Re: Discussion regarding latency

[Marcus Müller]

Hi Jiya,

That's going to be the sum of:


On the transmitter's site:
- actual time it takes for each packet to be computed in your GNU Radio blocks
- instantaneous amount of samples in buffers, due to the
  · time it takes to schedule a block that has data to work on,
  · Operating system thread scheduling
- host platform OS / driver overhead (non-deterministic, depends on hardware, interrupt 
load, general CPU utilization, driver…)
- Hardware connection latency (USB, Ethernet, PCIe,…)  (depending on the link, 
non-deterministic, and might range from 100s of ns to ms)
- SDR Hardware latencies (relatively static, but not constant nor identical across 
configurations, typically depends on the kind of processing done in-device)

Then, the actual RF channel (so, speed of light, essentially, considering all relevant 
multipath/dispersive effects).

On the receiver's side, the same, basically, in reverse

- SDR hardware
- Hardware connection
- OS / driver
- buffer fillage
- computation time

As you can see, there's a lot of nondeterminism in that. It's hard to tell what the main 
contributor is there – but most commonly it's dominated by the buffer fillage due to 
processing times and CPU time competition.

You can however manage things quite a bit

1. (transmit side) Some SDRs (most prominently, all the Ettus USRPs) support "timed 
commands", which allow to specify a specific time for transmission to happen – you'd put 
that far enough into the future for the nondeterministic transmitter computer latency is 
reliably over at that point
2. (GNU Radio data processing) Derek & Matt have written latency manager [1] to bound 
latency with a credit-based flow control scheme


Best regards,
Marcus

[1] 
https://www.gnuradio.org/grcon/grcon19/presentations/Managing_Latency/Matt%20Ettus%20-%20Managing%20Latency.pdf
https://www.youtube.com/watch?v=jq0RewceCwc


On 01.06.23 07:16, Jiya Johnson wrote:
>   Good morning,
>        I would like to know *how to calculate the latency if we are sending a signal or a 
> data packet from source to receiver in GNU Radio*.
>
>                              Thanks in Advance.