On 11/3/06, Jan Kiszka <[EMAIL PROTECTED]> wrote:
Yes, excuse me, 22 us
So, the time my heartbeat hold the cable standing on ethereal (and considering jitters==0) Is actually 22us? Or is it 22+19?
Teresa Noviello wrote:
> Hi. I have a master, 10.0.0.4 and a slave 10.0.0.5.
>
> Starting rtnet with "rtnet start" on the slave 10.0.0.5, i have seen that
> time difference with the master is about 19microsec (min 19, max 20).
You mean the calibration result in the kernel log? That is the time it
takes (averaged) from the master writing its current time into a frame
to the slave comparing that value with its local time after receiving
the related frame.
>
> With ethereal i do LAN sniffing, running ethereal on rteth0 on master,
> 10.0.0.4.
If you sniff on one node participating in the network, you will face an
inherent offset between timestamps reported for outgoing and incoming
packets. The former are stamped *before* registering with the hardware,
the latter after reception (including potential IRQ jitters).
>
> With ethereal i see TDMA Synchronisation, Heartbeats and so on...
(Hope you read in the docs about the different protocols behind TDMA
sync and RTcfg heartbeat. In this context, those heartbeat frames should
be considered as an arbitrary payload for TDMA.)
>
> Just to understand about timing: i see the tdma Synchronisation at time
> 0.608994 and the following heartbeat of the slave 10.0.0.5 to the master at
> time 0.609136. The difference is 142 microsec, so the heartbeat of slave
> 10.0.0.5 arrives at master after 142 microsec since the synchronisation.
> Now
> considering the tdma.conf i have written gives slot
> 0 0 to master
> and slot
> 0 120 to slave
> (the cycle is 1000)
>
> i can say that the heartbeat start at 120microsec after synchronisation, so
> the real time the heartbeat (60 bytes) hold the cable is
> 142 - 120 = 42 microsecs.
(guess you mean 22 us)
Yes, excuse me, 22 us
>
> Now, i want to understand: i do have to add the delay (19 microsecs) of the
> slave 10.0.0.5 to this time to calculate the real time the heartbeat holds
> the cable? What is the relation of this delay on the times i do snif with
> ethereal, on master?
In the unloaded, almost jitter-free case you actually see that packet
travelling delay here again, but now the other way around
(slave-to-master). Additionally, you face the wake-up latency of a timed
task on the slave, because the calibration doesn't account for this.
Jan
So, the time my heartbeat hold the cable standing on ethereal (and considering jitters==0) Is actually 22us? Or is it 22+19?
--
Teresa Noviello
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