Difference between HDLC and SDLC

Oct 21, 2011 by

Difference between HDLC and SDLC

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  1. daniel

    SDLC and HDLC are two communication protocols used globally. SDLC is used in computer networks and it was created by IBM. HDLC is also a form of data link protocol that was created by ISO. Learn how these two communication protocols differ by reading this article.

    1. Development

    HDLC was created when IBM had submitted the SDLC to several standards committees. It is a synchronous protocol regardless of the simple fact that various aspects of SDLC have been omitted. HDLC is therefore considered like a better alternative of SDLC. The frame format of SDLC is also seen in HDLC. SDL C was created in 1975 by IBM to be largely utilized in SNA environments and it was the first ever of its type. In fact, it exceeded the character oriented and synchronous protocols in speed flexibility and efficiency.

    2. Support

    HDLC supports full duplex and synchronous operation just like SDLC. It has the option of 32 bit and it cannot support the Hub or Loop configurations, which are obvious differences from the earlier SDLC. Conversely SDLC supports various link technologies and types like unbounded and bounded media, full duplex and half duplex transmission capabilities, packet switched and circuit switched networks and also multipoint links.

    3. Transfer modes

    The major difference is that HDLC can support three modes of transfer as compared to one found in SDLC. The first mode is the NRM where secondary nodes are incapable of communicating with primary nodes until permission is granted. This is essentially the mode of transfer used largely in SDLC. Furthermore, there is the ARM that enables a secondary node to communicate even without permission from the primary. Lastly, the ABM introduces combined nodes where communication occurs between these specific nodes only.

    In short, HDLC has better functionality than SDLC and is therefore commonly used in network protocols.

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