[OLPC-devel] Re: [One Laptop Per Child] #14: Time synchronization

[Zarro Boogs per Child]

#14: Time synchronization
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 Reporter:  jg      |        Owner:  ksankar   
     Type:  task    |       Status:  assigned  
 Priority:  normal  |    Milestone:  rev1 final
Component:  distro  |   Resolution:            
 Keywords:          |  
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Comment (by ksankar):

 Replying to [comment:4 krstic]:
 > No, permanent connectivity may certainly not be assumed.
 * No GPS assumed. Actually we do not need that kind of accuracy anyway
 * NTP through intermittent internet connection (or a server which is
 connected to the internet) is assumed, to setup the time whenever
 possible/needed
 * We can also ask the kid “Looks like I have lost my time, can you tell me
 what time it is” to get the current time.
 * Should provide a sync button in configuration settings somewhere – to
 sync with NTP – either with a school machine or with NTP via internet
 * Should provide distributed precision time sync API for applications that
 require precision time sync
 * We should not wake up just to sync and also not waste too much
 bandwidth, energy for time sync. So will be very conservative and if apps
 require time sync, they can call the API. I assume the apps that require
 precision time sync most probably will be using the network anyway and so
 a few more sync messages will not be that bad on energy consumption
 * Sync & accuracy two dimensions, we will provide both primitives
  * Sync thru IEEE 1588 and accuracy thru NTP or user input
 * Accuracy budget
  * Milliseconds – possible
  * Micro seconds – probable
  * Nano seconds – impossible
 * Need to experiment and document the accuracies and sync we can achieve,
 especially with the mesh network primitives
  * I will do that
 * When sync in a mesh, will assume one of the machines as master
 * Will use some algorithm to select the master clock (say the one with the
 highest IP) and all in that mesh will sync with that
 * Will use wisdom of crowds for sync i.e. if 5 machines show 3:00 PM and
 one shows 12:00 AM, we assume that one is wrong. It is possible to arrive
 at wrong time by this, for example kids from out of state visit a friend.

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Ticket URL: <http://dev.laptop.org/ticket/14#comment:5>
One Laptop Per Child <http://laptop.org/>