> -----Original Message-----
> From: Paul Koning [SMTP:[EMAIL PROTECTED]]
> Sent: Tuesday, November 23, 1999 11:31 AM
> To: [EMAIL PROTECTED]
> Cc: [EMAIL PROTECTED]; [EMAIL PROTECTED];
> [EMAIL PROTECTED]; [EMAIL PROTECTED];
> [EMAIL PROTECTED]
> Subject: Re: [rtl] Networking With RT-Linux
>
> >>>>> "Kulwinder" == Kulwinder Atwal <[EMAIL PROTECTED]> writes:
>
> Kulwinder> What can be done is that real time networking protocols
> Kulwinder> can be added. ... Where deterministic behavior has to
> Kulwinder> be guaranteed and there is no room for error.
>
> Those are dangerous words, because they are often associated with a
> nasty but common misconception.
>
> ALL networks have non-zero error rates.
[BIG SNIP]
[Norm Says:]
Well,,, Not quite.
There are a large class of networks that are designed to have
"errors", networks like Ethernet that work by collision-detection.
But there are a larger class of networks that have "no errors"
during normal operation, for example networks that work on master-slave
protocols and token-ring networks. {Also MIL-1553x MUXBUS networks which
I'm heavily involved with}
The consideration of whether master-slave networks are
"deterministic" or not depends on your point of view, i.e. whether you're
the master or the slave.
In a MIL-1553x network, the Bus Controller (the "master") usually
operates on a fixed schedule of messages (with time-slots for exceptional
messages) and can assume that any particular scheduled message will be
transmitted on the bus during it's allocated time-slot. In addition, there
are fairly rigid timing constraints for data transmission and status reply
so that it's quite possible to compute an exact time interval within which
the message will be on the bus.
GPIB is also master-slave, but not deterministic by design because
the timing restrictions on the "talker" or listener" (the slaves) is
significantly looser (millisecond rather than microseconds) but still there
is a computable maximum time interval (ignoring the complication that a
talker-slave can transmit as much data as it wants on the bus).
Token-ring networks are inherently non-deterministic (within sloppy
limits, I suppose, depending on the specific network protocols governing how
long messages can be and how many messages a station can transmit before
passing the token to the next station),
But there is assumed to be (under normal operation) no errors during
network operation because there is (again under normal operation) no
possibility for collision.
Norm
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