▁It ▁exists ▁in ▁the ▁user ▁plane ▁only . ▁It ▁is ▁located ▁above ▁the ▁Radio ▁Link ▁Control ▁( RL C ), ▁a ▁layer ▁ 2 ▁responsible ▁for ▁mapping ▁logical ▁channels . ▁It ▁is ▁similar ▁to ▁ 8 0 2 . 1 1 ' s ▁L LC ▁layer ▁which ▁supports ▁mult im ode ▁operations ▁and ▁it ▁works ▁in ▁three ▁different ▁modes : ▁ ▁a ) ▁Trans parent ▁▁ ▁b ) ▁Un ack now led ged ▁data ▁transfer ▁▁ ▁c ) ▁A cknow led ged ▁data ▁transfer . ▁Its ▁main ▁function ▁is ▁to ▁deliver ▁" Cell ▁Broadcast " ▁messages ▁to ▁its ▁upper ▁layer ▁such ▁as ▁NAS . ▁ ▁Other ▁functions ▁specified ▁in ▁[ 3 G PP ▁T S ▁ 2 5 . 3 0 1 ] ▁are : ▁ ▁Storage ▁of ▁Cell ▁Broadcast ▁Mess ages . ▁ ▁Tra ffic ▁volume ▁monitoring ▁and ▁radio ▁resource ▁request ▁for ▁CBS . ▁ ▁Sch ed uling ▁of ▁B MC ▁messages . ▁ ▁Trans mission ▁of ▁B MC ▁messages ▁to ▁UE . ▁ ▁D elivery ▁of ▁Cell ▁Broadcast ▁messages ▁to ▁upper ▁layer ▁( N AS ). ▁ ▁Ex cept ▁for ▁broadcast / mult ic ast ▁it ▁oper ates ▁in ▁transparent ▁mode ▁as ▁per ▁[ B MC - ST D ]. ▁On ▁the ▁u pl ink ▁B MC ▁it ▁requires ▁Un ack now led ged ▁mode ▁of ▁data ▁transfer ▁from ▁R LC . ▁ ▁References ▁▁ ▁[ B MC - ST D ] ▁ 3 G PP ▁T S ▁ 2 5 . 3 2 4 ▁V 8 . 0 . 0