Re: [Discuss-gnuradio] How to get multipe rx_time tags while receiving continuously

[Miklos Maroti]

Hi Harry,

You never stop the receiver on node B and C, right? You should not
observe anything like that if you do not have dropped packets. Are you
using USRP2's?

Miklos

On Sat, Nov 2, 2013 at 3:05 AM, Harry Zhang <address@hidden> wrote:
> Tom,
>     Thanks for your reply.
>     I got a weird problem when using rx_time tags. I have three nodes, node
> A sends 10 packets within 0.2 sec ,stops for 1sec sends 10 packets ,
> stops..., sends....,stops....  . Node B and C receive it and record the
> receive time using (rx_time+ sample_count*sample_rate).  Considerating the
> clock offset between B and C, the difference of B and C's receive time must
> remain stable. But every time after A stops for 1sec, the receive time's
> difference varies several hundreds of microsecond. I'm stumped by this
> problem.
>     Could you give me some advice. Thank you in advance.
>
> Harry
>
> 2013/11/1 22:26, Tom Rondeau wrote:
>>
>> On Thu, Oct 31, 2013 at 3:46 AM, Harry Zhang <address@hidden> wrote:
>>>
>>> Hi,
>>> As far as I know rx_time tag is associated with the first sample of a
>>> receive stream. If I wanna get multiple rx_time tags while receiving
>>> continuous data, should I stop and issue a new stream again and again
>>> for getting more rx_time tags.
>>> Thank you.
>>
>> Harry,
>>
>> We want to minimize tags through the flowgraph since it adds overhead.
>> The UHD driver only sends an rx_time tag whenever one is necessary.
>> That means that if there is a chance that the host has become
>> unsynchronized, it sends an updated tag. So there's one at the
>> beginning of the stream to set the initial time. Then, if a dropped
>> packet or overflow are detected, it sends a new rx_time tag.
>>
>> Between time tags, you can count samples and you know the sample rate,
>> so you know the time of every sample based on that initial rx_time tag
>> (to within the tolerance of the sample clock on the USRP).
>>
>> Tom
>>
>
>
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