单项选择题
Which layer 2 protocol encapsulation type supports synchronous and asynchronous circuis and has built-in security mechanisms?（）
A.Frame Relay B.HDLC C.x.25 D.PPP
多项选择题 Which two options are valid WAN connectivity methods?（）
单项选择题 At which layer of the OSI model dose PPP perform?（）
单项选择题 Which protocol is an open standard protocol framework that is commonly used in VPNs,to provid secure end-to-end communication?（）
