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Leased lines

These are usually referred to as a point-to-point or dedicated connection. A leased line is a pre-established WAN communications path that goes from the CPE through the DCE switch, then over to the CPE of the remote site. The CPE enables DTE networks to communicate at any time with no cumbersome setup procedures to muddle through before transmitting data. When you’ve got plenty of cash, this is really the way to go because it uses synchronous serial lines up to 45 Mbps. HDLC and PPP encapsulations are frequently used on leased lines; I’ll go over them with you in detail in a bit.


High-Level Data-Link Control (HDLC)was derived from Synchronous Data Link Control (SDLC), which was created by IBM as a Data Link connection protocol. HDLC works at the Data Link layer.It wasn’t intended to encapsulate multiple Network layer protocols across the same link the HDLC header doesn’t contain any identification about the type of protocol being carried inside the HDLC encapsulation. Because of this, each vendor that uses HDLC has its own way of identifying the Network layer protocol, meaning each vendor’s HDLC is proprietary with regard to its specific equipment.

The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, bit-oriented,Data Link layer protocol. It specifies an encapsulation method for data on synchronous serial data links using frame characters and check sums. HDLC is a point-to-point protocol used on leased lines. No authentication can be used with HDLC. HDLC is the default encapsulation used by Cisco routers over synchronous serial links.


Point-to-Point Protocol (PPP)is a pretty famous, industry-standard protocol. Because all multi protocol versions of HDLC are proprietary, PPP can be used to create point-to-point links between different vendors’ equipment. It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol and allows authentication and multi link connections to be run over asynchronous and synchronous links.

Point-to-Point Protocol (PPP) remember that it’s a Data Link layer protocol that can be used over either asynchronous serial (dial-up) or synchronous serial(ISDN) media. It uses Link Control Protocol (LCP) to build and maintain data-link connections. Network Control Protocol (NCP) is used to allow multiple Network layer protocols(routed protocols) to be used on a point-to-point connection.

Well, the basic purpose of PPP is to transport layer 3 packets across a Data Link layer point-to-point link, and it’s non proprietary. Since PPP can encapsulate several layer 3 routed protocols and provide authentication, dynamic addressing, and callback, PPP could be the best encapsulation solution for you instead of HDLC.

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