SDLC - Synchronous Data Link Control
The SDLC protocol was developed by IBM to allow IBM hosts to communicate over WAN connections to a mainframe.
SDLC can be used with point-to-point and multipoint connections between remote hosts and a central mainframe.
Cisco routers can interface with the mainframe and pass SDLC traffic via point-to-point serial links.
Cisco routers can encapsulate SDLC data frames into IP datagrams so that SDLC communications can traverse non-SDLC networks.