tailieunhanh - AD HOC NETWORKS Technologies and Protocols phần 9

xem xét các vấn đề của thiết bị đầu cuối ẩn do không nghe RTS / CTS tin nhắn. Trong kịch bản, cho B bắt đầu một truyền D. Sau đó, E có thể bắt đầu truyền dữ liệu với G hoặc ngược lại. Lưu ý rằng mặc dù B có thể được trong phạm vi định hướng của nút G, | Medium Access Control with Directional Antennas 209 Figure . A Scenario to Understand the Problems with DMAC consider the problem of hidden terminals due to unheard RTS CTS messages. In the scenario let B initiate a transmission to D. Subsequently E might initiate a data transmission to G or vice versa. Note that even though B might be in the directional range of node G it does not receive this CTS message. Thus upon the completion of its communication with D B might attempt a transmission to node G thereby causing a collision at E. Note that carrier sensing does not help here since B cannot physically sense the communications between E and G. Thus when involved in directional communications a node might miss out on hearing some of the RTS or CTS messages. Upon the completion of its communication it might initiate new transmissions that would interfere with the communications related to the missed RTS CTS messages. The second problem that we consider is the problem of hidden terminals due to asymmetry in gain. In order to discuss this problem we once again refer to the example in Figure . We consider an example wherein node B iniates a communication with node E. The handshake is achieved by the exchange of a DRTS and a DCTS message from B and C respectively . If A is in the omnidirectional reception mode it is possible that it does not hear the DCTS message sent by node E. Note that the total antenna gain in this case is G Go- Once the data communication between nodes B and F begins let us assume that node A wishes to initiate a communication with node B clearly it is unaware of the communication already in progress . Node A now sends an RTS directionally in the direction of node B. Node E s antenna is beamformed to receive in the direction of A. The antenna gain between nodes A and E is now G2d since both the transmission and the reception are directional. Thus it is possible that 210 Use of Smart Antennas in Ad Hoc Networks node A s signal now reaches node