ID phát sóng gửi tin nhắn (u, Pmax) tại tối đa truyền tải điện năng Pmax 2b. Hàng xóm phát hiện khi nhận được tin nhắn (v, Pmax) từ nút v N (u) = N (u) ∪ {v} xác định cường độ tín hiệu nhận được và lưu trữ thông tin này 3. Trao đổi danh sách đã ra lệnh cho các nước láng giềng sau khi tất cả các tin nhắn từ | 168 LEVEL-BASED TOPOLOGY CONTROL COMPOW is based on the idea of using a common power level for the nodes which is the minimum power such that the resulting network is fully connected. Although this idea is neat from a theoretical point of view and it leads to the design of a relatively simple protocol for joint TC and routing it has the disadvantage of setting the nodes transmit power on the basis of a global property of the communication graph that is connectivity. As thoroughly discussed in Chapter 9 relying on global network properties should be avoided in distributed TC in order to reduce the message overhead needed to build and maintain the communication graph and to design a protocol that can quickly react to changes in the network topology. The case of COMPOW well illustrates this point. As for the message overhead caused by COMPOW we observe that proactively maintaining one routing table for each power level requires a considerable message exchange the overhead increase with respect to the case of no TC only one routing table is maintained is h-fold where h is the number of power levels used by the wireless card equipping the nodes in practical situations h can be as high as 10 . Although the authors of Narayanaswamy et al. 2002 argue that the message overhead due to the need of maintaining multiple routing tables is a marginal fraction of the available IEEE bandwidth we believe that in practice dealing with this considerable message overhead might be problematic. The problem of slow reaction to topology changes in COMPOW is more dramatic. Consider the situation depicted in Figure at a certain instant of time t the nodes in the network use the common power level P2 which is the minimum level necessary to u P3 u V P2 P .P2 P2 P2 t e t 2e Time Figure Problems caused by the slow propagation of new neighbors information in COMPOW at time t network nodes use the common power level P2 only the power level of node v and of its immediate .
