lub interface, and protect the lub link from congestion collapse and overload. Flow Control (FC) In the case of HSDPA, as the traffic is mainly transmitted from the RNC to the Node B over the lub link, the transmission buffer (MAC-hs buffer) in the Node B requires flow control function to be applied between RNC and Node B for controlling the amount of traffic each user allowed to send to the base station to avoid buffer overflow. The purpose of flow control is to allocate the lub resources for each individual user according to their available air interface throughput. The user under good radio conditions can get more lub allocation as the data move fast through the air interface and thus the filling level of the MAC-hs queue remains low. On the other hand, when the buffer starts to get filled due to poor radio conditions, the flow control will try to slow down the transfer speed for that user. The flow control mechanisms are implemented in MAC-hs and FP layers. The flow control mechanism uses the frame protocol for the purpose of flow control over the lub interface and adapts the per-user lub flow rate to the air interface capacity. The MAC-hs user buffers at the Node B have to be monitored continuously to guarantee the data availability and avoid buffer overflow. Flow control is not a part of the 3GPP specifications. So there are a variety of flow control schemes used by different vendors and network operators. Two example flow control schemes are [WTG+06, WL*06]: 1. ON/OFF flow control; 2. Credit based flow control, i.e. Provided Bit Rate (PBR) based Flow Control (also called Enhanced Flow Control). The ON/OFF flow control mechanism is considered to be the most simple mechanism which can be applied for the purpose of flow control for HSDPA traffic on the lub. The flow control monitors the filling levels of the MAC-hs buffers for each user flow. It uses two thresholds to control the MAC-d flow rate over the lub. When the upper limit is exceeded data flow over the lub link is stopped and when the buffer limit rea