單項選擇題

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

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多項選擇題

Which two options are valid WAN connectivity methods?（）

A.PPP
B.WAP
C.DSL
D.Ethernet
E.L2TPv3

單項選擇題

At which layer of the OSI model dose PPP perform?（）

A.Layer 3
B.Layer 4
C.Layer 2
D.Layer 5