[aprssig] ARPS physical layer

[Pentti Gronlund]

Derek Love writes:

> Thanks for the info guys, maybe I should be a bit more specific.
> 
> I have the APRS 2.0 spec, I know about HDLC, NRZI, byte reversal, bit
> stuffing, CRC/FCS generation and stop/start (7E) flags.
> 
> However, I come from a PMR / Data radio environment, so my first questions
> are:
> What's the preamble sequence and length?
> What's the sync sequence and length?

Derek,

This is where I went, too, but packet radio was already there. Had been for
many years :)

PMR specs like MPT1327/1343 tend to specify the turn-around time, so they can 
also give exact specifications for preamble and sync sequences.

This wasn't the case with packet radio in the 80's. Our digipeater was based
on a late-60's Salora, and the mailbox was using a Regency from the same era. 
Both were using large coil relays for TX/RX changeover and were notoriously 
slow in both directions.

So there had to be a possibility to adjust the preamble length "TXDELAY". Many
APRS stations still adhere to standards of those days without any reason.

Benjamin OH3BK
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