Some basic question from iDirect HUB teleport.

i am a new hub Station Engineer. we are using iDx 3.1. I have
some question.

1. what is frame length? (we see from iBuilder that Frame length is 125ms. What does it means?) 
2. How does iDirect calculate its time slots?
3. iDirect 3.1 introduce 2D-16 State FEC type instead of TPC. When we create B/W profile  we see some FEC Type.

(like, 2D16S-170byte 3/4, 2D16S-100byte 3/4) what is the significant of 170 byte and 100 byte.

Thanks is Advance.