Difference between ppp0 vs wwan0

I am Running Raspian on a RaspberryPi. When I plug in a GSM modem I see two interfaces - *wwan0* & *ppp0*. - wwan0 exists even when the GSM modem is plugged but not connected - ppp0 exists only when the GSM modem is connected ### Questions 1. What is the difference between wwan0 and ppp0, and why do I see ppp0 in addition to wwan0? 2. Why is the IP address assigned to ppp0 and not wwan0 after a connection is established?