改進的IEEE 802.11分散協調退讓機制

Abstract

在現今無線設備需求的增長下，無線區域網路 (WLAN) 的使用已變得更為廣泛。而現今較廣為使用的區域無線網路標準為 802.11。此標準訂定了兩種無線設備存取傳輸介質的行為模式：分散協調機制 (Distributed Coordination Function) 及點協調機制 (Point Coordination Function)，分別使用在競爭或非競爭的情形下。 而在競爭模式下，分散協調機制主要是使用一個退讓時間 (backoff time) 做存取的依據。每個無線設備只有在傳輸介質閒置了一段固定的時間加上退讓時間後才可傳輸資料。此退讓時間是由競爭窗口 (Contention Window) 隨機選出，而競爭窗口就決定了退讓時間的範圍。當傳送發生碰撞時，無線設備必須將其競爭窗口加兩倍，以加倍過後的競爭窗口來選出重新傳送的退讓時間。但這種每次發生碰撞時即放大其競爭窗口的方式對傳輸量會造成相當的不良影響。因此我們提出了一個新的退讓機制，利用減緩無線設備的競爭窗口放大過程來提升傳輸量。我們利用 NS-2 來模擬我們提出的新機制，實驗結果顯示，當無線設備的數量少時，此新機制與傳統的分散協調機制不分軒輊；但在無線設備的數量增加時，新機制的傳輸量則明顯優於傳統機制。這種新機制的方式也相當簡單，使得它相當容易實作在真正的無線設備上。

With a rapid growing rate of wireless requirements, wireless LAN (WLAN) is becoming more and more popular. The main standard which is used today for WLAN is IEEE 802.11 standard. The standard defined two protocol functions in both contention-based and contention-free structures: Distributed Coordination Function (DCF) and Point Coordination Function (PCF). The major coordination way in contention-based structure is using a mechanism called backoff timer. After the channel idle for a period of time plus a backoff time can a mobile station transmit data frame. The backoff time is randomly chosen from a contention window (CW) that defines the range for the backoff time. While a transmission collision happened, the station doubles its CW in order to avoid another collision. But it is not efficient to double the CW every time the channel has collision. So we proposed a new backoff mechanism allowing stations to transmit data with slow increasing CW, use it to enhance the throughput. We use NS-2 to simulate the proposed mechanism, and the results show that the throughput of this new mechanism is the same as DCF when the station number is small, but is better than DCF when the station number becomes larger.