does not mean the same data rate being used on the lub and Iu-PS for that particular user. Because in the HSPA network, the radio resources and code are shared by all users in the cell, thus the average bit rate for a single user in a loaded cell is clearly lower. Thus the average traffic on the lub interface is usually less than the peak rate on the radio interface. For example, with HSDPA the radio interface peak rate supported by the UE is 7.2 Mbps, but the service data rate over the lub interface can be limited, e.g. to 1 Mbps. Therefore, the buffering in the Node B is required to allow high peak rate of the air interface (over a short period of time) while limiting the bit rate on the lub interface in line with the leased transport network bandwidth and the negotiated QoS parameters. Since HSDPA and HSUPA enable the mobile users to transmit data with high peak data rate over a short time period, this certainly also leads to a bursty nature of the traffic on the lub interface. Moreover, due to the increased user throughput and system capacity offered by the HSPA to transfer high-speed packet data, the carried traffic amount and traffic variations on the lub interface are also much higher than that in the UMTS Rel99 system. It is important to have a sufficient, non-congested transport at the lub interface. To fulfill this target, it is necessary to apply traffic control techniques on the lub interface to pursue for maximum bandwidth utilization while guaranteeing the QoS to the end user and also reduce the operator A.9 Introduction for HSPA (HSUPA/HSDPA) cost. The next section introduces HSDPA and HSUPA specific traffic control functions which aim to improve the transport network performance, and prevent overload situations on the lub interface. HSPA-Specific Traffic Control Functions HSDPA and HSUPA introduce two essential traffic control schemes to apply in the lub interface: flow control (only for HSDPA) and congestion control techniques. They are designed to reduce the burstiness of carried traffic on the 