Point Coordination Function

PCF is an alternative access method implemented on top of the DCF。 The operation consists of polling with the centralized polling master (point coordinator)。 The point coordinator makes use of PIFS when issuing polls。 Because PIFS is smaller than DIFS, the point coordinator can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses。

As an extreme, consider the following possible scenario。 A wireless network is configured so that a number of stations with time-sensitive traffic are controlled by the point coordinator while remaining traffic, using CSMA, contends for access。

The point coordinator could issue polls in a round-robin fashion to all stations configured for polling。 When a poll is issued, the polled station may respond using SIFS。 If the point coordinator receives a response, it issues another poll using PIFS。 If no response is received during the expected turnaround time, the coordinator issues a poll。 If the discipline of the preceding paragraph were implemented, the point coordinator would lock out all asynchronous traffic by repeatedly issuing polls。 To prevent this situation, an interval known as the superframe is defined。 During the first part of this interval, the point coordinator issues polls in a round-robin fashion to all stations configured for polling。 The point coordinator then idles for the remainder of the superframe, allowing a contention period for asynchronous access。

At the beginning of a superframe, the point coordinator may optionally seize control and issue polls fora give period of time。 This interval varies because of the variable frame size issued by responding stations。 The remainder of the superframe is available for contention-based access。 At the end of the superframe interval, the point coordinator contends for access to the medium using PIFS。 If the medium is idle, the point coordinator gains immediate access, and a full superframe period follows。 However, the medium may be busy at the end of a superframe。 In this case, the point coordinator must wait until the medium is idle to gain access; this results in a foreshortened superframe period for the next cycle。

4。 UWB technology and other short-range wireless communications technology is compared

As personal communication consumer electronics industry in the rapid development, short-range wireless communication field of all kinds of new skills ,art, new methods to emerge in endlessly, toward faster and more convenient and more safe and effective etc。 The new technology in the development of the Intel access, information home appliances, mobile office, industrialization and other fields has been widely used。 Among them, ultra-wideband (UWB) Wide Band, the mk-ultra technology is in after 1990's developed a kind of with high potential new wireless communication technology, it was listed as one of the ten future communication technology。

4。1 uwb wireless communication 

UWB (the mk-ultra Wideband, ultra-wideband) technology is currently has been widely studied a new wireless communication technology, it has become a high-speed wireless personal nets (WPAN first choice of the technology。 UWB refers to the signal bandwidth than 500MHz or is the signal bandwidth and center frequency ratio is more than 25% of communications technologies。 With the common use of continuous carrier communication mode is different, use of UWB wireless signals center frequency 4。1 GHz, bandwidth of 1。4 GHz, spectral range is very wide, but transmission power is very low。 Communications speed in 250Kbit ~ between 10Mbit/SEC。 In 250Kbit/SEC transmission speeds can ensure as communication distance。 In short (13m below) a has great advantage, highest transmission speed can reach 1Gb/S。 While the traditional narrowband technology in long distance, low speed transmission dominant。

Ultra-wideband (UWB) technology is initially applied to develop the radar oriented, it is generally thought that it belongs to a kind of carrierless communications technologies。 February 2002, the United States the FCC formally its suspension。 Currently ultra-wideband (UWB) technology is being integrated into the family cinema and portable products, mainly used for video and audio signal of wireless transmission。 Ultra-wideband (UWB) since when published has been regarded as bluetooth technology substitute, and other wireless technologies such as WLAN, bluetooth etc compared ultra-wideband (UWB), with low power waste, high bandwidth and low complexity, the advantages of low cost, can completely satisfy short family entertainment application requirements。 1。2 ultra-wideband performance characteristics。 模糊PLC系统的伺服机构英文文献和中文翻译(15):http://www.youerw.com/fanyi/lunwen_99783.html