At the minimum level most networks sends individual packets of data and the network needs each packet to follow an exact format dictated by the hardware, which is known service paradigms or Interface paradigms.
There are two kinds of service paradigms:
CONNECTION-ORIENTED:
It is same as the telephone system: endpoints create and maintain a connection as long as they have data to interchange.
CONNECTIONLESS:
Same as postal system: endpoints put data to transmit into a packet and deliver to network for delivery.