CCNA®: Cisco® Certified Network Associate: Fast Pass, Third Edition

8.1. Describe different methods for connecting to a WAN

Basically, Cisco just supports HDLC, PPP, and Frame Relay on its serial interfaces, and you can see this with the encapsulation ? command from any serial interface (your output may vary depending on the IOS version you are running):

Corp#config t
Corp(config)#int s0/0/0
Corp(config-if)#encapsulation ?
  atm-dxi      ATM-DXI encapsulation
  frame-relay  Frame Relay networks
  hdlc         Serial HDLC synchronous
  lapb         LAPB (X.25 Level 2)
  ppp          Point-to-Point protocol
  smds         Switched Megabit Data Service (SMDS)
  x25          X.25

Understand that if I had other types of interfaces on my router, I would have other encapsulation options, like ISDN or ADSL. And remember, you can't configure Ethernet or Token Ring encapsulation ...