When I want to send a pkt which is 126 Octet long, I get a message
from [at86rf2xx]:
[at86rf2xx] error: packet too large (2 byte) to be send
IEE802.15.4 MAX length is 127 so it should be sent.
#define IEEE802154_FRAME_LEN_MAX (127U) /**< maximum frame length */
I checked source code of at86rf2xx driver and I think I understand the
magic number +2 in the if condition but I don't know why this is
checked.
at86rf2xx_netdev.c:110:
/* current packet data + FCS too long */
if ((len + ptr->iov_len + 2) > AT86RF2XX_MAX_PKT_LENGTH) {
DEBUG("[at86rf2xx] error: packet too large (%u byte) to be
send (iov_len %d, i %d, count %d)\n",
(unsigned)len + 2, ptr->iov_len, i, count);
return -EOVERFLOW;
}
+2 mean two FCS octets?
In the samr21 datasheet, 37.3 Frame Check Sequence (FCS) [1]:
For a frame with a frame length specified as N (3 ≤ N ≤ 127), the FCS
is calculated on the first N-2 octets in the Frame Buffer, and the
resulting FCS field is transmitted in place of the last two octets
from the Frame Buffer.
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.
So while I think we should remove the +2 test and let the possibility
to send packet up to 127 octets and I don't understand the part in
datasheet: "the last two bytes are replaced by the internally
calculated FCS". This mean that a part of data is erased? (for 5
octets or 127)
If you take a look at figures 37-1 and 37-2 in the data sheet you linked you can see that IEEE 802.15.4 allows up to 127 bytes for the MPDU and this includes the FCS.
So if the driver would allow writing 127 bytes into the frame buffer the last two bytes would indeed be overwritten which is undesired I guess.
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?
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.
Personally, I never tried what happens if you remove this check for 127-2 and
fully fill the frame buffer but I would imagine that exactly that happens, the
last two bytes are overwritten by the FCS and thus lost, unless it already was
the exact same checksum.
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
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).
> Hi Baptiste,
>
>
>> 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:
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
In your example Alexandre:
I think when the transmitter starts sending the FRAME -> going into
BUSY. The transceiver will make some:
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.
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?
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:
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.