HDLC is used on point-to-pont serial links suc as T1s, DS3 and now

Packet-over-SONET (POS). It has a low overhead in terms of the

additional bytes required over and above the layer 3 payload. There is

no authentication protocol for HDLC.

PPP was and is heavily used in async dial-up and provide for

authentication using PAP or CHAP protocol. Now there is also PPP over

Ethernet (PPPoE).

A variation of PPP known as multi-link PPP (MLPPP) is quite popular for

loading balancing parallet circuit like T1's.

regards

shivlu