【正文】 監(jiān)聽(tīng)機(jī)制把傳輸?shù)臎Q定權(quán)分布到所有節(jié)點(diǎn)。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(wú)爭(zhēng)用服務(wù)。我們首 先考慮一個(gè)稱(chēng)為幀間間隔（ InterFrame Space,IFS）時(shí)延。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(tīng)媒體，如果媒體依然是空閑的，則可 以傳輸。在超幀時(shí)間的開(kāi)始部分，點(diǎn)協(xié)調(diào)器以循環(huán)方式向所有配置成輪詢(xún)的站點(diǎn)發(fā)布輪詢(xún)。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(zhēng)用的接入。點(diǎn)協(xié)調(diào)在發(fā)布輪詢(xún)時(shí)采用 PIFS。 2。否則，該站點(diǎn)必須等待直到當(dāng)前的發(fā)送結(jié)束。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。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。接入點(diǎn)的功能相當(dāng)于網(wǎng)橋。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。 不需要許可證的操作 ：用戶(hù)希望購(gòu)買(mǎi)和運(yùn)行的是這樣的無(wú)線(xiàn)局域網(wǎng)產(chǎn)品，它們不需要專(zhuān)門(mén)為局域網(wǎng)所使用的頻帶而申請(qǐng)?jiān)S可證。對(duì)于移動(dòng)用戶(hù)和自組無(wú)線(xiàn)網(wǎng)絡(luò)來(lái)說(shuō)，可能需要滿(mǎn)足這個(gè)要求。 自組網(wǎng)絡(luò) 自組網(wǎng)絡(luò)（ ad hoc work）是為了滿(mǎn)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(méi)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。 建筑物的互連 無(wú)線(xiàn)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(wú)線(xiàn)的也可以是有線(xiàn)的。大多數(shù)老建筑中已經(jīng)鋪設(shè)了足夠的 3 類(lèi)電纜，而許多新建筑里則預(yù)埋了 5 類(lèi)電纜。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。 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 wireless LANs, this is easily acplished through the use of control modules that connect to both types of LANs. There may also need to be acmodation for mobile users and ad hoc wireless works. Service area. A typical coverage area for a wireless LAN may be up to a 300 to 1000 foot diameter. Battery power consumption. Mobile workers use batterypowered workstations that need to have a long battery life when used with wireless adapters. This suggests that a MAC protocol that requires mobile nodes to constantlymonitor access points or to engage in frequent hand