[Discuss-gnuradio] reporting control/status responses for inband signaling?

[George Nychis]

This is a perfect question for Eric, but unfortunately he is on 
vacation.  So I'm hoping that maybe Brian or someone can answer who was 
involved in the initial design of the inband USB packets :)

The way we have the low-level control/status packets designed, I'm not 
sure how the application can ever get a response back correctly.

Heres how we have them defined:
http://gnuradio.org/trac/browser/gnuradio/trunk/usrp/doc/inband-signaling-usb#L116

They use the same packet header as data packets, except the channel is 
fixed to 0x1f so they can be deciphered as CS packets easily in the 
FPGA.  If we do this, how are we ever supposed to properly determine 
what application to send the response back to?

The current chain to get a CS packet to the USRP is like this:
Application -> usrp_server -> USRP

It's very straight forward, but sending responses back is a problem:

USRP -> usrp_server -> ?

How does usrp_server, which parses the USB packets, know which 
application to send the CS response to?  The packets used to be demuxed 
by channel since channels are owned by specific applications, and 
therefore the channel is read to determine the application.  In this 
case, the channel says absolutely nothing about the application that 
sent it.

So for example, the way the ports are setup:
http://gnuradio.org/trac/browser/gnuradio/branches/developers/gnychis/inband/usrp/host/lib/inband/usrp_server.mbh

usrp-tx includes usrp-low-level-cs so that I can send a PING over a TX 
channel that I own, and usrp-rx includes usrp-low-level-cs so that I can 
receive the PING response over an RX channel I own.  So, sending the 
PING works just great, but again how will usrp_server know what RX 
channel to place the response on?  The packets don't seem setup for this.

It would make more sense if we had a single bit that specified it was a 
CS packet, and then the channel specifies which RX channel to send the 
response on.

Maybe I'm missing something?

- George