A WAN link is generally a serial link, which means that the data is transmitted serially on the wire as electrical or optical signals depending upon the media type, and there is no contention as in Ethernet to set the channel to avoid any collision of frames.
The default data link encapsulation on a serial interface for a Cisco router is the High-Level Data Link Control (HDLC) protocol, which is a Cisco proprietary protocol for sending data on synchronous serial links. Cisco HDLC also provides a simple control protocol called Serial Line Address Resolution Protocol (SLARP) to maintain serial link keepalives, which helps in detecting the link state as being up or down.
Each router sends periodic keepalives ...
