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Re: [Discuss-gnuradio] udp source - endianness anomaly?

From: Josh Blum
Subject: Re: [Discuss-gnuradio] udp source - endianness anomaly?
Date: Sun, 11 Oct 2009 16:41:23 -0700
User-agent: Thunderbird (X11/20090817)


The udp source is just memcopying bytes from the udp socket to a output vector, it has no concept of what the data is and therefore cant be byteswapping it.

This may facilitate the need for a htonx block and a ntohx block for our gr short and int types, but the udp blocks are simple as can be; that being a good thing.


Richard Clarke wrote:
No indeed, that's what I'm saying. The AVR32 platform is sending out
everything in network byte order but the GNU Radio udp source doesn't appear
to be interpreting it from network byte order (big endian) into the host
byte order (little endian) correctly. My current work around to this is to
artificially over ride the byte ordering of the transmitted payload data at
the transmitting AVR32 end, to compensate for the apparent problem at the
GNU Radio host end. I'm sure if there was really a problem with the code at
the GNU Radio end it would have been picked up long ago, so my assumption is
that I've missed something obvious, or I need to look more suspiciously at
the LWIP code (v1.3.0). However, any ideas are welcomed.


2009/10/12 David I. Emery <address@hidden>

On Mon, Oct 12, 2009 at 11:16:11AM +1300, Richard Clarke wrote:
Hi All,

I was wondering if anyone has had any issues with the interpretation of
shorts by the GNU Radio udp source function, when the shorts are
transmitted  from a big endian based platform? In my situation I am
transmitting UDP packets comprised of 16 bit samples from an AVR32 (big
endian) which utilises the LWIP open source TCP/IP stack. On my GNU Radio
destination PC (Intel Pentium D CPU, little endian) I construct a simple
flowgraph (using GRC/GNU Radio v3.2.1) with a UDP source set to interpret
incoming data as shorts, into the short to float block and then straight
into the GUI scope. I'm receiving the UDP packets OK which I assume means
that all the protocol header info is being interpreted with the correct
endianness, but the waveform displayed in the scope is corrupt, until
is, I manually re-order the LSB and MSB of the transmitted samples at the
AVR32 end. Normally the lower level network code would take care of byte
reordering as required to match network byte order to the relevant host
order, however this doesn't appear to be happening correctly on the GNU
Radio side. I must be missing something simple here, can anyone shed some

        Is there a good reason to have packets in something other than
network byte order (to match the IP headers) ?

 Dave Emery N1PRE/AE, address@hidden  DIE Consulting, Weston, Mass
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole -
celebration of what could have been, but wasn't and is not to be now


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