 congestion between Node B and the RNC, which requires careful planning of the lub interface. A careful flow control mechanisms needs to be introduced in order to avoid a strong reduction of data rates due to ATM discards, which would generate RLC retransmissions and TCP window reduction. The understanding in 3GPP is that Node B is in control of the flow, SO it will send capacity allocation messages to the RNC. Node B knows the status of the buffers in the RNC from the capacity request, and uses this message to modify the capacity at any time, irrespective of the reported user buffer status. Two messages are defined for lub flow control, as shown in Figure B.5. The HS-DSCH capacity request procedure allows the RNC to request HSDSCH capacity by indicating the user buffer size in the RNC for a given priority level (CmCH-PI, or common transport channel priority indicator). The HS-DSCH capacity allocation is used by Node B to allocate resources for a given flow. It includes a number of parameters: the number of MAC-d PDUs that the RNC is allowed to transmit for the MAC-d flow (HSDSCH credits), the associated priority level indicated by the CmCH-PI, the maximum MAC-d PDU length, the time interval during which the HSDSCH credits granted may be transmitted (HSDSCH interval) and the number of subsequent intervals that the HSDSCH credits granted may be transmitted (HSDSCH repetition period). Node B CAPACITY REQUEST CAPACITY ALLOCATION Figure B.5 Flow control messages in lub Handset Capabilities Table B.4 HSDPA UE capabilities Max number Inter-TTI Maximum Number of Maximum Supported of HSPDSCH interval TB size (bits) soft bits data rate Category modulations codes (memory size) (Mbps) 16-QAM and QPSK 19 200 16-QAM and QPSK 28 800 16-QAM and QPSK 28 800 16-QAM and QPSK 38 400 16-QAM and QPSK 57 600 16-QAM and QPSK 67 200 16-QAM and QPSK 14411 115 200 16-QAM and QPSK 14411 134 4 400 16-QAM and QPSK 172 800 16-QAM and QPSK 27 952 172 800 14400 There are different possible approaches for the flow control algorithm, but they nee