# Length tag key in ofdm\_channel\_allocator

**URL:** <https://www.ruby-forum.com/t/length-tag-key-in-ofdm-channel-allocator/235745>\
**Category:** GNU Radio\
**Created:** [July 12, 2013, 11:48am UTC](https://www.ruby-forum.com/t/length-tag-key-in-ofdm-channel-allocator/235745 "2013-07-12T11:48:17Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [July 12, 2013, 11:48am UTC](https://www.ruby-forum.com/t/length-tag-key-in-ofdm-channel-allocator/235745/1 "2013-07-12T11:48:17Z")

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hi,

i am trying to use the flowgraph “Core elements of an OFDM transmitter”  
in  
the ofdm documentation which is there at Gnuradio-3.7 API. i got the  
following error.

FATAL: Missing a required length tag on port 0 at item #0  
thread[thread-per-block[1]: \<block ofdm\_carrier\_allocator\_cvc (14)\>]:  
Missing length tag.

how to overcome this…  
what to mention in length tag key in ofdm channel allocator… by default  
we  
have “packet\_len” in that field.

thanks,  
lingeswar