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Different Transmission States of SDLC

  • Last Updated : 05 Aug, 2020

Synchronous Data Link Control (SDLC) is generally discipline for serial-by-bit information transmission over channel of data communication. Whether switched or non-switched, physical communication channel is always considered to be constant once it is been established or developed. Communication channel over this transition channel is basically assumed to be not permanent. On this communication channel, there are basically three different types of transmission state that can be present or exist.

SDLC might exist in one of these transmission states as given below :

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Let us understand in detail :

  1. Transient State –
    This state usually arises when station is preparing itself to transfer and not yet reached steady state. This state also follows the poll of secondary station and come before the actual transfer of control data or information. This state also exists before and after initial transmission and also after each line turnaround.


  2. Idle State –
    The circuit is basically considered to be in Idle state by secondary station just after 15 or more consecutive logic 1s that must have been received. There is also no transmission of control information or data during this state. A link connection configuration that is being used is required to determine and identify the particular link station action in this state as given below :

    Link ConnectionPrimary Link StationSecondary Link Station
    Half-duplex point-to-pointCarrier offCarrier off
    Duplex point-to-pointAIl 1’sAIl 1’s
    Half-duplex multipointCarrier offCarrier off
    Duplex multipointAIl 1’sCarrier off


  3. Active State –
    It is a non-idle, non-transient state. In this state, there is possibility of transmission of control information and data. This state basically presents whether primary or one of the secondary stations is transferring information or control signals. In the following figure, it is clear that there is no exchange of information or data, but the line is held in this state.

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