Page "High-Level Data Link Control" Paragraph 11

A mildly different version is also used as the control channel for E-carrier ( E1 ) and SONET multichannel telephone lines.

Some vendors, such as Cisco, implemented protocols such as Cisco HDLC that used the low-level HDLC framing techniques but added a protocol field to the standard HDLC header.

More importantly, HDLC is the default encapsulation for serial interfaces on Cisco routers.

It has also been used on Tellabs DXX for destination of Trunk.