tailieunhanh - Communication Systems for the Mobile Information Society phần 9

WLAN mặt khác không có một giải pháp thanh toán tiêu chuẩn hóa. Điều này là do thực tế rằng đối với nhiều kịch bản như thế nào cho nhà và sử dụng văn phòng, mà các tiêu chuẩn WLAN ban đầu được hình thành, không có hóa đơn là cần thiết. | 284 Communication Systems for the Mobile Information Society As several subscriber stations might be necessary to forward packets from a distant device the mesh base station requires the help of all subscriber stations between itself and the sender of a packet to ensure QoS attributes like a guaranteed bandwidth or latency. QoS thus has to be ensured on a packet-by-packet basis. Each packet contains QoS service parameters in the header from which a receiving subscriber station can deduct how to handle the packet . how quickly it has to forward the packet to the next hop. This ensures that packets with higher priority are sent first in case several packets are waiting to be transmitted. Mesh network devices use a slightly different addressing scheme than devices in a standard network as shown in Figure . Each subscriber station has a 16-bit node ID. To form the mesh a connection is established to all subscriber stations each using a unique 8-bit link ID. Afterwards a broadcast message is sent over all links to inform the neighboring devices of the node ID and the number of hops that separate the subscriber station from the mesh base station. Adaptive Antenna Systems In order to minimize the costs of network deployment the transmission capacity of a base station should be as high as possible to serve as many users as possible. In practice the capacity of a base station is limited by factors such as the available bandwidth per base station modulation and coding schemes interference caused by neighboring base stations as well as the distance of the wireless clients. Capacity can be increased if subscribers are not moving and directional antennas are installed on the rooftop pointing into the direction of the base station. In this case lower power and better coding schemes can be used by the base station compared to moving subscribers with small omni-directional antennas. While systems like UMTS HSDPA and CDMA1x allow subscribers to roam freely the .

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