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[aprssig] More efficient use of channel capicity through shorterpackets

scott at opentrac.org scott at opentrac.org
Thu Oct 12 13:38:52 UTC 2006


And after TXD, you've got a minimum of 20 bytes of data - flag (1),
desination (7), source (7), control (1), pid (1), fcs (2), and flag (1).
Each byte is 8 bits, though bit stuffing can increase the number of bits
sent.  Ignoring bit stuffing, 20 * 8 = 160.  160 / 1200 bps = 133.3 msec.  I
think the tail is hardcoded as two extra flag characters on the OpenTracker.
Never had any trouble with that, at least not on VHF.

Also, I think you'll find that most ham rigs are slower to key up than
commercial mobiles.  Some are really bad, at 200 msec or more.  Though I
still have no idea what possessed Kenwood to hard-code the TXD at half a
second on the TM-D700 - I know of no radio that REQUIRES such a long TXD.
Unless maybe you're trying to catch a receiver in power save mode or
something...

Scott
N1VG

> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org 
> [mailto:aprssig-bounces at lists.tapr.org] On Behalf Of Mark Earle
> Sent: Thursday, October 12, 2006 6:04 AM
> To: ron.stordahl at digikey.com; TAPR APRS Mailing List
> Subject: Re: [aprssig] More efficient use of channel capicity 
> through shorterpackets
> 
> Ron Stordahl * wrote:
> >
> > I realize there is both a TXD (at the head of the 
> transmission) and a 
> > TXTail (at the end).  Between these is data as required by the 
> > specification.  Part of that data is of fixed length and other 
> > variable length.  For 
> 
> TXD is two things: Enough time for the transmitter to key up 
> and make RF 
> (the time between asserting PTT, and having full, stable 
> output). Part 
> two is, how long it takes the slowest, worst-case, RECEIVER 
> in range, to 
> open it's squelch, and not "chop off" part of the packet on 
> the way to 
> the attached TNC.
> 
> You can use crystal controlled, PIN DIODE switch transmitters 
> for fast 
> ptt to rf out, but the slowest RECEIVER hampers you. Especially true 
> where folks use squelched audio, as they do at 1200 in many 
> cases. Using 
> discriminator audio eliminates the squelch system delay. All 
> receivers, 
> though, require SOME time to start acting on an incoming 
> signal. It can 
> be close to zero, but there is some delay.
> 
> So TXD is a compromise for both transmitters (which you can 
> control at 
> your site or station) and receivers, which you have no control over.
> 
> -- 
>       ) )    de WA2MCT Mark
>      ( (     Echolink 99190      Grid Square EL17HQ
>       ) )    You will be assimilated... oooh, coffee!!
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