HDLC

HDLC HDLC (High Level Data Link Control)- HDLC was created by the ISO, but it was implemented different by each individual company. So each device may not be totally compatible. HDLC provides support for both PPP and multipoint services over synchronous serial data links and ISDN interfaces. HDLC is the default serial encapsulation method on Cisco routers. There are four different transfer modes that HDLC supports, they are:

NRM (Normal Response Mode): Allows a secondary device to communicate with the primary device when the primary device requests it.

ARM (Asynchronous Response Mode): Allows either primary or secondary device to initiate communication.

ABM (Asynchronous Balance Mode): Allows a device to work in what is called a combined mode which means it can works as either the primary or secondary device.

LAPB (Link Access Procedure, Balanced): Allows circuit establishment with both DCE and DTE.

HDLC (High-Level Data Link Control) Default encapsulation used by all Cisco routers over synchronous serial links. HDLC is bit oriented and operates on the Data Link layer it also is a low overhead connection oriented, point to point protocol. A draw back of HDLC is that it won’t connect to other routers, HDLC implementation is proprietary. Vendors do this because they needed a way to encapsulate multiple network layer protocols. So what do you do if you do need to connect to another router. You then use PPP. In the next section we will go over PPP.