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Re: [Discuss-gnuradio] building carrier sense in the FPGA and UHD

From: Darren Long
Subject: Re: [Discuss-gnuradio] building carrier sense in the FPGA and UHD
Date: Sun, 04 Mar 2012 15:55:18 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.2) Gecko/20120216 Thunderbird/10.0.2

In the amateur radio world, AX.25 "packet radio" terminal node
controllers supported  KISS mode, which left the CSMA and HDLC framing
in the TNC but offloaded the state-machine for connection management to
the host CPU stack. 

KISS merely provided a way to forward the frame metadata and payload
over the serial link between the TNC and the host.  With buffering at
each end of the serial link to the TNC, knowing exactly when a specific
frame of data has been transmitted on air is impossible and this causes
some grief in the Linux kernel's internal implementation of the state
machines for connected mode AX.25, specifically when scheduling

To be fair, we are talking about fairly large buffers and fairly low
bit-rates on tx here - 1200 baud typically, but nugatory retransmission
due to channel state backoff without backpressure is a bugger.

I assume that we would want a control plane that can provide that kind
of feedback, and would want some way of tagging transmission data units
as requiring some kind of notification of transmission on-air. 

I'm very new in the gr world so can't talk with authority on anything
other than provide vague waffle such as this for your consideration.


Darren, G0HWW

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