I noticed strange behavior with some TOS code I was working on
recently. I thought it was a protocol error, but it turned out that
the cause was receiving packets with erroneous data. It is my
understanding that the PHY layer CRC should provide adequate
protection from erroneous packets (i.e., the crc is checked at the PHY
layer and invalid packets are discarded and the likelihood of an
erroneous packet having a valid CRC is extremely low). To investigate
this further, I wrote a simple application that sends out packets
formated as follows:
#define JUNK_LEN 16
typedef struct data{
uint16_t junk[JUNK_LEN];
uint16_t crc;
}data_t;
uint16_t j;
for(j=0; j< JUNK_LEN; j++){
pkt->junk[j] = call Random.rand16();
}
// JUNK_LEN is in words, convert to bytes
pkt->crc = call Crc.crc16(&pkt->junk[0], JUNK_LEN<<1);
On the receive side (in the receive event handler), I then verify that
the application-level CRC is correct. Below is the typical output
(nodes transmit randomly around 3 packets per second). "rx" is the
lifetime total of receive events signaled. "err" is the number of
those with an application level crc error. "lenerr" is the number
where the length was incorrect (I have never seen this happen). I see
crc errors with both with the DMA and software SPI radio interfaces
using telosb motes. Just looking at the data, it seems DMA has fewer
CRC errors (possibly suggesting a software problem?). Has anyone else
seen this problem or looked into it further? I can send the test code
if anyone is interested. For now, I guess I'm going to include an
app-level CRC in my data packets.??
Alan
2011-04-14 10:10:52,420 - printf - INFO - (9004): TICK (tx (last 60
sec): 197, rx: 73386, err: 1, lenerr: 0)
2011-04-14 10:10:53,690 - printf - INFO - (9026): TICK (tx (last 60
sec): 205, rx: 76273, err: 1, lenerr: 0)
2011-04-14 10:10:53,960 - printf - INFO - (9015): TICK (tx (last 60
sec): 189, rx: 74755, err: 4, lenerr: 0)
2011-04-14 10:10:55,200 - printf - INFO - (9006): TICK (tx (last 60
sec): 194, rx: 73377, err: 5, lenerr: 0)
2011-04-14 10:10:55,220 - printf - INFO - (9017): TICK (tx (last 60
sec): 199, rx: 74364, err: 4, lenerr: 0)
2011-04-14 10:10:56,930 - printf - INFO - (9028): TICK (tx (last 60
sec): 180, rx: 78493, err: 1, lenerr: 0)
2011-04-14 10:10:56,980 - printf - INFO - (9008): TICK (tx (last 60
sec): 207, rx: 77481, err: 0, lenerr: 0)
2011-04-14 10:10:59,270 - printf - INFO - (9010): TICK (tx (last 60
sec): 180, rx: 78353, err: 4, lenerr: 0)
2011-04-14 10:11:00,520 - printf - INFO - (9019): TICK (tx (last 60
sec): 188, rx: 76016, err: 1, lenerr: 0)
2011-04-14 10:11:01,620 - printf - INFO - (9012): TICK (tx (last 60
sec): 185, rx: 77847, err: 1, lenerr: 0)
2011-04-14 10:11:03,270 - printf - INFO - (9021): TICK (tx (last 60
sec): 201, rx: 73821, err: 1, lenerr: 0)
2011-04-14 10:11:05,600 - printf - INFO - (9014): TICK (tx (last 60
sec): 180, rx: 74860, err: 1, lenerr: 0)
2011-04-14 10:11:06,010 - printf - INFO - (9023): TICK (tx (last 60
sec): 196, rx: 76486, err: 1, lenerr: 0)
2011-04-14 10:11:06,280 - printf - INFO - (9003): TICK (tx (last 60
sec): 194, rx: 76595, err: 0, lenerr: 0)
2011-04-14 10:11:07,961 - printf - INFO - (9016): TICK (tx (last 60
sec): 208, rx: 73878, err: 1, lenerr: 0)
2011-04-14 10:11:08,270 - printf - INFO - (9025): TICK (tx (last 60
sec): 193, rx: 69613, err: 1, lenerr: 0)
2011-04-14 10:11:09,360 - printf - INFO - (9005): TICK (tx (last 60
sec): 189, rx: 79283, err: 1, lenerr: 0)
2011-04-14 10:11:10,180 - printf - INFO - (9018): TICK (tx (last 60
sec): 192, rx: 75160, err: 0, lenerr: 0)
2011-04-14 10:11:10,340 - printf - INFO - (9027): TICK (tx (last 60
sec): 204, rx: 78511, err: 1, lenerr: 0)
2011-04-14 10:11:12,650 - printf - INFO - (9009): TICK (tx (last 60
sec): 183, rx: 77493, err: 0, lenerr: 0)
2011-04-14 10:11:12,940 - printf - INFO - (9029): TICK (tx (last 60
sec): 205, rx: 72160, err: 2, lenerr: 0)
2011-04-14 10:11:13,410 - printf - INFO - (9007): TICK (tx (last 60
sec): 180, rx: 76247, err: 1, lenerr: 0)
2011-04-14 10:11:14,930 - printf - INFO - (9020): TICK (tx (last 60
sec): 213, rx: 77005, err: 2, lenerr: 0)
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