Using Header/Payload Demux

GNU Radio
1

Hi,

I’m wondering if anyone can take a look at my grc file, which uses the
header/payload demux to try to obtain a payload - but I’m currently
getting
incorrect values from the tags. For instance packet_len should be 10,
but
the value is mostly 1.

I’ve attached the file.

If you can’t open it from the email i’ve also uploaded to -
http://www.anfractuosity.com/files/mux2.grc

Kind Regards,

Chris

2

On 01/18/2014 09:46 PM, chris 0 wrote:

http://www.anfractuosity.com/files/mux2.grc
Hey Chris,

nice to see people using this block! Outside of the OFDM scope, it
hasn’t yet received too much attention.

A couple of comments:

  • The CRC block increases the packet length by 4. This means packet_len
    should be 14. I ran the simulation, and that’s what it was – so it
    seems your code is working. However, the repacker thinks ‘14 bits’, and
    will then produce 1 byte at the output (because floor(14/8)==1, and we
    have that ‘floor’ because the alignment is set to ‘output’). That’s why
    you see 1 item at the first tag debug.
  • In your setup, there is no mechanism that tells the receiver that ‘14’
    actually means ‘14 bytes’, not ‘14 items’. The payload length is
    actually 14*8==112! This means the stream crc check block will never
    get the right data to work on.

The latter problem is easy to fix in your case. Simply use a
tagged_stream_multiply_length block after the demux, before the
repacking.

In the OFDM case, this is more complicated, because the number of OFDM
symbols and the number of data symbols is not necessarily an integer
multiple of one another. Here, a more elaborate solution is used: The
packet header formatter object is inherited from and modified to produce
both tags at the payload output.

Apart from that, you’re model is correct, and a good simple example for
the demuxer!

A hint for debugging: Use more tag debugs, with different names, not
only at the rx path, but also on tx. This way, you can identify problems
very quickly.

MB

3

Hi Martin,

First of all, many thanks for your last reply, it was extremely helpful!

I’m just having a little bit of a problem with your suggestion to scale
the
tag length, which
I’m doing using a length scalar of 8.

Tag Debug: Rx Bytes
Input Stream: 00
Offset: 0 Source: n/a Key: packet_len Value: 14
Offset: 0 Source: n/a Key: packet_num Value: 0
Offset: 1 Source: n/a Key: packet_num Value: 1
Offset: 1 Source: n/a Key: packet_len Value: 112
Offset: 3 Source: n/a Key: packet_num Value: 2
Offset: 3 Source: n/a Key: packet_len Value: 112
Offset: 5 Source: n/a Key: packet_num Value: 3
Offset: 5 Source: n/a Key: packet_len Value: 112
Offset: 7 Source: n/a Key: packet_num Value: 4
Offset: 7 Source: n/a Key: packet_len Value: 112
Offset: 8 Source: n/a Key: packet_num Value: 5
Offset: 8 Source: n/a Key: packet_len Value: 112
Offset: 10 Source: n/a Key: packet_num Value: 6
Offset: 10 Source: n/a Key: packet_len Value: 112
Offset: 12 Source: n/a Key: packet_num Value: 7
Offset: 12 Source: n/a Key: packet_len Value: 112

For some reason the first packet_len is still 14, rather than 112, which
is
confusing me.

Thanks a lot.

Chris

4

On 01/25/2014 09:30 PM, chris 0 wrote:

Offset: 7 Source: n/a Key: packet_num Value: 4
Offset: 7 Source: n/a Key: packet_len Value: 112
Offset: 8 Source: n/a Key: packet_num Value: 5
Offset: 8 Source: n/a Key: packet_len Value: 112
Offset: 10 Source: n/a Key: packet_num Value: 6
Offset: 10 Source: n/a Key: packet_len Value: 112
Offset: 12 Source: n/a Key: packet_num Value: 7
Offset: 12 Source: n/a Key: packet_len Value: 112

For some reason the first packet_len is still 14, rather than 112, which
is confusing me.

Hi Chris,

this is, in fact, correct behaviour.
HPD → All lengths are 14
Multiply tag → All lengths are 112
Repacker → Finds first tag, uses that for processing and scales it
back. You can test this by adding more tag debugs.

An easy solution would be to pipe the OUTPUT of the initial repacker
into both the header generator and the payload modulator. This way the
number of symbols gets coded into the header. Note that this length is
also used in the HPD to slice the payload out of the rx’d stream.

See: http://i.imgur.com/1ciVDjM.png

I’m beginning to realize that maybe the default header generator needs a
scaling value as well. sigh

MB

5

Hi Martin,

Thanks a lot for all your help! I just added the CRC checker, and it
works
perfectly!

I’d never have been able to figure that out by myself.

Kind Regards

Chris