在非排程自動省電模式中調整工作站的喚醒時間

Abstract

隨著無線區域網路的普及，各種不同的應用也隨之出現，舊有的IEEE 802.11 無法無線多媒體對於品質服務 (QoS) 的需求， 因此提出IEEE 802.11e ，對原有的MAC 進行增修，滿足了即時服務的要，也針對原先舊有的 IEEE 802.11 省電機制提出改善，分別於Enhanced Distributed Channel Access (EDCA) 與 Hybrid Controlled Channel Access (HCCA) 中提供非排程省電自動傳 送模式 (U-APSD)，及排程省電自動傳送模式 (S-APSD) 兩種省電機制。

在舊有的IEEE 802.11 省電模式，接收Beacon 訊框，察看是否有資料需要接收時，當要傳送或接收資訊時，先要傳送PS-Poll 通知Accesses Point (AP) 才能開始，但Beacon 的間隔約為100 ms，並不適合需QoS 資料的應用，IEEE 802.11e 的省電機制改進了此項缺點，使得QoS 的需求在省電模式下一樣可以滿足外，也強化了省電效能，對擁有無線網路功能並採用非排程省電自動傳送模式的行動裝置，如手機與個人行動助理，能獲得更長的使用時間。

在非排程省電自動模式傳送中，由於沒有排程的緣故，當系統服務多個工作站時，容易有overhearing 或碰撞的情形發生，本篇論文針對非排程自動省電模式，提供一個調整喚醒時間機制，藉由調整每個工作站的喚醒時間，減少工作站傳輸資料時所需的清醒(awake)的時間，並避免之後可能發生碰撞的機會，對於擁擠的系統，或是喚醒時間接近的工作站，提供更好的省電效能與通道的使用效率。

For the population of WLAN, there are several kind of different services are applied in WLAN. The legacy IEEE 802.11 is lack of supporting QoS service for wireless multimedia communication. So IEEE 802.11e, which modifies MAC protocol and new power save mode called Automatic Power-Save Delivery (APSD), is proposed for QoS service. There are two kind of APSD in 802.11e which are Scheduled APSD (S-APSD) used in Hybrid Controlled Channel Access (HCCA) and Unscheduled APSD (U-APSD) used in Enhanced Distributed Channel Access (EDCA).

In legacy 802.11 power save mode, STA should wake up periodically to listen Beacon frame to know if there are frames to receive. When STA want to transmit or receive frames, it should send PS-Poll to AP first. For QoS data, it is too long to wait for a Beacon interval which is about 100 ms. In APSD, AP will tell the ‘Service Start Time’ and ‘Service Interval’ to the STA. STA can know when to wake up according to these information and don’t list to Beacon frame anymore. For mobile devices like phones and personal digital assistants (PDAs), APSD can make mobile devices has longer lifetime.

In U-APSD, there is no scheduling for wake up time so more collisions and overhearing happen when system is congestion. We propose a scheme by delaying wake-up time of STAs to reduce overhearing and the collision probability. This can provide better power efficiency and enhance channel efficiency when STA number is larger and wake-up times of STAs are close.