Basic Frame Structure of SDLC

Synchronous Data Link Control (SDLC) is generally linked layer protocol that is used with Systems Network Architecture (SNA) environment. SNA is proprietary networking architecture of IBM that is developed in 1974. SDLC also supports huge variety of typologies and different types of data links.

Examples include point-to-point links, multipoint links, switched networks, packet networks, etc. It also uses primary station-secondary station model of data communication. On an SDLC data link, all of data and control transmission are simply organized and managed in specific format that is basically known as transmission frame. Transmission frame is also known as SDLC frame or just a frame.



Transmission frame simply carries or transfer control information and data of user among transmitting or sending station and receiving station. It allows and gives permission to receiving station :

  • To determine starting and end of frame.
  • To identify whether frame is made or intended for that particular station.
  • To identify actions that are needed to be performed with received information or data.
  • To detect and identify error occurred during transmission in received frames.
  • To acknowledge its frames receipt to transmitting station.

Frame Format :
Transmission frame of each of SDLC has specific frame format. Each frame is made up of following fields :




  1. Flag Field –
    Flag (F) is beginning frame that represents beginning of frame. This field is used to initiate and terminate occurrence of error by regular checking.

  2. Address Field –
    Address (A) filed follows just after beginning flag. It is used to identifies and determines secondary station that is transmitting frame. This is done because frame contains information or data regarding group address, specific address. Broadcast address, etc.

  3. Control Field –
    Control (C) field follows just after address field. It is used to specify functions of particular frame.
    This field can be present in three types of format as given below :

    • (i). Unnumbered (U) format :
      It is required to perform various functions such as to establish disconnect link, to report some procedural errors, to transfer or transmit data especially when location or address of data in frame sequence is not needed to be checked.

    • (ii). Supervisory (S) format :
      It is required to perform various functions such as to acknowledge received frames, to convey ready or busy conditions, report frame numbering errors, etc. This format does not contain any information field.

    • (iii). Information (I) format :
      It is required to perform various functions such as to transfer data or information, to control sequence in which frames are needed to sent and number of frames.
  4. Information Field –
    Information (I) field follows just after control field. This field is an optional field that mainly contains information data. The data is needed to be transmitted on data link is present in this field.

  5. FCS Field –
    Frame Check Sequence (FCS) field follows just after information field. This field especially allows and grants permission to receiving stations to simply ensure and check transmission accuracy of frame. This field simply checks received frame for any kind of error that might have been occurred by link connection.

  6. Ending Flag Field –
    This field indicates ending of frame.

Attention reader! Don’t stop learning now. Get hold of all the important CS Theory concepts for SDE interviews with the CS Theory Course at a student-friendly price and become industry ready.

My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.


Article Tags :
Practice Tags :


Be the First to upvote.


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.