[Discuss-gnuradio] packet_header_default formatter packet length - why constant?

[Steven Knudsen]

Hi,

I have created a packet header that subclasses gr::digital::packet_header_default and so far so good.

In trying to understand the base class behaviour, I created a simple header packet generation GRC project that multiplexes a header with a payload.

The header_formatter is defined as digital.packet_header_default(32,legnth_tag_key) where length_tag_key is set to “packet_len”; this is the same as many examples, obviously.

The header packet generator works fine with this, generating a 32 byte header with 1 bit of header data per byte. This makes sense since I left the default bits_per_byte in the header_formatter, which implies a BPSK modulator is used for the header.

However, if I change the header_formatter to digital.packet_header_default(32,legnth_tag_key,num_tag_key,header_mod.bits_per_symbol()) where num_tag_key is “packet_num” and header_mod is defined as digital.constellation_qpsk(), which means now 2 header bits per byte are encoded, the header generator still outputs 32 bytes and a packet_len = 32. Shouldn’t 16 bytes be output and packet_len = 16?

Accounting for the bits per symbol would make sense a) because that is consistent with the packet_header_default constructor bits_per_byte argument and b) it would be consistent with the behaviour of other blocks, such as Repack Bits, that are commonly used with packet blocks.

Anyone have an explanation? Perhaps I am not following GNU Radio philosophy in this approach?

BTW, in my packet header class I simply override the packet_header_default::header_len() to return the right number of bytes. That seems right to me :-)

Thanks!

steven

 

Steven Knudsen, Ph.D., P.Eng.

www. techconficio.ca

www.linkedin.com/in/knudstevenknudsen

All the wires are cut, my friends
Live beyond the severed ends.  Louis MacNeice