MAC Access Modes

最新推荐文章于 2025-03-26 16:50:58 发布

RichardYSteven

最新推荐文章于 2025-03-26 16:50:58 发布

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分类专栏： WLAN 文章标签： access function service random each 网络

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本文介绍了WLAN中三种主要的接入无线介质的方式：DCF（分布式协调功能）、PCF（点协调功能）和HCF（混合协调功能）。DCF是基于标准CSMA/CA访问机制的基础，类似于以太网，在发送前检查无线链路是否空闲，并采用随机退避来避免碰撞。PCF提供了无竞争的服务，通过特定的点协调器确保介质无冲突地使用，通常位于接入点内。HCF允许站点维护多个服务队列，平衡无线介质的访问以支持更高质量的服务需求。

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Access to the wireless medium is controlled by coordination functions。对WLAN来说，station接入无线网络有三种方式。

DCF （ distributed coordination function ）一般都用这种

The DCF is the basis of the standard CSMA/CA access mechanism. Like Ethernet, it first checks to see that the radio link is clear before transmitting. To avoid collisions, stations use a random backoff after each frame, with the first transmitter seizing the channel. In some circumstances, the DCF may use the CTS/RTS clearing technique to further reduce the possibility of collisions.

PCF （ point coordination function ）

The point coordination function provides contention-free services. Special stations called point coordinators are used to ensure that the medium is provided without contention. Point coordinators reside in access points, so the PCF is restricted to infrastructure networks. To gain priority over standard contention-based services, the PCF allows stations to transmit frames after a shorter interval. The PCF is not widely implemented and is described in Chapter 9.

HCF （ hybrid coordination function ）

Some applications need to have service quality that is a step above best-effort delivery, but the rigorous timing of the PCF is not required. The HCF allows stations to maintain multiple service queues and balance access to the wireless medium in favor of applications that require better service quality. The HCF is not fully standardized yet, but is being produced as part of the eventual 802.11e specification.