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The Goal of a Wireless Network
The convergence of voice, video and data services is the ultimate goal of many communication service providers. To achieve this goal, technologies associated with the traditional voice dominated network are replaced by newer technologies that accommodate the bandwidth demands of today's consumer. Access schemes, such as Frequency Division Multiple Access (FDMA) and Frequency Division Duplex (FDD), were regarded as innovative technologies when first applied to the requirements of the traditional voice network. Today, however, there are other technologies on the market that allow for the performance required to meet the high bandwidth demands and the dynamic nature of the current network that must deliver voice, video, Internet and data services efficiently.

FDD and TDD
Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are the two most prevalent duplexing schemes used in fixed broadband wireless networks. FDD, which historically has been used in voice-only applications, supports two-way radio communication by using two distinct radio channels. Alternatively, TDD uses a single frequency to transmit signals in both the downstream and upstream directions.

In fixed wireless point-to-point systems that use FDD, one frequency channel is transmitted downstream from a radio A to radio B. A second frequency is used in the upstream direction and supports transmission from radio B to radio A. Because of the pairing of frequencies, simultaneous transmission in both directions is possible. To mitigate self-interference between upstream and downstream transmissions, a minimum amount of frequency separation must be maintained between the frequency pair.

In fixed wireless point-to-point systems that use TDD, a single frequency channel is used to transmit signals in both the downstream and upstream directions.

Data Symmetry
FDD systems utilize channel plans that are comprised of frequencies with equal bandwidth. Since each channel has a fixed bandwidth, the channel capacity of each frequency also is fixed and equal to that of all other channels in the frequency band. This makes FDD ideal for symmetrical communication applications in which the same or similar information flows in both directions, such as voice communications.

TDD operates by toggling transmission directions over a time interval. This toggling takes place very rapidly and is imperceptible to the user. Thus, TDD can support voice and other symmetrical communication services as well as asymmetric data services. TDD also can handle a dynamic mix of both traffic types. The relative capacity of the downstream and upstream links can be altered in favor of one direction over the other. This is accomplished by giving a greater time allocation through time slots to downstream transmission intervals than upstream. This asymmetry is useful for communication processes characterized by unbalanced information flow. An obvious application for this technique is Internet access in which a user enters a short message upstream and receives large information payloads downstream.

FDD can be used for asymmetric traffic. However, in order to be spectrally efficient, the downstream and upstream channel bandwidths must be matched precisely to the asymmetry. Since Internet traffic is bursty by nature and the asymmetry is always changing, the channel bandwidth cannot be precisely set in FDD. In this respect, TDD is more efficient. Furthermore, channel bandwidths typically are set by the FCC or limited by the functionality of available equipment. As a consequence, users of FDD systems do not have the option to vary channel bandwidths dynamically in the upstream and downstream directions.

Spectrum Efficiency
Frequency spectrum is an increasingly scarce commodity. This scarcity drives the need to optimize the use of available bandwidth. FDD systems operate on the principle of paired frequencies. A channel plan is devised that is comprised of downstream and upstream channels, typically defined by the FCC, ITU, or other governing body. FDD channel plans maintain a guardband between the downstream and upstream channels. The guardband is required to avoid self-interference and, since it is unused, essentially is wasted spectrum.

In contrast, TDD systems require a guard time (instead of a guardband) between transmit and receive streams. The TX/RX Transition Gap (TTG) is a gap between downstream transmission and the upstream transmission. This gap allows time for the base station to switch from transmit mode to receive mode and subscribers to switch from receive mode to transmit mode. During this gap, the base station and subscriber are not transmitting modulated data but are simply allowing the base station transmitter carrier to ramp down, the TX /RX antenna switch to actuate, and the base station receiver section to activate.

Conclusions
The above discussion has highlighted the differences and some significant advantages of TDD over FDD. These advantages can be summarized as follows:

FDD is an older scheme that was best suited for applications, such as voice, that generate symmetric traffic, while TDD is best suited for bursty, asymmetric traffic, such as Internet or other datacentric services.

In TDD, both the transmitter and receiver operate on the same frequency but at different times. Therefore, TDD systems reuse the filters, mixers, frequency sources and synthesizers, thereby eliminating the complexity and costs associated with isolating the transmit antenna and the receive antenna. An FDD system uses a duplexer and/or two antennas that require spatial separation and, therefore, cannot reuse the resources. The result is more costly hardware.

TDD utilizes the spectrum more efficiently than FDD. FDD cannot be used in environments where the service provider does not have enough bandwidth to provide the required guardband between transmit and receive channels.

TDD is more flexible than FDD in meeting the need to dynamically reconfigure the allocated upstream and downstream bandwidth in response to customer needs.

TDD allows interference mitigation via proper frequency planning. TDD requires only one interference-free channel compared with FDD, which requires two interference-free channels.

In summary, TDD is a more desirable duplexing technology that allows system operators to receive the most from their investment in spectrum and telecom equipment, while meeting the needs of each individual customer.

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A special characteristic of this model is the Xerox Phaser Toner. Its density enables the printer to use less toner per page, leading to a reduced cost per page printed. This toner also enables the use of high quality colors, comparable to professional print jobs. In addition, the toner also mixes well, making sure that the color-matching is as accurate as needed.

Laser printers are an expensive investment for any company. The Phaser 6360 toner is a perfect example. The printer itself can cost over $1,000, and the toner can cost between $100 and $200. There are other kinds of laser printers that may not be quite as pricey. They are certainly a worthwhile investment to any company in terms of quality.

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Electronic equipment repair? Yes it is true if you wanted to. Everyone has their passion since when they are young and some begin to know it at the later age. There are many kinds of interest and passions such as wanted to be a singer, dancer, artist, lawyer, doctor, police, scientist and so forth. But in this article, Im talking about the passion of the electronics. Once you are certain that electronics is the field that you are going to venture in, then do your best to acquire the necessary knowledge about electronics-look beyond the dollar signs. Find a job you are passionate about and you will increase your chances of having a long and successful career.

Do not give in to pressure from parents or other forces or base your decision on the money will make. You may get a good paying job but you may not necessarily be happy. If your work does not match your values, interests and personality, it will not be long before you feel the strain. You need the passion in you to keep yourself going on bad days.

Sometimes being passionate about your career is not good enough. You should consistently plan and prepare to ensure that your technical skills and abilities are on par with your career ambitions. Lets take this for example, if you want to be a good electronic repairer, you need to stay ahead of the learning curve and update yourself on new technical issues.

Ok, lets come back the topic of You can do all kinds of electronic equipment repair. Why I want to bring up the word Passion because only passion that can help you to repair the equipment. Passion in you will tell you not to give up and continue to find the answers for the electronic problems. Passion in you will drive you to even a greater level of learning and understanding of electronics troubleshooting. Passion in you that motivates you to come out with new ideas of repairing and passion in you will help to guide you on what are the next steps you should make.

If you do not have the passion for electronics, I guess you will find this article boring and immediately exit this article as fast as I sent it to you. Well, I hope you are not because I presumed you love electronics otherwise you would not have subscribed to become the ERG member.

First you must know how to test and measure electronic components. There is no point for you even if you could able to identify the defective circuit in the equipment if you dont know how to check and measure basic electronic components. Assuming you have locate that the vertical section was at fault causing one horizontal line in the display, but because you are weak in testing electronic components, voltage checking or even scope probing, your percentage of able to repair the Monitor would be not high. In order to overcome this problem, you must be able to test and check electronic components accurately by doing lots of practical test and read more information about electronic components either from the repair books or from the internet. You must pass this test first before going further. Once you had grasped the techniques of accurately testing electronic components, you have won 50 % of the battle.

Second, no matter what kind of electronic equipment you are repairing it will always start from how this electronic equipment works. If you dont know how the equipment function or work, you will have the disadvantage and may completely cant repair it. Take a look at this example, if the Monitor problem is in the horizontal section and you conclude that the problem is in the vertical section, you will lost countless of hours trying to find the faulty parts where actually the caused of the problem is in the horizontal section. In other words, if you do not know how electronic equipment works you will not know at where you want to begin the repair.

This scenario happened very common to beginners where they just started out in the repair field. In order to solve this problem, get the electronic board back home or whatever place you find convenience where you can slowly analyse the electronic circuits. Ask yourself these questions such as why this board need to have the optoisolator in the power supply? Is the optoisolator functions are for overvoltage or overcurrent protection and etc? What is the part number of the optoisolator? What is inside this optoisolator and how to test it to confirm whether it is good or bad? What would happen if the optoisolator developed a short circuit? Will it cause the power supply to shutdown, produce low power or even power blink?

Hope you get what I mean. By asking questions yourself, you are actually training your mind to understand more about electronics (in the above case is the optoisolator). Using this way on other parts of the circuits in the board and Im sure one day you will definitely truly understand the electronic equipment that you are trying to repair. If you cant find the answer, refer to electronics books, electronic repair forum or just surf the internet from different search engine (we have google.com, yahoo.com, msn.com, ask.com and etc) to get the answer you want.

Third, you need a schematic diagram to back up on what you are trying to understand on that particular electronic equipment. If you want to understand how LCD Monitor works then you should buy or download free LCD Monitor schematic diagram and start to practice how each of the circuits works. Break the circuits into different sections thus we have the power supply section, inverter, LCD panel, mainboard and etc. Study and understand each of these circuits and start to ask questions as explained in the above. Similarly, this example is applicable to you if you want to repair Plasma or LCD TV or whatever electronic equipment.

Fourth, investing in you-Invest in yourself. Notice that the first rule of investment isnt invest in your business or anything else other than yourself. While investing and reinvesting into your repairing business is important, investing in yourself is your first priority of investing. You may ask What to invest in yourself? When I said, invest in yourself, I really meant that you should invest in your education (electronic repair courses), testing equipment, electronic repair books (either eBooks or physical books as long as the information are very helpful), schematic diagrams, tools and other necessary things making yourself better than you were yesterday. I think that by not investing in yourself by saving the little money you have actually had robs you of your technical knowledge (that you should know) or even financial independence! Why allow the excuse of not having money steal your technical knowledge (that you should add more) from you? In the technical line, one should have to go forward and not backward or still for such a long time. That kind of still mindset not only didnt bring improvement to your technical knowledge but also your life.

There was a saying which I found it to be true-There are three types of persons in the world:

First-The one who make things happen.

Second- The one who wait for things to happen

Third- The one who doesnt know what happen.

I wish that you are not in the third category. You should be in the first category where you make things happen and not to wait for things to happen. Make your decision now as not to wait for things to happen. If you found that there are websites selling information like the training manuals, schematic diagrams or even repair tips that can help you to understand more about the electronic equipment you are repairing then buy it. Let me tell you another fact is that no one will walk to you and show you how this or that functions in full details you just have to make things happen by finding your own answers from the information you had bought. How many times have you post questions in the repair forum and you did no get the reply? If you are the one in the second category (wait for things to happen) I can guarantee that even after many years in the electronic repair line you will still loose out to a beginner who has the mindset of making things happen. Those beginners have the mindset of moving forward, invest (time and money), never give up, and willing to learn.

Conclusion-Do you believe now that you can do all kind of electronic equipment repair? I believe you can if you carefully follow the steps that I had explained above. The reason I write this article is to tell you that do not limit yourself, give yourself a chance to change! People just dont like changes, they like their routine things but if you are able to make changes in your mindset, you can actually go further of what you are doing RIGHT NOW! Think about it my friend.

Jestine Yong is a electronic repairer and a writer and if you want to become a professional in testing electronic components please visit his website at http://www.TestingElectronicComponents.com