Rev 1                    AVAILABILITY OF SEVENTY
                 9600 BAUD PACKET CHANNELS ON TWO METERS
                      Bob Bruninga WB4APR @ WB3V.MD

     Unbelievable?  Not really!  With the advent of the latest 9600 baud
packet radio modems, there is an un-exploited mechanism for opening up
dozens of half duplex data channels without ANY impact on existing voice and
data bandplans.  Read this proposal thoroughly before jumping to any
conclusions.  First you must consider two apparently un-related facts:

TINY PACKETS:  With the new 9600 baud packet modems, not only is there
potential for higher speeds, but the packets are also 8 times shorter than
conventional packets.  This means that each packet occupies the channel for
less than 1/10 of a second at a time.  If another signal appears on the data
channel, then the presence of the new signal can be detected in 0.1 second.

AVAILABLE CHANNELS:  Throughout the two meter band, in every corner of the
country, there are almost 70 FM channels assigned to single VOICE receivers
that, in general, only use their single frequency about 1% to 40% of the
time.  If a mechanism could be designed to permit these single receivers to
continue to use their frequencies on a primary basis, at any time, with
priority access and control, then the rest of the time (60% to 99%) they
could use their channel for moving digitial data.  This mechanism could more
than double the bandwidth available in the two meter band!  (I used 2 meters
only to get your attention.  One hundred twenty channels on the 440 band is
the actual target because of the availability of 2 Watt 9600 baud UHF data
radios for under $150 each)

VOICE/DATA CHANNEL SHARING:  In the past, attempts to share voice and packet
have all failed, not because it is a bad idea, but because it has not been
done properly.  Under a very unique set of conditions, however, voice and
data can easily share a narrowband FM channel, IF:

    1)  VOICE has priority at ALL TIMES and all voice operations
        are completely transparent to the voice user
    2)  DATA can never interrupt or attempt to use a busy channel
    3)  VOICE users can pre-empt/interrupt data instantly at ANY time
    4)  VOICE users do NOT hear packets or in any way are
        encumbered by shared use.
    5)  NO MODIFICATIONS TO ANY voice radio is required

In other words, voice users do not even know that the channel is shared with
data.  To make data use of a channel work on a secondary basis (behind the
scenes) with NO impact on voice usage, there are additional requirements:

    1)  There must be ONLY one voice RECEIVER on the channel, and it MUST
        be able to hear EVERY voice user on the channel.
    2)  This single voice receiver and its conventional COR or un-modified
        squelch circuit must have total control over channel use.
        
THE BEST KEPT SECRET:  Think for a moment about the input frequency of a
typical voice repeater.  There is only one receiver listening, and it can
hear EVERYONE that desires to use the repeater.  If the repeater RECEIVER
does not hear anyone using the channel, then IT alone can decide to use the
channel itself for data!  Now if the data is transmitted in 0.1 second
bursts, FROM THIS SITE (on the input channel), with a pause in-between to
listen for voice users, then no one will be denied access to the voice
repeater for any longer than 0.1 second!  Also, while the repeater is
transmitting data on its input frequency, nothing is being transmitted to
anoy users on the output!  In this manner, we can TRANSMIT data FROM this
repeater site at 9600 baud on the input frequency WHENEVER THE REPEATER IS
NOT OTHERWISE BEING USED FOR VOICE.

MAKING A DATA LINK:  Now combine one such voice repeater with another
operating in the same manner, and you have a two-way data link between these
two sites.  Not only is this a FULL DUPLEX channel, but it also operates
with NO HIDDEN transmitters, and NO CONTENTION, because there is only ONE
transmitter on each such channel.  From a data perspective, each repeater
site is a data node that transmits on ONE assigned frequency and can LISTEN
on as many additional channels as desired.  This is an ideal multi-node
backbone network for passing traffic point-to-point over long distances!

HARDWARE:  To avoid even momentary delays during normal repeater
conversations, the external carrier detect of the modem is not only driven
by the COR of the repeater receiver, but also by the HANG-TIME of the
repeater transmitter as an indication that the repeater is engaged in a
conversation.   This way, data will only be transferred after the repeater
has been unused by voice users for a while.  

FORWARDING:  Obviously, this type of channel will NOT support any traffic
where an impatient human is on one end.  Also, only repeater-site-to-
repeater-site links are possible, because of the necessity for the COR of
the repeater voice receiver.  This channel, however, is ideal for off-hour
BULK forwarding between multi-channel level-4 nodes under computer control.

BAUDRATE:  Although I suggested off the shelf 9600 baud radios, the use of
15 KHz repeater channels in most areas of the country might require slowing
to 4800 baud to be certain that all energy is contained within 12 KHz. 
Also, the $139 TEKK 2 watt UHF data radio was actually designed commercially
for 4800 baud, and performs excsllently at that rate.  Also, to avoid co-
channel interference, operating at power levels above 2 watts is probably
not advisable.  Fortunately, 9600 baud data sounds just like white noise,
and usually will NOT open the squelch of another repeater anyway.  Finally,
to experiment with this concept, you can actually try conventional 1200 baud
during off hours.  You can limit packets to a single frame (about .8 second
instead of the usual 3 to 5 seconds), by setting MAXFRAME to 1 instead of
the default 7.  By adding an additional hang-time timer, so that the DATA
mode is only activated when the repeater is unused for over 3 minutes, a new
voice in-the-night will probably not even notice a maximum of 0.8 second
delay.   Comments?