hello all
i have a simple GMSK modulation tx and rx chain in gnuradio.data is read from a file,and recieve demod into another file, this model works well
i would like to add convolutional encoding/decoding,and this is my grc,

i got some problem
Using Volk machine: sse4_2_32
INFO: tried to set frame to 12000; max possible is 4096
INFO: cc_decoder: tried to set frame to 12000; max possible is 4096
FATAL: Missing a required length tag on port 0 at item #0
thread[thread-per-block[3]: <block fec_tagged_encoder (3)>]: Missing length tag.
i think that i am not using the fec in a correct way , so i want to ask is there someone who can give me some instruction about how to add a fec into gmsk,or how to use fec in gnuradio.
thank you
--Ekko