Difference Between HDLC and PPP


Both HDLC and PPP are data link layer protocols. HDLC (High-Level Data Link Control) is a communication protocol used at the data link layer of computer networks, developed by ISO (International Organization for Standardization), and was created out of IBM’s SDLC (Synchronous Data Link Control). PPP is a data link layer protocol based on HDLC and is very similar to HDLC. Both are WAN (Wide Area Network) protocols and work well to connect point-to-point leased lines.

What is HDLC?

HDLC came in to existence only when IBM submitted SDLC to various standards committees and one of them (ISO) modified SDLC and created HDLC protocol. HDLC is regarded as a compatible superset of SDLC. It is a bit-oriented synchronous protocol. HDLC supports synchronous, full-duplex operation. HDLC has an option for 32-bit checksum and HDLC support the Point-to-point and Multipoint configurations. HDLC identifies “primary” node type, which controls others stations that are called “secondary” nodes. Only a primary node will control the secondary nodes. HDLC supports three transfer modes and they are as follows. First one is the Normal Response Mode (NRM) in which secondary nodes cannot communicate with a primary until the primary has given permission. Secondly, the Asynchronous Response Mode (ARM) allows secondary nodes to talk without primary’s permission. Finally, it has Asynchronous Balanced Mode (ABM), which introduces a combined node, and all ABM communication happens between these kinds of nodes only.

What is PPP?

As mentioned above, PPP is a data link layer protocol based on HDLC, and is very similar to HDLC. It is used for the direct communication between two nodes. Transmission encryption privacy, authentication and compression are provided by PPP. Authentication is provided by PAP (Password Authentication Protocol) and more commonly by CHAP (Challenge Handshake Protocol) protocols. It is used for various types of networks that are made up of different physical mediums such as trunk line, fiber optics, serial cable, cellular telephone and phone line. It is very popular among ISPs (Internet Service Providers) as a medium for providing the customers with dial-up access to the Internet. To provide DSL (Digital Subscriber Line) services to their customers, service providers use Point-to-Point Protocol over Ethernet (POPoE) and Point-to-Point Protocol over ATM (POPoA), which are two encapsulated forms of PPP. PPP is used for both synchronous and asynchronous circuits. It works with different network protocols such as IP (Internet Protocol), IPX (Internetwork Packet Exchange), NBF and AppleTalk. Broadband connections also use PPP. Although PPP was designed somewhat after the original HDLC specifications, PPP includes many additional features that had been available only in proprietary data link protocols at that moment of time.

Although, HDLC and PPP are very similar WAN data link layer protocols used for point-to-point communications, they do have their differences. Unlike HDLC, PPP is not proprietary when used in a Cisco router. Several sub-protocols make up the functionality of PPP. PPP is feature-rich with dial-up networking features and is used heavily by ISPs to provide Internet to their customers. Unlike HDLC, PPP can be used with both synchronous and asynchronous connections.