【正文】 在這種情況下，點(diǎn)協(xié)調(diào)器必須等待直到媒體空閑并獲得接入。超幀剩余的時(shí)間用于基于爭用的接入。在超幀時(shí)間的開始部分，點(diǎn)協(xié)調(diào)器以循環(huán)方式向所有配置成輪詢的站點(diǎn)發(fā)布輪詢。點(diǎn)協(xié)調(diào)在發(fā)布輪詢時(shí)采用 PIFS。如果媒體在這段時(shí)間內(nèi)都是空閑的，則站點(diǎn)采用二進(jìn)制指數(shù)退避策略等待一段時(shí)間后再監(jiān)聽媒體，如果媒體依然是空閑的，則可 以傳輸。 2。我們首 先考慮一個(gè)稱為幀間間隔（ InterFrame Space,IFS）時(shí)延。否則，該站點(diǎn)必須等待直到當(dāng)前的發(fā)送結(jié)束。點(diǎn)協(xié)調(diào)功能（ Point Coordination Function, PCF）是一個(gè)集中式 MAC 算法，用于提供無爭用服務(wù)。當(dāng)某些數(shù)據(jù)是時(shí)間敏感的或者是高 優(yōu)先級(jí)的時(shí)，這種方法特別有用。分布式接入?yún)f(xié)議類似于以太網(wǎng)，采用載波監(jiān)聽機(jī)制把傳輸?shù)臎Q定權(quán)分布到所有節(jié)點(diǎn)。在這種情況下，該站點(diǎn)的數(shù)據(jù)交付需要尋址功能，能識(shí)別出該站點(diǎn)的新位置。分發(fā)服務(wù)的正常運(yùn)行需要該 ESS 內(nèi)所有站點(diǎn)的信息，而這個(gè)信息是由與關(guān)聯(lián)（ association）相關(guān)的服務(wù)提供的。接入點(diǎn)的功能相當(dāng)于網(wǎng)橋。自從 FCC 開放了這個(gè)波段以來，在擴(kuò)頻無線局域網(wǎng)中的應(yīng)用就越來越普遍。其中有些產(chǎn)品運(yùn)行的頻率需要 FCC 的許可證，而其他一些產(chǎn)品則使用了不需要許可的波段。目前所有無線局域網(wǎng)產(chǎn)品都可歸為以下三個(gè)大類之一： 紅外線（ IR）局域網(wǎng) ：紅外線局域網(wǎng)的一個(gè)蜂窩只能限制 在一個(gè)房間里，因?yàn)榧t外線無法穿過不透明的墻。 不需要許可證的操作 ：用戶希望購買和運(yùn)行的是這樣的無線局域網(wǎng)產(chǎn)品，它們不需要專門為局域網(wǎng)所使用的頻帶而申請(qǐng)?jiān)S可證。 傳輸健壯性和安全性 ：除非涉及合理，無線局域網(wǎng)很容易受到干擾并且容易被竊聽。對(duì)于移動(dòng)用戶和自組無線網(wǎng)絡(luò)來說，可能需要滿足這個(gè)要求。以下是一些無線局域網(wǎng)最 終要的要求： 吞吐量 ：媒體接入控制協(xié)議應(yīng)當(dāng)盡可能地有效利用無線媒體以達(dá)到最大的容量。 自組網(wǎng)絡(luò) 自組網(wǎng)絡(luò)（ ad hoc work）是為了滿足某些即時(shí)需求而臨時(shí)而建立的一種對(duì)等網(wǎng)絡(luò)（沒有中央服務(wù)器）例如，有一群職員，每人帶著一臺(tái)膝上電腦或掌上電腦，會(huì)聚在商務(wù)會(huì)議室或課堂上。 漫游接入 漫游接入提供局域網(wǎng)和帶有天線的移動(dòng)數(shù)據(jù)終端之間的無線鏈接，如膝上型電腦和筆記本電腦。 建筑物的互連 無線局域網(wǎng)技術(shù)的另一種用途是鄰樓局域網(wǎng)之間 的連接，這些局域網(wǎng)可以是無線的也可以是有線的。在以上這些情況下，無線局域網(wǎng)向人們提供了一個(gè)有效且更具吸引力的選擇。大多數(shù)老建筑中已經(jīng)鋪設(shè)了足夠的 3 類電纜，而許多新建筑里則預(yù)埋了 5 類電纜。但是，無線局域網(wǎng)的這個(gè)動(dòng)機(jī)被以下一系列的事件打消。原因包括成本高、數(shù)據(jù)率低、職業(yè)安全方面的顧慮以及需要許可證。 this results in a foreshortened superframe period for the next cycle. 無線局域網(wǎng)技術(shù) 最近幾年，無線局域網(wǎng)開始在市場(chǎng)中獨(dú)霸一方。 and small offices where installation and maintenance of wired LANs is not economical. In all of these cases, a wireless LAN provides an effective and more attractive alternative. In most of these cases, an anization will also have a wired LAN to support servers and some stationary workstations. For example, a manufacturing facility typically has an office area that is separate from the factory floor but which must be linked to it for working purposes. Therefore, typically, a wireless LAN will be linked into a wired LAN on the same premises. Thus, this application area is referred to as LAN extension. CrossBuilding Interconnect Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a pointtopoint wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single pointtopoint link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN. Nomadic Access Nomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop puter or notepad puter. One example of the utility of such a connection is to enable an employee returning from a trip to transfer data from a personal portable puter to a server in the office. Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may move around with their portable puters and may wish access to the servers on a wired LAN from various locations. Ad Hoc Networking An ad hoc work is a peertopeer work (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop puter, may convene in a conference room for a business or classroom meeting. The employees link their puters in a temporary work just for the duration of the meeting. Wireless LAN Requirements A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs: Throughput. The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity. Number of nodes. Wireless LANs may need to support hundreds of nodes across multiple cells. Connection to backbone LAN. In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wirel