Synchronous Data Link Control (SDLC) Loop Operation

Loop configuration is basically a communication link that is used to transmit data or information from one link station to another link station conferring the entire loop. At transmit port of the primary station, link connection gets initiated, connects one or more than one secondary station in a serial fashion, and after then ends back at the receive port of the primary station. In the following, we can see the loop that contains total of five secondary stations.



SDLC Loop Operation :

  1. Primary Station Transmitting :
    Primary station basically transfers data frames or packets to one or more secondary stations. Each of the frames that is transmitted or transferred contains address of station to which the frame is directed i.e., secondary station. When data frames get transmitted successfully by the primary station, it then follows the last flag of the last frame along with total eight consecutive logic 0’s i.e., known as turnaround sequence and then followed by continuous logic 1’s i.e., known as go-ahead sequence (01111111).



    In turn, every secondary station decodes the address field of each data frame and then removes these data frames addressed to them. Secondary station is basically presented as repeaters for any of the down-line secondary stations. They pass data frames to down-loop stations. This station basically uses CFGR command to cause many diagnostic operations to be performed by a secondary station.





  2. Secondary Station Transmitting :
    Secondary station cannot perform task on their own. They cannot transmit or transfer any of the data frames of their own unless and until they receive a data frame with P bit set. The initial or first secondary station that basically receives a data frame addressed to it with the P bit set alters or converts the seventh logic 1 in the go-ahead sequence to a logic 0, therefore developing a flag. If anyhow it does not convert the go-ahead sequence that is received after the turnaround into a flag, then secondary station lose the opportunity to transmit or transfer data frames.



    Each and every response frame from secondary stations are generally put or inserted just after the turnaround sequence or after a secondary response from an up-loop station. The cycle is only completed successfully when primary station receives its own turnaround sequence, a series of response frames, and also go-ahead sequence. Secondary station only has to stop sending the transmission to abort or end the transmission without any requirement of its own abort sequence. During the transmission, if secondary station receives eight contiguous 0-bits, it should abort or end or terminate its transmission and this termination is known as shut-off sequence. Primary station is responsible for establishment of this shut-off sequence.



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