tailieunhanh - Voic 802.11e over phần 2

(CDMA) trong cách tiếp cận của nó. CDMA sử dụng các biến đổi toán học phức tạp để đưa truyền nhiều vào một tàu sân bay duy nhất, OFDM mã hóa một truyền đơn thành nhiều sóng mang con. | Alternative Access 13 CDMA in its approach. CDMA uses complex mathematical transforms to put multiple transmissions onto a single carrier OFDM encodes a single transmission into multiple subcarriers. The mathematics underlying the code division in CDMA is far more complicated than in OFDM. OFDM devices use one wide-frequency channel by breaking it up into several component subchannels. Each subchannel is used to transmit data. All the low subchannels are then multiplexed into one ast combined channel. Carrier Multiplexing When network managers solicit user input on network build-outs one of the most common demands is for more speed. The hunger for increased data transmission has driven a host of researchers to search for ways to increase the speed of their technologies. OFDM takes a qualitatively similar approach to multilink PPP When one link is not enough use several in parallel. OFDM is closely related to plain old frequency division multiplexing FDM . Both divide the available bandwidth into slices called carriers or subcarriers and make those carriers available as distinct channels for data transmission. OFDM boosts throughput by using several subcarriers in parallel and multiplexing data over the set of subcarriers. Traditional FDM was widely used by first-generation mobile telephones as a method for radio channel allocation. Each user was given an exclusive channel and guard bands were used to ensure that spectral leakage from one user did not cause problems for users of adjacent channels 5 p. 199 . MAC Concepts and Architecture The IEEE MAC layer is common to all IEEE PHY layers and specifies the functions and protocols required for control and access. The MAC layer is responsible for managing data transfer from higher level functions to the physical media. Figure earlier in this chapter illustrates this relationship to the OSI model. MAC Layer Services Devices using the IEEE PHY and MAC as part of a WLAN are called stations.