HDLC vs PPP - Key Differences Explained

This page highlights the core differences between HDLC (High-Level Data Link Control) and PPP (Point-to-Point Protocol).

We know that unlike a Local Area Network (LAN), which is typically used within a single organization, a Wide Area Network (WAN) connects different organizations across cities and countries.

Given the security concerns that arise when data leaves company premises and traverses links between different networks, data is encapsulated into frames before crossing the WAN link.

Several encapsulation protocols exist within WANs. For instance, leased lines (between two routers) and circuit-switched networks often use HDLC, PPP, and SLIP protocols. Packet-switched networks, on the other hand, might use X.25, Frame Relay, and ATM protocols.

HDLC - High-Level Data Link Control Protocol

• HDLC operates at Layer 2 (the data link layer). It’s also used for synchronous PPP connections.
• It’s a bit-oriented protocol.
• On the transmit side, HDLC receives data from the application layer and delivers it to the receiver at the other end of the link.
• On the receive side, HDLC accepts data and delivers it to the higher-level application layer.
• Both sides of the HDLC modules exchange control information, encoded within a frame.

As shown in the figure above, an HDLC frame contains a start flag, address, control, information, FCS (Frame Check Sequence), and an end flag.

HDLC frames begin and end with a specific flag pattern: 01111110. The Information field contains the data to be transmitted. The FCS is a 16-bit or 32-bit sequence used for error detection. The Control field indicates the purpose or function of the HDLC frame. It can be one of three types: an information frame, a supervisory frame, or an unnumbered frame.

Because HDLC uses the standard pattern 01111110 to mark the start and end of a frame, it can sometimes confuse the receiver. To avoid this transparency problem, bit stuffing is employed. In bit stuffing, a 0 is added after every five consecutive 1s on the transmit side, and this 0 is deleted on the receive side.

Broadly speaking, HDLC is a reliable protocol, often using selective repeat or go-back-N mechanisms. It supports full-duplex communication and includes a flow control mechanism to adjust the window size based on the receiver’s capabilities. HDLC relies on physical layer clocking and synchronization for sending and receiving frames.

PPP - Point to Point Protocol

• It’s a byte-oriented protocol.
• PPP operates at Layer 2 and Layer 3. It utilizes the HDLC frame format at Layer 2 and IPv4 and IPv6 formats at Layer 3. Besides IP, it also supports other network layer protocols.
• It features built-in security mechanisms such as PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol).
• It incorporates a CRC-16 field in the frame for error control.
• It also addresses the transparency problem by employing the same start and end flags as HDLC.

A PPP frame consists of a start flag, address, control, protocol, information, check, and end flag.

The Protocol field indicates the upper-layer protocol to which the frame is delivered (e.g., PPP-LCP, IP, IPCP). The Info field carries the upper-layer data. The Check field contains the CRC-16 value used for error detection.

PPP utilizes two core protocols: LCP (Link Control Protocol) and NCP (Network Control Protocol). LCP handles link establishment, while NCP ensures that IP and other protocols are transmitted across the PPP link.

PPP supports two authentication protocols: PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol). PAP uses a simple two-way handshake during the initial link establishment phase. It’s not considered a strong authentication protocol because passwords are sent in clear text. CHAP provides better authentication, utilizing a three-way handshake mechanism.

In general, PPP offers more features compared to HDLC, but HDLC protocol is faster.

Tabular Difference Between HDLC and PPP

The following table outlines the key differences between HDLC and PPP protocols.