[Discuss-gnuradio] forecast method for HDLC transmit block

[Ed Criscuolo]

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