单项选择题
Which one of the WAN data-link protocols below is used for out-of-band signaling?（）
A. HDLC B. NCP C. PAP D. LAPD E. LAPB F. None of the above
多项选择题 Which of the following correctly describe characteristics of PPP?（）
多项选择题 What are some common layer 2 encapsulations for a WAN link?（）
单项选择题 Which WAN service utilizes two different encapsulation types (one for data and one for signaling) on its data link layer?（）
