In order for the communication shown in the figure above to function, the data formats must be "packed" into one another:

• the protocol information of the individual layers is stored around the actual user data - from the highest to the lowest layer.
• The protocol information consists of a header (e.g. with address information) and, for some protocols, a trailer.
• During the data flow through the individual layers, the data is "packed" on the sending communication system and later "unpacked" on the receiver in reverse order.