| Abstract: | 在多步的無線隨建即連網路(Multihop Wireless Ad Hoc Networks)環境中，隱藏節點(Hidden Terminal)問題對以競爭方式為設計要點的MAC機制，可說是影響甚大。眾所熟知的解決隱藏節點問題為RTS與CTS封包交換機制。然而隱藏節點問題仍存在某些運作場景中。例如在IEEE 802.11多步的無線隨建即連網路中，若兩個無線通訊裝置(Mobile Host)彼此相互隱藏，且亂數倒數計數不同，RTS控制封包仍可能發生碰撞。若運作場景為高流量或是密度較高的多步的無線隨建即連網路此控制封包碰撞問題將顯更加嚴重。此外電量控制機制(Power Control)可用於節省無線通訊裝置的電量消耗。然而若是無線通訊裝置採用剛好的電量通訊，則隱藏節點問題仍將會發生。甚至在以時槽分割(TDMA)為基礎的無線隨建即連網路中，如果無線通訊裝置選擇不恰當的時槽作為傳輸資料，隱藏節點問題仍會發生。因此本論文將在以下三種場景中著重探討。第一：IEEE 802.11多步的無線隨建即連網路。第二：以電量控制機制為基礎的多步無線隨建即連網路。第三：以時槽分割為基礎的無線隨建即連網路。
在IEEE 802.11多步的無線隨建即連網路中，RTS封包的碰撞不但會造成接下來的CTS與ACK封包的碰撞，甚至造成網路傳輸被抑止的現象。在本論文中我們提出了一個只需一個頻道(Channel)與收發器(Transceiver)的改善機制。此一改善機制可大幅的改善RTS碰撞的問題，進而減少重傳資料時所需的成本。
在以電量控制機制為基礎的多步無線隨建即連網路中，電量控制機制是一普遍的節省無線通訊裝置的電量消耗並進而延長無線隨建即連網路的生命週期(Lifetime)機制。本論文提出並驗證了一個因採電量控制機制所引發的隱藏節點問題。此外本論文亦提出一機制以解決因採電量控制機制所引發的隱藏節點問題。
在以時槽分割為基礎的無線隨建即連網路中，如果無線通訊裝置只參考一步鄰居的時槽使用資訊來做時槽選擇的依據，則無線通訊裝置將可能選擇到不恰當的通訊時槽，進而造成隱藏節點問題。本論文亦提出一時槽限制策略以決定通訊對所需的時槽。
如上所述，在多步的無線隨建即連網路上隱藏節點問題是一重要的議題。雖然RTS與CTS交換機制用於解決隱藏節點問題，然而隱藏節點問題仍會發生於IEEE 802.11多步的無線隨建即連網路、以電量控制機制為基礎的多步無線隨建即連網路、以時槽分割為基礎的無線隨建即連網路中。此外本論文亦提出了改善機制以解決在此三種場景中所發生的隱藏節點問題。在未來我們將著重於另一著名問題曝露節點(Exposed Terminal)問題，以進一步的改善網路效能，並在傳輸上能更有效率與更貼近現實網路環境。
Hidden terminal problem is a notorious problem in contention-based MAC protocol for multihop wireless ad hoc networks. RTS/CTS exchange is the well-know solution to hidden terminal problem. However, hidden terminal problem still happens in some particular situations. For example, in IEEE 802.11 ad hoc network, for two mobile hosts (MHs) hidden from each other, RTS frames are also possible to collide with each other, even that their backoff counters are different. The situation will be getting worse in high traffic load or in a dense network. On the other hand, power control is a common and popular mechanism used to save energy for MHs in wireless ad hoc networks. Unfortunately, hidden terminal problem is very likely to happen if the exactly power is used to transmit between the sender and the receiver. In addition, on TDMA-based mobile ad hoc networks, if an MH selects an unsuitable slot to send, the hidden terminal problem will occur as well. As a result, the major focus of this dissertation is to solve hidden terminal problem for the three environments: 1) IEEE 802.11-based wireless ad hoc networks, 2) power-control-based wireless ad hoc networks, and 3) TDMA-based mobile ad hoc networks.
In IEEE 802.11-based wireless ad hoc networks, RTS collisions not only result in the following CTS or ACK collisions, but also induce false blocking problem, even dead locks of transmissions. In the dissertation, an improvement mechanism protocol is also devised, which only uses a single channel and one transceiver to reduce RTS collisions. This improvement mechanism provides a type of fast collision detection and decreases the probability of RTS collisions, which is benefit for RTS/CTS exchange scheme. Meanwhile, this improvement mechanism can reduce the retransmission cost and have lower control overhead than that of IEEE 802.11 DCF.
Power control mechanism which can save power consumption of station and prolong the network lifetime is a common technology in power-control-based wireless ad hoc networks. However, collisions are very likely to happen if exact power level is used. Thus, the dissertation identifies a power control induced hidden terminal problem and takes the interference range into consideration to propose a collision avoidance power control MAC scheme which uses the appropriate power to exchange packets, instead of the exact power in order to resist the interference of other stations for avoiding interference due to the reduced power in power control mechanism.
On TDMA-based mobile ad hoc networks, if an MH depending on one-hop neighboring information may select an unsuitable slot to send, the hidden terminal problem will occur as well. In this dissertation, a Slot Inhibited Policies is proposed to determine which slots are valid to use in a communication link.
As mentioned above, hidden terminal problem is a serious and important issue in multihop wireless ad hoc networks. Although RTS/CTS exchange mechanism is used for avoiding hidden terminal problem, hidden terminal problem may still happen in IEEE 802.11-based, power-control-based, or TDMA-based ad hoc networks. Consequently, this dissertation proposes some mechanisms to avoid the hidden terminal problems in these three particular situations. In the future, we will study and focus on another famous problem, exposed terminal problem, to improve the network performance. Thus, the protocols will be more effective and practical for real situations. |
