tailieunhanh - Satellite networking principles and protocols phần 7

So với việc chậm trễ tuyên truyền, sự chậm trễ trong phân khúc mặt đất là không đáng kể. Đệm trong mô-đun phân khúc mặt đất có thể gây ra biến thể của sự chậm trễ, bị ảnh hưởng bởi tải giao thông trên bộ đệm. Hầu hết các biến thể được gây ra trong bộ đệm TIM-ATM. | 194 Satellite Networking Principles and Protocols Compared to the propagation delay the delay within the ground segment was insignificant. Buffering in the ground-segment modules could cause variation of delay which was affected by the traffic load on the buffer. Most of the variation was caused in the TIM-ATM buffer. It caused an estimated average delay of 10 ms and worst-case delay of 20 ms. Cell loss occurred when the buffer overflowed. The effects of delay delay variation and cell loss in the system could be controlled to the minimum by controlling the number of applications the amount of traffic load and allocating adequate bandwidth for each application. Satellite bandwidth resource management The TDMA system was used with frame length of 20 ms which was shared by the earth stations. Each earth station was limited to the time slots corresponding to the allocated transmission capacity up to maximum 960 cells equivalent to Mbit s . The general TDMA format is shown in Figure . There are three levels of resource management RM mechanisms. The first level is controlled by the network control centre NCC and allocates the bandwidth capacity to each earth station. The allocation is in the form of burst time plans BTP . Within each BTP burst times are specified for the earth station which limit the number of cells in bursts the earth stations can transmit. In the CATALYST demonstrator the limit is that each BTP is less than or equal to 960 ATM cells and the sum of the total burst times is less than or equal to 1104 cells. The second level is the management of the virtual paths VPs within each BTP. The bandwidth capacity that can be allocated to the VP is restricted by the BTP. The third level is the management of the virtual channels VCs . It is subject to the available bandwidth resource of the VP. Figure illustrates the resource management mechanisms of the bandwidth capacity. Each station is allocated a time slot within the burst time plan. Each