DLMS/COSEM HDLC LLC Sublayer & Information Frame (I-Frame) Sliding Window Flow Control
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High-Throughput HDLC Communication via Information Frames
For bulk data transfer over serial links, HDLC Information Frames (I-Frames) enable continuous bidirectional data transmission using sliding-window flow control.
I-Frame Control Field Format
Bit 0: 0 (Identifies I-Frame) Bits 1-3: N(S) - Send Sequence Number (0 to 7) Bit 4: Poll/Final (P/F) bit Bits 5-7: N(R) - Receive Sequence Number (0 to 7)When
window_size_transmit> 1 is negotiated in SNRM/UA, the client can transmit multiple I-frames consecutively without waiting for individual acknowledgments, drastically improving profile read throughput. -
If an I-frame is received with an out-of-order sequence number $N(S)$, the receiver responds with a Reject (REJ) supervisory frame (
Control: 0x09), instructing the transmitter to retransmit from frame $N(R)$.
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