RQ, etc. Figure A.6 and Figure A.7 illustrate the user plane protocol architecture of HSDPA and HSUPA [3GP05b, 3GP06a], highlighting the HSDPA/HSUPA-specific additions and their location in the protocol stacks. As shown in Figure A.6, HSDPA introduces a new MAC entity, MAC-hs (hs for high speed), at the base station (Node B). The RNC retains the MAC-d (d for dedicated), but the only remaining function is transport channel switching and all other MAC functionalities such as scheduling and priority handling are moved to the MAC-hs located in the Node B which resides close to the radio interface. The main reason is that the scheduler is able to exploit rapid variations in the interference level and the channel conditions, and HARQ with soft combining also benefits from the rapid retransmissions as this reduces the cost of retransmissions. The RLC layer, however, stays mainly unchanged. It is worth mentioning that even though HSDPA has introduced fast physical layer retransmissions using HARQ at the Node B, there is still the RLC layer handling the retransmissions in case the physical layer operation fails transmission in RLC acknowledge mode (AM). Using RLC unacknowledged mode (UM), retransmissions only take place in the physical layer. An example is a VoIP call where the RLC layer transmissions from the RNC would take too long. A.9 Introduction for HSPA (HSUPA/HSDPA) MAC-d MAC-d HS-DSCH HS-DSCH MAC-hs MAC-hs Layer 2 Layer 2 Physical Physical Layer Layer Layer 1 Layer 1 Node B Figure A.6: HSDPA protocol architecture Figure A.7 shows the HSUPA protocol architecture and highlights the main modifications done in the MAC layer compared to the previous release. To allow UMTS system to support the HSUPA enhancements, a new MAC entity, the MAC-e, is introduced in the UE and Node B. In the Node B, the MAC-e is responsible for support of fast hybrid ARQ retransmissions and scheduling, while in the UE, the MAC-e is responsible for selecting the data rate within the limits allocated by the scheduler of the Node B MAC-e. For si