I can't say if it is a time division model, as it isn't multiplexing as such.
To the best of my knowledge, sub-ifs are dependant upon different frame type (ethernet) or different DLCI (frame relay) etc... Generally speaking, multiple encapsulation can be used on a physcial interface and each encap divided into logical interfaces. There is no "timeslot" that a packet falls into when sending/receiving on a sub-if.
Mick.
