WAN Technologies (PPP, HDLC, WAN Access)
PPP
PPP is a layered data-link protocol: LCP establishes, configures and tests the link; NCPs (IPCP for IPv4, IPv6CP for IPv6) negotiate network-layer parameters; authentication is optional. Its phases are link dead → establish (LCP) → authenticate → network (NCP) → terminate. PAP is a two-way handshake sending the password in clear text; CHAP is a three-way challenge/response using MD5, so the password never crosses the link and re-challenges can occur. PPP supports multilink and loopback detection, which HDLC does not.
HDLC
HDLC is a simple, reliable-framing protocol with no authentication and no network-layer negotiation. Vendors extended it incompatibly, so HDLC links normally work only between devices of the same vendor. On Huawei VRP the default encapsulation on a serial interface is PPP, not HDLC - a classic question.
WAN access options
Leased lines (E1/CEP, serial) give fixed, symmetric bandwidth. xDSL rides the local copper pair to a DSLAM. PON (EPON/GPON) uses OLT–splitter–ONU with a shared fibre tree. Carrier Ethernet/MPLS VPN provides L2 or L3 VPNs with QoS, and SD-WAN overlays internet links with policy-based path selection. Match the technology to the requirement: authentication and dynamic addressing → PPPoE; fixed point-to-point → PPP/HDLC over leased line; branch interconnect with SLA → MPLS or SD-WAN.
Worked example
When a PPPoE client cannot come up, check the order: physical link and dialer rule, then LCP negotiation, then PAP/CHAP credentials, then IPCP address assignment - a wrong password stops the link at the authenticate phase, so display ppp interface reports an authentication failure rather than an IP problem.