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Seven Control Elements are defined 

4 for DL

•Timing Alignment (8bits): Sent to provide initial and periodic time synchronization to the UE for UL

•DRX Command (8 bits): Initiates discontinuous reception mode at UE

•UE Contention Resolution Identity (48bits): Used during RACH procedure to resolve possible contention b/w multiple UEs trying to simultaneously access the network 

•MCH Scheduling Information MAC Control Element

3 for UL

•UE Buffer Status Reports (8 or 24bits): Reports UE buffer occupancy for UL scheduling

•UE Power Headroom (8 bits): Reports UE transmit power compared to maximum or if the UE is currently power limited

•C-RNTI (16 bits): Identifies a UE when sending information over CCCH