Timeslot allocation

Dear colleagues, Could you please tell me when I am wrong in my assumptions? My story is about beacon channel. (fcch,sch,bcch) which is required for MS to find our network. OK? On the pictures it's drawn like we transmit BCCH on C0T0. So DL TS1-7 can be used for i.e. TCH. So if we want just to create bts which can be found by MS and that's all, we may not transmit on TS1-7 at all. OK? When we are talking about CalypsoBTS we say that we must transmit several burst in a row. So here I am a bit confused. So if we want to just transmit bcch messages we can transmit every TS0 and rest on TS1-7. Why not? Why I can see in manuals and videos pictures like Ttt_R_tt? Why do I need these dummy "t" bursts? Why one burst is "t" and another is "T"? Sure I understand that we must receive something for normal BTS operation like RACH and need some time to switch Tx/Rx. But why we just cannot use mask like T_R_T_R_ ...? I understood that we cant use mask TTT_RR because calypso phone was not hacked to receive two bursts in a row. Thanks in advance!