RE: [Discuss-gnuradio] Timestamp value

[Ulrika Uppman]

Tim,

thanks for your reply.

 

I thought the packets always had the same size (specified by U2_MAX_SAMPLES), or am I wrong here? I also do get the very same timestamp diff when running rx_streaming_samples with N= 400, 4000, 6000, 60000 etc.

 

I don't quite follow you on what will happen when calling the handler multiple times? I print the timestamps in usrp2_impl handle_data_packet which I thought would be run every time (once) a new ethernet packet arrives which isn't a control packet?

 

(And yes the timestamps are treated as uint32's.)

 

Regards,

/Ulrika

---

From: Tim Pearce [mailto:address@hidden
Sent: Thursday, February 18, 2010 9:35 PM
To: Ulrika Uppman
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] Timestamp value

Ulrika,

I agree with how you think the timestamps are generated -- it seems to work for me that way anyway!

I did it with a custom source block that added the counter*decimation rate after the first sample, the trap I fell into there is that (particuarly at lower decimation rates) rx_*_handler() can be called multiple times per instance of it.

Are the timestamps being treated as UINT32's?

400 samples is quite low, I think the packets are usually bigger than that (I might be wrong, its been a while since I looked into that).

Cheers,

Tim

On Thu, Feb 18, 2010 at 4:10 PM, Ulrika Uppman <address@hidden> wrote:

Hi,
I wonder how the timestamps are being generated for each ethernet-packet sent from the USRP2 to the host? My initial idea about how it works was that timestamps are generated at 100MHz (same as the samples) and then the timestamp associated with the first sample in an ethernet data packet will be put in the metadata which could then be unpacked in the host. I then would assume that the next packet after the first one will have a timestamp value that is proportional to the number of samples per packet times the decimation rate. However I get timestamp values that increase much much more for each received packet, so I wonder if my idea of how timestamps are generated is wrong?

I run the stable 3.2 version of gnuradio on Ubuntu 9.04 and I have an USRP2 with the RFX2400. (I also was going to try the gnuradio trunk but I got problems with building, see my other post "Error on make from git development trunk"). I tried both an old version of the fpga bin-file and one that I just recently downloaded (but both gave the same result).

I put some printouts in usrp2_impl.cc in the handle_data_packet function and the output when I run rx_streaming_samples then looks like this:
./rx_streaming_samples -f 2457e6 -d 16 -N 400 -v
...................................................................................
Daughterboard configuration:
 baseband_freq=2456250000.000000
      ddc_freq=-750000.000000
 residual_freq=-0.016764
      inverted=no

USRP2 using decimation rate of 16
Receiving 400 samples

ts_in = 1435221596, ts_last = 0, diff = 1435221596
ts_in = 2560802396, ts_last = 1435221596, diff = 1125580800
ts_in = 3367616092, ts_last = 2560802396, diff = 806813696
ts_in = 4174429788, ts_last = 3367616092, diff = 806813696
ts_in = 686341724, ts_last = 4174429788, diff = 806879232
ts_in = 1493155420, ts_last = 686341724, diff = 806813696
ts_in = 2283192156, ts_last = 1493155420, diff = 790036736
ts_in = 3090005852, ts_last = 2283192156, diff = 806813696
ts_in = 3896819548, ts_last = 3090005852, diff = 806813696
ts_in = 408731484, ts_last = 3896819548, diff = 806879232

Copy handler called 2 times.
Copy handler called with 2968 bytes.

Elapsed time was 0.000 seconds.
Packet rate was 100000 pkts/sec.
Approximate throughput was 148.40 MB/sec.
Total instances of overruns was 0.
Total missing frames was 0.
...................................................................................

ts_in is the timestamp found in the metadata of the packet just received, ts_last is the one from previous packet and diff is just the difference them between. Since there seems to be no missing frames I'm guessing the big value of diff can't be related to lost packets?
If I try different decimation rates, I see no obvious relation between the difference in value between two timestamps...

Do anyone know why the difference in timestamp value between received packets is so big? What am I missing here?

Thanks,
/Ulrika

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