802.11技術規格筆記

1.規格簡述

802.11a : Freq 5 GHz, Max speed 54 Mbps, PHY OFDM(Orthogonal frequency division multiplexing)

802.11b : Freq 2.4 GHz, Max speed 11 Mbps, PHY HR/DSSS(High Rate/Direct Sequence Spread Spectrum)

802.11g : Freq 2.4 GHz, Max speed 54 Mbps, PHY OFDM

 

2.IEEE 802.11 specification

Including 3 layer : LLC (Logical Link Control), MAC(Media Access Control), PHY(Phyical layer)

There are one MAC and two PHY layer in 802.11

Component : Station, Access point, Wireless media (RF), Distribution system (Bridge)

Network type (Basic Service Set) : 

  -> Independent BSS (ad hoc bss) : Peer to peer network

  -> Infrastructure BSS : Station type network

  -> ESS (Extended Service Set) : 以骨幹網路將BSS串聯在一起提供服務,並擁有相同的SSID (Service Set Identifier)

 

Wi-Fi Multimedia (WMM) and WMM Power Save modes. WMM adds Quality of Service (QoS) capabilities to Wi-Fi networks. WMM Power Save is an improvement to the 802.11e amendment adding advanced power management functionality to WMM. WMM Power Save is based on Unscheduled Automatic Power Save Delivery (U-APSD), which, like WMM, is based on Enhanced Distributed Channel Access (EDCA). U-APSD is a solution well suited for dynamic environments where Wi-Fi is generally deployed and allows clients supporting U-APSD to download all or some of the frames buffered by the access point during unscheduled service periods.

關於WME

考慮到基於時間的數據傳輸的QoS重要性，在IEEE正式批准802.11e標準(預計在2005年上半年)之前，已經有一些無線設備製造商開始在實施能基本滿足近期需要的QoS機制，這些大都是基於EDCF的，最有代表性的就是由Wi-Fi聯盟開發的WME(Wireless Mutimedia Enhancements)技術。WME技術並不是除802.11e標準之外的又一個標準，它實際上只是部份地使用了EDCF的標準，所以WME技術是802.11e的一個子集，或者說是在802.11e正式批准之前的過渡標準。一旦802.11e被批准，採用WME技術的設備都可以升級到802.11e標準。

另外，Wi-Fi聯盟還提出了無線計畫媒體(WSM，Wireless Scheduled Multimedia)技術，這是基於HCF機制的增強QoS的技術。Wi-Fi聯盟將初步使用WME，當802.11e標準最終批准之後才有可能採用WSM。