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Groups > comp.arch.embedded > #28069

Re: clock synchronization over IP

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On 09/21/18 15:58, Robert Wessel wrote:
> On Fri, 21 Sep 2018 17:12:00 +0300, Dimiter_Popoff<dp@tgi-sci.com>
> wrote:
>
>> On 21.9.2018 ?. 17:00, Chris wrote:
>>> On 08/25/18 01:54, gp.kiwi@gmail.com wrote:
>>>> We need to synchronize clocks (so we can synchronize audio) on
>>>> physically separated controllers that are connected via ethernet that
>>>> may or may not be connected to the internet.  The controllers have
>>>> ARM9 LPC3250 SOM and embedded Linux.  We need to synch with no more
>>>> than 50 ms difference but preferably less.  Synchronization needs to
>>>> continue working between any two controllers regardless of the failure
>>>> of any other controller.
>>>>
>>>> Does anyone have an idea what is the easiest way to do this?
>>>
>>> Read all the replies about ntp etc, but perhaps
>>> look at this another way ?. Rather than each client
>>> *requesting* time, how about a server / node
>>> broadcasting time to the net, with each client
>>> locking on to that ?. Not sure if there are any
>>> standards for it, but may be be worth having
>>> a look.
>>>
>>> That would get rid of any client / server request
>>> delays altogether, with sync offsets determined
>>> entirely by the client software, probably down
>>> to microseconds...
>>>
>>> Chris
>>>
>>
>> I think the idea is excellent. Broadcast a packet - on the same
>> Ethernet branch (as is the case of the OP I think) there will be
>> no skew to talk about; then have all recipients confirm to the
>> server they got the time, and once the server receives all confirmations
>>from all active hosts (it will need to know which these are) it safely
>> (via tcp) tells each host the broadcast time it got is now universally
>> valid.
>
>
> Unless you're using one of the shared media versions of Ethernet,
> which is somewhat unlikely these days, although not impossible, or if
> your "switch" is really a hub (aka a repeater), also unlikely, except
> in really inexpensive gear, there's no guarantee that the switch isn't
> going to introduce variable latency in forwarding broadcast frames to
> the various ports.  In fact the switch will have to introduce skew if
> on of the ports has queued transmission traffic, and the others don't,
> or if the ports are not all running at the same speed.
>
> Slew can be introduced at the receiving nodes as well based on how
> likely they have any queued receive traffic, either in the NIC or in
> OS buffers.  A question would be how close to the wire the received
> frame can be timestamped.
>
> That might also leave all the nodes, except the time server,
> synchronized, unless the time server can also receive the broadcast,
> which is not a universal capability (the sending node often cannot).
> That's not an issue if the time server node doesn't need to be
> synchronized with the other nodes, but I'd generally assume that being
> the time server was just a function of one of the nodes doing "normal"
> work.
>
> So there's a lot to be said for an interactive approach.

Agreed, but the spec is to synchronise audio, which is a hard real time
requirement. You might have to consider other issues as well, such as
embedded Linux real time response time. That said, if you have control
over the network topology, you can specify which switches and hubs
are permissable and design around that. Lowest cost that meets the
spec is usually the aim and the simplest protocol that gets the job
done can also minimise latency and hardware requirements :-)...

Chris

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clock  synchronization over IP gp.kiwi@gmail.com - 2018-08-24 17:54 -0700
  Re: clock  synchronization over IP Paul Rubin <no.email@nospam.invalid> - 2018-08-24 18:28 -0700
  Re: clock  synchronization over IP wzab01@gmail.com - 2018-09-15 09:00 -0700
    Re: clock  synchronization over IP gp.kiwi@gmail.com - 2018-09-15 15:08 -0700
      Re: clock  synchronization over IP George Neuner <gneuner2@comcast.net> - 2018-09-16 08:49 -0400
        Re: clock  synchronization over IP gp.kiwi@gmail.com - 2018-09-17 01:45 -0700
          Re: clock  synchronization over IP George Neuner <gneuner2@comcast.net> - 2018-09-17 11:30 -0400
            Re: clock  synchronization over IP gp.kiwi@gmail.com - 2018-09-17 15:04 -0700
              Re: clock  synchronization over IP George Neuner <gneuner2@comcast.net> - 2018-09-17 22:10 -0400
                Re: clock  synchronization over IP graeme.prentice@gmail.com - 2018-09-18 04:23 -0700
                Re: clock  synchronization over IP Andrew Smallshaw <andrews@sdf.org> - 2018-09-18 13:19 +0000
                Re: clock  synchronization over IP Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> - 2018-09-19 18:56 +0300
                Re: clock  synchronization over IP upsidedown@downunder.com - 2018-09-22 11:51 +0300
          Re: clock synchronization over IP Clifford Heath <no.spam@please.net> - 2018-09-18 21:10 +1000
  Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-21 15:00 +0100
    Re: clock  synchronization over IP Dimiter_Popoff <dp@tgi-sci.com> - 2018-09-21 17:12 +0300
      Re: clock  synchronization over IP Robert Wessel <robertwessel2@yahoo.com> - 2018-09-21 09:58 -0500
        Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-21 16:26 +0100
      Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-21 16:02 +0100
    Re: clock  synchronization over IP George Neuner <gneuner2@comcast.net> - 2018-09-21 17:15 -0400
      Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-21 22:35 +0100
        Re: clock  synchronization over IP "gp.kiwi@gmail.com" <graeme.prentice@gmail.com> - 2018-09-21 15:16 -0700
          Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-22 00:11 +0100
            Re: clock  synchronization over IP "gp.kiwi" <graeme.prentice@gmail.com> - 2018-09-21 16:35 -0700
              Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-22 14:19 +0100
              Re: clock  synchronization over IP gnuarm.deletethisbit@gmail.com - 2018-09-22 07:22 -0700
            Re: clock  synchronization over IP graeme.prentice@gmail.com - 2018-09-21 19:40 -0700
        Re: clock  synchronization over IP George Neuner <gneuner2@comcast.net> - 2018-09-21 22:17 -0400
          Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-22 13:58 +0100
          Re: clock synchronization over IP Brett <ggtgp@yahoo.com> - 2018-09-22 18:50 +0000
    Re: clock  synchronization over IP upsidedown@downunder.com - 2018-09-22 11:56 +0300
      Re: clock  synchronization over IP gnuarm.deletethisbit@gmail.com - 2018-09-22 07:31 -0700
        Re: clock  synchronization over IP upsidedown@downunder.com - 2018-09-22 20:02 +0300
          Re: clock  synchronization over IP gnuarm.deletethisbit@gmail.com - 2018-09-22 14:40 -0700
    Re: clock  synchronization over IP gnuarm.deletethisbit@gmail.com - 2018-09-22 07:06 -0700
    Re: clock  synchronization over IP Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> - 2018-09-22 20:16 +0300
      Re: clock  synchronization over IP Chris <xxx.syseng.yyy@gfsys.co.uk> - 2018-09-22 22:22 +0100

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