nel for the HSPDSCH, and may be allocated a data rate of 64, 128 or 384 kbps. The primary modulation is QPSK and the SF can be from 4 to 512. Soft handover is supported for both DPCHs. B.3.2 MAC Layer Split As a result of new functionalities to be carried by Node B in HSDPA, the MAC layer is split into two entities. While MAC-d remains in the RNC in the same way as for R99, MAC-hs is located in Node B to allow rapid retransmission of NRT data. Main Changes in HSDPA MAC-d is responsible for mapping between logical channels and transport channels, selection of and appropriate transport format and handling priorities. It also has to identify UEs in the common transport channels and multiplex/demultiplex upper layer PDUs and to measure the traffic volume. Ciphering for the transparent mode RLC is also managed by MAC-d. MAC-hs is responsible for packet scheduling, link adaptation and layer 1 error correction and retransmission (HARQ). Due to this split in the functionality of the MAC layer, the user data buffers, which used to be in the RNC, are moved to Node B. This makes the introduction necessary of a flow control mechanism in the lub interface, in order to avoid the buffer overflow and throughput degradation due to buffers becoming empty. MAC-d schedules the number of RLC PDUs according to the credits granted by MAC-hs at each interval of 10 ms, and the aggregated rate of the HSDPA connections is controlled by the rate control implemented in MAC-hs. The MAC-d PDUs are framed into FP-HSDSCH frames, while a maximum number of 16 MAC-d flows per BTS are supported. B.3.3 Adaptive Modulation and Coding (AMC) Scheme Link adaptation (LA) is the key feature to the success of HSDPA, since there is no power control in HSPDSCH, and it is used to adapt HSPDSCH to different radio conditions. If LA does not work properly, cell capacity is lost and other techniques such as fast scheduling will not work. Link adaptation is done by changing the modulation and number of codes. The UE signals informa- tion to the network about the hi