Như vậy, số lượng tồn đọng n phụ thuộc vào tỷ lệ đến, cũng như số lượng các trạm đang hoạt động trong mạng. Công nghệ máy tính tốc độ giống như một xe trượt tuyết tên lửa trên một hồ nước đông lạnh. Một máy tính mà bạn mua ngày hôm nay là đảm bảo không bị lỗi thời trong ba năm, một con khủng long trong năm năm, | 246 Broadband Powerline Communications Networks 100 Network with fast resignaling Undisturbed Lightly disturbed J Heavily disturbe E D 10 iz Undisturbed Network without fast resignaling J Lightlyj disturbed 1 r Heavily dilsturbe 1 _ __ _ _ __ 50 100 150 200 250 300 350 400 450 100 150 200 250 300 350 400 450 Number of stations Number of stations Figure Mean signaling delay - rare requests average packet size 1500 bytes Integration ofARQ in Reservation MAC Protocols As described above in the case of reservation MAC protocols a network station starts transmission of data segments belonging to a user packet . IP packet by using a particularly allocated portion of the transmission resources. After a network station starts transmitting the data segments it can happen that one or more segments are disturbed. In previous investigations simple retransmission of the whole packet is applied if at least one segment of the packet is disturbed. However in communications systems with higher BER it is more efficient to retransmit smaller data units Sec. . Therefore ARQ is applied to retransmit erroneous segments and not the whole packet. In the case of Go-back-N ARQ mechanism the base station has knowledge of the number of requested segments and can discover if there are some erroneous or missing data segments on the receiving side. In this case it sends a negative acknowledgment NAK to the sending station including the sequence number of the last received segment. Thus the sending station has to retransmit only the data segments with the higher sequence number. If the Selective-Reject ARQ mechanism that achieves the best performance from among different ARQ mechanisms is applied the sending station retransmits only the erroneous data segment. Each of the ARQ variants described in Sec. can be applied together with reservation MAC protocols. However because of the applied per-packet reservation method the affected station is not .