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The standards of IEEE 802.16 define the unsolicited grant service (UGS) for real-time applications of stringent delay requirement. In this paper, using a discrete-time GI-D-c model to describe the behaviors of UGS in 802.16 networks is presented. The performance metrics of mean number of PDUs, PDU queueing delay, and bandwidth utilization subject to the change in traffic load or allocated bandwidth are investigated. The results verify that if the required bandwidth of the real time traffic is more than the granted capacity, the mean number of PDUs in the queue will grow unexpectedly as the traffic becomes saturated. As more granted bandwidth is offered, the queue length will keep empty until the traffic load is significant. Once the load becomes saturated, the queue is vast filled immediately. Similar performance indication for PDU queueing delay can be derived under various bandwidth allocations. The experimental results also show that the bandwidth utilization is linearly increased along with the traffic load.