Which Layer 2 protocol encapsulation type supports synchronous and asynchronous circuits and has built-in security mechanisms?

C

The correct answer is C. PPP (Point-to-Point Protocol).

PPP is a Layer 2 protocol that is commonly used for connecting remote networks over serial interfaces, such as synchronous and asynchronous circuits. It provides a standardized method for encapsulating packets over a point-to-point link and supports a range of network layer protocols, including IP, IPX, and AppleTalk.

One of the advantages of PPP is that it has built-in security mechanisms. For example, it can use authentication protocols like PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol) to verify the identity of the remote peer before allowing network traffic to pass. This helps prevent unauthorized access to the network and ensures that only authorized devices can communicate over the link.

PPP also supports features like error detection and correction, link quality monitoring, and link configuration negotiation. These capabilities make it a reliable and flexible protocol for connecting remote networks and devices.

In comparison, HDLC (High-level Data Link Control) is a similar Layer 2 protocol that is widely used in Cisco networking environments. However, it is primarily designed for use on synchronous links and does not have the same level of flexibility and security features as PPP.

X.25 is an older protocol that was widely used for connecting remote networks over leased lines. However, it has largely been replaced by newer technologies like Frame Relay and PPP.

Frame Relay is a Layer 2 protocol that is commonly used for connecting remote networks over high-speed, packet-switched networks. While it has some security and quality of service features, it is generally less flexible and less secure than PPP, which is why PPP is often preferred for point-to-point connections.