# Find a Packet in a bit stream **URL:** \ **Category:** GNU Radio\ **Created:** [November 12, 2014, 10:05am UTC](https://www.ruby-forum.com/t/find-a-packet-in-a-bit-stream/233028 "2014-11-12T10:05:08Z")\ **Posts on this page:** 5\ **Page:** 1
**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:** [November 12, 2014, 10:05am UTC](https://www.ruby-forum.com/t/find-a-packet-in-a-bit-stream/233028/1 "2014-11-12T10:05:08Z")
Hi, How to tell my receiver to identify a specific packet through a received bit stream? I transmit a custom packet (with header, payload, crc16). Is there any block in gnuradio that I can use for this purpose? I had a look on “Packet Header parser” and “parser sink” blocks, but I did’t understand how to use them. Any direction or examples will be very helpful for me. Thank you Daniel ---
**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\ **Post date:** [November 12, 2014, 10:57am UTC](https://www.ruby-forum.com/t/find-a-packet-in-a-bit-stream/233028/2 "2014-11-12T10:57:40Z")
Hi Daniel, I think looking at the gr-digital/examples/OFDM example flow graphs will help you. Greetings, Marcus ---
**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\ **Post date:** [November 12, 2014, 11:25am UTC](https://www.ruby-forum.com/t/find-a-packet-in-a-bit-stream/233028/3 "2014-11-12T11:25:09Z")
Thank you Marcus, I need something like the header/payload demux block. Can you help my understand how it works? The trigger input value can be the starting byte of my packet? In header\_data input should I insert the expected header? Is it possible to have a variable length payload? Instead of defining the length of the frame is it possible to set a byte as the end of my frame? Thank you in advance! Daniel On Wednesday, November 12, 2014 11:56 AM, Marcus M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: Hi Daniel, I think looking at the gr-digital/examples/OFDM example flow graphs will help you. Greetings, Marcus ---
**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\ **Post date:** [November 12, 2014, 11:38am UTC](https://www.ruby-forum.com/t/find-a-packet-in-a-bit-stream/233028/4 "2014-11-12T11:38:53Z")
Hi Daniel, On 11/12/2014 11:23 AM, Daniel B. wrote: > I need something like the header/payload demux block. > Can you help my understand how it works? > Well, depends 😉 First, I’d point you to the obvious source, the > documentation for header\_payload\_demux [1] and especially the “Detailed > Description” section. That will explain your following quistions: > The trigger input value can be the starting byte of my packet? > (kind of) > In header\_data input should I insert the expected header? > (no) > Is it possible to have a variable length payload? > (yes) > Instead of defining the length of the frame is it possible to set a byte as the > end of my frame? > (no, I think that’s the job of the header to define. Packeted > transmission with end-of-transmission symbols don’t really make sense, > imho) header\_payload\_demux does really what its name promises: Given a sample stream and information about packet starts (either via input stream or by using trigger\_tag\_key tags). The packet detection is something you’ll have to do “upstream” in the flow graph. I really like to do that, so here I’m pointing you at the official documentation for packeted data transmission. Greetings, Marcus > Thank you in advance! > Daniel [1] > **[GNU Radio Manual and C++ API Reference: gr::digital::header\_payload\_demux...](https://www.gnuradio.org/doc/doxygen/classgr_1_1digital_1_1header __payload__ demux.html)** [2] [http://gnuradio.org/doc/doxygen/page\_packet\_data.html](http://gnuradio.org/doc/doxygen/page_packet_data.html) ---
**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\ **Post date:** [November 12, 2014, 12:08pm UTC](https://www.ruby-forum.com/t/find-a-packet-in-a-bit-stream/233028/5 "2014-11-12T12:08:43Z")
Hi Marcus, thank you again for your useful recommendations. Cheers! On Wednesday, November 12, 2014 12:37 PM, Marcus M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: Hi Daniel, On 11/12/2014 11:23 AM, Daniel B. wrote: > I need something like the header/payload demux block. > Can you help my understand how it works? > Well, depends 😉 First, I’d point you to the obvious source, the > documentation for header\_payload\_demux [1] and especially the “Detailed > Description” section. That will explain your following quistions: > The trigger input value can be the starting byte of my packet? > (kind of) > In header\_data input should I insert the expected header? > (no) > Is it possible to have a variable length payload? > (yes) > Instead of defining the length of the frame is it possible to set a byte as the > end of my frame? > (no, I think that’s the job of the header to define. Packeted > transmission with end-of-transmission symbols don’t really make sense, > imho) header\_payload\_demux does really what its name promises: Given a sample stream and information about packet starts (either via input stream or by using trigger\_tag\_key tags). The packet detection is something you’ll have to do “upstream” in the flow graph. I really like to do that, so here I’m pointing you at the official documentation for packeted data transmission. Greetings, Marcus > Thank you in advance! > Daniel [1] > **[GNU Radio Manual and C++ API Reference: gr::digital::header\_payload\_demux...](https://www.gnuradio.org/doc/doxygen/classgr_1_1digital_1_1header __payload__ demux.html)** [2] [http://gnuradio.org/doc/doxygen/page\_packet\_data.html](http://gnuradio.org/doc/doxygen/page_packet_data.html)