Common WAN protocols include PPP, HDLC, and Frame Relay. There are many others, of course, but you need to focus on those included in the CCNA syllabus.
Point-to-Point Protocol (PPP) can be used when you have a Cisco device connecting to a non-Cisco device. PPP also has the advantage of including authentication. It can be used over a number of connection types, including DSL, circuit-switched, and asynchronous/synchronous connections.
Cisco’s High-Level Data Link Control (cHDLC) is its implementation of the open standard version of HDLC. HDLC requires DTE and DCE and is the default encapsulation type on Cisco routers (serial interfaces). Keepalives are sent from the DCE in order to check link status.
As already discussed, Frame Relay is a packet-switching technology which has become less popular in recent years, as DSL has become both more affordable and more readily available. It works at speeds from 56Kbps to 2Mbps and builds virtual circuits every time a connection is required. There is no security built into Frame Relay (but see Farai’s comment below). Frame Relay will be covered in more detail later.
Farai says – “Frame Relay commonly uses Permanent Virtual Circuits (PVCs), which are always present, although it can use Switched Virtual Circuits (SVCs), which are created on demand. A PVC is a type of Virtual Private Network (VPN). However, some people run PPP over Frame Relay (PPPoFR) to allow for PPP security for Frame Relay connections.”