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principles of network and system administration phần 3

p / người sử dụng i nhu cầu chức năng. Đây là số lượng băng thông anh ta sẽ muốn N để mua nếu giá mỗi đơn vị băng thông được trang Theo giả định của chúng tôi trên u i, xi. p / giảm khi tăng p. Hãy để chúng tôi giả sử rằng ở một mức giá từ 0 khách hàng sẽ tổng hợp muốn mua nhiều hơn so với | 58 NETWORK TECHNOLOGY for the call. The network uses signalling to implement controls for accepting the call choosing the route of the virtual circuit or tree reserving resources and setting parameters in the tables of the intermediate switches. The virtual circuit may be visualized as a virtual pipe or branching pipe that is dedicated to the connection s traffic stream. Appropriate resources are allocated for this pipe so that the traffic stream receives the specified quality of service. As we will see these pipes may not require a fixed bandwidth. Instead they may inflate and deflate in time according to the bursts of data sent through the connection. Since such fluctuations cannot be determined a priori and occur on fast timescales their contract traffic parameters bound the maximum duration and frequency of such inflation and deflation and the maximum bandwidth consumed during each period of peak operation. These bounds are expressed by leaky buckets in the traffic contracts of the connections. The network uses statistical models for the behaviour of such pipes to decide how many can be handled simultaneously. These issues are investigated in Chapter 4 where we present a methodology for deriving effective bandwidths for such contracts. Signalling is the most complex part of ATM. It is common for network operators to disable ATM s full signalling or to use a simpler implementation. It is common to use only permanent virtual circuits or virtual paths bundles of virtual circuits which are set by network management rather than by signalling on customer request. These connections remain in place for months or years. They are mainly used to make permanent connections between the networks of an enterprise that has many physical locations or to connect Internet routers when Internet is run on top of ATM . This an example of a wholesale service in which bandwidth is sold in large contracts to large customers and other network operators ISPs . ATM specifies five native .