High-Level Data Link Control (HDLC) started in the 1970s. IBM Synchronous Data Link Control (SDLC) was created in the mid-1970s for Systems Network Architecture (SNA). SDLC is a bit-oriented synchronous data-link protocol that the ISO modified into HDLC. The Consultative Committee for Telegraph and Telephone (CCITT), now the ITU-T, modified HDLC to create Link Access Procedure (LAP) and Link Access Procedure Balanced (LAPB) for X.25. IEEE modified HDLC, too, for its IEEE 802.2.
HDLC is the default WAN encapsulation type between two Cisco devices. Example 9-12 clears the counters and displays the output of show interfaces serial 0 for r6.
r6#clear counters serial 0 Clear "show ...