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By The Numbers |
A cellular industry body expects India to have some 492 million mobile phone users by the year end, compared with 392 million at the end of March.
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Exams alert |
Karnataka PGCET – 2009: July 12th, 2009; Medical Biotechnology Entrance Tests (AIIMS, New Delhi)– July 19th, 2009; Rajasthan Pre Engineering Test -2009: July 20th, 2009. |
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Thus Spake |
Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for. |
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Interesting Facts |
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Ambulances were developed by Napoleon's surgeon in his Italian company of 1796-97.
The world's most expensive water is heavy water used as moderator in nuclear reactors.
TCDD is a man-made chemical which is 150,000 times more deadly then cyanide.
Saccharin is 500 to 700 times more sweeter than sugar. |
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Did you know |
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The word "dollar" has its origins in the Roman Empire: A mining hole in the mountains of Bohemia produced so much silver it became the official source of coinage for the entire Holy Roman Empire. The mine was in a valley called Joachimsthal, and the coins came to have the same name: "Joachimstalers". Over time this became shortened to "Talers" and over more time, the American pronunciation of the word became the name for the currency that you would like to have in your pocket. The $ sign was designed in 1788 by Oliver Pollock, a New Orleans businessman, using a combination of Spanish money symbols.
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Mac OS X |
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Mac OS X is a uniquely powerful development platform, bringing a 32-bit and 64-bit architecture and multiprocessor capability to the desktop and server arenas. It provides an extremely productive high-level programming environment, Cocoa, combined with the full power of real UNIX, as well as a host of open source web, scripting, database, and development technologies. The built-in Xcode tools, combining time-tested stability and performance, standards-based technologies, and a remarkable user interface, make Mac OS X an amazingly multifaceted development platform. Mac OS X delivers revolutionary technologies like Spotlight, Dashboard, Automator, Core Data, Core Animation, Core Image, and many others. These exciting additions to the modern, UNIX-based foundation make Mac OS X the most advanced operating system available. |
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Smile Please..! |
Bill's clock is.... |
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Hillary Clinton died and went to heaven. As she stood in front of Saint Peter at the Pearly Gates, she saw a huge wall of clocks behind him. She asked, "What are all those clocks"?
Saint Peter answered, "Those are Lie-Clocks. Everyone on Earth has a Lie-Clock. Every time you lie, the hands on your clock will move.
"Oh," said Hillary, "whose clock is that"?
"That's Mother Teresa's. The hands have never moved indicating that she never told a lie".
"Whose clock is that"?
"That's Abraham Lincoln's clock. The hands have only moved twice telling us that Abe only told 2 lies in his entire life".
"Where's Bill's clock"? Hillary asked.
"Bill's clock is in Jesus' office. He's using it as a ceiling fan".
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The crowded store
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It was the day of the big sale. Rumors of the sale (and some advertising in the local paper) were the main reason for the long line that formed by 8:30, the store's opening time, in front of the store.
A small man pushed his way to the front of the line, only to be pushed back, amid loud and colorful curses. On the man's second attempt, he was punched square in the jaw, and knocked around a bit, and then thrown to the end of the line again. As he got up the second time, he said to the person at the end of the line...
"That does it! If they hit me one more time, I won't open the store!"
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Boarding from what gate
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At the airport for a business trip, I settled down to wait for the boarding announcement at Gate 35. Then I heard the voice on the public address system saying, "We apologize for the inconvenience, but Delta Flight 570 will board from Gate 41".
So my family picked up our luggage and carried it over to Gate 41. Not ten minutes later the public address voice told us that Flight 570 would in fact be boarding from Gate 35.
So, again, we gathered our carry-on luggage and returned to the original gate. Just as we were settling down, the public address voice spoke again: "Thank you for participating in Delta's physical fitness program".
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From The Editors Desk |
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Heartiest Greetings!
In this issue of Youniverse, we have presented an article on "Mac OS X", the article provides Overview, Best Graphics on a Desktop, Advanced Developer Tools, Versions, Software, Hardware & Conclusion.
Our regular section on Exam Alerts informs you of the important dates of the upcoming entrance examinations. The section on "Complex simplicities" provides an introduction to the concepts of XNU, Asymmetric digital Subscriber Line (ADSL) & Time Division Multiple Access (TDMA) .
More >>
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Complex Simplicities |
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XNU is the computer operating system kernel that Apple Inc. acquired and developed for use in the Mac OS X operating system and released as free and open source software as part of the Darwin operating system. XNU is an acronym for X is Not Unix.
Kernel design: Like some other modern kernels, XNU is a hybrid, containing features of both monolithic and microkernels,
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Asymmetric Digital Subscriber Line (ADSL) |
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Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. A splitter or microfilter allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the central office, typically less than 4 kilometres (2 mi), but has been known to exceed 8 kilometres (5 mi) if the originally-laid wire gauge allows for farther distribution. |
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Time division multiple access (TDMA) |
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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 his 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. It is also used extensively in satellite systems, and combat-net radio systems. |
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Interesting Facts |
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Ambulances were developed by Napoleon's surgeon in his Italian company of 1796-97. |
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The world's most expensive water is heavy water used as moderator in nuclear reactors. |
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TCDD is a man-made chemical which is 150,000 times more deadly then cyanide. |
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Saccharin is 500 to 700 times more sweeter than sugar. |
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Solid carbon di oxide is called dry ice, because when it melts it does not change into liquid but vaporizes directly. |
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Ice does not melt when kept in liquid ammonia. |
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Quick silver is not silver, but it is another name of mercury. It is so heavy that piece of iron floats on its surface. |
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The longest regularly formed English word is Praetertranssubstantiationalistically which contains 37 letters. |
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The first McDonald's restaurant opened for business in 1952 in Edinburgh, Scotland, and featured the McHaggis sandwich. |
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The typewriter was invented by Hungarian immigrant Qwert Yuiop, who left his "signature" on the keyboard. |
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In 1843, a Parisian street mime got stuck in his imaginary box and consequently died of starvation. |
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In ancient Greece, children of wealthy families were dipped in olive oil at birth to keep them hairless throughout their lives. |
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The Venezuelan brown bat can detect and dodge individual raindrops in mid-flight, arriving safely back at his cave completely dry. |
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Did you know |
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The word "dollar" has its origins in the Roman Empire: A mining hole in the mountains of Bohemia produced so much silver it became the official source of coinage for the entire Holy Roman Empire. The mine was in a valley called Joachimsthal, and the coins came to have the same name: "Joachimstalers". Over time this became shortened to "Talers" and over more time, the American pronunciation of the word became the name for the currency that you would like to have in your pocket. The $ sign was designed in 1788 by Oliver Pollock, a New Orleans businessman, using a combination of Spanish money symbols. |
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The $ sign is used in many countries other than the United States, including the use for the Argentine peso, Brazilian real, Cape Verde escudo, Chilean peso, Colombian peso, Cuban peso, Dominican peso, Mexican peso, Tongan pa'anga and Uruguayan peso. Other countries that trade in their currency as dollars are Australia, Bahamas, Canada, Liberia and others the silver coin first struck as a "Guldiner" in 1486 in Tyrol. The name "Taler" was first given to the silver coin of the same size struck in Joachimstal in Bohemia. |
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Eating with a fork considered scandalous: Forks were first used in the Middle Ages, but eating with one was considered scandalous. In the 11th Century, when a Greek princess died shortly after introducing forks at her wedding with a Venetian Doge (chief magistrate) Domenico Selvo, it was perceived as divine punishment. |
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While forks were a regular feature on the tables of nobles in Italy since the 11th Century, and used in France in the 14th Century, it was introduced in England only in 1611 by Thomas Coryat through his book "Coryat's Curdities Hastily gobbled up in Five Months Travels in France, Savoy, & Italy". Even then, he was mocked about promoting the use of forks and called "Furcifer", meaning fork-bearer. The upper classes of Spain were using forks in the 16th Century, as could be told from a large assortment of forks that were recovered from the wreck of La Girona, which sank off the coast of Ireland in 1588. In 1630, Governor Winthrop of Massachusetts had the first and only fork in colonial America. Forks, mostly being two-tined, were known as "split spoons". Although there are examples of four-and five-tined forks from the before the 1600s, the four-tined fork became popular only in the late 1800s. |
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The Largest Archipelago in the World with 13500 Islands: When Columbus set sail in 1492, he was searching for a shorter route to the Indies, a group of islands now known as Indonesia. Indonesia, Columbus's original goal, is the largest archipelago, or chain of islands, in the world. This island chain stretches 3,500 miles, about the distance from New York to San Francisco, and includes more than 13,500 islands. |
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Only about 1,000 of these islands are inhabited, and most of them are small. But some Indonesian islands are among the largest on earth. The island of New Guinea, part of which belongs to the nation of Indonesia, is the second largest island on earth. Another island that belongs partly to Indonesia, Borneo, is the third largest island on earth. The Indonesian island of Sumatra is the sixth largest on earth, Celebes is the 14th largest, and Java is the 16th largest! |
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Karate Invented In Japan: Karate, the art of self-defense employing chopping blows with the side of the hand, is a Japanese invention. Actually, karate was not invented by the Japanese. It was invented by the people of Okinawa, as a means of defense against the Japanese. Okinawa, an island about 350 miles from the coast of Japan, has had a long history of invasion by other peoples, including the Chinese and Japanese. Centuries ago, the people of Okinawa developed the art of karate. Which means "open hand'' in Japanese, as a means of fighting Japanese invaders without the use of weapons. Karate did not reach Japan itself until 1916. |
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Mac OS X |
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| Overview |
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Mac OS X is a uniquely powerful development platform, bringing a 32-bit and 64-bit architecture and multiprocessor capability to the desktop and server arenas. It provides an extremely productive high-level programming environment, Cocoa, combined with the full power of real UNIX, as well as a host of open source web, scripting, database, and development technologies. The built-in Xcode tools, combining time-tested stability and performance, standards-based technologies, and a remarkable user interface, make Mac OS X an amazingly multifaceted development platform. Mac OS X delivers revolutionary technologies like Spotlight, Dashboard, Automator, Core Data, Core Animation, Core Image, and many others. These exciting additions to the modern, UNIX-based foundation make Mac OS X the most advanced operating system available. |
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| Best Graphics on a Desktop |
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Mac OS X is built around a powerful, integrated stack of graphics technologies, including OpenGL, Core Animation, and Core Image. These provide a solid foundation for application developers to create great applications. Mac OS X's multithreaded graphics layer handles application windowing, 2D and 3D drawing, animation, and multimedia. Together, the subsystems of the graphics layer provide fast, elegant graphics to the operating system and to your application, making possible cutting-edge user interface features. The stunning visual effects that you see in the Mac OS X Desktop are generated using the very same technologies available to your application. For example, the Desktop uses Core Image to provide the mirrored shelf under the Dock, shadowed icons, and the transparent menu bar. The Dock uses Core Animation for the popup behavior of Stacks, those piles of files that expand into a stack or grid when clicked. Time-based graphics on Mac OS X are provided by QuickTime 7, which enables developers to manipulate, enhance, and store video, sound, animation, graphics, text, music, and even 360-degree virtual reality. And for web streaming, QuickTime gets the job done in real-time.
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| Advanced Developer Tools |
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Mac OS X provides you with a full suite of free developer tools to prototype, compile, debug, analyze, and optimize your applications, speeding up your development cycle. Xcode 3 includes a robust Integrated Development Environment (IDE) for writing applications, libraries, and drivers, and a complete set of developer documentation. Underlying Xcode is GCC 4, an Apple-optimized version of the popular open source compiler, plus the GNU Debugger (GDB). Xcode also includes Interface Builder, an easy-to-use graphical editor for designing and managing your application’s user interface. Whether you code in Objective-C, C/C++, or another popular language, Xcode can handle it. Xcode’s performance tools, including Instruments and Shark, help you analyze and debug your code, gather metrics, identify and eliminate bottlenecks in your code, and provide a mental model of the inner workings of your code, giving you the information you need to make your program run faster. Mac OS X works with open source tools, too. From Ant to Ruby, Python, Perl, and PHP, open source tools and languages let you work faster on Mac OS X. |
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Versions |
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Mac OS X versions are named after big cats. Prior to its release, version 10.0 was code named "Cheetah" internally at Apple, and version 10.1 was code named internally as "Puma". After the immense buzz surrounding version 10.2, codenamed "Jaguar", Apple's product marketing began openly using the code names to promote the operating system. 10.3 was marketed as "Panther", and 10.4 as "Tiger". "Leopard" is the name for the current release, version 10.5. The forthcoming version 10.6 is named "Snow Leopard". "Panther", "Tiger" and "Leopard" are registered as trademarks of Apple, but "Cheetah", "Puma" and "Jaguar" have never been registered. Apple has also registered "Lynx" and "Cougar" as trademarks, though were allowed to lapse. Computer retailer Tiger Direct sued Apple for its use of the name "Tiger". On May 16, 2005 a US federal court in the Southern District of Florida ruled that Apple's use does not infringe on Tiger Direct's trademark. |
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On March 24, 2001, Apple released Mac OS X v10.0 (internally codenamed Cheetah). The initial version was slow, incomplete, and had very few applications available at the time of its launch, mostly from independent developers. While many critics suggested that the operating system was not ready for mainstream adoption, they recognized the importance of its initial launch as a base on which to improve. Simply releasing Mac OS X was received by the Macintosh community as a great accomplishment, for attempts to completely overhaul the Mac OS had been underway since 1996, and delayed by countless setbacks. Following some bug fixes, kernel panics became much less frequent. |
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Later that year on September 25, 2001, Mac OS X v10.1 (internally codenamed Puma) was released. It had better performance and provided missing features, such as DVD playback. Apple released 10.1 as a free upgrade CD for 10.0 users, in addition to the US$129 boxed version for people running only Mac OS 9. It was discovered that the upgrade CDs were actually full install CDs that could be used with Mac OS 9 systems by removing a specific file; Apple later re-released the CDs in an actual stripped-down format that did not facilitate installation on such systems. On January 7, 2002, Apple announced that Mac OS X was to be the default operating system for all Macintosh products by the end of that month. |
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On August 23, 2002, Apple followed up with Mac OS X v10.2 "Jaguar", the first release to use its code name as part of the branding. It brought great performance enhancements, a sleeker look, and many powerful enhancements (over 150, according to Apple), including Quartz Extreme for compositing graphics directly on an ATI Radeon or Nvidia GeForce2 MX AGP-based video card with at least 16 MB of VRAM, a system-wide repository for contact information in the new Address Book, and an instant messaging client named iChat. The Happy Mac which had appeared during the Mac OS startup sequence for almost 18 years was replaced with a large grey Apple logo with the introduction of Mac OS X v10.2. |
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Mac OS X v10.3 "Panther" was released on October 24, 2003. In addition to providing much improved performance, it also incorporated the most extensive update yet to the user interface. Panther included as many or more new features as Jaguar had the year before, including an updated Finder, incorporating a brushed-metal interface, Fast User Switching, Exposé (Window manager), FileVault, Safari, iChat AV (which added video-conferencing features to iChat), improved Portable Document Format (PDF) rendering and much greater Microsoft Windows interoperability. Support for some early G3 computers such as "beige" Power Macs and "WallStreet" PowerBooks was discontinued. |
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Mac OS X v10.4 "Tiger" was released on April 29, 2005. Apple stated that Tiger contained more than 200 new features. As with Panther, certain older machines were no longer supported; Tiger requires a Mac with a built-in FireWire port. Among the new features, Tiger introduced Spotlight, Dashboard, Smart Folders, updated Mail program with Smart Mailboxes, QuickTime 7, Safari 2, Automator, VoiceOver, Core Image and Core Video. The initial release of the Apple TV used a modified version of Tiger with a different graphical interface and fewer applications and services. On January 10, 2006, Apple released the first Intel-based Macs along with the 10.4.4 update to Tiger. This operating system functioned identically on the PowerPC-based Macs and the new Intel-based machines, with the exception of the Intel release dropping support for the Classic environment. Only PowerPC Macs can be booted from retail copies of the Tiger client DVD, but there is a Universal DVD of Tiger Server 10.4.7 (8K1079) that can boot both PowerPC and Intel Macs. |
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Mac OS X v10.5 "Leopard" was released on October 26, 2007. It was called by Apple "the largest update of Mac OS X". It brought more than 300 new features. Leopard supports both PowerPC- and Intel x86-based Macintosh computers, however support for the G3 processor was dropped and the G4 processor required a minimum clock speed of 867 MHz. The single DVD works for all supported Macs (including 64-bit machines). New features include a new look, an updated Finder, Time Machine, Spaces, Boot Camp pre-installed, full support for 64-bit applications (including graphical applications), new features in Mail and iChat, and a number of new security features. Leopard is an Open Brand UNIX 03 registered product on the Intel platform. It is also the first BSD-based OS to receive UNIX 03 certification. |
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Mac OS X v10.6 "Snow Leopard" was announced on June 9, 2008 at WWDC. Rather than delivering new functionality, Snow Leopard will focus on stability and performance improvements. It is expected to be released in September of 2009. It will feature Microsoft Exchange Server support, new 64-bit technology capable of supporting greater amounts of RAM, QuickTime X, advanced GPU performance with OpenCL, better use of multi-core processors through Grand Central Dispatch, and SquirrelFish JavaScript interpreter, improving the JavaScript rendering speed of Safari by over 50%. The Developer Preview released at WWDC has a version number of 10.6, removes support for the PowerPC architecture, and requires an Intel CPU. |
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Mac OS X used to support the Java Platform as a "preferred software package" in practice this means that applications written in Java fit as neatly into the operating system as possible while still being cross-platform compatible, and that graphical user interfaces written in Swing look almost exactly like native Cocoa interfaces. Traditionally, Cocoa programs have been mostly written in Objective-C, with Java as an alternative. However, on July 11, 2005, Apple announced that "features added to Cocoa in Mac OS X versions later than 10.4 will not be added to the Cocoa-Java programming interface". |
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Since Mac OS X is POSIX compliant, many software packages written for the BSDs or Linux can be recompiled to run on it. Projects such as Fink, MacPorts and pkgsrc provide pre-compiled or pre-formatted packages. Since version 10.3, Mac OS X has included X11.app, Apple's version of the X Window System graphical interface for Unix applications, as an optional component during installation. Up to and including Mac OS X v10.4 (Tiger), Apple's implementation was based on the X11 Licensed XFree86 4.3 and X11R6.6. All bundled versions of X11 feature a window manager which is similar to the Mac OS X look-and-feel and has fairly good integration with Mac OS X, also using the native Quartz rendering system. Earlier versions of Mac OS X (in which X11 has not been bundled) can also run X11 applications using XDarwin. With the introduction of version 10.5 Apple switched to the X.org variant of X11. |
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For the early releases of Mac OS X, the standard hardware platform supported was the full line of Macintosh computers (laptop, desktop, or server) based on PowerPC G3, G4, and G5 processors. Later versions discontinued support for some older hardware; for example, Panther does not support "beige" G3s, and Tiger does not support systems that pre-date Apple's introduction of integrated FireWire ports. Mac OS X v10.5 "Leopard", introduced October 2007, has dropped support for all PowerPC G3 processors and for PowerPC G4 processors with clock speeds below 867 MHz. With the introduction of the MacBook Air and later the "unibody" MacBook, which lack any FireWire ports, Leopard does not require an integrated FireWire port. Tools such as XPostFacto and patches applied to the installation disc have however been developed by third parties to enable installation of newer versions of Mac OS X on systems not officially supported by Apple. This includes a number of pre-G3 Power Macintosh systems that can be made to run up to and including Mac OS X 10.2 Jaguar, all G3-based Macs which can run up to and including Tiger, and sub-867 MHz G4 Macs can run Leopard by removing the restriction from the installation DVD or entering a command in the Mac's Open Firmware interface to tell the Leopard Installer that it has a clock speed of 867 MHz or greater. Except for features requiring specific hardware (e.g. graphics acceleration, DVD writing), the operating system offers the same functionality on all supported hardware. |
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PowerPC versions of Mac OS X prior to Leopard retain compatibility with older Mac OS applications by providing an emulation environment called Classic, which allows users to run Mac OS 9 as a process within Mac OS X, so that older applications run as they would under the older operating system. Classic is not supported on Intel-based Macs or in Mac OS X v10.5 "Leopard", although users still requiring Classic applications on Intel Macs can use the SheepShaver emulator to run Mac OS 9 on top of Leopard. |
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Conclusion: The Uniquely Powerful Platform |
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Mac OS X has always been friendly to an international audience. Every major release of Mac OS X ships simultaneously in sixteen languages. To support this capability, Mac OS X provides conversion utilities to manage locales, dates, currencies, and measurement systems in a consistent manner. Mac OS X includes Unicode tools to handle text systems used around the world. And, by packaging an application’s executable code, libraries and resource files into single binary, both internationalized and localized software versions can launch dynamically from a single application icon. Finally, Mac OS X is not only a great development platform, but it’s also fun to use when you’re not programming. You only need one machine to code in Cocoa, UNIX, and many other languages, as well as manage your digital lifestyle with industry-leading applications like iTunes. Mac OS X seamlessly and elegantly bridges the modern developer’s professional and personal lifestyle on one powerful machine. |
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XNU |
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XNU is the computer operating system kernel that Apple Inc. acquired and developed for use in the Mac OS X operating system and released as free and open source software as part of the Darwin operating system. XNU is an acronym for X is Not Unix.
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Kernel design: Like some other modern kernels, XNU is a hybrid, containing features of both monolithic and microkernels, attempting to make the best use of both technologies, such as the message passing capability of microkernels enabling greater modularity and larger portions of the OS to benefit from protected memory, as well as retaining the speed of monolithic kernels for certain critical tasks. Currently, XNU runs on ARM, x86, x86-64 and PowerPC based processors, both single processor and SMP models.
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The core of the XNU kernel, Mach, was originally conceived as a simple microkernel. As such, it is able to run the core of an operating system as separated processes, which allows a great flexibility but this often reduced performance because of time consuming kernel/user mode context switches and overhead stemming from mapping or copying messages between the address spaces of the microkernel and that of the service daemons. Mach provides kernel threads, processes, pre-emptive multitasking, message-passing (used in inter-process communication), protected memory, virtual memory management, very soft real-time support, kernel debugging support, and console I/O. The Mach component also allows the OS to host binaries for multiple distinct CPU architectures within a single file (such as x86 and PowerPC) due to its use of the Mach-O binary format. |
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I/O Kit is the device driver framework, written in a subset of C++. Using its object-oriented design, features common to any class of driver are provided within the framework itself, helping device drivers be written more quickly and using less code. The I/O Kit is multi-threaded, Symmetric multiprocessing (SMP)-safe, and allows for hot pluggable devices and automatic, dynamic device configuration. Many drivers can be written to run from user-space, which further enhances the stability of the system; if a user-space driver crashes, it will not crash the kernel. However, if a kernel-space driver crashes it will crash the kernel. Examples of kernel-space drivers include Parallels, Eye TV and the Apple USB driver. |
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Asymmetric Digital Subscriber Line (ADSL) |
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Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. A splitter or microfilter allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the central office, typically less than 4 kilometres (2 mi), but has been known to exceed 8 kilometres (5 mi) if the originally-laid wire gauge allows for farther distribution. |
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The distinguishing characteristic of ADSL over other forms of DSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for consumers to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require high speed in the other direction. |
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Currently, most ADSL communication is full-duplex. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD), or time-division duplexing (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user. |
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The DSL modem will make a plan on how to exploit each of the bins sometimes termed "bits per bin" allocation. Those bins that have a good signal-to-noise ratio (SNR) will be chosen to transmit signals chosen from a greater number of possible encoded values (this range of possibilities equating to more bits of data sent) in each main clock cycle. The number of possibilities must not be so large that the receiver might mishear which one was intended in the presence of noise. Noisy bins may only be required to carry as few as two bits, a choice from only one of four possible patterns, or only one bit per bin in the case of ADSL2+, and really noisy bins are not used at all. If the pattern of noise versus frequencies heard in the bins changes, the DSL modem can alter the bits-per-bin allocations, in a process called "bitswap", where bins that have become more noisy are only required to carry fewer bits and other channels will be chosen to be given a higher burden. The data transfer capacity the DSL modem therefore reports is determined by the total of the bits-per-bin allocations of all the bins combined. Higher signal-to-noise ratios and more bins being in use gives a higher total link capacity, while lower signal-to-noise ratios or fewer bins being used gives a low link capacity. |
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Time division multiple access (TDMA) |
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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 his 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. It is also used extensively in satellite systems, and combat-net radio systems. |
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TDMA characteristics: |
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Shares single carrier frequency with multiple users |
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Non-continuous transmission makes handoff simpler |
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Slots can be assigned on demand in dynamic TDMA |
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Less stringent power control than CDMA due to reduced intra cell interference |
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Higher synchronization overhead than CDMA |
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Advanced equalization may be necessary for high data rates if the channel is "frequency selective" and creates Intersymbol interference |
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Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA |
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Frequency / slot allocation complexity |
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Pulsating power envelop: Interference with other devices |
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A major advantage of TDMA is that the radio part of the mobile only needs to listen and broadcast for its own time slot. For the rest of the time, the mobile can carry out measurements on the network, detecting surrounding transmitters on different frequencies. This allows safe inter frequency handovers, something which is difficult in CDMA systems, not supported at all in IS-95 and supported through complex system additions in Universal Mobile Telecommunications System (UMTS). This in turn allows for co-existence of microcell layers with macrocell layers. |
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A disadvantage of TDMA systems is that they create interference at a frequency which is directly connected to the time slot length. This is the buzz which can sometimes be heard if a TDMA phone is left next to a radio or speakers. Another disadvantage is that the "dead time" between time slots limits the potential bandwidth of a TDMA channel. These are implemented in part because of the difficulty in ensuring that different terminals transmit at exactly the times required. Handsets that are moving will need to constantly adjust their timings to ensure their transmission is received at precisely the right time, because as they move further from the base station, their signal will take longer to arrive. This also means that the major TDMA systems have hard limits on cell sizes in terms of range, though in practice the power levels required to receive and transmit over distances greater than the supported range would be mostly impractical anyway. |
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From The Editors Desk |
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| Kayalvizhi M.S |
| Email - kayal@mindlogicx.com |
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| Heartiest Greetings! |
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In this issue of Youniverse, we have presented an article on "Mac OS X", the article provides Overview, Best Graphics on a Desktop, Advanced Developer Tools, Versions, Software, Hardware & Conclusion.
Our regular section on Exam Alerts informs you of the important dates of the upcoming entrance examinations. The section on "Complex simplicities" provides an introduction to the concepts of XNU, Asymmetric digital Subscriber Line (ADSL) & Time Division Multiple Access (TDMA) .
We hope that you would find the information presented in this issue of Youniverse interesting and useful. |
We welcome your thoughts, views, comments and suggestions to share information as knowledge. |
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