Figure 2 shows the concept of temporal synchronization

Figure 3 shows 3-layer synchronization reference model

Figure 4 shows 4-layer synchronization reference model

Data objects can be classified in the following way

Time dependent relational	Natural	Simultanous recording of voice and video through VCR
	Synthetic	A voice annotated slide show
Presentation and application lifetime	Persistent object	Exist for the duration of the application
	Non-persistent object	Created dynamically and discard when obsolete
	Transient object	Presented for a short duration
	Static object	Presented for the entire duration
Continuous or discrete	Continuous	Sequence of discrete data elements
	Discrete	Each discrete data element