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HDLC:
HDLC stands for High-Level Data Link Control protocol. Like the two other WAN protocols mentioned in this article, HDLC is a Layer2 protocol (see OSI Model for more information on Layers). HDLC is a simple protocol used to connect point to point serial devices. For example, you have point to point leased line connecting two locations, in two different cities. HDLC would be the protocol with the least amount of configuration required to connect these two locations. HDLC would be running over the WAN, between the two locations. Each router would be de-encapsulating HDLC and turning dropping it off on the LAN.HDLC performs error correction, just like Ethernet. Cisco’s version of HDLC is actually proprietary because they added a protocol type field. Thus, Cisco HDLC can only work with other Cisco devices.HDLC is actually the default protocol on all Cisco serial interfaces. If you do a show running-config on a Cisco router, your serial interfaces (by default) won’t have any encapsulation. This is because they are configured to the default of HDLC.
PPP:
Point to Point Protocol (PPP) it is used for most every dial up connection to the Internet. PPP is based on HDLC and is very similar. Both work well to connect point to point leased lines. The differences between PPP and HDLC are:PPP is not proprietary when used on a Cisco routerPPP has several sub-protocols that make it function.PPP is feature-rich with dial up networking featuresBecause PPP has so many dial-up networking features, it has become the most popular dial up networking protocol in use today. Here are some of the dial-up networking features it offers:Link quality management monitors the quality of the dial-up link and how many errors have been taken. It can bring the link down if the link is receiving too many errors.Multilink can bring up multiple PPP dialup links and bond them together to function as one. Authentication is supported with PAP and CHAP. These protocols take your username and password to ensure that you are allowed access to the network you are dialing in to.
At first need to consider3 types of WAN connections:
1. Leased Line: That is dedicated bandwidth between2 locations(eg.T1 and E1)
Protocol: HDLC
2. Circuit Switched: On Demand Bandwidth between2 locationseg dialup modems
Protocols: PPP, ISDN
3. Packet Switched: Shared but guaranteed between locations.
Protocols: Frame Relay,ATM
The major differences are:
a. HDLC is cisco propritery and PPP is not.It means you use HDLC all of your devices must be Cisco.
b. Usually PPP is used for dialup and HDLC is used for leased line between two points.
PPP is based on HDLC but it has some other features.Here are some of the dial-up networking features it offers:
Link quality management monitors the quality of the dial-up link and how many errors have been taken. It can bring the link down if the link is receiving too many errors.
Multilink can bring up multiple PPP dialup links and bond them together to function as one.
Authentication is supported with PAP and CHAP. These protocols take your username and password to ensure that you are allowed access to the network you are dialing in to.
HDLC only works on synchronous serial links
HDLC does not support authentication
HDLC is the standard on cisco devices for the encapsulation type over serial links
Cisco's implementation of HDLC adds a2 byte Type field to the header
HDLC does not support error detection
PPP does work on both synchronous and asynchronous
PPP allows for both PAP and CHAP authentication
PPP does support error detection only at layer2, error correction is done by a high level
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