即時嵌入式系統之混合模式軟體媒介層延遲評估平台

Abstract

近年來，由於下一代無線通訊的需求，投入SDR 的研究日益增加，SDR 是實體層中一項以軟體取代硬體功能的技術，而在SDR 之上的協定也被認為必須能夠在不同媒 體存取機制間適時地切換，SoftMAC 是一項可以滿足此需求的技術，其為一具有可透視性、彈性且低價的軟體元件，然而，IEEE 802.11 要求在一個傳輸中訊框間隔必須非常 精確，這限制了此協定無法完全以軟體實現，為了解決這個問題，我們需要一個可以同時傳送硬體和軟體回應的平台，如此便能夠量測兩者之間的時間差，由於時間緊要的控 制訊框通常是由硬體處理，而且只能在監聽模式下才可被軟體接收，因此，我們提出一個方法，在驅動程式中混合一般與監聽模式，再者，為了克服這個障礙，可利即時嵌入 式系統回應時間短的優點，然而，即時嵌入式系統的架構不同於個人電腦，其處理器未配有硬體亂數產生器，而作業系統核心也過於老舊無法支援mutex 銓鎖，我們發現硬體 亂數產生器可由軟體取代，而mutex 栓鎖可利用spinlock 代替，儘管這些取代方式在個人電腦上是可行的，但驅動程式仍無法如預期般的在嵌入式系統上運作，所以全部的實 驗皆實做在個人電腦上，實驗結果顯示軟體可在一般模式下接收控制訊框，我們鼓勵研究者投入即時SoftMAC，並在SoftMAC 上驗證新的協定而非使用模擬器，例如：ns simulator。

In the past few years, researchers have devoted to developing software defined radio (SDR) significantly due to the demand of next generation wireless communications. The technique replaces hardware components with software in the physical layer. The protocol above SDR is also considered to be able to alter adaptively for different media access control (MAC) mechanisms. SoftMAC, a transparent, flexible and low cost software component for data link layer MAC protocol, fulfills this objective. However, IEEE 802.11 requires rigid interframe space during a transaction. It restrains the protocol unable to be implemented completely in software. In order to cope with the requirement, we need a platform that delivers the responses both hardware-wise and software-wise to evaluate the time difference between the two approaches. Since the responses for time-critical control frame are usually manipulated by hardware, it can only be received by software in monitor mode. Therefore, we proposed a mechanism that mixes station and monitor modes in Linux wireless driver. Furthermore, to overcome the barrier, real-time embedded system is demanded for developing since it generates responses rapidly. Nevertheless, the embedded system architecture is quite different from desktops. The processor is not capable of hardware random number generator, and the operating system kernel is outdated that mutex lock is not supported. We discovered that the hardware random number generator can be replaced by software; mutex lock can be substituted by spinlock. Notwithstanding these replacements have been proved feasible on desktops, the driver still cannot work completely as expected on target machines. Thus, we have done the experiments totally on desktops. The experiment results show that control frames can be received by software in station mode. We encourage the researchers to involve in real-time SoftMAC and verify their designs on SoftMAC but not on simulator, e.g. ns simulator.