Which Layer 2 protocol encapsulation type supports synchronous and asynchronous circuits and has
built-in security mechanisms?
A.
HDLC
B.
PPP
C.
X.25
D.
Frame Relay
Explanation:
High-Level Data Link Control (HDLC) – HDLC is the default encapsulation type on point-to-point,
dedicated links, and circuit-switched connections. It is used typically when communicating between
two Cisco devices. It is a bit-oriented synchronous data link layer protocol. Point-to-Point Protocol
(PPP) – Provides router-to-router and host-to network connections over synchronous and
asynchronous circuits. PPP was designed to work with several network layer protocols, such as IP,
and IPX. PPP also has built in security mechanisms such as PAP and CHAP X.25/Link Access
Procedure, Balanced (LAPB) – ITU-T standard that defines how connections between DTE and DCE
are maintained for remote terminal access and computer communications in public data networks.
X.25 specifies LAPB, a data line layer protocol. X.25 is a predecessor to Frame Relay. Frame Relay –
Industry standard, switched data link layer protocol that handles multiple virtual circuits. It is a nextgeneration to X.25 that is streamlined to eliminate some of the time-consuming processes (such as
error correction and flow control) that were employed in X.25.
