一種用於802.11s無線網狀網絡中的新式防撞設計

Abstract

Wireless Mesh Networks (WMNs) have emerged as the next generation of backbones networks in Wireless Local Area Network (WLAN). IEEE has therefore been on the front edge to develop a standard for WMN, which they did by adding a new amendment in the current 802.11 standard: the 802.11s amendment. 802.11s allows traditional wireless access points to act as router at the layer 2 level, by carrying and routing data from devices to gateways, normally out of range of these devices. 802.11s comes with its own routing protocol (HWMP) as well as some new features such as EDCA for channel access to the medium and MCCA, an optional feature that allows nodes to reserve and coordinate their access to the medium in order to avoid collision among them. However, since MCCA is optional, not all nodes within the mesh network have it enabled. This can results in nodes being unaware of any reservations made by other nodes, thus trying to access the channel at the same time. This disparity can create collision and reduce the mesh network performance. We propose a new solution to this issue, by creating different planes of communications that will use different non-overlapping channels to transmit data. In every plane, all nodes should have the same features enabled. Therefore nodes without MCCA enabled will not interfere with nodes with MCCA. In that way we can avoid these collisions and hope to increase the network performance by a factor of 3. Since a new solution brings some new trade-offs, tests have also been made to quantify these trade-offs and see their impacts on the performance.

無線網狀網絡(Wireless Mesh Networks, WMN)已經成為無線局域網路(WLAN)的下一代骨幹網路。電子電機學會(IEEE)也因此成為無線網狀網路標準制定的先驅，而他們制定的標準，是透過在目前的802.11標準中添加一個新的修正：802.11s。 802.11s中允許傳統的無線接入點(AP)在網路第二層作為路由器，將數據由設備端路由至閘道器端(通常超出設備本身的通訊範圍)。 802.11s有自行規範的路由協議（HWMP），以及一些新的功能，如EDCA(通道接入)以及MCCA(一個選擇性功能，允許節點保留並協調其通訊，以避免節點間的碰撞。 然而，由於MCCA是選擇性的，不是所有網狀網絡中的節點都啟用此功能。這可能使得某些節點完全不知道任何其他節點保留的通道，從而試圖在同一時間訪問通道。這種資訊落差會造成碰撞，並減低網狀網絡的性能。 針對此議題，我們提出了一種新的解決方法，通過建立不同的通訊平面(plane)，並使之使用非重疊的不同通道來傳輸數據。在每一個通訊平面中，所有節點應該啟用相同的功能。因此沒有MCCA功能的節點不會干擾有MCCA的節點。這樣一來，我們就能避免這些碰撞，並希望能提高網絡的性能為三倍。由於新的解決方案帶來了一些新的權衡，我們也做了一些測試，來量化這些權衡和對性能的影響。