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 next-generation 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.