# Packet Header Generator **URL:** \ **Category:** GNU Radio\ **Created:** [November 14, 2014, 9:15pm UTC](https://www.ruby-forum.com/t/packet-header-generator/233065 "2014-11-14T21:15:20Z")\ **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 14, 2014, 9:15pm UTC](https://www.ruby-forum.com/t/packet-header-generator/233065/1 "2014-11-14T21:15:20Z")
I’m trying to use the Packet Header Generator block (not the Default one) from within GRC to generate a packet header formatted to my own specification in order to match an existing protocol. I assume that an appropriate “formatter object” can be submitted to this block to instruct it to build an arbitrary header, but I’m having difficulty finding any documentation or example of using a formatter object. All I found points to the packet\_header\_default::header\_formatter method, which composes a fixed format that is of no use to me. Any help point me in the right direction? Am I just missing it or is there no “generic” header formatter capability? @(^.^)@ Ed ---
**Author:** ![Ed\_Criscuolo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ac8455/32.png) [@Ed\_Criscuolo](https://www.ruby-forum.com/u/Ed_Criscuolo)\ **Post date:** [November 14, 2014, 10:51pm UTC](https://www.ruby-forum.com/t/packet-header-generator/233065/2 "2014-11-14T22:51:04Z")
On Fri, Nov 14, 2014 at 3:14 PM, Ed Criscuolo [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > method, which composes a fixed format that is of no use to me. > > Any help point me in the right direction? Am I just missing > it or is there no “generic” header formatter capability? > > @(^.^)@ Ed Hey Ed, Funny enough, I was just working on this today. I’m adding a similar packet header generator for async (PDU-based) packets. The concept is similar with a default base class that you’d overload for your own purposes for packet formatting. I’ll be documenting this behavior more in the blocks themselves, and this should eventually become the basis for another level of our tutorials. I have it working here in simulation, but I know there’s going to be some trouble going over the air. I suspect I’ll have something that’s is worth sharing and reasonably documented early next week. Tom ---
**Author:** ![Ed\_Criscuolo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ac8455/32.png) [@Ed\_Criscuolo](https://www.ruby-forum.com/u/Ed_Criscuolo)\ **Post date:** [November 15, 2014, 5:50pm UTC](https://www.ruby-forum.com/t/packet-header-generator/233065/3 "2014-11-15T17:50:07Z")
On 11/14/14 4:49 PM, Tom R. wrote: > ``` > arbitrary header, but I'm having difficulty finding > > ``` > > some trouble going over the air. I suspect I’ll have something that’s is > worth sharing and reasonably documented early next week. > > Tom OK. Meanwhile, is there some clever Python hack I can use in the interim? I’m trying to get a timetag in a specific format (64-bit integer, LSB=250 picoSeconds) into the header I’m generating. It has to increment by 0x0000000000028000 for each packet. For example, for my sequence number field, I needed a 16-bit unsigned number that skipped over any value where the last 5 bits are ‘1’, and rolled over at 2\*\*16. I used a repeating short vector source with the Python list comprehension [s for s in range(0,32768)+range(-32768,0) if s%32 != 31] as the vector entry in the GR block. This got merged with the other header fields via stream mux block. It worked great. But for the timetag, I don’t have the rollover which allowed me to pregenerate a a fixed (large) vector. Any ideas? @(^.^)@ Ed ---
**Author:** ![Ed\_Criscuolo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ac8455/32.png) [@Ed\_Criscuolo](https://www.ruby-forum.com/u/Ed_Criscuolo)\ **Post date:** [November 17, 2014, 9:49pm UTC](https://www.ruby-forum.com/t/packet-header-generator/233065/4 "2014-11-17T21:49:04Z")
On 11/14/2014 10:49 PM, Tom R. wrote: > Funny enough, I was just working on this today. I’m adding a similar > packet header generator for async (PDU-based) packets. The concept is > similar with a default base class that you’d overload for your own > purposes for packet formatting. I’ll be documenting this behavior more > in the blocks themselves, and this should eventually become the basis > for another level of our tutorials. > > I have it working here in simulation, but I know there’s going to be > some trouble going over the air. I suspect I’ll have something that’s is > worth sharing and reasonably documented early next week. Ed, if you check out the OFDM code, is uses its own custom packet header generator. Follow those examples to see how it’s done (or wait until Tom has his stuff ready). Cheers, M ---
**Author:** ![Ed\_Criscuolo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ac8455/32.png) [@Ed\_Criscuolo](https://www.ruby-forum.com/u/Ed_Criscuolo)\ **Post date:** [November 18, 2014, 5:15pm UTC](https://www.ruby-forum.com/t/packet-header-generator/233065/5 "2014-11-18T17:15:02Z")
On Mon, Nov 17, 2014 at 3:47 PM, Martin B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > > Ed, > > if you check out the OFDM code, is uses its own custom packet header > generator. Follow those examples to see how it’s done (or wait until Tom > has his stuff ready). > > Cheers, > M Hey Ed, You can check out my work-in-progress branch here: [https://github.com/trondeau/gnuradio/tree/packet\_handling](https://github.com/trondeau/gnuradio/tree/packet_handling) It has what you need to add headers to PDUs in two different styles. The default is our normal access code plus the frame length repeated twice. The second does the same header plus a 16 bit frame counter. The second is partly to show how to manage the inheritance. Note that this is a WIP and some commits will likely get squashed and reworked to clean up the history before merging. This concept is based off of Martin’s OFDM packet header generators/formatters, but these are now classes designed around PDUs. The default async formatter works well inserted directly into our chat app from the Guided Tutorials after the CRC32. As an aside, one project that I have for 3.8 is to clean up our digital modulation support. We have a lot of blocks that have a lot of overlap in their capabilities, but are designed for very small differences in the application. I’d like to try to reduce the overlap and remove some duplication. Tom