[aprssig] More efficient use of channel capicity through shorterpackets
scott at opentrac.org
scott at opentrac.org
Thu Oct 12 08:38:52 CDT 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
> -----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
> 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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