Hi Baptiste,
On 17 May 2017, at 23:22 PDT(-0700), Baptiste Clenet wrote:
I re-do my example again started at 0:
Example:
at86rf2xx_tx_load(dev, ptr->iov_base, length=5, 0);
FCS is calculated on bytes 0, 1 and 2 and bytes 3 and 4 are replaced
by FCS
at86rf2xx_tx_load(dev, ptr->iov_base, length=126, 0);
FCS is calculated on bytes 0, to 123 and bytes 124 and 125 are
replaced by FCS
Just ran a test using the txtsnd command in the default example which
sends raw 802.15.4 frames and I didn't see any of my payload overwritten
by FCS bytes
In your example Alexandre:
I think when the transmitter starts sending the FRAME -> going into
BUSY. The transceiver will make some:
init_fcs();
for (b:each_bytes_to_tx) { <--- 127 - 2
send(b);
calc_fcs(b);
}
send_fcs(); <--- 2 bytes
then you need to be sure you send 127 - 2 bytes out
You send to the radio driver N bytes, then the transceiver calculates
FCS on this N bytes and after sending these N bytes, send the
calculated FCS. So you say that transmitter appends FCS to the given
frame.
This is the expected behavior with AUTO_CRC on, yes.
Now, I'm helping to port OpenThread on RIOT and OpenThread stack gives
sometimes to the radio driver a frame of 126 bytes. My team asks them
how is it possible:
Their answer:
The IEEE 802.15.4 frame length includes the FCS bytes at the end of
the frame. The radio driver should simply update the last two bytes
of the frame rather than extending it.
If I understand well, RIOT stack does not include FCS in its
ieee802154 layer and OpenThread stack includes it?
Am I right?
You are right.
For using OpenThread with RIOT the quick fix would be giving the OT
buffer to RIOT with the buffer length - 2.
Best, Thomas
2017-05-17 21:26 GMT+02:00 Alexander Aring <alex.ar...@gmail.com>:
Hi,
On Wed, May 17, 2017 at 07:56:05PM +0200, Baptiste Clenet wrote:
2017-05-17 17:47 GMT+02:00 Thomas Eichinger <tho...@riot-os.org>:
Hi Baptiste,
On 17 May 2017, at 1:14 PDT(-0700), Baptiste Clenet wrote:
According to their example:
Example:
A frame transmission of length five with TX_AUTO_CRC_ON set, is
started with a Frame Buffer write access of five bytes (the last
two
bytes can be omitted). The first three bytes are used for FCS
generation; the last two bytes are replaced by the internally
calculated FCS.
Even a five bytes frame would have its last two bytes overwritten.
Is
my understanding correct?
So I don't understand why the driver limits the frame length to
127-2?
The at86rf2xx driver doesn't limit the *frame length* to 127-2
octets
because the
FCS is part of the frame sent out. The driver just makes sure that
no
payload
data is overwritten by the FCS.
Every 802.15.4 frame has two bytes of FCS. So if we didn't use
TX_AUTO_CRC_ON
we would have to calculate the checksum in software and write it
into the
frame buffer, appended to the header, sequence number, addresses
and payload
we
want to send. All together (FCF + seq_num + addrs + payload + FCS)
this can
be 127 bytes max.
Now RIOT's at86rf2xx driver uses TX_AUTO_CRC_ON thus we don't have
to
calculate
the FCS, it's appended to the rest of the frame.
I don't think it is appended to the frame but it really replace the
last two bytes of the frame
Example:
at86rf2xx_tx_load(dev, ptr->iov_base, 5, 0);
FCS is calculated on bytes 1, 2 and 3 and bytes 4 and 5 are replaced
by FCS
starting here at 0 or 1? :S
Replaced by FCS? I suppose this function loads frame into
framebuffer,
the FCS isn't calculated there. See below.
at86rf2xx_tx_load(dev, ptr->iov_base, 126, 0);
FCS is calculated on bytes 1, to 124 and bytes 125 and 126 are
replaced by FCS
So the stack which give the frame should give a frame 2bytes longer
to let the transceiver calculate the FCS.
This is why IMO this check is useless.
After reading more the datasheet, it's not clear because it is
written:
On transmission the radio transceiver generates and appends the FCS
bytes during the frame transmission. This
behavior can be disabled by setting register bit TX_AUTO_CRC_ON = 0
(register 0x04, TRX_CTRL_1).
I think when the transmitter starts sending the FRAME -> going into
BUSY. The transceiver will make some:
init_fcs();
for (b:each_bytes_to_tx) { <--- 127 - 2
send(b);
calc_fcs(b);
}
send_fcs(); <--- 2 bytes
then you need to be sure you send 127 - 2 bytes out. If you disable
---
now if you disable AUTO_CRC then you can load 127 bytes into
framebuffer
and hopefully last 2 bytes are FCS (or doesn't need to be, but then
you
running out-of-spec).
- Alex
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Baptiste
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