Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

Automated Meter Reading and SCADA Application 231IdleTokenWaiting for anwser(2) Response to rootResponse (1)(3) Token sentFig. 4. Polling scheme 2.Where– CT 1 - Collect Time for schedule 1.– N - Network nodes.– PTS - Polling Token Size.– AT S- Answer Token Size.Second collecting algorithm is shown in figure 4. An application layer in allthe nodes receives the order. This node passes the token to one of its children,and waits for a response. When it is received, (figure 4(1)) the node sends datato the root (figure 4(2)), and it passes the collect order to the next child (figure4(3)). Only when there are no more children to be polled, will the node answerwith its own data. Only one token is being passed between parent and children, sono path header information is necessary, and the answer token is simultaneouslyflowing to the root. To prevent a lost token, a timeout period guarantees thetoken passes to the next child.Using the previous scenario, we can compute an approximate collecting timevalue for that schedule. Neglecting initial transmissions and second order effects,the majority of transmissions are caused by answer token passing. Polling tokenpassing occurs simultaneously and so does not have any significant effect. In thisway:CharSizeCT 2 = N · ATS · NT ·(8)BaudRateThe ratio between CT 2 and CT 1 :CT 2CT 1=N · ATS · NTN · PTS · NT + N · ATS · NT = 11+ PTSAT SUsual PTS and AT S values allow a relative decrement of about 70 per cent.A greater reduction can be reached by using multiple tokens. If collisions or(9)