How does Packet Header Generator module handle tags

GNU Radio
1

Hi Experts,
I was reading tx_ofdm.py. I found this module use
‘set_tag_propagation_policy(TPP_DONT)’ to handle tags by itself, but I
cann’t find the code that used to handle tag in this module.
I knew this module will call ‘packet_header_ofdm.header_formatter()’ and
‘packet_header_default.header_formatter()’, but I also cann’t find the
handle tag code in these functions.
Anyone can help me figure out where does this module process tags?

Thanks
Tiankun

2

Hi Tiankun,

what’s your GNU Radio version [1]? Where did you find that tx_ofdm.py?
There is no tx_ofdm.py in my GNU Radio source tree (in none of v3.7.4,
v3.6.5 or v3.4.2). There is a tx_ofdm.grc, which can be used to generate
a tx_ofdm.py, but that does not (and shouldn’t) contain a
“set_tag_propagation_policy” line.

Greetings,
Marcus

[1] you can find out by running “gnuradio-config-info -v”

3

Hi Marcus,
My tx_ofdm.py is gen by tx_ofdm.grc, there are a
‘packet_headergenerator_bb’ module in it. This module use
‘set_tag_propagation_policy(TPP_DONT)’ to handle tags by itself.
I didn’t find any code has touch tags in this module, but when I connect
it to Tag Debug module, I can find the tags output.
In my opinion, if one module use ‘set_tag_propagation_policy(TPP_DONT)’,
and it does not handle it, tags will not pass down.

Thanks
Tiankun

于 2014年07月20日 21:15, Marcus Müller 写道:

4

Hi Tiankun,

On 20.07.2014 15:50, hutiantian wrote:

Hi Marcus,
My tx_ofdm.py is gen by tx_ofdm.grc, there are a
‘packet_headergenerator_bb’ module in it. This module use
‘set_tag_propagation_policy(TPP_DONT)’ to handle tags by itself.
Ah ok!
I didn’t find any code has touch tags in this module, but when I
connect it to Tag Debug module, I can find the tags output.
This is indeed strange. However, packet_headergenerator_bb is a subclass
of tagged_stream_block [1].

In my opinion, if one module use
‘set_tag_propagation_policy(TPP_DONT)’, and it does not handle it,
tags will not pass down.
That’s generally correct, but there’s the tagged stream block exception:
[1], “A note on tag propagation” tells us that one most of the time
would use TPP_DONT to handle all non-length tags, whilst length tags are
always passed forward.

Hope that helped!

Greetings,
Marcus

[1] http://gnuradio.org/doc/doxygen/page_tagged_stream_blocks.html

5

Before myths about tagged stream blocks start perpetuating the webs:
TPP_DONT on a tagged stream blocks does the same as on any other type of
block; however, TSB tags still get through.

A closer inspection of packet_headergenerator_bb (either by reading the
source code or the manual page
(GNU Radio Manual and C++ API Reference: gr::digital::packet_headergenerator_bb Class Reference)
reveals that the actual processing is done in a separate kernel, the
header formatter object, which is user-defined. See
packet_header_default for an example.

M

6

Hi Martin,
What’s the TSB tags? Does the tags have the different types? I think the
tags just a pair of key-value.

Thanks
Tiankun

于 2014年07月21日 17:54, Martin B. 写道:

7

Hi Marcus,
What’s the “non-length tags” mean? the “length tags” you mentioned is
that input parameter, right? I only find this one tags.
When I remove ‘set_tag_propagation_policy(TPP_DONT)’, this module seems
still works fine. Why should we need set this flag in this module?

Thanks
Tiankun

于 2014年07月20日 22:54, Marcus Müller 写道:

8

On 07/21/2014 02:04 PM, hutiantian wrote:

Hi Martin,
What’s the TSB tags? Does the tags have the different types? I think the
tags just a pair of key-value.

http://gnuradio.org/doc/doxygen/page_tagged_stream_blocks.html

M