extra timeslots are the timeslots which are not occupied by OML, RSL or TCH.
if those TS are not “extra” then they are nothing, they are lost. You might as well define them as extra, don’t you think?
if you have 2 Abis links, i think you can have much more than only 2 extra TS.
recommendation: this is a dimensioning job ! nobody is doing that in your company ?
ok, I recommend you to define 6 extra timeslots per BTS.
… what do you do with that number…? Please let me know.