六级模拟试题2.doc

1 六级模拟测试(2) Beijing 2008: The First 4G Wireless Olympic Games? About half a million years ago, Peking man lived in Zhoukoudian, in the southwestern suburbs of what is now Beijing. If you have been to Beijing more recently, or are at all familiar with modern China, then you know this ancient city is going to host the most modern, high-tech Olympic Games ever in 2008. With technology available today, and a vision for what Beijing could be in 2008, there is an opportunity for the hosts to make the city's munications infrastructure- in the words of the Olympic motto- -" swifter, higher, stronger. " The Path to 4G Beijing has the good fortune of looking at previous generations of works and avoiding the same mistakes as it prepares for 2008. First generation (1G) wireless munications--the brick-like analog phones that are now collector's items-introduced the cellular architecture that is still being offered by most panies today. Second generation (2G) wireless supported more users within a cell by using digital technology, which allowed many callers to use the same multiplexed channel. But 2G was still primarily meant for munications, not data, except some very low data-rate features, like short messaging service (SMS). So-called 2. 5G allowed carriers to increase data rates with a software upgrade at the base transceivers stations (BTS), as long as consumers purchased new phones too. Third generation (3G) wireless offers the promise of greater bandwidth, basically bigger data pipes to users, which will allow them to send and receive more information. All of these architectures, however, are still cellular. Cellular architecture is sometimes referred to asa "star architecture", because users within that cell access mon, centralized base station. The advantage is that given enough time and money, carriers can build works, which most of the big carriers have done. Some of the disadvantages include a singular point of failure, no load balancing, and spectral inefficiencies.