Which encapsulation protocol is used to transmit data over serial links?

The High-Level Data Link Control (HDLC) protocol is used as the encapsulation method for serial links. This protocol is the open standard HDLC compared to the native Cisco proprietary version. The Point-to-Point Protocol over Ethernet (PPPoE) is widely used with digital subscriber lines (DSLs) and WiMax wireless services. Multiprotocol Label Switching is a WAN connectivity method. X.25 is a legacy WAN protocol used to transmit data.