he triggering of the upgrade or downgrade for a user connection. The decision should be made based on the activity of the user connection, which can be observed through monitoring the utilization of the RLC buffer located at both RNC and UE sides. The measurement of buffer utilization is illustrated in Figure 3.10. The buffer utilization is monitored based on a configurable time interval (e.g. multiples of 500 ms), which is denoted as "Monitoring Period". The time scale of the monitoring period is divided into frames. In each frame the RLC buffer content is observed. If the RLC buffer is empty within a frame, then it is counted as empty, otherwise as used. Buffer utilization is defined as the ratio between the used frames and all frames. The triggering of RAB upgrade or downgrade is determined by the thresholds of the buffer utilization: if the buffer utilization is above the upgrade threshold value, e.g. 80%, then the system assumes that the corresponding user's activity is high and therefore tends to upgrade its current RAB to the next higher RAB rate from the available RAB set given in the above table. If the buffer utilization is below a downgrade threshold, e.g. 20%, then the system assumes that the user's activity is low and then tends to downgrade to the next lower RAB rate. used frame (RLC buffer is not empty) empty frame (RLC buffer is empty) N frames Monitoring period Figure 3.10: Evaluating the Buffer Utilization Furthermore, in order to avoid frequent BRA triggering a "Waiting Time" is executed between two subsequent upgrades or downgrades. The waiting timer starts after an upgrade or downgrade, and before the timer expires no more upgrades or downgrades of the RAB rate are allowed. Upgrade and downgrade events in uplink and downlink are triggered separately, i.e. there are independent RAB triggers on uplink and downlink. If upgrade in one link direction, either uplink or downlink, also requires an upgrade in the other link direction according to the defined RAB settings on both uplink and downlink gi