無線網路之排程自動省電模式之重排演算法研究

Abstract

將可攜式的裝置與無線區域網路技術整合時的其中一個重大挑戰，即有限的電池壽命。通常手持電話的實體大小不會很大，而這些移動裝置的規模也限制了它的電力資源，所以將電力消耗達到最小化是一個關鍵的議題。IEEE 802.11e的推出了一個進階的省電架構，以改善802.11的Power Save Mode (PSM)，命名排程自動省電模式Scheduled Automatic Power Save Delivery (S-APSD)。在 S-APSD當中, Access Point (AP) 會固定週期定期地傳送緩衝在AP的資料給Station (STA) ，所以AP不需要發送poll給每個STAs。並且為了提高省電的效率，則AP必須避免Service Periods (SPs)重疊的情形發生。所以當排定Scheduled Events (SEs)的Service Start Time (SST)的時候，更要注意如何排程。 在OAS-APSD以及我們之前的研究，LCS-APSD，這兩者在選擇SEs的SST 時的做法皆是使用貪心法，因此只能達到局部的最佳解，甚至可能無法達到整體全局的最佳解。因此為了使省電的效能達到最好，我們提出了新的解決方法。我們的想法是重新安排現有的SEs以進一步減少SP 重疊的機率。並且從數值分析的結果發現，我們所提出的演算法更能提供更好的省電效能。

One of the major challenges to integrate the wireless LAN technology into portable devices is the limited battery life. Power consumption minimization is a critical issue since the size of mobile devices also limits its power resources. The IEEE 802.11e introduced an advanced architecture to improve 802.11 power saving management, named Scheduled Automatic Power Save Delivery (S-APSD). In S-APSD, the access point (AP) periodically delivers buffered traffic to PS Stations (STA) and therefore prevents the need to poll for each frame by STAs. It is clear that the AP has to avoid the overlapping of Service Periods (SPs) to increase the power saving performance. The OAS-APSD [3] and our previous work, LCS-APSD [4] select the Service Start Times (SSTs) of the Scheduled Events (SEs) in a greedy fashion for the S-APSD. However, to achieve the global optimum for power saving, it may require another method to obtain a better solution. Our idea is to rearrange the existing SEs to further minimize the chance of SP overlapping. From the numerical results, we find that the proposed scheduling algorithms provide better power saving performance.