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Broadband Powerline Communications Networks Design phần 8

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Để ổn định một giao thức ALOHA rãnh, nhiệm vụ của các thuật toán Pseudo-Bayesian để ước tính số lượng tồn đọng hoặc mạng lưới trạm n cố gắng để truyền tải các gói dữ liệu mới hoặc truyền lại các gói dữ liệu va chạm. Sau đó, mỗi gói tin sẽ được truyền với xác suất P = min {1, 1 / n}. | 188 Broadband Powerline Communications Networks During operation of a communications system the network conditions are permanently changed. This is caused by a varying number of subscribers that actively used the network service by various service using the network resources with a changing intensity dynamic traffic characteristics of different services varying activity of individual subscribers and so on. Particularly in a network operating under unfavorable noise conditions the available data rate in the network can frequently change in accordance with current disturbance behavior. All these factors directly influence the data transmission in a network and can cause degradation of QoS in the network. To reduce the possible QoS degradation in a network efficient CAC mechanisms Sec. 5.4.3 can be implemented to limit the number of admitted connections in the network e.g. users various data connections etc. . However in spite of the usage of such mechanisms the QoS degradation for particular services already admitted in a network has to be managed by the MAC layer. It is possible to control data throughput and transmission delays of the connections existing in the network by tuning parameters of the MAC protocols in accordance with the current network conditions. Also by a control of the transmission delays it is possible to influence blocking and dropping probability as well as the packet losses. Thus if the QoS degradation for a particular connection or user or service is observed this connection has to be preferred until its QoS level becomes satisfied. Of course the privileged connection must not be carried out to handicap other connections in the network. The temporary preferential treatment of connections with the degraded QoS can be ensured by assigning them to a service class with higher priority for a while. So the same mechanisms discussed for the contention and the arbitration protocols for the priority realization described above can be applied for the .