 <video>
tag).
Adobe Flash Streaming (RTMP) and HTTP Dynamic Streaming are supported by the Flash plugin. Apple HTTP Live
Streaming is supported by RealPlayer for Android, and by the
operating system in Android 3.0 (Honeycomb).
Additional hardware support
Android can use
video/still cameras, touchscreens, GPS, accelerometers,
gyroscopes,
barometers,
magnetometers,
dedicated gaming controls, proximity
and pressure sensors, thermometers,
accelerated 2D bit blits (with hardware orientation, scaling,
pixel format conversion) and accelerated 3D graphics.
Multi-touch
Android has native
support for multi-touch which was initially made available in
handsets such as the HTC Hero. The feature was originally disabled at
the kernel level (possibly to avoid infringing Apple's patents on touch-screen
technology at the time). Google has since released an update for the Nexus One
and the Motorola Droid which enables
multi-touch natively.
Bluetooth
Supports A2DP,
AVRCP,
sending files (OPP), accessing the phone book (PBAP), voice dialing and sending
contacts between phones. Keyboard, mouse and joystick (HID) support is available in Android
3.1+, and in earlier versions through manufacturer customizations and
third-party applications.
Video calling
Android does not
support native video calling, but some handsets have a customized version of
the operating system that supports it, either via the UMTS
network (like the Samsung Galaxy S) or over IP. Video
calling through Google Talk is available in Android 2.3.4 and later.
Gingerbread allows Nexus S to place Internet calls with a SIP
account. This allows for enhanced VoIP dialing to other SIP accounts and even
phone numbers. Skype 2.1 offers video calling in Android 2.3, including front
camera support.
Multitasking
Multitasking of
applications, with unique handling of memory allocation, is available.
Voice based features
Google search through
voice has been available since initial release. Voice actions for calling,
texting, navigation, etc. are supported on Android 2.2 onwards.
Tethering
Android supports tethering,
which allows a phone to be used as a wireless/wired Wi-Fi hotspot.
Before Android 2.2 this was supported by third-party applications or
manufacturer customizations.
Screen capture
Android supports
capturing a screenshot by pressing the power and volume-down
buttons at the same time. Prior to Android 4.0, the only methods of capturing a
screenshot were through manufacturer and third-party customizations or
otherwise by using a PC connection (DDMS developer's tool). These alternative
methods are still available with the latest Android.
External storage
Most Android devices
include microSD slot and can read microSD cards formatted with FAT32,
Ext3fs
or Ext4fs
file system. To allow use of high-capacity storage media such as USB flash drives
and USB HDDs,
many Android tablets also include USB 'A' receptacle.
Storage formatted with FAT32
is handled by Linux Kernel VFAT driver, while 3rd party solutions
are required to handle other popular file systems such as NTFS,
HFS Plus
and exFAT.
Security
An example of app permissions in
Android Market.
Android applications run in a sandbox, an isolated area of the operating system that does not
have access to the rest of the system's resources, unless access permissions are
granted by the user when the application is installed. Before installing an
application, Play Store
displays all required permissions. A game may need to enable vibration, for
example, but should not need to read messages or access the phonebook. After
reviewing these permissions, the user can decide whether to install the
application. The sandboxing and permissions system weakens the impact of
vulnerabilities and bugs in applications, but developer confusion and limited
documentation has resulted in applications routinely requesting unnecessary
permissions, reducing its effectiveness.
Several security firms have released
antivirus software for Android devices, in particular, AVG
Technologies, Avast!,
F-Secure, Kaspersky,
McAfee and Symantec.
This software is ineffective as sandboxing also applies to such applications,
limiting their ability to scan the deeper system for threats.
Privacy
Android smartphones have the ability
to report the location of Wi-Fi
access points, encountered as phone users move around, to build databases
containing the physical locations of hundreds of millions of such access
points. These databases form electronic maps to locate smartphones, allowing
them to run apps like Foursquare, Latitude, Places, and to deliver location-based ads.
Third party monitoring software such
as TaintDroid,
an academic research-funded project, can, in some cases, detect when personal
information is being sent from applications to remote servers.
In March 2012 it was revealed that
Android Apps can copy photos without explicit user permission, Google responded
they "originally designed the Android photos file system similar to those
of other computing platforms like Windows and Mac OS. we're taking another look
at this and considering adding a permission for apps to access images. We've
always had policies in place to remove any apps on Android Market that
improperly access your data."
