On 20/10/2011 10:25 AM, David Barton wrote:
I have been troubleshooting an issue with possible packet relflections
and cannot figure out the cause. I am running tunnel.py on two USRP2s
that are cabled together with a 20dB attenuator between them. The
settings I am using on both sides for tunnel.py are:
Tx Gain: 15 dB
Rx Gain: 10 dB
Carrier Threshold: -80
Rx Tunnel Freq: 400 MHz
Modulation: GMSK
Bit Rate: 1Mb/sec
When I use VLC to stream a video from computer A to computer B over
the USRP link it works ok but there are alot of reflected packs being
recorded by computer A. The same thing happens when I try to stream
from computer A to computer B. This also occurs when I use iperf to
test the link. Strangely, though there are NO reflected packets when I
ping between the computers.
Below is a paste of some of the output from computer A. I put in a
timestamp on the left of when events occur. I also put in an explicit
statement to print out when tunnel is backing off and for how long. I
added sequence number to make it blatantly obvious that the computer
is receiving its own packet. Any packet with a sequence number
beginning originates from computer A. If the packet originated from
computer B it shows up at RX_packet=none. As it shows computer A is
receiving its own packets!
[ 63.61 ] Tx: seq_no = 100054 | len(payload) = 1448
[ 63.61 ] Tx: seq_no = 100055 | len(payload) = 186
[ 63.61 ] Tx: seq_no = 100056 | len(payload) = 1310
[ 63.61 ] Tx: seq_no = 100057 | len(payload) = 1448
[ 63.61 ] Tx: seq_no = 100058 | len(payload) = 1021
[ 63.61 ] Backing off for 0.001 sec
[ 63.62 ] Backing off for 0.002 sec
[ 63.62 ] Backing off for 0.004 sec
[ 63.63 ] Backing off for 0.008 sec
[ 63.64 ] Backing off for 0.016 sec
[ 63.64 ] Rx: ok = True | seq_no = 100054 | len(payload) = 1448
[ 63.66 ] Backing off for 0.032 sec
[ 63.64 ] Rx: ok = True | seq_no = 100055 | len(payload) = 186
[ 63.66 ] Rx: ok = True | seq_no = 100056 | len(payload) = 1310
[ 63.66 ] Rx: ok = True | seq_no = 100057 | len(payload) = 1448
[ 63.67 ] Backing off for 0.064 sec
[ 63.67 ] Rx: ok = True | seq_no = 100058 | len(payload) = 1021
[ 63.72 ] Tx: seq_no = 100059 | len(payload) = 1448
[ 63.72 ] Tx: seq_no = 100060 | len(payload) = 70
[ 63.72 ] Tx: seq_no = 100061 | len(payload) = 1448
[ 63.72 ] Tx: seq_no = 100062 | len(payload) = 150
[ 63.72 ] Tx: seq_no = 100063 | len(payload) = 1448
[ 63.72 ] Tx: seq_no = 100064 | len(payload) = 248
[ 63.72 ] Backing off for 0.001 sec
[ 63.72 ] Backing off for 0.002 sec
[ 63.73 ] Backing off for 0.004 sec
[ 63.74 ] Backing off for 0.008 sec
[ 63.75 ] Backing off for 0.016 sec
[ 63.74 ] Rx: ok = True | seq_no = 100060 | len(payload) = 70
[ 63.75 ] Rx: ok = True | seq_no = 100061 | len(payload) = 1448
[ 63.76 ] Backing off for 0.032 sec
[ 63.75 ] Rx: ok = True | seq_no = 100062 | len(payload) = 150
[ 63.76 ] Rx: ok = True | seq_no = 100063 | len(payload) = 1448
[ 63.76 ] Rx: ok = True | seq_no = 100064 | len(payload) = 248
[ 63.78 ] Tx: seq_no = 100065 | len(payload) = 1448
[ 63.78 ] Tx: seq_no = 100066 | len(payload) = 566
[ 63.78 ] Tx: seq_no = 100067 | len(payload) = 1448
[ 63.78 ] Tx: seq_no = 100068 | len(payload) = 987
[ 63.78 ] Backing off for 0.001 sec
[ 63.79 ] Backing off for 0.002 sec
[ 63.79 ] Backing off for 0.004 sec
[ 63.79 ] Backing off for 0.008 sec
[ 63.8 ] Backing off for 0.016 sec
[ 63.8 ] Rx: ok = True | seq_no = 100066 | len(payload) = 566
[ 63.82 ] Backing off for 0.032 sec
[ 63.82 ] Rx: ok = True | seq_no = 100067 | len(payload) = 1448
[ 63.82 ] Rx: ok = True | seq_no = 100068 | len(payload) = 987
[ 63.84 ] Tx: seq_no = 100069 | len(payload) = 1448
[ 63.84 ] Tx: seq_no = 100070 | len(payload) = 1448
[ 63.84 ] Tx: seq_no = 100071 | len(payload) = 1448
[ 63.84 ] Tx: seq_no = 100072 | len(payload) = 321
[ 63.84 ] Tx: seq_no = 100073 | len(payload) = 1448
[ 63.84 ] Tx: seq_no = 100074 | len(payload) = 855
[ 63.84 ] Backing off for 0.001 sec
[ 63.84 ] Backing off for 0.002 sec
[ 63.85 ] Backing off for 0.004 sec
[ 63.85 ] Backing off for 0.008 sec
[ 63.86 ] Backing off for 0.016 sec
[ 63.87 ] Backing off for 0.032 sec
[ 63.86 ] Rx: ok = True | seq_no = 100070 | len(payload) = 1448
[ 63.89 ] Rx: ok = True | seq_no = 100071 | len(payload) = 1448
[ 63.89 ] Rx: ok = True | seq_no = 100072 | len(payload) = 321
[ 63.89 ] Rx: ok = True | seq_no = 100073 | len(payload) = 1448
[ 63.9 ] Backing off for 0.064 sec
[ 63.9 ] Rx: ok = True | seq_no = 100074 | len(payload) = 855
Does anyone have any idea why this could be occurring? I thought
maybe it would be a physical reflection but dont understand why ping
packets would not be reflected in that case.
Thanks,
Dave
There's generally only about 45dB of isolation available between TX and
RX, so if you're running in full-duplex, your receiver will see its
own transmissions.
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