I'm building a set of blocks to implement the HDLC link-layer functions for a spacecraft communication system. These blocks deal with IP packets from/to the stack on one side, and a bitstream to/from the modulator/demodulator on the other side.
The receive block is already done and working. It takes a bitstream from the demodulator, finds the frame, un-bitstuffs it, extracts the IP packet payload and shoves it into the network stack. But I've run into a snag doing the transmit part. HDLC is a synchronous serial protocol. It has to keep clocking bits out at a fixed rate no matter what. When there are no packets to transmit, it outputs "flag" bytes (0x7E) continuously until there is more packet data to transmit. The problem is that this makes the output independent of the input. I'm struggling with how to implement a forecast method to deal with this. The "how-to-write-a-block" tutorial states that complex forecast methods are possible, but gives no examples. I've gone through the code of many forecast implementations, but all of them seem to be simple decimators (N:1) or interpolators (1:N). I need something more complicated. When there is a packet of "N" bytes ready on the input, the number of output bits produced will be : ( Bytes x 8 x Bitstuffing_factor ) + header_size + crc_size but when there's NO packet data ready on the input, the number of output bits produced will simply be 8 (one flag byte). Is this sort of forecast method possible? Where can I find some code examples of complicated forecast functions, or a more detailed description of the use of the forecast method? Or am I overthinking this, and the simple answer is to set ninput_items_required to Zero, since I always have something to output? @(^.^)@ Ed _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
