Re: Digest Number 5174

Hi,

 Before most of you were born I used a LAN product, called Planet, by RACAL
to connect 2 mainframes and a few minicomputers to a Cambridge Ring fibre
optic network of a couple of hundred terminals.  The problem you face is
similar to the one that Planet had to overcome in that it was possible for
"packets" from two stations could collide on the network. The planet system
implemented a system called "exponential back off" . Now I am pretty rusty
now however I seem to recall it worked a bit like this. The system
monitored the time it took from a packet to be transmitted to the receipt
of an acknowledgement that the packet was received. If the system did not
receive the acknowledgement within the expected time it assumed that the
packet was corrupted. So it knew it needed to re transmit it. But it did
not do this immediately, it waited a little before sending it again. If the
system did not receive an acknowledgement, it waited a longer time before
retransmitting it. And so on. There would come a point at which the system
would decide that the "receiver" was not there, record some sort of error
message and give up.

Given that this technology was used in the mid 1980s, when micro
controllers were not really around and microprocessors were mostly 8 bit
with limited capabilities compared with what we have now I am pretty sure
this would be quite easy to implement on an MSP430 system.

Hope this helps,

Steve G0XAR

On 10 June 2015 at 10:00, <[email protected]> wrote:

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>    1 Message
> Digest #5174
>      1a
>  Re: Random number generator in assembler <#14ddcb332202c9e7_1a> by
> "Peter Grey" petertgrey
>
>   Message
>  1a   Re: Random number generator in assembler
> <https://groups.yahoo.com/neo/groups/msp430/conversations/topics/52414;_ylc=X3oDMTJyNTM3ZzhpBF9TAzk3MzU5NzE1BGdycElkAzIzNDI2MjkEZ3Jwc3BJZAMxNzA1MDA1Mzc4BG1zZ0lkAzUyNDE0BHNlYwNkbXNnBHNsawN2bXNnBHN0aW1lAzE0MzM5MjY4Mzc->
>   Tue Jun 9, 2015 6:38 pm (PDT) . Posted by:   "Peter Grey" petertgrey
> <[email protected]?subject=Re%3A%20Random%20number%20generator%20in%20assembler>
>   Hi David
>
> Thanks for the very informed reply. I had decided to use the ID to encode a
> send time. Lucky for me I do not have a busy network. I am also looking at
> the RSSI prior to a send to avoid possible collisions. In another project I
> may get the RF traffic will increase so I need to do a little more in
> relation to collisions. I will take a better look at the hash function you
> mentioned.
> I have volume I & II of Knuth. I agree it is not light reading but very
> interesting to see how he does things.
>
> Thanks
>
> Peter
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> Sent: Tuesday, 9 June 2015 4:15 PM
> To: [email protected]
> Subject: Re: [msp430] Random number generator in assembler
>
> Hi,
>
> Knuth is the ultimate reference for many algorithms, but The Art of
> Computer
> Programming is not light reading. If you are not heavily into theoretical
> computing, don't bother trying to use them as sources for something like
> this.
>
> What you have to do is think about the /real/ problem, not jump to a
> solution and try to implement it. You don't need a random number generator
> - so trying to make something that will generate good quality high entropy
> pseudo-random numbers will mean extra effort and extra risk of getting
> things wrong.
>
> You have a bunch of nodes that are going to send sometime between 1200 and
> 1300. That's a period of 3600 seconds. Each has a unique node number
> between 1 and 65000. If the radio transmission takes less than
> 50 ms, then your solution is easy - wait (id * 50 ms) before sending.
> That will also guarantee that there won't be collisions, which would
> otherwise always be a risk with random delays. Case closed.
>
> If send times are longer than 50 ms, there will always be a risk of
> collisions. You will need some way to spread out the nodes amongst
> appropriately timed slots - but it does not need to be random. In fact, it
> is better if it is /not/ random. Use a hash function on the ID with an
> initial fixed salt value. If two nodes collide, they should each pick a
> different salt value - they will not collide again. (In theory, they could
> now collide with other nodes - there is no limit to the maximum length of
> such collision chains, but in practice the likelihood of collision
> decreases
> at each step unless you have too many nodes.) Once the nodes have settled
> on
> their slot numbers, stick to them.
>
> An easy hash function is something like crc16(idLo ++ idHi ++ saltLo ++
> saltHi), since there's a fair chance you already have code for a crc
> calculation for adding checksums to your telegrams.
>
> mvh.,
>
> David
>
> On 09/06/15 03:28, 'Peter Grey' [email protected] [msp430] wrote:
> >
> >
> > Thank you to all who replied. It is appreciated. I am going to have a
> > look at SLAA338 and I found I have a few books by Donald Knuth with
> > references to random number generation. It will keep me busy over a
> > few evenings!
> >
> >
> >
> > Peter
> >
> >
> >
> > *From:*[email protected] [mailto:[email protected]]
> > *Sent:* Tuesday, 9 June 2015 1:42 AM
> > *To:* [email protected]
> > *S ubject:* Re: [msp430] Random number generator in assembler
> >
> >
> >
> >
> >
> > XOR-SHIFT is one technique, wikipedia describes it .
> >
> > Brian
> >
> > 'Peter Grey' [email protected] <mailto:[email protected]>
> > [msp430] wrote:
> >
> >
> >
> > Does anyone know of a simple way of randomising some data in
> > assembler. I want to send a RF signal between noon and 1300 hours.
> > There are quite a few transmitters and I do not want them to all
> > send off at once. Each has a unique ID numbered from 1-65000.
> >
> >
> >
> > TIA
> >
> >
> >
> > Peter< o>
> >
> >
> >
> > Peter Grey
> >
> >
> >
>
> ------------------------------------
> Posted by: David Brown <[email protected]>
> ------------------------------------
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