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Difference Between HDLC and SDLC


HDLC (High-Level Data Link Control) and SDLC (Synchronous Data Link Control) are two protocols that provide point to multipoint interconnection between computers. The main difference between HDLC and SDLC is actually their origin. SDLC was developed by IBM for use with their computers. They eventually moved for it SDLC to be standardized by governing bodies like ISO and ANSI. ISO adopted SDLC but renamed it to HDLC but introduced a number of changes that make it distinct. Because of this, HDLC is actually a standard protocol that was used by many hardware makers while SDLC is not but is still used in some IBM hardware.

As an improvement over SDLC, ISO introduced a new feature to HDLC known as Asynchronous Balanced Mode, which is more commonly referred to as ABM. ABM is considered to be more superior, and is thus used more often, than the older Normal Response Mode (NRM) and Asynchronous Response Mode (ARM). ABM makes the master-slave relationship in other modes irrelevant. Either point can initiate the connection, unlike on the other modes where only the master can initiate a connection. HDLC also made it possible to use packet sizes that are multiples of bit octets. SDLC can only have packets that are 8, 16, 32, and so forth packet sizes. The ability to use differently sized packets give added flexibility in designing certain designs.

Aside from added features, ISO also decided to remove certain procedures and messages that they deemed to be unnecessary. One such message is the test message. This is used to determine that the lines are functioning correctly and that packets can be sent reliably across them. Despite that, HDLC is still considered to be a superset of SDLC.

SDLC is quite old and has since been replaced by HDLC and Advanced Data Communication Control Procedures or ADCCP, which is the version standardized by ANSI, due to the latter’s superiority. There are also a number of competing protocols that serve as alternatives to SDLC and HDLC.


1.HDLC is actually adopted from SDLC
2.HDLC is a standard protocol while SDLC is not
3.HDLC has the Asynchronous Balanced Mode feature while SDLC does not
4.HDLC supports frames that are not multiple of bit-octets while SDLC does not
5.HDLC removed some procedures that were present in SDLC

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1 Comment

  1. Word usage such as; “is considered to be more superior” and “in designing certain designs”, show the writer’s inability to properly articulate his ideas. He needs to work on his writing skills so that he will become “more better”.

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