[Discuss-gnuradio] gmsk+fec

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