

        ================================================================

               D A S (DTMF ACCESSORY SQUELCH) APPLICATION NOTE 2

                            SELECTIVE GROUP CALLING

                                  951105/Iss-1
        ================================================================


                               Paul Newland, ad7i
                              Post Office Box 205
                               Holmdel, NJ  07733
                                 ad7i@tapr.org


           ----------------------------------------------------------
           |        Copyright (c) 1995 by Paul Newland, P.E.        |
           |   All Rights Reserved.  This guide may be reproduced   |
           | by radio amateurs for their own personal use, provided |
           | this copyright notice is included in any reproduction. |
           ----------------------------------------------------------

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      INTRODUCTION
      ------------------------------------
        DAS (DTMF Accessory Squelch) acts like a switch connected in
        series between the speaker output of your VHF or UHF transceiver
        and an external speaker.  DAS will monitor a radio channel for
        you, with the speaker switch open so the speaker remains silent,
        until someone sends one of the DTMF sequences you have selected.
        When DAS hears your your personal Touch-Tone ID sequence on the
        radio channel it will light a LED, sound an buzzer and close the
        series speaker switch so that you can hear the audio of the
        calling station.

        An introductory discussion of DAS was published in //QST//,
        December, 1995, pages 25-31.  The "DAS Configuration Reference
        Guide", available from the TAPR FTP file server as well as the
        ARRL BBS//1, provides a more complete description of each
        programming option, the purpose of the option and how the various
        options may interact.

        This document, DAS Application Note 2, "Selective Group Calling",
        describes how select PER ID sequences for various stations that



      DAS-AN2: SELECTIVE GRP CALLING  -2-                     951105/Iss-1


        might be used in some sort of network, with DAS serving as the
        selective calling decoder.


      WILD CARD CHARACTER
      ------------------------------------
        Most hams are now familiar with PCs and computer operating
        systems.  Many of these systems allow the use of star (*) and
        query (?) as wild card characters.  In computer operating systems
        * is often used to represent one or more positions of any symbol
        while ? is often used to represent just one position of any
        symbol.

        For example, consider a computer file system that contained the
        six files:  DOG DOGBARK, DOGBONE, DOGHAIR, DOGGONE & DOGPARK.
        Issuing the command "SELECT DOG*" would select all six files.
        Issuing the command "SELECT DOGB*" would select only DOGBARK and
        DOGBONE.  Issuing the command "SELECT DOG?" would select no files
        while issuing the command "SELECT DOG?ARK" would select only
        DOGBARK and DOGPARK.

        In these computer operating system cases ? means the system will
        accept only one character at the site of the ? but that character
        can be any symbol, while * means the system will accept any number
        of characters at the site of the * and those characters can be any
        symbol.

        In the DAS system, the * symbol can serve as a wild card symbol in
        the same way that the ? symbols serves as a wild card for computer
        operating systems -- it represents a single character position of
        any value.  DAS does NOT consider a DTMF * to be like the wild
        card * as used in a computer system.

        For example, if a DAS decoder was set up to make use of Wild Cards
        (STAR IS WILD CARD CHARACTER set to Active) then a decoder
        programmed for a sequence of "123456" will respond to "12*456",
        "123**6", "1***56", or even "******".  However, it will not
        respond to "123*", or "1*", etc.


      SELECTING ID SEQUENCES
      ------------------------------------
        When selecting ID sequences for selective group calling
        applications, it's important to determine the different dimensions
        you may want to use for selective group calling.  For example,
        geographical location is one example.  Perhaps you have stations
        in several townships or counties.  Further, you might want to
        divide each of of those areas into sub areas (like a tic-tac-toe
        board: NW, NC, NE, WC, CC, EC, SW, SC, SE, where NW stands for
        northwest, etc., and C stands for central).

        Another dimension might be functional purpose of station.  In the
        case of communications backup or auxiliary order-wire of a RACES
        group, there might be shadow stations at law enforcement



      DAS-AN2: SELECTIVE GRP CALLING  -3-                     951105/Iss-1


        locations, fire fighter stations, paramedic stations,
        transportation depots, material/supply depots, etc.

        Yet another dimension might be type of station.  You might have
        fixed stations, portable stations (mobile trailers that can be
        placed for several days but could be moved on 30 minutes notice),
        automobile stations, highly mobile stations (motorcycle, bicycle)
        and even pedestrian stations.

        So, taking these factors into account, consider how a useful,
        workable system might be developed.  Below is a concrete, although
        hypothetical, example for demonstration purposes.  Consider a
        multi-township RACES network with DAS servicing as a selective
        calling device.  The selective calling feature may not always be
        needed at all stations (stations with only one communications
        channel may want to monitor all voice traffic on that channel, all
        the time) but for some stations, perhaps where one or two
        operators are responsible for operating 10 communications devices,
        selective calling will be a must in order to keep the background
        "din" to a reasonable and workable level.  PER ID sequences for
        stations on this RACES net will be constructed as follows:

        The first digit is always 2 (an arbitrary choice) to differentiate
        this system from any others (which might use 3-8 is the first
        digit of their sequences).  The second digit will be 1-5 and will
        designate township.  The third digit will be 1-9 and will
        designate location within the township by means of 9 sector
        (tic-tac-toe) grid.  The fourth digit will be 1-9 and will
        indicate the function performed by that station's location.  The
        fifth and final digit will be 1-6 and will indicate the type of
        station.

        Putting this all together, assign digits as follows:

        Second Digit, Townships:

             Name            Symbol
             --------        ------
             Washington        1
             Lincoln           2
             Hamilton          3
             Jackson           4
             Franklin          5

        Third Digit, Sectors within Townships:












      DAS-AN2: SELECTIVE GRP CALLING  -4-                     951105/Iss-1


             Name            Symbol
             --------        ------
             North-West        1                 |      |
             North-Central     2            NW-1 | NC-2 | NE-3
             North-East        3            ------------------
             West-Central      4                 |      |
             Central-Central   5            WC-4 | CC-5 | EC-6
             East-Central      6                 |      |
             South-West        7            ------------------
             South-Central     8            SW-7 | SC-8 | SE-9
             South-East        9                 |      |

        Fourth Digit, Functional Operation:

             Name            Symbol
             --------        ------
             Law Enforcement   1
             Fire Fighters     2
             Paramedics        3
             Materials &
               Supplies        4
             Shelter           5
             Transportation    6
             Communications    7

        Fifth Digit, Mobility of Station

             Name            Symbol
             --------        -------
             Fixed             1
             Portable          2
             Automobile        3
             Motorcycle        4
             Bicycle           5
             Pedestrian        6

        Now for actual sequence assignments.

        For the station covering the fire department in Hamilton township
        in the east-central sector by means of a ham operating out of his
        car in the parking lot, that station would be assigned PER ID
        sequence 23623.

        For the station covering the City of Lincoln shelter in the south
        east sector that is using an HT and was dropped there by the
        police (so he's on foot, at best), that station would be assigned
        PER ID sequence 22756.

        There are a couple of things to note at this point.  First, note
        that there is a good possibility of repeated digits.  Don't worry
        about that -- the problems caused by repeated digits, if any, are
        infrequent problems.  Second, note that under these assignment
        tables it's possible that two or more stations could be assigned
        the same code.  This isn't a problem either.  The goal is not to



      DAS-AN2: SELECTIVE GRP CALLING  -5-                     951105/Iss-1


        ensure that everyone has a unique ID sequence -- the goal is to
        allow any station to open up groups of speakers with just one
        command sequence.  It's better to have a stations SELCAL device be
        activated, determine by monitoring that the call is not of
        interest and press the CLR button than it is to miss a call.  It's
        better to skew the system performance toward false positive
        detection than toward missing calls altogether.


      CONFIGURING DAS FOR WILD CARD
      ------------------------------------
        First and foremost, any DAS decoder that's used as part of this
        selective group calling system must have the STAR IS WILD CARD
        CHARACTER option set to ACTIVE.  To do this, press and hold the
        CLR button until the STA lamp is winking.  This indicates that you
        are in the CONFIG mode.  Send DTMF "*4041#".  Observe the command
        accepted flash code (GRP lamp flashing).  Press and release CLR.
        You have now configured DAS to accept the * (star) as a wild card
        character.


      OPERATING THE SYSTEM
      ------------------------------------
        Net control would have a list of the PER ID sequences used for
        each station.  To send a bulletin to several stations net control
        could send the PER ID sequence for each station, voice the
        bulletin and then as for a QSL from each station.

        In the group calling scenario only one DTMF sequence would be
        needed.  For example, let's say that there has been an earthquake
        in Hamilton.  Standard telecommunications facilities are out of
        service or overloaded.  The local RACES group has been called out
        and are on site at their respective posts.  The water department
        has notified authorities that water pressure to fire hydrants in
        the northern sectors of Hamilton is likely to be interrupted for
        several hours.  This will affect the ability of fire fighters to
        contend with fire.  Net control has been asked for forward a
        bulletin to all fire stations.

        In this hypothetical situation, net control needs to contact all
        stations shadowing fire fighters in all sectors of Hamilton
        (central and southern sectors need to have this information
        because they could be called north to help with fire fighting
        because of low water resources).

        Net control could send the DTMF code for every station shadowing
        fire fighters to deliver this message, but that might take several
        minutes of air time.  Net control could also use LiTZ, but that
        would wake up everyone and most of the stations monitoring the
        channel don't need to be interrupted from their non-radio duties
        to determine that they don't need to copy this bulletin.  Instead,
        net control can determine the group call sequence needed to get
        all stations shadowing fire fighters.  To do this, form the
        sequence digit by digit.  The first digit is hard coded at "2".



      DAS-AN2: SELECTIVE GRP CALLING  -6-                     951105/Iss-1


        The second digit is township and we only want Hamilton so the
        second digit would be set to "3".  The third digit is sector and
        we want all sectors so set that to "*" (star) so that all sectors
        will be selected.  The fourth digit is fire fighters and that's
        all we want so that digit would be "2".  The fifth digit is
        station mobility and we don't care about that so that digit would
        be "*" so that all types will be selected.

        Thus, the group sequence for this transmission would be "#23*2*#".
        Use the # to start the sequence to ensure that all decoders are
        ready to copy.  This is only important if a decoder may have heard
        a DTMF signal in the last 2 seconds (it's not likely, but it's
        possible).  End the sequence with # so that the decoder will check
        the data immediately, and not wait for the 2 second timeout.  This
        avoids the problem of someone else accidently sending DTMF right
        after you finish your transmission and causing the decoders to get
        confused.

        After sending the DTMF code you should do whatever your standard
        procedure requires for delivering a bulletin to many stations.
        That might be a roll call before sending the bulletin, send the
        bulletin and then collect a "roger" from each station.  Another
        alternative would be to simply tell everyone to get ready to copy,
        wait 10 seconds, send the bulletin and then collect the individual
        "rogers", providing fills where needed.

        Consider the case where you wanted to call all shadows at law
        enforcement as well as the fire fighters.  In this case you would
        need to send two DTMF sequences.  The first would be "#23*2*#" for
        the fire fighters and the second would be "#23*1*#" for law
        enforcement.  Of course, these could be combined into one
        concatenated sequence as "#23*2*#23*1*#".


      SEMI-REMOTE CONFIGURATION
      ------------------------------------
        Although DAS provides for remote configuration, there is another
        concept called semi-remote configuration that can be useful in
        RACES/AREAS situations.  In semi-remote configuration the DAS unit
        is placed in the CONFIG mode by the local operator pressing and
        holding the CLR button.  However, all configuration commands are
        given by another remote station over the air.

        If at any time there is a question regarding the configuration of
        particular DAS unit in the field the net control station (or any
        knowledgeable user on channel, with the approval of net control)
        should re-configure the unit himself (or herself).  To do this,
        net control would instruct the operator at the site with the
        questionable DAS unit to set the monitor switch to the MON
        position (with the monitor switch set to MON no matter how DAS is
        configured the operator at that site can continue to receive
        verbal instructions from net control).  Next, net control tells
        the operator to press the CLR button for 5 seconds and then
        release the button.  After that net control confirms by voice with



      DAS-AN2: SELECTIVE GRP CALLING  -7-                     951105/Iss-1


        the operator that the STA LED is winking.  This confirms (well,
        98% likely) that the DAS microcomputer is operating correctly and
        that the unit is now in the CONFIG mode.  Net control can now
        issue the configuration command to set the unit back to defaults
        (*30*) and have the operator confirm that the STA LED flashed
        several times (command accepted flash code).  Next, net control
        configures the PER ID sequence (*nnnnnn#) and confirms with the
        local operator.  Next, net control sets the STAR IS WILD CARD
        CHARACTER option ACTIVE by sending "*4041#" and confirms with the
        local operator.  Next, net control exits the CONFIG mode by
        sending the exit command (*0#) and confirms with the local
        operator.  Finally, net control can send several PER ID sequence
        and have the operator confirm that the PER LED is flashing and
        that the horn sounded.  Do this again but use a "*" at any
        position in the PER ID sequence to confirm that STAR IS WILD CARD
        CHARACTER option is ACTIVE (if * works one place in the PER ID
        sequence it's 99.99% likely it will work at any and all positions
        of the PER ID sequence).


      CONCLUSION
      ------------------------------------
        My examples are strictly hypothetical and may not be useful in
        your application.  However, I think they do serve to demonstrate
        the utility of using DAS for selective group calling.


      NOTES
      ------------------------------------
        1.   TAPR FTP File Server ftp.tapr.org, look in /tapr/das.  TAPR
             on the Web at "http://www.tapr.org".  ARRL BBS, +1 860 594
             0306, 8-N-1, look in the FILES area, search on DAS.


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