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principles of network and system administration phần 7

Những ưu và khuyết điểm của chương trình bằng cách sử dụng các phép đo tốt hơn là gì? Một mặt, phí chính xác hơn có thể phản ánh việc sử dụng hiệu quả và trong ý nghĩa này được công bằng hơn. Mặt khác, có tăng chi phí cho đo lường và kế toán. | 208 CHARGING GUARANTEED SERVICES What are the pros and cons of schemes using finer measurements On the one hand the charge may more accurately reflect effective usage and in that sense be fairer. On the other hand there are increased costs for measurement and accounting. The obvious question is whether such costs are justified by the accuracy of the resulting tariff this accuracy being reflected into providing better price stability and fairness. Experimental results suggest that a substantial improvement is achieved by the simple time-volume tariffs compared to the flat rate tariffs and that further refinements may not be worth the added complexity. Many network operators even consider the measurement of time and volume a burden they would rather avoid. 8.3.3 An Example of an Actual Tariff Construction We consider in more detail the construction of tariffs of the form aT bV for simple contracts that specify only the peak rate h of the source. Let aon-off m be the value of the effective bandwidth computed using 8.12 and showing the dependence on m where for simplicity we take t 1. We provide formulas for the coefficients a m and b m . Since these are coefficients of the tangent to aon-off - at m simple algebra gives esh - 1 b m h 7 T a m on-off m - mb m 8.13 s h m esh 1 J The tariff fm M a m b m M is shown in Figure 8.1. M VIT is the measured average rate of the connection. As we have said fm M is a rate of charge that depends upon the rate M. Hence over a period T the charge is T X a m b m M a m T b m V. It is not essential to provide the user with a continuum of tariff choices the function on-off m may be well approximated by a small number of tangents say q especially if the capacity C is large compared to the peak rate h. Observe that m simply labels a linear function and that the presentation of tariff choices may be entirely couched in terms of a miI h s t and b miI h s t i 1 . q where we now also make explicit the dependence of the tariff coefficients on the