This object represents the method used to group VCs into
an STM-1 signal. Applicable only to SDH.
au3Grouping: STM1<-AU-3<-TUG-2<-TU-12<-VC12 or
STM1<-AU-3<-TUG-2<-TU-11<-VC11.
au4Grouping: STM1<-AU-4<-TUG-3<-TUG-2<-TU-12<-VC12 or
STM1<-AU-4<-TUG-3<-TUG-2<-TU-11<-VC11.
The initial value is au3Grouping(2) for SDH and
notApplicable(1) for SONET.