Data call in with mISDN & Asterisk


I set up one Asterisk with mISDN,and let others using the data call in feature so that can go to the internet through it,Now I finished it successfully,it can  let users dial in with ppp encapsulation,get the ip address and browse the internet.
Anyway,a little confused:

As I used PPP,why must I set the HDLC=yes in the configuraion?

We know that both PPP and HDLC are layer 2 protocol.Is it just a asterisk magic switch or the data encapsulation is really using HDLC way?

Thanks in advance.