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HDLC vs PPP | difference between HDLC and PPP

This page on HDLC vs PPP describes basic difference between HDLC and PPP protocols.

We know that unlike LAN which is used in a single organization, WAN is used to connect different organizations across the cities and countries. WAN stands for Wide Area Network. As there is a security concerns when data leaves the company premises and transport across the links between different networks, the data is encapsulated into frames before they cross the WAN link.

WAN Encapsulation Protocols

There are different networks exist in WAN. They use different encapsulation protocols. For example, leased line (between two routers) and circuit switched network use HDLC, PPP and SLIP protocols. Packet switched networks use X.25, Frame relay and ATM protocols.

HDLC-High Level Data Link Control Protocol

• HDLC operate at layer-2 i.e. data link layer. It is also used for synchronous PPP connections.
• It is bit oriented protocol.
• On transmit side, HDLC receives data from application and delivers it to the receiver on the other side of link.
• On receive side, HDLC accepts data and delivers to high level application layer.
• Both side of HDLC modules exchange control information, encoded into a frame.

HDLC Frame Structure

As shown in figure-2, HDLC Frame contains start flag, address, control, information, FCS, end flag. HDLC frame consists of start and end flags with pattern 01111110. Information contains data to be transmitted. FCS is Frame Check Sequence of 16 bit or 32 bit in size used for error detection. Control field indicates purpose or function of the HDLC Frame. It has three types viz. information frame, supervisory frame or unnumbered frame.

As HDLC uses standard pattern i.e. 01111110 to mark start and end of the frame, it may often confuse the receiver and hence to avoid this transparency problem bit stuffing is done. In bit stuffing, 0 is added after every five 1s at the transmit side and 0 is deleted at the receive side.

Broadly HDLC is a reliable protocol (uses selective repeat or go-back-N). It uses full duplex communication. It has flow control mechanism as it adjusts window size based on capability of the receiver. It uses physical layer clocking and synchronization to send and receive frames.

PPP-Point to Point Protocol

• It is a byte oriented protocol.
• PPP operate at layer-2 and layer-3. It uses HDLC frame format at layer-2 and uses IPv4 and IPv6 formats at layer-3. Other than IP, it supports other network layer protocols also.
• In addition, it also has built in security mechanism such as PAP and CHAP.
• It has CRC-16 field in the frame which helps in error control.
• It also suffers from transparency problem as it uses same start and end flags as used in HDLC frame.

PPP Packet Structure

PPP Frame consists of start flag, Address, control, protocol, info, check, end flag. Protocol indicates upper layer protocol to which frame is delivered viz. PPP-LCP, IP, IPCP etc. Info field carries upper layer data. Check field is CRC-16 used for error detection.

• PPP uses two protocols viz. LCP and NCP. LCP takes care of establishment of link. NCP makes sure that IP and other protocols are transmitted across the PPP link.

• PPP supports two authentication protocols viz. PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol) .
• PAP uses simple 2 way handshake used at initial link establishment phase. It is not strong authentication protocol as passwords are sent across in clear text.
• CHAP provides better authentication as it uses 3-way handshake mechanism.

In general PPP provides more features compare to HDLC but HDLC protocol is fast.

Tabular difference between HDLC and PPP

Following table mentions difference between HDLC and PPP protocols.

Operates at layer-2 (i.e. Data link layer) Operates at layer-2 and layer-3 (i.e. network layer)
bit oriented protocol byte oriented protocol
It does not have method to detect the errors. It uses FCS to detect the errors while transmitting the data.
HDLC protocols have two types viz. ISO HDLC and Cisco HDLC It uses HDLC format as defined by ISO.
It supports both synchronous and asynchronous links. Supports synchronous, asynchronous, HSSI(high speed serial interface), ISDN links
It used to perform encapsulation of data without using other encapsulation protocols. PPP can not encapsulate data without the help of other encapsulation protocols such as HDLC, SDLC(synchronous data link control)
It does not support authentication i.e. it fails to provide authentication between two nodes. It supports authentication using protocols such as PAP and CHAP
It provides a frame format which contains a proprietary field. The other 6 fields are similar to PPP protocol frame fields. ISO HDLC do not have proprietary field and hence has only 6 fields. It provides a frame format which contains a protocol field. The other 6 fields are similar to HDLC frame field.
It fails to check for quality of a link established. It uses link control protocol(LCP) to check for quality of the established link.
HDLC frame format PPP vs SLIP

Difference between SLIP and PPP

Refer SLIP vs PPP which mentions difference between SLIP and PPP protocols.

Networking related links

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What is an IP address
network switch
network bridge
Hub and Types of Hub
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