> It's just a separate module. Each machine has an application
> called LocalHub which uses Multicast to communicate with
> MasterHub. When an update is required ( a train leaves a
> station for instance ) that is sent as a message from a
> LocalHub to MasterHub, which relays it to all the other
> LocalHub's (It's a frame relay thing - the MasterHub has the
> wide bandwidth link :) ). If that train is going through the
> station of a receiving LocalHub, the LocalHub updates that
> record in a local copy of the database. Simple replication,
> and so only required refreshes go over the wire, so the
> whole thing works with just 64kBaud links at each station.
> In the gaps each hour, LocalHub downloads another hours
> worth of train movements and the whole thing just keeps
> rolling.
>
> All the local applications work into the local database, and
> talk to LocalHub, which relays the messages. The activity to
> LocalHub is also used to update the network status, so the
> control centre knows what is going on. If a link is lost,
> the local displays can be maintained, using a simple timed
> depart, and the database just gets rebuilt when the
> connection is restored.
>
> (p.s. while we had to pay for licences the slaves were
> simple dBase engines - with Firebird that ceased to be a
> problem ;) )
>
> --
> Lester Caine
> -----------------------------
> L.S.Caine Electronic Services

Sounds very nice Lester.. (hope they don't rely on this for signalling :-) )
Alan