Remote dial-up services choice: Acme needs remote employees to dial in; the solution must support multiple routed protocols, user authentication, and callback to reduce long-distance charges. Which protocol is the best fit?

Introduction / Context:
Dial-up remote access historically relied on protocols that could carry multiple Network layer protocols, authenticate users, and offer enhancements like callback. Point-to-Point Protocol (PPP) became the de-facto standard.

Given Data / Assumptions:

• Remote employees connect over dial-up.
• Multiple routed protocols must be supported.
• User authentication is required.
• Callback support desired to control toll charges.

Concept / Approach:
PPP provides Link Control Protocol (LCP) for link setup and options (including callback) and Network Control Protocols (NCPs) for carrying multiple Layer-3 protocols (e.g., IPCP for IP, IPXCP for IPX back in the day). PPP also supports PAP and CHAP for authentication.

Step-by-Step Solution:

Verification / Alternative check:
Documentation for PPP lists multilink, authentication, encryption options, and callback control in various implementations.

Why Other Options Are Wrong:

• 802.1: IEEE category, not a specific dial-up protocol.
• Frame Relay: WAN service, not dial-up RAS protocol.
• HDLC: Cisco proprietary on serial links; lacks standardized authentication/callback/NCP suite.
• PAP: Only an authentication method; does not define the whole data link.

Common Pitfalls:
Confusing PPP (the link protocol) with PAP/CHAP (authentication mechanisms used by PPP); assuming HDLC suffices for multi-protocol RAS.

Final Answer:
PPP